1 Introduction
Aggregation is a relatively stable state formed by the arrangement of single molecules under certain temperature and pressure conditions, which shows certain commonality in many research fields
[1]. Among them, organic polymer optoelectronic functional materials are widely used in optoelectronic devices, information security, life and health and other fields in the form of thin films, nanoparticles, solid powder and other aggregation forms
[2]. "How to achieve the aggregation of regulatory molecules?"? How does aggregation affect the performance of optoelectronic functional materials? How to carry out molecular design from the perspective of molecular aggregation regulation? It is an important research content to improve material properties and rational molecular design.
Molecular aggregation phenomena exist widely, such as crystalline state, liquid state and even liquid crystal transition state of macromolecules or small molecules. The structure of molecular aggregation is difficult to characterize, but it significantly affects the properties of materials. For example, slowly cooled polyester sheet (PET) is brittle, while rapidly cooled biaxially stretched polyester film shows good toughness. Similarly, for small molecular luminescent materials, although the molecular structure determines the basic properties of the material, the difference of molecular aggregation structure also leads to different or even opposite properties of the compound
[3⇓~5]. The photophysical properties of single molecule are diametrically opposite to those of its aggregation state, which enlightens us that besides the molecular structure itself, the study of the structure-property relationship of its aggregation state should also be fully concerned. The exploration of the law of single molecule-aggregation state-photoelectric properties will promote the further understanding and transformation of the material world in chemical science, which is the key to the development of high-performance photoelectric materials
[6].
The aggregation forms of organic molecules are rich and diverse, and the study of the composition and structure of different aggregation forms is the difficulty and breakthrough point in the study of structure-activity relationship. At present, the structure of molecular aggregation in crystals can be well characterized and studied by X-ray single crystal diffraction, including the packing mode of molecules and the interaction between molecules. On this basis, force, light, heat and medium molecules in the environment can also achieve partial regulation and functional response to molecular aggregation, which provides an effective way to explore the structure-activity relationship (Figure 1). In 2018, our research group proposed the concept of MUSIC (Molecular Uniting Set Identified Characteristic) based on a large number of studies on the regulation of Molecular aggregation state. In analogy to musical notes, atoms are covalently linked in specific sequences to form organic molecules, just as different notes form melodies, and organic molecules form aggregates in different forms, just as melodies form music. Through the optimization and regulation of molecular structure and aggregation mode, the aggregated molecules produce excellent properties, which is the unique "music" of the microcosm. This concept vividly emphasizes the importance of molecular aggregation to optoelectronic properties, and special aggregation structures can even reflect unique properties beyond single molecules. Among them, organic room temperature phosphorescence (RTP) shows high dependence on the molecular aggregation structure and fast response. In recent years, we have systematically studied the structure-activity relationship among molecular structure-aggregation behavior-luminescent properties
[7,8].
Phosphorescence is a slow photoluminescence process, and for organic materials, there is a spin-forbidden from singlet state to triplet state and a large energy level difference, so RTP has a large Stokes shift and a long luminescence lifetime, and has good application prospects in the fields of biological imaging, photodynamic therapy, information encryption and anti-counterfeiting
[9]. However, for most pure organic RTP materials, RTP can hardly be observed in the monomolecular state (such as dilute solution or as a guest dispersed into the host), because the molecule is prone to non-radiative transition in the solution state, resulting in energy dissipation, and the excited triplet state is easily quenched by water, oxygen, etc. In the aggregated state, the rigid environment of the molecule can effectively alleviate the above problems, so it is easy to show long afterglow in crystals or films (aggregated state)
[10]. Moreover, the coupling between molecules can effectively stabilize the excited triplet state, and realize the optimization and improvement of phosphorescence color change, lifetime and efficiency (Fig. 2).
In this paper, RTP is used as a macroscopic functional display of molecular aggregation structure changes, and the composition and core influencing factors of molecular aggregation structure are systematically discussed in combination with the related work of our group in recent years, and on this basis, the study of aggregation is extended to dynamic stimulus response processes and complex systems. The study of aggregation science will effectively improve the controllability and predictability of the design and synthesis of optoelectronic functional materials, and promote the further development of the field of functional materials.
2 Study on the molecular aggregation structure of organic room temperature phosphorescent materials
2.1 Intrinsic Mechanism and Performance Regulation of Molecular Aggregate Structure Formation
2.1.1 Molecular conformation
The study of molecular aggregation includes the analysis and regulation of its arrangement and intermolecular interaction, and the optimization of its performance. The change of molecular structure, especially the regulation of molecular conformation, is an important way for endogenous fine regulation of aggregation state. At room temperature, the thermal motion of organic molecules makes multiple conformations coexist dynamically. For organic compounds with multiple dominant conformations, through the regulation of crystallization process, such as solvent, growth rate, etc., homogeneous polycrystals with different molecular conformations can be obtained, reflecting different intermolecular interactions and stacking modes. A Phenothiazine derivative CzS-CN containing a benzonitrile substituent was prepared by our research group, and a needle-like crystal a was cultivated in a mixed solvent of dichloromethane, methanol, and ethyl acetate, a block-like crystal B was cultivated in a single solvent of dichloromethane, and a plate-like crystal C was cultured in a mixed solvent of methylene chloride, n-hexane, and methanol.Their RTP properties show large differences, with phosphorescence lifetimes of 266 ms (crystal A), 41 ms (crystal B), and 32 ms (crystal C) and luminescence quantum yields of 22.6% (crystal A), 17.8% (crystal B), and 6.9% (crystal C), respectively (Fig. 3)
[11]. Single crystal analysis shows that the molecular conformation and arrangement of the three crystals are quite different. The adjacent molecules in crystal A form a rigid aggregation structure through strong π-π interaction and intermolecular hydrogen bonding, which effectively suppresses the non-radiative transition, thus achieving a long lifetime and a high quantum yield. The intermolecular interaction in crystal B is obviously weakened, and the phosphorescence lifetime and quantum yield show a decreasing trend. In contrast to the staggered packing of crystal A and crystal B, the antiparallel packing is observed in crystal C, which is associated with the enhanced planarity of the molecular conformation. The theoretical calculation shows that this conformation is not conducive to the intersystem crossing process, so that crystal C shows the weakest room temperature phosphorescence properties. Recently, we designed and synthesized the compound PTZ-3Cl (Fig. 4) by modifying phenothiazine, and successfully obtained two polycrystals, in which the phenothiazine group is in the quasi-equatorial (eq) and quasi-axial (ax) conformations, respectively
[12]. Under the irradiation of 365 nm ultraviolet lamp, PTZ-3Cl-eq and PTZ-3Cl-ax crystals showed blue and green fluorescence, respectively. After the ultraviolet lamp was turned off, PTZ-3Cl-eq had no afterglow, while PTZ-3Cl-ax had a long green afterglow of about 0. 3 s, and the phosphorescence quantum yield could reach 12. 68%. Different molecular conformations endow the compounds with different electronic properties and excited state energy levels. The energy gap between T
1 and T
2 of PTZ-3Cl-eq crystal is large (0.51 eV), which hinders the internal conversion from T
2 to T
1, resulting in the absence of room temperature phosphorescence. For PTZ-3Cl-ax crystal, the smaller energy gap between different triplet States is beneficial to the transition of triplet excitons, and the π-π coupling between adjacent molecules can stabilize the excited triplet States, thus reflecting the efficient RTP performance. In other words, the cooperation of the energy level characteristics and the packing mode leads to the conformation-dependent RTP property. Interestingly, the eq conformation of phenothiazine can be converted to the ax conformation under dichloromethane fumigation, and the ax conformation can also be converted to the eq conformation by heating (80 ℃), and the presence or absence of RTP properties of phenothiazine can be converted between the two polymorphs.
The difference of molecular aggregation behavior can not only change the performance of RTP, but also produce different optical functions. 9,9-Dimethylacridine (DMAC) is an anthracene compound similar to phenothiazine, and its derivatives can also exhibit folded or planar conformations, but the two conformations rarely coexist
[13]. Our research group prepared the compound DBTDO-DMAC by connecting DMAC with dibenzothiophene sulfone unit (DBTDO) through a carbon-nitrogen single bond, and cultivated two kinds of homogeneous polycrystals (crystal G and crystal Y) by controlling the volatilization rate of dichloromethane and n-hexane mixed solvent (Figure 5A)
[14]. In crystal G, the DMAC group is almost in a planar conformation, the molecules are in a staggered packing, the intermolecular interaction is weak, and only a weak room temperature phosphorescence phenomenon occurs. Under the stimulation of external force, the molecular packing mode of crystal G is changed, accompanied by Mechanochromism (MC). In crystal Y, the DMAC group is in a folded conformation, and the molecules are in an antiparallel packing. The π-π interaction and the intermolecular hydrogen bonding interaction not only stabilize the molecular conformation, but also inhibit the non-radiative transition. The room temperature phosphorescence lifetime of crystal Y is increased from 13 μs (crystal G) to 3. 17 ms.
In addition, our research group also designed and synthesized a formate-substituted triphenylamine derivative TPA-o-3COOMe, and the bulk crystal A was cultivated in dichloromethane single solvent, and the plate crystal B was cultivated in dichloromethane and n-hexane mixed solvent (Figure 5B)
[15]. Crystal A exhibits visible yellow room temperature phosphorescence (lifetime of 33 ms), while crystal B exhibits Mechanolumine luminescence (ML). Single crystal analysis shows that there are two conformations in crystal A, and the molecules form a close antiparallel arrangement. There are six conformations in crystal B, and the irregular loose packing makes the interaction between adjacent molecules relatively weak, which is not conducive to the generation of room temperature phosphorescence properties, but the overall asymmetry gives it piezoelectric effect, thus it has mechanoluminescence properties.
2.1.2 π-π interaction
According to classical perturbation theory, when two π systems interact with each other, the corresponding frontier molecular orbitals will split and recombine to form new molecular highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
[16]. The electrons of the coupled π system redistribute after being excited to form a new excited state. On the other hand, the specific discrete π-π interaction is beneficial to suppress the non-radiative transition, promote the generation of room temperature phosphorescence and prolong the phosphorescence lifetime. In general, we use adjacent molecules as a simplified model for the study of aggregation structure to explore the π-π interaction between molecules and their aggregation behavior.
In 2020, our research group reported the dimethyl isophthalate-substituted phenothiazine oxide derivative CS-2COOCH
3 w ith luminescent properties such as fluorescence, thermally activated delayed fluorescence (TADF), and room temperature phosphorescence, revealing the room temperature phosphorescence excitation process and the first attempt to reveal the special role of the molecular dimer (Figure 6A)
[17]. The compound exhibits blue fluorescence under an ultraviolet lamp, and a blue afterglow of more than 1 s can be observed after the ultraviolet lamp is turned off. Photoluminescence measurements show that the fluorescence emission peaks of the crystal are located at 405 and 505 nm, and the corresponding lifetimes are 1.27 and 10.6 μs, respectively, indicating TADF characteristics. The RTP emission peak is at 430 nm, and the phosphorescence lifetime is 91 ms (Fig. 6B, C). When CS-2COOCH
3 was doped into poly (methyl methacrylate) (PMMA) with a mass fraction of 5%, the maximum emission wavelength was only 420 nm, and the fluorescence lifetime was 0. 97 μs. Based on the fluorescence lifetime and quantum yield of the CS-2COOCH
3, the authors estimated the radiative (k
r) and non-radiative (k
nr) fluorescence rates of the compound in different States, and demonstrated that the short wavelength fluorescence emission (405 nm) and room temperature phosphorescence (430 nm) of the crystal originate from isolated single molecules, while the long wavelength fluorescence emission (505 nm) originates from excimers generated by neighboring molecules through π-π interactions.
In general, the room temperature phosphorescence spectrum of most organic molecules shifts to the long wave direction due to π-π stacking in the aggregated state, which leads to the lowering of the excited state energy level (Fig. 6e). Substituent regulation is an effective way to optimize the π-π interaction between adjacent molecules, and then adjust the photoelectric properties of compounds. Our research group has designed and synthesized a series of phenothiazine oxide derivatives with different electronegative substituents. With the substituents changing from methoxy and methyl with electron-donating effect to bromine, chlorine and fluorine with electron-withdrawing characteristics, the π-π distance between molecules in the corresponding compound crystals decreases from 4.251 251Å to 3.677 677Å, and the RTP lifetime increases from 88 ms to 410 ms (Fig. 6f)
[7]. The experimental results show that the introduction of electron-withdrawing substituents can effectively reduce the repulsion between π systems, promote π-π interaction, and stabilize the excited triplet state, thus promoting the growth of RTP lifetime.
2.1.3 Hydrogen bond
The classical hydrogen bond is an intermolecular interaction in the form of X-H … Y, in which a hydrogen atom is covalently bonded to an X atom with a large electronegativity and then approached by an Y atom with a large electronegativity (Fig. 7A). Hydrogen bonding can enhance the rigidity of molecular aggregation structure, prolong the lifetime of RTP and improve the brightness of RTP by inhibiting the non-radiative transition of molecules, and even realize the room temperature phosphorescence effect of non-aromatic systems. For example, the non-aromatic small molecule cyanoacetic acid (CAA) can form a layered stacking and network hydrogen bond system through intermolecular O-H … O hydrogen bonds and dipole-dipole interactions, which promotes the coupling between functional groups and effectively inhibits the molecular motion, thus realizing the long afterglow luminescence in the crystalline state with a lifetime of 862 ms (Fig. 7B)
[18]. For common aromatic chromophores, such as phenylboronic acid derivatives (PBA series), the lifetime can be extended from 0.13 s to 2.24 s by changing the strength and number of O-H … O and C-H … O hydrogen bonds formed between boronic acids and between boronic acids and alkoxy groups (Fig. 7C)
[19].
In addition, there are weak hydrogen bonds such as C-H … N, C-H … O, C-H … π, and C-H … X (X = F, Cl, Br, I) between nitrogen and oxa-aromatic rings, as well as common building blocks such as carbonyl and halogen, which play an important role in coordinating the fine structure of molecular packing. As shown in Fig. 8A, different substituents were introduced to the 9,9-dimethylanthracene nucleus (XCO) to promote the intermolecular interaction, especially the C-H … O hydrogen bonding, which realized the fine control of the intermolecular distance and the optimization of the arrangement mode, and greatly improved the room temperature phosphorescence lifetime from 52 ms to 602 ms
[20].
In weak hydrogen bonding, the alkyl chain also plays an important role. As shown in fig. 8B, in the PIth-C1 ~ PIth-C8 series compounds, different alkyl chains from methyl to octyl are introduced to the aromatic group of phenanthroimidazole (PI)
[21]. These derivatives all show RTP emission in the long wavelength region (520 ~ 634 nm), but only PIth-C5 and PIth-C6 can observe obvious orange-red afterglow by naked eyes. In addition, PIth-C3 and PIth-C4 also have mechanoluminescent properties. Based on the negligible electronic effect of the alkyl chain and the slight influence on the luminescent properties of the molecule, the change of these photophysical properties mainly comes from the regulation of the molecular aggregation structure. With the extension of the alkyl chain, the dihedral angle between the thiophene ring and the planar phenanthroimidazole (PI) skeleton increases from 34.94 ° to 58.42 ° and then decreases to 18.66 °, showing a trend of first increasing and then decreasing. At the same time, their packing pattern gradually changed from crossed to anti-parallel, and finally showed parallel packing. Among them, PIth-C5 and PIth-C6 show a symmetrical head-to-tail stacking mode, and the C-H … π interaction is significantly increased, which effectively suppresses the non-radiative transition caused by the vibration of the long alkyl chain, and PIth-C6 achieves a red RTP with a lifetime of 14. 54 ms. However, the RTP of PIth-C7 and PIth-C8 was relatively weak, and the lifetime was shortened to 0. 02 and 0. 49 ms, which was mainly attributed to the weakening of the intermolecular interaction by the steric hindrance of the alkyl chain and the intensification of the non-radiative transition of the alkyl chain itself with the increase of the chain length. On the other hand, the difference of the stacking mode of the alkyl chain itself can also realize the effective regulation of the π-π coupling between molecules. We have observed the odd-even effect of the RTP properties of this series of compounds by introducing alkyl chains with different lengths from methyl to hexyl (CS-C
n ) to the nitrogen atom of phenothiazine: the RTP lifetimes of phenothiazine derivatives with odd-carbon alkyl substituents are longer than those of the adjacent compounds with even-carbon alkyl substituents
[22]. Single crystal analysis showed that odd-carbon alkyl-substituted compounds have stronger π-π interactions (Figure 8 C). In addition, a C-TPA compound with an RTP lifetime of 30 ms was obtained through the ring locking effect of the alkyl p-triphenylamine unit (TPA), and its RTP lifetime was further extended to 184 and 288 ms by the introduction of methoxy (C-TPA-OMe) and methyl (C-TPA-Me) substituents (Fig. 9a)
[23]. Theoretical calculations show that for C-TPA and its derivatives, although the energy level difference between the excited singlet (S
1) and triplet (T
1) States is larger than 0.3 eV,However, both the S
1state and the high-energy triplet state (T
n) have charge transfer (CT) characteristics, and the energy level difference between them is small, so the ISC process occurs in the form of
1CT-
3CT, and finally transitions to T
1.
At the same time, the change of substituents also adjusts the intermolecular interaction and stacking mode. The strong intermolecular interaction in C-TPA-OMe crystal can effectively suppress the non-radiative transition, resulting in a luminescence quantum yield of 83. 55%, and the unique layered fishbone stacking of C-TPA-OMe endows it with a longer phosphorescence lifetime (288 ms).
The position of the substituent also has an important effect on the photophysical properties of the compound. Benzophenone is a common unit for constructing RTP molecules, and BP-o-BO, BP-m-BO, and BP-p-BO can be prepared by introducing borate at the ortho, meta, and para positions, respectively (Fig. 9b)
[24]. The position difference of substituents affects the energy levels and intermolecular hydrogen bonds of the compounds, and then affects the generation and radiation process of triplet excitons, thus realizing the regulation of luminescence quantum yield and RTP lifetime. The (Φ
A=0.6%,τ=53 ms)RTP performance of BP-o-BO(Φ
A=0.5%,τ=63 ms) and BP-m-BO shows obvious differences. Two kinds of crystals with different luminescence properties can be obtained from BP-p-BO: crystal A(Φ
A=11.3%,τ=17.1 ms) and crystal B(Φ
B=6.9%,τ=3.3 ms). Crystal structure analysis shows that the intermolecular hydrogen bonds in crystal A are significantly increased, which can effectively suppress the non-radiative transition, thus effectively improving the quantum yield and lifetime of RTP.
2.2 Study on the structure-activity relationship of organic room temperature phosphorescent materials
How to realize the full wavelength emission from blue to red, ultra-long phosphorescence lifetime and brighter phosphorescence emission of RTP materials is the main development direction and research focus in this field. In order to achieve different emission colors, different structural units are usually selected: phenylboronic acid compounds are usually used to prepare blue-green RTP materials, phenothiazine and dibenzothiophene derivatives are usually used to prepare yellow-green RTP materials, and naphthoimide compounds are usually used for RTP materials with redder wavelengths. Rational molecular design is the key to obtain efficient RTP materials. However, it is impossible to explain the difference of luminescence properties of different crystals of the same compound only from the perspective of molecular design.
The study of structure-activity relationship of "molecular structure-aggregation behavior-luminescence properties" is very important for further understanding of the above phenomena. Firstly, through the study of the properties of polymorphs, our group discussed the multiple changes of intermolecular interaction, stacking mode and excited state energy level caused by the difference of molecular conformation.Furthermore, the adjustment of π-π interaction by different electronegative units, the effect of hydrogen bonding on the rigidity of molecular aggregation, and the effect of alkyl chain on molecular packing are discussed. A series of studies have shown that for RTP materials, in addition to the inherent characteristics of the molecule itself, the optimization of molecular packing mode, effective π-π coupling and strong intermolecular interaction have a great impact on its luminescent properties, giving it unique photophysical properties beyond single molecules.
In general, the difference of molecular conformation changes the excited state energy level and ISC process, and affects the generation of triplet exciton. The intermolecular interaction effectively stabilizes the triplet exciton and reduces the energy loss of the non-radiative transition; The change of molecular aggregation structure can change the interaction between molecules, realize the regulation of energy levels, and then optimize the color, lifetime, quantum yield and so on of RTP materials. The study of the structure-property relationship of "molecular structure-aggregation behavior-luminescence property" is the basis of molecular design and optimization of RTP materials, and also inspires the study of other photophysical processes. Therefore, only focusing on the chemical structure of molecules has been unable to meet the in-depth study of a variety of luminescent properties. In order to better understand this process, we must pay attention to the structure of the molecule itself, and at the same time, we must study the structure and composition of the substance aggregation state and its structure-activity relationship at multiple levels through different ways and means.
3 Dynamic Regulation of Molecular Aggregation
The structure of molecular aggregation state can be regulated by molecular conformation, intermolecular interaction and stacking mode, which reveals the structure-property relationship of molecular aggregation state and shows its "static" regulation process. External stimuli (such as force, light, electric field, etc.) Can also cause changes in molecular aggregation, resulting in unique photophysical properties, and this "dynamic" regulation process has also attracted more attention.
3.1 Force stimulus response mechanism
Force stimulation mainly refers to the stimulation of materials by forces such as friction, tension, compression and impact. When an external force is applied, the crystal is broken, and the aggregation structure of the molecule changes significantly, mostly reflected in the planarization of the distorted conformation, the transformation from crystalline state to amorphous state, etc., accompanied by the change of the excited state energy level of the organic molecule, resulting in the change of photoluminescence color, that is, the mechanochromism (MC) phenomenon
[25,26]. Our group has designed and synthesized a series of pyrene derivatives, which have different aggregation modes due to the steric and electronic effects of substituents, and also have different response behaviors to force stimulation (Fig. 10)
[27].
The aldehyde substituted compound PPCHO was transformed from a non-emissive crystalline state (λPL=424 nm,ΦPL=0.05%) to a bright cyan microcrystalline state (λPL=461 nm,ΦPL=20.07%) by grinding. High pressure experiments were further used to explore that, unlike the grinding process, high pressure does not destroy the arrangement of crystals, but can reduce the distance between molecules and make the molecular conformation more planar. As shown in Fig. 10 d, e, with the increase of pressure from 0.00 GPa to 17.01 GPa, the luminescence of the crystal is red-shifted and gradually weakened; When the high pressure is gradually released to the normal pressure, the emission wavelength is blue-shifted and gradually enhanced, and the emission of the final state is red-shifted and brighter than that of the initial state without pressure, which is closely related to the generation of excimers.
In addition to mechanochromism, force can be used as a stimulus to directly stimulate the luminescence of compounds, namely mechanoluminescence (ML). This phenomenon was discovered and reported by France Bacon as early as 1605. However, limited by the detection conditions and the weak luminescence intensity of organic electroluminescent materials, the internal mechanism has not been unified. It is generally believed that lattice defects, special molecular packing and piezoelectric effect are the key to the generation of mechanoluminescence
[28].
Aggregation-induced luminescence (AIE) units such as tetraphenylethylene and triphenylamine are mostly used as the host framework of force-responsive materials to achieve high luminescence of compounds in the solid state and to give the molecular conformation changeable under force stimulation. In 2016, our research group designed and synthesized a tetraphenylethylene derivative TMPE, and cultured Cp crystals and Cc crystals from dichloromethane/methanol and dichloromethane/n-hexane mixed solvents, respectively, which had similar blue fluorescence, but only the former showed ML performance (Fig. 11A)
[29]. Single crystal analysis shows that the ethylene bonds of adjacent molecules in Cc crystal are parallel to each other, while the adjacent molecules in Cp crystal are nearly perpendicular to each other. The tighter packing of Cp crystals and their more intermolecular interactions (C-H … π and C-H … O) greatly reduce the non-radiative transitions under force stimulation, resulting in ML performance. In addition, the maximum emission wavelength of Cp crystal and Cc crystal is red-shifted from 429 nm and 420 nm to the same 489 nm after grinding, and the similar mechanochromic properties are attributed to the transformation of molecules from crystalline state to amorphous state.
How does the aggregation structure affect the properties of organic mechanoluminescent materials? The discussion of structure-activity relationship is the key
[30,31]. We prepared 10 TPA-X compounds with triphenylamine as the host and different substituent units (Figure 11 B)
[32]. It is found that the non-centrosymmetric single crystal structure is the key to the mechanoluminescence performance, and the rigidity of the crystal effectively reduces the energy loss caused by the non-radiative transition, which is conducive to the production of bright mechanoluminescence. However, if the molecular packing is too tight, the good structural stability makes it difficult for external stimulation to change its aggregation state, but it can not produce mechanoluminescence. After that, we further introduced the thiophene group with self-assembly properties to prepare the tetraphenylethylene derivative tPE-2-Th (Figure 11C)
[33]. The compound shows extremely sensitive blue mechanoluminescence due to the non-centrosymmetric herringbone-like layer packing and the close interlayer packing, and the quantitative relationship between the pressure and the luminescence intensity of the organic mechanoluminescence material is established for the first time.It can be applied to information encryption, impact alarm, heartbeat detection and so on, which effectively promotes the development of organic electroluminescent materials and expands their applications.
Multiple dynamic photophysical properties can also be produced during mechanical stimulation. For example, mechanical force can promote the coupling between adjacent molecules of phenylboronic acid ester derivative DPP-BO, enhance its ISC process, and successfully realize the first organic mechanophosphorescence (Fig. 12A)
[34]. The brominated fluorene derivative BrFlu-CBr exhibits three different luminescence properties under mechanical stimulation: a cyan mechanoluminescence is initially observed, which comes from the fluorescence-phosphorescence dual emission of the crystal; After the external stimulation is stopped, white force-induced phosphorescence is displayed; With continued force stimulation, the crystal gradually turned into a powder, exhibiting only blue force-induced fluorescence (Fig. 12 B)
[35]. In-depth study of the single crystal structure and theoretical calculations show that the strong Br … Br interaction promotes the ISC process of the molecule, resulting in force-induced phosphorescence.The color change of mechanoluminescence is mainly attributed to the different composition of fluorescence and phosphorescence, which is related to the destruction of the regular molecular arrangement in the crystal under force stimulation and the weakening of some intermolecular interactions. Under the stimulation of force, in addition to the change of intermolecular interaction, it can also produce the change of molecular conformation. We took phenothiazine compounds with ax and eq conformations as the research objects, and used their conformational variability to explore the effect of force stimulation on the aggregation structure.
Under continuous force stimulation, the mechanoluminescence of phenothiazine derivative FCO-CzS changes from blue to white and then to yellow. Referring to the photoluminescence properties, the change in the mechanoluminescence process is mainly attributed to the conformational transition of phenothiazine from ax to eq under continuous force stimulation (Fig. 12C)
[36]. In addition, the molecular conformational transformation under force stimulation can even realize the process of organic room temperature phosphorescence from scratch. As shown in Figure 12d, the crystal of phenothiazine derivative CzS-Ph-3 F only has blue fluorescence, and the molecular conformation is changed by grinding, and an obvious green afterglow appears, showing a unique force-induced room temperature phosphorescence effect
[37].
3.2 Light stimulus response mechanism
Stimulation produced by infrared, ultraviolet, visible light, X-ray and other light sources belongs to the broad category of light stimulation. Among them, ultraviolet-visible light is the main excitation source (200 ~ 400 nm) of organic optoelectronic functional materials, and it is of great significance to explore its influence on the structure and properties of molecular aggregation (Fig. 13A).
In previous studies, we have found that the introduction of electron-withdrawing substituents is beneficial to the π-π stacking of phenothiazine oxide derivatives and promotes their room temperature phosphorescence properties. Further, CF
3 substituents with stronger electron-withdrawing ability were introduced to the benzene ring to prepare CS-CF
3 compounds (Fig. 13B)
[7]. Unexpectedly, the CS-CF
3 crystal showed almost no RTP phenomenon. When the crystal is irradiated by an ultraviolet lamp for 5 min, it will produce a visible green afterglow with a lifetime of 299 ms. This is the first report of light-induced organic room temperature phosphorescence. The crystal of photoinduced room temperature phosphorescence can be restored to the initial state within 2 H by the thermal motion of molecules. The comparison of the single crystal structure of CS-CF
3 before and after UV irradiation for 5 min shows that the distance between the benzene rings in the adjacent molecules in the crystal is reduced from 3. 999 999 Å to 3. 991 991 Å, which preliminarily proves that the molecules have a slight movement under the irradiation, changing the structure of the molecular aggregation state. Combined with other experimental results, it can be confirmed that light can regulate the structure of molecular aggregation and the properties of materials. We also designed and synthesized a triphenylamine derivative TPA-B, which also achieved a yellow afterglow from scratch after the crystal was irradiated by an ultraviolet lamp for 20 min, and the phosphorescence lifetime increased from 5.32 ms to 211.13 ms (Fig. 13C)
[38]. The analysis of the crystal before and after UV irradiation at room temperature shows that the molecular conformation of the crystal is more distorted, the distance between adjacent molecules is reduced, and the C-H … O and C-H … π interactions are significantly enhanced, which can effectively inhibit the non-radiative transition of the excited state, thus achieving a lasting RTP effect.
In addition to adjusting the aggregation state composition of the molecule, our research group has also realized a sensitive photoinduced deformation process by using the structural changes of intramolecular proton transfer and trans isomerization in the excited state of Schiff base compounds, and the doped film can achieve a light-driven contraction of up to 70% and a bending angle of about 141 ° (Fig. 14A)
[39]. In addition, light stimulation can also trigger the photochemical reaction of compounds, which can lead to significant changes in the structure and composition of the aggregation state, and produce different photophysical properties. For example, the two benzene rings on the same side of the triphenylethylene double bond can be closed under light excitation and show a change in their own color, that is, photochromism. We selected this unit as the skeleton and combined different substituents to prepare six triphenylethylene derivatives, which realized the multifunctional photoresponse characteristics of photochromism, photodeformation and room temperature phosphorescence (Fig. 14B)
[40]. Among them, TPFBr2 with trifluoromethyl and dibromo substituents has the highest absorbance after UV irradiation, and changes from white crystals to red crystals, which has the most obvious photochromic phenomenon. The PET film (2.5 wt%) containing TPMBr2 with methyl and dibromo substituents has the largest photoinduced deformation, with a bending angle of 130 °; TPMBr containing methyl and monobromo substituents has the longest RTP emission with a lifetime of 297 ms. These properties are closely related to molecular conformation, intermolecular interactions, and packing patterns. As the photoactive unit of the photocyclization process, the dihedral angle of the two benzene rings on the same side of the triphenylethylene double bond significantly affects the planarization of the molecular conformation, which, combined with the intermolecular interaction, affects the photocyclization process of triphenylethylene derivatives. As shown in Figure 14b, the dihedral angle between the photoactive benzene rings of TPM is 64 °, and the twisted structure is not conducive to the photocyclization reaction, but the intermolecular interaction is weak, which gives the flexibility of molecular rotation and shows a certain color change caused by photocyclization. The photoactive benzene ring of TPF has a dihedral angle of 62 °, and the strong intermolecular interaction (C-H … F) hinders the molecular motion, making it difficult for the photocyclization process to occur. The photoactive benzene ring in TPMBr2 has the smallest dihedral angle (54.54 °), which is the most prone to photocyclization and shows the most obvious photochromic properties. Moreover, the promotion of bromine atoms to the ISC process, the C-H … Br interaction between molecules, and the compact packing form are conducive to the generation and stability of triplet excitons, thus having RTP performance. In addition, short-wavelength ultraviolet light can also promote the production of organic molecular free radicals. Recently, we designed and synthesized three triphenylamine derivatives, DPANa, DNaAPh, and TNaA, and doped them into the PMMA host. As shown in Figure 14C, DNaAPh and TNaA will form cation radicals under the continuous irradiation of 395 nm ultraviolet light, and the change of absorption will change the film from colorless to green
[41]. At the same time, ultraviolet irradiation can also dissipate triplet oxygen, effectively inhibit the quenching of triplet excitons by oxygen, and endow the material with RTP performance
[42]. The DPANa in this system can achieve efficient ISC, showing light-activated room temperature phosphorescence (Φ
p=3.47%,τ
p=122 ms) in combination with the dissipation effect of ultraviolet light on oxygen.
3.3 Electric field stimulation response
Electric field poling is widely used in the field of organic second-order nonlinear optical (NLO) materials. Under an applied electric field, the film is heated to the glass transition temperature of the polymer, so that the chromophore, which is an effective building unit, tends to be arranged in a non-centrosymmetric manner with the same dipole moment. The obtained film with NLO activity has great application prospects in the fields of ultrafast optical switching, high-speed optical modulator, high-density optical data storage, etc. (Fig. 15A). In general, the generation of ideal noncentrosymmetric arrangements is severely hindered by the presence of strong dipole-dipole interactions between polar chromophores. How to optimize the aggregation structure of second-order nonlinear materials to improve their optical effects?
For most organic second-order nonlinear optical materials, the introduction of "suitable spacer" can partially enhance the macroscopic second-order nonlinear optical effect of the poled film and enhance the stability of the directional non-centrosymmetric arrangement of chromophores in the poled film. The concept of a suitable spacer originates from the principle of "site separation", which is closely related to the interaction between chromophores and can be regulated by the shape of chromophores. The principle of "site separation" points out that a group which does not affect the first order hyperpolarizability β of the chromophore can be introduced into the chromophore as a spacer through chemical modification to change its space conformation.O as to weaken the strong dipole-dipole electrostatic interaction between chromophores, thereby improving the macroscopic second-order nonlinear optical effect of the material. On this basis, our research group has designed and synthesized a series of side chain, main chain and pendant polymers, which introduce different sizes of spacer groups at different chromophores and different connection positions.The structure-activity relationship between the size of the spacer and the macroscopic second-order nonlinear optical effect of the material was systematically studied, and the concept of "suitable spacer" was proposed through the summary and induction of a large number of experimental facts. That is, in a particular polymer, for a given chromophore and attachment site, there will be a most suitable group that allows the material to obtain the most effective macroscopic second-order nonlinear optical coefficient. However, the commonly used spacer groups are non-polar and have little contribution to the macroscopic second-order nonlinear optical effect. In order to further improve the second-order nonlinear effect of the material, we proposed the concept of "spacer chromophore", that is, a chromophore with a lower β value (i.e., spacer chromophores) can be used as a spacer for another chromophores with a larger β value (i.e., main chromophore).It can reduce the electrostatic interaction between chromophores and improve the degree of directional and orderly arrangement of chromophores in the polarization process. At the same time, chromophores with lower β values can still contribute to the macroscopic second-order nonlinear optical effect. In addition, the optical transparency of the polymer is also improved due to the lower beta value of the spacer chromophore. This can also be considered as an effective way to solve the contradiction between "nonlinearity" and "transparency" in the nonlinear field
[43].
For example, a series of NLO molecules M1 ~ M5 were synthesized through the "click" reaction, which use triazole as the spacer group and contain two different chromophores of FTC (cyanothiophene) and Azo (nitroazo) (Fig. 15C)
[44]. In the film, with the increase of temperature, the movement of chromophores is activated by the flexible chain as the constraint center, which shows a non-centrosymmetric arrangement under the action of electric field polarization. These five molecules all showed large NLO performance and excellent depolarization stability, and the d
33 value of M1 was up to 384 pm/V. In addition, we innovatively synthesized a new type of Janus dendrimer with triazole as the spacer group and nitroazobenzene as the chromophore (Fig. 15d)
[45]. The asymmetric character of the Janus dendrimer exactly fulfills the requirement for a noncentrosymmetric arrangement of chromophores in NLO polymers, and three dendrimers, D-13 N, D-17 N, and D-21 N, exhibit very high NLO performance: the NLO coefficient of D-13 N reaches 299 pm V
-1, the highest value for polymers containing only azo chromophores.
Two-terminal locking refers to the covalent connection of both ends of the donor-acceptor of multiple chromophores. Compared with the single-terminal locking chromophores, the two-terminal locking chromophore can fix the spatial orientation of the chromophore more thoroughly because it locks both ends of the chromophor at the same time. For example, the head and tail ends of the chromophore are bound together by a flexible chain to form a ring structure R1 (fig. 15B)
[46]. The molecule contains two cyanothiophene (FTC) units, which can effectively overcome the strong dipole-dipole interaction between molecules and arrange in a molecule in almost the same direction under the constraint of the molecular chain, contributing to the improvement of the macroscopic second-order nonlinear optical effect. The d
33 value of R1 is 562 pm V
-1, which is twice that of the "H" type contrast molecule H1. The stability of the directional non-centrosymmetric arrangement of chromophores in the poled film is also very good. This is attributed to the "double-stranded locking" structure of R1, which partially fixes the orientation of the chromophore, and its circular structure can also weaken the dipole-dipole interaction between molecules.
3.4 Environmental stimulus response
Environmental media can also affect the photophysical properties of substances. For example, the quenching effect of oxygen and water on the triplet exciton of RTP materials, and the introduction of acid or other solvent molecules can change the intermolecular interaction, so that the compounds show different luminescent properties.
Our research group skillfully combined triphenylethylene and thiophene units to prepare a molecule with thermal response, tPE-5-MeTh (Fig. 16A)
[47]. The P1 crystal with ML properties and the P2 crystal without ML properties were obtained by cultivation in different solvents, respectively. They have completely different packing modes and intermolecular interactions. In the non-centrosymmetric P1 crystal, the strong intermolecular interactions are conducive to the formation of an effective hydrogen bond network, and the whole crystal is layered and tightly packed. In the centrosymmetric P2 crystal, the molecules are more loosely packed antiparallel. Interestingly, after heating the compound to 90 ° C, the slowly cooled crystal has the same ML properties as P1, while the crystal obtained after heating to 95 ° C and cooling is similar to P2, without ML. This phenomenon is related to the fact that the compound has two phase transition temperatures at 87 and 92 ℃. The heat treatment successfully realized the rearrangement of the aggregated structure, and the presence or absence of ML properties was used as its macroscopic functional display.
The equilibrium state of molecular packing can also be broken by the introduction of medium molecules, such as water, acid, etc. The medium does not simply penetrate into the interstices of molecular packing, but forms new intermolecular interactions with the target molecules, inducing the target molecules to form completely different packing patterns, thus changing their properties. In 2020, our group designed and synthesized three phenothiazine derivatives with ortho-, meta- and para-substituents based on pyridine units with acid-base response characteristics. Among them, the CzS-o-py crystal exhibited an ultra-bright yellow RTP emission (Φ
P=15.36%,τ
P=52 ms), and the RTP disappeared after fumigation with hydrochloric acid vapor, showing the characteristics of acid stimulation response (Fig. 16b)
[48]. The crystals of CzS-o-py and the mixture of CzS-o-py and hydrochloric acid were obtained by solvent evaporation method. The packing of CzS-o-py crystal is relatively tight, while in CzS-o-py + HCl crystal, HCl and H
2O molecules participate in the molecular arrangement, forming a relatively loose packing mode and weak intermolecular interaction, which directly leads to the disappearance of RTP. HCl molecules inside the molecules can be effectively removed by NH
3 fumigation again, and the molecules are restored in a stacking manner and show persistent RTP.
In addition, we also designed and synthesized an organic pyridinium CPBBr compound, which has good solubility in both water and organic phases. Under the irradiation of ultraviolet lamp (365 nm), the compound emits weak dark blue fluorescence, and when it is placed in a sealed container containing saturated water vapor for 30 min, the fluorescence completely changes to bright turquoise (Fig. 16c)
[49]. The initial crystal CPBBr-W was obtained by solvent evaporation, and the green crystal CPBBr-G was obtained by adding a small amount of water to the mixed solution. Compared with the crystal structure, it is found that the molecular conformation of CPBBr is changed by the participation of water molecules in the crystal lattice, and the main difference in conformation comes from the flexible rotation of the benzyl moiety, which leads to the change of the crystal from orthorhombic (W) to triclinic (G), forming a completely different molecular packing mode. Moreover, the incorporation of water molecules into the CPBBr-G lattice greatly enhances the interaction between molecules and promotes the increase of luminescence intensity. This environmental stimulus response also exists in the multi-component system. We covalently bonded the arylboronic acid phosphorescent group with the polyvinyl alcohol polymer matrix to prepare the hydrothermal stimulus-responsive RTP material DPP-BOH-PVA (Figure 16D)
[50]. The rigid environment provided by the hydrogen bond and B — O covalent bond between the arylboronic acid and polyvinyl alcohol makes the RTP lifetime of the polymer film as long as 2. 43 s and the phosphorescence quantum yield of 7. 51%. However, due to the good water absorption of PVA, the hydrogen bonding interaction between adjacent DPP-BOH-PVA chains is destroyed when water is present, which reduces the rigid environment of the system and leads to the quenching of RTP instead. Intermolecular hydrogen bonds can be reconstructed by heating the membrane to remove water molecules, thereby restoring RTP performance.
In a word, the introduction of environmental medium molecules in the system, on the one hand, realizes the adjustment of molecular conformation and packing mode, on the other hand, affects the interaction between molecules. As mentioned above, these factors can lead to significant changes in the photophysical properties of materials, which further demonstrates the importance of the study of the relationship between the structure and properties of molecular aggregates and the universality of the concept of "MUSIC" in different photophysical processes and systems.
4 Regulation of Molecular Aggregation in Complex System
In a single-component system, whether the packing mode is regulated endogenously by structure, conformation, or intermolecular interaction, or by force, light, or electric field as an external stimulus, the fine regulation of molecular aggregation state can be achieved. Through the systematic discussion of structure-property relationship, it will not only promote the optimization of material properties, but also effectively guide the study of aggregation state of complex systems.
The aggregation state regulation of host-guest system mainly depends on the adjustment of molecular structure or the change of host-guest doping ratio. A series of phenothiazine (CzS, guest) and phenothiazine oxide (CS, host) derivatives with different alkyl and aromatic substituents were designed and synthesized by our group. With the substitution of substituents (R) from methyl, ethyl and alkyl,The RTP emission with long lifetime and high efficiency, up to 55%, was achieved by host-guest doping (M) with benzene ring substituents modified by methoxy, methyl, hydrogen, chlorine and fluorine
[51]. As shown in Figure 17A, taking the M-CH
3 as an example, different compositions show different RTP. The phosphorescence lifetimes of CS-CH
3 and CzS-CH
3 themselves are 245 and 1.01 ms, respectively, and the quantum yields are ≤ 1%. The co-crystal (M-CH
3) was formed by mixing at a mass ratio of 100:1, and its phosphorescence quantum yield was increased to 20%. The M-CH
3 nanocrystals constructed by the top-down approach via F127 embedding showed a long afterglow lasting 25 min in water. By adjusting the molecular structure of M cocrystal with substituents, the phosphorescence quantum yield can be increased from 20% to 55% and the RTP lifetime can be changed from 29 ms to 53 ms. Single crystal analysis shows that the host CS-R and the guest CzS-R have different structures but similar molecular conformations, which is beneficial to the formation of charge-separated States in their cocrystals. At the same time, the similar crystal structure of eutectic M-R and host CS-R is beneficial to maintain the rigid environment of the crystal itself. Moreover, in the co-crystal M-R, there are π-π interactions between the neighboring molecules of the host CS-R. These factors work together to make the co-crystal exhibit high phosphorescence quantum yield and long RTP lifetime. In addition, the change of doping ratio makes the luminescence color and efficiency significantly different (Fig. 17b), which further indicates the influence of aggregation composition on the material properties. In the same year, we designed three guest structures (Pph, BPph, DBPph) with different degrees of conjugation by adding one or two benzene ring units to the benzene rings on both sides of phenothiazine oxide, and prepared three RTP cocrystal :Pph-C(Ф
P=0.37%,τ
P=98 ms),BPPh-C(Ф
P=3.33%,τ
P=980 ms),DBPph-C(Ф
P=1.13%,τ
P=334 ms) with triphenylphosphine oxide (OPph
3) as the host (Figure 17 C)
[52]. The good crystallinity of the host provides a suitable rigid environment for the guest, which effectively suppresses the non-radiative transition of the guest. The difference of RTP performance of cocrystals is closely related to the energy transfer efficiency and the different aggregation modes caused by the molecular conjugated structure.
There is also a dynamic regulation process in the subject-object system
[53]. In 2020, our research group constructed a stimulus-responsive host-guest RTP system using resonance energy transfer (FRET) (Fig. 17d)
[54]. The host molecule (DMAP) has good rigidity, and the guest molecule (Cdp) has phosphorescence emission at low temperature. The RTP brightening process of the system can be realized by grinding with the mass ratio of 100 ∶ 1 of host-guest doping. This is because the force stimulation shortens the distance between the host molecule and the guest molecule, and the close packing makes the energy of the host molecule effectively transferred to the guest, thus showing good RTP performance, with a phosphorescence quantum yield of 4. 04% and a RTP lifetime of 768 ms. Heating can also turn on the RTP emission of the mixed sample. No significant RTP was observed when DMAP and Cdp samples were mixed at 25 ° C. With the increase of heating temperature, the FRET process between DMAP and Cdp was promoted, showing a green afterglow visible to the naked eye. After heating at 110 ℃ for 10 min, the phosphorescence quantum yield of the host and guest is 3. 16%, and the phosphorescence lifetime is 756 ms. When the guest material was replaced by N, N, N, N-tetramethylbenzidine (Tmb), the co-crystal showed bright blue phosphorescence with a long RTP lifetime of 2. 08 s and a phosphorescence quantum yield of 13. 4% when the host-guest ratio was 400 ∶ 1, which was one of the excellent RTP systems with high efficiency and long lifetime at that time. The authors further introduced additional receptors into the system and successfully achieved the change of afterglow color from blue to yellow. In view of the response of FRET and other processes to heat, the binary or ternary host-guest system can enhance the RTP effect by promoting the interaction between molecules through heating
[55].
The study of molecular aggregation state and structure-activity relationship promotes the optimization of different material properties, and effectively improves the controllability of the design and synthesis of optoelectronic functional materials. In various optoelectronic devices, the molecular aggregation structure also has a great influence on the device performance. For example, in perovskite solar cell (PSCs) devices, the film morphology of the hole transport layer is a key factor affecting its charge transfer and interface properties. The regulation of its morphology mainly depends on the different aggregation States caused by the change of molecular structure
[56]. The photoelectric conversion efficiency of the device can be improved from 18. 64% to 20. 16% by adjusting the peripheral alkyl chain of the Spiro derivative as a commonly used hole transport material
[57]. Recently, carbazole (Cz), dibenzocarbazole (DBC) and tetrabenzocarbazole (TBC) were used as building units, and the close packing of molecules was used to effectively improve the hole mobility, and the corresponding perovskite solar device efficiency increased from 18% to 20%
[58].
Classical organic light-emitting diodes (OLEDs) generally have a sandwich-like structure, in which a light-emitting layer is added between the cathode and anode, and a variety of functional material layers (such as electron injection layer, transport layer, hole injection layer, transport layer, etc.) Are introduced to optimize the device structure
[59]. Organic light-emitting layer is the core part of OLED device, which usually adopts host-guest doped multi-component system. On the one hand, it can reduce exciton quenching and weaken efficiency roll-off; On the other hand, it can optimize the energy level matching, balance the transmission of carriers, and ensure the maximum utilization efficiency of carriers. However, due to the complex spatial structure, the structure-property relationship of the device is still unclear, and its performance is mostly regulated by the type, position, and number of substituents
[60]. Recently, we designed and synthesized a series of blue-emitting materials with diphenyl sulfone as the acceptor and acridine derivatives as the donor, providing a simple way to optimize the device performance through the subtle adjustment of the side alkyl chain (Figure 18 A, B)
[61]. Among them, the DAc-C2 device (24.1%) has the best external quantum efficiency, which is 42% higher than that of DAc-C1 (17.0%). The external quantum efficiencies of the DAc-C3, DAc-C4 and DAc-C5 devices are 20.1%, 14.6% and 12.6%, respectively. Although it is impossible to clearly characterize the molecular aggregation mode of the light-emitting layer after doping and deposition, the packing mode of the molecules in the crystal state can still provide a corresponding reference to partially explain the different performance of the device.
The threefold six-membered ring structure of the acridine unit is prone to form a pseudo-planar bent structure. The dihedral angles of the two phenyl groups in acridine were selected to evaluate the planarity of the conjugated system, and the results showed that the alkyl substitution had a significant effect on the conformation of acridine and produced different packing modes. The authors propose that the changes in molecular conformation and packing affect the intermolecular interactions, and then affect the intramolecular charge transfer process: the acridine unit of DAc-C1 is bent, the adjacent molecules are stacked head-to-head, and there is a strong S = O … H interaction, which makes it difficult for intramolecular charge transfer to occur; The two conformations of DAc-C2 are disordered and tightly packed, which promotes intermolecular charge transport. The acridine configuration of DAc-C3 is relatively planar, and the hydrogen bonding interaction between DAc-C3 and the sulfone group is beneficial to intermolecular charge transfer, but the packing of DAc- C3 is relatively loose. The two different molecular configurations of DAc-C4 have different energy levels, and the planar conformation is beneficial to improve the emission efficiency of the material; The acridine of DAc-C5 is relatively bent, the molecule adopts loose head-to-edge packing, and the long alkyl chain blocks the intermolecular interaction, resulting in poor intermolecular charge transfer ability. Through the detailed analysis of the stacking mode of the compound, the influence of alkyl chain regulation on the device performance is partially explained, which provides a new way to achieve high-performance blue light-emitting materials.
5 Sum up
Molecular aggregation is a wonderful phenomenon. At present, chemical research has gradually changed from focusing on single molecules to molecular aggregation science. Single molecule forms different packing modes through a variety of intermolecular interactions, and different packing modes will affect the intermolecular interactions, which complement each other and play an important role in the generation and regulation of the unique properties of the aggregation structure. Molecular aggregation science is an important science to study this process, and it is also an important part of condensed matter chemistry.
In this review, organic room temperature phosphorescence is used as a macroscopic functional display of the change of molecular aggregation structure, the formation and regulation of molecular aggregation structure are systematically discussed, and the significant impact of stacking mode on performance is deeply analyzed. On this basis, the regulation of structure-activity relationship of materials is extended from static to dynamic, such as the use of force, light, electric field, environment and other stimuli to regulate the accumulation of molecules. Among them, mechanoluminescence, photoinduced room temperature phosphorescence and other dynamic processes further show that the macroscopic properties of organic optoelectronic functional materials depend on the aggregation form of materials to some extent. Through a large number of experiments and analysis, the dominant molecular aggregation structure of various photoelectric functional materials has been preliminarily established. For example, the strong intermolecular interaction and π-π stacking are beneficial to room temperature phosphorescence due to the suppression of non-radiative transitions and the influence of energy levels. Large dipole moment and non-centrosymmetric stacking are beneficial to mechanoluminescence, which effectively improves the controllability and predictability of the design of optoelectronic functional systems.
Therefore, the packing mode of aggregated molecules is an important part in the design of functional materials. In 2018, based on this change from single molecule to aggregation state, our research group proposed the concept of "MUSIC" to visualize the aggregation behavior of molecules with MUSIC, emphasizing the importance of molecular aggregation state regulation. At present, although the development of aggregate state science has made great progress, it still faces many challenges. (1) There are different conformations in the same molecule, and different packing modes cause great changes in properties. This provides a good discussion of the relationship between structure, but also brings new challenges-the control, separation and utilization of different conformations. (2) Due to the limitation of test methods, the change of accumulation during stimulation is still a difficult problem. For example, for mechanochromic materials, the force stimulation makes them change from crystalline state to amorphous state, and the accumulation mode of amorphous state can only be obtained by simulation, at the same time, the change of this dynamic process is difficult to capture. It limits the further discussion of the structure-property relationship of the material. (3) For complex systems, it is difficult to clearly characterize their internal packing modes, but through the analysis of the structure-activity relationship of the corresponding core materials, it is proved that the aggregation mode plays an important role in the performance of different systems. This inspires us to pay more attention to the "MUSIC" of molecules, adopt or develop appropriate technical means to establish accurate structure-packing-property relationships, and further provide a good theoretical basis for the optimization and practical application of materials.
National Natural Science Foundation of China(22122504)
National Natural Science Foundation of China(51973162)
National Natural Science Foundation of China(21734007)
PDF
