1 Introduction
Circularly polarized luminescence (CPL) has been the subject of intensive research due to its potential applications in novel optoelectronic devices, smart sensors, high-resolution 3D displays, information technology, etc
[1⇓~3][4,5][6,7][8⇓~10]. So far, researchers have developed a variety of CPL materials, mainly including rare earth metal-based coordination compounds, chiral organic molecules and other chiral small molecule systems, as well as chiral luminescent supramolecular assemblies
[11⇓~13][14⇓⇓⇓⇓~19][20⇓⇓⇓⇓⇓⇓⇓⇓⇓~30]. Supramolecular assembly is a process in which structural motifs spontaneously form ordered nanostructures by reversible noncovalent intermolecular bonding, such as hydrogen bonding, ionic dipole, coordination or coordination bonding, van der Waals force, host-guest interaction, π-π stacking, and hydrophilic-hydrophobic interaction
[31,32]. Therefore, supramolecular assemblies have become an attractive candidate for the development of CPL systems due to their adjustable and modifiable structures, simple and easy synthetic methods, and controllable and reversible assembly methods. There are three main methods to achieve circularly polarized luminescence in self-assembled supramolecular systems: (1) self-assembly of chiral luminescent molecules; (2) chiral transfer between chiral and achiral luminescent molecules; (3) Symmetry breaking in achiral luminescent molecules. Different from the stable covalently linked chiral luminescent small molecules, the supramolecular assemblies obtained by non-covalent bonding can exhibit reversible structures and morphologies under external stimulation, thus showing more flexible and variable circular dichroism (CD) and CPL characteristics. More attention has been paid to the reversible conversion of the excited state chirality of the assembly under external stimulation, because this stimulus-responsive CPL material has more advantages in many applications, such as information storage, which not only has the property of lossless readout, but also can significantly increase the amount of information storage. Stimulus-responsive chiral luminescent supramolecular assemblies usually possess at least two interconvertible stable States with different properties. This conversion between different States can be achieved by isomerization, redox reactions, electron transfer, coordination, assembly-decomposition, etc. Each state can be triggered by physical or chemical stimuli, including light irradiation, temperature, solvent, pH, and metal ions
[33,34][35,36][37,38][39,40][41,42]. When the responsive chiral supramolecular system switches between different States, it presents different CD and CPL signals.
The luminescence, chirality, structure, morphology and other characteristics of the assembly will change significantly when the environment changes or the assembly process is stimulated by certain external factors. The responsiveness of the chirality of the excited state of the assembly can be divided into three types according to the way in which the chiral signal is changed. The first one is the "on-off" type, in which the chiral signal switches between "on" and "off" States under various external stimuli. The second is the "amplification" type, a type of chiral assembly system that can reversibly enhance or attenuate the chiral signal in response to external stimuli. The third is the "inversion" type, in which the chiral signal changes from "positive" to "negative" and vice versa. Stimulus-responsive chiral supramolecular systems with CPL properties Unlike many other responsive systems based solely on absorption or fluorescence changes, changes in the chiral signal (CD and CPL) can serve as a new way of output. On the other hand, chirality inversion at the molecular level requires cleavage to form covalent bonds, whereas chirality inversion at the supramolecular level often occurs through a delicate balance of multiple noncovalent bonds. Therefore, researchers have constructed many stimuli-responsive chiral supramolecular systems, which not only provide a simple method to change the packing mode of assemblies by adjusting the aggregation driving force through external stimuli, but also endow supramolecular luminescent materials with stimuli-responsive properties as smart supramolecular materials
[43⇓~45]. With the advent and advancement of the intelligent era, the demand for intelligent materials is growing, and the research of circularly polarized luminescence intelligent materials is becoming more and more urgent.As a kind of smart materials, stimuli-responsive self-assembled luminescent system has unique advantages, which combines the advantages of supramolecular chemistry and smart optical materials, and shows excellent application prospects.
In this paper, we review the recent progress in stimuli-responsive chiral supramolecular assembly systems with CPL properties based on different external stimuli, including light, solvent, pH, temperature, and metal ions. The discussion in this paper mainly focuses on the changes of chiral signals in the ground and excited States of chiral supramolecular systems under different external stimuli. In addition, the definitions of ground state chirality and excited state chirality are briefly explained. Finally, the current shortcomings and future development prospects of the excited state chiral response of supramolecular chiral assemblies are prospected. It is hoped that this review will contribute to the design and exploration of future stimuli-responsive circularly polarized luminescent materials, while promoting the development of multidisciplinary related applications.
2 Definitions of CD and CPL
In general, natural light and most other common visible light can be considered to be unpolarized, with their vibration planes not limited to a fixed direction but uniformly distributed in all directions, and their polarization changing rapidly over time. Unpolarized light can be described as a mixture of two independent opposite polarities, each of half intensity. That is, unpolarized light is fully polarized in every direction, but becomes polarized when one beam has more energy than the other parts. When the plane of vibration of light is restricted to a fixed direction, the light is called plane polarized light or linearly polarized light. Natural light can be converted into circularly polarized light by a combination of a linear polarizer and a quarter-wave plate. The main disadvantage of this method is that more than half of the energy of the light is lost after passing through the various wave plates. In addition to being accessible by photophysical methods, circularly polarized light can also be obtained directly from chiral materials with active luminescence properties. When the chiral system has circularly polarized luminescence, the molecule is excited from the ground state to the excited state when it is excited by unpolarized light or electron injection, and when the excited state undergoes a radiative transition back to the ground state, the light emitted by the system is circularly polarized light. Based on these concepts, the mechanism of circular dichroism spectroscopy and circularly polarized luminescence measurement can be simply explained.
The mechanism of the CD and CPL spectral measurements is shown in Figures 1A and 1B, respectively. CD, a form of ground state chirality, is one of the most widely used techniques to study the chiroptical properties of materials. When the left and right circularly polarized light with the same wavelength and intensity are absorbed by the same chiral system, the light intensity transmitted through the system will be different. CD is defined as the difference of the absorption ability of chiral molecules to circularly polarized light with different handedness, which can be expressed as:
Where the absorption to the observed left- and right-handed circularly polarized light is defined as AL and AR, respectively. To exclude the effect of sample concentration and optical path length, the absorption asymmetry factor is usually defined as:
Where A is the absorbance of the equivalent mean of A
L and A
R. The asymmetry factor is often called the anisotropy factor or simply the g-factor
[46].
When a chiral molecule or a molecule in a chiral environment is excited by unpolarized light, the CPL spectrum tester detects the difference between the intensity of left-handed circularly polarized light and right-handed circularly polarized light emitted by the excited sample, that is:
Where IL, IR represent the intensity of the left-handed circularly polarized light and right-handed circularly polarized light in the luminescence of the measured chiral sample, respectively. Since the evaluation of the absolute values of I and ΔI is difficult, the intensity of circularly polarized luminescence is usually evaluated with the luminescence asymmetry factor glum, which can be defined as the difference between the intensities of left- and right-handed circularly polarized luminescence divided by the average luminescence intensity, as follows:
The theoretical value of g
lum ranges from -2 to + 2
[47]. For non-circularly polarized light emitting systems, g
lum=0; For pure left-handed and right-handed circularly polarized luminescence systems, the luminescence asymmetry factors are + 2 and -2, respectively. For most chiral luminescent systems, the luminescence asymmetry factor is between + 2 and -2. For small organic molecules in solution, small g
lum values usually occur between the 10
-5~10
-3. However, when the chiral luminescent system exhibits aggregation or condensation state, the g
lum value increases to the range of 10
-3~10
-1.
3 Different stimulus responsiveness based on excited state chirality of supramolecular assemblies
Supramolecular self-assembly provides a powerful solution for precisely regulating the arrangement of assemblies and enhancing the g
lum value of their circularly polarized luminescence. Supramolecular systems are composed of structural motifs spontaneously assembled by weak non-covalent interactions, which are dynamic and responsive to external stimuli in nature, so they have unique advantages in chirality control compared with monomolecular systems. Chiral optical signals such as CD and CPL of CPL supramolecular assemblies can be reversibly switched between two or more States under external stimuli such as light, temperature, solvent, pH and metal ions, which promotes their potential applications in chiral sensing, optical display, information storage, asymmetric catalysis and other fields
[48,49]. Responsive chiral supramolecular assemblies also have the advantages of design flexibility and ease of operation. More importantly, through these strategies, the chirality control is extended from the molecular level to the supramolecular and nanoscale levels, thus improving the feasibility of circularly polarized luminescent materials based on supramolecular assemblies for practical applications.
3.1 Illumination
Illumination is a non-destructive external stimulus, which has the advantages of easy processing and fast readout, so it has been widely used in chiroptical switchable systems. Light-driven supramolecular chiral assemblies are based on the combination of photochromic compounds and chiral sensing. Photochromism is defined as "the molecular structure of a compound (A) changes under the action of light of a certain wavelength and intensity to obtain a product B, resulting in a significant change in its absorption spectrum, while B can be reversibly restored to A under the action of light of another wavelength or heat". The ingenious combination of chiral information and photochromism paves the way for the fabrication of various light-driven circularly polarized luminescence smart materials.
Spiropyran (SP) is a very important photochromic compound, which has the characteristics of thermal response, light response, acid response and ion response. Under UV irradiation, SP undergoes a ring-opening reaction and is converted into a colored cyanine state (MC). When irradiated with visible light or heated, the MC morphology switches back to the colorless SP morphology
[50]. The reversible transformation of the two States of spiropyran is accompanied by significant changes in color, geometric structure and charge distribution, resulting in distinct photophysical properties of SP and MC, which lays a good foundation for the further application of spiropyran dyes. For example, when the SP unit is modified on the side chain of poly (L-glutamic acid), the polypeptide undergoes a huge conformational change from a random coil to a helix under the alternating action of white light and dark conditions, accompanied by a change in the CD signal. Therefore, a chiral optical switch system for the interaction between a chiral polymer and a photochromic spiropyran was successfully obtained
[51].
A pair of enantiomeric glutamate gelling agents (SP-LG and SP-DG) modified by SP were synthesized by Liu et al
[52]. Although the molecule does not show chiral signals in solution, the supramolecular chiral gel formed by the self-assembly process can show reversible changes in CD and CPL signals under the alternate irradiation of ultraviolet and visible light (Fig. 2a – e).
Meanwhile, the nanostructure of SP-LG or SP-DG gel (from helical structure to disordered nanofibers) was also reversible under alternating UV and visible light stimulation. Therefore, the enantiomeric supramolecular gel has the characteristics of both a quadruple photoswitch and a chiral photoswitch with reversible UV absorption, fluorescence, CD, and CPL signals. In addition, they further made xerogel on glass into a rewritable material. Under alternating UV and visible light illumination, this rewritable material can be printed efficiently in 30 cycles without significant reduction in contrast and resolution. The chiral self-assembly gel system shows durability in repeated light cycles, and has the potential to be applied to multiple logic switches and memory devices in the future.
Xing et al. Reported a chiral vesicle system with light-tunable CD and CPL chiroptical signals
[53]. As shown in Fig. 3, the cyanostyrenes and the cholesteryl groups of the chiral center, which act as photoresponsive luminophores, respectively, are connected by carbonate ester bond conjugation (CSC), which provides the chiral center and lipophilic structure for the formation of flexible vesicle membranes. The structural units self-assemble to form vesicles with CD and CPL signals. At this time, the g
lum value of CPL reached the order of 10
-2. Under UV irradiation, the cyanostyrene in the vesicles was photoisomerized, the Cotton effect was enhanced, and the g
abs factor was increased by 3 times, from 1.5×10
-3 to 4.5×10
-3, while the luminescence asymmetry factor of CPL showed strong fatigue resistance, and the g
lum was still maintained at the order of 10
-2. Furthermore, they loaded a hydrophobic fluorescent dye (Nile Red) into the vesicle membrane to realize the chirality and energy transfer from the chiral assembly to the achiral receptor Nile Red (NR), resulting in a red CPL with a high g
lum value, and the ultraviolet resistance of the vesicle was further enhanced after loading the fluorescent dye. This study provides a new idea for the construction of smart response chiral luminescent materials in aqueous media.
3.2 Temperature
The conformation and intermolecular interactions of thermoresponsive assemblies often exhibit the property of temperature-responsive changes. For example, chiral assembly structures constructed by hydrogen bonding interactions are particularly sensitive to temperature. The behavior of CPL is also affected by temperature. When the temperature is gradually cooled, the structural units form ordered chiral assemblies by self-assembly, and then the chiral direction and emission wavelength of CPL are adjusted by heating to dissolve the assemblies.
Haino et al. Reported a spiral assembly that can respond to external temperature to achieve CPL tuning
[54]. They designed and synthesized chiral cyclopenta dithiophene germanium (DTG) derivatives, which were observed in methylcyclohexane (MCH) when the temperature was lowered, and the dipole-dipole interaction of chiral phenyl isoxazole could drive DTG molecules to a two-stage assembly process, including the first nucleation stage and the subsequent growth stage. Correspondingly, CPL signals with different emission wavelengths and different chiral directions were observed (Figure 4A). A weak positive CPL signal at 566 nm appeared when the temperature was decreased from the initial 40 ° C to 20 ° C. Upon further cooling to 10 ° C, the CPL signal at 566 nm was enhanced, while a new negative signal was observed at 472 nm. Upon further cooling to -10 ° C, the positive CPL signal disappeared, and only a negative signal was detected at 514 nm. Therefore, the positive signal (g
lum~8×10
-4) of CPL at 566 nm was completely reversed to the negative signal (g
lum~-3×10
-4) at 514 nm during the temperature drop from 20 ° C to -10 ° C (Fig. 4 B). Similar changes were also observed in the CD signals of the S- and R-enantiomers. The change of chirality signals of the ground state and excited state of the luminescent system indicates that the assembly behavior determines the chirality direction of the assembled system. Therefore, the chiral signal can be effectively regulated by controlling the assembly process at different temperatures.
Similarly, Ihara et al. Reported that l-glutamine-functionalized phenylanthracene (g-PA) showed solvent- and temperature-cooling rate-dependent CPL (Fig. 5)
[55]. No CPL signal was observed in pure tetrahydrofuran (THF) solution of g-PA. When the organogel was formed in hexane/tetrahydrofuran (50:1) mixed solution, it showed a clear CPL signal of excimer at 526 nm with a g
lum value of ca. 3.2×10
-3. Thus, the formation of the gel assembly enhances the chirality transfer from the l-glutamine unit to the anthracene excimer. As the temperature increases from 10 ℃ to 60 ℃, the excimer emission peak changes to the monomer emission peak (426 nm) accompanied by a decrease in the g
lum. Moreover, the change of the chiroptical signal in the gel state is able to be reproduced in the solid polymer film. By adding a small amount of Nile red dye, the system can achieve white light emission. This color-tunable CPL material has a wide range of applications in displays, OLEDs, spectral conversion films, and sensors.
3.3 Solvent
Solvent plays a vital role in the molecular self-assembly process as a bridge between the bonding monomolecule and the nanoassembly. The change of solvent usually alters the self-assembly process, resulting in the formation of morphologies with different dimensions, thus presenting different chiroptical properties.
Liu et al. Reported that the same chiral luminescent small molecule (R- or S-SPAn) can self-assemble into nanoassemblies with different dimensions in different solvents (Figure 6A)
[56]. SPAn can be well dissolved in THF and aggregated with the addition of the poor solvent water. When the volume fraction of water (fw) increases from 50% to 90%, the aggregation process of small molecules will produce different size morphologies, from zero-dimensional (0 D) nanospheres to two-dimensional (2D) nanosheets to three-dimensional (3D) laminated nanosheets. The X-ray diffraction (XRD) spectra of different nanomorphologies show that SPAn polymerizes and self-assembles with an increasing trend during the addition of water. The selected area electron diffraction (SAED) results further confirmed that the 2D flakes and 3D nanoflakes were generated on the same crystal structure. These results indicate that the obtained 2D and 3D nanostructures have highly ordered molecular packing and a certain degree of crystallinity. In addition, the luminescence of SPAn nanostructures changes from purple to cyan during the nanosize enlargement of the assemblies, which is attributed to the excimer formation of anthracene units in the SPAn structure during aggregation. More interestingly, self-assembled nanostructures constructed using the same chiral fluorescent molecule showed different chiroptical properties. As the water volume fraction increases, the g
lum values range from 10
-4 for 0 D nanospheres to 10
-3 for 2D nanosheets to 10
-2 for 3D nanosheets. The increased g
lum value can be attributed to the strong luminescence of the aggregation-induced excimer formation. In this study, the correlation between CPL intensity and the morphology of luminescent nanostructures was discussed, and then the controllable adjustment of chiral optical properties was realized, which provided a new idea for the preparation of controllable chiral organic nanomaterials.
In addition to chiral luminescent small molecules, achiral molecules can also self-assemble into different morphologies by changing the composition of the solvent. As shown in Fig. 6 B, Duan et al. Reported that the C
3 symmetric molecule BTANM could be self-assembled into nanoassemblies of various sizes and morphologies by simply adjusting the solvent
[57]. A mixed solvent of DMF and water is adopted to regulate the molecular assembly process; As the proportion of DMF/H
2O increases, the morphologies of the assemblies change from distorted nanowires to trumpet-like nanoparticles to distorted nanowires. C
3 symmetric molecules are first assembled into nanoribbons, which are the basis for the construction of high-dimensional nanostructures. In solvents with low DMF/H
2O ratios, large amounts of water would cover the exposed carbonyl groups of BTANM and prevent further aggregation. When the water content decreases, hydrogen bonding will force the nanoribbons to roll up into trumpet-shaped nanoparticles with lower surface area. When the DMF/H
2O ratio reaches 6 – 7, a small amount of water molecules cannot cover all the carbonyl groups on the nanoribbon surface, causing it to further twist into a thicker nanohelix. The CPL signal increased with the increase of DMF content. The maximum g
lum value was 0.021, indicating a positive correlation between the nanometer-sized morphology and the luminescence asymmetry factor. This work simply modulates the supramolecular nanostructure by the solvent, which in turn modulates the CPL signal and provides a deeper understanding of the emergence of supramolecular chirality in achiral molecules.
3.4 pH
Many self-assembly systems can be driven reversibly by adjusting the pH, and the wavelength and intensity of the assembly CPL can be further adjusted
[58].
Duan and Liu et al. Prepared a proton-triggered CPL switch — a co-assembly gel system composed of an achiral fluorophore and a chiral gelator (Fig. 7)
[59]. They found that the orthogonal assembly or co-assembly of achiral perylene diimide (PBI) with chiral gelator (LBG/DBG) could be tuned by solvent. The chiral gel and PBI can be co-assembled in ethanol to form a gel. However, only a very weak fluorescence can be observed, and the CPL signal can not be detected. After exposure of the cogel to an acidic environment, strong fluorescence and CPL signals could be observed. If the cogel is exposed to ammonia, both the fluorescence and the CPL disappear. While the orthogonal assembly can be performed in toluene. The co-assembled gel forms chiral nanofibers through chirality transfer, and the protonation of the co-assembled nanofibers can light up the circularly polarized luminescence. In this way, cumbersome organic synthesis and purification can be avoided, so the supramolecular gelation of chiral gelators and achiral dyes provides a new method for the preparation of CPL active gel materials.
Liu and Zhang et al. Recently reported that photoacid-functionalized dyes also exhibit pH-dependent circularly polarized luminescence behavior
[60]. As shown in Fig. 8, the non-luminescent molecule 8-hydroxy-1,3,6-pyrene trisulfonate (HPTS) and chiral glutamic acid derivative (LG) can be co-assembled to form a supramolecular gel, and the emission wavelength and g
lum value of circularly polarized light can be adjusted by changing the solvent and pH value. LG can self-assemble into nanotube structures in pure DMF solvent and DMF/H
2O mixed solvent. The nanotube structure was preserved after doping HPTS in LG (DG) gel. At the same time, the acquisition of CD and CPL signals proved that the chirality of LG or DG was transferred to the HPTS chromophore, and the chirality direction of HPTS was consistent with that of LG or DG. When the assembly was exposed to an acidic environment, a negative CPL signal appeared at 430 nm, whereas in an alkaline environment, the CPL was red-shifted to 520 nm. This is caused by intermolecular excited state proton transfer from the acidic group in HP TS to neighboring solvent molecules. In addition, the solvent polarity also affects the fluorescence and the corresponding chiral signal. Thus, the CPL emission of LG (DG)/HPTS nanotubes exhibits a dual response to solvent polarity and acidity. This study provides a simple self-assembly route to obtain responsive CPL materials that switch between different wavelengths.
3.5 Metal ion
In general, metal ions with different stable valences are introduced into the assembly system. Metal ions with different stable valences can change the conformation, chemical composition, electronic structure, assembly behavior, etc. Metal-ligand interaction has been proved to be a simple method to adjust the performance of CPL.
Liu et al. Reported a coordination-driven pyrene-histidine supramolecular system that can generate a switchable CPL signal
[61]. As shown in fig. 9, the coordination and dissociation between the imidazole unit and Zn (II) play an important role in the self-assembly process. It is worth noting that by using zinc (II) to coordinate with the imidazole group, the morphology of the assembly changes from a nanofiber structure to a nanosphere, accompanied by a change in the direction of the CPL signal. The reason for this phenomenon is that this coordination interaction changes the stacking mode of the chromophore from T-shaped stacking to π-π stacking. The reversible reversal of supramolecular chirality can be repeated for at least 4 cycles by alternately changing the external stimulus. Extensive spectroscopic data combined with X-ray crystallographic analysis and DFT calculations suggest that the change in packing mode between T-shaped and π-π stacking is the underlying cause of the supramolecular chirality switch. This work not only provides deep insights into how coordination and packing modes affect the chirality of excited States in chiral assembly, but also provides new avenues for the development of switchable CPL supramolecular materials.
Ni
2+ and Zn
2+ have similar effects on the assembly behavior of organic molecules and the properties of CPL through complexation. Feng et al. Reported that a chiral phenylalanine-derived hydrogel (LPF) and an achiral coumarin derivative (G) could be co-assembled to form a nanofibrous hydrogel with controllable chirality (Fig. 10)
[62]. The chiral direction of the composite hydrogel CPL can be changed by changing the nitrogen (N) steric position in the pyridine ring of the coumarin derivative. The chirality of CPL can be further tuned by the metal ion due to its unique directionality and packing mode (Ni
2+, Ag
+), and the tuning results are influenced by the steric position of N and the 4-methyl substituent on the coumarin derivative. Moreover, the controllable supramolecular chirality and nanostructures (e.g., nanohelix, nanohelix, and nanotube) demonstrate that the chirality of CPL is largely determined by the co-assembly manner of LPF and achiral coumarin derivatives rather than the inherent molecular chirality. Hydrogen bonding, coordination interactions, and steric hindrance are key factors in CPL conversion. This study contributes to a better understanding of the relationship between intrinsic molecular chirality and supramolecular chirality, and through the rational design of chiral molecules and well-chosen fluorophores, a method for effectively tuning CPL properties through co-assembly has been developed, with potential applications in optoelectronic devices and smart chiroptical materials.
3.6 Other
In addition to the above common stimuli-responsive self-assembled luminescent systems, the excited state chirality of supramolecular assemblies can also respond to other unusual external stimuli, such as host-guest interactions, mechanical forces and magnetic fields
[63,64][65,66][67,68]. Most of them introduce functional groups with specific stimulus responsiveness in the design process of assembly motifs. When the corresponding external stimulus is given, the configuration or structure of the functional groups will change, and the assembly will dynamically reassemble under the action of the external stimulus, so that the chiral self-assembly luminescent system is responsive to the external stimulus.
The binding-dissociation of the host-guest interaction can act as a switch for excimer molecule formation, chirality transfer, chirality inversion, and modulation of the CPL state. Feng et al. Inverted the chirality of G-quadruplex DNA (G4 DNA) by alternating the addition of potassium ions (K
+) and the cryptand (Fig. 11)
[63]. Human telomeric G4 DNA and an achiral fluorescent dye (ThT) achieve CPL emission by co-assembling to transfer chirality from the G4 template to the binding molecule. A right-handed CPL(490 nm,g
lum can be induced on the parallel G4 structure (about 0.5×10
-3~1.5×10
-3,10℃); While the left-handed CPL is generated on the antiparallel G4. Parallel-aligned G4 DNA can also be changed to antiparallel alignment by the addition of K
+. In addition, low temperature (10 ℃) is beneficial to the ordered assembly of G4 DNA and ThT to achieve the emission of circularly polarized light. Higher temperature (10 ~ 70 ℃) dissolved the assembly motifs and inhibited the production of CPL. Therefore, the supramolecular assembly of G4 DNA and ThT realizes the switching of chirality and intensity of circularly polarized light under host-guest interaction, which provides a new idea for the study of optical technology based on variable G4.
The application of mechanical force in chemical systems, whether mechanical grinding, sliding, or stirring, has attracted a great deal of attention because it meets the requirements of clean processing, environmental friendliness, and solvent-free reactions. With the development of supramolecular chemistry, the concept of using mechanical methods to induce chemical transformations has been extended to the disruption and formation of noncovalent interactions of assemblies, that is, interactions that affect the structure and stability of molecular assemblies. Yamashita et al. Demonstrated that the physical chirality of the vortex can be transferred to the molecular chirality under mechanical stirring, thus realizing the emission of circularly polarized light (Fig. 12)
[65]. The achiral hydrogel embedded with rhodamine B dye exhibited a mirror-image CPL signal induced by mechanical stirring, with counterclockwise (CCW) stirring inducing a positive CPL signal, and conversely, clockwise (CW) stirring inducing the formation of a negative CPL signal, which disappeared for the gel formed without any stirring. Therefore, when the sample is slowly cooled from the sol state to the gel state, the responsiveness of the multifunctional material to the degree of polarization is controlled by the stirring direction of the eddy current. This study can confirm the transfer of chiral information from mechanical stirring to molecular chirality by detecting the CPL signal of gel molecules, which has potential applications in advanced chiral optical materials such as chiral optical memory devices, light-emitting devices and multi-functional sensors in the future.
Magnetic field is also relatively suitable for regulating the state of supramolecular assemblies because it does not require contact and is relatively easy to achieve. Stimulated by an external magnetic field, magnetic particles can assemble into one-dimensional (1D) periodic chains, 2D sheets, 3D photonic crystals, or complex self-assembled superstructures. Law et al. Reported the design and synthesis of magnetically responsive circularly polarized luminescent complexes showing exceptional photophysical properties (quantum yield up to 31% and g
lum value 0.24 higher) by introducing chiral substituents on the macrocyclic scaffold (Fig. 13)
[67]. The circularly polarized light emitted from the chiral Eu
III complex shows the characteristic of magnetic response. When the strength of the applied magnetic field increases from 0 T to 1.4 T, the g
lum increases, and there is a linear relationship between the 20%,g
lum and the strength of the magnetic field. These CPL magnetically responsive Eu
III C omplexes, due to their higher sensitivity and resolution, provide a potential development for CPL imaging applications.
4 Conclusion and prospect
The stimuli-responsive intelligent supramolecular circularly polarized luminescence system has structural dynamic controllability, and the introduction of functional groups with stimuli-responsive and luminescent properties on the basis of chiral supramolecules endows chiral supramolecular assembly with intelligent responsiveness. At the same time, with the discovery of a large number of functional groups, more stimulus-responsive CPL organic materials will be developed, which will provide more excellent choices for the development and development of intelligent optoelectronic materials. In this paper, the switchable behavior of CPL materials is classified according to different environmental stimuli, including light radiation, temperature, solvent, pH value and metal ions. The photoresponsive CPL supramolecular self-assembly is mainly composed of functionalized groups and chiral motifs, such as photochromic spiropyran and chiral units through covalent attachment. Most acid-base responsive CPL switches regulate the emission wavelength through protonation/deprotonation processes, while recent responsive CPL materials regulate the system by controlling protonation-induced electron transfer. Metal ions modify CPL behavior by forming highly oriented and rigid complexes with ligands, which determine the associated electronic band structure, molecular conformation, and helical orientation. Solvent and temperature-based CPL switches are widely used because of their ease of operation, and the response process is usually regulated by controlling the assembly process, adjusting solvent polarity, solvent-solute interaction forces, or by controlling the cooling process.
To sum up, various types of external stimuli for responsive CPL materials, on the one hand, can change the related dynamic assembly behavior by adjusting the molecular or electronic structure, on the other hand, can be achieved by adjusting the external interaction between chiral and achiral components. Although many studies have achieved remarkable results in this field, there are still many challenges in this field. First, in order to meet the needs of practical applications, the responsive CPL system needs to possess high luminescent quantum efficiency and glum value. Although energy transfer in assembled systems has been demonstrated to be an effective way to amplify glum values, including F Förster resonance energy transfer, radiative energy transfer, and photon up-conversion, the glum values (10-2~10-1) obtained for most systems are difficult to expand for further applications. Therefore, how to obtain the maximum glum value with enhanced luminescence quantum efficiency still requires the improvement of the basic properties of the responsive CPL system and the comprehensive design and adjustment of the arrangement of chiral emitters. Additionally, that switch between the multiple States of the CPL is steady-state. The stability of each state is important to improve reproducibility and fatigue resistance. In addition to the design of functional molecular components, the attachment to chiral units is also very important, requiring both perfect coupling between chiral and functionalized building blocks and balancing the use of various non-covalent bonds, such as hydrogen bonds, π-π stacking, and electrostatic interactions; Or the functionalized functional group is coupled with the chiral unit through coordination, and the multiple non-covalent interactions between the structural units are utilized to increase the stability of the assembly. Therefore, how to use the interaction between supramolecules to construct more highly ordered and stable nanostructures remains to be explored. Finally, in practical applications, although responsive CPL systems are expected to be made into materials or devices, and the huge library of structural and functional molecules can be used as the basis for building smart CPL materials, these molecules do not always show their inherent properties in the final materials. Chirality transfer or control from the molecular level to nanoscale and macroscopic materials thus remains a challenging problem.
National Natural Science Foundation of China(21773103)
National Natural Science Foundation of China(22172041)
National Natural Science Foundation of China(52173159)
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