1 Introduction
In recent years, organic light-emitting materials have been widely used in various fields such as biological detection, sensing and optoelectronic devices
[1⇓~3]. However, most luminescent materials suffer from aggregation-induced luminescence quenching (ACQ) due to strong π-π stacking between molecules
[4]. In general, the ACQ effect is prevalent in conventional organic light-emitting materials having a rigid framework. Since F Föster discovered the above phenomenon in 1954, the problem has puzzled scientists for more than 60 years. Take the classical pyrene molecule, which emits bright blue light in tetrahydrofuran solution. However, when a large amount of water is added, pyrene molecules form exciplexes with strong π-π interactions, resulting in luminescence quenching, which is extremely unfavorable for practical applications. In order to overcome the problem of ACQ, scientists have adopted a series of physical and chemical strategies to inhibit the aggregation of luminescent molecules, but these methods are contrary to the spontaneous aggregation of molecules when they are concentrated, so they get half the result with twice the effort.
In 2001, Tang Benzhong's group of Hong Kong University of Science and Technology accidentally discovered the phenomenon that the luminescence of polyphenyl substituted Silole derivatives was enhanced when they were aggregated
[5]. Unlike pyrene, which has a planar structure, these molecules have a very twisted propeller configuration. In solution, the peripheral benzene ring can rotate around the central thiole ring, so the energy of the excited state is dissipated in the form of non-radiative transition, so that the molecule does not emit light. When aggregated, the intramolecular rotation is restricted by means of intermolecular and hydrophobic interactions, resulting in strong emission. Because the above luminescence phenomena are induced by aggregation, Professor Tang Benzhong first proposed the scientific concept of "aggregation-induced luminescence" (AIE). Later, on the basis of a large number of experiments and theories, Tang Benzhong's team proposed the working mechanism of AIE of intramolecular motion limitation (RIM)
[6]. As a result, the problem of ACQ has been fundamentally solved. So far, according to the working mechanism of AIE, researchers have reported a number of AIE molecular systems. In addition, they can now obtain AIE materials with specific properties through simple design according to problem orientation and application needs, which have been widely used in organic light-emitting diodes (OLED), bioimaging, stimulus response, sensors and optical waveguides
[7].
Because of the importance of AIE in theory and practical application, the concept of AIE has attracted the attention of scientists in different fields all over the world since it was proposed. According to the statistics of Web of Science, since 2001, both the number of published papers and the number of citations of AIE-based research have increased exponentially, which shows its research vitality and influence. Therefore, AIE has become a research hotspot in the fields of luminescent materials and photophysics, and has been listed as the second of the top ten research frontiers in the field of chemistry by the report "Research Frontiers 2015" jointly published by the Documentation and Information Center of the Chinese Academy of Sciences and Thomson Reuters. In view of the original and leading achievements of AIE research in material design, mechanism exploration and application development, Academician Tang Benzhong's team won the first prize of the National Natural Science Award in 2017. In addition, AIE was selected by the International Union of Pure and Applied Chemistry (IUPAC) as one of the top ten emerging technologies in the field of chemistry in 2020. In view of the above outstanding achievements, Academician Tang Benzhong organized Chinese scholars active in the field of AIE to jointly write this review, aiming to systematically introduce the mechanism, derivative fields, properties and applications of AIE-related material systems to readers, and to introduce the research progress in this field in a simple way, so as to jointly promote the further development of AIE-related fields.
2 Aggregation-induced luminescent material
After 20 years of development, AIE materials have been greatly developed, and the types of AIE materials have continued to grow. The family of AIE molecules (AIEgens) grows from pure hydrocarbons to heteroatom-containing compounds, from small molecules to large molecules, from organic to inorganic or organometallic compounds, and from single components to organic co-crystals. This section aims to demonstrate several classes of AIEgens systems, and careful exploration of various new AIEgens will help to reveal the AIE process and hopefully inspire the synthesis of more AIEgens.
2.1 AIE small molecule
Small molecule systems are still the main research objects in the field of AIE, with clear structure and relatively simple chemical, electrical and photophysical properties, which provide strong support for the analysis of the relationship between structure and properties and the exploration of mechanism. For example, tetraphenylethylene (TPE) is a typical hydrocarbon AIEgen, which is often used as a skeleton to construct AIE compounds, and then applied in various fields
[8]. The pure hydrocarbon AIEgens are simple in structure and easy to synthesize, and can be used as an excellent carrier for AIE mechanism research. However, the category of pure hydrocarbon systems is limited, and the inclusion of heteroatoms into AIEgens will enrich the category of AIE systems and expand the scope of AIE research. Due to the wide variety of heteroatoms, the generated heteroatom AIEgens have good structural diversity. Therefore, two pure hydrocarbons (9,10-distyrylanthracene and tetraphenylbenzene) and heterocyclic compounds (tetraphenylpyrazine and polyarylpyrrole) will be selected for detailed description in this section.
2.1.1 9, 10-Distyrylanthracen
9,10-distyrylanthracene (DSA) and its derivatives are a kind of typical AIE molecules, which have attracted much attention due to their high aggregated fluorescence quantum efficiency, easily controllable molecular structure and excellent optical properties. In 1991, Fabian et al reported the synthesis of DSA derivatives and their photophysical properties in solution
[9]. Subsequently, Prasad et al. Showed that the nanoaggregates of the aminostyryl-substituted DSA derivative 1 had enhanced fluorescence
[10]. In 2009, Tian et al. Synthesized a series of DSA derivatives and took the lead in the research of DSA derivatives in the field of AIE
[11]. They found that the dilute solution of DSA-like compounds was almost not luminescent, and its fluorescence quantum efficiency was less than 0.8%, while the crystals showed strong fluorescence emission, and the luminescence efficiency (Ф
F) was as high as 18.7% to 50.8%. The results show that DSA derivatives have highly distorted molecular conformations. In solution, the vibrational and rotational motion between the anthracene ring and the adjacent vinyl group in the molecule leads to a significant reduction in the radiative transition rate, resulting in a weak fluorescence emission. In the aggregated state, the molecules show strong fluorescence emission due to the tight packing and weak intermolecular interactions, which limit the intramolecular vibrational and rotational motion
[11⇓~13].
In addition, researchers have obtained DSA derivatives such as small molecules, dendrimers, oligomers and polymers with different luminescent properties by chemical modification, taking advantage of the easy modification of DSA molecular structure. More importantly, the distorted molecular structure and multiple modification sites of DSA derivatives provide large free space and abundant weak intermolecular interactions for their aggregation state.It lays the structural and material foundation for the regulation of aggregation structure, and shows broad application prospects in the fields of optoelectronic devices, intelligent response, ion detection and biosensing. We will introduce the research progress of DSA derivatives with excellent AIE characteristics from the aspects of molecular structure design, aggregation structure regulation and functional applications (Figure 1).
2.1.1.1 DSA material
(1) Small organic molecules
The DSA molecule is an organic conjugated small molecule fluorophore which is formed by connecting the 9 and 10 positions of a central anthracene ring with a peripheral benzene ring through a carbon-carbon double bond. Because the luminescence center of DSA is concentrated on the central benzene ring and the adjacent double bond part, the luminescence characteristics of DSA molecules can be regulated and controlled by modifying the peripheral groups. For example, a variety of DSA derivatives 7 to 10 can be obtained by halogen substitution and different substitution positions. It is found that the crystals exhibit red-shifted emission as the atomic number of substituted halogens increases
[14]. Organic crystals with different optical properties can be obtained by adjusting the length of alkyl chains in amino-substituted DSA derivatives 11 ~ 15
[15].
By introducing fluorine atoms, it is easy to form strong intermolecular C-H … F interaction, which effectively regulates the molecular aggregation structure and is conducive to the formation of micro-nano structures with different morphologies. Tian et al. Prepared micro-nano structures based on DSA derivatives 2 and 20 ~ 22 by reprecipitation method, and obtained ribbon-shaped nanocrystals with strong fluorescence emission
[16].
By introducing acid and base sensitive groups (such as hydroxyl, amino, etc.), derivatives 16 and 17 show distinct luminescent properties in different pH solutions, which can be applied to fluorescent pH sensing
[17].
By introducing water-soluble functional groups or specific recognition groups and utilizing the unique AIE luminescence characteristics of DSA, it can realize the "lighting" non-labeled detection of a variety of ions and biological macromolecules
[18⇓~20]. For example, a thymine group (compound 26) which can selectively recognize the Hg
2+ is introduced, when the Hg
2+ exists in the solution, the Hg
2+ and the compound 26 can form a coordination complex in an aggregation form to realize lightening type fluorescence sensing
[18].
It is worth mentioning that Tian and Xu took the lead in combining DSA derivatives 27 ~ 31 with graphene oxide/carbon nanotubes to construct a highly sensitive "lighting" fluorescent aptamer sensor, which realized the specific detection of DNA, single nucleotide (SNP), adenosine triphosphate (ATP) and other biomolecules
[18,21⇓⇓~24].
Taking advantage of the easy modification of DSA, Tian and Xu et al. Reported that oligomers 32 ~ 34 with AIE characteristics, the rigidity of molecular structure and the increase of steric hindrance affect the freedom of movement of the central double bond, and the molecules show different fluorescence characteristics of aggregation state
[25]. Among them, the fluorescence quantum yield (PLQY) of 33 was the highest in THF/H
2O solution with 90% water content, and it could self-assemble into efficient luminescent nanorings.
D-π-D DSA molecules 35 and 36 with phenothiazine as the electron donor group formed distorted intramolecular charge transfer (TICT) States in polar solvents, showing red-shifted emission and low fluorescence quantum yield. When a large amount of water was added, the molecules formed aggregates, which limited the intramolecular torsion and eliminated the TICT state, showing blue-shifted emission and enhanced emission
[12].
Dendrimer is a kind of macromolecule with dendritic structure. Tian and Xu et al reported a series of dendrimers AnG0, AnG1 and AnG2 with DSA as the center and triphenylamine as the peripheral group, which have the characteristics of aggregation-induced luminescence and large two-photon absorption cross section. With the increase of generation, the molecules change from AIE to aggregation-enhanced luminescence (AEE)
[26]. The fluorescence quantum yield of AnG2 nanoparticles is 85%, and the two-photon absorption cross section is as high as 5180 GM.
Tian, in collaboration with Meldrum et al., reported copolymers based on DSA derivatives37
[27]. By changing the molar ratio of AIE fluorophores in the copolymer, the fluorescence quantum yield of the copolymer can reach 13%. Subsequently, 2,2,2-trifluoroethyl methacrylate (TFEMA), a hydrophobic segment, was introduced to obtain a highly efficient luminescent copolymer 39 for bioimaging
[28]. The polymer is self-assembled into nanoparticles in aqueous solution with a fluorescence quantum yield of up to 40%, which is expected to be applied to biological fluorescence imaging and
19F nuclear magnetic resonance imaging.
In addition, the introduction of folic acid groups can obtain amphiphilic copolymers 38 and self-assemble into polymer quantum dots (Pdots)
[29]. Pdots have high fluorescence quantum efficiency (27%) and can be used for specific fluorescence imaging of HeLa cells.
2.1.1.2 The structure and function of DSA
(1) High-efficiency light-emitting crystal
The relationship between structure and properties is a fundamental scientific issue in chemistry and materials science. The luminescent properties of aggregates are closely related to the aggregate structure, and the electronic transitions of molecules are strongly dependent on the interaction and arrangement of the transition dipoles of adjacent molecules. In the typical dimer exciton model, there are mainly H-aggregation modes in which the transition dipoles are arranged in face-to-face parallel and J-aggregation modes in which the transition dipoles are arranged in head-to-tail parallel. Among them, the lowest excited state transition of H-aggregation is forbidden, which is not conducive to fluorescence emission. While the lowest excited state transition of J-aggregation is allowed, which is favorable for fluorescence emission. The theory also predicts a possible stacking mode of cross-arrangement of transition dipoles. The interaction between transition dipoles of adjacent molecules is very weak, and the transitions between the two split excited States and the ground state are optically allowed, which is most conducive to fluorescence emission.
Inspired by the butterfly structure, Tian and Xu proposed a molecular design strategy of inserting wings into rigid molecules, developed a DSA derivative BDPVA molecule (compound 24) with a "butterfly-like" structure, and obtained a cross-dipole stacking structure conducive to luminescence
[30]. As shown in Figure 3A, the molecules with two independent conformations form a 1D molecular column (crossing angle of 67 °) along the B axis in a cross-stacking mode. The distorted molecular structure and the intermolecular C — H … π interaction contribute to the formation of a cross-arrangement stacking mode of transition dipoles, which effectively avoids fluorescence quenching caused by aggregation and improves the solid-state luminous efficiency of the crystal material (60%). Moreover, the BDPVA single crystal has excellent amplified spontaneous emission (ASE) characteristics with a half-peak width of 12 nm, a maximum net gain of 69 cm
-1, and a threshold of 207μJ·cm
-2·pulse
-1.
In addition, they further developed the butterfly-like molecule BDFVA (compound 25)
[31]. As shown in fig. 3 B, the G phase of the green fluorescence forms a J-aggregate stacking mode with a tilt angle of 25.9 ° and a Ф
F of 43%. The B phase of the blue fluorescence forms a cross-dipole stacking mode with a crossing angle of 73.5 ° and a Ф
F of 60%. Compared with the G phase, the molecules in the B phase are more distorted, and the more abundant interactions can restrict the vibration and rotation of the molecules to a greater extent. The B-phase crystals with cross-arrangement of transition dipoles show excellent ASE performance, lower threshold, narrower FWHM and higher gain than the G-phase crystals, and are expected to be used in solid-state lasers.
By changing the peripheral substituents, the interaction between molecules and the packing structure in the crystal can be effectively regulated, thereby regulating the optical properties of the crystal
[32,33]. As shown in Figure 3C, the butterfly-like molecule 23 (BDTVA) crystal containing four methyl substituents shows a uniaxially oriented layer-by-layer stacking structure, which is more conducive to the propagation of light. Therefore, the crystal has excellent waveguide and polarized luminescence properties, and its loss coefficient and polarization contrast are 2.75 cm
-1 and 0.72, respectively
[34].
The molecular stacking pattern can be effectively controlled by controlling the supramolecular interaction through the co-crystal strategy, so as to realize the control of the optical performance. Tian and Xu et al. Obtained two different crystal forms of co-crystals by slow evaporation method, and the significant difference of their optical properties is closely related to the packing structure and intermolecular interaction
[35]. There is a strong π-π interaction between two adjacent molecules of C1 crystal (Ф
F=2%) with weak green emission and the formation of local H-aggregates, while the host molecules of C2(Ф
F=34%) with bright yellow emission are J-aggregated and there is no obvious strong intermolecular interaction.
(2) Dynamic control of aggregate structure
External pressure stimulation is an effective means to regulate the aggregation structure. The application of mechanical force can change the packing pattern and luminescent properties of molecules, which attracts researchers to explore the relationship between mechanical force, structure and properties, so as to develop new materials, and try to apply this smart material to the fields of fluorescent sensors, security inks, inkless printing and memory chips
[36].
In 2012, Tian et al. First reported the in-situ observation of the luminescence properties of DSA derivative BP2VA substituted by bis-ortho-pyridine under high pressure, revealing the mechanism of the change of aggregation structure and luminescence properties under pressure stimulation from the molecular level
[37]. The fluorescent color of that pow is changed from green to yellow unde anisotropic grinding; Under the stimulation of isotropic static pressure, the fluorescence color can be changed to red, and a piezochromic shift of up to 124 nm can be achieved. As shown in Figure 4A, they obtained three crystal forms with different aggregation structures by solvent diffusion method: C1 (J-aggregation), C2 (H-aggregation), C3 (face-to-face stacking dimer). Combining the single crystal structures of different aggregation States with photophysical properties, it is clarified that the transition from green to red fluorescence emission can be attributed to the enhancement of π-π interaction between adjacent anthracene planes, exciton coupling, and the increase of orbital overlap between adjacent molecules.
In addition, Xu and Chi et al. designed and synthesized a series of organic small molecules and dendrimers by introducing TPE, hexylphenothiazine, carbazole, etc. On the DSA body, and changed the molecular packing structure of the aggregated state through pressure or grinding to realize the control of the luminescent color
[38⇓~40].
By introducing an alkyl chain to the peripheral benzene ring of DSA, the length and substitution position of the alkyl chain can be changed, and the piezochromic behavior of the aggregation state can also be regulated
[41,42].
Tian and Xu et al. also conducted in-depth research on the acid stimulation response color change of DSA diffraction materials, analyzed the influence of protonation process on molecular orbital energy level and luminescence characteristics from the perspective of crystal structure, clarified the exact binding mode between proton and proton acceptor in the aggregation state, and developed a series of solid acid photochromic materials
[43⇓~45].
It is worth mentioning that asymmetric protonated crystals can be obtained by self-assembly of DSA derivatives in mixed solution with HCl, while symmetric protonated crystals can be obtained by self-assembly in mixed solution with H
2SO
4 (Fig. 4B)
[45]. The asymmetric protonated BP4VA-1H crystal shows a further red-shifted emission than the symmetric protonated BP4VA-2H crystal due to the enhancement of dipole moment and the close packing between molecules. This kind of protonated crystal provides a bridge for understanding the internal relationship between supramolecular interaction, aggregation structure and luminescence properties in different protonated States.
In order to solve the problem of photoisomerization of spiropyran (SP) molecules in the aggregated state, Tian and Xu proposed a new molecular design strategy for the construction of solid-state fluorescent optical switches. DSA and SP were linked in a non-conjugated manner through flexible ester bonds to obtain a new solid-state photostimuli-responsive material DSA-2SP
[46][47]. As shown in Fig. 4C, under the stimulation of ultraviolet light, SP was transformed into the paracyanine (MC) configuration, the initial yellow DSA-2 SP powder was transformed into purplish black, and the fluorescence color was changed from yellow to red. Its color and fluorescent color can be restored to the initial state after heating stimulation. The results show that there is an energy transfer process from DSA ligand to MC ligand between DSA-2MC molecules, which causes the fluorescence color of DSA-2MC powder to change from yellow to red after UV irradiation. In addition, the DSA ligand has a distorted molecular configuration, which creates a larger free space for the photoisomerization of SP, and further forms an intramolecular hydrogen bond with the MC ligand, which stabilizes the MC configuration and improves the stability of the ring-opening configuration. The DSA-2SP material of which the aggregation state structure and performance are regulated and controlled through light stimulation can be applied to the fields of anti-counterfeiting coatings and super-resolution imaging pigments.
Organic materials with reversible fluorescence switching in response to multiple stimuli are very rare. Due to the lack of packing structure in amorphous powders, it is an effective strategy to reveal the underlying mechanism of luminescence transition at the molecular level through the aggregation structure of different polymorphs. Tian and Xu et al. reported two polymorphic forms of BPS3A (G phase and O phase), which showed fluorescence color change under mechanical grinding, heating, and acid-base stimulation
[48]. Among them, both the J-aggregated G phase (green) and the H-aggregated O phase (orange) exhibit red-shifted emission upon mechanical grinding stimulation. Subsequently, after heating at 150 ℃ for 5 min, the ground G phase can be partially restored to the initial emission, while the ground O phase is almost converted to the initial emission of the G phase. Powder X-ray diffraction (XRD) results show that the O phase can be transformed into the G phase by grinding and heating. Upon fumigation with HCl vapor, the G and O phases exhibit red-shifted emission due to pyridine N protonation and the formation of strong hydrogen bonds between Cl
- and protonated molecules. While when fumigated with diethylamine, the diethylamine vapor captures protons from the protonated pyridine, the two crystal phases show blue-shifted luminescence. However, the crystal structure of the G phase shows that the nitrogen atom of the pyridine group can form strong C — H … N interactions with neighboring molecules, and in the protonation process, the incoming HCl can destroy the initial interactions and lead to the destruction of the crystal phase. On the contrary, because there is no C — H … N interaction in the O phase, the initial intermolecular interaction will not be destroyed by protonation and deprotonation, so the O phase remains crystalline.
Subsequently, a multi-stimulus-triggered crystal-to-crystal phase transition between the two luminescent co-crystal polymorphs was achieved by a supramolecular self-assembly strategy (Figure 4D)
[49]. Two kinds of BP4VA-FIB eutectic Ⅰ (yellow needle-like crystals) and eutectic Ⅱ (green bulk crystals) were successfully obtained by controlling the crystallization solvent and changing the molar ratio of different components. The yellow fluorescent polymorph I can be transformed into the green polymorph II by THF vapor fumigation or heating, and the polymorph II can be restored to the yellow fluorescence by grinding. The directional hydrogen and halogen bonds in crystal I can be affected by the enhanced intramolecular vibration and rotation during heating and the polar solvent (THF). At the same time, the change of molecular orientation promotes the formation of non-directional π-π interaction, which drives specific molecular reorganization and leads to phase transition. Due to the presence of the pyridine group and the Cl — N halogen bond, the cocrystals Ⅰ and Ⅱ were protonated by HCl vapor, which resulted in the red shift of fluorescence to orange/red, respectively.
2.1.2 Tetraphenylbenzene
Compared with TPE and DSA, tetraphenylbenzene (TPB) has no double bond, which can completely exclude the possibility of AIE caused by cis-trans isomerization. TPB is helical, and its peripheral benzene ring is connected with the middle benzene ring through a single bond, so it can rotate around the middle benzene ring in solution to consume energy. However, in the aggregate state, the intramolecular rotation is limited, and then the AIE effect occurs. TPB confirms the limited intramolecular rotation mechanism of AIE again
[50]. In addition, the twisted molecular configuration makes its luminescence located in the violet region, which is very suitable for the construction of deep blue AIEgens as AIE motifs. Therefore, most of the current work based on TPB focuses on the construction of deep-blue AIEgens and their applications in undoped deep-blue OLED's
[51⇓⇓~54].
In 2015, Qin and Tang et al. Designed and synthesized A donor-acceptor (D-A) structure molecule (TPB-AC) based on tetraphenylbenzene, that is, the nitrile group and diphenylamine were introduced into the para-benzene ring of TPB only through Suzuki reaction, so that its solid powder luminescence was located at 450 nm
[55]. Because of its high solid-state luminous efficiency, they used it as a light-emitting layer to prepare undoped devices. However, the efficiency of the above device is low, and the CIE y value is also greater than 0.1 (fig. 5A). In 2018, Ma et al realized the preparation of TPB-AC-based non-doped deep blue devices through device structure optimization, making its external quantum efficiency reach 7.0% and CIEy value reach 0.08
[56]. However, there is still a shortage of design strategies for such deep-blue AIEgens.
Subsequently, Qin et al. constructed an AIE luminescent material, CN-TPB-TPA, by increasing the number of benzene rings in the direction of the molecular dipole moment on the basis of TPB-AC (Fig. 6a, B)
[54]. In addition, the horizontal dipole orientation of the molecule can be effectively improved by increasing the number of benzene rings in the long axis direction of the transition dipole moment of the molecule. Therefore, CN-TPB-TPA also exhibits a high horizontal dipole orientation, with a value of 83.6% (fig. 6C). Because of its high solid-state luminous efficiency and excellent horizontal dipole orientation, the CN-TPB-TPA-based undoped device exhibits excellent device performance, with an EQE as high as 7.26%, exceeding the 5% efficiency limit of common fluorescent OLEDs (fig. 6d). The theoretical calculation shows that the intermolecular crossing process of the high energy T
3→S
1 and the intermolecular interaction may also be the reason for the high performance of the undoped device. In addition, due to the balanced hole and electron transport ability of CN-TPB-TPA, the preparation of WOLEDs was realized by using CN-TPB-TPA as a deep blue light-emitting layer and an orange phosphorescent host material. The white device exhibits a low turn-on voltage (2.8 V) and excellent device performance (EQE = 19.1%, CE=58.0 cd·A
-1, and PE=60.7 lm·W
-1) (fig. 6e). These results demonstrate the great potential of this new class of luminescent materials for the fabrication of undoped deep blue and hybrid white OLEDs.
Qin et al. Further replaced the triphenylamine group with carbazole derivatives with weaker electron-donating ability, and designed and synthesized a new blue-violet AIEgen, TPBCzC1-TPBCzC3
[52]. Decreasing the intensity of ICT makes the luminescence of TPBCzC1-TPBCzC3 blue shift, and at the same time ensures its efficient solid-state luminescence quantum efficiency. In addition, they also adjusted the emission wavelength of the luminescent molecule by changing the connection position of carbazole, so that the solid-state emission wavelength of the luminescent molecule was in the violet to dark blue region, and the solid-state emission quantum efficiency was close to 100% (Fig. 7A). The EQE of the undoped TPBCzC1 based OLED is 4.34%, CIE (0.160, 0.035), which is the first reported undoped AIEgen based OLED with CIEy less than 0.046. Meanwhile, the maximum EQE of the OLED based on TPBCzC2 also reaches 4.78%, and the CIE coordinates are (0.159, 0.060) (Figure 7 B ~ d). This work provides a practical molecular design strategy for blue-violet OLEDs, which is essential for the next generation of ultra-high definition display applications.
2.1.3 Tetraphenylpyrazine
In addition to AIE molecules of pure hydrocarbons, some heteroatom systems have also been developed. In fact, the concept of AIE is summarized from the very common luminescence phenomena of silole derivatives containing heteroatoms. Due to the wide variety of heteroatoms, the resulting heteroatom AIEgens also have great structural diversity. This subsection aims to discuss an emerging heteroatom AIEgens, tetraphenylpyrazine (TPP). TPP, like HPS and TPE, has a distorted molecular configuration. This makes it easy to dissipate the excited state energy non-radiatively due to intramolecular motion in solution, thus not emitting light, but in the aggregation state, the radiation transition channel is opened due to the limitation of molecular motion, and the fluorescence enhancement is realized.
2.1.3.1 Synthesis of TPP
The synthesis of TPP is very simple and can generally be prepared by cyclization of benzoin or benzil with diphenylethylenediamine (Fig. 8)
[57]. The reaction involved simple substrates, mild reaction conditions and convenient separation of products, which laid the foundation for obtaining more TPP derivatives. Compared with TPE, TPP has better light/thermal stability because there is no double bond in TPP center, which is not easy to photoisomerization and thermal oxidation. Compared with HPS, the pyrazine ring in the center of TPP is not easy to decompose in alkaline atmosphere, so it has better chemical stability. In addition, TPP also has certain electron affinity. Qin and Tang et al. Obtained AIE molecules TPP-4M, TPP-2p, TPP-2PM, TPP-2MP and TPP-2MPM by introducing phenyl π bridge or methoxy weak electron donor on TPP, and the maximum emission of these TPP derivatives covers the whole blue light region due to different conjugation effects and D-A effects (Fig. 9)
[57].
With the help of the electronic effect of TPP, Qin and Tang continued to modify different numbers of triphenylamine strong electron donor units on TPP, and prepared AIE molecules TPP-TPA, TPP-2 TPA and TPP-4 TPA (Fig. 10)
[58]. Because these molecules have strong D-a effect and flexible molecular configuration, they show TICT effect and AIE effect. These AIE molecular solid-state powders emit sky blue, green, and yellow-green light, respectively. TPP-TPA has a higher Ф
F(35.2%), and the maximum emission of the three-layer structure device prepared by TPP-TPA is 482 nm, and it has a low development voltage (3. 7 V), a high maximum brightness (17459 cd·m
-2) and a maximum external quantum efficiency (2. 88%). In addition, thanks to the D-π-a structure of these molecules, they also have two-photon fluorescence properties. They studied their two-photon absorption cross sections in dichloromethane solution under laser excitation at 700 ~ 900 nm. TPP-TPA and TPP-2 TPA have similar two-photon absorption cross sections, while TPP-4 TPA results are significantly larger (up to 480 GM). This indicates that increasing the number of electron donors is one of the important factors affecting the two-photon fluorescence properties of AIE molecules with D-π-a structure.
2.1.3.2 Application of TPP derivatives
Modifying the active groups on the TPP will increase the stimulus response performance of the AIE molecule. Tang et al. Obtained the AIE molecule TPP-PDCV by modifying the dicyanoethylene group on TPP
[59]. TPP-PDCV has the structure of D-a, which emits orange light at 565 nm due to the TICT effect in DMSO/buffer mixture. When the H
2S is added, the D-a effect of the molecule is destroyed due to the nucleophilic addition of the H
2S to the double bond in the TPP-PDCV. At the same time, the formed disulfide compound TPP-2PS is easy to aggregate due to its weak polarity and strong molecular rigidity, which gradually weakens the orange-yellow emission and strengthens the short-wavelength blue AIE emission. This ratiometric fluorescence detection of H
2S was accomplished within 10 min and was quantified over the concentration ranges of 0 to 75 μM and 150 to 225 μM with a detection limit of 0.5 μM (fig. 11). In addition, the fluorescent probe has good selectivity relative to common anions and biological sulfhydryl. The crystal powders of TPP-PDCV which emit green light and orange light respectively can also be obtained by recrystallization in acetone and toluene. Most AIE materials have the phenomenon of red shift of luminescence stimulated by mechanical force, but this orange-yellow powder has abnormal mechanical color change behavior: its luminescence color shifts from orange-yellow blue to green under grinding.The reversible mechanochromic behavior is attributed to the destruction of the orange-yellow crystalline powder and the recovery of the crystalline form after recrystallization.
In addition, the isomerization effect of AIE molecules will have a significant impact on its functionality. Tang and Qin et al. obtained three AIE isomers of TPP-p-TPE, TPP-m-TPE and TPP-o-TPE by modifying triphenylethylene units at the para, meta and ortho positions of TPP benzene ring, and deeply discussed their structure-activity relationship (Fig. 12)
[60]. The molecular configuration of these three AIE isomers evolves from extension to folding, resulting in the gradual deterioration of molecular conjugation, which leads to the gradual blue-shift and decrease of their luminescence wavelength and efficiency. TPP-p-TPE and TPP-m-TPE can be used to fabricate three-layer electroluminescent devices because of their high solid-state luminescent efficiency (54. 2% and 34. 2%, respectively). The results show that although the emission of TPP-m-TPE is close to pure blue light, TPP-p-TPE has significantly higher maximum brightness and maximum external quantum efficiency, which indicates that the performance of TPP-m-TPE can be improved by increasing molecular conjugation. In addition, they studied the impact of this isomerization effect on the performance of the molecule as a nanoscale fluorescent probe. It was found that for picric acid (PA), the fluorescence of the probe was significantly quenched by static quenching mechanism. For Ru
3+, the dynamic quenching energy has a certain fluorescence quenching effect on the probe. Whether dynamic quenching or static quenching, good molecular conjugation is conducive to increasing the affinity of molecules for analytes and improving the performance of probes. At the same time, increasing the folding of molecules can also improve the performance of probes, because the folding structure is conducive to improving the porosity of aggregates, thereby improving their ability to accommodate analytes. Finally, they obtained TPP-m-TPE organic porous crystals accommodating different solvent molecules through single crystal growth, and found that the folded structure of TPP-m-TPE helped to produce ordered one-dimensional nanochannels in the crystal, and its porous structure was not affected by guest molecules.
Qin and Tang et al. Also prepared polytriazoles containing TPP units based on copper-catalyzed click polymerization (fig. 13)
[61]. The results show that the AIE effect of TPP can be well maintained when it is derived into polymers. This is mainly due to the distorted TPP units in the polymer, which can promote the molecular motion in solution and dissipate the excited state energy in the form of non-radiation, while preventing intermolecular π-π stacking in the aggregation state, effectively preventing fluorescence quenching. In contrast, when two phenyl groups in the TPP unit of the polymer were replaced by nitrile groups, the ACQ effect was observed. This is because the nitrile group is a strong electron-withdrawing group. On the one hand, the introduction of the nitrile group promotes the generation of the TICT effect of the luminescent unit, which makes it emit light in solution, on the other hand, it makes the luminescent unit more planar, which promotes the formation of the π-π stack effect. The above results illustrate that the selection of structurally distorted TPP units is the key to the design of AIE polymers.
TPP is similar to TPE in structure and has good symmetry, so it has broad prospects in structural chemistry. Tang et al. prepared a TPP-cage containing TPP units based on TPP-4OH (fig. 14)
[62]. TPP-4OH does not emit light in solution due to intramolecular motion, while TPP-cage emits blue fluorescence with a quantum efficiency of 34.3% in solution. Theoretical studies show that the rotation barrier of the benzene ring in TPP-cage is significantly higher than that of TPP-4OH due to the crowded arrangement, and the restriction of intramolecular motion promotes the generation of fluorescence. In addition, the TPP-cage can provide a lipophilic cavity for trapping the ACQ molecule pyrrolopyrroledione (DPP), which emits orange-yellow light, to form a host-guest complex for white light emission, indicating that the ACQ effect of DPP can be inhibited through this host-guest interaction, and the simultaneous utilization of blue and orange-yellow light can be realized. Then, they found that by doping the host-guest complex into polyethylene glycol and coating it on the ultraviolet lamp sheet, long-term and stable white light illumination could be achieved. In addition, TPP is an achiral molecule, while TPP-cage has chirality, which is due to the steric effect of the molecular cage forcing the benzene ring on TPP to adopt a clockwise or counterclockwise helical arrangement.
Researchers have also used TPP in the study of MOF materials.Yin et al., based on TPP-4COOH by reacting with lanthanide ions (Eu
3+,Tb
3+ and Gd
3+) were coordinated to prepare Eu
3+-MOF, Tb
3+-MOF, and Gd
3+-MOF
[63]. The results show that different lanthanide ions have significantly different effects on the luminescence behavior of MOF. Eu
3+-MOF emits dual fluorescence at 420 and 630 nm from TPP and Eu
3+, respectively (fig. 15A). In contrast, Gd
3+-MOF emits only fluorescence from TPP, while Tb
3+-MOF has only weak Tb
3+ emission while maintaining TPP emission. This is due to the fact that the TPP ligands in different MOFs have different energy level differences between singlet and triplet States, as well as between ligand triplet and lanthanide ion excited States, which determine different intersystem crossing and energy transfer efficiencies (Fig. 15b). The emission wavelengths of these MOFs are blueshifted compared to TPP-4COOH, because TPP-4COOH needs to adjust its configuration to fit the MOF lattice. In addition, the luminescent quantum efficiency of TPP-4COOH in the powder state is 12.9%, while the luminescent quantum efficiency of TPP-4COoh in the Eu
3+-MOF is as high as 60.8%, which indicates that the luminescent efficiency of the material can be significantly improved through the coordination effect. Later, they also found that the dual emission of Eu
3+-MOF could be used to design a proportional fluorescent probe for the detection of citrulline. When citrulline was added, the emission of TPP in Eu
3+-MOF was significantly enhanced, while the emission of Eu
3+ remained unchanged, which was attributed to the hydrogen bond interaction formed by the hydrogen in citrulline and the nitrogen atom in TPP, thus limiting the rotation of the phenyl ring in TPP (Figure 15C). In order to verify the influence of this interaction, they prepared another MOF by coordinating tetracarboxyl-substituted TPB with Eu
3+, and found that it failed to have a ratiometric fluorescence response to citrulline, indicating that functional luminescent materials can be easily designed by using the structural characteristics of TPP (Fig. 15d).
2.1.4 Polyarylpyrrolo
Pyrrole, as a typical aromatic heterocyclic ring, is widely distributed in natural products, and has rich functional modifiability and adjustable properties, which has attracted great attention and made significant progress in synthesis and application
[64].
2.1.4.1 Relationship between molecular structure and photophysical properties
Previous studies have shown that the substitutional position of the phenyl group has an effect on the luminescence properties of polyphenyl pyrrole. The substituent at the 1-position of the pyrrole ring acts as a rotor and dissipates the excited state energy. Substituents at 2 and 5 positions increase molecular conjugation and lock conformation; However, the 3,4 position substitution gene has a distorted conformation to avoid the close packing of molecules, which is conducive to the fluorescence emission of aggregated state
[65⇓⇓⇓~69].
Based on this, Dong et al. synthesized a series of coumarin-based polyarylpyrrole (MAP) derivatives MAP1-4 (Fig. 16) by introducing a natural compound coumarin as a rotor into the 1-position of the pyrrole ring to suppress the non-radiative energy dissipation of the excited state by virtue of its large size
[70]. The results showed that MAP1, MAP2 and MAP4 all exhibited AEE characteristics, and the conversion of coumarin from ACQ to AEE was successfully realized. However, MAP3 exhibits ACQ properties, which are different from the AIE properties of 1,3,4-triphenylpyrrole
[71]. The single crystal structure analysis shows that, unlike the large torsion angle of 69.2 ° at position 1 in MAP2, MAP3 has a very small torsion angle (9.7 °) between the coumarin group and the pyrrole ring due to the lack of steric hindrance of H atoms at positions 2 and 5. The more planar configuration makes MAP3 form intermolecular π-π stacking of 3.438 and 3.435 435 Å in the aggregate state, thus showing ACQ properties. Secondly, the experimental results also show that the maximum emission wavelength of MAP2 (λ
em=537 nm) is more red-shifted than that of MAP4(λ
em=519 nm) and MAP1(λ
em=511 nm), which is attributed to the enhanced conjugation of MAPs by substituents at 2 and 5 positions. This indicates that the substituents at positions 2 and 5 play an important role in regulating the luminescent properties of MAPs when large planar conjugated substituents are introduced at position 1.
Near-infrared AIE compounds are being applied in the fields of biological imaging and diagnosis and treatment due to their unique advantages, and pyrrole, as a new core skeleton of AIE system, has also attracted wide attention.Dong et al. Regulated the fluorescence intensity and emission wavelength by introducing different donor, acceptor and linker units, and clarified the influence of structure on its photophysical properties, thus providing theoretical guidance for the realization of NIR performance of polyarylpyrroles
[72,73][74]. The results show that when the 3 position of pyrrole is connected with a strong electron-withdrawing indole acceptor, the three D-π-a type NIR compounds MAP5 ~ 7 with AIE characteristics are synthesized by introducing different donor groups at the 2 and 5 positions of pyrrole. Due to the strong electron-donating ability of triphenylamine, the solid state maximum emission wavelength of MAP5 ~ 7 is red-shifted from 600 nm to 724 nm. When a stronger electron-withdrawing group furane structure was introduced at position 3, the maximum emission wavelength of MAP11 was 866 nm, and the tail of MAP11 extended to 1200 nm and entered the near infrared region II, due to the best electron-donating effect of N, N-dimethylaniline
[75].
In addition to the D-π-a strategy, the introduction of fused rings to increase the degree of conjugation is also the second commonly used strategy to obtain long wavelength emission. To this end, Dong et al. Introduced a series of fused rings at the 2,5 positions of pyrrole, and obtained six MAP13 ~ 17 compounds with AEE characteristics
[76]. The analysis shows that the condensed ring at 2,5 position of pyrrole is a "double-edged sword" for the luminescent properties of dyes: it is beneficial to increase the emission wavelength, and the longest solid emission is MAP15, which can reach 729 nm; At the same time, due to the introduction of condensed rings, it is easy to lead to π-π interaction, which is not conducive to solid-state emission, resulting in low fluorescence quantum yield, of which MAP17 is the highest (6.56%).
It is one of the common strategies to obtain long fluorescence emission by enhancing the degree of conjugation, but this strategy will also increase the π-π interaction between molecules, which is not conducive to the fluorescence emission in the aggregated state. Therefore, they tried to use alkyl chains with different lengths to regulate the regularity of near-infrared MAP derivatives MAP18-23 in the aggregation state and inhibit the intermolecular π-π interaction in the aggregation state, so that MAP20 could obtain a higher solid fluorescence quantum yield (32.86%, 663 nm)
[77].
Compared with pyrrole, polyarylpyrrole (MAPP) has greater coplanarity and higher electron enrichment, which endows polyarylpyrrole (MAPP) with significantly different photophysical properties from MAP. They analyzed the single crystal structures of MAPP1 (fig. 17) with AIE properties obtained in different culture environments. The results show that the solvent, molecular conformation, intermolecular forces and molecular packing mode will work together to cause MAPP1 to have three different single crystal forms and emission wavelength :A(λ
em=480 nm), B(λ
em=504 nm) and C(λ
em=540 nm).Moreover, the above three single crystals can be transformed into each other under certain conditions, that is, single crystal C can be transformed into single crystal B after fumigation with chloroform for 12 H, and then into single crystal A after fumigation for another 12 H, which indicates that MAPP1 has adjustable AIE performance
[78]. On this basis, the research team synthesized 13 MAPP derivatives, and found that MAPP2 with electron-withdrawing groups at positions 1 and 4 and electron-donating groups at positions 2 and 5 had AIE characteristics (Fig. 17). Both single crystal analysis and theoretical calculations confirm that the AIE property is achieved by suppressing the TICT effect and the π-π interaction in the solid state
[79]. Three D-a compounds MAPP4 ~ 6 were constructed by further changing the pyrrole nucleus and introducing electron-withdrawing groups to the para-position of the benzene ring at the 2,5 position. Compared with MAPP3 without electron-withdrawing groups, MAPP4 ~ 6 exhibited ACQ properties because the electron-withdrawing groups stiffened the rotation of the benzene rings at positions 2 and 5, resulting in a smaller torsion angle between the benzene rings and the pyrrole core and an enhanced coplanarity
[80].
2.1.4.2 Application
Among a series of MAP derivatives, the photothermal conversion efficiency (PCE) of MAP11 nanoparticles (MAP11 NPs) prepared by Dong et al. Using amphiphilic polymers to encapsulate MAP11 is as high as 72%, which shows excellent photothermal performance. Under laser irradiation, MAP11 NPs can rapidly heat the tumor site of mice to 64 ℃ within 10 min, and even completely ablate the tumor of mice within two weeks without affecting the physiological state of mice, thus achieving efficient and safe photothermal therapy effect (Fig. 18).
The molecular dynamics simulation proves that MAP11 in the aggregation state shows loose packing, and the molecules have a larger free motion space, which leads to more dissipation of the excited state energy in the form of non-radiative transitions, showing a high PCE. In addition to the efficient photothermal conversion performance, the narrow singlet-triplet energy gap of MAP11 is conducive to the intersystem crossing process, showing a high efficiency of ROS generation, which further enhances the therapeutic effect on tumors. Therefore, this design strategy of introducing small rotor donors into the molecular structure provides a new idea for the design of small molecule phototherapeutic agents and a new perspective for the design of multifunctional therapeutic cancer platforms
[81].
Based on the MAPP structure, Dong et al. designed and prepared a series of new liquid crystal molecules with AIE properties (Figure 19 a). Among them, MAPP with shorter middle alkyl group has more significant AIE properties, while increasing the length of alkyl chain gives MAPP more obvious liquid crystal (LC) properties, and belongs to the unidirectional liquid crystal phase, showing smectic LC phase texture (Fig. 19b).
The 2D GIWAXS analysis of MAPP-C6 and MAPP-C12 by synchrotron X-ray (Fig. 19 C ~ e) shows that MAPP-C6 and MAPP-C12 have a highly ordered molecular arrangement layered structure in the LC phase (Fig. 19 f ~ H), and based on the molecular model and density functional theory (DFT) calculation, the relationship between the molecular configuration and its AIE, LC characteristics is further revealed
[82].
At the same time, they also studied MAPP3-6 in depth. The results show that the emission wavelength of the compounds can be controlled by changing the electron-withdrawing ability of the para-substituents on the 2,5-benzene ring, and the formation of liquid crystal phase can be promoted, because only MAPP5 and MAPP6 with strong electron-withdrawing ability can be assembled into liquid crystal by heat treatment, which provides a reference for the optimization design of performance. Because of its good liquid crystallinity, MAPP6 showed a significant temperature-stimulated response luminescence phenomenon, with a light-on luminescence in the temperature range of 135 ~ 185 ℃, accompanied by a change in emission wavelength (Fig. 20A, B). Temperature-dependent XRD shows that MAPP6 induces a phase transition from solid state to liquid crystal state and then to isotropic liquid state when heated to this temperature range, which destroys the π-π interaction between molecules in the original state. Corresponding to the sensitivity to temperature, MAPP6 also showed the properties of mechanochromism and fluorescence enhancement, the quantum yield increased from 0. 1% to 4. 3% after fully grinding, and the emission wavelength red-shifted from 610 nm to 648 nm.Its mechanism is that mechanical force induces MAPP6 to change from a short-range ordered layered structure to another long-range ordered layered structure, and after grinding, the intermolecular π-π stacking is weakened, so the fluorescence is significantly enhanced. These properties and functions lay a foundation for guiding the design and application of new sensing materials and devices.
Based on the above, the research results obtained so far clearly show that the developed polyarylpyrrole has become an important part of the AIE system, and has great potential in biological imaging, diagnosis and treatment, as well as intelligent sensing, and is expected to become an "AIE star molecule".
2.2 AIE eutectic system
In the expansion of AIE luminescence system, researchers have developed different types of AIE molecules, among which a class of molecular co-crystals based on supramolecular self-assembly has become a hot research topic due to its low cost, simple design method and easy preparation. Although the emission color can be tuned by chemical modification of common AIE units (such as TPE derivatives and hexaphenylsilyl), the AIE systems with tunable polychromatic emission properties are still relatively limited
[83⇓~85][86][87]. In addition, most of the studies on AIE fluorophores have focused on single-component systems, and if the AIE phenomenon and characteristics can be extended to multi-component systems, a large class of new efficient luminescent materials can be developed.
In recent years, with the in-depth study of multi-component eutectic systems, the influence of eutectic systems is not only expanding in the fields of pharmaceutical, ferroelectric, fluorescence, optical waveguide, nonlinear optics and electronic devices, but also further enriching the characteristics of AIE in multi-component systems and obtaining materials with different luminescent colors
[88][89][90][91][92][93]. This section focuses on the research progress of eutectic materials with AIE effect.
2.2.1 AIE eutectic
A eutectic is a solid that is composed of two or more different molecules in a certain stoichiometric ratio through the recognition and assembly of intermolecular non-covalent bonds (hydrogen bonds, halogen bonds, π-π, charge transfer forces) and combined in the same lattice
[94]. Due to the synergistic effect and collective effect between different molecules in the eutectic, novel properties not possessed by a single component can be realized
[95]. Based on supramolecular chemistry and crystal engineering, new functional cocrystals with AIE luminescence can be realized by selecting appropriate coassembly units for cocrystallization and utilizing different intermolecular interactions. In cocrystals, the typical intermolecular interactions are hydrogen bonding, halogen bonding, π-π interaction, and charge transfer (CT) interaction, respectively. In addition, the cocrystal formed by π-π interaction, with its planar structure and parallel packing, makes the excited state energy dissipate in a non-radiative form, which easily leads to fluorescence quenching
[96]. Therefore, at present, according to the interaction between molecules, the cocrystals with AIE properties are mainly divided into two types: hydrogen bond/halogen bond cocrystals and charge transfer cocrystals.
2.2.1.1 Hydrogen bond/halogen bond AIE cocrystal
Hydrogen bonding and halogen bonding are important non-covalent forces in supramolecular chemistry and crystal engineering, which are widely used in the design of functional solid systems. Among them, different types of halogen/hydrogen bond donors and acceptors can adjust the packing mode of cocrystal molecules, thus affecting the properties of cocrystals
[97⇓~99]. At present, hydrogen bond/halogen bond co-crystals with AIE performance have also been developed to improve the problem of single emission performance of single component. For example, Yan et al. reported a series of halogen-bonded and hydrogen-bonded co-crystals of 4- (1-naphthylethylene) pyridine (NVP) groups, with co-assembly units of 1,4-diiodotetrafluorobenzene (A), 4-bromo-2,3,5,6-tetrafluorobenzoic acid (C), and 4-benzoylbenzoic acid (D), respectively, and the synthesized co-crystals showed typical AIE characteristics (Fig. 21)
[100]. When CH
3OH and water were selected as the solvent pair, it was observed that the fluorescence emission intensity of the samples was significantly enhanced with the increase of the concentration of water as the poor solvent, and the samples in the aggregated state showed different emission colors, from purple, blue, green to cyan, and the colors became more and more obvious with the increase of water content, which corresponded to the formation process of the eutectic. They believe that, on the one hand, the process of forming cocrystals is the process of coaggregation between different self-assembled molecular units; On the other hand, due to the promotion of hydrogen bonds or halogen bonds, the cocrystal has a distorted compact structure and deviates from face-to-face stacking, which can be fixed to avoid the fluorescence quenching caused by the formation of H aggregation. In addition, the fluorescence of the eutectic at low temperature and in the condensed state is enhanced, indicating that the molecular rotation in the condensed state is hindered, which also confirms the AIE characteristics of the eutectic again.
2.2.1.2 Charge-transfer AIE eutectic
Since the researchers first discovered the cocrystal of tetrathiafulvalene and 7,7,8,8-tetracyanoquinodimethane (TTF-TCNQ) in 1973, the CT cocrystal has attracted much attention and become an opportunity for researchers to explore the physicochemical properties different from those of single materials
[101]. The charge-transfer interaction usually occurs in the molecular system composed of donor (D) and acceptor (A), and the degree of transfer depends on the ionization potential of the donor in the eutectic, the electron affinity of the acceptor, and the electrostatic Coulomb interaction. In addition, the emission of D-A structure system is helpful to further expand to the long wavelength region, which makes it more widely used in the optical field
[102]. In order to obtain A wide range of AIE systems, D-A structures are commonly constructed by designing electron-donating and electron-withdrawing groups, such as connecting acceptor functional groups to donor molecules such as tetraphenyl butadiene, carbazole and pyrrole, resulting in intramolecular charge transfer, which makes electrons jump from the lowest energy molecular orbital to the empty high energy molecular orbital and reduces the HOMO-LUMO gap
[103]. However, this method also has some problems, for example, the design of AIE donor-acceptor molecules requires complex chemical synthesis steps, and the cycle is long; The addition of steric hindrance groups may distort the conformation of chromophores and reduce the electron transport or hole transport ability of chromophores. The main advantage of the co-crystal is that it is easy to prepare, and the host and guest small molecules can be used to design the charge transfer AIE co-crystal.
9-Acetylanthracene is stacked into planar and rigid structures by strong π-π interactions, and decays in the form of non-radiation in the excited state to form excimers, resulting in fluorescence quenching. However, after self-assembly with 1,2,4,5-tetracyanobenzene, which is a small molecule with very weak luminescence and no AIE performance, the charge transfer state co-crystal can be formed, the fluorescence quantum yield is increased to 45.52%, and the AIE effect of orange luminescence is produced in the form of aggregation state
[104]. Yan et al. selected acetone-water as the solvent to study its AIE performance, as shown in Fig. 22, when the water content is 95%, the fluorescence intensity at the emission position 563 nm is increased by about 338 times compared with that in pure acetone solution, showing a typical AIE phenomenon
[104]. The analysis of the crystal structure shows that the eutectic is connected by strong C — H … N hydrogen bonds and has strong rigidity, and the configuration deviates from the conjugate plane. In addition, the π-π interaction becomes weaker (the spacing is larger than 3.66 Å), the overlap is smaller, and the non-radiative decay is suppressed, resulting in the enhancement of the fluorescence of the aggregate state.
2.2.1.3 Other types of AIE eutectic
In addition to the above-mentioned small organic molecule cocrystals with AIE properties, some other types of supramolecular cocrystals have also been reported. Chen et al. Studied the assembly of salicylaldehyde azine (CA-SAA) with carboxyl functional group and GaOOH nanoparticles to form AIE properties, and used the hydrophobic effect between organic-inorganic molecules to drive the formation of aggregates
[105]. Liu et al. Reported the AIE system assembled by melamine (TCP) molecule and bispyridine-functionalized naphthalenediimine (ND) molecule through non-covalent interaction, and the AIE system self-assembled by Schiff base-rhodamine B
[106].
2.2.2 Application of AIE eutectic
Eutectics with AIE properties not only retain the excellent properties of crystals, but also have AIE activity, which is an ideal choice for the preparation of multi-functional materials. It is well known that AIE materials have been widely used in optoelectronic devices, chemical sensing, biological imaging and detection, and many other fields. With the deepening and expansion of AIE materials research, the newly developed AIE eutectic also has important practical application potential.
2.2.2.1 Drug delivery and release
The AIE effect can track the dynamic trajectory of intracellular drug carriers, but its release behavior is not yet known. Fluorescence resonance energy transfer (FRET) is a method used to monitor the dynamic process of visualization in imaging systems
[107]. The FRET effect between the nano-carrier and the drug is used to study the release process of the carrier in cells, and when the drug is released, the FRET effect between the carrier and the drug-loaded disappears, so as to achieve better therapeutic effect. In view of this, Wang et al. Successfully prepared a supramolecular drug carrier H-PAMAM-ss-mPEG/A-CD (HG/CD) loaded with doxorubicin (DOX), which has both AIE and FRET effects, based on the host-guest interaction between polyethylene glycol (PEG) and α-cyclodextrin (α-CD).The monitoring of the dynamic trajectory of the drug carrier and the drug release process proves that the supramolecular assembly system has a good therapeutic effect on tumors, and the host-guest self-assembly, as a new drug delivery technology, has broad application prospects (Figure 23)
[108].
2.2.2.2 Biological Imaging
Fluorescent probe is an effective technology to realize cell imaging and visual monitoring. Researchers have developed different types of probes by using the interaction between fluorescence and organisms. Among them, AIE materials are favored by people in the field of biological imaging because of their high emission efficiency and good stability
[109]. As shown in Fig. 24, Liu et al. reported an AIE ternary assembly system with near-infrared emission and two-photon excitation, which was assembled by a TPE derivative (TPE-2SP) modified by methoxy and vinyl pyridinium, cucurbituril [8] (CB [8]) and hyaluronic acid (HA-CD) modified by β-cyclodextrin.Combined with the AIE characteristics of TPE and the inclusiveness of cucurbituril and cyclodextrin, the synthesized supramolecular system has two-photon absorption characteristics, which can penetrate deep into tissues, reduce background interference, improve the signal-to-noise ratio of imaging, and avoid protein absorption in vivo. It has been successfully applied to mitochondrial targeted imaging, opening up a new way for the study of biological imaging
[110].
2.2.2.3 Optical Sensing
It is well known that cyano-diphenyl derivatives have obvious AIE effect and are usually used in the synthesis of AIE compounds. In addition, due to the existence of slip planes, they show intelligent switching response under external stimuli (temperature, pressure)
[111⇓~113]. In 2018, Yan et al. selected 4-bis (1-cyano-2-styryl) benzene and 2,3,5,6-tetrafluoro-4-hydroxybenzene for self-assembly. As shown in Fig. 25, the eutectic not only retains the AIE and piezochromic properties of the main unit 4-bis (1-cyano-2-styryl) benzene, but also has a certain response function to acid and alkali vapors, which has potential application value in optical sensors
[114].
Different from the traditional luminescent co-crystal molecules, which can cause fluorescence quenching in the aggregation state, AIE co-crystal overcomes the problem of fluorescence quenching and has broad application prospects in the fields of optoelectronic devices, sensors and biological imaging. The mechanisms of AIE characteristic cocrystals reported at present mostly focus on intermolecular non-covalent bonding forces (hydrogen bonding and charge transfer) or J-shaped stacking structure, which inhibit non-radiative transitions, and the core is still the restriction of intramolecular motion. However, the phenomenon of AIE is often caused by the synergy of multiple factors, so it is necessary to further improve and study the mechanism of AIE performance in eutectic, so as to make it more systematic, which has guiding significance for researchers to further design and develop AIE eutectic.
At present, compared with other types of AIE materials, cocrystals with AIE luminescent properties are still in the preliminary stage of development, and the number is small. In addition, the application of AIE eutectic is mainly focused on basic research, and there is still a long way to go to realize its practical application value. Therefore, the next step is to explore and develop more new AIE materials, improve their mechanisms, and achieve real applications.
2.3 AIE polymer
Compared with AIE small molecule and AIE co-crystal system, AIE polymer has its unique advantages, such as good functional composite, processability and film-forming properties, which can meet the diverse needs. In addition, the intermolecular and intramolecular interactions of AIE polymers usually endow the system with synergistic effects, showing more excellent properties than small molecule systems or even small molecule systems
[115⇓~117]. Since the first AIE polymer was reported in 2003, the related fields have developed rapidly, especially in the past five years, AIE polymers have shown vigorous vitality in the fields of biological imaging, sensing and diagnosis and treatment
[118][119,120]. However, compared with the rapid development of small molecule AIE materials, the development of AIE polymers is still insufficient, and there are still many scientific problems to be solved, which also means that the development potential of this field is infinite. This section focuses on the research progress of AIE polymers in the field of biology.
2.3.1 Biological Imaging
AIE polymers have the advantages of both polymers and AIE materials, and their properties are often not only a simple stack of the two properties, but also the synergistic effect of intramolecular and intermolecular interactions, which makes them often have the properties that pure polymers and AIE materials do not have
[121]. In addition, the good processability and functional composability of AIE polymers, combined with their excellent photophysical properties, enable them to respond to environmental changes with high sensitivity and specificity, and achieve selective and responsive bioimaging
[122⇓~124].
2.3.1.1 Selective imaging
The backbone and side chains of AIE polymers can be designed and structurally regulated to achieve selective imaging according to the structural and metabolic differences of different organelles, cells or tissues. The detection and judgment of cell activity is of great significance for testing the effect of drugs and guiding treatment. Qin et al. Designed a new type of AIE polymer according to the great differences in the surface structure and charge state of cells and microbial cells in different States. The polymer has unique optical properties endowed by the conjugated skeleton of AIE properties, which makes it have high luminous efficiency and sensitivity in the aggregation state of physiological environment
[125]. The introduction of oligo (ethylene glycol) side chains can increase the water solubility, reduce the non-specific interaction and improve the selectivity of the system. As shown in fig. 26, this polymer selectively binds to living cells in the presence of mammalian cells (living cells, apoptotic cells, and dead cells), microorganisms (bacteria and fungi), and the like. Under the same conditions, the selectivity and biocompatibility of the commercial living cell dye calcein and the small molecule model compound of the polymer are not as good as those of the AIE polymer. In addition, osteoblasts maintain the balance of human bone, and their proliferation and differentiation ability are closely related to fracture healing. Therefore, the detection of osteoblast differentiation degree is of great significance in the study of orthopedic diseases. Zhang and Qin et al prepared water-soluble AIE conjugated polymers by introducing EDTA groups with strong chelation with calcium ions into the side chain, which realized the specific imaging of osteogenic differentiation cells and the dynamic, real-time and sustainable observation of osteogenic differentiation process
[126]. It is worth pointing out that the polymer can detect a significant increase in fluorescence signal on the 7th day of osteogenic differentiation, while under the same conditions, the classic alizarin red staining can detect osteogenic differentiation nodules on the 14th day. The system provides a new solution strategy for the detection of osteogenic differentiation of cells and the study of related processes. In addition to specific imaging at the cell level, AIE polymers have also achieved highly selective imaging at the organelle level. Ma et al. Constructed a series of hyperbranched AIE polymers with side chains carrying a large number of positive charges, and achieved specific imaging of cell nuclei based on their strong cell penetration ability
[127]. In addition, at the tissue level, Cai et al. Successfully developed an AIE conjugated polymer with near-infrared emission by regulating the skeleton structure, and prepared a nano-bionic robot system for crossing the blood-brain barrier by coating its surface with a natural killer cell (NK) membrane.Finally, the specific recognition of NK cell membrane surface protein and tumor cell membrane surface receptor is enriched to glioma, which provides a new tool for the diagnosis and treatment of brain tumors and other brain diseases
[128].
2.3.1.2 Responsive Imaging
The changes of cell metabolites and biomacromolecules are often closely related to some diseases. Therefore, the rapid and highly sensitive detection of pH, temperature, reducing and oxidizing environment and metabolites in cells/tissues can provide important reference information for disease diagnosis and treatment
[129⇓⇓~132]. The level of CO
2 in tumor cells is higher than that in normal tissues. Based on this, Wang et al. introduced AIE motif and CO
2 response group into the side chain of the polymer at the same time, and prepared a breathing block polymer for detecting intracellular CO
2 metabolism (Fig. 27)
[133]. The polymer self-assembles into vesicles of 150 nm in water due to its amphiphilicity. After the vesicles are treated with CO
2, the hydrophobicity of the vesicles increases due to the protonation of the amino moiety of the CO
2 responsive group, and the vesicles swell to 200 nm with an increase in fluorescence. When the system was further treated with N
2, the deprotonation of the responsive group would lead to the shrinkage of vesicles and the weakening of the fluorescence of the system, which proved that the process had good repeatability. Based on this, the polymer was used to detect the level of CO
2 in cells, and the differentiation of normal cells and tumor cells was achieved by monitoring the intracellular fluorescence intensity, which is expected to develop into an effective tumor diagnosis strategy. Intracellular temperature can often provide an important basis for pathological research and disease diagnosis. Saha et al. Synthesized a non-traditional luminescent AIE polymer, which did not contain traditional luminescent units, and the lowest critical solution temperature (LSCT) was very close to the human body temperature, and the fluorescence intensity increased with the increase of temperature after LSCT
[21]. Base on that characteristic of the polymer, the efficient detection of the intracellular temperature is realized. In addition, AIE-based polymers have also achieved the detection of intracellular pH, ATP and other ions, and most of the AIE polymers have shown better performance than the corresponding small molecules in related applications.
2.3.2 Biological diagnosis and treatment
AIE polymers can not only be used for highly sensitive and specific biological imaging due to their excellent photophysical properties, but also have good application prospects in drug/gene delivery, disease therapy and other applications due to their diverse structures, tunable functions and excellent photodynamic/photothermal properties
[134].
2.3.2.1 Photodynamic therapy
Photodynamic therapy (PDT) has shown unique advantages in the treatment of cancer because of its good space-time controllability and low drug resistance
[135]. Different from traditional photosensitizers, AIE polymers not only have a good ability to sensitize the generation of reactive oxygen species (ROS), but also their photosensitivity will be enhanced with the formation of aggregates, which has been applied to the integration of diagnosis and treatment
[136,137]. Liu et al. prepared a series of AIE polymers by copper-catalyzed atom transfer radical polymerization using different microorganisms as templates based on the structural differences of cell walls of different microorganisms (Fig. 28)
[138]. The polymer was collected from the surface of the microorganism by washing with NaCl solution, and the number average molecular weight was measured to be around 22,000. Imaging experiments show that AIE polymers prepared by using Gram-positive bacteria as templates can selectively bind to Gram-positive bacteria, and this selective imaging is also applicable to other types of microorganisms. Further, the AIE polymer has a good ability to sensitize the generation of ROS, and can efficiently kill bound microorganisms without generating drug resistance. In the aspect of PDT-based microbial infection therapy, Qin et al. constructed a novel AIE conjugated polymer.The molecular design concept of that AIE conjugate polymer is mainly based on the follow points: a D-pi-a structure and a TPE group are introduced into the skeleton of the conjugated polymer to endow the obtain polymer with super ROS generation capability in the aggregation state, and the ROS generation capability is even more than 10 times of that of a small molecule model compound and a commercial photosensitizer Ce6
[139]; By introducing the 4-azido perfluorobenzoate radical, the polymer can effectively generate free radicals under ultraviolet irradiation; Placing the quaternary ammonium salt group in the middle of the side chain, rather than at the end, allows the polymer to have a balanced hydrophilicity and hydrophobicity, enabling specific binding to microorganisms. The research proves that the polymer can be selectively combined with microorganisms only under the condition that the microorganisms and the mammalian cells exist at the same time, and a cytotoxicity experiment shows that the polymer basically has no toxicity to the mammalian cells; Under the same conditions, the small molecule model compound can efficiently combine with microorganisms and mammalian cells, shows high toxicity to mammalian cells, and verifies good antibacterial performance in vitro and in a small animal model. The results of blood biochemical analysis and tissue section study show that the polymer has very good biological safety. Therefore, based on the excellent antibacterial properties and good biocompatibility of the novel AIE conjugated polymer, it has broad application prospects in the treatment of microbial infections and other fields. It can be seen that AIE polymers have better therapeutic effects in the field of PDT-based diagnosis and treatment compared with small molecule photosensitizers and traditional photosensitive materials.
2.3.2.2 In situ diagnosis and treatment
In-situ preparation of AIE polymers in physiological environment and realization of their diagnostic and therapeutic functions have attracted more and more attention in the field of diagnosis and treatment
[140]. Qin et al. Constructed a spontaneous polymerization reaction in living cells and prepared a polymer with a molecular weight of 7300
[141]. As shown in fig. 29a, the carbonyl-activated acetylenic monomer can spontaneously react with the TPE-containing diamine monomer in the cell, realizing the "light-up" type cell imaging. It is worth pointing out that intracellular polymerization can kill cells by destroying tubulin and actin structures in situ. Therefore, based on spontaneous alkyne-amine click polymerization, an artificial "lab-in-cell" is constructed, which can realize in-situ polymerization and kill tumor cells by one-step method, and is a new type of "reaction therapy", which has broad application prospects in biomarkers and anti-tumor. Xia and Lou et al. Constructed AIE oligomers in situ at the validation site based on a hydrogen peroxide and peroxidase dual-responsive AIE probe (TT)
[142]. As shown in fig. 29b, in the absence of hydrogen peroxide and peroxidase, the TT probe had a high dispersibility in the aqueous phase and did not emit light; In the presence of hydrogen peroxide and peroxidase, the TT probe can polymerize in situ under the catalysis of hydrogen peroxide and peroxidase to obtain the corresponding oligomer and emit a strong fluorescent signal. The polymerization reaction in the cell will damage the mitochondria of the cell and eventually lead to apoptosis. Experiments prove that the TT probe can be used for the specific detection of H
2O
2 and peroxidase in cells, and finally realizes the selective killing of only inflammatory cells under the condition of almost no influence on the activity of normal cells based on the level difference of hydrogen peroxide and peroxidase in normal cells, inflammatory cells and tumor cells. Therefore, the probe shows great potential in the diagnosis and treatment of inflammatory diseases. In situ construction of AIE polymers with diagnostic and therapeutic effects has great prospects in the field of biological diagnosis and treatment.
2.3.2.3 Drug/nucleic acid delivery
In addition to the therapeutic effect of AIE itself, AIE polymers are often used as therapeutic carriers to deliver drug molecules, proteins and nucleic acids, which can effectively release these molecules based on tumor microenvironment response and other factors, and ultimately achieve delivery tracking and disease treatment
[143,144]. Li et al. constructed a photoactivatable AIE polymer prodrug for the combination therapy of tumors
[145]. As shown in Fig. 30, TPE and cisplatin prodrug structures were introduced into the polymer backbone by covalent linkage to prepare an AIE polymer responsive to light and reducing substances, and they developed a functional nanomaterial by encapsulating doxorubicin (DOX) with the polymer. Because of the ACQ effect of DOX and the FRET between TPE and DOX, almost no fluorescence signal can be detected when the nanoparticles are intact. However, when the nanoparticles were irradiated, the polymer skeleton was broken, and Pt (Ⅳ) was activated to Pt (Ⅱ) with therapeutic effect. TPE and DOX were dissociated from the polymer skeleton and nanoparticles, respectively, and emitted bright fluorescence. Therefore, based on the above nanomaterials, the combination therapy of Pt (II) and DOX activated by light on tumor cells was realized, and the drug release process could be dynamically monitored by detecting the changes of TPE and DOX fluorescence signals. Loh et al. Developed a polymer with AIE properties and prepared a thermosensitive hydrogel based on it. The hydrogel was injected into the tumor site in situ to achieve efficient drug release from the tumor site
[146]. In addition to drug delivery, Liu et al. Also developed a photoactivated AIE polymer rich in positive charges for DNA loading and delivery, and achieved efficient killing of tumor cells
[147]. It can be seen that AIE polymers have broad application prospects in the field of biological diagnosis and treatment.
2.4 AIE organometallic complex
AIE materials have been widely used in the field of luminescent materials because of their high solid-state luminescent efficiency. However, the current research in this field is still focused on small organic molecules, which are facing challenges such as difficult synthesis, short luminescence lifetime and short wavelength excitation. Compare with small organic molecules, transition metal complexe have richer photophysical properties due to that fact that the metal center heavy atom effect of the transition metal complexe promotes the formation of a luminescent triplet state and is assisted by a pi conjugated ligand,It is considered to be an effective strategy to construct new luminescent functional materials with long lifetime (millisecond level), large Stokes shift (hundreds of nanometers) and high quantum yield
[148⇓~150]. Among them, the cluster molecules with precise atomic structure based on copper, silver, gold and other coin metals often show the characteristics of long wavelength and long lifetime, which are also called coin metal clusters. The discovery of new metal clusters with AIE properties has significantly improved the luminescence quantum yield of such compounds, greatly expanded their application prospects, and become the focus of attention of researchers. In addition, with the rapid development of supramolecular chemistry and coordination chemistry, coordination-directed self-assembly has also been used to construct supramolecular organometallic complexes with controllable shape and size. The ruthenium (Ⅱ) and iridium (Ⅲ) complexes, coinage metal clusters and supramolecular organometallic complexes with AIE characteristics were reviewed in this part.
2.4.1 Ruthenium (Ⅱ) and Iridium (Ⅲ) Complexes with Aggregation-induced Luminescence
Organometallic complexes with AIE properties have been reported. The metal centers include main group metal elements (lead, antimony, bismuth, tellurium, etc.) And transition metal elements (copper, gold, ruthenium, iridium, platinum, rhenium, etc.). Their applications also cover organic light-emitting diodes, chemical sensors, biological imaging, disease therapy and other fields
[18⇓~20]. In this part, we will give an overview of ruthenium/iridium complexes with aggregation-induced luminescence.
2.4.1.1 Luminescence Mechanism and Design of AIE Metal Complex
With the development of the study on aggregation-induced luminescence of organic molecules, the mechanism of aggregation-induced luminescence has experienced a step by step development from the restriction of intramolecular rotation to the restriction of intramolecular motion. RIM has also been used to partially explain the AIE phenomenon of metal complexes, but this field is still in its infancy because the excited States of metal complexes are more abundant than those of small organic molecules. Taking octahedral ruthenium (II) and iridium (III) complexes as examples,Its common excited States include
1LC(Ligand centered),
1ILCT(Intra-ligand charge transfer),
1MLCT(Metal to ligand charge transfer),
1LLCT(Ligand to ligand charge transfer) and
3MC(Metal centered).Triplet States such as
3ILCT,
3MLCT,
3LLCT, and
3LMCT(Ligand to metal charge transfer)
[151]. In addition, the aggregation of platinum (Ⅱ) and palladium (Ⅱ) complexes with planar configuration is often accompanied by the appearance of MMLCT (metal-metal-to-ligand charge transfer) excited States
[152⇓⇓⇓~156].
The introduction of groups with AIE properties is a common method for the construction of AIE metal complexes. Chao et al. Grafted triethylamine to the end of iridium (III) complexes, and constructed AIE-iridium (III) -based photosensitizers and mitochondrial tracer probes, respectively
[157,158]. Su et al. Developed trinuclear AIE-iridium (Ⅲ) photosensitizer with triethylamine as the core
[159]. In addition, the introduction of flexible groups, multi-hydrogen bonding groups and other strategies have also been reported, and the detailed structure will be summarized in the next part. Overall, the universality of these strategies still needs to be verified, and the structure-activity relationship of AIE metal complexes needs to be summarized.
2.4.1.2 AIE ruthenium (Ⅱ) complex
Ruthenium (Ⅱ) complexes with AIE properties are rarely reported. In 2010, Chao et al. reported Ru (II) complexes [Ru(bpy)
2(pzta)]
2+(pzta=6-(pyrazin-2-yl)-1,3,5-triazine-2,4-diamine,bpy=2,2'-bipyridine, complex 56, Figure 31)
[160]. This is the first ruthenium (Ⅱ) complex with AIE property. In acetonitrile solution, complex 56 has very weak luminescence (Ф
F=0.1%), while in acetonitrile/toluene mixed solution containing 80% poor solvent toluene, the complex exhibits bright red luminescence visible to the naked eye. The results of dynamic light scattering (DLS) showed that the complex spontaneously aggregated into nanoparticles with an average particle size of 736.2 nm. X-ray single crystal diffraction confirmed that the N atom of the main ligand pzta pyrazine ring, which was not coordinated to the metal center, was connected to the H atom of the other bipyridine molecule through hydrogen bonds during aggregation, forming a new quadrilateral prism along the α axis. Interestingly, when the pyrazine ring in the main ligand pzta is replaced by a pyridine ring, a quinoline ring or an isoquinoline ring, the intermolecular hydrogen bond cannot be formed, and the AIE property of the complex also disappears
[161]. On the one hand, this indicates that the formation of intermolecular hydrogen bonds inhibits the non-radiative transition pathways and promotes the AIE properties of the complexes.
In 2018, Khatua et al. reported the Ru (II) complex Ф
F=1.1% (complex 57, phen = 1,10-phenanthroline, Figure 31), which has weak emission (Ф
F=1.1%) in acetonitrile solution, while increasing the water content in the acetonitrile-water mixed system can significantly enhance its luminescence
[162]. When the water content in the mixed system is 90%, the quantum yield of complex 57 is increased by about 2. 5 times (Ф
F=3.4%), while dynamic light scattering, scanning electron microscopy, transmission electron microscopy, and atomic force microscopy all indicate that complex 57 aggregates to form nanoparticles under this condition, and the average particle size of Zeta potential is 372. 4 nm. The results of X-ray diffraction show that there are π-π interactions, intermolecular hydrogen bonds and halogen bonds in the aggregates. Because it can connect with cell nucleolar ribosomal RNA by halogen bond and gather to emit light, it realizes the in situ tracing of nucleolar ribosomal RNA.
In 2020, Deng, Zou and Yang reported two Ru (II) complexes with AIE properties [Ru(bpy)
2(Hphbtz)]
+[163]. They detected the luminescence of complexes 58,59 in water-tetrahydrofuran mixed solvents with different proportions, and the luminescence intensity increased significantly with the increase of water proportion. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) show that the complexes aggregate in the mixed solvent of water and tetrahydrofuran. Since they did not provide evidence, the aggregation mechanism of complexes 58,59 is not clear; However, due to the presence of the tetrazole group, hydrogen bonding may play a role in it. The nanoparticles formed by the aggregation of the complexes 58 and 59 can penetrate the cell membrane to enter the tumor cells, and a large amount of ROS substances are generated after illumination, so that the anti-tumor effect is realized.
2.4.1.3 AIE iridium (Ⅲ) complex
In 2008, Huang et al. reported that complexes Ir(ppy)
2(DBM) and Ir(ppy)
2(SB) (complexes 60, 61, DBM = 1,3-diphenyl-1,3-propanedione, SB = 2- (naphthalen-1-yliminomethyl) -phenol, ppy = 2-phenylpyridine, fig. 32) exhibit AIE properties in acetonitrile-water mixed solvents as well as in the solid state
[164]. Through crystal structure and DFT calculation, they believe that the main ligand benzene ring and auxiliary ligand ppy form intermolecular π-π stacking during aggregation, which reduces the π
* orbital energy of ppy ligand and makes the
3MLLCT(Metal-to-ligand-ligand charge transfer) become the lowest emission energy state, that is, the aggregation luminescence comes from the excimer formed by cyclic metal auxiliary ligand. Shortly after this work was published, Park et al. Questioned this mechanism, arguing that the mechanism of AIE must be derived from RIM
[165]. To this end, they synthesized complexes 62-65 (fig. 32), which are structurally similar to complex 61, in which the auxiliary ligand of complexes 64,65 is dfppy (dfppy = 2- (2,4-difluoro-phenyl) pyridine). Park et al. Suggest that if the formation of ancillary ligand excimers is indeed the mechanism of aggregated luminescence for complexes 60, 61, the emission wavelengths of complexes 62 and 63 (or 64 and 65) with the same ancillary ligand should be the same. However, the spectral results show that the red shift of the solid state phosphorescence of complexes 64 and 65 is caused by the replacement of the auxiliary ligand by dfppy, which indicates that the luminescence change is directly related to the structure of the auxiliary ligand. In addition, the complex also exhibits strong emission in the frozen CHCl
3 solvent, and its phosphorescence intensity decreases rapidly and reversibly near the melting point of the solvent. Based on these results, Park et al. Suggested that the mechanism of AIE in these complexes is the inhibition of rapid non-radiative decay by the rotation restriction of N-aryl ring bonds at aggregation or low temperature. In response to this query, they then replaced the N-aryl heterocycle in the ligand with a hydroxyacetophenone derivative (complexes 66-68, Fig. 32), and characterized the AIE properties of complexes 66-68 by solid-state and low-temperature spectroscopy
[166]. Because the rotation of single bond in single molecule has been limited by conjugation, the theory of aggregation or low temperature limiting single bond rotation can not explain its aggregation luminescence. The crystal structure confirmed the intermolecular π-π stacking of 66 ~ 68, indicating that the excimer induced by π-π stacking is the origin of the aggregate luminescence. This well-known controversy indicates that the luminescence mechanism of AIE metal complexes remains to be further elucidated.
Although the RIM mechanism can not explain the aggregated luminescence of complexes 60, 61 and 66 ~ 68, it has been successfully applied to the design of other AIE iridium (Ⅲ) complexes. Acharya and Laskar et al. introduced triphenylphosphine and its derivatives into iridium (III) complexes (complexes 69 ~ 80, Figure 33), and constructed a series of iridium (III) complexes with AIE properties, which were successfully applied in luminescent materials, small molecule detection (CO
2, H
+, OH
-, Hg
2+, etc.), biological imaging and other fields
[167⇓~169][168,170⇓~172][173⇓~175]. The molecular packing analysis of the crystal structures and the DFT calculation results reveal that RIM is the main source of the AIE activity of these complexes: there is intermolecular C — H … π interaction between the triphenylphosphine benzene ring and the rigid bipyridine derivative ligand of the adjacent molecule during aggregation, which severely limits the internal rotation of these triphenylphosphine benzene rings
[176].
Su et al. designed a series of iridium (III) complexes with AIE activity (complexes 81 ~ 91, Fig. 34) by using triazolyl pyridine derivatives as ligands and introducing a flexible carbon chain from one end of the N atom of the triazole group
[177⇓⇓~180]. By adjusting the length of the carbon chain and grafting carbazole, phenothiazine and dihydroacridine derivatives with AIE activity at the other end of the carbon chain, the relationship between the structure of the complexes and their AIE activity and emission wavelength was explored. The results show that the complexes usually have strong luminescence in dilute solution when the carbon chain end is not grafted with AIE groups, which is mainly due to the
3MLCT and
3LLCT of the complexes. On the contrary, the HOMO orbital electron cloud of the complex concentrates on the AIE group, and the HOMO → LUMO transition of the complex is mainly
3ILCT in dilute solution, showing weak emission or even no emission.
The construction of polynuclear iridium (III) complexes with AIE active groups as the core is another strategy to prepare AIE active complexes by RIM mechanism. Bryce, Su, and Xu et al. Synthesized a series of binuclear, trinuclear, and polynuclear iridium (III) complexes (complexes 91-97, Fig. 35) with p-di (N-vinyl) benzene, m-triphenylbenzene, triphenylamine, and tetraphenylethylene as cores, respectively
[181⇓⇓~184].
Chao et al. Developed a series of AIE-iridium (III) complexes by introducing triphenylamine structure at the end of the complexes
[157]. Experiments such as co-incubation with commercial organelle dyes and ICP-MS showed that the series of complexes could cross the cell membrane and selectively aggregate on the mitochondria, thereby lighting up the mitochondria (complex 98, fig. 36). By adjusting the auxiliary ligand, complex 99 can produce a large amount of singlet oxygen when illuminated after aggregation, which is an excellent mitochondria-targeted photodynamic diagnostic and therapeutic agent (fig. 36)
[158]. In addition, they also constructed two examples of mitochondria-targeted photosensitizers with AIE activity by introducing a freely rotatable benzene ring on the imidazole ring (complex 100, fig. 36)
[185]. In addition to the above examples, many AIE active iridium (III) complexes have been reported and partially applied in the fields of light-emitting diodes, small molecule detection, disease diagnosis and treatment, etc
[186⇓⇓⇓⇓⇓⇓~193].
2.4.2 Coinage metal cluster
Coin metal cluster is a kind of cluster molecule with precise atomic structure based on copper, silver, gold and other coin metals. The photoluminescence (PL) of metallic nanoclusters is characterized by long wavelength and long lifetime. However, the practical application of luminescent metallic nanoclusters is restricted by their low quantum efficiency
[194,195]. The discovery of new metal clusters with AIE properties has significantly improved the luminescence quantum yield of such compounds, greatly expanded their application prospects, and become the focus of attention of researchers
[196].
From Au (I) -thiol polymer, Xie et al. Found that the polymer dispersed in water does not emit light, but it can be induced to aggregate by solvent or cation to produce strong luminescence, and its luminescence intensity and color are related to the degree of aggregation
[196]. In 2012, the first Au (I) -thiol compound with AIE characteristics was reported, providing new insights into the origin of the luminescence of coin-metal nanoclusters. When the volume fraction of ethanol is less than 75%, the Au (Ⅰ) -thiol compound solution does not emit light; As the volume fraction of ethanol increases, the compounds aggregate and the fluorescence of the solution gradually increases (Figure 37). With the change of aggregation degree, the emission wavelength shifts from 630 nm to 565 nm, which is due to the fact that the metallophilic interaction between compounds is more dominant than that within compounds during the aggregation process.
Subsequently, Xie et al. Used a series of glutathione (GSH) -protected water-soluble gold nanoclusters ( [Au
10SR
10], [Au
15SR
13], [Au
18SR
14] and [Au
25SR
18]
-) as models to reveal the effect of surface Au (I) -thiol units on gold nanoclusters
[197]. The results show that the emission wavelength range of gold nanoclusters can be changed from the visible region to the near-infrared II region by changing the length of the Au (I) -thiol unit (fig. 38). In addition, by reducing the length of Au (I) -thiol units on the surface of gold nanoclusters, the luminescence of gold nanoclusters can change from phosphorescence with AIE characteristics to fluorescence induced by Au (0) core.
Wang et al. Are also one of the early researchers to study metal nanoclusters with AIE activity
[198,199]. They successfully prepared a series of AIE copper nanoclusters with good luminescence and stability by using functional organic ligands such as glutathione and penicillamine. In 2013, they used non-luminescent nanocrystals as raw materials to prepare luminescent nanoclusters with good water solubility and high stability, showing typical AIE characteristics. The cluster has a strong luminescence intensity in the aggregation state and a weak luminescence intensity in the solution state. The mechanism study shows that the AIE effect of the copper nanocluster is due to the restriction of intramolecular motion in the aggregated state
[198]. Subsequently, they successfully prepared another luminescent copper nanocluster. The fluorescence intensity of the cluster decreased linearly with the change of pH from 5.0 to 6.2. When the pH value was 4.0, the fluorescence intensity was about 100 times higher than that when the pH value was 6.2, so it was successfully used for pH detection (Fig. 39). Meanwhile, the fluorescence of the copper nanocluster can be effectively quenched by H
2O
2, and the copper nanocluster is successfully used for biosensing
[199].
It is a simple and effective method to construct metal nanoclusters with AIE activity by using the AIE performance of ligands to endow clusters with AIE characteristics. In 2019, Zang et al. Successfully synthesized cyclic Au
3, Ag
3 and Cu
3 cluster molecules with AIE activity by using 3,5-dimethyl-4- (4- (1,2,2-triphenylvinyl) benzyl) -1H-pyrazole (DTBP) with classical AIE group TPE structure as peripheral protection ligand
[200]. DTBP ligand has almost no fluorescence in the solvent with good solubility, but it shows strong blue-green fluorescence with the addition of water, indicating that the ligand has AIE properties. Taking the Ag
3 cluster as an example, when the poor solvent isopropanol was added to the tetrahydrofuran solution of the Ag
3 cluster without fluorescence, the fluorescence of the cluster gradually increased (fig. 40). In addition, the Ag
3 cluster in the crystalline state also exhibits similar blue-green fluorescence emission with a quantum yield of 12%. The mechanism study shows that the strong aggregated or crystalline luminescence is due to the restriction of intramolecular motion, which suppresses the non-radiative transition of excitons.
Crystalline-induced luminescence (CIE) is a derivative phenomenon of AIE. In 2017, Zhu et al. Reported the first bimetallic nanocluster Au
4Ag
13(DPPM)
3(SR)
9 with CIE properties
[201]. The cluster as a whole is a wind-like fan-shaped structure (Fig. 41), showing strong red luminescence in the crystalline state, in marked contrast to the weak luminescence in the amorphous state and the absence of luminescence in the solution state. The structure analysis shows that the compact C — H … π interaction is helpful to restrict the intramolecular vibration and rotation, which can effectively enhance the radiative transition of the crystal state and lead to its strong luminescence. Since non-covalent interactions such as hydrogen bonding, π … π stacking and cation … π interactions are ubiquitous and can be easily regulated by changing the chemical environment, it is expected that more CIE nanoclusters will be developed in the future to further explore the relationship between their structures and optical properties.
In 2018, Zhu et al. proposed a novel AIE mechanism of metal nanoclusters: inducing photoluminescence by limiting the dissociation-aggregation process of ligands in the clusters
[202]. Taking Ag
29(BDT)
12(PPh
3)
4 nanoclusters as an example, the nanoclusters have almost no fluorescence in solution, but show obvious red luminescence in crystalline or solid state. By adding the triphenylphosphine ligand to the N, N-dimethylformamide solution of Ag
29(BDT)
12(PPh
3)
4 nanoclusters, the dissociation-aggregation process of the triphenylphosphine ligand in the clusters was limited, the solution luminescence intensity was obviously enhanced, and the luminescence quantum yield was increased from 0.9% to 11.7% (Figure 42).
In 2015, Zhang et al. prepared copper nanoclusters with AIE characteristics by using dodecanethiol as an organic ligand
[203]. By forming a compact and ordered assembly, the cluster exhibits strong emission at 490 nm with a quantum efficiency of 6. 5%. By changing the self-assembly structure from the compact ribbon structure to the loose sheet structure, the luminescence intensity gradually decreased, and the wavelength red-shifted from 489 nm to 547 nm. This result indicates that the luminescent color of the copper nanocluster assembly can be affected by the self-assembly strategy (fig. 43a). The mechanism study shows that the self-assembly induced luminescence enhancement of copper nanoclusters can be attributed to the improvement of the interaction between ligands and adjacent clusters, which reduces the energy of non-radiative transition and makes radiative transition the main way. Subsequently, Zhang et al. Further used ethanol to regulate the self-assembly process of copper nanoclusters, revealing the important contribution of metal defects to the AIE performance of clusters
[204]. In the presence of ethanol, the Cu nanoclusters rapidly assembled into ultrathin nanosheet structures, facilitating the formation of abundant metallic defect surfaces (Fig. 43b ~ d). The surface with abundant metal defects increases the ratio of Cu (Ⅰ) to Cu (0) in the nanosheets, which affects the Ligand-to-metal-metal charge transfer (LMMCT) process and promotes the radiative transition of excitons.
Interestingly, some metal nanoclusters with AIE characteristics can realize the transition from fluorescence to phosphorescence in different States
[205,206]. Sun et al. prepared a novel water-soluble nanocluster (NH
4)
9[Ag
9(mba)
9], and the aqueous solution of the cluster exhibited relatively low luminescence intensity at room temperature, which was hardly observed by the naked eye
[206]. However, in the frozen state, the cluster shows a strong yellow fluorescence. When the poor solvent ethanol is added to the aqueous solution of the clusters, the solution undergoes a gelation reaction, the clusters in the solution aggregate and self-assemble to form a highly ordered fibrous structure, accompanied by a significant enhancement of luminescence. The results show that the enhanced luminescence is attributed to phosphorescence. That is, during the aggregation process, (NH
4)
9[Ag
9(mba)
9] self-assembled into a fibrous structure, showing a clear AIE effect, and the luminescence lifetime increased by 144 times with the transition from fluorescence to phosphorescence (Figure 44).
Chiral metal clusters with AIE characteristics can also show excellent Circularly polarized luminescence (CPL) characteristics. Inspired by the idea of propeller-like design of organic AIE system, Zang et al. Reported a pair of propeller-like chiral trinuclear copper clusters, which showed typical AIE characteristics
[207]. The copper cluster is synthesized by using axially chiral R/S-2,2 '-bis (diphenylphosphine) -1,1' -binaphthyl (R/S-BINAP) as a ligand and tert-butyl copper sulfide as a copper source through a simple one-pot method. In which that R/S-BINAP ligand are arranged around the core helix of the metal cluster, forming a unique trilobal fan helix structure (fig. 45 a). The cluster has obvious AIE characteristics in the system of dimethyl sulfoxide and water. When the water volume fraction is lower than 40%, there is no photoluminescence. When the water volume fraction is higher than 50%, the emission peak at 605 nm appears and increases with the increase of water content (Fig. 45b, C). The results show that the luminescence of the clusters originates from the metal cluster center (MCC) mechanism and the metal-to-ligand charge transfer (
3MLCT) mechanism, which can be effectively adjusted by restricting the intramolecular rotation, thus changing the luminescence of the clusters. Due to the chiral structure and AIE characteristics of the cluster, it shows excellent circularly polarized luminescence properties in the aggregate state, and the luminescence anisotropy factor reaches 2×10
-2. In addition, the clusters also show good photoinduced ROS generation ability, which can be used as photosensitizers in PDT.
In 2020, Zang et al. successfully obtained and characterized a pair of atomically precise chiral copper alkynyl clusters [Cu
14(R/S-DPM)
8](PF
6)
6(R/S-Cu
14) by using a pair of chiral alkynyl ligands (R/S) -2-diphenyl-2-hydroxymethylpyrrolidone-1-propyne (R/S-DPM) with multiple benzene rings
[208]. The cluster shows obvious AIE characteristics and CD signals with mirror symmetry in solution, but no photoluminescence. The experiment shows that the addition of poor solvent can induce the aggregation of chiral copper alkyne clusters, showing bright red luminescence and CPL signal (fig. 46). With the increase of poor solvent content, the signal intensity of CPL gradually increased with the enhancement of luminescence intensity, but the luminescence anisotropy factor did not change significantly, and its value was 1.0×10
-2. Due to the AIE characteristics and good biocompatibility, the cluster material was successfully used for bioimaging. The results show that the electrostatic interaction, C — H … π, C — H … F and O — H … F hydrogen bonding interaction can restrict the rotation and vibration of the ligand molecule to a certain extent, and enhance the radiative transition in the aggregated and crystalline States, which indicates that the AIE properties of the enantiomeric cluster are due to the restriction of intramolecular motion mechanism.
In addition, Zang et al. prepared two pairs of gold nanocluster enantiomers L/D-[Au
4(C
9H
8S
2N)
4] and L/D-{[Au
4(C
6H
10S
2N)
4]
3}
n with AIE properties by using chiral thiazolidinethione ligands
[209]. L/D-{[Au
4(C
6H
10S
2N)
4]
3}
n showed no fluorescence in N, N-dimethylformamide dilute solution, and showed obvious luminescence enhancement with the increase of water content in poor solvent (Figure 47). When the water volume fraction is 60%, the L/D-{[Au
4(C
6H
10S
2N)
4]
3}
n exhibits bright green luminescence with a luminescence quantum efficiency of 41.4%. In addition, L/D-[Au
4(C
9H
8S
2N)
4] also showed typical AIE properties, which exhibited blue luminescence with a maximum luminescence quantum efficiency of 14.4% with the addition of poor solvent. It is shown that the intercluster metallophilic interaction is an important driving force for the AIE properties, and is also the main reason for the significant differences in the emission wavelengths and luminescent quantum efficiencies of the aggregated and crystalline clusters between L/D-[Au
4(C
9H
8S
2N)
4] and L/D-{[Au
4(C
6H
10S
2N)
4]
3}
n. Due to their characteristic CPL properties (luminescence anisotropy factor 7.0×10
-3), these nanoscale metal clusters have been used to successfully construct circularly polarized organic light-emitting diodes.
Similar to monometallic clusters, some bimetallic clusters also exhibit typical AIE properties, in which the metallophilic interaction involving copper, silver, and gold plays an important role in the stability of the cluster structure and endows it with excellent optical properties depending on its structure
[210⇓⇓~213]. In addition, the intermolecular or intramolecular non-covalent interactions are easily affected by the external environment, which is beneficial to adjust the optical properties of the clusters. Chou et al. prepared bimetallic nanoclusters [Au
2Cu
2(C
2OHC
5H
8)
4]
2 and [Au
2Cu
2(C
2OHC
5H
8)
4]
n[213]. Due to the formation and dissociation of the metal-metal bonds in the clusters, they exhibit different structures in different solvents, resulting in a significant change in the luminescence intensity. The [Au
2Cu
2(C
2OHC
5H
8)
4]
2 is composed of two nearly planar Au
2Cu
2 units, and the [Au
2Cu
2(C
2OHC
5H
8)
4]
n is a polymer composed of two types of Au
4Cu
4 units.One of the Au
4Cu
4 units has a structure similar to a [Au
2Cu
2(C
2OHC
5H
8)
4]
2. The quantum efficiency of [Au
2Cu
2(C
2OHC
5H
8)
4]
n is 25%, while that of [Au
2Cu
2(C
2OHC
5H
8)
4]
2 is only 10%. The theoretical calculation shows that the increase of the emission intensity of the cluster is closely related to the increase of the metal-metal bond. Then, Chou et al. Prepared bimetallic nanoclusters with AIE properties by a simple one-pot method
[212]. In the crystalline state, the Au
2Cu
2(C
2OC
13H
11)
4 units aggregate into a one-dimensional chain structure, and each Au
2Cu
2 unit has a strong gold-copper bond. In addition to the metallophilic interaction and π-C ≡ C-Cu coordination, the O-H … O hydrogen bonding interaction between alkynyl ligands also plays an important role in stabilizing the chain structure. The structure exhibits extremely high luminescent quantum efficiency (95%) in the crystalline state and almost no fluorescence in solution, reflecting its AIE properties (fig. 48). The mechanism study shows that in the solution state, the metallophilic interaction in the one-dimensional chain structure is destroyed and the Au
2Cu
2(C
2OC
13H
11)
4 unit is obtained. In the crystalline state, the one-dimensional chain polymer is driven by the metallophilic interaction and the non-covalent interaction, so that the luminescence intensity of the cluster is significantly increased. The emission intensity of the structure can also be significantly reduced by fine-tuning the alkynyl substituent, such as the use of a less sterically hindered isopropyl-alkynyl ligand. Therefore, in this system, the diversity of aggregated structures driven by metallophilic interactions is the key to regulate the luminescent properties.
In recent years, metal clusters with AIE characteristics have become a very important class of luminescent nanoclusters. How to efficiently prepare metal nanoclusters with AIE characteristics and deeply understand the relationship between their structures and properties remains a major challenge. In addition, in the field of metal nanoclusters, besides the known mechanisms, are there more mechanisms to initiate AIE performance? Solving these problems can not only provide a practical strategy for the design and synthesis of more clusters with strong luminescent quantum efficiency, but also help to promote the application of AIE effect in the field of nanoclusters.
2.4.3 Supramolecular Organometallic Complexes with AIE Properties
In recent decades, coordination-directed self-assembly has been proven to be an effective strategy for the construction of Supramolecular coordination complexes (SCCs) with controllable shape and size in the rapid development of Supramolecular and coordination chemistry
[214⇓~216]. By combining a series of different organic ligands and complementary Metal salts, a variety of supramolecular metal-organic complexes have been constructed, including Metallacycles, Metallacages and metal-organic frameworks (MOFs)
[217⇓⇓⇓⇓~222]. Importantly, functional materials based on supramolecular metal-organic complexes have shown certain advantages in synthesis, design, functional integration and application.
On this basis, the two fields of AIE and coordination-directed self-assembly are naturally integrated to construct AIE active supramolecular coordination complexes. In the past decade, many research groups at home and abroad have successfully developed a series of new supramolecular organometallic complexes with AIE properties
[223⇓⇓~226]. TPE has been widely used in the construction of metal-organic complexes with AIE properties because of its simple synthesis and easy modification
[227⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓⇓~240]. In this section, we review the recent progress in the construction and properties of TPE-based AIE metal-organic complexes, including AIE metal-organic macrocycles, metal-organic cages and metal-organic frameworks, and their synthesis, properties and applications in fluorescence sensing, cell imaging and therapeutic diagnostics, and luminescent materials. Due to the limitation of space, this section only selects some representative research work to summarize.
2.4.3.1 Metal-Organic Macrocycles with AIE Properties
Metal-organic macrocycles usually have discrete and precise two-dimensional (2D) supramolecular structures, which can provide a very clear supporting framework for the precise positioning of various functional groups, especially organic fluorophores
[241,242]. In addition, since the metal macrocyclic skeleton will be positively charged after the formation of the metal-ligand coordination bond, electrostatic interactions may occur with negatively charged compounds
[243]. Using these characteristics, Yang et al. reported the first TPE functionalized hexagonal metal-organic macrocycle 101 in 2015, and realized the selective detection of heparin, an anticoagulant drug (Fig. 49)
[244]. Metalorganic macrocycle 101 is prepared by self-assembly of a 120 ° TPE containing donor and a 120 ° platinum metal salt containing acceptor. It is worth noting that there are six positive charges in the core framework of the organometallic macrocycle 101. When the negatively charged heparin sodium was added to the mixed solvent of acetone and water of organometallic macrocycle 101, the fluorescence emission intensity increased with the increase of the content of heparin sodium (fig. 49b). These results indicate that metallo-organic macrocycle 101 and heparin sodium aggregate under the induction of electrostatic interaction, thus restricting the intramolecular movement of TPE units. In addition, atomic force microscopy (AFM) was used to study the morphology of the mixture of organometallic macrocycle 101 and heparin sodium, and the AFM image showed that the mixture had a beaded chain morphology with a height distribution ranging from 1.0 to 4.0 nm (fig. 49c). This result further supports the complexation of metallo-organic macrocycle 101 and heparin sodium under electrostatic interaction. Interestingly, metallo-organic macrocycle 101 can selectively recognize heparin sodium. For example, when other biomolecules such as sucrose, glucose, lactose, human serum albumin and bovine serum albumin were added to the metallo-organic macrocyclic 101 solution, no significant change in emission intensity was observed.
Subsequently, Stang and Niu et al. reported a metal-organic macrocycle 102 based on TPE units, which can construct metal-organic biohybrid materials with tobacco mosaic virus (TMV) through multiple electrostatic interactions (fig. 50)
[245]. By simply mixing the aqueous solution of TMV with the DMSO solution of organometallic macrocycle 102, the system showed significant fluorescence enhancement. Transmission electron microscopy (TEM) studies showed that the TMV changed from a one-dimensional rod-like morphology to a three-dimensional micron-scale bundle-like superstructure, which was due to the close arrangement of TMV after the addition of metal organic macrocycle 102. The results indicate that the organometallic macrocycle 102 can act as a "molecular glue" to bind the monodisperse TMV together through multiple electrostatic interactions. In addition, taking advantage of the dynamic reversibility of the coordination bond, tetrabutylammonium bromide (TBAB) was added to the organometallic macrocycle 102 and TMV hybrid complex, and the organometallic macrocycle 102 was assembled uncontrollably, thus realizing the controlled release of TMV. These results indicate that the organometallic macrocycle 102 can not only be used as an AIE-based fluorescent sensor to probe the stepwise self-assembly process, but also be used to fabricate stimuli-responsive organometallic biohybrids.
In 2015, Stang, Huang and Yan et al. designed and synthesized 120 ° bipyridine ligand 103 containing TPE units, and then successfully constructed a series of novel multi-TPE metal-organic macrocycles 109 ~ 113 with different shapes and sizes through coordination-directed self-assembly (Fig. 51)
[246]. The study of AIE properties showed that with the increase of hexane content in dichloromethane/hexane mixed solution, the fluorescence of organometallic macrocycles 109 ~ 113 showed significant enhancement, while ligand 103 had no AIE properties under the same conditions. Further studies show that the luminescent properties of these multi-TPE organometallic macrocycles are closely related to their structural factors, such as size, shape, location of TPE and distance between them. For example, in the aggregated state, the fluorescence quantum yield values of the three TPE containing hexagonal metal-organic macrocycles 101 and 110 are higher than those of the six TPE containing hexagonal metal-organic macrocycles 112 and 113. Besides the structural factors, the counteranions also have important effects on the photophysical properties of organometallic macrocycles with AIE properties. For example, in this work, the fluorescence quantum yields of organometallic macrocycles follow the
>OTf
->
in both solution and aggregate States.
In 2019, Li and Yang et al. reported a class of rosette-like metal-organic assemblies with tunable fluorescence emission characteristics
[247]. They first designed and synthesized three generations of TPE ligands 114 ~ 116 containing multiple terpyridine (TPY) units, and when these ligands were assembled with Cd
2+, three generations of rosette ring assemblies 117, 118 and 119 were obtained in high yields (Fig. 52). It should be noted that when the Cd
2+ is assembled with ligand 114, the system forms a mixture of trimer, tetramer, pentamer, and hexamer macrocycles rather than a single product (fig. 52A). However, coordination-directed self-assembly of ligand 115 or 116 with Cd
2+ resulted in the formation of a bilayer hexamer 118 and a trilayer heptamer 119, respectively (figs. 52b and 52c). This may be because multi-terpyridine units can provide more geometric constraints in self-assembly through multivalent interactions, leading to the formation of the most favorable thermodynamic products. Interestingly, rosette-shaped assemblies show photophysical properties that some monomers do not possess. For example, bilayer hexamer 118 exhibits interesting room-temperature white light emission in acetonitrile/water mixed solvents.
Recently, Yang et al. reported a case of liquid crystal functionalized AIE metal-organic macrocycle, and successfully realized the application research of holographic image storage (Fig. 53)
[248]. They designed and synthesized a new type of 120 ° TPE containing bipyridine ligand 120, which modified the dendritic vinylidene liquid crystal unit. Using the coordination-directed self-assembly of ligand 120 and metal salt receptor 121 or 122, the construction of rhombohedral metal-organic macrocycle 123 and hexagonal metal-organic macrocycle 124 was achieved (fig. 53 a). Spectroscopic studies show that ligand 120, organometallic macrocycles 123 and 124 all have AIE properties in acetone/water mixed solvents. Subsequently, they systematically studied the liquid crystal properties of the assembly system, and the results showed that the two metal-organic macrocycles showed smectic A liquid crystal phase at room temperature, while the ligands showed rectangular columnar liquid crystal phase. Finally, based on the liquid-crystal-functionalized AIE metal-organic macrocyclic ring 123, a series of color holographic images with high definition and high brightness were obtained (Fig. 53b ~ f) with the theme of "Annual Surplus", and the holographic storage of color images was realized through the liquid-crystal-functionalized supramolecular metal-organic macrocyclic ring.
Artificial Light-harvesting systems (LHSs) based on AIE organometallic complexes can not only help people understand plant photosynthesis, but also have a wide range of applications in the fields of solar energy development, optoelectronic materials and chemical sensing. Recently, Yi and Zhang et al. successfully constructed a supramolecular artificial light harvesting system of cascade FRET through AIE metal-organic macrocycle (Fig. 54)
[249]. They aggregated the obtained metallo-organic macrocycle 127 containing TPE (fig. 54a) in a water/methanol mixed solvent system to form dispersed nanoparticles, and used the aggregated nanoparticles as donors to load the fluorescent dye Eosin Y (Eosin Y) as an acceptor to construct a primary artificial light harvesting system; Sulforhodamine 101 (SR101) was further introduced as a secondary receptor to construct an artificial light harvesting system with a cascade FRET process (fig. 54b). Interestingly, the artificial light-harvesting system showed significantly enhanced photocatalytic performance in the C — H bond alkylation reaction.
2.4.3.2 Metal-Organic Cages with AIE Properties
In 2015, Stang and Huang et al. designed and synthesized a novel tetrapyridyl TPE derivative 128. The ligand was mixed with dicarboxylate sodium salt ligand 129 or 131 and 90 ° metal salt receptor 130 in a ratio of 1:2:4, and two tetragonal prismatic metal-organic cages 132 and 133 were constructed through three-component coordination bond-directed self-assembly, which is the first example of metal-organic cage with AIE properties (Fig. 55a)
[250]. At a concentration of 10.0.m u.M, the fluorescence intensity of the organometallic cages 132 and 133 is enhanced by more than about 20 times compared with that of the ligand. Interestingly, by simply increasing the ratio of n-hexane in dichloromethane, the fluorescence emission color of the metallo-organic cage 132 changes from yellow to blue to cyan, with a significant fluorescence enhancement. In addition, the organometallic cage 133 exhibits various emission colors in different polar solvents, for example, white light emission in tetrahydrofuran (fig. 55b). This work realizes the room-temperature white light emission of a single metal-organic cage in a single solvent, which has broad potential applications in fluorescent probes, biological imaging agents, optoelectronic devices and so on.
In 2019, Zhang et al. successfully constructed a supramolecular artificial light harvesting system based on a tetragonal prismatic metal-organic cage, and used it in the study of photocatalytic cross-coupling hydrogen evolution reaction (Fig. 56)
[251]. They first designed and synthesized a TPE-based tetracarboxylic acid sodium salt ligand 134, and then constructed a square prismatic metal organic cage 136 through coordination-oriented self-assembly with a bipyridine ligand 135 modified with an oligoethylene glycol chain and a 90 ° metal salt receptor 130. At a concentration of 10 μM, the organometallic cage 136 aggregates in aqueous phase to form spherical nanoparticles with high fluorescence emission and a particle size of about 140 nm. The water dispersible nanoparticles can encapsulate fluorescent dye Eosin Y to construct an artificial light harvesting system with TPE metal organic cage 136 as a donor and Eosin Y as an acceptor. Since Eosin Y, as a photosensitizer, can catalyze the C-H arylation and cross-coupling hydrogen evolution reaction of heteroarenes, the obtained artificial light-harvesting system was used as a cross-coupling photocatalyst. Compared with Eosin Y alone, this artificial light-harvesting based catalytic system showed better catalytic activity in the cross-coupled hydrogen evolution reaction.
In 2016, Su et al. reported a class of TPE functionalized silver-phosphine tetragonal prismatic metal organic cages (Fig. 57)
[252]. TPE ligand 137 and AgBF
4 were mixed in a 1:2 ratio to give discrete metal-organic cages 138, the structure of which was determined by X-ray crystallography (fig. 57 a). Due to the restricted intramolecular motion of the TPE units in the organometallic cage 138, it has high fluorescence emission efficiency in both dilute and aggregated States. Interestingly, it was shown that the four silver centers of the metallo-organic cage 138 can be used to bind guest ions or alkene compounds with a concomitant decrease in fluorescence emission (fig. 57b, C). For example, the fluorescence emission of the metallo-organic cage 138 is quenched in the presence of I,
, and trans and cis stilbenes. Therefore, this work provides an example of a metal organic cage that can be used for molecular sensing of AIE.
In 2020, Liu and Zhao designed and constructed a highly stable zirconium cluster metal organic cage based on TPE unit, and successfully applied it to live cell imaging in vitro
[253]. They first synthesized two 120 ° dicarboxylate ligands 139 and 140 with TPE unit as the core skeleton, and then constructed two zirconium cluster metal organic cages 141 and 142 with different sizes through coordination-oriented self-assembly with highly stable trinuclear zirconium clusters (Fig. 58a), showing excellent AIE properties (Fig. 58b, C). It should be pointed out that the two metal organic cages 141 and 142 based on zirconium clusters have high structural stability and low cytotoxicity, which can be used for in vitro living cell imaging and provide a new material basis for further research in cancer cell diagnosis, drug delivery and photodynamic therapy.
The TPE unit can also be introduced into the outside or inside of the metal organic cage through chemical modification, thereby realizing the construction of the AIE metal organic cage. Recently, Stang and Yan et al. designed and synthesized bipyridine ligands 143 and 144 of TPE functionalized at different sites, and then prepared M
12L
24-type metal organic cages 145 and 146 by coordination-oriented self-assembly with Pd(CH
3CN)
4(OTf)
2 or Pt(CH
3CN)
4(OTf)
2 (Figure 59 a)
[254]. The molecular simulation results show that the TPE group of the metallo-organic cage 145 is outside the metallo-organic cage, showing a higher degree of freedom and flexibility, compared with the TPE unit 146 inside the cage cavity, so they have different emission characteristics. Interestingly, the absorption curves of the four metal-organic cages show that their optical absorption properties are affected by the species of the coordinated metal ions, regardless of the location of the TPE. For example, the metallo-organic cages containing the same metal ion have almost the same characteristic absorption band, while the absorption of the metallo-organic cages 145b and 146b based on the Pt ion is red-shifted (fig. 59b).
In addition, the emission spectra showed that the Pd ion based metal organic cages 145a and 146a exhibited significant fluorescence quenching. Notably, the endofunctionalized metalloorganic cage 146b exhibited stronger fluorescence emission than the exofunctionalized 145b, which was attributed to the additional intramolecular motion restriction of the endofunctionalized TPE unit in the cage cavity-confined environment (fig. 59c). These results indicate that the confinement effect of metal-organic cages can significantly improve the fluorescence emission properties of TPE motifs.
2.4.3.3 Metal-Organic Frameworks with AIE Properties
Metal-organic frameworks (MOFs) are a class of one-dimensional, two-dimensional or three-dimensional rigid extended structures formed by metal ions or clusters coordinated with organic ligands. In the past decades, MOFs have become one of the most rapidly developing research fields in chemistry, materials, biomedicine and other disciplines. MOFs usually have organic – inorganic hybrid characteristics, high specific surface area, tunable porosity, and variable chemical structures and properties, which exhibit many potential applications in gas storage and separation, catalysis, magnets, sensing, drug delivery, and cancer therapy.
In 2011, Dinc Dincet al. Reported a MOF 148 based on TPE units
[255]. The MOF was formed by self-assembly of ligand 147 and Zn(NO
3)
2·6H
2O in a mixed solvent system of N, N-diethylformamide (DEF) and ethanol (fig. 60 a). By X-ray single crystal diffraction analysis, MOF 148 is a secondary building unit with a paddle-wheel-like Zn
2(O
2C
-)
4 bridged by ligands 147 to form an interlaced two-dimensional sheet structure. In MOF 148, the nearest intermolecular phenyl-phenyl distance is 4.753753 Å, which is more than 11 Å larger than the shortest phenyl-phenyl distance (3.44 Å) of common TPE derivatives. Interestingly, MOF 48 maintains its fluorescence performance even at up to 350 ° C (fig. 60b)
[256]. MOF 148 was shown to lose only 50% of its initial fluorescence intensity at 190 ° C by temperature-dependent fluorescence emission studies. This result is quite different from the behavior of most TPE-based luminescent systems, which usually do not emit light at high temperatures. In addition, the unique function of MOF 148 enables it to selectively detect gaseous ammonia at 100 ° C (Figure 60c). When the activated MOF 148 was exposed to different vapors (e.g., ammonia, triethylamine, ethylenediamine, DEF, and water vapors) at 100.d egree. C., only ammonia caused a significant change in the emission maximum.
Zhou et al. reported a novel MOF 150, which was self-assembled based on TPE organic ligand 149 and ZrCl
4 in the system of benzoic acid and DEF (Figure 61a)
[257]. The photophysical properties of TPE-based organic ligand 149 show that it exhibits yellow fluorescence emission in the solid state at room temperature and ambient atmosphere with a quantum yield (Ф
F) of 30.0%. However, due to the rigid structure of the fluorophore in MOF 150, its fluorescent emission color changes from yellow to blue. In addition, the Ф
F value of MOF 150 increased to 99.9% under argon atmosphere and at room temperature (fig. 61b, C). When trifluoroacetic acid (TFAA) was used as a competing reagent instead of benzoic acid, another novel TPE-based MOF 151 was successfully prepared using the same ligand and metal salt (fig. 62a)
[258].
The initially prepared MOF 151 was a white powder (151-W) and emitted a bright blue fluorescence at 470 nm. However, when 151-W was compressed with a glass slide or treated with 6.0 M HCl solution, the TFA group coordinated to the Zr6 cluster was lost and its color changed from white to yellow (151-Y). The fluorescence emission of 151-Y is at 538 nm, appearing as green fluorescence, which is red-shifted by nearly 70 nm compared to 151-W (Figure 62b). In addition, the changes of color and luminescence are completely reversible (Figure 62c), which is a class of reversible pressure-induced fluorescent color-changing MOFs with potential applications in photocatalysis and sensing.
In addition, Liu, Tang, and Zhao et al. reported a 2D layered MOF constructed from a novel TPE ligand, in which two phenyl rings without carboxylate groups can be confined by molecular interaction with the analyte
[259]. Thus, the two-dimensional MOF shows responsive turn-on fluorescence to various volatile organic compounds. Li et al. designed and synthesized a new luminescent MOF, which can be used as a chemosensor for efficient and selective optical detection of mycotoxins via a luminescence quenching mechanism
[260].
3 AIE mechanism
AIE breaks the classical judgment of "concentration quenching luminescence", changes people's traditional understanding of luminescent materials, and opens up a new thinking mode for the development of organic luminescent materials. After 20 years of development, AIE has achieved many innovative research results in theoretical mechanism, molecular design, functional materials, optoelectronic and biological devices, which makes it a frontier hotspot in the field of chemistry and materials research. Due to the variety of organic molecules, the variability of their chemical structures and the diversity of their intramolecular motion, the electronic excited States of molecules have a variety of decay pathways, and the decay time scales span a wide range (such as picoseconds, nanoseconds and even seconds). Isolated organic molecular systems can move freely, resulting in complex and diverse photochemical and photophysical processes. If multiple organic molecules are brought together by intermolecular interactions, stable organic molecular aggregates are formed. Then its electronic excited state will decay under the "confined" conditions of various intermolecular interactions, and many decay pathways will inevitably be changed, blocked or even cut off. Therefore, as we have observed, the same molecule exhibits completely different photophysical behaviors in different States, such as ACQ, AIE, aggregation-induced delayed fluorescence, organic room temperature phosphorescence, and force-stimulus-responsive luminescence. However, there is no unified AIE mechanism that can reasonably explain the AIE phenomenon in all organic systems, based on the fact that there are many decay pathways, long decay time span and strong environmental dependence of the excited state of organic molecules. This section briefly summarizes several popular AIE mechanisms and their applicable regime categories. This will contribute to the understanding of AIE, and then benefit the structural innovation and the development of new applications of AIE system.
3.1 Microscopic mechanism of AIE
After absorbing a photon, an isolated organic molecule generates a high-energy excited state R*,R* from the ground state R, which can not exist stably, and quickly returns to the low-energy ground state by releasing energy (Fig. 63). The energy dissipation pathways are: (1) kept in the resonance region, the R* undergoes radiative transitions (rate kr) and nonradiative transitions induced by vibrational relaxation (NR-VR, rate or );(2) Beyond the resonance region, the R* undergoes a large structural change to generate P*, which decays to P (NR-ISO with reradiation and non-radiation at a rate of ); Or reach the potential energy surface crossing MECP (NR-MECP, with the rate ), and then rapidly decay without radiation to P or R (both and can be written as ). These energy dissipation processes coexist and compete with each other. Therefore, the luminescence of an organic molecule must satisfy the following conditions at the same time: (1) the molecular excitation energy is in the visible region; (2) effective radiation process; (3) The radiative rate cannot be much smaller than the sum of all non-radiative rates.
Organic molecular aggregates have a wide variety of interactions, such as hydrogen bonding, halogen bonding, π-halogen bonding, π-π, cation-π, anionic -π
+, dipole-dipole, ion-dipole, and so on. These interactions are essentially represented by four forces: electrostatic, diffusive, repulsive, and inductive. They can change the geometric structure (vibrational type, vibrational energy, vibrational coupling, etc.), electronic structure (electronic configuration and properties, electronic coupling, spin-orbit coupling, electronic transition dipole moment, etc.) And electron-vibrational coupling of the excited state of organic molecules.Thereby affecting various energy dissipation paths of the molecular excited state, and causing the organic aggregate to show completely different photophysical behaviors from the isolated molecule
[261]. As a result, various AIE mechanisms have emerged, such as J-aggregate luminescence mechanism, limited intramolecular motion, blocked radiationless channel, elimination of photoisomerization process, excited-state intramolecular proton transfer, blocked conical intersection pathway, crystal-induced conversion of dark state to bright state, Herzberg-Tell type electron-phonon coupling induced luminescence, etc
[262][84,118][263][264][265][266][267][268]. Below, we will briefly review several popular AIE mechanisms and their applicable system characteristics.
3.1 Luminescence mechanism of J-aggregate
In the early stage, most of the luminescent systems studied were organic planar conjugated molecules, which formed aggregates with typical face-to-face stacking (H-aggregates) or head-to-tail stacking (J-aggregates). According to the in-phase superposition and antiphase cancellation of the transition dipole moment of the Kasha exciton model (fig. 64, taking the dimer as an example), the lowest electronic excited state of the H-aggregate is an antisymmetric exciton state of a transition dipole forbidden state, resulting in luminescence quenching; The lowest electronic excited state of the J-aggregate is a transition-dipole-allowed symmetric exciton state (the transition dipole moment of the dimer is 1.414 times that of the single molecule), which leads to the enhancement of luminescence. The luminescence enhancement of J-aggregates explains the phenomenon that the luminescence of some organic planar molecules is enhanced after aggregation, which was once considered to be the main reason for the luminescence enhancement of organic molecules aggregation
[269]. However, it can not give a reasonable explanation for the aggregation-induced luminescence phenomena of flexible systems or partially planar conjugated systems or even some organic planar conjugated systems. On the one hand, the luminescence properties of organic molecular aggregates are determined by the competition between the intramolecular reorganization energy (electron-vibrational coupling) λ and the intermolecular excitonic coupling J. In flexible and partially planar conjugated systems, J is usually much smaller than λ (J/λ < 0.17 at room temperature), and the spectral shape is mainly determined by the intramolecular electron-vibration coupling rather than the intermolecular exciton coupling
[270]. On the other hand, in some organic planar conjugated systems, not only the intermolecular exciton coupling is large, but also the degree of intermolecular charge transfer is serious, which will lead to multiple coupling between the exciton coupling state and the charge transfer state. In this case, the H-aggregate may exhibit a luminescence enhancement phenomenon similar to the J-aggregate, and the J-aggregate exhibits a luminescence reduction phenomenon similar to the H-aggregate, as shown in fig. 64
[271⇓~273].
3.2 Mechanism of restricted intramolecular motion
In 2003, Tang et al. Used a series of internal chemical methods and external physical methods to systematically study the effects of intramolecular steric hindrance, solution viscosity, rising and falling temperature and other factors on the luminescence intensity, efficiency and lifetime of silole compounds, and proposed and confirmed the intramolecular rotation restriction (RIR) mechanism
[84]. With the development of AIE material molecules, rotor-free molecular systems also show obvious AIE phenomena. In 2014, Tang and Qian et al. compared the photophysical properties of TPE with rotor and TPE derivatives without rotor (THBDBA and BDBA) by experimental measurements and theoretical calculations
[118]. They found that the intramolecular torsional vibrations and their reforming energies in the rotorless TPE derivatives are very sensitive to the environment. In solution, the distorted structure of the excited States of these molecules changes greatly, which makes the reforming energy of the distorted vibration very large and provides an efficient energy dissipation channel for the radiationless transition process. In the aggregation state, the "confined" environment formed by the interaction between molecules greatly limits the change of the molecular distorted structure, which makes the reforming energy of the distorted vibration smaller and effectively blocks the non-radiative transition channel, so that the radiative transition process is dominant and the molecules show strong fluorescence. This is the intramolecular vibration confinement (RIV) mechanism. Therefore, they further proposed a more universal mechanism of intramolecular motion limitation (RIM) with both intramolecular rotation and vibration limitation
[118]. This mechanism is easily understood and accepted by people. The decay of the excited state of a molecule to its ground state is a process of excessive electronic energy dissipation, either in the form of light (radiation) or in the form of heat (no radiation). The restriction of intramolecular motion indicates that the change of molecular structure is restricted in the process of molecular decay from the electronic excited state to the ground state, so the heat energy consumed will be greatly reduced. In this way, the radiation process will be highlighted, and the molecular system will show significant luminescence. At present, the RIM mechanism has been widely recognized and plays an important role in guiding the structural design and practical application of AIE molecules
[274⇓~276]. In conclusion, the RIM mechanism can explain the sensitivity of various radiationless transition processes to aggregation, but further elaboration is needed to explain the AIE phenomenon in some rigid systems.
3.3 Non-radiation channel blocking mechanism
In 2005, Liu, Zhu and Shuai et al. Jointly studied the structure, electronic state, luminescence and transmission properties of three silole derivatives from both experimental and theoretical aspects
[277]. They experimentally measured the luminous efficiency of three compounds in different States, and theoretically calculated the radiative and non-radiative transition rates. The results show that the weak luminescence of DMTPS in solution is mainly caused by the large nonradiative rate. In the solid state, the molecular packing blocks the radiationless channel for the rotation of the surrounding benzene rings within the molecule, resulting in enhanced luminescence. The theoretical predictions of radiative and nonradiative rates for these compounds are confirmed by later experimental measurements. In 2007, in order to predict the nonradiative transition rate more accurately, Shuai and Peng et al developed the theory of multimode coupling nonradiative transition rate, which realized the quantitative prediction of the luminous efficiency of complex organic molecules and successfully explained the AIE phenomenon of TPBD
[278][263]. The aggregation or lowering of the temperature blocks the radiationless channel generated by the diolefin skeleton distortion and the surrounding benzene ring rotation of the TPBD, resulting in luminescence enhancement. Moreover, the coupling between the rotational modes of the benzene ring (Duschinsky rotational effect) has the strongest environmental sensitivity and largely determines the change in the radiationless decay rate. In 2011, Lin et al. studied the influence of aggregation on several physical parameters in the radiationless rate formula through the ONIOM method, and concluded that aggregation suppressed the change of the molecular structure of o-DPDBF, reduced the Huangkun coefficient and the reforming energy, thus slowing down the radiationless decay
[279]. In 2012, Shuai et al. realized the quantitative prediction of luminous efficiency of molecular aggregates for the first time by combining molecular mechanics/quantum mechanics, radiation and non-radiation rate theory of multi-mode coupling, and revealed the AIE mechanism of indole derivatives (CPEI): intermolecular hydrogen bond interaction blocked the non-radiation transition channel caused by low-frequency vibration mode, resulting in strong fluorescence
[280]. In the same year, Shuai et al. realized the hybrid quantum/classical non-adiabatic dynamics method in the framework of tight-binding density functional theory, and simulated the excited-state radiationless decay process of "open" and "closed" AIE-active and non-AIE-active cyclopentadiene derivatives (o-DPDBF and c-DPDBF) in real time
[281]. The results show that the low-frequency torsional mode of AIE molecule is effectively coupled to the electronic state and receives energy, and rapidly decays to the ground state without radiation. However, this mode of non-AIE molecules is limited by chemical bonds, and the radiation-free channel is blocked, thus emitting strong fluorescence.
Subsequently, Shuai and Peng et al. Have successively realized the multi-form and multi-scale simulation calculation of photophysical properties of compounds in solution, crystalline and amorphous States, and comprehensively investigated the intrinsic luminescence properties of molecules in different environments.The relationship between experimental measurability and microscopic physical quantities is constructed, and experimental schemes such as "resonance Raman spectroscopy to detect electron-vibration coupling", "isotope to detect vibration-vibration coupling", "aggregation size effect of luminescence intensity to detect non-radiative rate" are proposed and confirmed by experiments
[282,283]. All these prove that the electron-vibration coupling from solution to solid state and the inhibition of radiationless transition due to the weakening of vibration-vibration coupling are the main reasons for aggregation-induced luminescence (Fig. 65). According to the structural characteristics of the molecule, the vibration modes involved include ring rotation, ring-closing distortion, bond bending, bond rocking and bond stretching vibration (Fig. 65)
[284,285]. This indicates that many organic molecular systems will show AIE phenomenon, which confirms its universal existence. Many research groups have explained a series of AIE, aggregation-induced phosphorescence, aggregation-induced delayed fluorescence and so on by this mechanism
[286⇓~288].
In addition, Shuai et al. Also constructed a calculation scheme of electron-vibration coupling projection to intramolecular coordinates, which clearly outlined the chemical structure fragments of strong coupling.Based on this, the chemical structure strategies of reforming, removing, recombining and changing the molecular aggregation mode and external environment to dissociate the electron-vibration coupling and vibration-vibration coupling are proposed, which has important guiding significance for the design of new AIE systems and the development of new applications
[289,290].
There are many types and ways of nonradiative decay of electronic excited States of organic molecules, and most of the processes discussed above are nonradiative processes induced by structural relaxation in the resonant region. In 2013, Blancafort and Li et al. proposed that the aggregation of o-DPDBF cut off the radiationless process induced by the conical intersection, which made the system emit light
[266]. Su and Liao et al studied the mechanism of Aggregation induced phosphorescence emission (AIPE) of iridium metal complexes by combining theory with experiment, and the limitation of non-radiative transition decay process is the source of strong phosphorescence of aggregates
[291]. Subsequently, several research groups found the crossing point of organic molecules with low energy in solution through theoretical calculation, and the crossing point rose sharply or did not exist in the solid state, which explained many phenomena (Fig. 67)
[292⇓⇓⇓⇓~297]. Some systems relax to a local stable point on the excited potential energy surface and undergo radiative and nonradiative transitions in the Frank-Condon region at this point, but if the radiative transition here is much smaller than the radiative transition rate, the system still does not emit light (Fig. 67)
[298]. Compared with the vibrational relaxation-induced nonradiative transition, the molecular structure changes (ring deformation, ring torsion, ring formation, ring opening, bond distortion, bond swing, etc.) Will be greater, resulting in a faster nonradiative transition rate, whether the conical intersection or isomerization occurs. However, in the solid rigid environment, the change of molecular structure is limited, the radiationless pathway caused by intersection or isomerization will be blocked, the radiation process will dominate, and the system will emit light.
In a word, the mechanism of radiation-free transition hindrance and the mechanism of intramolecular motion mentioned above are the same and interrelated. The former is elaborated from the microscopic process of excited state decay, while the latter is analyzed from the change of molecular structure, which maps a clear relationship between structure and performance. The degree of change of molecular structure determines the speed of radiationless transition, so the scope of application of the two is similar.
3.4 Mechanism of aggregation-induced radiative transition
With the abundance of AIE systems, the luminescence phenomena of some novel aggregation-induced luminescence systems can not be reasonably explained by the restriction of intramolecular motion and radiationless channels. Because the lowest electronic excited state of these systems is the transition dipole-forbidden electronic state (often referred to as the dark state), the radiative transition rate is almost zero. Excited States and dark States are mostly formed due to the complete spatial separation of the wave functions of the electronic excited state and the ground state or the opposite phase of the function, which can not have any effective overlap. Because there are many kinds of organic molecules, there are different characteristics of orbitals, different degrees of electronic correlation and different properties of electronic excited States in an organic molecule. Intermolecular interactions can change the orbital and electronic correlation characteristics, and then change the nature and energy level order of the excited state, and realize the transition between the dark state and the bright state
[261]. For example, Fig. 68, in the D-A molecular system, the polar solution induces the TICT state to be the lowest excited state to quench the luminescence state, while the aggregation restricts the torsion between D and A to inhibit the formation of the TICT state, thus forming the locally excited luminescence state
[299]. In the lone-pair system, the electrostatic interaction will greatly reduce the n orbital, but has little effect on the π orbital, which will greatly raise the
1(n,π
*) or
1(σ,π
*) of the electronic state and slightly reduce the
1(π,π
*) of the electronic state.The energy sequence conversion between the
1(n,π
*)) or the
1(σ,π
*) and the
1(π,π
*)) is realized, so that the
1(π,π
*)) is converted from the second excited state in the solution to the first excited state in the aggregation state, and strong light is emitted
[300].
The first excited state of quantitative calculation is the transition dipole forbidden dark state whether in solution or in solid state. However, the molecular vibration will induce a second-order electric dipole moment, which will lead to an effective radiative transition. The mechanism of aggregation-induced radiative transition can be applied to the systems with symmetry, the systems in which n or σ orbitals participate in the electron transition, and the systems with complete charge transfer characteristics
[301].
The above four mechanisms are all in accordance with the lowest electronic state luminescence of Kasha rule. The first mechanism is that the intermolecular exciton coupling is equivalent to the intramolecular electron-vibration coupling, and the two cooperate to determine the luminescent properties of the system. In the other three mechanisms, the excitonic coupling between molecules is ignored, and the electronic, vibrational and coupled luminescence properties of a single molecule in different environments are discussed. The weak interaction between molecules can change the geometric structure, electronic structure, vibrational structure, electron-vibration coupling properties, vibration-vibration coupling and so on, and then change the radiative and non-radiative pathways to determine the luminescence properties. Among them, the core changes are the shape of the potential energy surface of the excited state and the decay path (fig. 69). In the solution, the nonradiative transition is dominated by various pathways. Under aggregation, the nonradiative transition is blocked, the radiative transition becomes stronger, and the system emits bright light.
4 Derivation of the AIE Concept
The AIE phenomenon has aroused great interest of researchers. In the process of studying AIE materials, researchers have also discovered some novel properties of AIE molecules, such as organic Room-temperature phosphorescence (RTP) and unconventional luminescence. These concepts are described below.
4.1 Organic room temperature phosphorescence
Room temperature phosphorescence (RTP) phenomena of pure organic compounds have been reported in the 19th and 20th centuries, including RTP phenomena of specific organic chromophores in gelatin, protein, agar, boric acid, sugar and other matrices, as well as RTP phenomena of tetraphenylmethane and its derivatives crystals, but the reports are generally scattered
[302⇓⇓~305][306]. In the 20th century, the research on organic phosphorescence focused more on the study of low temperature mechanism, for example, Lewis and Kasha clearly attributed phosphorescence to the radiative transition from triplet excited state to singlet ground state in 1944.In 1949, McClure proposed the heavy atom effect of enhancing intersystem crossing. In 1950, Kasha proposed that n-π
* transition could promote the generation of triplet excited state. El-Sayed et al developed it into the widely accepted El-Sayed rule
[307][308][309][310].
Since the concept of AIE was put forward, this field has made rapid development, and one of the most important research directions is pure organic RTP, which has developed rapidly in the past 10 years.During this period, on the one hand, first-principles calculations were used to assist the mechanism study, on the other hand, people paid unprecedented attention to the great application prospects of pure organic aggregate RTP materials in anti-counterfeiting, sensing, bio-imaging, light-emitting devices and other fields. The key to realize organic RTP is to promote intersystem crossing and suppress the non-radiative transition of triplet exciton. The main strategies reported to promote intersystem crossing in new material systems include the above-mentioned heavy atom effect, El-Sayed rule, and aggregation-induced intersystem crossing, charge transfer state (including charge transfer state containing sp3 linkers), hyperfine coupling, etc. Material morphology covers small molecules, macromolecules, single-component and multi-component aggregates.
4.1.1 Crystallization induced phosphorescence and aggregation induced phosphorescence
Restriction of intramolecular motion is the main reason for the AIE effect. In the solution state, active intramolecular motion, such as rotation and vibration, dissipates the energy of the excited state through non-radiative decay pathways, so that the luminophore does not emit. However, in solids, intramolecular motion is hindered or RIM process is activated, which converts the luminous source into a strong emitter
[311].
AIE molecules generally exhibit fluorescence emission and rarely phosphorescence. The reason is that for pure organic compounds, the triplet exciton is spin-forbidden, and the triplet exciton is easy to inactivate, so the phosphorescence efficiency of pure organic chromophores is very low at room temperature, and it is generally believed that they can not emit phosphorescence at room temperature
[312]. In 2010, Tang et al. Discovered a remarkable crystal-induced luminescence (CIE) process, in which the luminophore fluoresces only for the crystalline state
[311]. The intramolecular motion of these light emitting sources remains active even in the amorphous state, and can only be suppressed by crystal packing arrangement, resulting in a new CIE effect. The intramolecular motion is also one of the main reasons for the non-phosphorescent properties of organic luminophores, and its excited singlet state is very likely to cross the transition between systems to the excited triplet state. It is conceivable that in the crystalline state, the nonradiative relaxation channel of the excited triplet state may be blocked by the RIM process, which in turn provides an opportunity to develop pure organic luminophores with efficient phosphorescence at room temperature, thereby further generating crystal-induced phosphorescence (CIP)
[313].
In addition to CIP, other aggregation modes can sometimes also reduce the non-radiative transition of the triplet state, resulting in a new luminescence mode, which we call aggregation-induced phosphorescence (AIP).
4.1.2 Organic room temperature phosphorescence system
In 2015, An et al. proposed the strategy of stabilizing triplet excitons by H aggregates, and synthesized a series of organic molecules with H aggregation structure through reasonable molecular design. The lowest triplet excited state (T
1) generated a triplet excited state (
) with lower energy under the stability of H aggregation, which successfully extended the phosphorescence lifetime. Among them, the longest phosphorescence lifetime could reach 1.35 s (Figure 70)
[314].
Porous organic framework materials have fascinating properties due to the atomically precise integration of repetitive building units. An et al. successfully obtained HOAFs (Hydrogen-bonded organic aromatic frameworks) with ultra-long organic room temperature phosphorescence properties by constructing π-π stacking in the molecular layer, with the longest phosphorescence lifetime of 79.8 ms (Fig. 71)
[315]. In addition, they combined a variety of Supramolecular interactions to synthesize SOF (Supramolecular organic frameworks) with ultra-long organic room temperature phosphorescence, with lifetimes of 398 and 762 ms, respectively (Fig. 71)
[316,317].
Self-assembly of melamine with aromatic carboxylic acids in aqueous solution can form SOFs (Figure 72a), effectively limiting the nonradiative decay of triplet excitons
[318]. On the basis of learning from molecular crystals and MOFs, An et al. Also constructed a new organic ionic crystal RTP material based on aromatic carboxylic acids (Fig. 72b)
[319]. By introducing ionic bonds (N
, Na
+, and K
+), a more rigid ionic crystal structure is constructed to fully suppress the non-radiative transition, thereby enhancing the intensity of room temperature phosphorescence.
Compared with small molecular crystal materials, polymer materials have the characteristics of flexibility and light weight, and have more advantages in practical application. In recent years, An et al. introduced ionic bonds into polymer material systems, and used the synergistic effect of polymer covalent bonds to limit the movement of chromophores and achieve efficient room temperature phosphorescence (Fig. 73a)
[320,321]. In addition, based on the polymer system, they realized a polymer material with adjustable phosphorescent color (Fig. 73b)
[322].
4.1.3 Triplet state regulation of organic room temperature phosphorescence
The triplet exciton of organic molecules has long lifetime and spin multiplicity, which makes it have outstanding value and significance in a variety of photophysical processes and related application technology research. Organic phosphorescence, which originates from the radiative transition of triplet exciton, is expected to become a new generation of luminescent materials in the fields of organic optoelectronics and biomedicine. However, organic phosphorescence is usually observed in solution at low temperature and under inert conditions, which seriously limits its practical application. The development of AIE has led to a conceptual revolution in organic phosphorescence and has bridged the study of molecules and aggregates. In recent years, room temperature phosphorescence (RTP) based on organic aggregates has made rapid development, and great achievements have been made in molecular structure design and aggregation behavior control. However, due to the high sensitivity and complexity of triplet excitons in photophysical processes, the existing studies have not yet clearly explained the mechanism and principle, and researchers have been committed to solving these problems.
Fig. 74 shows a simplified Jablonski diagram, from which it can be seen that in order to obtain efficient organic phosphors for room temperature phosphorescence and improve the ФP, the following three conditions can be met from the perspective of open source and throttling: (1) there must be an effective intersystem crossover (ISC) from the lowest singlet S1 to the triplet state (Tn,n≥1), so as to increase the number of triplet excitons; (2) The rate of phosphorescence decay from the lowest triplet T1 to the ground state S0 is fast, competing over nonradiative decay and quenching processes; (3) Suppress or reduce the nonradiative decay and quenching processes of T1. The realization of persistent room temperature phosphorescence requires three key steps: (1) generation of sufficient triplet excitons; (2) Inhibition of unwanted inactivation processes (reduction of Кnr and Кq);(3)T1) has a slow Кp. It can be seen that there is an obvious conflict between improving the ФP of organic room temperature phosphorescent materials and τp, and a fast Kp is needed to improve the efficiency.While extended life requires a slow Кp, systems with high ФP(>20%) have short τp(<10 ms), while systems with long τp(>1 s) generally have low ФP. An increase in τp generally means a decrease in ФP, at which time a strong SOC is needed to achieve rapid КISC and efficient room temperature phosphorescence, but at this time it will accelerate the Кp, thus reducing the τp.Because SOC has the same effect on the increase of КISC and the increase of Кp, it is very challenging to obtain pure organic room temperature phosphorescent materials with high efficiency and long lasting performance at room temperature.
Phosphorescence is a spin-forbidden radiative decay process, which involves intersystem crossing from the excited singlet state to the excited triplet state and radiative transition from the excited triplet state to the ground state. From the phosphorescence lifetime formula , where, τp is the phosphorescence lifetime, kr is the radiative transition rate constant, and knr is the non-radiative transition rate constant. Therefore, in order to increase the phosphorescence lifetime τp without affecting the phosphorescence efficiency, the nonradiative transition rate constant (knr) of the triplet exciton should be reduced as much as possible. We briefly summarize some effective strategies for controlling the lifetime of organic phosphorescence, including different material systems ranging from small organic molecules, cocrystals to polymers.
4.1.3.1 Life-time control of organic room temperature phosphorescence
Based on the strategy of stabilizing triplet excitons with H-type aggregates in 2015, An et al. Expanded a series of ultra-long organic room temperature phosphorescent materials with H-type aggregates. For example, in 2018, the phosphorescence lifetime was significantly improved by constructing intermolecular dihydrogen bonds (Fig. 75), increasing the rigidity of molecules and weakening the non-radiative transition process of triplet excitons
[323]. A series of fluorophenylboronic acid derivatives were designed around hydrogen bonding (fig. 76). Triplet excitons are stabilized by H-aggregation, and strong intermolecular forces limit the rotation of molecular rotors, which successfully prolongs the phosphorescence lifetime. Among them, the longest phosphorescence lifetime can reach 2. 50 s, which is the longest luminescence lifetime of the single-component pure organic phosphor reported at that time
[324].
Tuning the electronic composition of the singlet and triplet States has an important effect on the phosphorescent properties. In 2016, He et al. proposed a rational design principle based on intrinsic molecular structure engineering, and a comprehensive study of molecular orbitals showed that the ideal excited state is a hybrid (n,π
*) and (π,π
*) configuration with a significant proportion, as shown in Fig. 77
[325]. Designing aromatic subunits in arylbenzenes can effectively tune the energy levels and orbital characteristics of triplet excitons. Experimental data show that a series of full-color pure organic phosphors with equilibrium lifetimes (up to 0.23 s) and efficiencies (up to 36.0%) can be realized under ambient conditions, demonstrating the effectiveness of this design principle.
In 2017, He et al. reported that a single pure organic phosphor, namely 4-chlorobenzoyldibenzothiophene, can emit white room temperature phosphorescence, and the coordinates of the International Commission on Luminescence are (0.33, 0.35), as shown in Figure 78
[326]. Experimental and theoretical studies show that the white light emission is produced by diphosphorescence, which is emitted from the first and second excited triplet States. This work also demonstrates the effectiveness of the strategy to realize metal-free pure phosphorescent unimolecular white emitters via dual room-temperature phosphorescence generated by mixing of low and high triplet States within the system, providing a certain strategy for white room-temperature phosphorescence luminescence.
In 2018, He et al. Realized the design principle of small energy gap between the lowest singlet and triplet States (ΔE
ST) and pure π-π
* configuration of the lowest triplet state (T
1) through structural isomerization, and obtained efficient and ultra-long RTP materials
[327]. The meta-isomer of carbazole-substituted methyl benzoate shows an ultralong lifetime of 795.0 ms with a quantum yield of 2.1%. The study of structure-property relationship shows that the different steric and conjugation effects produced by the ester substituents at different positions are responsible for the small and pure π-π
* configuration of T
1, as shown in fig. 79.
He et al. In 2019 mentioned a strategy to improve the phosphorescence efficiency through intramolecular triplet-triplet energy transfer (TTET), as shown in fig. 80
[328]. Intersystem crossover processes are facilitated by the binding of (bromo) dibenzofuran or (bromo) dibenzothiophene to carbazole, providing intramolecular triplet bridges as well as near-quantitative triplet – triplet energy transfer to repopulate the lowest triplet state of carbazole. All these factors contribute to the design strategy of efficient phosphorescence to improve the efficiency of organic persistent room-temperature phosphorescence through intramolecular TTET.The four molecules designed in this paper all show efficient and persistent luminescence, with the best Ф
P of 41% and lifetime of 0. 54 s under ambient conditions. Of course, heavy atoms, small energy gap and spin-vibration coupling mechanism effectively promote the ISC process, which provides a certain molecular design strategy for the development of room temperature phosphorescent materials.
In 2019, An et al. achieved a 19-fold increase in phosphorescence lifetime by introducing a d-p π bond into the phenothiazine system (fig. 81a)
[329]. The introduction of the d-p π bond increases the (π,π
*) component of the T
1state, which effectively reduces the radiative transition rate (k
r), and the intermolecular interaction is also enhanced, and the non-radiative transition rate (k
nr) is weakened, resulting in a significant increase in the phosphorescence lifetime (Figure 81 B).
The above reports have improved the lifetime of phosphorescence emission through effective molecular design, which provides a theoretical support for the construction of ultra-long phosphorescent materials.
4.1.3.2 Efficiency Control of Organic Room Temperature Phosphorescence
Improving the phosphorescence efficiency of organic room-temperature phosphorescent materials is helpful to realize the application of such materials in the fields of high-performance optoelectronic devices and the like. Introducing heavy atoms can effectively improve the ISC process and achieve efficient phosphorescence emission. In 2018, An et al. reported a series of isomers containing bromine atoms (p-BrTCz, m-BrTCz, and o-BrTCz)
[330]. Among them, m-BrTCz can effectively form π-type halogen bonds, thus having higher phosphorescence efficiency (fig. 82 a). In 2019, they introduced bromine atoms (PDBCz) on the PDCz molecule, which improved the phosphorescence efficiency by an order of magnitude through the intramolecular space heavy atom effect, with an efficiency of 38.1% (Fig. 82b)
[331]. In 2019, they prepared a series of organic long afterglow materials (p-1, m-1, o-1) by introducing chromophores at different sites of carborane (Fig. 82c)
[332]. P-1 exhibits the highest phosphorescence efficiency and the longest phosphorescence lifetime (7. 1%, 168 ms) due to its stronger intermolecular force and tighter molecular packing structure, which can effectively suppress the non-radiative transition of triplet excitons.
In addition, based on the rule that the intersystem crossing rate is inversely proportional to the energy level difference (ΔE
ST) between the singlet excited state and the triplet excited state, Zhang et al. realized an ultra-long-lived pure organic RTP by introducing an intramolecular Charge transfer state (CT) into a 1,8-naphthalimide derivative (fig. 83)
[313]. When the substituent on the N atom of 1,8-naphthalimide is weak, the π-π
* transition is dominant, and the degree of space overlap between π and π
* orbitals is high, so the energy level difference between
1(π-π
*) state and
3(π-π
*) state is large, the intersystem crossing is difficult, and the RTP is weak. When the N atom is covalently attached to a stronger electron-donating group, the highest occupied molecular orbital (HOMO) is distributed on the N-donating group, while the lowest unoccupied molecular orbital (LUMO) is distributed on the naphthalimide ring, and the two are separated in space.The CT state has smaller ΔE
ST and higher intersystem crossing efficiency, so the CT state can mediate the intersystem crossing of molecules to realize the step-by-step energy transfer process of the
1(π-π
*)→
1CT→
3CT→
3(π-π
*), thereby enhancing the RTP efficiency of the whole molecule. The efficiency of yellow RTP ( ~ 541 nm) was also increased with the increase of N-substituent of naphthalimide derivatives.The efficiency of yellow RTP (
~ 541 nm) was increased by more than 40 times when the N-substituent was trimethylbenzene compared with n-butyl, and the RTP lifetime was as long as 0. 23 s. The N-substituent containing alcoholic hydroxyl can be used as a lactide ring-opening polymerization initiator to obtain polylactic acid with RTP property and good biocompatibility, and is expected to be applied in the field of biological imaging. At the same time, this work also proved that the RTP of polylactic acid obtained by using naphthalimide derivatives as initiators was significantly enhanced when the N-substituent was a stronger electron-donating group than when the N-substituent was a non-electron-donating group.
Because red/near-infrared luminescence has better biological tissue penetration, it has a better application prospect in the field of imaging. Based on this, Zhang et al continued to use the strategy of CT state-mediated intersystem crossing to replace 1,8-naphthalimide with 1,4,5,8-naphthalenetetracarboxylic diimide, which could reduce the energy level of
3(π-π
*) to the red region.The red/NIR RTP was realized by
1(π-π
*)→
1CT→
3CT→
3(π-π
*) energy transfer process with the main emission peak in the red region and the emission peak tailing to the NIR region, and the absolute quantum yield was about 5% and the lifetime was more than 30 ms
[333].
In the later work, Zhang et al. proved the universality of the CT-state-promoted RTP strategy by using thioflavin derivatives
[334]. The thioflavin derivative is covalently attached to the polar polyurethane, and the initial polyurethane does not show visible RTP because the transition of the thioflavin derivative itself is dominated by π-π
*; When protonic acid is added to the polyurethane, any one of the two N atoms on the thioflavin derivative is protonated and becomes a charge acceptor, which induces the generation of intramolecular CT state and reduces the ΔE
ST, thereby promoting the generation of yellow RTP. Density functional theory calculations also show that the ΔE
ST of the two protonated isomers of thioflavin derivatives decrease from 0. 97 eV before protonation to 0. 43 eV and 0. 32 eV, respectively.
For some pure organic molecules with close spacial distance and good planarity between charge Donor (D) and Acceptor (A), it is difficult to obtain RTP even if the intramolecular CT state is generated. This is because the HOMO and LUMO still have a high degree of spatial overlap, resulting in large molecular ΔE
ST and difficulty in intersystem crossing. Based on this, Zhang et al. Used sp
3 atoms as connectors between appropriate D and A to obtain “D-sp
3 connector-A "structure, which can increase the space distance between HOMO and LUMO and reduce the ΔE
ST. At the same time, D and A form a twisted conformation, which can enhance spin-orbit coupling. The change of spin angular momentum caused by electron turnover during intersystem crossing can be compensated by the change of orbital angular momentum to ensure the conservation of angular momentum
[335]. Accordingly, they demonstrated this idea in a series of “D-sp
3 "link-a" structures, obtaining efficient RTP.
For example, when the N atom of phthalimide is covalently linked to dimethoxybenzene through a methylene group, dimethoxybenzene acts as the charge donor, the phthalimide ring acts as the acceptor, and the methylene group acts as a sp
3 linker, the resulting molecule has an ultra-long (523 ms) yellow RTP (absolute quantum yield of 3. 5%) in the aggregated state; If the dimethoxybenzene is replaced by phenyl, the electron-donating property of dimethoxybenzene is weakened, the effective intramolecular CT state can not be formed, and the molecule basically does not show RTP in the aggregation state
[335].
Zhang et al. Extended the sp
3 linker to sulfur atom, and also obtained efficient pure organic RTP. As shown in Figure 84, when one end of the sulfur atom is cyanobenzene and the other end is methoxybenzene, the "D-S-A" structure is formed, which can produce the intramolecular CT state, and the aggregation state shows bright yellow RTP (the total quantum yield is up to 20%, and the phosphorescence lifetime is greater than 80 ms). Based on this, they also designed RTP "lighting" proton probe, that is, one end of sulfur is methoxybenzene, the other end is quinoline ring (DSQ) or pyridine ring (DSP), its own aggregation state does not show RTP, but when the quinoline ring and pyridine ring are protonated respectively, they form "D-S-a" structure, the aggregates show bright RTP, the lifetime is 10.2 and 58 μs respectively, and the total quantum yield is as high as 10.7% and 5.7% respectively. It is worth noting that DSP has a good application prospect in the quantitative detection of acidic substances because the linearity of RTP increases with the increase of hydrogen chloride concentration in hydrogen chloride gas
[336].
In addition, Zhang et al. constructed a multi-degree-of-freedom molecular system with single-component aggregation-induced dual RTP properties by using sp
3 linkers
[337]. Five “D-sp
3 linkers-a structures, TPA1, TPA2, TPA3, TPA4 and TPA5, were constructed by using the "paddle-like" triphenylamine (TPA) motif as the charge donor, methylene or oxygen atoms as the sp
3 linkers, and cyanobenzene, benzophenone and acetophenone as the charge acceptors, respectively. These molecules hardly emit in solution because of more rotational and vibrational degrees of freedom.In solid state, the intramolecular motion is limited, and the "paddle-like" structure of triphenylamine and the twisted configuration provided by sp
3 linkers restrict the π-π stacking of molecules, so the solid-state luminescence efficiency is very high, the RTP quantum yield is as high as 9. 5%, 10.0%, 8.2%, 13.3% and 13. 6%, respectively, and the lifetime is as long as 340 ms. A theoretical explanation can be obtained by combining the variable-temperature spectrum and the density functional theory calculation data as follows: the lowest triplet excited state (T
1) potential energy surface of the “D-sp
3 connector-a 'structure may be composed of the respective local potential energy surfaces of D and a (namely the
of the triphenylamine group and the
of the electron-withdrawing group),The molecular vibration at higher temperature (E. G., room temperature) are sufficient to overcome that energy barrier between the
and the
,Most of the
can be converted to
before emission, showing mixed phosphorescence peaks with unclear structure. With the decrease of temperature (such as 77 K), the molecular motion is limited.The
can not overcome the energy barrier to convert to the
, so the possibility of U
as an emissive state increases, showing a clear phosphorescence peak of UN
NK
. This work provides another idea for the development of aggregation-induced single-component multiplex RTP molecules, which are expected to be used in the field of temperature detection.
4.1.3.3 Color Control of Organic Room Temperature Phosphorescence
Colorful luminescent properties play an important role in expanding applications in the fields of sensing, optoelectronic devices and information encryption. At present, most of the phosphorescent emissions are mainly concentrated in the yellow-green region, while there are few reports on pure organic phosphorescent materials emitting in the blue and red regions. As shown in Fig. 85a, An et al. adjusted the organic phosphorescence from 515 nm to 587 nm based on the H-type aggregate structure, and obtained green, yellow and red ultra-long phosphorescent materials
[314]. After that, they replaced the planar carbazole molecule with a twisted iminodibenzyl molecule, which weakened the conjugation between the molecules and achieved a blue shift in the phosphorescence spectrum (Fig. 85b)
[312]. In addition, in the process of constructing the ionic crystal, by changing the type of the countercation, the twist angle of p-toluic acid was increased from 6.62 ° to 13.20 °, the main peak of phosphorescence emission was red-shifted from 456 nm to 508 nm, and the phosphorescence color changed from sky blue to green (Fig. 85c)
[319].
In addition to the method of changing molecular structure or counterions, An et al. Have also realized the control of phosphorescence color by adjusting different luminescent centers in a single material component
[338]. As shown in figs. 86a, B, the phosphorescence color may change from blue to green as the excitation wavelength is changed. Experiments and theoretical simulations show that the two emissions correspond to the monomolecular and aggregated phosphorescence, respectively. Subsequently, in 2020, they developed a series of urea derivatives with excitation-dependent properties
[339]. Uch as hydantoin molecules, the afterglow emission color of the material changes from blue to yellow-green by changing the excitation wavelength (fig. 86c, d).
Similarly, An et al. Have also achieved colorful phosphorescence emission in polymer systems
[320]. In 2019, they used the strategy of ion locking to achieve the multi-color emission of polymers at 77 K. The phosphorescence emission of PSSNa red-shifted from 441 nm to 568 nm as the excitation wavelength red-shifted (fig. 87 a). Also, based on the amorphous PVP-S, a wider range of color adjustability can be achieved
[321]. At 77 K, with the red shift of the excitation wavelength, the ultra-long organic phosphorescence can be adjusted from blue to red, and the corresponding phosphorescence emission peak shifts from 452 nm to 600 nm (Fig. 87b). In 2020, through radical polymerization, they obtained a polymer film PDNA with adjustable color at room temperature
[322]. As shown in Figure 87c, d, the ultra-long organic phosphorescence color of PDNA red-shifts from blue (445 nm) to yellow (547 nm) as the excitation wavelength red-shifts. The above strategy has certain universality, and the phosphorescence can be adjusted from blue to green by changing the type of the chromophore.
4.1.3.4 Excitation Wavelength Control of Organic Room Temperature Phosphorescence
At present, most of the organic room temperature phosphorescent materials are based on the material system excited by ultraviolet light. In order to further expand the application field of organic room temperature phosphorescent materials, it is a hot research topic to design and prepare organic room temperature phosphors with the properties of X-ray excitation and visible light excitation in recent years. In 2021, An et al. proposed a universal design strategy to realize efficient pure organic room temperature phosphorescence, introduced halogen heavy atoms into organic phosphorescent molecules, and prepared a series of efficient room temperature phosphorescent materials (Fig. 88a)
[340]. The introduction of heavy atoms improves the X-ray absorption capacity of the material, and converts the triplet exciton from a dark state to a bright state, so that the triplet exciton participates in radioluminescence, which effectively enhances the radioluminescence performance of the pure organic scintillator (Fig. 88b), and successfully realizes the imaging of the pure organic material under X-ray excitation. The scintillator material has excellent light stability and extremely low detection limit (33 nGy·s
-1), which reflects the great application potential of organic phosphorescent materials in the fields of X-ray detection, imaging and the like.
In addition, in 2017, An et al. constructed a series of organic phosphorescent molecules based on carbazole (Fig. 89)
[341]. CPhCz and BPhCz have multiple interactions between molecules in the xy plane, resulting in a red-shift of the excitation spectrum. At the same time, such molecules have strong H-stacking in the z-axis, stabilizing the triplet exciton (Figure 89 a). As a result, such materials exhibit organic long afterglow luminescence properties with visible light excitation (fig. 89b).
4.1.3.5 Dynamic Control of Organic Room Temperature Phosphorescence
Dynamic intelligent response material is a new type of material which can sense external stimuli and make corresponding feedback behavior. The combination of dynamic response properties and ultra-long organic phosphorescent materials is expected to provide a broader platform for expanding the application of organic electronics. In 2018, An et al. Reported a series of photoactivated dynamic organic room temperature phosphorescent compounds, which have significant changes in the intramolecular torsion angle under the stimulation of light sources.Thereby leading to different intermolecular forces, so that the material shows adjustable phosphorescence lifetime under continuous ultraviolet light excitation, and the phosphorescence property of the material can be recovered after the ultraviolet light is removed
[342]. Later, they developed crystal-dependent organic room temperature phosphorescent compounds
[343]. By volatilizing in different solvents, they grew two crystal forms, block and rod, respectively. The two crystals show different response properties to UV light. The intermolecular π-π interaction in the bulk crystal is relatively weak, and the molecules are more likely to move under the stimulation of ultraviolet light, so the bulk crystal is more sensitive to ultraviolet light than the rod-shaped crystal and has faster response speed.
4.1.3.6 Host-guest doped organic room temperature phosphorescence
Although the above-mentioned “D-sp
3 linker-a "" structure based on dimethoxybenzene and phthalimide groups has the aggregation-induced RTP property, in the synthesis of phthalimide derivatives with bromine atom substitution (BrBID) on the benzene ring, only the product obtained by using DMF as the solvent has yellow RTP, and the product obtained by using ethanol as the solvent does not show any visible RTP.Combined with column chromatography, high performance liquid chromatography separation and single crystal X-ray diffraction analysis, it was found that a trace amount of by-product X, whose Br atom was replaced by N, N-dimethyl, was obtained when DMF was used as solvent.The quantum yield of the host guest two component aggregates could be as high as 25.4% when the mass fraction of X was 1%. No RTP was observed when X was doped in BrPID (N atom of bromophthalimide was directly covalently linked to dimethoxybenzene), and no phosphorescence was observed when X was dissolved in 1,6-dibromohexane and cooled to 77 K, indicating that the heavy atom effect is not the main reason for the host-guest RTP phenomenon of X/BrBID. Because the molecular size of X is similar to that of BrBID, combined with density functional theory calculations, we propose the following mechanism: the n orbitals of N, N-dimethyl in X partially overlap with the σ
* orbitals of C-Br bonds in nearby BrBID molecules.That is to say, the charge transfer of n electrons to σ
* orbitals may participate in the excited state and promote intersystem crossing and RTP, which is similar to the role of CT state in promoting RTP in the external heavy atom effect described by Zhang et al
[344].
In another sp
3 linker binary aggregate system, they found that naphthalimide derivatives MONI, MSNI, and MNNI with low triplet energy levels, as guests, could show efficient ultra-long lifetime RTP in phthalimide derivative host BrBI (Fig. 91). When the mass fraction of the guest is 1%, the RTP lifetimes of the binary aggregates MONI @ BrBI, MSNI @ BrBI and MNNI @ BrBI are as long as 243. 07 ms, 42.48 ms and 131. 77 ms, respectively, the quantum yields are as high as 17. 4%, 12.6% and 8. 3%, respectively, and the maximum emission peaks are 554 nm, 599 nm and 623 nm, respectively. Importantly, when the mass fraction of MSNI is as low as 10 ppb, MSNI @ BrBI can still display a long afterglow visible to the naked eye. The RTP phenomenon of the host-guest system can be explained by triplet-triplet energy transfer, that is, the triplet energy of the aggregate of the excited host molecule is transferred to the guest molecule with lower triplet energy level, and the triplet excited state of the guest molecule is used as the final emission state to emit the observed RTP
[345].
The RTP system of pure organic host-guest binary aggregates has attracted much attention recently. For example, Tang et al. Reported the host-guest RTP system based on the cluster exciton theory model, and Liu et al. Reported the host-guest RTP phenomenon caused by carbazole isomers, which proved the universality of the pure organic host guest RTP phenomenon
[346⇓~348][347][348].
4.1.4 Application of Organic Room Temperature Phosphorescent Materials
4.1.4.1 OLED
OLEDs play an important role in the lighting and display industry, and phosphorescent materials are ideal materials for the new generation of OLEDs, because they can use 25% singlet excitons and 75% triplet excitons at the same time, which is theoretically more efficient than fluorescent OLEDs, and is of great significance for energy saving, emission reduction and sustainable development. However, most of the current industrialized phosphorescent OLED materials still rely on precious metals with high cost and toxicity, such as platinum and iridium, and the luminescent materials need to be doped in a specific matrix as an emission layer, resulting in complex manufacturing processes. In order to solve these problems, Zhang et al. Designed a class of pure organic molecules with aggregation-induced RTP properties, on the one hand, the intersystem crossing was enhanced by the combination of charge transfer state and n-π
* transition between dicarbazole and benzophenone units.At the same time, the twisted conformation of the molecule is used to inhibit π-π stacking and luminescence quenching, and the resulting molecule has a double phosphorescence phenomenon in the solid state, with a lifetime of microseconds, and a total RTP quantum yield of up to 64% (Fig. 92)
[349]. Using the bulk of these pure organic molecules directly as the OLED emitter layer, the resulting device exhibited an external quantum efficiency of 5.8%, exceeding the theoretical limit for fluorescent devices and breaking the record for pure organic phosphorescent OLED devices, demonstrating the great potential of pure organic aggregation-induced RTP in developing highly efficient OLEDs.
4.1.4.2 Biological imaging and therapy
An et al. prepared Phosphorescent nanodots (PNDs) by coating hydrophobic ultra-long organic phosphors (Fig. 93a) with hydrophilic Hollow mesoporous silica nanoparticles (HMSNPs), which still maintained efficient phosphorescence emission in aqueous solution and significantly improved the in vivo bioimaging performance
[350]. The synthesis scheme is shown in fig. 93b, and a series of PNDs have been successfully prepared by this method. Taking DBCz-BT-PNDs as an example, HeLa cells and zebrafish were cultured with PNDs, and it was found through experiments that the biological toxicity of PNDs to HeLa cells and zebrafish was very low (Fig. 93c). On this basis, phosphorescence imaging of both was successfully realized (Fig. 93d, e)
[351].
In addition, An et al. continued to explore DBCz-BT-PNDs as photosensitizers for photodynamic therapy and antimicrobial therapy of HeLa cells
[352]. As shown in fig. 94a, with the increase of light irradiation time, the generated
1O
2 can lead to oxidative apoptosis of cancerous cells, so that the tumor volume and cell area in the treatment group are significantly reduced (fig. 94b) and the body weight of mice is stable (fig. 94c), which proves that PNDs have effective PDT effect. In addition, they also reported the application of pure organic PNPs in photodynamic antimicrobial therapy, and the sterilization effect of PNPs was proved in cell and in vivo experiments.
4.1.4.3 Volatile organic compounds sensing
Volatile organic compounds (VOC) exist widely in our daily life and do great harm to human health and the environment. Therefore, the detection of VOC is of great significance. An et al. Developed a phosphorescent material with good selective response to chloroform in 2018
[353]. Through cyclic fumigation and heating, the combination of chloroform molecules and TDP molecules (Fig. 95) can be regulated, and then the on-off regulation of organic long afterglow luminescence signal can be realized. Based on this, the corresponding properties of the material can be fully utilized, and the material can be applied to the visual detection of chloroform with high selectivity, good reversibility and high sensitivity.
4.1.4.4 Information encryption
Organic phosphorescent materials can be used in the field of information encryption by using their unique and rich properties. Based on the long-lived emission feature, "2014" made with RTP molecules can be clearly recognized by the naked eye after the excitation is turned off (fig. 96 a)
[314]. Based on RTP materials that respond to excitation light, An et al. Further demonstrated their applications in multi-color display (Figure 96b), multi-level data encryption (Figure 96c), and logic gates (Figure 96d)
[338][342][343].
4.2 Atypical luminescence
Organic light-emitting materials have attracted much attention due to their unique photophysical properties and important applications in organic light-emitting diodes (OLED), organic lasers, optical waveguides, bio-imaging, photodynamic therapy, biological and chemical sensing and other fields. Most of the traditional organic light-emitting compounds have large conjugated structures such as aromatic (heterocyclic) rings or alternating single bond-multiple bonds. However, in recent years, another class of compounds has attracted great interest due to their unique structures and endogenous luminescence properties
[354⇓⇓⇓~358]. Compared with traditional luminescent materials, these compounds only contain aliphatic amine, carbonyl group (C = O), ester group (COO), amide group (CONH), silicon-oxygen bond (Si — O), unsaturated bond (such as C = C, C ≡ N) and other groups (Fig. 97), but they can also emit light under certain conditions. The atypical luminescent compound can be a small molecule or a polymer; It can be a natural product, a biomolecule, or a synthetic compound (fig. 98)
[359⇓~361]. Generally, their synthesis is more convenient than traditional luminescent materials, and they are easy to modify. In particular, many atypical luminescent compounds have good hydrophilicity, environmental friendliness, excellent biocompatibility and other advantages, which make them have broad application prospects in biology, medicine and other fields. At the same time, atypical luminescent compounds usually also exhibit concentration-enhanced luminescence and AIE characteristics, which are more conducive to their practical applications. In addition, these compounds also have universal phosphorescence emission and even ultra-long-life room temperature phosphorescence (p-RTP) emission properties (fig. 98), which make them applicable to sensing, high-end anti-counterfeiting, confidentiality and other fields.
In recent years, more and more attention has been paid to atypical luminescent compounds, and important progress has been made in system expansion, performance improvement, property control and mechanism exploration, which will be briefly summarized below.
4.2.1 Classification of atypical luminescent compounds
In recent years, more and more attention has been paid to atypical luminescent organic/polymer materials. Many common synthetic macromolecules, biomacromolecules and even small molecules have been found to have intrinsic fluorescence or phosphorescence, while exhibiting AIE behavior. Unlike traditional fluorescent compounds containing π-conjugated benzene rings or heterocycles, such organic/polymer materials usually contain non-conjugated or small conjugated functional groups rich in n electrons or π electrons, such as amino groups, hydroxyl groups, carbonyl groups, ester groups, acid anhydrides, ureido groups, sulfonic acid groups and cyano groups (Fig. 97), i.e. atypical chromophores. These atypical fluorescent organic/polymer materials can emit bright intrinsic fluorescence or phosphorescence in the aggregated state. According to the difference of molecular weight, atypical luminescent materials can be roughly divided into two categories: Nonconventional luminescent polymers (NLPs) and atypical luminescent small molecules.
4.2.1.1 Atypical luminescent polymer
Due to the absence of aromatic ring structure, NLPs usually have the advantages of easy preparation, good chain flexibility, stable structure, hydrophilicity and biocompatibility, which make them ideal candidates for biological and medical applications. NLPs can be further subdivided into dendritic/hyperbranched polymers and linear polymers, and the early studies on NLPs mostly focused on the former.
Dendrimer and hyperbranched polyamidoamine (PAMAM) are the first reported synthetic NLPs, and then various dendrimer/hyperbranched NLPs have been widely studied, such as hyperbranched polyaminoester (PAE), polyetheramide (PEA), polyesteramide ether (PEAE), polyamide acid (PAA), polyethylene imine (PEI), polycarbonate, polysiloxane, polyether, polyaminoether ester and dendrimer polyurea, all of which have intrinsic fluorescence emission. The fundamental reason for the fluorescence emission of tree-like and hyperbranched NLPs is that the atypical chromophores in the molecular chain are in a confined space, which on the one hand leads to the clustering of each other, resulting in the overlap and sharing of electron clouds, thus realizing the expansion of electron conjugation and delocalization, and making them easier to be excited; On the other hand, due to the restriction of its motion, the probability of energy dissipation caused by non-radiative transition of excited molecules is reduced, which leads to the increase of the probability of energy release by fluorescence emission.
Due to the advantage of topological structure, the internal atypical chromophores of tree-like/hyperbranched NLPs are easier to cluster, so they have become the research object that attracts the attention of researchers earlier. Recently, more and more linear NLPs have been reported to have intrinsic fluorescence or phosphorescence emission. According to the composition of functional groups, these NLPs can be roughly divided into the following five categories.
(1) polyamides and polyimides. Uch as polyacrylamide (PAM), poly N-isopropylacrylamide (PNIPAM), polyamide, polyvinylpyrrolidone (PVP), polyvinylcaprolactam (PNVCL) and polysuccinimide.
(2) polycarboxylic acid, polyanhydrides, and polyester. Uch as polyacrylic acid (PAAc), polymaleic anhydride (PMAh), polyitaconic anhydride (PITA) and the like.
(3) sulfur-containing polymer. Uch as sulfonated acetone-formaldehyde polycondensates (SAF), poly (4-vinylpyridine) butane-1-sulfonate (PVP-S), polythioethers, polysulfoxides and polysulfones, perfluorosulfonic acid ionomers (PFSI) and the like.
(4) copolymer. Uch as poly (maleic anhydride-vinyl acetate) (PMV), poly (maleic anhydride-vinylpyrrolidone) (PMVP), poly (itaconic anhydride-vinylpyrrolidone) (PIVP), polyitaconic anhydride vinylcaprolactam (PIVC) and other binary or ternary copolymers.
(5) Other polymers. Uch as linear polyethyleneimine (PEI), polyurethane, polyethylene glycol (PEG), polyacrylonitrile (PAN), polyurea, polyimine furan (PIFs), oligomeric siloxane and the like.
In addition, many biological macromolecules, such as cellulose and its derivatives, starch, chitosan, alginate and other carbohydrates, polypeptides, polyamino acids and proteins, have also been found to have intrinsic fluorescence or phosphorescence emission properties.
4.2.1.2 Atypical luminescent small molecular
Compared with NLPs, atypical small molecules are not as good as the former in processing, flexibility and other properties, but they widely exist in nature and organisms, and their sources are abundant, and their structures are clear and easy to design, which brings convenience to the mechanism exploration of atypical luminescent compounds.
Similar to the classification of NLPs, atypical luminescent small molecules can be roughly divided into the following three categories according to the type of chromophore.
(1) oxygen-containing atypical luminescent small molecules. Such as various monosaccharides (fructose, xylose, galactose) and pentaerythritol.
(2) Atypical luminescent small molecules containing nitrogen. Uch as hexaldoxime, amino acid, polypeptide and the like. Among them, small molecules containing amide groups have excellent photophysical properties, such as urea, hydantoin, acrylamide and various synthetic small molecules.
And (3) atypical luminescent small molecule containing sulfur, phosphorus, boron, halogen and that like. Uch as thiourea, cysteine, boric acid derivatives, phosphoric acid derivatives, halogenated alkanes and the like.
4.2.2 Luminescence Mechanism of Atypical Luminescent Compound
As a new class of luminescent materials, the luminescence mechanism of atypical luminescent compounds has attracted the interest of many researchers. However, the early reported systems are usually just accidental discoveries. The understanding of the mechanism of different systems is not the same, and even for the same system, there is a wide range of controversy. This results in a lack of effective guidelines for the design of atypical luminescent compounds. Taking the representative PAMAM system as an example, the mechanisms of oxidation, the formation of unsaturated hydroxylamine and the aggregation of carbonyl groups have been proposed to explain
[357,358][362][363]. In these studies, people paid more attention to the effects of oxidation, peripheral substituents (— OH, —NH
2, — COOH), topological structure of compounds (linear, branched, dendritic), solution pH and other factors, but lacked comparative studies under different concentrations, different aggregation States and different temperatures, which easily led to one-sided conclusions and was soon proved not to be the case by follow-up studies. For biomolecular systems, mechanism disputes also exist. For example, for L-Lys system, researchers believe that the side group of —NH
2 is very important, while for polypeptide and protein system, although there are a lot of amide bonds in its structure as PAMAM, Shukla et al believe that its luminescence is due to the formation of hydrogen bonds and the delocalization of peptide bond electrons along the hydrogen bond skeleton
[364][365].
Therefore, the exploration of the mechanism can help people to deepen the understanding of the photophysical process of atypical luminescent compounds and its influencing factors, reveal the electronic interaction and electronic process behind the phenomenon, uncover the mystery of self-luminescence in biological systems, and guide researchers to discover and develop more new luminescent materials.
In 2013, Yuan and Tang et al observed blue photoluminescence from rice, and studied the photophysical properties of starch and its analog cellulose, the main component of rice, in the aggregated state
[366]. Considering that starch and cellulose do not contain large conjugated structures, but only contain hydroxyl, ether bonds and other groups, their intrinsic luminescence should come from the clustering of these groups. When the distance between two electron-rich atoms is less than the sum of their Van der Waals radii (d<r
1+r
2), it can be considered that there is an electronic interaction between them, thus realizing the sharing of electrons and the overlap of electron clouds.The range of conjugation and electron delocalization is extended, i.e., Through-space conjugation (TSC) is formed, which induces energy level splitting and reduces the energy gap, thereby promoting electronic transition and producing luminescence (Fig. 99).
Subsequently, based on the systematic study of the photophysical properties of PAN, Yuan et al. further explained its photophysical behavior with the above mechanism, and demonstrated the crucial influence of atypical chromophoric cluster aggregation on luminescence, and named it "cluster-induced luminescence" (CTE)
[367]. Tang et al. Proposed and verified the "Cluster Luminescence" (CL) mechanism through the study and theoretical calculation of the luminescence behavior of the homopolymer of maleic anhydride (PMAh) and the alternating copolymer of maleic anhydride with vinyl acetate and trimethyl-1-pentene, respectively. The above concept of "Cluster Luminescence" can be further extended to metal nanoclusters, aromatic aggregates and other systems
[368][369].
It should be clear that the "cluster" referred to by CTE mechanism is different from the "aggregate" or "particle" formed by polymer or small molecular compound. "Cluster" refers to an intramolecular or intermolecular aggregation structure of a plurality of atypical chromophores and is not limited to aggregation of atypical luminescent molecules. Different from the intramolecular conjugation, TSC is the conjugation of two or more atypical chromophores through overlapping and sharing of electron clouds when they are close to each other, which does not depend on the formation of actual covalent bonds, and non-covalent interactions play an important role in it. The effect of TSC is related to the type, quantity, relative position and distance of atypical chromophores. Due to the differences in the type, quantity and degree of spatial conjugation of atypical chromophores involved in the formation of "clusters", there may be many identical "clusters" in an "aggregate" or "particle", or there may be many different "clusters", that is, different luminescent species. The coexistence of multiple emitters is the basis for the design of multicolor tunable luminescence of atypical luminescent compounds.
As the real luminescence center, the formation and conformational hardening of the cluster group are very important for luminescence. According to the CTE mechanism, the formation of "clusters" with TSC effect by the intramolecular/intermolecular interaction of atypical chromophores is the fundamental reason for the intrinsic luminescence of atypical luminescent substances. First, polymers or small molecular compounds are able to adopt appropriate molecular conformations to achieve TSC action of atypical chromophores. Generally speaking, the more atypical chromophores involved in clustering, the greater the degree of spatial conjugation, the stronger the fluorescence emission, and the higher the degree of red shift. Second, the widespread intramolecular/intermolecular interactions help to enhance the conformational rigidization of the luminescent clusters, thereby suppressing the nonradiative decay and promoting luminescence.
4.2.3 Photophysical Properties of Atypical Luminescent Compounds
The core of atypical luminescent compounds is the cluster, so its photophysical properties are also around the cluster characteristics, mainly showing two phenomena of concentration-enhanced luminescence and condition-dependent luminescence.
Firstly, with the gradual formation of clusters and the gradual enhancement of TSC, from discrete state (dilute solution) to aggregated state (concentrated solution or solid), atypical luminescent compounds generally show obvious concentration-enhanced luminescence properties. Secondly, due to the different degree of clustering of chromophores in atypical luminescent compounds, the coexistence of multiple emitters will lead to significant excitation wavelength-dependent luminescence, which provides convenience for the design of multi-color tunable luminescence. Thirdly, due to the different responsiveness of different cluster species to external conditions, the change of external environment (such as temperature, pressure, pH, etc.) Will cause the change of the luminescence contribution of different cluster species, thus realizing the adjustment of the apparent luminescence of atypical luminescent compounds and their application in the sensing field.
4.2.4 Modulation of photophysical properties of atypical luminescent compounds
Due to the lack of conjugated structure, the luminescence of atypical luminescent compounds is mostly concentrated in the short wavelength region, and the quantum yield is usually low. Therefore, the exploration of new atypical luminescent materials with long-wavelength emission and high quantum yield is not only of great significance in understanding their luminescence mechanism, but also helpful to expand their application scope.
4.2.4.1 Luminous wavelength adjustment
In recent years, some progress has been made in the development of atypical luminescent compounds with long-wavelength emission, and a variety of NLPs and atypical luminescent small molecules with green, yellow and even red luminescence have been reported.
Li et al. reported sulfonated acetone-formaldehyde condensates (SAF) and sulfonated ethylenediamine-acetone-formaldehyde condensates (SEAF) that emit green fluorescence in solution, and SEAF even emits yellow fluorescence in the solid state
[370]. The strong hydrogen and ionic bonding interactions within/between the polymer chains play an important role in the red shift of the fluorescence emission.
Yan et al. Synthesized a series of aliphatic hyperbranched polyesters with blue light emission by adjusting the proportion of raw materials. The fluorescence emission intensity increased with the increase of the molecular weight of the polymer, and the emission was adjustable in different solvents
[371]. Recently, they prepared a new hyperbranched polysiloxane containing carbon dots (HBPSi-CDs) by hydrothermal treatment of a hyperbranched polysiloxane
[372]. Under the excitation of 340 nm ultraviolet light, it emits two kinds of fluorescence with the maximum emission wavelength at 414 nm and 532 nm, respectively, so the overall fluorescence emission is close to white light. The results show that the 414 nm emission originates from the HBPSi skeleton structure and the 532 nm emission originates from the carbon dot structure excited by 414 nm light.
Qiao et al. Found that maleic anhydride-vinyl acetate alternating copolymer (PMV) was esterified in strong base or directly reacted with strong base to produce PMV derivatives, which could emit a variety of color fluorescence from blue to red
[373]. The strong ionic bonding makes the conformation of the clusters extremely rigid, resulting in a red-shift in fluorescence emission and an increase in fluorescence emission. Recently, they also hydrothermally treated PMV and found that with the increase of hydrothermal time, the emission color of PMV derivatives changed from blue to yellow, the maximum emission wavelength could reach 608 nm, and the absolute quantum yield could reach 17.05%.
Wang et al. Copolymerized maleic anhydride (MAh) and itaconic anhydride (ITA) with N-vinylcaprolactam (NVC) and N-vinylpyrrolidone (NVP), respectively, to prepare a variety of NLPs with lactam structure and cyclic anhydride structure. They found that the molecular weight of these copolymers had an important effect on their fluorescence emission behavior, and the fluorescence emission in high concentration solution and solid state also had a red shift
[374,375]. They further synthesized polyitaconic anhydride (PITA) and poly (1-octene-co-itaconic anhydride) (POITA) and tested their fluorescence behavior, demonstrating that, at least in some cases, increasing the flexibility of the chain is beneficial to enhancing the photoluminescence of unconventional light-emitting polymers
[376]. The possible reason is that the better flexibility of polymer chains can make the molecular chains adopt more suitable conformations during molecular aggregation, so that atypical chromophores can aggregate into clusters with strong spatial conjugation effect.
Recently, Guo et al. Synthesized two kinds of zwitterionic oligomers with different fluorescence emission behavior by controlling the concentration of zwitterionic monomers, which were synthesized by the reaction of 2- (dimethylamino) ethyl methacrylate with itaconic anhydride at room temperature
[377]. The maximum emission wavelength of the oligomer synthesized at high concentration is 712 nm.
Yuan's team has conducted a series of studies on atypical luminescent small molecules with long-wave emission and color tunability
[378]. By introducing a C = C bridge to the dianhydride CHDA with a stereoconfiguration or aminating it to form an amide, its luminescent behavior can be selectively tuned (Figure 100). Aft that C = C bridge bond is introduced, the TSC effect is enhance, the electron delocalization expansion is promoted, and the red shift of luminescence can be realized; The improvement of quantum efficiency can be achieved by ammoniating anhydride into amide, introducing strong hydrogen bonds and improving conformational rigidity. In addition, the two modified diamide molecules with bridge bonds also show good tunable luminescence properties with the change of external pressure. On the other hand, they obtained polymorphs of oxaanhydrides and oxaimides with different emission wavelengths by using different solvents
[379]. Although the structural difference is small, a small amount of solvent molecules changes the microenvironment of the cluster, which provides a new idea for the design of tunable luminescence.
4.2.4.2 Adjustment of luminous intensity or quantum efficiency
The core of improving the luminescence intensity of atypical luminescent compounds is to expand the range of TSC and promote electron delocalization; The second is to use the strong intermolecular interaction to inhibit the non-radiative transition pathway, thus promoting luminescence.
Yuan et al. Found that the hydantoin (HA) molecule has a good planar structure
[380]. There are extensive steric conjugation interactions and multiple hydrogen bonds between carbonyl groups and nitrogen heteroatoms in the crystal. The synergy of these non-covalent interactions can make it have extremely high fluorescence efficiency (87.5%) and long-lived room temperature phosphorescence efficiency (21.8%). In addition, Yuan et al. Introduced C = C bridge bond into CHDA molecule to enhance TSC, and introduced hydrogen bond interaction to suppress non-radiative transition by ammoniating anhydride into amide, which could improve the quantum efficiency from 3.1% to 13.5%
[378].
In addition to atypical luminescent small molecules, some research groups have found that enhanced and red-shifted luminescence can also be obtained by post-processing NLPs. Wang et al. Found that the fluorescence emission of hyperbranched PAMAM was gradually enhanced and red-shifted with the increase of thermal oxidation time
[381]. Wang et al. Found that the oxidation product of linear polymer polyvinylpyrrolidone (PVP) after hydrolysis in alkaline solution would emit strong fluorescence, and its fluorescence intensity was about 1000 times that of the original PVP
[382]. Yuan et al. Obtained a new atypical fluorescent polymer polythioether through Michael addition polymerization, and oxidized the thioether to different degrees to obtain sulfoxide (S = O) and sulfone (O = S = O) groups, which promoted the TSC effect of the oxidized polymer (polysulfoxide and polysulfone) and improved the quantum yield of the polymer
[383]. Zhao et al. reported a new class of NLPs based on dynamic covalent crosslinking, polyhydroxycarbamates
[355]. This kind of polymer can emit bright blue fluorescence after heat treatment at high temperature, and the fluorescence intensity increases with the increase of heating time. They believe that reversible exchange occurs between carbamate groups and hydroxyl groups in the polymer chain during heat treatment, which leads to the formation of dynamic covalent crosslinking between polymer chains, promotes the clustering of carbamate groups, inhibits non-radiative decay, and thus produces fluorescence. Ou et al. Found that the fluorescence emission intensity of polyacrylic acid (PAA) solution could be increased by more than two orders of magnitude after thermal annealing
[356].
From the above, it can be seen that there are two main ways to develop atypical luminescent compounds with long wavelength emission and high quantum yield.
(1) Promote TSC. The introduction of various electron-rich atypical chromophores makes it easier for space conjugation to occur between them and promotes electron delocalization and transition.
(2) increase that degree of conformational rigid-hardening. For polymers, strong physical crosslinking can be introduced, and for small molecules, strong interaction networks can be constructed, planar structures can be designed to promote molecular stacking, so as to harden the conformation of luminescent clusters formed by atypical chromophores and inhibit non-radiative transitions.
(3) The NLPs were oxidized, crosslinked and partially carbonized. New, more atypical chromophores may be introduced into the polymer, while chemical or physical crosslinking may also be introduced, promoting clustering and stiffening.
At present, some progress has been made in the study of atypical luminescent polymers and small molecules with long wavelength emission and high quantum efficiency.More and more atypical luminescent polymers and small molecules with excellent properties will be developed, and their applications in various fields will also be developed.
5 Stimulus response characteristics of materials
Stimulus-responsive organic light-emitting materials are a kind of smart materials whose luminescent properties, such as luminescent color and luminescent intensity, change significantly under external stimuli. There are many types and forms of external stimuli, such as light, electricity, magnetism, heat, force, pH and so on. In these external stimuli, force stimuli are very easy to obtain, because force is around us. Mechanochromic luminescent materials, whose luminescent color changes obviously under the action of external force
[36,384]; The other is mechanoluminescent materials, which emit light directly under the action of external forces, unlike traditional luminescent materials, which need to emit light under the excitation of ultraviolet light. In this part, we will mainly introduce the mechanoluminescent materials, the mechanoluminescent materials and some other stimulus-responsive materials.
5.1 Mechanoluminescent discoloration
At present, it has been found that some organic luminescent materials, such as fluorescence, phosphorescence, thermal activated delay fluorescence and long afterglow, have the property of electroluminescence. The concept of "mechanoluminescent discoloration" is different from the concept of piezochromism of materials proposed by Sch Schönberg et al. In 1958. Piezochromism refers to the change of color of materials under pressure, which is essentially the change of absorption spectrum of materials, not the change of emission spectrum. It is well known that for materials used for detection and other purposes, their emission spectra are less disturbed and more sensitive than their absorption spectra, so they have important potential applications in the fields of stress sensing, information storage, commodity anti-counterfeiting and light-emitting devices.
There are two ways to change the color of molecular luminescence by force: one is through the change of chemical structure; Second, through the change of aggregation structure. The former is the chemical reaction of material molecules under the action of external force, that is, the fracture of old bonds and the formation of new bonds, which essentially means that different molecules formed before and after the force emit different colors of light. The latter is that under the action of external force, the packing mode, molecular conformation or intermolecular interaction of material molecules have changed, thus affecting the electronic transition energy level in molecular orbitals, resulting in the difference of luminescence color before and after the force. Although it is the easiest way for researchers to adjust the color of luminescence by changing the chemical structure, it is difficult to predict the chemical reaction at the molecular level by external stimulation in the solid state, which is often difficult to carry out due to the low conversion rate of the solid state reaction. Moreover, in the solid state, most chemical reactions are often irreversible, so it is difficult for these compounds to be used as smart response materials working in a switching mode. Therefore, up to now, there are very few successful examples of achieving mechanoluminescent discoloration by changing the chemical structure of molecules. In these few successful examples, the mechanochromic luminophor is usually introduced into the main chain or crosslinked network of the polymer, and the ring-opening reaction of the mechanochromic luminophor on the polymer chain occurs under the traction of external force, for example,The Spiropyran mechanochromic luminophor is changed into a Merocyanine structure under the action of an external force, and the conjugated structure of the Spiropyran mechanochromic luminophor is obviously changed, so that the fluorescence emission color is shifted to a longer wavelength emission direction, that is, the emission spectrum is red-shifted (fig. 101)
[385]. However, it is obvious that the realization of this method is not only difficult in synthesis, but also very limited in application.
The luminescent properties of organic molecules in solid state are closely related to the stacking mode of molecules, the flexibility of conformational changes and the interaction between molecules. Therefore, any stimulus, including an external force, that causes a change in the above factors will affect the energy level of the electron transition, that is, the energy gap (ΔEg), thereby affecting the wavelength of the emitted light. Moreover, the change of molecular packing mode and conformation can often be restored by other stimuli such as thermal annealing or solvent induction, so as to achieve the reversible change of emission spectrum. Therefore, it may be easier to achieve mechanoluminescent discoloration by changing the physical aggregation state than by chemical methods. At present, dye-doped polymers, metal ion complexes and organic small molecular compounds have been reported based on the change of physical aggregation state.
For example, in 2002, Weder et al. mixed styrene nitrile derivatives 152 and 153 into low-density linear polyethylene (LLDPE) respectively to make dye-doped polymer films, and then carried out a tensile test on the films, and found that the fluorescence emission wavelength of the films had an obvious blue shift (Figure 102)
[386].
However, on the whole, there is little correlation among the reported mechanoluminescent compounds, which are basically isolated events, and there is no rule to follow, so it can not be used for the design of mechanoluminescent compounds. By 2010, these compounds were still relatively few. The number of such compounds reported is less than 100, which greatly limits the application of such materials. In addition, a general luminescent material has a high luminous efficiency in a dilute solution, but in a high concentration or solid state, the luminous efficiency is greatly reduced or even no light is emitted due to the occurrence of the ACQ effect. Therefore, it is difficult for general luminescent materials to produce electroluminescence in solid state due to ACQ effect, which is also an important reason for the scarcity of electroluminescence materials in quantity.
In contrast to materials having the ACQ effect, AIE molecules exhibit high luminous efficiency in the solid state. This kind of material can solve the above two problems that lead to the scarcity of mechanoluminescent materials: it can be assumed that if AIE materials have mechanoluminescent properties, it will be very easy to be observed, because AIE materials emit very strongly in the solid state; In addition, AIE molecules can be designed, such as triphenylethylene, TPE, silacyclopentadiene, DSA, cyanostilbene and so on, which can be used as the core group of AIE to construct AIE molecules. Therefore, the reported AIE molecules are very large in variety and quantity. From the reported results, most of the AIE compounds have a certain response to external stimuli, such as organic solvent atmosphere, heat and so on. In fact, although the above-mentioned mechanochromic compounds 152 and 153 reported by Weder et al. In 2002 are not mentioned as AIE molecules, from the molecular structure, it is clear that they are very typical AIE molecules containing cyanostilbene AIE core groups, which were reported by Park et al
[387]. Since 2010, Chi's research team has found that most of the AIE compounds have obvious mechanoluminescent properties at the same time. This common discovery makes people realize that AIE materials are likely to become an important source of mechanoluminescent materials, which has triggered an upsurge in the study of mechano-stimulated luminescence response of AIE materials
[384].
5.1.1 Micromolecule mechanoluminescence color-changing material
At the beginning of the concept of AIE, it has been recognized that AIE molecules show changes in luminescent properties when they are stimulated by organic solvents, heat and other external stimuli. Tang et al., for example, report several AIE compounds, such as dye 154, 155 and 156, which have luminescent switching properties between crystalline and amorphous States
[388][389][390]. Dye 155 thin film deposited on a quartz plate emits green light (emission wavelength 508 nm), however, after thermal annealing at 100 ℃ for 5 H, its fluorescence emission spectrum (PL) is blue-shifted to 474 nm, accompanied by a more than three-fold increase in emission intensity. This phenomenon is called the crystal-enhanced emission (CEE) effect or morphology-dependent emission. Tang et al. Further report on CEE luminescent compound 156, which emits very weakly in the amorphous state and very strongly in the crystalline state
[391]. This interesting luminescence phenomenon can be switched by solvent fumigating the amorphous sample and cooling the hot melt of the crystalline sample (the switching ratio is more than 40 times). In 2008, in the process of studying the AIE mechanism, Tang and Zou found that the luminescence intensity of amorphous film of compound 154 would be enhanced under the action of static pressure
[391]. With the increase of pressure, the luminescence intensity increases rapidly by 9% (104 atm pressure), but with the further increase of pressure, the luminescence intensity decreases gradually, which should be the first clear report on the effect of force on the luminescence intensity of AIE molecules. In 2010, Park et al. Reported a multi-stimulus two-color luminescence switch of an aggregation-enhanced luminescence (AEE) compound cyanostilbene derivative (157), which can realize the reversible conversion of blue and green two-color fluorescence under the stimulation of pressure, temperature and solvent atmosphere
[392]. According to Park et al., intermolecular hydrogen bonds such as C — H … N and C — H … O allow the formation of molecular sheets with bidirectional shear sliding ability, that is, metastable green G-phase and thermodynamically stable blue B-phase.
Because the local dipoles of the G-phase are stacked in an antiparallel manner, excimer emission is easily formed, while the local dipoles of the B-phase are stacked in a head-to-tail manner, the exciton interaction is enhanced, and the excimer formation is weakened, so that the blue emission of a single molecule is shown. However, at that time, it was not fully recognized that there was a relationship between AIE molecules and force-stimulated luminescence response performance. Almost at the same time, Chi et al. Synthesized and reported a series of mechanoluminescent (also known as pressure-induced fluorescent (PFC)) molecules based on AIE compounds in 2011, such as compound 158, and pointed out that mechanoluminescent discoloration should be a common feature of most AIE molecules
[393]. Chi et al. Proposed that the mechanism of mechanoluminescent discoloration of these materials is that AIE molecules have distorted molecular conformation and loose packing in the crystalline state, which makes these molecules easy to change their aggregation state under the action of external forces, thus leading to the change of molecular luminescent properties. Chi et al. Named this kind of luminophore as "piezochromic aggregation-induced luminescence (PAIE) compound" at that time because this kind of molecule has the properties of both mechanoluminescence and AIE. The discovery of this commonness makes the stimulus-responsive organic light-emitting materials become regular and make them enter a period of rapid development. Since then, a large number of mechanostimulus-responsive AIE luminescence systems have been reported.
In 2011, Chi et al. reported a mechanoluminescent color-changing zinc-containing AIE complex (159), which extended the mechanoluminescent color-changing phenomenon to metal complexes
[394]. In 2012, Su et al. reported the mechanochromic properties of an AIE phosphorescent iridium (III) complex (160), also known as mechanochromic phosphorescent materials
[395]. In 2013, Xu and Tian et al. studied the mechanoluminescent properties of AIE compound 161 under different pressures, and for the first time attempted to quantitatively analyze the relationship between mechanoluminescent discoloration and applied pressure
[37]. For this AIE molecule, they proposed a mechanism for pressure-induced enhancement of intermolecular interactions. In 2013, Chi et al. Reported an AIE luminophor 162 with multi-stimulus single-photon and two-photon fluorescence switching characteristics, which extended the mechanoluminescence color change to the field of two-photon absorption and emission
[396]. In 2014, Zhang et al reported that red and near-infrared fluorescent molecules with CEE activity had three-channel solid-state fluorescent switching characteristics, which extended the emission wavelength of mechanoluminescent AIE to the near-infrared region
[397]. In 2015, Chi et al. Reported some AIE luminescent molecules with fluorescence/phosphorescence (163) or fluorescence/thermally activated delayed fluorescence (164, 165) dual emission
[398⇓~400]. By that variation of the relative intensity of the twin emission under the force stimulus, a luminescence color from orange to violet is obtain and traverses the white light region in a straight line, a phenomenon very similar to a color palette,The palette requires at least two pigments to produce different colors, but this material can only achieve the adjustment of different luminous colors by a single component. In 2015, Tang et al. Reported the force-stimulated luminescence response of long afterglow material or ultra-long room temperature phosphorescence (pRTP) compound 166
[401]. In 2016, Chi et al. found by grinding pRTP compound 167 that, in addition to the change of emission spectrum under the irradiation of ultraviolet lamp, after the ultraviolet lamp is turned off, the pRTP of this material no longer emits, forming a dual-channel force stimulated luminescence response. If this material is used for encryption and anti-counterfeiting, it can achieve single-component dual encryption and anti-counterfeiting function, greatly improving the anti-counterfeiting level
[402].
Dicarbonyl fluoroboron compounds are a kind of fluorescent dyes with intramolecular charge transfer (ICT) properties, which have strong luminescence in both solid and solution States, while TPE is a kind of group with AIE properties commonly used in the construction of organic light-emitting materials. In 2020, Yang and Cui introduced the TPE group into the dicarbonyl fluoroboron dye to obtain the compound 168, and the tetrahydrofuran (THF) solution of the compound 168 has weak luminescence. When water is added into the tetrahydrofuran (THF) solution, the fluorescence gradually increases with the addition of water, indicating that 168 has AIE performance
[403]. Compound 168 has different molecular packing morphology due to the different dihedral angles between the benzene ring, the dicarbonyl fluoroboron group and the TPE group, the intermolecular π-π interaction, the dipolar interaction, and the C — H … π and C — H … F hydrogen bonding.In the same solvent environment, four different crystals of compound 168 can be grown, and the amorphous powder can be obtained by rapid precipitation, thus obtaining a strong luminescent aggregate with luminescent colors of green, yellow, orange and red (fig. 104). Further studies have found that different aggregates can be transformed by external stimuli, such as mechanical grinding, pressure, temperature, solvent fumigation, etc., which can induce reversible changes in the molecular configuration and packing form of metastable States, thus realizing the color control of single chromophoric aggregates and the transformation between aggregates. For example, all of the three crystals can change into the red amorphous state (168-R) after fully grinding, while the green and orange crystals and the red amorphous powder can get the yellow aggregate state after solvent fumigation or high temperature treatment, which indicates that the crystal 168-Y is in the most thermodynamically stable state. It is worth noting that the luminescence of 168-G aggregate state with obvious intermolecular interaction is blue-shifted than that of 168-Y with weak interaction (fig. 104). The theoretical calculation shows that the fluorescence of the four different crystals all comes from the first excited singlet state. The HOMO orbital becomes more stable due to the intermolecular interaction in 168-G, which increases the HOMO-LUMO energy difference and leads to the blue shift phenomenon. However, the intermolecular interaction in 168-O makes the LUMO orbital more stable, which reduces the HOMO-LUMO energy difference and leads to the red-shift phenomenon.
5.1.2 Polymeric electroluminescence photochromic material
Research on mechanoluminescence of AIE polymers has also made progress. For example, in 2017, Chi et al. reported that AIE polymer 169 can also change its luminescent color under the action of force. The luminescent color changes from yellow (541 nm) to red (602 nm) before grinding, and the maximum luminescent wavelength is red shifted by 61 nm
[404]. However, due to the poor crystallinity of polymer molecules, the initial sample needs to be slowly precipitated or fully annealed to make the sample have a certain degree of crystallinity, and the obvious mechanoluminescence discoloration of the sample can be observed after grinding. Therefore, there are few reports on the mechanoluminescence response of AIE polymers.
5.2 Mechanoluminescence
Mechanolumine luminescence (ML) refers to the phenomenon of light emission from an object under the action of an external force. Due to its novel luminescence mechanism, ML has great application prospects in the fields of stress sensing, structural flaw detection, display, biological imaging and so on. At present, great progress has been made in the research of luminescence mechanism and application of inorganic ML materials, such as zinc manganese sulfide and rare earth doped aluminate. In contrast, the ML luminescence mechanism of organic compounds is more complex, lacking a clear molecular design strategy, and the development speed is relatively slow. In recent years, it has been shown that the molecular arrangement, spatial conformation and weak interaction between molecules in the aggregated state of organic compounds have a tremendous impact on the photophysical properties of the aggregated state. In addition, the electronic interaction between aggregated molecules can enhance the efficiency of intersystem crossing of triplet excitons, which makes organic compounds show phosphorescent properties. ML also belongs to the aggregated state luminescence phenomenon, so from the point of view of molecular packing, more luminescent properties of organic ML can be revealed from the molecular level. This part will briefly summarize the recent progress of organic ML, which is divided into five parts: force-induced fluorescence, homopolycrystalline, force-induced phosphorescence, doped ML and a special class of force-induced luminescent HOF materials.
5.2.1 Force induced fluorescence
Due to the small SOC effect, fluorescence is the most common luminescence mode of organic compounds, which is produced by the radiative transition of singlet excitons. Therefore, the vast majority of organic MLs are fluorescent. Studies have shown that piezoelectric effect is the main reason for the production of organic ML, and statistical data also show that most of the organic crystals that can produce ML belong to piezoelectric crystals. That is, ML is more easily observed in non-centrosymmetric crystals with polar space groups, while non-polar space group crystals tend not to produce ML. In 2015, Tang et al. Found that organometallic copper complexes with AIE properties can produce bright ML under pressure, which is the earliest reported AIE ML compound
[4]. Most of the pure organic ML compounds reported later have AIE properties, because the AIE properties avoid the annihilation of aggregated excitons, which is conducive to the generation of ML. From the point of view of molecular structure, organic ML compounds usually introduce polar groups containing heteroatoms to enhance the polarity of molecules. According to the types of functional groups, organic ML compounds are mainly divided into carbonyl, sulfonyl, triphenylamine, tetrastyrene and so on. In addition, due to the complexity of the molecular structure of organic compounds, such classification is not strict, and it is possible for multiple functional groups to appear in a ML molecule.
Carbonyl and sulfone groups have moderate electron-withdrawing ability and are usually connected with electron donor units containing N atoms, which are good acceptor units for building organic light-emitting molecules. In recent years, with the in-depth study of TADF materials, carbonyl and sulfone groups have been widely used in the design of TADF molecules. As shown in Fig. 106, due to the distorted molecular conformation, the five ML compounds containing carbonyl and sulfone groups reported at present, SPFC, DEAC-DPS, mono-DMACDPS, DBT-BZ-DMAC and FCO-CzS, have AIE, TADF and ML properties at the same time. The ML spectrum is basically coincident with the PL spectrum of the molecule, and the ML has extremely high brightness and can be clearly seen in sunlight
[405][406][407][408][409]. From the point of view of molecular packing, these compounds have asymmetric crystal structure and very close molecular arrangement. For example, DEAC-DPS, its methyl, propyl, butyl and pentylacridine substituted analogues have no ML, only the ethyl substituted product has ML properties, indicating that ML is closely related to the molecular structure. For FCO-CzS, yellow fluorescence and AIE properties were exhibited in Water/THF solution at 99% Water content (fig. 106b). As phenothiazine undergoes a transition from a planar conformation (Quasi-equatorial) to a vertical conformation (Quasi-axial) during grinding, the blue light in the ML spectrum gradually decreases and the yellow light gradually increases (fig. 106c). Correspondingly, the ML color changes from blue to white and finally to yellow (Figure 106d).
In addition, Chi et al. Designed and synthesized the first flexible organic luminescent molecule DOS (170) with selective expression of chromophores by using flexible phenothiazine groups and rigid phenoxazine groups to construct asymmetric molecules.Under different conditions, 10 different luminescent crystals with the maximum emission wavelength ranging from 469 to 583 nm can be obtained, and the luminescent color covers the whole visible light region
[410]. The phenoxazine-dominated donor-O group crystal exhibits high-energy emission, TADF characteristic, mechanoluminescence characteristic, and mechanoluminescence discoloration red shift, and the phenothiazine-dominated donor-S group crystal exhibits low-energy emission, room temperature phosphorescence characteristic, force-free luminescence characteristic, and mechanoluminescence discoloration blue shift (Fig. 107). Therefore, they explored the application of DOS in the fields of VOCs detection, colorful dynamic display and sensing by using the characteristics of DOS multiple stimulus response luminescence.
Triphenylamine is a classical electron donor with strong electron-donating properties. After further connection with aromatic groups, it can produce a larger dipole moment in the molecule and enhance the polarity of the molecule, which is very conducive to the production of ML. Li et al. reported a series of triphenylamine-based ML compounds
[411]. As shown in Figure 108a, the molecular polarity is further enhanced after the triphenylamine is connected to the benzaldehyde group of the electron acceptor, and the aldehyde group can also effectively enhance the intermolecular hydrogen bonding interaction. The obtained product TPA-1BA has a polar Pca2
1 space group, emits strong green fluorescence with high ML brightness, and can also be observed under sunlight (Figure 108b). The introduction of halogen atoms can also enhance the interaction between molecules, such as TPA-CHO-2R (R = F, Cl, Br), which directly connects triphenylamine with aldehyde groups and halogen atoms. The three compounds have symmetrical Pbca and asymmetric Pc, Pc space groups, which can produce strong ML
[412]. The products TPA-pR and TPA-mR (R = F, Cl, Br), in which the polar group methoxy was introduced into triphenylamine and the halogen atoms were introduced into the para and meta positions of the phenyl substituent, also showed ML properties
[413]. Among them, TPA-pF has a centrosymmetric space group P2
1/c1, and its ML is relatively weak, while the other five compounds have an asymmetric space group C1c1, and the intensity of ML increases significantly (Figure 108c), which can produce dark blue ML. These results also indicate the important influence of molecular space group on the properties of ML.
TPE is a typical AIE compound, in which four benzene rings form a very distorted spatial conformation around the central vinyl group, which effectively inhibits the tight π-π stacking between aggregated molecules and enhances the solid-state luminescence efficiency. The introduction of polar groups into TPE can increase the molecular polarity of TPE derivatives, enhance the interaction between molecules, and form close packing. At present, a large number of TPE derivatives are found to have ML properties. For example, introducing aldehyde group, amino group, ester group, methoxy group, alkynyl group, phenanthroimidazole group and cyano group into TPE can produce ML
[414][415][416][417][418][418][419]. Some of these compounds are more special, with unique molecular structure and electronic properties. For example, Z-BFBM-TPE introduces two methoxy and F substituents at the cis position of TPE, respectively, which enhances the dipole moment and polarity of the molecule, so it has ML properties, while the corresponding trans-substituted E-BFBM-TPE has smaller dipole and no ML properties. Heteroatom-containing groups are usually introduced to enhance the polarity of the molecule, while TETPE is a pure hydrocarbon compound with centrosymmetric space group C2, which increases the charge density of the substituent and the electropositivity of the terminal hydrogen atom by forming an alkynyl group, thus increasing the charge interaction and hydrogen bonding interaction between molecules, resulting in ML. DMPBPTZ is more special. The B — N and C = C bond in the center of the molecule are isoelectronic to each other, so it also has the property of C = C bond. DMPBPTZ has remarkable AIE properties, and the electron donor is connected to the acceptor in such a way that the molecule forms an asymmetric arrangement of polarity, resulting in ML properties
[420]. TPE-2-Th and tPE-3-Th are also variants of tPE structure. The asymmetric molecular structure endows these two compounds with better asymmetry, while the introduction of thiophene group enhances the electron-donating properties and molecular polarity, resulting in close molecular arrangement and ML properties
[421].
5.2.2 Polymorph
Polymorphism is common in organic compounds, but it is special for ML, because the compounds producing ML have the same molecular structure, but the molecular arrangement in the crystal is different, which is of great significance to reveal the luminescence mechanism of ML. The ML homopolycrystalline compounds that have been found are also consistent with the judgment of ML crystal form, that is, polar space group crystals are more likely to produce ML and have more compact molecular arrangement, while non-polar space group crystals are often unable to produce ML. As shown in Figure 110a, after the TPE was introduced into the aldehyde thiophene group, the compound P
4TA with two homogeneous polycrystals was obtained, both of which were non-central polar P (2) 1 space group, the bulk C
b crystal emitted blue fluorescence, and the columnar C
g crystal emitted green fluorescence, but only the C
g crystal could produce bright green ML
[422]. The two crystals were also able to undergo crystal phase transformation, and the C
b phase crystal could transform into the C
g phase (C
bf) after dichloromethane fumigation, showing ML properties. After the introduction of 4 methoxyl groups to the TPE group, TMPE was obtained which also had two polymorphs
[423]. Among them, the bulk C
p crystal with polar P2
1(c) space group emits fluorescence at 420 nm, while the columnar C
c crystal with non-polar C2 space group emits fluorescence at 429 nm. Only the C
p crystal is capable of producing a bright, deep blue ML, whereas the C
c crystal is not. The analysis of single crystal structure shows that the polar C
p crystal has stronger intermolecular C — H … π and C — H … O hydrogen bonding, which leads to more compact molecular packing, and the intermolecular slip is blocked by the rigid crystal structure, so it is easier to produce ML. The structural variant of tPE, TPE-5-MeTh, is also polymorphic, and only the non-centrosymmetric P1 phase crystal can produce ML, while the centrosymmetric P2 phase crystal cannot
[424]. In other polycrystals, both crystals have ML properties, and the ML spectrum depends on the luminescence spectrum of the crystal. As shown in fig. 110b, CDpP has two crystal phases of B and G, the luminescence of CDpP-B and ML are both blue, and CDpP-G is both green
[425]; In the two crystal phases of TBIM, the fluorescence of TBIMG and ML are both blue, while the fluorescence of TBIMG is green
[426]. TPA-o-3COOMe shows more peculiar properties. The centrosymmetric crystal phase A shows RTP phenomenon but no ML property, while the non-centrosymmetric crystal phase B, on the contrary, can not produce RTP, but can produce strong ML (Fig. 110c). The different photophysical properties of homogeneous polycrystals are closely related to the arrangement of molecules in their crystals
[427].
5.2.3 Force-induced phosphorescence
Generally speaking, the SOC coefficient of organic compounds is very small, and the triplet exciton is easily quenched by molecular vibration and oxygen, which makes it difficult for organic compounds to produce phosphorescence at room temperature. However, recent studies have shown that RTP is ubiquitous in solid organic compounds. The ML may have phosphorescent properties when the organic compound has the ability to produce both RTP and ML. In fact, in 1974, Zink et al. First reported the phenomenon of phosphorescence of phthalic anhydride and acenaphthene, which consists of fluorescence and phosphorescence
[428]. Recently, phosphorescent ML compounds have been reported. As shown in fig. 111a, DPP-BO, CX, CzS-C
2 , and BrFlu-CBr all exhibited dual fluorescence and phosphorescence MLs, and long-lived phosphorescence MLs were observed even after the force stimulation was stopped
[429][430][427]. Taking DPP-BO and CzS-C
2H
5 as an example (Figure 111b, C), their ML spectra contain fluorescence and phosphorescence components, and the phosphorescence properties depend on the close packing of crystalline molecules, but due to the short phosphorescence lifetime, the phosphorescence can not be observed after the cessation of force stimulation. In 2018, Chi et al. reported ML compounds with more significant phosphorescent properties
[431]. As shown in fig. 111d, the phthalimide derivatives ImF and ImBr have polar molecular structures and non-centrosymmetric space groups, and both have RTP and ML properties. ImF has a short RTP lifetime (τ = 35.3 ms), while ImBr has a long RTP lifetime (τ = 102.1 ms). Therefore, the spectrum of ImF is similar to that of PL, while that of ImBr is more peculiar. After being stimulated by force, ML changes from fluorescence to fluorescence-phosphorescence dual luminescence. After stopping force excitation, ML does not disappear, but forms long afterglow luminescence. Therefore, under the action of stress, the stress contact point of ImBr shows blue ML, while the ML at the contact position shows yellow band afterglow, which can last for about 1 s.
5.2.4 Doped ML
Impurities have special effects on the photophysical properties of crystals, and often a small amount of impurities can affect or even change the microscopic crystal phase and luminescent properties of crystals. From the point of view of luminescence mechanism, due to the low concentration of impurities in organic crystals, they can not emit light directly under the action of force, but they can become the luminescence center of crystals by FRET, showing the luminescence characteristics of impurities. In fact, the inorganic ML systems reported so far are almost all composed of doped composite materials, such as copper or manganese doped zinc sulfide (ZnS: Cu, ZnS: Mn), rare earth doped ceramic materials (SrAl
2O
4:Eu,Dy), etc., and their ML properties are almost entirely dependent on the nature of the dopant. However, for organic ML compounds, due to the lack of systematic molecular design strategies, the species of ML molecules are few, the high-brightness ML molecules are rare, and the luminescent colors are not rich enough, which is not conducive to the practical application of ML. However, the doping method can effectively solve this dilemma, and colorful MLs can be obtained by exciting guest fluorescent molecules with host MLs. For example, carbazole derivatives Cz-alkyl-6 and NPC can produce dark blue ML (Figure 112a), and the ML spectrum overlaps with the absorption spectrum of some organic fluorescent compounds, which can be used as an excitation source to excite organic molecules to emit light
[432][433]. The two compounds can be used as non-polar solvents to dissolve organic compounds after melting, and produce MLs with corresponding colors after being doped with fluorescent molecules such as blue light, green light, yellow light, red light and the like, and the spectra of the MLs are consistent with those of the PLs, so that high-brightness MLs with various colors are successfully obtained. In 2019, Tang et al. Prepared a more interesting host-guest dopant
[347]. As shown in fig. 112b, the host materials PCP, PA, and DCB have the properties of ML and RTP, and when the host and guest molecules NA are doped at a ratio of 1%, they all produce yellow RTP. More interestingly, when stimulated by force, the adulterant can not only produce immediate dark blue ML, but also produce long-lived delayed yellow ML, showing a unique force-stimulated long-lived phosphorescent ML.
5.2.5 Mechanoluminescent HOF material
In recent years, a new class of porous materials, Hydrogen-bonded organic frameworks (HOFs), has attracted more and more attention. HOFs are porous materials constructed from organic units interconnected by hydrogen bonds
[410]. HOFs have many inherent advantages, such as low cost, easy purification, easy regeneration, good water stability and thermal stability
[434]. Therefore, HOFs have broad application prospects in gas storage/separation, proton conduction, molecular recognition, optics and other fields
[435]. In 2017, Chi et al. Designed and synthesized TPE4N (171), a TPE derivative based on nitro substitution, and constructed HOF structure. Its single crystal structure contains two holes with different sizes, which are 5.855 855 Å × 5.855 855 Å (α hole) and 7.218 218 Å × 7.218 218 Å (β hole)
[436]. There is a nitrobenzene group which can rotate freely in the α hole, and the excited state energy is dissipated in the form of radiationless transition, so the crystal hardly emits light. When the intramolecular benzene ring rotation in the HOF structure is suppressed by grinding the crystal or cooling the crystal to 77 K, the crystal luminescence can be greatly enhanced. After reconstitution of the HOF structure by annealing or solvent fumigation, its luminescence can be quenched again (fig. 113).
The results of this study not only obtained the HOF structure of force stimulus response, but also intuitively explained the mechanism of AIE, which has important academic value. In 2018, Tang et al. Also found in their research that the change of luminous intensity of TPE4N (171) crystal before and after being stressed has the characteristics of high contrast, high sensitivity, good reversibility and fast response. They applied it to information storage and tactile sensing. When they pressed the film with their thumbs, the fluorescence in the touch area was immediately "lit up" to produce a fingerprint pattern visible to the naked eye
[437].
In 2020, Hisaki et al. reported a triaxially-braided HOF structure based on TPE derivative 172
[438]. Amorphous sample 172-amorph with yellow emission was obtained by grinding. Its maximum emission wavelength was red-shifted from 477 nm to 547 nm, and the fluorescence quantum yield increased from 21.6% to 55.5%. They believed that this was due to the collapse of the low-density framework structure, which resulted in the close arrangement of molecules and the inhibition of the rotation of aromatic groups.
Chi et al. Designed and synthesized TPE-4pn (173) by adding a benzene ring on the basis of the aforementioned compound TPE4N (171), and proposed the concept of ultra-high flexible HOFs with permanent pores, and explored its performance and possible application fields
[439]. Under different external stimuli, through the adjustment of molecular conformation and assembly mode, the HOF structure formed can undergo multiple reversible structural transformations among large-size pore structure, small-size pore structure and non-pore structure.Including shape memory behavior and single crystal-single crystal transition behavior, as well as achieving reversible structure and luminescence color transition upon mechanical force stimulation (fig. 115).
Chi et al. Successfully prepared the first HOF compound TPE-4PCOM (174) with mechanoluminescence properties
[440]. In the further study, it is found that the acetophenone group in the molecule can not only act as a hydrogen bond acceptor, but also play a vital role in the mechanoluminescence phenomenon of the HOF structure. Under external stimulation, the benzene ring of TPE group and the benzene ring of acetophenone group can rotate around the C-C single bond, resulting in the change of the relative position of methyl and oxygen atoms in acetophenone group. Further, in molecule 174, the orientation of the oxygen atom in the acetophenone group with respect to the plane of the ethylene double bond is changed. The different rotation angles of the acetophenone group have a great influence on the dipole moment of the molecule 174, and then have a great influence on its supramolecular dipole moment. When the orientation of the oxygen atoms in the acetophenone group tends to be the same, the supramolecular dipole moment of the framework structure increases sharply, so that a strong electric field can be generated under the stimulation of external forces, which is conducive to the realization of mechanoluminescence properties in HOFs. Dichloromethane can specifically align the orientation of the oxygen atoms in the acetophenone group to induce the mechanoluminescence effect, which they call the guest-induced respiration effect accompanied by the controllable behavior of the mechanoluminescence activity (Fig. 116).
Chi et al. Then designed and synthesized the X-shaped molecule TPE-4PhOH (175) by changing the above acetyl group into a phenol group
[441]. It is found that the molecule 175 can be used as a raw material for the molecular weaving process, and the surrounding molecules can be assembled into cyclic or chain-like aggregates through the O — H … O hydrogen bonds between the hydroxyl groups. When each X-shaped molecule 175 is connected with two adjacent molecules by four pairs of intermolecular O-H.. O hydrogen bonds, a one-dimensional long chain with rhombic holes can be formed. Molecules 175 on long chains in different directions can be inserted into these diamond-shaped holes and interweave with each other to form an interlocking pattern, thus obtaining the first woven HOF structure constructed by one-dimensional long chain interlocking (fig. 117). The HOF structures obtained by molecular weaving in different solvents have different structures and properties. Under the action of mechanical force, the crystal structure of 2D-90 and 2D-82 is destroyed and gradually transformed into an amorphous state, the intermolecular interaction is destroyed, and the excited state can relax to a lower energy level, so that the luminescence of the amorphous state sample is red-shifted and red-shifted to a yellow region. When the samples were fumigated with EA and MeOH respectively, the amorphous state could be transformed into their respective crystalline States.
In addition to HOF, there are other organic frameworks such as metal-organic framework (MOF) and covalent organic framework (COF), and there are also some reports on the mechano-stimulated luminescence response based on MOF. For example, the MOF structure formed by ligand molecule 172 and zirconium ion reported by Su et al. Can produce one-photon, two-photon and three-photon absorption luminescence under the action of force
[442].
5.3 Some other stimuli-responsive properties
In addition to the research on mechanochromic and mechanoluminescent materials, researchers have also reported some luminescent materials with other stimuli-responsive properties. For example, in 2018, He et al. proposed a new design strategy for high-performance On-Off mechanically responsive luminescent materials by controlling the ISC process using the molecular stacking effect (Fig. 118). Nitro groups were introduced into the distorted AIE molecules to facilitate the efficient ISC process. Theoretical calculations show that the torsion angle and the number of nitrophenyl groups are the key factors to regulate the ISC process. Mechanical stimulation can affect the conformation of the molecule, resulting in different solid-state morphologies at different on-off transitions of luminescence. Experiments demonstrate high contrast and sensitive switching MRL performance, as well as reversibility and fast response behavior, which provides a possibility for the application of pure organic room temperature phosphorescent materials in optical information storage and tactile sensors
[437].
In addition, He et al. reported a new photoresponsive luminescent material (PRL) in 2021, which has reversible conformation and fluorescence switching properties in the solid state (Fig. 119)
[443]. The interesting luminescence properties of TPE-4N can be reversibly regulated by rapid phototriggering and thermal annealing. Small molecular conformational changes can be attributed to the corresponding PRL properties, where the crystalline and amorphous States confer efficient and weak ISC processes to turn on and off emission, respectively. Crystals, powders and thin films in different solid States exhibit considerable photoresponse behavior, which has the advantages of high contrast, fast response, good durability and reversibility because the light stimulation is non-contact and non-destructive. In addition, the excellent film quality and high performance PRL behavior make it possible to be applied to stealth anti-counterfeiting and micro-resolution dynamic optical data storage.
6 Application of AIE materials
AIE materials can make full use of the natural aggregation of molecules to inhibit the non-radiative dissipation of the excited state by restricting the intramolecular motion.Because of its high aggregation luminescence efficiency, AIE materials have been widely used in optoelectronic devices, chemical sensing, biological detection and imaging and other frontier cross-cutting fields.
6.1 Application of AIE in Evaluation of Inorganic Phase Dispersion of Composites
Polymer materials are widely used in agriculture, industry, national defense and other fields because of their high strength, high toughness and high temperature resistance
[444,445]. In order to further optimize polymer processing technology, reduce processing costs and improve performance, researchers have constructed a variety of organic-inorganic composites through physical doping and blending methods
[446,447]. Wherein inorganic nano materials (such as silicon dioxide, two-dimensional layered montmorillonite, hydrotalcite and the like) are used as fillers to be added into organic polymers (such as polyethylene, polyvinyl chloride, silica gel and the like) to construct an organic-inorganic composite material,The mechanical properties, oxidation resistance and smoke suppression of the polymer are improved, and the performance of the inorganic-organic composite material greatly depends on the dispersion degree of the inorganic filler in the organic polymer
[448⇓~450]. Therefore, it is very important to study the dispersion state of inorganic fillers for the study of the structure-activity relationship of composites and the design and construction of high-performance composites.
In order to characterize the dispersion of inorganic phases in composites, electron microscopy is often used to observe them
[451⇓~453]. However, the use of electron microscopy to observe the sample preparation process is complex, the sample is damaged, and only individual positions can be evaluated after slicing, which is only suitable for small area window evaluation, and the results obtained can not represent the macro-dispersion state of inorganic fillers. In addition, although the dispersion of inorganic phase can be studied by calculation simulation, the calculation of theoretical simulation is cumbersome and limited in practicability
[454]. With the rapid development of nanomaterials in various fields, the current characterization methods can no longer meet the needs of the evaluation of the dispersion of composites, and it is urgent to develop a simple, practical and non-destructive method to evaluate the dispersion of inorganic fillers in inorganic-organic composites.
In recent years, confocal laser scanning microscopy (CLSM) has been widely used in chemical, biological and medical fields
[455,456]. Ultra-fast and non-invasive acquisition of three-dimensional images is the biggest feature of CLSM. At the same time, compared with the penetration depth of electron beam, the penetration of laser beam is stronger, which can reach tens of microns. In addition, the scanning range of CLSM in the two-dimensional plane can reach hundreds of microns to achieve large-scale fluorescence imaging
[457,458]. These characteristics have led researchers to use CLSM to characterize the structure of materials. It is assumed that if the inorganic phase can emit light, CLSM can be used to evaluate the dispersion of the inorganic phase in the composite material. However, the traditional fluorescent probes are quenched in concentrated solution or in the aggregated state, and the evaluation of the structure and properties of polymer materials is mostly carried out in the aggregated state or solid state, which greatly limits the application of fluorescent probes in material characterization.
Solving this problem and realizing the design and synthesis of AIE molecules and their effective modification of inorganic phases are the key to the characterization of inorganic phase dispersity by fluorescence imaging.
6.1.1 Three-dimensional Fluorescence Imaging and Qualitative Evaluation of Inorganic Phase Dispersion
The inorganic phase dispersed in the organic polymer is usually hydrophilic on the surface. In order to make it effectively dispersed in the organic polymer, it is often modified by surfactants
[459,460]. In principle, fluorescent surfactants can directly modify the inorganic phase to achieve organic modification of the inorganic phase and make it have strong fluorescence, and then evaluate the dispersion of the inorganic phase in organic-inorganic composites by fluorescence imaging technology. Lu et al. effectively achieved this goal by designing and synthesizing anionic surfactant tetraphenylethylene-dodecyl sodium sulfonate (TPE-SDS) and cationic surfactant tetraphenylethylene-dodecyltrimethylammonium bromide (TPE-DTAB) with AIE properties (Fig. 120)
[461]. They first studied the self-assembly process of TPE-SDS surfactants, and the results showed that under different ionic strengths, the synthesized AIE surfactants would self-assemble into assemblies with different morphologies, and the fluorescence of these assemblies would not be quenched, but enhanced. Confocal fluorescence imaging technique was used to capture the morphology information of the assemblies, and the results were completely consistent with the conventional in-situ small-angle X-ray scattering technique to monitor the transition process of the micellar aggregates from spherical to worm-like. These results demonstrate the successful synthesis of AIE molecules and the feasibility of fluorescence imaging in the solid-state characterization of materials.
In addition, they utilized the synthesized AIE surfactant TPE-DTAB to modify the layered nanofiller montmorillonite (MMT). By comparing the X-ray diffraction (XRD) patterns, infrared spectra and potential changes before and after MMT modification, it can be seen that TPE-DTAB has successfully modified MMT (Figure 121). Fluorescence spectrum analysis showed that MMT powder did not have any fluorescence, but MMT modified by TPE-DTAB had strong cyan fluorescence, and the emission wavelength was about 490 nm, which was consistent with the emission wavelength of the original TPE-DTAB. Meanwhile, because the TPE-DTAB/TPE-SDS molecule has the characteristic of a surfactant molecule, the modified hydrotalcite or MMT is well dispersed in a petroleum ether solvent and shows good hydrophobicity. This shows that AIE surfactants can realize the organic modification and fluorescent labeling of layered nanomaterials.
They also prepared organic-inorganic composites by mixing AIE surfactant-modified MMT with polyvinyl chloride, and observed the dispersion of layered nanofillers by transmission electron microscopy and fluorescence imaging. It is clear that transmission electron microscopy images are limited to a few microns. In contrast, the fluorescence imaging technique allows for a comprehensive evaluation of the dispersion level of layered nanofillers with an observation range of several hundred microns. More importantly, fluorescence imaging allows fast and nondestructive tomography to observe the three-dimensional dispersion of MMT in organic-inorganic composites: 30 images in the two-dimensional plane of X and y were acquired at the same position and different z-axis depths.The three-dimensional spatial distribution of the TPE-DTAB modified MMT in the organic matrix PVC was obtained by three-dimensional reconstruction of the 30 images (Fig. 122). The size of the image is 600 μm × 600 μm, and the real dispersion state of montmorillonite in polyvinyl chloride can be observed on a mesoscopic scale.
The imaging technique described above is based on the pre-modification of the inorganic material before it is added to the organic matrix. In order to promote the visual evaluation of fillers in polymer matrix, Lu et al. Further developed the "post-dyeing localization identification" method: no fluorescent molecules were introduced in the process of composite membrane preparation, and only the fillers were labeled with fluorescence during imaging evaluation. The method mainly adopts a molecular targeting recognition technology, and based on the specific combination between a probe molecule and a target molecule, the in-situ recognition of the target molecule is realized under the condition of maintaining the original characteristics of the target molecule.
Boronic acid groups have been widely used in molecular recognition, supramolecular assembly and other fields due to their targeted recognition of hydroxyl groups
[462,463]. Inorganic fillers commonly used at present, such as hydrotalcite (LDHs), MMT, silica, etc., have abundant hydroxyl structures on their surfaces. When tetraphenyl ethylene diborate (TPEDB) with AIE activity is specifically combined with LDHs powder, it can be found that the combined complex has strong blue-green fluorescence at 470 nm (Fig. 123); The fluorescence intensity increased with the increase of the concentration of LDHs. When the content of hydrotalcite is 1.8 mM, the fluorescence intensity of the hydrotalcite can reach 25 times of that of the original TPEDB solution. This is mainly because TPEDB is immobilized on the hydrotalcite layer, so that the non-radiative transition is limited, thereby enhancing its fluorescence signal
[464].
They also tried to disperse LDHs with a mass ratio of 5% in polyethylene (PE), prepare PE-LDHs composites by hot pressing, and dye the composites in TPEDB solution. Through CLSM observation, the composite material with LDHs inorganic phase showed a large number of cyan fluorescence spots, while the PE without LDHs did not show this fluorescence phenomenon after dyeing. In order to prove the accuracy of this method, they used quantum dots (QDs) to pre-dye LDHs by electrostatic interaction, and prepared PE-5% (QD @ LDH) composites. Then the composite material is put into TPEDB solution for staining, and the imaging images of the two fluorescent molecules are respectively acquired through CLSM double channels. As shown in fig. 124, LDHs pre-stained by QDs exhibited red fluorescence, while LDHs post-stained by TPEDB exhibited cyan fluorescence in the PE composite. When the two images were overlapped, it was found that the staining points were almost completely coincident. It can be seen that the developed AIE molecular post-dyeing recognition can achieve the accurate location of LDHs in composites. This technology is not only simple to operate, but also only introduces probe molecules into polymer imaging, which does not affect the preparation process and performance of polymers, and realizes in-situ recognition and visual analysis of inorganic phases in polymers in a non-destructive state.
6.1.2 Three-dimensional Fluorescence Imaging and Quantitative Evaluation of Inorganic Phase Dispersion
The above work provides a reliable method for the qualitative analysis of the dispersion degree of inorganic fillers. In order to further realize the quantitative analysis of the dispersion degree of the inorganic phase in the composite material, three-dimensional imaging is carried out based on the CLSM technology, and further calculation simulation is carried out on the imaging analysis.Three-dimensional quantitative analysis of inorganic phase dispersion in composites can be carried out by probability analysis of inorganic phase dispersion and calculation of particle spacing, which provides a basis for the study of structure-activity relationship of composites.
Lu et al. performed data mining on 3D macroscopic fluorescence images of the dispersion of AIE surfactant-modified layered nanofillers in PVC (Fig. 125), used 3D image analysis software to obtain the 3D information of hydrotalcite fillers, and used the centroid point to replace the inorganic phase with irregular shape
[465]. Firstly, Pearson chi-square test was used to evaluate the dispersion of inorganic phase, and the smaller the χ
2,χ
2 value of the test statistic was, the more uniform the dispersion of inorganic phase was. Secondly, the two-dimensional plane density of the sample was analyzed, and the centroid points were projected into each two-dimensional coordinate system (XY plane, YZ plane and XZ plane). For a sample with absolutely uniform dispersion, the density of its centroid point appearing at any position in the two-dimensional coordinate plane is the same, so the closer to this assumption, the more uniform the inorganic phase dispersion is. Finally, one-dimensional numerical characteristic evaluation is carried out, the centroid point is projected to a one-dimensional coordinate system (X axis and Y axis) to evaluate the dispersion degree of the montmorillonite, a dimensionless parameter, a correlation coefficient CV is introduced, and the value of the correlation coefficient CV is the ratio of the standard deviation (SV) to the average value (MV).Calculate the CV, SV and MV values corresponding to the X axis, Y axis and Z axis respectively. The CV and SV values are usually used to represent the fluctuation range of the data. The smaller the value is, the smaller the fluctuation range is and the more uniform the dispersion is. Based on this, four dispersity levels of excellent, good, medium and poor were specified for different composites, and the quantitative evaluation of their three-dimensional dispersity was realized.
Furthermore, by evaluating the distance between two adjacent particles in three-dimensional space, it is expected to realize the study of the structure-activity relationship of composites
[466]. In the PE-LDHs composites, with the LDHs content increasing from 0.4% to 1.6%, the average spacing of LDHs inorganic phase particles decreased from 16.2 μm to 7.8 μm, and the dispersion tended to be uniform. However, when the content of LDHs exceeds 1.6%, the average spacing increases abnormally, which is mainly due to the agglomeration of LDHs in the system, resulting in larger spacing and wider distribution (Fig. 126). However, the mechanical properties of the composites are consistent with the change of the average spacing of inorganic particles, showing a trend of first increasing and then decreasing. It can be seen that the smaller the average space distance of inorganic particles is, the more uniform the dispersion is, and the better the macroscopic properties are. The establishment of this method provides a strong support for the quantitative analysis of the relationship between the structure and properties of composites.
6.2 AIE Materials for Organic Electroluminescent Diodes
Organic light-emitting diodes (OLEDs) are light-emitting devices based on organic semiconductors, which have the characteristics of planar luminescence, abundant material sources, adjustable color, ultra-thin, soft light, large area and flexibility.It is considered to be a new generation of display and lighting materials, and shows great application prospects in display, lighting, wearable devices, medical detection and visible light communication
[467]. In order to meet the application requirements, OLEDs are required to have the basic characteristics of high efficiency, long life and low cost, in which materials play a very important role. So far, three generations of organic light-emitting materials have been developed: the first generation of fluorescent materials, the second generation of phosphorescent materials and the third generation of thermally activated delayed fluorescent materials. Although the phosphorescent materials containing noble metals have a high efficiency of 100% exciton utilization, their high price and short lifetime of blue phosphorescent materials limit their application. Although thermally activated delayed fluorescent materials (TADF) have the advantages of low cost and high efficiency, the problems of efficiency roll-off and stability at high brightness have not been well solved. In comparison, although fluorescent materials show low efficiency due to the limitation of spin statistics theory, their unique advantages of long life and low cost make them still the most valuable material system for application. Especially in recent years, with the proposal of new principles and the successful synthesis of related materials, the efficiency of fluorescent organic light-emitting materials has exceeded the theoretical limit of 25% singlet exciton emission.Through the effective conversion of triplet excitons to singlet excitons, nearly 100% exciton utilization can also be achieved in fluorescent organic light-emitting materials, which provides a new hope for the application of fluorescent organic light-emitting materials in OLEDs.
The electroluminescence (EL) of OLEDs usually includes four processes: carrier injection, carrier transport, exciton formation by carrier recombination and exciton radiation decay. Therefore, the external quantum efficiency ηext (EQE) of the device can be written as:
Where ηint is the internal quantum efficiency, ηout is the output coupling efficiency, γ is the balance factor of electrons and holes, χST is the number of excitons formed per unit charge (singlet and triplet States), and ηPL is the fluorescence quantum efficiency of the material film. It can be seen that the efficiency of the device not only requires the material to have the characteristics of efficient exciton utilization to achieve 100% exciton emission, but also requires the material to have high fluorescence quantum efficiency in the thin film state. Due to the existence of concentration bursting effect, traditional fluorescent organic light-emitting materials often show low fluorescence quantum efficiency in the thin film state, which can not be completely solved even if doped in the host material, but also greatly increases the difficulty of device preparation process and the stability problem caused by phase separation. Because the AIE material shows a high efficiency luminescence characteristic of nearly 100% quantum efficiency in the solid-state thin film state, it can be used as an undoped light-emitting layer to prepare high-efficiency OLEDs, which solves the problem of concentration quenching well and shows great application prospects.
In this section, the fluorescent OLEDs prepared by AIE materials are introduced firstly, and then four kinds of AIE devices with high efficiency are discussed according to the mechanism of high efficiency electroluminescence. Finally, the latest research results of AIE materials in white OLEDs are introduced to show the versatility and broad application prospects of AIE materials.
6.2.1 OLEDs based on AIE material
The application research of OLEDs based on silole derivatives of AIEgens started earlier. As early as reporting the AIE phenomenon of silole derivatives, Tang et al. had already explored the preparation of undoped OLEDs devices with the device structure of ITO/CuPc/TPD/MPPS/Alq
3/LiF/Al, and the maximum brightness, power efficiency (PE), and current efficiency (CE) reached 9234 cd·m
-2, 12.6 lm·W
-1, and 12 cd·A
-1, respectively
[468]. In addition to the unique AIE properties, silole derivatives generally have good electron transport properties due to their σ
*-π
* conjugation characteristics
[469]. In 2005, Liu and Zhu et al. Measured that the electron mobility (μ
e) of some silole derivatives is about 10
-6cm
2·V
-1·s
-1, which is similar to that of the typical electron transport material Alq
3[470]. Therefore, silole derivatives can also be used as bifunctional materials for light emission and transmission in OLEDs devices. For example, Zhao et al. modified benzimidazole and phenanthroimidazole groups at the 2,5-site of silole to obtain a series of n-type AIE materials with μ
e up to 10
-4cm
2·V
-1·s
-1, showing excellent electron transport capability
[471]. Using these n-type AIE materials for both electron transport and light emission in OLEDs devices, they fabricated OLEDs with dual organic functional layers with simple structure, with maximum CE, PE and EQE of 13.30 cd·A
-1, 14.51 lm·W
-1 and 4.25%, and the efficiency is close to that of traditional OLEDs devices with separate electron transport layers.
Compared with silole derivatives, the chemical synthesis of AIEgens based on TPE derivatives is simpler, so it is favored by many researchers. With TPE or triphenylethylene (TriPE) as the core, researchers have controlled the properties of TPE derivatives by selecting substituents with different properties
[472]. As early as 2007, Tang et al. Prepared TPE and TPE-2Ph by McMurry reaction, and observed their AIE phenomenon
[473]. The Ф
F of TPE and TPE-2Ph reached 50% and 90%, respectively, in the aggregated state (tetrahydrofuran/water mixed solution, water volume fraction f
w=90 vol%). The performance of their undoped OLEDs is quite different. The maximum brightness of undoped OLED based on TPE is only 1800 cd·m
-2, and the maximum EQE is only 0.4%. While the maximum brightness and the highest EQE of the undoped OLED based on TPE-2Ph are improved to 10680 cd·m
-2 and 2.56%.
In order to further explore the structure-property relationship of AIE molecules based on TPE, Tang et al. Proposed the molecular design idea of "turning stone into gold", modified four TPE groups with AIE characteristics on the typical ACQ molecule pyrene, and developed a new blue light material TTPEPy
[474]. Due to the introduction of AIE group into ACQ molecule, TTPEPy exhibits the properties of AEE, and its fluorescence quantum yield in solution and powder solid state is 9. 5% and 70%, respectively, and it has good thermal stability, which is suitable for the preparation of vapor-deposited non-doped OLEDs devices. By optimizing the device structure, the high efficiency of the electroluminescent device is achieved, and the maximum CE, PE and EQE reach 12.3 cd·A
-1, 7.5 lm·W
-1 and 4.95%, respectively, which are obviously improved compared with the previous work. Furthermore, they modified TPE with thiophene and benzothiadiazole groups to obtain four molecules (BTPE, BTPETTD, BTPETD and BTPEBTTD) emitting from blue to red light
[475]. The undoped OLEDs based on these molecules were fabricated with the structure of ITO/NPB (60 nm)/emitter (20 nm)/TPBi (10 nm)/Alq3 (30 nm) and LiF (1 nm). The maximum EL efficiency of the undoped OLEDs based on BTPETTD reached 6.4 cd·A
-1 and 3. 1%.
The modification of TPE with different groups can not only change the photophysical properties of materials, but also regulate the electrical properties of materials. For example, Zhao et al. Introduced diphenylamine and dimesityl boron units into TPE to construct "D-a" molecular structure, and obtained two molecules, TPE-NB and TPE-PNPB
[476]. Among them, TPE-PNPB has excellent hole transport properties, and can be used as both the light-emitting layer and the hole transport layer in the device, with the maximum device efficiency of 16.2 cd·A
-1, 14.4 lm·W
-1, and 5.35%. Subsequently, they also followed a similar idea to prepare dimesityl boron-TPE derivatives, and obtained TPE-DB, TPE-BPDB and TPE-TPDB by adjusting the number of benzene rings between dimesityl boron and TPE
[477]. These molecules all have good electron transport ability, and TPE-DB is used in both electron transport and luminescence processes in OLEDs devices, and a green bilayer device with a maximum EQE of 4.6% is obtained. Similarly, n-type AIE materials can also be fabricated using benzimidazole, and the maximum brightness of green bilayer OLEDs devices exceeds 100 000 cd·m
-2, with a maximum efficiency of 5.8%, 14.6 lm·W
-1, and 16.8 cd·A
-1[478].
6.2.2 AIE-OLEDs with High Exciton Utilization
Although the above AIE materials based on silole and TPE have very high fluorescence quantum yield in the solid state, the efficiency of their electroluminescent devices is still not ideal, and the highest EQE is only about 5%. This is due to the fact that 25% of singlet excitons and 75% of triplet excitons are generated when holes and electrons are injected into the light-emitting layer in OLEDs. Among them, the radiative transition of triplet excitons is usually spin-forbidden, and the exciton utilization rate of traditional AIE materials in OLEDs is only 25%. Therefore, improving the exciton utilization of AIE materials in OLEDs is the key to further optimize the efficiency of AIE-OLEDs.
In general, researchers have used new luminescent materials with triplet-triplet annihilation (TTA), Hybridized local and charge-transfer (HLCT), Thermally activated delayed fluorescence (TADF), phosphorescent emission and other characteristics to improve the exciton utilization of OLEDs devices, so that the maximum EQE of devices reaches 20% ~ 30%, which is better than traditional fluorescent materials.
(1) OLEDs based on AIE-TTA material
Compared with traditional fluorescent OLEDs, OLEDs constructed with TTA materials have long device life and relatively high device efficiency, which is considered to be an excellent way to break through the theoretical limit of 5% of pure fluorescent OLED device efficiency
[479]. However, high TTA upconversion efficiency is limited to some special materials, such as anthracene derivatives and rubrene, due to strict energy level requirements. However, due to the ACQ effect of these materials, they often need to be incorporated into an appropriate host material. However, for doped devices, it is difficult to obtain high-performance blue OLED by controlling the TTA process. More importantly, doping the device makes the device fabrication process more complex. Therefore, the development of anthracene compounds with efficient solid-state luminescence is very advantageous for the construction of high-performance undoped blue TTA-OLEDs. Combining PAH compounds with AIE characteristics with TTA function is an effective way to achieve high device efficiency. According to the AIE molecular design concept, triphenylamine with distorted structure was used to modify ACQ molecule, thus realizing the preparation of novel AIEgens
[4]. In addition, Konishi et al. demonstrated that the frontier orbitals of diarylamine and anthracene are perpendicular to each other when the donor plane is twisted, and that the transition between molecular orbitals with relatively orthogonal directions may also promote the conversion of
3(TT) to S
1[480].
Based on the above analysis, Qin et al. designed and synthesized two compounds: TPA-An-Ph and TPA-An-mPhCz
[481]. The triphenylamine and carbazole groups can promote the hole transport of the device, thereby improving the performance of the device; The meta-substitution strategy can achieve high luminescence efficiency of the compound. Since TPA-An-Ph and TPA-Ph-mPhCz have AIE characteristics, they exhibit high photoluminescence quantum efficiency in a thin film state. Because of their high solid-state luminous efficiency and excellent thermal stability, undoped OLEDs can be fabricated using them as light-emitting layers. The maximum external quantum efficiency (EQE) of the undoped OLED based on TPA-An-Ph is 4.51%. The maximum EQE of the undoped TPA-An-mPhCz-based OLED is 8. 10%, and the device efficiency is nearly doubled. The TTA + AIEgens strategy provides a new platform for building efficient non-doped blue OLEDs (Figure 128).
(2) OLEDs based on AIE-HLCT materials
Giving AIEgens to the HLCT process is also an idea to break through the exciton utilization of fluorescent materials. The excitons in HLCT materials have the characteristics of both Locally excited (LE) and Charge transfer (CT) excitons. Among them, the characteristic of the LE state can ensure the high luminescence efficiency of the material, while the characteristic of the CT state is conducive to the realization of the reverse intersystem crossing (T
n→S
n) process of the high-energy state, thereby improving the exciton utilization. For example, Wang et al. Constructed six AIE-HLCT blue light molecules using phenanthroimidazole (PPI) group, cyano group and TPE motif, and regulated the components of CT state and LE state by changing the sites of cyano group and TPE motif in the molecules
[482]. The existence of TPE units in the molecule endows these materials with obvious AIE properties, so that these materials all show high fluorescence quantum efficiency in the solid state, and then non-doped sky-blue OLEDs devices with maximum brightness, EQE, CE and PE of 31 070 cd·m
-2, 7.16%, 18.46 cd·A
-1 and 16.32 lm·W
-1 are obtained, respectively, and the utilization of electrogenerated excitons reaches 48% with low efficiency roll-off. They also constructed AIE-HLCT materials (TNZPPI, TNZtPPI and TNZ2tPPI) emitting from red to near infrared using PPI-based, naphthothiadiazole-based and triphenylamine-based motifs, and prepared non-doped/doped OLEDs on this basis, with the maximum EQE reaching 2.48%/6.83% and the maximum utilization of electrogenerated excitons exceeding 80%, which illustrates the effectiveness of AIE-HLCT design ideas
[483].
Undoped blue OLEDs using AIE material as the light-emitting layer show high efficiency and low efficiency roll-off characteristics. The device efficiency basically exceeds the theoretical limit of 5%, and the best device efficiency even reaches 8.5%, which fully demonstrates the potential application value of AIE as an OLEDs material
[484]. In order to reveal the physical mechanism of efficient electroluminescence of AIE materials, Ma and Tang et al studied a blue AIE molecule, TPB-AC, by magnetoluminescence measurement, theoretical calculation and transient fluorescence (PL) spectroscopy
[485]. The structure of TPB-AC molecule is shown in Figure 129. The fluorescence quantum efficiency of the material film is as high as 98.4%, the triplet energy level is 2.48 eV, the emission wavelength is 448 nm, and the exciton lifetime is 3.25 ns, which is a typical AIE molecule. Multilayer OLEDs using pure TPB-AC as the light-emitting layer have the highest EQE of 7.0% by optimizing the electron transport layer, and the maximum EQE is 6.3% at 1000 cd·m
-2 brightness, which shows very low efficiency roll-off. The CIE coordinates is (0.15,0.08), which shows good deep blue emission. Fig. 130 shows the horizontal orientation characteristics of the TPB-AC molecule. It can be seen that the molecule has a good molecular orientation (up to 79%), which greatly helps to improve the light output coupling efficiency of the device, and is also one of the main reasons for the high efficiency of the device.
More interestingly, TPB-AC blue OLEDs show a very special magnetic electroluminescence (MEL) characteristic (Fig. 131). The MEL intensity first increases and then decreases with the magnetic field, and the slope at high magnetic field does not change with the bias current, which is completely different from the traditional fluorescent OLEDs. It also excludes the TTA process and indicates a new electroluminescence mechanism.
Obviously, TPB-AC blue OLEDs must have generated an additional exciton process, which makes more triplet excitons transfer to singlet excitons, thus improving the device efficiency and leading to this special MEL lineshape. To explore the origin of this extra exciton, they performed calculations on the energy levels of the TPB-AC molecule, as shown in Figure 132. It can be seen that, similar to traditional fluorescent molecules, the T
1 of TPB-AC is 0. 37 eV lower than that of S
1, which effectively suppresses the RISC process from T
1 to S
1. However, the T
2 of TPB-AC is high and very close to the S
1, so it is easy to form RISC from the T
2 to the S
1, which realizes the efficient Dexter energy transfer, making the triplet exciton participate in the luminescence of the singlet exciton.The efficient RISC process from T
2 to S
1 is further confirmed by the transient fluorescence decay characteristics of TPB-AC films and the MEL characteristics of TPB-AC blue OLEDs at different temperatures
[485]. This implies that the physical nature of the efficient electroluminescence of TPB-AC is due to the thermal exciton process on the T
2, which also provides a new basis for the validity of the hybrid local and charge transfer (HLCT) luminescence principle.
It is further found that the position of the excited triplet T
n(n≥2) relative to S
1 plays a decisive role in the exciton conversion, thus affecting the efficiency of the device. Fig. 133 shows the molecular structures of two AIE blue-emitting molecules, CN-TPB-TPA and TPBzC1, and the EQE and MEL characteristics of the blue-emitting OLEDs prepared with CN-TPB-TPA and TPBzC1. Although the fluorescence quantum efficiency of CN-TPB-TPA (93.2%) is lower than that of TPBzC1 (99.9%), it can be seen that the maximum EQE of CN-TPB-TPA device reaches 8.5%, which is almost twice as high as that of TPBzC1 (4.5%).Accordingly, they also show completely different MEL characteristics. The high efficiency CN-TPB-TPA device also shows an increasing and then decreasing MEL line shape, while the low efficiency TPBzC1 device shows a normal positive Lorentzian MEL line shape, indicating the difference in exciton conversion efficiency between the two devices. In order to reveal the physical reason for this large difference, the energy levels of the two AIE molecules were also calculated, as shown in Figure 134. The S
1 of CN-TPB-TPA is located between T
2 and T
3,While the S
1 of PBzC1 is located between the T
3 and the T
4. Studies have shown that excitons on the excited triplet T
n usually have three dynamic processes: RISC from T
n to S
n, internal conversion (IC) from T
n to
and non-radiative transition.According to Kasha's rule, the IC rate from T
n to
increases rapidly with the increase of n, which will inhibit the RISC process from T
n to S
n and cause a large exciton loss
[13]. It can be seen that the triplet exciton of PBzC1 comes from the higher T
4 energy level. Obviously, the efficiency of this transition is very low, and the exciton can not be effectively utilized, so that the device shows low efficiency. Therefore, when designing AIE materials, the relative position of excited triplet energy levels must be considered, which should also be the basic principle of molecular design of efficient AIE materials.
However, the exciton energy loss from S1 and Tn to still exists in CN-TPB-TPA molecule. In order to further improve the device efficiency and prove the existence of such energy loss, Ma et al. Further designed CN-TPB-TPA/mCBP:The device structure and its energy level and exciton transfer process are respectively shown in figs. 135a and 13b, wherein DMPPP is a TTA up-conversion material, and its low T1 energy level can well receive the exciton from the T2 and T1 of CN-TPB-TPA, and finally transfer the lost exciton to CN-TPB-TPA through its TTA up-conversion characteristics, so that the lost exciton is further utilized. It can be seen that the luminescence of the device still originates from CN-TPB-TPA (Fig. 135c), but its efficiency has been significantly improved, with the maximum EQE reaching 10.2% (Fig. 135d), which undoubtedly provides a new idea for the design and synthesis of efficient AIE materials.
(3) OLEDs based on AIE-TADF material
TADF materials are also a class of luminescent materials that can achieve high exciton utilization
[486]. Usually, by designing the distorted intramolecular electron donor (D) -electron acceptor (A) structure, researchers can separate the HOMO and LUMO orbitals in the molecule and achieve a smaller S
1-T
1 energy difference (ΔE
ST), thus achieving a rapid upconversion process of T
1→S
1, I. E. Reverse intersystem crossing (RISC)
[487]. Similarly, combining the idea of AIE with TADF can also achieve high quantum yield and high exciton utilization at the same time, which has become a hot spot in the field of electroluminescence application of AIEgens.
Zhao et al. Have carried out a lot of research work on the development of new AIE-TADF materials
[488]. In their early work, they first synthesized a series of luminescent molecules DPS-PXZ, DBTO-PXZ, DPS-PTZ and DBTO-PTZ with D-A-D symmetry based on diphenylsulfonyl (DPS) and dibenzothiophenyl oxide (DBTO) as electron acceptors (A) and phenoxazinyl (PXZ) and phenothiazinyl (PTZ) as electron donors
[489]. The D-a structural distortion in these molecules not only suppresses their fluorescence quenching in the aggregated state, but also significantly separates their HOMO and LUMO electronic clouds, endowing these materials with significant AIE and TADF properties. Therefore, this kind of material has potential application value in the development of high efficiency undoped OLEDs.
In order to further develop the luminescent materials of AIE-TADF system, Zhao et al. Developed new materials based on the design idea of asymmetric D-A-D 'structure
[408]. This design strategy can expand the diversity of materials, and thus regulate the optical and electrical properties of luminescent molecules. Among them, DBT-BZ-DMAC is a typical example, in which the dimethylacridine group (DMAC) and benzoyl group form a distorted D-a structure, which endows the molecule with AIE and TADF characteristics. The dibenzothiophene group on the other side of the molecule may be beneficial to improving the carrier transport performance of the molecule. The maximum CE, PE and EQE of the undoped OLEDs based on DBP-BZ-DMAC reach 43.3 cd·A
-1, 35.7 lm·W
-1 and 14. 2%, respectively, and the efficiency roll-off is only 0. 46% at 1000 cd·m
-2 brightness. They replaced the DMAC motif with PXZ and PTZ motifs to obtain two other AIE-TADF molecules (DBT-BZ-PXZ and DBT-BZ-PTZ), and the maximum EQE of the non-doped device can also reach 9.7%, which is far higher than the theoretical efficiency of the traditional fluorescent OLEDs device
[490].
Zhao et al. Also tried to introduce carbazole substituents into the molecular structure and synthesized CP-BP-PXZ, CP-BP-PTZ and CP-BP-DMAC
[491]. The fluorescence quantum yields of these three molecules were only 2.2% ~ 18.2% in solution state, but increased to 45.3% ~ 67.4% in thin film state, showing typical AEE characteristics. They also found that these molecules have weak luminescence and short fluorescence lifetime (nanosecond) in solution, while in the Aggregation state, the luminescence is significantly enhanced and shows a significant delayed fluorescence phenomenon, which is called aggregation-induced delayed fluorescence (AIDF) (Fig. 136). They used these AIDF molecules to fabricate non-doped OLEDs devices, and achieved high efficiency of 59.1 cd·A
-1, 65.7 lm·W
-1, and 18.4%, with almost no efficiency roll-off at 1000 cd·m
-2 brightness.
AIDF materials can be endowed with different photophysical properties by further functional group modification. For example, introducing chlorine atoms at different substitution sites of CP-BP-PXZ to promote the RISC process of the material can improve the performance of undoped AIDF-OLEDs to 76.6 cd·A
-1, 75.2 lm·W
-1, and 21.7%, and that of doped AIDF-OLEDs to 100.1 cd·A
-1, 104.8 lm·W
-1, and 29.1%
[492]. For another example, the introduction of host materials (such as mCP, CBP, etc.) In the AIDF material structure has made further breakthroughs in the performance of undoped OLEDs devices, reaching the levels of 69.0~72.9 cd·A
-1, 75.0~81.8 lm·W
-1 and 21.4-22.6%
[493]. These AIDF-OLEDs devices all exhibit superior integration and performance, and are stable over a broad brightness range.
When Zhao et al. studied fluorene-based AIDF materials DMF-BP-PXZ, DPF-BP-PXZ and SBF-BP-FXZ, they used nanosecond transient absorption spectra to confirm that the AIDF phenomenon of these molecules was the result of the suppression of the internal conversion process and the enhancement of the ISC process (Fig. 137)
[494]. These materials show excellent performance in both undoped and doped OLEDs devices, with a maximum EQE of 14.3% and an efficiency roll-off of less than 10% at 19.4%,1000 cd·m
-2 brightness, respectively. They then developed three molecules with similar structures, DMF-BP-DMAC, DPF-BP-DMAC and SBF-BM-DMAC
[495,496]. The solid state quantum yields of these materials are further improved, and the prepared undoped and doped OLEDs devices achieve high efficiencies of 20.1% and 24.5%, respectively. The SBF-BP-DMAC can also be used as a phosphorescent host to prepare orange phosphorescent devices and warm white devices, and the EQE is as high as 26.8% and 21%, respectively.
Zhao et al. Also developed a class of luminescent materials BDMAC-XT and BDPAC-XT with xanthones as electron acceptors and dimethylacridine and diphenylacridine as electron donors. The oxygen bridging structure in xanthones can improve the rigidity of molecules, thereby improving the solid-state thin film luminescent quantum yield of the materials. Meanwhile, these molecules have significant AIDF characteristics
[497]. The maximum brightness of the undoped device fabricated based on BDMAC-XT is as high as 117 900 cd·m
-2, and the maximum CE, PE, and EQE can reach 74 cd·A
-1, 67 lm·W
-1, and 21%. The external quantum efficiency of the device has an efficiency roll-off of 0% from the maximum to a brightness of 1000 cd·m
-2. In addition, the two materials can also be used as hosts to prepare high-efficiency orange (Ir(tptpy)
2acac) and red (Ir(piq)
2acac) phosphorescent OLED devices. Its maximum CE, PE and EQE are as high as 80 cd·A
-1, 105 lm·W
-1, 25% and 86 cd·A
-1, 101 lm·W
-1, 26%,1000 cd·m
-2 brightness respectively, and the efficiency rolls down to 3.8%.
In addition, Zhao et al. fabricated a sky-blue emitting material DCB-BP-SFAC with AIDF characteristics based on carbonyl and 10-H-spiro [acridine-9,9 '-fluorene], and the maximum EQE of the undoped and doped devices were 14.8% and 23.5%, respectively, and the luminous efficiency of the fully fluorescent SEL-WOLED based on the modified material was 47.1 cd·A
-1, 46.2 lm·W
-1, and 19.1%, respectively
[498]. The above work shows that AIDF materials can not only be used as guest luminescent materials to prepare high-performance OLED devices, but also can be used as host materials to effectively inhibit the quenching of electroluminescence and prepare high-performance fluorescent or phosphorescent OLED devices.
Furthermore, they also developed Through-space charge transfer (TSCT) AIDF materials and solution-processable AIDF materials, and made good progress in device preparation
[499][500].
The design and development of AIE materials with TADF has also gradually attracted the interest of many scholars (Fig. 138). Yang et al. developed two blue-light molecules, o-ACSO2 and m-ACSO2, based on the TADF material DMAC-DPS
[501]. O-ACSO2 and m-ACSO2 not only have the properties of AIE and TADF, but also have good solubility, which enables them to be used to prepare undoped blue-sky OLEDs by solution processing, and the peak electroluminescence efficiency reaches 37.9 cd·A
-1, 23.8 lm·W
-1 and 17.2%. They also used naphthyridine or cyanonaphthyridine as electron acceptors to synthesize yellow-green ND-AC and red CND-AC with AIE and TADF properties. The efficiency of doped OLEDs reached 58.1 cd·A
-1, 50.7 lm·W
-1 and 16.8%, and the maximum efficiency of undoped OLEDs reached 38.5 cd·A
-1, 30.2 lm·W
-1 and 12.0%
[502].
Choi et al. First synthesized a molecule BPCP with AIE-TADF characteristics, and then connected BPCP with the host material structure in a non-conjugated manner to obtain a new molecule BPCP-2CPC
[503]. The light color of BPCP-2CPC is similar to that of BPCP, and the former shows a higher film fluorescence quantum yield, indicating that the non-radiative transition channel in its aggregated state is well suppressed. The non-doped OLED device of BPCP-2 CPC was fabricated by solution processing, and CE and EQE reached 40 cd·A
-1 and 13.4%, which were higher than those of the OLED device based on BPCP (CE/EQE=22.3 cd·A
-1/7.46%.
Wang et al. designed and synthesized an AIDF material PXZ2PTO using phenothiazine oxide as an electron acceptor, and applied it to the fabrication of undoped and doped OLEDs devices with peak CE/PE/EQE of 44.9 cd·A
-1/35.2 lm·W
-1/16.4% and 43.8 cd·A
-1/32.0 lm·W
-1/16.3%, respectively
[504]. In addition, they also successfully obtained two materials, 34AcCz-PM and 34AcCz-Trz, by using a new acridine-carbazole electron donor for the construction of AIE-TADF molecules
[505]. The efficiency of the doped device based on 34 AcCz-Trz is 73.3 cd·A
-1/22.6%, and the efficiency of the undoped device is 45.2 cd·A
-1/14.1%.
Chi et al. developed a green AIE-TADF material 4,4-CzSPz with an asymmetric structure based on a diphenylsulfone acceptor, a carbazole donor, and a phenothiazine donor
[506]. Undoped and doped devices were fabricated via a simple OLEDs device structure, with CE/PE/EQE up to 61.2 cd·A
-1/38.4 lm·W
-1/20.7%, respectively. They further synthesized 2Cz-DPS, an isomer of 4,4-CzSPz, and the photophysical properties of the two compounds are similar. The non-doped OLED device based on 2Cz-DPS shows superior efficiency (CE/PE/EQE=82.3 cd·A
-1/51.8 lm·W
-1/28.7%), but the efficiency stability at high brightness still needs to be improved (EQE=8.4%@300 cd·m
-2).
The TADF molecules TAT-BP and TAT-2BP developed by Tong et al. Also have AIE properties
[507]. The solution-processed non-doped OLED with TAT-2BP has a low turn-on voltage of 2.5 V and peak electroluminescence efficiencies of 32.3 cd·A
-1, 33.0 lm·W
-1, and 9.8%. He et al. Prepared BPI-PhPXZ and BPI-PhDMAC by using the distorted biphenyl diimide units as electron acceptors, and observed the TADF and AIE properties at the same time, and the EQE of the doped devices prepared from them was as high as 18.1%
[508]. You et al. Constructed the AIDF molecule DMAC-BPI based on the seven-membered ring electron acceptor of N- (4- (tert-butyl) phenyl) -1,1 '-biphenyl-2,2' -dicarboximide (BPI), and fabricated a simple non-doped OLEDs device with the maximum PE and EQE up to 59.7 lm·W
-1 and 24.7%
[349]. To sum up, it is a wise molecular structure design strategy to combine the high solid-state quantum yield of AIE materials with the high exciton utilization of TADF materials.
Wang et al. Prepared Ac3TRZ3 and TAc3TRZ3 with both AIDF and TSCT effects using hexaarylbenzene as the core, and the maximum EQE of the solution-processed undoped device prepared using Ac3TRZ and TAc3TRZ3 as the light-emitting layer reached 3.5%, while the maximum EQE of the doped device reached 14.2%
[509].
Circularly polarized light-emitting OLEDs (CP-OLEDs) have attracted much attention due to their potential applications in optical data storage, 3D display, and so on. Tang et al. Prepared four molecules S-BN-CF, S-BN-CCB, S-BN-DCB and S-BM-AF by combining the group with circular polarization characteristics with AIDF
[510]. The devices based on the above devices all have circular polarization characteristics, and the maximum EQE reaches 9. 3%. This work provides a new idea for the preparation of high efficiency CP-OLEDs.
It is not difficult to see that the research of AIE-OLEDs with high efficiency is gradually improving. However, in order to really make AIE-OLEDs from the laboratory to practical, in-depth and systematic research on the working life of devices is still needed.
(4) OLEDs based on AIE-Ph material
For some organic complexes containing noble metals (such as platinum and iridium), the interaction between metal atoms and organic ligands in their molecules can produce prominent spin-orbit coupling (SOC) effect, which can lead to the intersystem crossing (ISC) process between the lowest singlet S
1 and the lowest triplet T
1, and realize the radiative transition of triplet exciton, resulting in phosphorescence. Zhou and Wu applied the design idea of AIE materials to the development of phosphorescent materials, and developed a series of platinum complex AIPE materials. The EQE/CE/PE of OLEDs devices prepared based on the solution processing process was as high as 28.4%/75.9 cd·A
-1/62.7 lm·W
-1[505][511]. On this basis, through molecular engineering to control the emission color of materials, they obtained red and green emission AIPE materials with EQE of 4.8% and 25.2%, respectively. Lee et al. Also reported deep-red AIPE materials based on iridium complexes, and prepared solution-processed OLEDs devices with high doping concentration, with the maximum EQE exceeding 7%
[512].
The results show that AIE materials can also be used as a good phosphorescent host to prepare phosphorescent OLEDs with high efficiency and low roll-off. Ma et al. successfully prepared high-efficiency red, orange and green phosphorescent OLEDs by doping Ir(ppy)
2(acac), Ir(tptpy)
2(acac) and Ir(dmdpprdmp)
2(divm) in AIE material TPB-AC as the light emitting layer. The electroluminescent spectrum and efficiency characteristics of the devices are shown in Figure 139a and B. The EQE efficiencies of the red, orange and green devices reach 26.1%, 27.3% and 21.0% respectively, and are still as high as 24.1%, 25.9% and 19.5% at 1000 cd·m
-2 brightness, which is an extremely high factor for phosphorescent OLEDs
[56]. It can be seen that using TPB-AC as the phosphorescent host material to prepare phosphorescent OLEDs not only reduces the working voltage and improves the efficiency, but also greatly improves the efficiency roll-off of the device at high brightness, which fully demonstrates the superior performance of AIE materials as the host material, and also expands the application of AIE materials.
6.2.3 White OLED's Based on AIE Materials
Hybrid fluorescent/phosphorescent white OLEDs prepared with AIE materials have also been successfully developed due to their efficient blue light emission and good host properties. The fluorescent/phosphorescent hybrid white OLEDs prepare by using that AIE material as a blue light emit and phosphorescent main body at the same time can not only eliminate the problem of exciton quenching caused by charges accumulate on an interface potential barrier in a light emitting layer and improve the efficiency and efficiency roll-off of the device, but also greatly simplify the device structure and have important practical application value. Ma et al. successfully prepared high-efficiency two-color and four-color fluorescent/phosphorescent hybrid white OLED s by using TPB-AC as the blue fluorescent light emitting layer, red, orange, and green phosphorescent iridium complexes doped TPB-AC as the long-wavelength phosphorescent light emitting layer, and using the multilayer device structure design without a spacer layer. The device structure and electroluminescent performance are shown in Figure 140
[56]. The results show that the spectrum of the two color light device is obviously from the blue emission of TPB-AC and the orange emission of Ir(tptpy)
2(acac), showing a good warm white light performance. The maximum power efficiency (PE) and EQE achieve 99.9 lm·W
-1 and 25.6%, respectively, and maintain 72.1 lm·W
-1 and 22.1% at 1000 cd·m
-2 brightness, which reach the level of fluorescent lamp. The four-color device also showed good comprehensive performance, covering the whole visible wavelength with a display index of more than 90. The maximum PE and EQE reached 60.7 lm·W
-1 and 25.3%, respectively, and 43.5 lm·W
-1 and 21.3% at the brightness of 1000 cd·m
-2, which is the best comprehensive performance of fluorescent/phosphorescent white OLEDs reported so far.
Ma et al. Also designed and prepared three-color fluorescent/phosphorescent hybrid white OLEDs with simpler structure by using blue TPB-AC and a green AIE material CP-BP-PXZ as blue and green fluorescent light emitting layers, respectively, and a red phosphorescent Ir(dmppr-mp)
2(divm) doped with TCTA as a red phosphorescent light emitting layer
[513]. Among them, CP-BP-PXZ is an AIE material with delayed fluorescence luminescence characteristics, which can theoretically achieve 100% exciton luminescence. The light-emitting layer structure and electroluminescent performance of the device are shown in Figure 141. The device shows good comprehensive electroluminescence characteristics, with a turn-on voltage as low as 2.4 V, a maximum PE and EQE of 50.5 lm·W
-1 and 20.5%, respectively, and 32.9 lm·W
-1 and 18.9% at 1000 cd·m
-2 brightness. In particular, the device shows stable spectral characteristics, and its color rendering index CRI is more than 90 in the whole brightness range, and the maximum can reach 93. The results show that AIE materials can be used to prepare high-performance white OLEDs with simpler structure.
6.3 Application of AIE Materials in Biological Field
AIE compounds are a new type of organic light-emitting materials, which successfully overcome the limitation of ACQ effect in the biological application of traditional fluorescent dyes, greatly improve the detection sensitivity and imaging photostability, and are conducive to fluorescence imaging. In addition, AIE materials have been proved to have excellent ROS generation ability, which has been widely used in the field of PDT guided by fluorescence imaging
[514]. This is due to the fact that the energy of the excited state in the aggregate state can flow to the intersystem crossing channel to produce ROS in addition to luminescence. In addition, compared with the unimolecular state, the molecules in the aggregated state have better singlet (S
1) and triplet (T
1) energy matching, and the singlet to triplet energy range (ΔE
ST) is smaller, which is conducive to the transfer of exciton energy from singlet to triplet, and promotes the ISC rate, thereby enhancing the efficiency of ROS generation, showing the characteristics of aggregation-induced reactive oxygen generation enhancement (AIG-ROS)
[515]. In addition to fluorescence imaging-guided PDT based on the limitation of intramolecular motion in the aggregated state, AIE materials can also be developed into Photothermal reagents with excellent photothermal conversion properties by actively promoting their intramolecular motion in the aggregated state, so as to achieve efficient photothermal therapy (PTT) guided by Photo acoustic imaging (PAI)
[516,517]. Furthermore, thanks to the abundant mobile groups in the molecular structure, AIE materials can be an excellent template for balancing energy dissipation. Through rational molecular design, AIE materials can also be developed into multifunctional photodiagnostic agents to achieve synergistic therapy guided by multi-modality imaging by regulating the degree of intramolecular motion of molecules in the aggregated state and achieving a balance between radiative and non-radiative transitions to maximize the use of excited state energy
[518].
Thanks to their excellent biological safety, excellent optical properties (high aggregate quantum yield, tunable emission wavelength, large Stokes shift, and good light stability), excellent photosensitivity, unique single-molecule versatility, and easy synthesis and functional modification, AIE materials have made great breakthroughs in the field of biology in recent years
[518]. In this part, the latest progress of AIE materials in the field of biology is summarized from two aspects: the design of AIE fluorescent probes and biological detection and imaging, and the biological diagnosis and treatment of AIE materials.
6.3.1 Design of AIE Fluorescent Probe and Its Application in Biological Detection and Imaging
In the past decades, fluorescent probes have been widely used in biological imaging of cells, tissues and animals, and in monitoring a variety of biological processes in complex environments. The researchers used fluorescent probes to achieve direct visualization of lesion sites and cells
[519]. Although traditional fluorescent molecules have been developed and widely used, their fluorescence is often weakened or quenched at high concentrations, which limits their practical applications
[520,521]. As mentioned above, in contrast to the ACQ effect, molecules with AIE characteristics exhibit weak fluorescence in the monomolecular state, while in the aggregated state, bright fluorescence can be emitted due to the restriction mechanism of intramolecular motion/rotation. In addition, these molecules also have some obvious optical advantages, including low background, long-term tracking ability, strong resistance to photobleaching and so on. Therefore, using AIE molecules to design fluorescent probes for the detection of biological targets and the imaging of biological samples has become a current research hotspot. In this section, several AIE fluorescent probes and their applications in the field of biological detection and imaging are listed and described.
6.3.1.1 AIE Probes Based on Modular Peptides
Peptides are usually composed of 20 natural amino acids and some non-standard amino acids, which play a very important role in biological functions
[522]. Diversified amino acid raw materials and their different combinations and arrangements can endow polypeptides with different physiological activities, such as tissue targeting, membrane penetration, endosomal escape, cellular transport, nuclear localization and so on
[523⇓~525][526⇓⇓⇓~530][531⇓⇓~534][535][536,537]. Modular peptides based AIE probes (MPAPs) can be used as functional modules to modify AIE probes with good biocompatibility, excellent biodegradability, good bioactivity and rich diversity. At the same time, the introduction of peptides can also improve the hydrophilicity of AIE molecules, which is conducive to its application in the field of biomedicine
[538⇓⇓⇓~542]. Modular peptides can be covalently linked by specific active groups on amino acid side groups and different AIE molecules, such as amide condensation reaction of amino and carboxyl, click chemistry reaction of alkynyl and azide, Michael addition reaction of sulfhydryl and maleimide, etc
[543,544]. It is of great chemical significance to choose an appropriate connection method to avoid complex chemical synthesis and improve the efficiency of modular design.
According to the different roles of AIE molecules in the probe, this part is divided into two parts: imaging and treatment.
(1) preparation as imaging trac
Because of the obvious optical advantages of AIE molecules, such as high luminous efficiency, large Stokes shift, good biocompatibility and strong photobleaching resistance, Lou et al.A plurality of MPAPs are designed and synthesized through a covalent modular polypeptide sequence, thereby realizing the tracking and monitoring of different types of molecules, including chemotherapeutic drugs, therapeutic genes and the like, and the imaging capability in a plurality of cell models
[545⇓~547].
Lou et al. designed and synthesized a "T" type probe TCNTP (Fig. 142) in order to accurately target the target cells and locate in the nucleus, and realize the long-term and low-toxicity tracking of tumor cells
[548]. The probe is obtained by covalently linking a functional polypeptide (TCNT) and a fluorescent imaging agent (PyTPE) through a copper-catalyzed azide and alkyne click reaction. TCNT consists of four modules, including a targeting cyclic peptide (cNGR) with high affinity for aminopeptidase N (CD13), a cell-penetrating peptide (CPP), a nuclear localization signal peptide (NLS), and a targeting peptide (RGD) that selectively interacts with integrin α
vβ
3. With the assistance of functional peptides, the TCNTP probe could selectively target the nuclei of A375 cells overexpressing both integrin α
vβ
3 and CD13, and showed strong yellow fluorescence. In addition, 78% of the cells were stained with TCNTP after the fifth culture cycle, while only 2% of the cells were stained with a commonly used commercial nuclear dye (Hoechst 33258). The experimental results show that the modular TCNTP probe can not only precisely target specific cell lines and transport biomolecules to the nucleus, but also achieve long-acting tracking transport.
Similarly, Lou et al. designed and reported a DOX-FCPPs-PyTPE probe in order to track the delivery pathway of anticancer drug doxorubicin (DOX) during tumor treatment
[549]. As shown in fig. 143, the probe is covalently bound by a chemotherapeutic drug (DOX), a cell-penetrating peptide (CPP), a matrix metalloproteinase-2 (MMP-2) responsive polypeptide (LGLAG) and a fluorescent imaging molecule (PyTPE). In the microenvironment of tumor cells, highly expressed MMP-2 can specifically recognize and cleave LGLAG peptide, thereby releasing DOX and PyTPE modules. The inherent red fluorescence of DOX and the yellow fluorescence of PyTPE can trace the transmission process of the probe in MCF-7 cells (high expression of MMP-2). The three-dimensional map further confirmed that the red fluorescence of DOX could be observed in the nucleus of MCF-7 cells after co-incubation with the probe for 2 H.
Subsequently, in order to track the delivery process of antisense oligonucleotides (ASO) in gene therapy, Lou et al. Constructed a TNC peptide probe for efficient loading, while tracking the therapeutic gene ASO
[550]. A cationic polypeptide consist of an integrin targeting peptide, a nuclear localization signal peptide, and a cell-penetrating peptide was covalently synthesize with hydrophobic PyTPE (fig. 144). TNCP self-assembles into nanocomposites through electrostatic interactions and electronegative ASO. Confocal microscopy experiments showed that the yellow fluorescence of PyTPE could be observed on the cell membrane within 5 min after the nanocomposites were co-cultured with cells, and it could last for more than 150 min, showing its excellent real-time and long-term imaging performance. Then, they used Cy5-labeled ASO (ASO-Cy5) to study the correlation coefficient between ASO-Cy5 and TNCP in cells. The results showed that the red and yellow fluorescence appeared in the cytoplasm within 1 H of incubation, and the fluorescence in the nucleus increased gradually within 4 to 12 H of incubation. However, the fluorescence overlap level decreased from 85.7% at 4 H to 73.8% at 12 H, indicating the intracellular disintegrative behavior of ASO-Cy5 and TNCP. With the help of the functional module of the polypeptide, the TNCP probe shows remarkable biocompatibility and specific nuclear imaging ability, and confirms the important role of the polypeptide structure in gene transmission.
In addition to common peptide functional modules such as targeting peptides and enzyme-responsive peptides, some peptide structures with chemical reactivity have also attracted great interest of researchers. Inspired by the cross-linking process of tyrosine under the action of hydrogen peroxide (H
2O
2) and myeloperoxidase (MPO), Lou et al. developed a TPE-Tyr (TT) probe consisting of an AIE molecule TPE and a dityrosine (Tyr) module
[142]. The TT probe can be activated by H
2O
2 and MPO to form a TT oligomer through tyrosine bond cross-linking, and the intramolecular motion of the TPE module is restricted, thereby generating intense blue fluorescence. Using the ability of TT probe to distinguish intracellular H
2O
2 and MPO levels, we can selectively image and inhibit inflammatory cells, so as to achieve early detection and timely intervention of inflammatory cells.
(2) as an imaging therapeutic agent
In addition to the obvious advantages of optical imaging, AIE molecules have recently been endowed with another performance through rational molecular design and structural optimization. Some AIE molecules can show strong photosensitivity in the aggregated state, that is, they can produce highly toxic ROS under light conditions. Therefore, photosensitizers based on AIE molecules can also show good prospects in cancer therapy, bacterial killing and so on.
Usually, a variety of therapeutic agents are integrated, using different therapeutic mechanisms and targets, which can reduce the disadvantages of a single treatment and improve the therapeutic effect of cancer. However, due to the different physical and chemical properties of different therapeutic agents, there is still a lack of effective methods to introduce them into cells in an "optimal" way at the same time. To solve this problem, Lou et al. designed a FC-PyTPA probe (Fig. 145). FC-PyTPA contains three parts: (1) hexadecyl chain, hydrophilic peptide segment and cell-penetrating peptide (FC); (2) AIE-based photosensitizer (PyTPA); (3) MMP-2 substrate peptide and cathepsin B substrate peptide
[551]. When the complex FC
siRNA-PyTPA of FC-PyTPA probe and siRNA reached the tumor vicinity, MMP-2 highly expressed in the tumor microenvironment cleaved the FC
siRNA-PyTPA into FC
siRNA and PyTPA,FC
siRNA, and PyTPA was internalized by macropinocytosis and caveolae-mediated endocytosis, respectively. Subsequently, FC
siRNA form nanofibers under the cleavage of cathepsin B in lysosomes, destroy the structure of lysosomes, and promote siRNA to escape from lysosomes and perform gene silencing. At the same time, they applied light at the right time to exert the PDT ability of PyTPA. The three therapeutic drugs (nanofiber, siRNA and PyTPA) are efficiently internalized by cells to exert their respective optimal therapeutic performance, thus effectively inhibiting tumor growth.
Lou et al. constructed a multifunctional modular probe T
CDTMP (Fig. 146) to solve the problem of postoperative residual and recurrence of ovarian cancer, and proposed the strategy of preoperative, intraoperative and postoperative segmented treatment
[552]. It consists of four functional parts: (1) targeting peptide TMTP1, which specifically binds to highly metastatic tumor cells; (2) TAT, which has the function of gene loading; (3) MMP-2 substrate peptide; (4) PyTPA for tumor imaging and PDT. Nanoparticles were formed from assembly by loading miR-145-5P or VEGF-siRNA. In vivo, preoperative use of T
CDTMP/miR-145-5p ensures that active targeting accumulates sufficiently in the tumor, increasing miR-145-5p expression in the tumor, thereby inducing tumor cell apoptosis. Intraoperatively, most of the tumor was resected, and a small amount of residual tumor was essentially eliminated by PDT mediated by T
CDTMP/miR-145-5p. After surgery, T
CDTMP/VEGF-siRNA was given antiangiogenic therapy, which delayed tumor recurrence. This MPAP-based treatment system can significantly improve the survival rate and reduce the recurrence rate, which provides a prospect for the surgical treatment of highly invasive ovarian cancer.
In addition to being used as a photosensitizer for tumor imaging therapy, Lou et al. Proposed that the inherent distorted stereochemical structure of AIE molecules can also be used as a small molecule scaffold to lay the foundation for the construction of probes with different modular arrangements. In order to compare the influence of different spatial arrangements of modules on the performance of probes, two kinds of probes (T-AIE-M, TM-AIE) with the same modules but different spatial arrangements were designed
[553]. AIE module is a derivative of TPE, which can be used for fluorescence localization and can produce ROS for tumor killing under light conditions. Polypeptide module T is composed of tumor-targeting peptide, MMP-2 substrate peptide and cell-penetrating peptide, which are used to promote the specific uptake of probes by tumor cells. Polypeptide module M is composed of α-helix pro-apoptotic peptide (HLAH) (fig. 147). By comprehensively comparing the cell selectivity, cell internalization efficiency, mitochondrial targeting, singlet oxygen production efficiency and mitochondrial membrane potential interference ability of the two probes, it was found that there were significant differences between the two probes in terms of specificity and toxicity. The double-bonded T-AIE-M has higher tumor cell specificity due to higher integrin affinity, while the single-bonded TM-AIE shows stronger toxicity due to longer helical structure. This phenomenon provides a reference for the construction of MPAPs in the future, and also provides a new idea of "rearranging existing modules" for optimizing the performance of multi-module probes.
6.3.1.2 Activated Detection and Imaging of Biomarkers Using Hemicyanine Based AIE Probe
Biomarkers refer to measurable biochemical indicators when the structure or function of systems, organs, tissues or (sub) cells changes, so biomarkers can be used to detect and diagnose diseases. In the process of disease occurrence and development, it will be accompanied by changes in the microenvironment (such as pH, redox status, etc.) Or abnormal expression of biomarkers (such as genes, ROS, small molecule metabolites, metabolic enzymes, etc.).Therefore, accurate detection of biomarkers is of great significance in disease diagnosis and efficacy monitoring
[554⇓⇓~557]. At present, serum detection is still the mainstream method of biomarker detection. However, in addition to the lesion area, many biomarkers also exist in other organs or tissues, which affects the specificity of detection and diagnosis. Therefore, in-situ detection of relevant biomarkers at the lesion site is helpful for accurate diagnosis of diseases and can avoid false positive diagnosis
[558,559]. As a non-invasive detection method, fluorescence imaging technology has the advantages of high sensitivity and real-time imaging. However, due to the strong light scattering of biological tissue, the fluorescence signal (especially the fluorescence of short wavelength, such as the fluorescence of visible light) decreases sharply with the increase of imaging depth, and the autofluorescence of biological tissue is mainly concentrated in the short wavelength region, which leads to the limited effect of fluorescence imaging in the short wavelength region
[560,561]. Because the light scattering and autofluorescence of tissue decrease exponentially with the increase of wavelength, in order to overcome the above defects, the strategy of extending the wavelength of fluorescence signal can be adopted, such as fluorescence imaging in the near infrared region I or even region II (NIR-I, NIR-II fluorescence imaging), on the other hand, photoacoustic imaging technology can be used. Photoacoustic imaging, as a new non-invasive imaging technology, has become one of the research hotspots in the field of biological detection imaging in recent years. Photoacoustic imaging is based on the principle of "light in and sound out", that is, near-infrared pulse laser is used to excite tissue samples, and ultrasonic signals generated by local thermal expansion of samples are collected for detection and imaging. Because of the low scattering of ultrasound signals in biological tissues, photoacoustic imaging can have high tissue penetration depth and spatial and temporal resolution
[562,563]. Among them, multi-spectral photoacoustic tomography (MSOT) uses multiple wavelengths of near-infrared laser to excite the sample and collect ultrasound signals from different light absorbers in the tissue.Photoacoustic signals generated by various light absorbers (such as endogenous light absorbers or exogenous probes) can be distinguished and obtained through the spectrum splitting function, and three-dimensional photoacoustic images can be obtained, which is helpful to accurately locate the lesion area through in-situ detection of biomarkers
[564,565].
Contrast agents or probes are one of the keys to ensure diagnostic accuracy when using near-infrared fluorescence and photoacoustic imaging to diagnose diseases in a non-invasive manner by detecting biomarkers. By reacting or combining with a specific biomarker, the activatable probe makes the signal of the probe molecule from "off" to "on", so the probe has a high signal-to-noise ratio and can be used in the specific detection and diagnostic imaging of disease biomarkers
[566,567]. Hemicyanine chromophore compounds are large conjugated systems composed of vinyl-linked aromatic ring structures, which have become widely used dyes because of their high extinction coefficient, flexible structural design, and easy control of absorption and emission wavelengths by adjusting the length of conjugated π bridges or the absorption (supply) capacity of substituents
[568⇓⇓~571]. However, such chromophores generally exhibit an ACQ effect, whereas bioassays are usually performed in an aqueous environment and the chromophores are often present in the form of aggregates, in which case probes based on such chromophoresmay give false-negative results. In addition, it is difficult to achieve high-quality fluorescence and photoacoustic dual-mode imaging due to the ACQ effect. In contrast to the ACQ chromophore, the AIE-based probe can overcome the above defects well and is beneficial for biological detection and imaging in the aqueous phase of physiological environment. Therefore, the hemicyanine activatable probe with AIE characteristics, which is designed with reasonable structure, shows great application advantages in the detection of biomarkers and the precise diagnosis of diseases through near-infrared fluorescence and photoacoustic dual-mode imaging.
(1) structural design of a hemicyanine activatable probe with AIE characteristic
Hemicyanine chromophore compounds are conjugated systems formed by connecting two aromatic groups with vinyl as the bridge, which usually have the D-π-a structure of electron donor-electron acceptor. Hemicyanine chromophores can be endowed with some beneficial properties by reasonable structural design or modification, such as extending the absorption and emission wavelengths, making them have AIE characteristics, and constructing activatable probes. The classical hemicyanine chromophore (In-Y) consists of indole salt (In) as the electron acceptor and xanthene (Y) as the electron donor. The absorption and fluorescence emission peaks of In-Y are at 684 and 703 nm, respectively (Table 1).
In this paper, we take In-Y as an example to illustrate the design strategy that can be used to construct hemicyanine activatable probe structures with AIE characteristics. Firstly, the introduction of "rotor" units In the molecular structure can endow the system with AIE characteristics, for example, the introduction of diphenylamine on the xanthene donor In In-Y and Q-Y (hemicyanines composed of quinolinium salts and xanthenes) to form In-Y-BA and Q-Y-BA can make the system show typical AIE characteristics, which is conducive to the emission of strong fluorescence in the aggregation state of the aqueous phase
[572,573]. Secondly, in the aspect of extending the absorption/emission wavelength, the absorption and/or emission wavelength can be red-shifted by extending the conjugation length of the molecule. For example, benzo-indole is used to replace indole as an electron acceptor to form a hemicyanine chromophore BIN-Y, whose absorption wavelength and fluorescence emission wavelength are extended to 705 and 715 nm, respectively
[574]. The absorption/emission wavelength can also be extended by using acceptors with strong electron-withdrawing ability, for example, the absorption wavelength and emission wavelength of hemicyanine FB-Y-BA formed by using strong electron-withdrawing fluoroboron heterocyclic acceptors are extended to 730 and 923 nm, respectively
[575]. A response element is introduced into a molecular structure, and the change of the electronic effect of a system induced by the reaction of the response element and a specific biomarker is utilized to realize the spectral change of the activation type of the biomarker, thereby constructing an activatable probe, for example,Activatable detection of enzyme biomarkers can be achieved by introducing a motif on the electron donor xanthene that responds to the enzyme biomarker, thereby weakening the donor's electrical activity, which is restored after the response (Table 2).
(2) Detection and imaging of biomarkers by hemicyanine activatable probes with AIE characteristics
(A) Detection and imaging of enzymatic biomarkers
Breast cancer has become one of the most common types of cancer and is one of the leading causes of cancer death in women, with one in eight women worldwide suffering from breast cancer
[576]. In the early stages, breast cancer is concentrated in the breast. If left untreated, it will continue to grow and may spread to lymph nodes and even invade distant organs, such as the lungs
[577,578]. Breast cancer is more likely to be cured if it is treated before metastasis occurs. However, once metastasis occurs, treatment becomes more difficult, and more than 90% of breast cancer mortality is caused by metastasis. Nitroreductase is a highly expressed biomarker in the hypoxic microenvironment of solid tumors, including breast tumors. Ouyang et al. Designed a nitroreductase-activated hemicyanine probe
[573]. As shown in fig. 148, the probe Q-NO
2 uses xanthene containing diphenylamine as a donor, quinoline salt as an acceptor, 4-nitrobenzyl as a recognition group of nitroreductase and a fluorescence quencher, and the probe molecules are aggregated in an aqueous medium to form a nanoprobe (NP-Q-NO
2). In the absence of nitroreductase, the nanoprobe exhibits essentially no fluorescence in the near-infrared region. In the presence of nitroreductase, the nitroreductase converts the strong electron-withdrawing nitro group into an amino group, and induces a self-elimination reaction to generate an electron-donating hydroxyl group. The generated activated probe, namely the chromophore Q-OH, has typical AIE characteristics, so it has strong photoacoustic and fluorescence signals. The results show that the nanoprobe NP-Q-NO
2 can be used to detect the nitroreductase in the breast tumor metastasis model to achieve continuous metastasis imaging of the breast tumor in situ to the lymph node and then to the lung, and can be used to monitor and track the effect of chemotherapy. In addition, three-dimensional MSOT imaging can accurately locate the focus of breast cancer in situ and track the metastatic path of tumors to lymph nodes and lungs.
Huang et al. designed a quinone oxidoreductase (NQO1) -activated hemicyanine probe
[572]. As shown in fig. 149, the probe molecule DHXI was prepared by using diphenylamine-containing xanthene as an electron donor, indole salt as an electron acceptor, and quinine propionic acid as a recognition motif, and DHXI was combined with recombinant human serum albumin (HSA) in solution through self-assembly to form a nanoprobe DHXI @ HSA. The nanoprobe can specifically respond to NQO1 in a canc cell, when that nanoprobe DHXI @ HSA reacts with the NQO1, a strong fluorescence quench element quinine propionic acid group on a probe compound is removed by the reaction to expose hydroxyl,It is suitable for in vivo fluorescence imaging, and can be used for multispectral photoacoustic tomography (MSOT) and near infrared (NIR) fluorescence imaging because of the absorption at 710 nm and fluorescence enhancement at 730 nm due to the change of internal charge transfer. The nanoprobe DHXI @ HSA was used to perform MSOT imaging and three-dimensional localization of the orthotopic liver cancer mouse model, and to guide the resection of the liver tumor.
(B) Detection of pH of biological microenvironment
The occurrence of some diseases will lead to the change of pH value in biological microenvironment (such as the change of pH value in cells), so it is of great significance to detect the pH value of biological microenvironment. Wang et al. Designed a hemicyanine probe that can detect the pH value in living cells
[579]. As shown in fig. 150, the probe has typical AIE characteristics with indole salt as the acceptor and rhodamine containing tetraphenylethylene as the donor. Under weak acid, neutral or alkaline conditions, the rhodamine unit is in a closed ring state; Under strong acidic conditions, the rhodamine unit is in an open-ring state, thereby improving the degree of conjugation of the molecule. When the pH value is less than 5.5, the probe mainly emits red fluorescence, and the emission peak is at 737 nm. When the pH value was greater than 5. 5, the probe mainly emitted green fluorescence, and the emission peak was at 510 nm. When the pH value increases from 3 to 7, the fluorescence of the probe at 510 nm increases gradually, while the fluorescence at 731 nm decreases gradually, so the probe can visually detect the pH value in the range of 3 to 7.
(C) Detection of small molecule biomarkers
Small molecule biomarkers are overexpressed in many diseases, such as hydrogen peroxide (H
2O
2), which is an important biomarker of oxidative stress and is closely related to the occurrence and development of inflammatory diseases.Glutathione (GSH) is overexpressed in some tumors, and sulfite is closely related to myocardial diseases, so the detection of small molecular biomarkers is of great significance for the diagnosis of some diseases. Xie et al. Designed a GSH-activatable hemicyanine probe
[580]. The probe takes indole as an acceptor, xanthene as a donor, and a disulfide bond as a response element of GSH, wherein the disulfide bond is connected to the xanthene through a carbonate bond. The absorption wavelength of the probe was 600 nm, and there was no fluorescence emission. After the probe reacted with GSH, the disulfide bond was broken, and the hemicyanine chromophore was released.The absorption wavelength was about 685 nm, and the fluorescence emission wavelength was about 705 nm, so the probe could realize near-infrared fluorescence and photoacoustic detection and imaging of GSH. Yu et al. designed a hemicyanine probe responsive to hydrogen peroxide
[581]. As shown in fig. 150a, the probe uses indole salt as the acceptor, xanthene as the donor, and borate ester as the responsive element of hydrogen peroxide. When hydrogen peroxide reacts with the probe, the borate bond is broken to release a hemicyanine chromophore, which emits strong near-infrared fluorescence in the aggregated state, thus realizing the detection of hydrogen peroxide. Zhang et al. Designed a cyanine probe that can respond to sulfite
[582]. As shown in fig. 151b, the probe with indole salt as acceptor and naphthopyran as donor showed aggregation-induced luminescence characteristics before and after response. In the absence of sulfite, the probe emits red fluorescence with an emission peak at 668 nm. In the presence of sulfite, the addition reaction of the probe with sulfite occurred, and the fluorescence of the activated probe was enhanced in the aggregation state, and the fluorescence was blue-shifted, emitting green fluorescence with a peak at 520 nm. Therefore, the probe can achieve ratiometric fluorescence detection of sulfite, and they successfully used the probe to detect sulfite in cells and zebrafish.
6.3.1.3 NIR two-region fluorescent probe
In the past few centuries, various biomedical imaging technologies have flourished, such as X-ray tomography (CT), Ultrasound imaging, magnetic resonance imaging (MRI) and positron emission tomography (PET), which have made human beings have a better understanding of life. However, each of the above imaging techniques has its own shortcomings, such as X-ray is more harmful to the organism, ultrasound imaging is contact imaging, the resolution is not high, and the accuracy of its results largely depends on the skill and experience of the operator, MRI can not be used for real-time imaging, PET is expensive and has a small amount of radiation. Fluorescence imaging, as a new in vivo imaging technique, has the advantages of high sensitivity and specificity, low cost, non-contact, non-invasive and good biological safety compared with the above imaging methods, and is of great value in the visualization of biological structures and physiological processes, as well as in the diagnosis of diseases.
For optical imaging technology, understanding the interaction between biological tissue and light is particularly important. Generally speaking, when only elastic scattering is considered, the scattering of biological tissue decreases with the increase of wavelength. The absorption of biological tissues mainly comes from water, fat, etc. In addition, the interference of autofluorescence can cause the degradation of imaging quality, which should be avoided as far as possible. For a long time, due to the limitations of fluorescent probes and equipment, single-photon fluorescence imaging mainly focuses on the visible band (Vis, 400 ~ 700 nm) and the traditional near-infrared biological window (NIR-I, 760 ~ 900 nm). However, the imaging quality is not satisfactory due to the high scattering and strong autofluorescence of living tissues in the Vis and NIR-I bands. In Recent years, near infrared region Ⅱ (NIR-Ⅱ, 900 ~ 1880 nm) photons have the advantages of large absorption, low photon scattering and low autofluorescence in biological tissues.As a result, the signal-to-background ratio and resolution of the fluorescence image in this band have been greatly improved, the visualization of small structures and rapid physiological processes and the early diagnosis of diseases have become possible, and the fluorescence imaging technology in this band has developed rapidly
[583⇓⇓⇓⇓⇓~589]. However, fluorescent probes in the second region of the near-infrared spectrum are still focused on inorganic materials such as single-walled carbon nanotubes, rare earth materials and quantum dots, and their biocompatibility, long-term biosafety, preparation difficulty and clinical transformation prospects are still inferior to those of organic materials
[590⇓⇓~593][594]. The fluorescence intensity of traditional organic fluorescent probes is greatly reduced due to the ACQ effect in the high concentration or aggregation state, and then the imaging quality decreases, which seriously limits their application in biological imaging. AIEgens has been widely used as an excellent exogenous probe for fluorescence imaging. Its large Stokes shift, high fluorescence brightness, excellent photostability, biocompatibility and easy preparation make AIEgens attractive to researchers in various fields. In the past few years, NIR-II emitting AIEgens have emerged, and their rich variety and unique optical properties have promoted the vigorous development of NIR-II bioimaging technology.
The absorption of most biological tissues mainly comes from water. Considering the light absorption of water (Fig. 152) and the light scattering of biological tissues, NIR-Ⅱ is divided into NIR-Ⅱa (1300 ~ 1400 nm) and NIR-Ⅱb (1500 ~ 1700 nm)
[595]. In recent years, NIR-Ⅱb biological imaging technology has been recognized by many researchers because of its high signal-to-background ratio and high resolution. Although the advantages of low scattering and low autofluorescence of NIR-Ⅱb are often mentioned, their advantages are mainly due to the strong absorption of light by water in biological tissues. It has been shown that for in vivo bioimaging with large optical path, the suppression of scattering background by the absorption of biological tissue promotes the NIR-II fluorescence imaging with high signal-to-background ratio
[596,597]. Based on this concept, to further play the role of water absorption in improving the imaging quality, it has been reported that ~ 1450 nm is a better NIR-Ⅱ bioimaging window than NIR-Ⅱb, and it is defined as NIR-Ⅱx (1400 ~ 1500 nm)
[598]. In order to achieve high-quality biological imaging, it is necessary to find fluorescent probes for NIR-Ⅱx emission. At present, the number of AIEgen whose emission wavelength is in the NIR-Ⅱ band is limited, and they are concentrated in the range of 900 ~ 1100 nm
[599,600]. However, NIR-IIx + NIR-IIb imaging technology using fluorescence "tailing" requires probes with high brightness and long wavelength emission to resist absorption losses in biological tissues.
Based on the band gap theory, due to the vibration overlap between the ground and excited States, the photo-induced Ф
F decreases as the emission wavelength moves toward the near infrared region. Therefore, increasing the absorption by increasing the planar conjugation length and achieving high Ф
F by using the twisted structure are the key points for the design of high-brightness AIE probes
[601⇓⇓⇓⇓⇓~607]. At present, there are two main methods to design NIR-II AIEgen with D-A-D structure: (1) enlarging the π-conjugation length to reduce the band gap; (2) The electron donor or electron acceptor effect is enhanced to reduce the band gap
[605⇓⇓~608]. At present, many NIR-II AIEgen with high brightness and long wavelength emission have been designed and applied to biological imaging. At the same time, a variety of AIEgen coating methods have been explored to improve biocompatibility and site-specific targeting
[609⇓~611]. In 2019, Liu et al. confirmed that the twisted backbone and twisted rotor played an important role in improving the Ф
F of AIE molecules through the design of isomers (Fig. 153)
[601]. NIR-Ⅱ emitting 2TT-mC6B has ACQ effect due to the planarity of thiophene-BBTD-thiophene, and the NIR-Ⅱ emitting AIEgen: 2TT-oC6B is obtained by changing the position of the hexyl group to twist the backbone. The 2TT-oC6 B nanoparticles (NPs) coated with DSPE-mPE
have a maximum absorption at 808 nm and a maximum fluorescence intensity at 1063 nm, and the Ф
F can reach 11%. In this study, neutrophil-coated 2TT-oC6B NPs were used to penetrate the blood-brain barrier, benefiting from the low scattering, high absorption and low autofluorescence characteristics of NIR-II, and the inflammatory sites of deep tissues were observed through the intact scalp and skull.
Thanks to the high Ф
F, long emission wavelength, high urine chemical stability and good biocompatibility of 2TT-oC6B nanoparticles, Du et al. Realized the precise identification of rabbit model ureter, ureteral injury detection and injury repair in NIR-II band (Fig. 154)
[612]. 2 TT-oC6 B nanoparticles can even be excited by conventional laparoscopic xenon lamps, which provides a new idea for clinical surgery guided by real-time fluorescence imaging.
In 2020, Liu et al. combined 2TT-mC6B and 2TT-oC6B to design an AIEgen with two electron donors that are both twisted and planar: 2 TT-m, oC6B (Fig. 155)
[605]. The AIEgen combines the advantages of the strong absorption of the planar structure and the high Ф
F of the twisted unit, achieves a 1.12×10
4·M
-1·cm
-1 in molar absorptivity at 808 nm, and shows good NIR-II fluorescence intensity and PCE. The 2TT-m, oC6B NPs coated with DSPE-mPEG
2000 have the maximum absorption at 777 nm and the strongest fluorescence at 1034 nm, which has important guiding significance for the design of contrast agents for photothermal therapy.
In 2020, Li et al. First used AIE NPs for NIR-Ⅱb biological imaging, verifying the possibility of AIE and NIR-Ⅱb strong combination to achieve high-quality biological imaging. In 2020, Liu et al. proposed a molecular design strategy of integrating a planar unit into a twisted skeleton to improve the brightness of AIEgens, and synthesized TT3-oCB, which retained the high Ф
F characteristics of AIEgens, and its larger π-conjugated planar unit could enhance the absorption. The high Ф
F and large absorption were integrated into one AIEgen (Figure 156) to achieve excellent NIR-IIb fluorescence intensity
[606]. The TT3-oCB NPs coated with Pluronic F-127 has a maximum absorption at 784 nm and a maximum emission at 1062 nm, and the molar absorptivity QY of the 1.12×10
4M
-1·cm
-1,NIR-Ⅱ at 793 nm is 4. 6%, and the QY of the NIR-Ⅱb band is 0. 031%. Using TT3-oCB NPs with high NIR-Ⅱb brightness, NIR-Ⅱb images of systemic blood vessels, femoral arteries and bladder in mice were obtained, and small blood vessels with a diameter of 0. 49 mm could still be distinguished by NIR-Ⅱ B imaging under the interference of strong autofluorescence in the liver, which once again verified the imaging superiority of NIR- Ⅱb band for small structures and deep tissues.
In 2021, Wu et al. realized NIR-Ⅱb biliary tract imaging, detection of biliary tract injury and other lesions on a rabbit model with the help of bright NIR-Ⅱb contrast agent TT3-oCB NPs (Fig. 157)
[613]. Therefore, TT3-oCB NPs with high brightness, high stability and high biocompatibility assisted NIR-Ⅱb band cholangiography provides a method for intraoperative non-radioactive cholangiography and diagnosis, showing great potential for clinical transformation.
Although the fatty side chain of TT3-oCB is beneficial to its separation from polyethylene glycol micelles under the interaction with proteins in vivo and further excretion from the body, the lack of efficient and specific tumor labeling is a huge challenge for imaging-guided therapy
[614]. In the future, TT3-oCB NPs can be efficiently targeted to tumors, inflammation and other lesions through a series of functional means, giving full play to the advantages of NIR-II band imaging-guided therapy, and further expanding the clinical application of AIE.
Since Qi et al. First used AIE NPs with high brightness NIR-I emission for NIR-II in vivo imaging in 2018, AIE and NIR-II have promoted each other, from rodent small animal models, large animal models to non-human primate models.With the help of macroscopic and microscopic imaging systems and the excellent optical properties of AIE nanomaterials, the structure and function of blood vessels, urinary system, reproductive system and digestive system were imaged, and the photodynamic and photothermal therapy and sensing of tumors and bacterial infections were realized
[587].
6.3.1.4 AIE NIR Two-Zone Three-Photon Imaging
Considering the interaction between biological tissue and light, the main factors affecting the penetration depth of light are absorption and scattering. Generally speaking, the absorption in biological tissues is mainly determined by water, and the elastic scattering leads to the deflection of the propagation direction of light in biological samples. Compared with ultraviolet and visible light, near-infrared photons have the advantages of low phototoxicity, low photon scattering and low autofluorescence in biological tissues, which is conducive to achieving greater penetration depth.Currently, high-brightness fluorescent probes and high-sensitivity detectors are concentrated in the visible light band, and multi-photon imaging technology emerges as the times require. As early as 1931, G. Göppert-Mayer and others proved the feasibility of two-photon luminescence in theory. Until 1961, Kaiser and others used laser to excite CaF
2:Eu
2+ crystals and observed the phenomenon of two-photon excitation for the first time
[615][616,617]. Over the past 30 years, near-infrared region I (NIR-I, 760 ~ 900 nm) excited two-photon fluorescence microscopy has revolutionized the study of structure and function in vivo, enabling high spatial resolution, neurophysiological recordings to reach unprecedented scale and depth in the highly scattering interior of the brain
[618]. However, the imaging depth of 1 mm is approaching the theoretical limit of the two-photon penetration depth of NIR-I excitation. To seek higher penetration depth and signal-to-background ratio, a three-photon microscope with excitation light located in the second region of the near infrared (NIR-II, 900 ~ 1880 nm) must be used. In 1996, Hell, Xu and Svoboda proposed the concept of three-photon scanning microscope
[619,620]. Since then, three-photon microscopic imaging with long-wavelength excitation has had an important impact in many fields such as neuroscience
[621,622]. Considering the water absorption and scattering of biological tissues, the biological windows of NIR-II suitable for multi-photon excitation are located at ~ 1300 and ~ 1700 nm, respectively (Fig. 158)
[623,624].
Three-photon fluorescence (3PF) imaging with large penetration depth and high signal-to-background ratio requires fluorescent probes with high brightness and stability. Various organic dyes have been designed and synthesized for multiphoton imaging, but most of them have planar structures and are prone to aggregation in biological environments, leading to ACQ effect. Under the irradiation of high-energy pulsed laser, which is required for 3PF imaging, organic small molecule dyes have poor photostability and are easily bleached, which leads to the decline of imaging depth and quality. AIE nanoparticles (AIE NPs) containing AIE molecules are promising multiphoton fluorescent probes
[625,626]. First of all, AIE NPs have strong multiphoton fluorescence: due to the restriction of molecular motion, the aggregated AIEgens (such as coated NPs) have a large photoluminescence quantum yield; Furthermore, the increase of the multiphoton absorption cross section can be achieved by simply increasing the number of molecules in the nanoparticles or by molecular structure design. It is reported that the three-photon absorption cross section of TPEPT NPs at 1550 nm is as high as 6.33×10
-78cm
6·s
2[627]; In addition, the thousands of AIE gens in AIE NPs make them highly resistant to photobleaching, which is beneficial for 3PF with large penetration depth and long tracking time
[628]. In recent years, several AIEgen have been applied to 3PF imaging technology
[629,630].
In 2015, Qian et al. reported TTF AIEgen with large conjugation length. Under 1560 nm femtosecond laser excitation, TTF in solution has two-photon, three-photon and even four-photon fluorescence. Interestingly, with the increase of TTF aggregation degree, the intensity of tripling and 3PF in TTF NPs increases. Based on this, Qian et al. proposed the views of "aggregation-induced tripling enhancement" and "aggregation-induced three-photon enhancement" (Fig. 159)
[631].
The rich nonlinear optical properties of TTF make it a potential application for 3PF imaging. In 2015, Zhu et al. Used nano-graphene oxide as a surface modifier to coat TTF (TTF-NGO NPs), and proposed that this method could be widely used in other AIEgens
[632]. 3PF imaging of mouse ear vessels with zebrafish was achieved using TTF-NGO NPs (fig. 160).
In 2016, Li et al. Used TTF NPs coated with DSPE-mPEG
2000 to verify its excellent photostability, low biological toxicity and chemical stability
[628]. On this basis, they injected TTF NPs into zebrafish embryos, and used 3PF imaging to track different growth stages of embryos for up to 120 H under 1560 nm femtosecond light excitation (Fig. 161), highlighting the anti-photobleaching advantage of AIE NPs as a multi-photon fluorescent contrast agent. In addition, 3PF angiography was performed by using TTF NPs to target zebrafish blood vessels, and the shape of the heart and blood vessels could be clearly observed.
In 2017, Wang et al. reported a TPATCN AIEgen with TPA as the electron donor (D) and DBFN as the electron acceptor (A), which has a typical D-A-D structure with the fluorescence emission peak at 642 nm
[633]. NPs coated with Pluronic F-127 were used to simultaneously collect signals generated by 3PF and frequency tripling under 1560 nm femtosecond light excitation, achieving an imaging depth of 875 μm for craniotomy of mouse cerebral vessels, and a signal-to-background ratio of up to 4.7 was maintained at 875 μm (Figure 162).
In order to further optimize the excitation band and 3PF intensity, Li et al. moved the excitation wavelength to 1300 nm, and used TPATCN NPs coated with Pluronic F-127 to achieve 3PF imaging with a high signal-to-background ratio at a depth of 1360 μm in the open craniocerebral blood vessels of mice, and the blood vessels of 9.4 μm can still be distinguished at 1360 μm, which is the maximum imaging depth of 3PF imaging excited by NIR-IIa (1300 ~ 1400 nm) at present (Fig. 163)
[634].
Because the skull is dense and scatters light greatly, traditional multi-photon imaging technology requires craniotomy or grinding the skull thin to observe the brain, which may lead to congestion, injury and other problems, so non-invasive observation is very important for the analysis of biological structures and physiological processes. With the help of the large penetration depth of NIR-II excitation light and the high 3PF brightness of AIE NPs, Wang et al. First realized the transcranial imaging in 2017 (Fig. 164)
[635]. Under the excitation of 1550 nm femtosecond laser, DCDPP-2TPA NPs with high fluorescence intensity and strong photobleaching resistance can be used for non-invasive in vivo cerebral vascular imaging in mice with an imaging depth of 300 μm, and small blood vessels of 2. 4 μm can still be observed at 300 μm, which provides a promising new idea for non-invasive in vivo biological observation in the future.
In 2020, Liu et al. Constructed binary AIE NPs, which achieved high 3PF absorption cross-section and Ф
F, and simultaneously received 3PF and third harmonic signals under 1610 nm femtosecond light excitation, with an imaging depth of up to 1680 μm
[591]. This is the maximum imaging depth that can currently be achieved with AIE NPs as the 3PF probe (Figure 165).
6.3.1.5 AIE light-harvesting fluorescent probe
Although AIE functional dyes can overcome the shortcomings of traditional dye molecular aggregation quenching, they still show high luminescence quantum yield in water environment. However, most of the AIE dye molecules have broad fluorescence emission bands with a half-peak width of more than 100 nm. This shortcoming limits the wide application of AIE dye molecules. For example, in biological multi-color imaging and co-localization experiments, the broad fluorescence emission signal of AIE dye will interfere with other fluorescent markers, and cross-color phenomenon of different fluorescence channels will occur
[636,637]. In the application of OLED materials, researchers must install additional filters to obtain pure color luminescence, which will seriously reduce the external quantum efficiency of the device
[638,639]. The emission bandwidth of AIE materials is mainly due to the fact that their molecules contain multiple aromatic groups that can rotate freely, and they will have different conformations and aggregation States in the process of aggregation, which determines that it is difficult to solve this problem by molecular design and modification. Therefore, it is of great significance to find a universal strategy to obtain narrow emission AIE materials. Based on this, Yang et al. Proposed a strategy to solve this problem by using light harvesting, by introducing fluorescent dyes with narrow emission as energy acceptors in the AIE system.When excited by light, AIE molecules will efficiently transfer the excitation energy to the energy acceptor, causing the luminescence of the acceptor, thus narrowing the emission band, greatly increasing the luminescence intensity, and the luminescence color can be easily controlled
[640]. As shown in Fig. 166, by introducing two quadruple hydrogen bonding units (UPy) on the AIE compound TPE as the energy donor (TPEH, TPEP, TPEDC) and the UPy diffractor with narrow emission BODIPY as the energy acceptor (GM, YM, RM, NIR-M), the energy donor and acceptor copolymerize through hydrogen bonding interaction to form a supramolecular polymer, which is further co-assembled in water to form nanospheres with a particle size of about 90 nm. Taking the AIE luminescent nanoparticles formed by copolymerization of TPEH and RM as an example, when the donor is selectively excited, it can efficiently transfer the excitation energy to the acceptor, causing the acceptor to emit light, so that the half-peak width of the AIE luminescent nanoparticles is reduced from 109 nm (TPEH) to 35 nm, accompanied by a 6-fold increase in fluorescence. The combination of other energy donors and acceptors can obtain AIE materials with narrowed emission band and enhanced luminescence, and AIE luminescent nanoparticles covering the whole visible light range can be obtained by changing the type and proportion of acceptors. The AIE nanoparticle cell imaging study found that the AIE nanoparticles without receptors had obvious fluorescence signals in the red, green and blue channels due to the spectral bandwidth, while the red-emitting AIE nanoparticles with light harvesting properties only had strong fluorescence signals in the red channel, which significantly improved the imaging effect. Light-harvesting nanoparticles have also shown excellent performance in in vivo imaging. The light-harvesting AIE material was used as a chemiluminescent illuminant for imaging of inflammatory sites, and good results were also achieved.
6.3.2 Application of AIE Materials in Biological Diagnosis and Treatment
The AIE material not only has excellent aggregation-state luminescent properties, but also has good ROS generation ability and certain photothermal properties. According to the Jablonski energy diagram, after absorbing energy to the excited state, the molecule can return to the ground state in three ways: (1) radiative transition, which can be used for fluorescence and phosphorescence imaging; (2) non-radiative dissipation generates heat, which can be used for photoacoustic imaging/photothermal therapy; (3) Intersystem crossing (ISC) to the triplet state produces phosphorescence or ROS, which can be used for PDT. Therefore, in this section, we mainly review the applications of these three pathways in the fields of imaging-guided tumor ablation, photoacoustic imaging/photothermal therapy, and chemosensitization/immunotherapy, respectively, by regulating the luminescence, photothermal, and ROS generation properties of AIE materials. In addition, we also summarize the latest research progress of AIE materials in the field of integrated optical diagnosis and treatment from three aspects: fluorescence imaging-guided PDT, inorganic materials-assisted multimodal optical diagnosis and treatment, and multimodal optical therapy based on a single AIE molecule.
6.3.2.1 Modulating Fluorescence Properties of AIE Materials for Imaging Tumor Resection
Thanks to the advantages of high sensitivity and safety, fluorescence imaging has been widely used in the basic research of imaging-guided tumor resection, and has been applied in clinical practice. However, the fluorescent molecules used in the current study not only emit fluorescence, but also have certain photothermal properties and ROS generation ability. How to inhibit these two pathways to maximize the fluorescence performance is the key to improve the imaging quality of fluorescent materials in surgical navigation.
To solve this problem, Gu et al. Proposed to amplify the fluorescence and ROS generation ability of AIE points by enhancing the internal restricted microenvironment of nanoparticles, thus greatly improving the effect of tumor photodiagnosis and treatment in vivo
[641]. They used bowlene-modified PEG (Cor-PEG) and DSPE-PEG-loaded AIE fluorescent molecules (TPP-TPA) to prepare nanoprobe Cor-AIE dots and DSPE-AIE dots with different intraparticle rigid microenvironments, respectively (Fig. 167), using bowlene-modified PEG (Cor-PEG). Compared with the DSPE-AIE dot, the Cor-AIE dot has a fluorescence quantum yield that is 4 times larger and a ROS generation that is 5.4 times larger. The structure of Cor-AIE dots provides a rigid environment inside the nanoparticles and strong interaction with fluorescent molecules, which limits the intramolecular rotation of fluorescent molecules, thus effectively inhibiting non-radiative decay.As a result, more excited state energy flows to the fluorescence channel and ISC process, which improves the fluorescence and ROS generation ability of AIE points, and promotes the effect of fluorescence imaging-guided tumor surgery and PDT of abdominal metastasis mice.
Although the fluorescence performance of AIE dots can be improved by introducing bowlene to enhance the internal restricted microenvironment of nanoparticles, the ROS generation ability of AIE dots is also improved, but the ROS generation ability is not necessary for imaging guided tumor resection, on the contrary, it also limits the further improvement of fluorescence performance. In order to solve this problem, Chen et al. Proposed to effectively close the intersystem crossing and thermal deactivation pathways of AIE molecules by host-guest doping of calixarenes with AIE molecules, so as to maximize the energy absorbed by the molecules for fluorescence emission
[642]. Calixarene, as a macrocyclic host, is a novel supramolecular nano-drug carrier developed in recent years, which has the advantages of multi-stimulus responsiveness, multi-modification tunability and low cytotoxicity
[643]. As shown in Fig. 168, if the three dissipation paths of the absorbed excitation energy are compared to three taps, the host-guest complexation between the calixarene and the AIE molecule obviously shuts off the intersystem crossing and non-radiative transition of the taps, allowing most of the water (absorbed excitation energy) to flow out as fluorescence. Thus, the interaction of calixarenes with AIE molecules severely limits the nonradiative transitions in which the AIE molecules absorb energy, and at the same time reduces the generation of ROS that are highly cytotoxic, providing supramolecular AIE nanoparticles of high fluorescence intensity. The results show that the fluorescence quantum efficiency of the supramolecular AIE nanoparticles can reach 0.72. The in vivo peritoneal cancer mouse model study shows that the signal to noise ratio of the fluorescence signal of the tumor tissue relative to the normal tissue can reach 48.5, and the in vivo toxic and side effects are quite low, which indicates that the material can be used as a superior fluorescence biological probe for high-sensitivity fluorescence imaging guided tumor surgical resection. This study introduces the concept of host-guest doping on the basis of AIE small molecules, which provides an ideal platform for the development of high-brightness AIE fluorescent probes.
Although fluorescence imaging has high sensitivity, the signal-to-noise ratio of fluorescence imaging still needs to be further improved due to the limitation of the body's own fluorescence background
[644]. In recent years, the development of long afterglow imaging technology has provided a new technical means to improve the signal-to-noise ratio of optical imaging. Due to the absence of real-time laser excitation, the long afterglow imaging avoids the fluorescence background of the organism itself and has a very high imaging signal-to-noise ratio
[645]. Schaap probes are a class of classical chemiluminescent molecules with excellent long afterglow luminescence properties
[646]. However, the benzoate molecule, which is the most critical in the luminescence process of the Schaap probe, also has ACQ effect, resulting in a low luminescence quantum yield of the Schaap probe in water. In response to this problem, Ni et al. introduced AIE fluorescent molecules into the Schaap-type nanoprobe (Fig. 169), and used the FRET effect of the Schaap-type nanoprobe and AIE fluorescent molecules to improve the quantum yield of luminescence. For the first time, they developed a near-infrared afterglow luminescence AIE nanoprobe (AGL AIE point) with ultra-long afterglow luminescence time for imaging-guided tumor resection
[647]. In vivo studies have shown that the near-infrared afterglow luminescence of AGL AIE dots not only has deeper tissue penetration ability, but also can be rapidly quenched in normal tissues such as liver, resulting in ultra-high tumor-liver signal ratio. Thanks to its high signal-to-noise ratio of long afterglow imaging, the AGL AIE point showed superior tumor surgical navigation in living mice.
6.3.2.2 Photodynamic therapy
Photodynamic therapy (PDT) is a new technology that uses specific wavelength light to excite photosensitizers to produce strong oxidative ROS, which can oxidize and destroy various biological macromolecules in tissues and cells, and cause irreversible damage or even death of cells in diseased sites
[648]. Photosensitizer is the key to PDT, and the efficiency of ROS generation directly affects the effect of PDT. According to the ISC rate formula, decreasing the ΔE
ST and increasing the spin-orbit coupling constant (SOC) can effectively improve the ISC rate of photosensitizer and promote the generation of ROS
[649]. In general, enhancing the D-a effect of molecules is an effective strategy to reduce ΔE
ST and increase the efficiency of ROS production. For example, Tang et al. Synthesized a series of AIE molecules with different D-a intensities, and confirmed that with the enhancement of molecular D-a effect, the generation efficiency of ROS also showed a gradual increase trend
[650]. Similarly, they successfully developed the first bifunctional near-infrared AIE molecule with Gram-positive bacteria targeted imaging and efficient PDT antibacterial effect for rat wound bacterial infection model by ordered enhancement of molecular D-A effect
[651]. On this basis, Tang et al. Recently reported a new strategy to construct high-performance photosensitizers by simultaneously reducing ΔE
ST and increasing SOC. They skillfully introduced non-heavy atom groups (carbonyl groups) with enhanced SOC and strong electron-deficient cyano groups into triphenylamine (TPA) derivatives with strong electron-donating ability.TPAIC, an AIE photosensitizer with large molar absorption coefficient, long absorption and emission wavelength, and excellent ROS generation efficiency, was successfully developed by precisely adjusting the ratio of cyano and carbonyl groups in the molecule and the electron-donating effect of the TPA part to regulate the ΔE
ST and SOC constant, and was used in tumor PDT guided by fluorescence imaging
[652]. The results show that TATIC has both a low ΔE
ST value and a high SOC constant, achieving a balance and optimization between the two, exhibiting optimal photosensitivity (Figure 170).
Organelles play an irreplaceable and unique role in the process of cell growth, metabolism and proliferation. Due to the limited lifetime and diffusion distance of ROS, organelle-targeted PDT can effectively amplify the effect of PDT. At present, researchers have developed a series of photosensitizers with different organelle targeting functions (such as cell membrane, mitochondria, lipid droplets, lysosomes) for photodynamic killing of cancer cells
[20⇓⇓⇓~24,653⇓⇓⇓~657]. Tang et al. Developed three AIE molecules with mitochondrial, cell membrane and lysosomal targeting functions at the same time, and combined them for the first time in multi-organelle targeted PDT
[658]. The results showed that the ingenious combination of three AIE photosensitizers effectively achieved the synergistic PDT efficacy of "1 + 1 + 1 > 3", which provided a new perspective and strategy for enhancing the PDT effect (Fig. 171).
In order to further realize the systemic delivery of AIE materials and in situ PDT of mouse tumor organelles in vivo, Tang et al. successfully constructed the first cell nucleus targeting PDT system based on type I AIE photosensitizer by introducing a specific acid-responsive nucleus targeting carrier and loading TTFMN, an AIE photosensitizer with type I ROS generation ability, and realized in situ PDT of mouse tumor nucleus (Fig. 172)
[659].
Some AIE materials not only have fluorescent properties, but also have good ROS generation ability. These materials are called AIE photosensitizers. AIE photosensitizers are usually used in tumor PDT because of the excellent killing ability of ROS to cells. However, its application in the field of chemosensitization and immunotherapy has been less studied.
In view of this blank field, Chen et al. studied in detail the effect of sensitizing chemotherapy by using an AIE-based integrated diagnostic and therapeutic material (TPE-Py-FFGYSA), and proposed for the first time that AIE photosensitizer could be used as a non-toxic and safe adjuvant to enhance the anti-tumor effect of paclitaxel, and successfully achieved the synergistic therapeutic effect of "0 + 1 > 1"
[660]. The photosensitizer consists of an AIE molecule TPE-Py, a tripeptide FFG self-assembly auxiliary unit, and a polypeptide YSA targeting EphA2 protein, a target of tumor therapy (fig. 173). It was found that by optimizing the experimental conditions, EphA2-targeted TPE-Py-FFGYSA could provide an intracellular oxidative environment that was insufficient to kill cancer cells through photocontrol of ROS production, thus significantly enhancing the cytotoxicity of paclitaxel on human prostate cancer cells and reducing the median lethal concentration from 75.9 nM to 7.8 nM. However, when TPE-Py-FFGYSA was used alone, the ROS produced by TPE-Py-FFGYSA did not activate the mitochondria-mediated apoptosis pathway under the same light conditions. In addition, the self-assembled auxiliary unit can better promote the fluorescence output of AIE molecules and achieve high-contrast imaging of tumor cells. This study not only expands the application scenarios of AIE photosensitizers, but also provides a good platform for improving the efficacy of traditional chemotherapy drugs.
Later, Chen et al. Further studied the immunological effects of AIE photosensitizers
[661]. Immunogenic cell death (ICD) is a form of apoptotic cell death characterized by the release of immunostimulatory injury-associated molecules (such as calreticulin exposed to the outside of the cell), and therapies based on this pathway may increase the sensitivity of cancers with low immunogenicity to immunotherapy
[662]. However, there are few reports on drugs that can cause ICD, and their effects are not satisfactory. To solve this problem, Chen et al. Developed a photosensitizer (TPE-DPA-TCyP) targeting mitochondria and first demonstrated that mitochondrial oxidative stress can effectively induce ICD
[661]. TPE-DPA-TCyP has D-π-a structure, which can effectively reduce the interaction between molecules in the aggregate by increasing the separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), thereby enhancing the ROS generation ability of molecules, and causing mitochondrial oxidative stress under light conditions, thereby inducing ICD in tumor cells (Fig. 174). Cell experiments show that the AIE photosensitizer can greatly enhance the translocation of calreticulin to the outside of an envelope and promote the release of ICD related factors such as adenosine triphosphate (ATP), high mobility group box 1 (HMGB1) and heat shock protein 70 (HSP70),Its therapeutic effect is better than that of chlorin E6 (Ce6), pheophorbide-a (PPa) and oxaliplatin, which can induce ICD. At the same time, its immune effect in vivo is better than that of Ce6 through tumor vaccine experiments. This article is the first to propose the relationship between mitochondrial oxidative stress and ICD, that is, mitochondrial oxidative stress can trigger ICD, and it is also the first to combine two emerging fields, AIE and ICD, and to prove that AIE is an ideal platform for the development of photosensitizer-based ICD inducers.
6.3.2.3 Photothermal therapy
Photothermal conversion materials refer to a class of materials that can convert absorbed light energy into heat energy
[663]. Organic photothermal materials have developed rapidly due to their good processability, rich variety, easy control of properties and good biocompatibility, and have emerged in the fields of fine chemicals, seawater desalination, disease diagnosis and treatment
[664,665]. Excited state molecular motion can theoretically promote non-radiative transition and convert absorbed light energy into heat energy, which can be used in the design of photothermal materials. However, in practical application scenarios, whether it is seawater desalination, photothermal devices, or disease diagnosis and treatment, organic photothermal materials are in a solid or aqueous working environment, mainly in the form of aggregates, thus inhibiting their molecular motion and heat generation properties
[666]. Therefore, how to overcome the limitation of molecular motion in solid or aggregated state is a common problem in the field of molecular machines and molecular motion, which is of great significance for the design of photothermal conversion materials based on molecular motion and more challenging solid-state molecular machines
[667].
(1) Intramolecular motion-induced photothermy (iMIPT)
By considering the working mechanism of AIE, Tang et al. Proposed that if the active excited-state molecular motion of AIE molecules in the solid state can be maintained, the light energy absorbed by AIE molecules can be converted into heat energy in theory, so that AIE can be switched from a luminescent material to an effective photothermal material
[668].
Figure 175a, B shows the design principle of AIE type molecules applied to photothermal materials. They proposed to use the long alkyl chain strategy commonly introduced in liquid crystal molecules and organic semiconductor materials to prevent the strong interaction between aromatic ring molecules.The aromatic ring molecules are in a relatively flexible and spatially isolated environment under the isolation of the long alkyl chain to maintain the molecular motion of the excited state so as to effectively convert light energy into heat energy
[669]. They named the design strategy of this photothermal material as Intramolecular motion-induced photothermy (iMIPT).
As shown in Figure 175, they synthesized two small organic molecules with near-infrared absorption and receptor-donor structure through molecular design: 2TPE-NDTA and 2TPE-2NDTA, and modified the TPE molecular rotor on the naphthalene diimide receptor. The long branched alkyl chain can reduce the interaction between molecules and create space for intramolecular group movement, and the flexible twisted TPE rotor can rotate freely in the space formed by the long alkyl branch chain,They verified the molecular motion in the solid state through theoretical calculation, ultrafast spectroscopy, solid-state nuclear magnetic resonance and other means, and found that their nanoparticles can dissipate almost all the absorbed light energy in the form of heat in the solid state and aggregation state (Fig. 175c). The temperature of 2TPE-2NDTA can be rapidly increased to 81 ℃ after 808 nm laser irradiation, and the PCE is as high as 54. 9%, which is superior to commercial methylene blue, indocyanine green and many semiconductor polymer photothermal agents
[670,671]. The good photothermal properties of iMIPT make it possible to be used for high-resolution and deep-level photoacoustic imaging of mice in vivo. For example, the photothermal agent 2TPE-2NDTA based on the iMIPT molecule has a better photoacoustic effect compared to the widely used photoacoustic imaging agent Methylene blue (MB) and semiconducting polymer nanoparticles (fig. 175d). This study first proposed the concept of iMIPT and confirmed its rationality in mechanism and feasibility in application.
In order to further verify the iMIPT mechanism and explore the effect of long alkyl chain on intramolecular motion, Tang et al. synthesized a series of benzobisthiadiazole-type iMIPT molecules with different chain lengths (Fig. 175)
[672]. The results show that the photothermal effect of the molecule increases with increasing chain length (fig. 175b), confirming the effectiveness of the long alkyl side chain strategy. Nanoparticles based on NIR14 had excellent photothermal anti-tumor effect, and the tumor volume was significantly reduced by intravenous injection of polymer-coated NIRb14-PAE/PEG nanoparticles combined with near-infrared light treatment (fig. 176c).
(2) Application of iMIPT in the field of biomedicine
The excellent photothermal conversion properties of iMIPT molecules have attracted the attention of the biomedical field, and its applications in photothermal synergistic therapy of coronavirus and photothermal sterilization have been reported by different research groups.
COVID-19 has seriously threatened the health of the global public. Clinically, acute respiratory distress syndrome and multiple organ damage occur in critically ill patients with rapid deterioration. This is because the patient's violent immune response to the virus can lead to a severe inflammatory syndrome (called "cytokine storm"), which can lead to death
[673]. Therefore, it is urgent to develop effective treatment strategies with antiviral and anti-inflammatory properties. As SARS-CoV-2 virus is sensitive to heat, Huang et al. prepared TN @ AM biomimetic nanoparticles (TN @ AM NPs) by coating iMIPT molecule 2TPE-2NDTA based nanoparticles with alveolar macrophage cell membrane (fig. 177)
[674][673]. The encapsulated alveolar macrophage cell membrane can specifically bind to the spike protein of coronavirus, which prevents the virus from attacking the host target, and the local high temperature generated by near-infrared illumination can effectively kill the virus. In addition, because the cell membrane has binding receptors for cytokines corresponding to the virus, it can absorb a large number of cytokines to effectively control the symptoms caused by the strong immune response after human infection with COVID-19. Finally, the photothermal effect of 2TPE-2NDTA molecules under near-infrared laser radiation can not only reduce the number of viruses in the lungs of mice, but also effectively reduce the inflammatory factors produced by the body, thus alleviating the growth of COVID-19 and reducing the severity of the disease.
Bacterial biofilm is a unified community. Extracellular polymeric substance (EPS) encapsulates bacterial cells, forming a physical barrier. In addition to a good barrier, the overall quorum sensing effect of bacteria also helps mature bacteria to develop strong resistance to antibiotics
[675,676]. Tang et al. Developed photothermal nanoparticles based on iMIPT molecules to kill bacteria and remove bacterial biofilms
[677]. When iMIPT NPs accumulate in a bacterial matrix, they generate heat that not only targets individual bacteria, but also destroys neighboring bacteria. At the same time, photothermal effect can also lead to the inactivation of proteins, polysaccharides, extracellular DNA and other adhesive components in the matrix, leading to the disintegration of mature biomembranes. As shown in Figure 178, using S. aureus as a model, irradiation with near-infrared laser (808 nm,1 W·cm
-1) for 10 min after the addition of 2TPE-2NDTA NPs could effectively remove the mature S. aureus biofilm, and the relative biomass was reduced by 75%. The use of near-infrared laser irradiation alone or the addition of nanoparticles alone had little effect on the bacterial biofilm (fig. 178b).
(3) Application of iMIPT in biosensing
At present, most of the reported PA probes have been in the "on" state, which limits their application in the field of specific detection
[678]. The loose aggregation and relatively free movement of iMIPT molecules in the solid state help them react with specific signaling molecules, thus changing the donor-receptor interaction to achieve specific "off-on" PA probes
[679][680]. As shown in Figure 179, the long aliphatic chain and the distorted triphenylamine structure guarantee the intramolecular motion in the excited state. The benzothiadiazole substituted by amino group in the OTTAB molecule can react with nitric oxide. The electron-withdrawing ability of the acceptor unit in the generated OTTTB is enhanced, the donor-acceptor interaction is enhanced, and the absorption peak is red-shifted, so that the excitation at 700 nm can realize "off-on". The nitric oxide-responsive photoacoustic probe has potential application in the early diagnosis of encephalitis.
6.3.2.4 AIE Materials for Multimodal Optical Diagnosis and Treatment
(1) Inorganic material-assisted multi-modal optical diagnosis and treatment
Limited by the high oxygen dependence of PDT, the hypoxic environment at the tumor site, and the limited life span and diffusion distance of ROS, PDT alone often fails to completely eliminate tumors. As another important means of phototherapy, PTT can produce high heat to kill diseased cells by photothermal conversion agents excited by light
[681,682]. The combination of PDT and PTT can achieve complementary advantages and achieve the effect of PDT-PTT synergistic therapy
[683,684]. On the other hand, although pure fluorescence imaging has the advantages of high sensitivity and fast response, it has the limitation of insufficient penetration depth and spatial resolution. In contrast, photoacoustic imaging can provide sufficient tissue penetration depth and high spatial resolution
[648]. Fluorescence imaging combined with photoacoustic imaging will be able to provide rich and accurate imaging information with imaging sensitivity, spatial resolution and penetration depth at the same time
[685]. Therefore, PDT guided by fluorescence imaging based on AIE photosensitizer supplemented by photothermal reagents with high PCE can realize the integration of various components, which is one of the directions to build a multi-modal diagnosis and treatment system.
For the first time, Tang et al. combined AIE photosensitizer with two-dimensional nano-photothermal material black phosphorus nanosheets, and prepared a multimodal diagnosis and treatment system assisted by inorganic materials based on AIE materials
[686]. In this system, the black phosphorus nanosheet provides a load platform for the AIE photosensitizer, which can effectively improve the stability of the black phosphorus nanosheet. By integrating the photothermal conversion performance of the black phosphorus nanoflake and the ROS generation ability of the AIE material, this multi-modal diagnosis and treatment system can effectively eliminate tumors in vivo experiments in mice and achieve a good "1 + 1 > 2" multi-modal synergistic therapeutic effect (Figure 180a). In addition, they further compounded AIE photosensitizer with water-soluble pillarene and gold nanorods, and successfully constructed a multi-stimuli-responsive multi-modal nano-hybrid diagnosis and treatment system guided by supramolecular self-assembly
[687]. The system has multiple stimulus responsiveness of pH, temperature and near-infrared light, which can realize the tumor PDT-PTT synergistic therapy guided by FLI-PAI, effectively eliminate tumors and inhibit tumor recurrence, and provide a new idea for multi-modal diagnosis and treatment of tumors (Fig. 180b).
(2) Multimodal optical diagnosis and treatment based on single AIE molecule
Compared with single-modality diagnosis and treatment, multi-modality diagnosis and treatment assisted by inorganic materials can greatly improve the anti-tumor effect of AIE materials. However, this "all-in-one" diagnosis and treatment system often has some shortcomings, such as complex components, poor stability, high toxicity of components, and low repeatability of preparation
[688]. Therefore, it is of great significance to design "one-for-all" multi-functional molecules with multiple functions to construct a single-component multi-modal optical diagnosis and treatment system. The key to the design of single-molecule multi-modality diagnosis and treatment system is to achieve the balance between the energy attenuation of molecular radiation and non-radiation transitions. As an excellent template for balancing energy dissipation, AIE materials are excellent candidates for building "one-for-all" multifunctional diagnostic and therapeutic systems. On the premise of retaining the radiative transition and ISC, part of the excited state energy is attenuated in the form of non-radiative thermal dissipation by enhancing the intramolecular motion in the aggregated state, so as to ensure the effective photoacoustic imaging and photothermal therapy effect, and realize the multi-modal diagnosis and treatment based on a single AIE molecule
[689⇓~691].
Tang et al. Reported an AIE molecule containing multiple rotors (fig. 181a), which, thanks to its extremely distorted conformation, is arranged haphazardly in the form of an amorphous state inside the nanoparticle.It greatly promotes the rotation of the rotor, greatly improves the non-radiative heat dissipation of the excited state energy, and shows excellent PAI-guided PDT-PTT synergistic therapeutic effect in tumor treatment
[692]. Subsequently, they constructed the first multi-modal diagnosis and treatment system based on single AIE molecule, which integrates FLI, PAI, PDT and PTT, by introducing an oscillator into the AIE molecular structure of D-A configuration to promote the intramolecular motion in the aggregation state. It can be called an "all-round player" on the optical diagnosis and treatment track (Fig. 181b)
[693].
Recently, Tang et al. Have successfully constructed an AIE multi-modal diagnosis and treatment system with the advantages of large molar absorption coefficient and strong light absorption capacity by regulating the substituent groups of AIE molecules with D-A configuration, which can improve the overall energy source on the basis of balancing various energy dissipation pathways
[694]. The increase in total allocable energy endows the system with long emission wavelength and high PCE and ROS generation efficiency, and successfully realizes multi-modality imaging-guided synergistic therapy including FLI, PAI, PDT and PTT.
7 Summary and prospect
Since the concept of AIE was put forward in 2001, Chinese scientists have made innovative research achievements in the design and preparation of AIE material system, the establishment and improvement of AIE theoretical system, and the development and expansion of AIE material application.A number of new luminescent materials with independent intellectual property rights have been produced, and a new theory of molecular aggregation luminescence has been put forward, which has realized the high-tech application of AIE materials in the fields of energy, environment and health. AIE has become a new and important research direction in the fields of chemistry and materials, and is the leading research field in the world for Chinese scientists. In this review, we discuss recent developments in the field of AIE and its derivatives. As a general platform for studying aggregate science, AIE has been continuously integrated with other research fields such as materials, biology, energy and environment, injecting new vitality into these fields. As more and more researchers begin to work in this field, more breakthroughs in basic research and applications are expected in the near future.
However, with the deepening of AIE materials research, there are still some new challenges.
(1) Although a large number of AIE materials have been reported, their unique luminescence properties need to be further studied and discussed, especially the luminescence enhancement mechanism of some AIE molecules in the aggregate state is still not very clear. Therefore, clarifying the relationship between molecular structure, aggregate structure and properties, and effectively regulating the structure of aggregates at the molecular level are expected to achieve excellent properties and new functions of materials. Therefore, the future research should further broaden their types, further clarify their photochemical and photophysical processes, so as to provide new ideas and theories for the development of efficient aggregated luminescent materials, and further discover new phenomena, understand new laws, propose new concepts and establish new theories.The new system lays a foundation for enriching the research connotation of material science, information engineering, biotechnology and other disciplines, and makes new contributions to life health, environmental protection, national defense security and technological innovation.
(2) The decay of molecular excited States is a dynamic process with a very large time scale span. At present, there is still a lack of accurate measurement methods to capture the geometric structure of the excited state of molecules, and it is impossible to separate the contribution of each electronic state and each way to the decay of the excited state. There is still a lack of advanced theoretical calculation methods of electronic structure in theory, and many disputes about the mechanism come from different results given by different calculation methods of electronic structure. Therefore, we should pay attention to the development of fine measurement instruments and the construction of measurement systems in experiments, pay attention to the development of electronic structure theory and methods in theory, and strengthen the close combination of the two, generate new knowledge, put forward new ideas, and promote the overall development of AIE.
(3) At present, remarkable progress has been made in the field of pure organic room temperature phosphorescent materials both in molecular design and application. However, as far as the mechanism is concerned, there is still a lack of comprehensive and in-depth understanding of the photophysical process of pure organic room temperature phosphors, and the luminescent behavior of some luminophores can not be fully explained. The above problems are challenging and require researchers to make unremitting efforts to develop and explore. In addition, considering the advantages and wide application space of pure organic phosphor materials, it will continue to be the focus and hot spot in the field of organic luminescent materials.
(4) At present, new progress has been made in the field of atypical luminescent compounds, especially in the rational design of molecules, the creation of new biomolecular systems, the study of room temperature phosphorescence and the regulation of photophysical properties of compounds. However, the luminescence mechanism, especially the photophysical process of the exciton, remains to be further explored. At the same time, there are still some deviations between the description and prediction of the properties of atypical luminescent compounds in theoretical chemistry and the experimental results, and the development of the above research directions is worth looking forward to. In the future, a further summary of the commonalities and characteristics of different systems in this field; The influence of compound structure, concentration, aggregation state, intramolecular and intermolecular interaction, external environment (pressure, temperature) on luminescence was clarified. It is very important to clarify the multi-level structure and activity of the electron-molecule-aggregate system combined with the study of ultrafast dynamics. The solution of the above problems will lay a solid foundation for further clarifying the luminescence mechanism, summarizing the influencing factors and control means of photophysical properties, and developing its application fields.
(5) After nearly 10 years of development, great progress has been made in both molecular design and mechanism research of force-stimulated luminescence responsive AIE materials. However, after the single crystal sample is ground (the commonly used method to study the mechanoluminescence and discoloration), the single crystal has been destroyed, and the real single crystal structure data of the final state can not be obtained. Even if the hydrostatic pressure method is used, the single crystal structure is not destroyed, and the real-time single crystal X-ray detection can not be carried out, so there is no direct evidence of the relationship between the force stimulus response and the aggregate structure. If we want to use the force-stimulated luminescence response AIE material for stress sensing, we must establish the quantitative relationship between the force and the luminescence performance. Although researchers have tried to quantitatively study the relationship between the force-stimulated luminescence performance and the force, the results are relatively superficial. In addition, the application research of force-stimulated luminescence responsive AIE materials needs to be further deepened and expanded, and a breakthrough in practical application is needed.
(6) The visualization of composite structures is also facing new opportunities and challenges: (a) how to obtain more stable and efficient AIE molecules to achieve high-resolution visual analysis of composite materials; (B) Simultaneous recognition and discrimination of multiple inorganic phases in composites by designing AIE molecules with different colors. Based on this, we hope to further expand the application of AIE molecules through the cooperation of material design and analysis in many fields, and provide assistance for the research of materials, energy, environment and other fields.
(7) It can be predicted that AIE materials will exhibit unlimited potential in the next generation of OLEDs, which will greatly reduce the cost of commercial applications of OLEDs and open up new directions for the research of OLEDs materials and devices. Therefore, it is urgent to develop AIE materials with high efficiency and long lifetime, and to optimize the device structure with higher exciton utilization, so as to further improve the efficiency and stability of the device and meet the application requirements. It is believed that if the concepts of TADF, TTA, HLCT and RTP are introduced into the design of AIE materials, the device efficiency of AIE materials will undoubtedly be greatly improved, and once the stability and lifetime are solved, it will bring a new generation of commercial material system to the OLEDs industry, realize independent innovation, and promote the development of OLEDs industry in China and even in the world.
(8) Although considerable progress has been made in the regulation of photophysical properties of AIE molecules, there are still some challenges in their biomedical applications. For example, most of the long-afterglow AIE nanoprobes used in current research are still long-afterglow molecule/AIE photosensitizer doped systems, so it is urgent to develop new small molecule probe systems with long-afterglow/AIE properties. In addition, the current strategies to improve the photoacoustic properties of AIE materials still focus on improving the photothermal properties of materials, and there are still few studies on regulating the thermoacoustic conversion efficiency. At the same time, most of the AIE photosensitizers used at present absorb visible light and have weak tissue penetration ability, so it is urgent to develop AIE photosensitizers with near-infrared absorption. The above problems provide a new direction for the application of AIE materials in the field of biomedicine.
(9) AIE materials still face some urgent problems and challenges in the process of practical clinical application. Firstly, the excitation wavelength of most AIE diagnostic and therapeutic materials that have been developed is still short, and these AIE materials are not good at the diagnosis and treatment of deep tumors due to the limited penetration depth of the matching excitation light source, so the AIE diagnostic and therapeutic materials with near-infrared absorption or double/multi-photon absorption characteristics need to be further developed. However, the development and research of multifunctional AIE molecules for multimodal diagnosis and treatment have just started, and their structure-activity relationship is not very clear, so the corresponding construction strategies and high-performance molecular libraries need to be further enriched. Finally, in order to achieve more accurate detection and diagnosis and more effective treatment, responsive AIE diagnostic and therapeutic materials based on tumor-specific markers need to be developed urgently. In addition, strengthening the strong combination of AIE diagnostic and therapeutic materials and functional nano-carriers is also an effective way to improve the effect of disease diagnosis and treatment. We have reason to believe that with the innovative development of AIE diagnostic and therapeutic materials with higher performance and the in-depth study of related structure-activity relationships, AIE materials will be further developed in the field of optical diagnosis and treatment, and its promotion in clinical practice is worth looking forward to.
(10) The application advantages of AIE NPs in the field of 3PF imaging have attracted wide attention and discussion, but there are still some problems to be solved: (a) The inherent characteristics of AIEgens ensure its high luminescence quantum efficiency, but the high fluorescence intensity is the result of the combination of large absorption cross-section and high ФF. Therefore, the design and synthesis of AIEgen with multi-photon absorption cross section is very important for multi-photon imaging, which is believed to make a further breakthrough in the depth and quality of 3PF imaging. (B) At present, AIE is limited to biological structure imaging in the field of 3PF imaging. The design and development of targeted AIE probes can mark specific tissues such as neurons and tumor cells, and analyze physiological processes and other biological structures, which is helpful to further expand the application of AIE in 3PF imaging.
National Natural Science Foundation of China(21788102)
National Natural Science Foundation of China(21977126)
National Natural Science Foundation of China(21907112)
National Natural Science Foundation of China(21778079)
National Natural Science Foundation of China(51961160730)
National Natural Science Foundation of China(51873092)
National Natural Science Foundation of China(81921004)
National Natural Science Foundation of China(21975238)
National Natural Science Foundation of China(21974008)
National Natural Science Foundation of China(21838007)
National Natural Science Foundation of China(21804006)
National Natural Science Foundation of China(51733010)
National Natural Science Foundation of China(21875019)
National Natural Science Foundation of China(21975020)
National Natural Science Foundation of China(21975021)
National Natural Science Foundation of China(21975061)
National Natural Science Foundation of China(22090050)
National Natural Science Foundation of China(21974128)
National Natural Science Foundation of China(21874121)
National Natural Science Foundation of China(52003257)
National Natural Science Foundation of China(21973099)
National Natural Science Foundation of China(21905113)
National Natural Science Foundation of China(21835001)
National Natural Science Foundation of China(51773080)
National Natural Science Foundation of China(21674041)
National Natural Science Foundation of China(51903163)
National Natural Science Foundation of China(21801169)
National Natural Science Foundation of China(21875069)
National Natural Science Foundation of China(21625202)
National Natural Science Foundation of China(21971023)
National Natural Science Foundation of China(51822303)
National Natural Science Foundation of China(52073172)
National Natural Science Foundation of China(21574015)
National Natural Science Foundation of China(52003228)
National Natural Science Foundation of China(92061201)
National Natural Science Foundation of China(21825106)
National Natural Science Foundation of China(21771021)
National Natural Science Foundation of China(21822501)
National Natural Science Foundation of China(22061130206)
National Natural Science Foundation of China(61975172)
National Natural Science Foundation of China(82001874)
National Natural Science Foundation of China(51620105009)
Natural Science Foundation of Guangdong Province(2021B1515020102)
Natural Science Foundation of Guangdong Province(2020B1515020011)
Natural Science Foundation of Guangdong Province(2016A030312002)
National Key R&D Program of China (Intergovernmental Cooperation Project)(2017YFE0132200)
National Key R&D Program of China(2017YFA0303500)
Natural Science Foundation of Guangdong Province(2019B030301003)
Science and Technology Foundation of Shenzhen City(JCYJ20190808153415062)
Fundamental Research Funds for the Central Universities(2020-KYY-511108-0007)
China Postdoctoral Science Foundation(2019M653036)
University of Science and Technology of China startup funds(KY2340000139)
Innovation and Technology Commission of Hong Kong(ITC-CNERC14S01)
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