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Structure, Assembly, and Function of (Latent)Chiral AIEgens Hai-Tao Feng,†,‡,§ Chenchen Liu,†,§ Qiyao Li,† Haoke Zhang,† Jacky W. Y. Lam,† and Ben Zhong Tang*,†,#
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Department of Chemistry, Hong Kong Branch of Chinese National Engineering Research Center for Tissue Restoration & Reconstruction, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, China ‡ Baoji AIE Research Center, College of Chemistry and Chemical Engineering, Baoji University of Arts and Sciences, Baoji 721013, China # Center for Aggregation-Induced Emission, SCUT-HKUST Joint Research Laboratory, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China ABSTRACT: The discovery of the photophysical phenomenon of aggregationinduced emission (AIE) by Tang in 2001 has drawn intense attention of scientists all over the world and created potential applications in various areas, such as biological probes, chemical sensors, and optoelectronic devices. Incorporation of AIE with chirality is a pioneering attempt to broaden the field of AIE research. Generally, chiral AIEgens are designed by attaching chiral units to an AIE-active building block. From another point of view, AIEgens with molecular rotors or vibrators are a kind of latent chiral molecule, whose chirality can be detected at a specific state by breaking the mirror symmetry. Aggregation as a bridge tethers molecular AIEgens and their macroscopic self-assembly. Through precise control of the chirality, well-defined helical architectures with amplified chiral signals are formed. In contrast to circularly-polarized luminescence at the molecular level, these aggregates with ordered packing always show enhanced performance in emission efficiency and dissymmetry factor. ing research interest in chemistry, physics, and biological area.7 The measurement of CD affords the ground state of absorption difference between left and right circularly polarized light. Its performance could be evaluated by the absorptive dissymmetry factor gabs = 2(εL − εR)/(εL + εR).8 Thanks to the availability of commercial spectrometer since the 1960s, numerous studies on the tertiary structures of proteins and chiral sensing have been carried out by CD. In contrast, CPL is a burgeoning research field because its commercial instruments are only available recently and it gives the excited-state information on differential emission of left and right circularly polarized light.9 The primary criterion for appraising CPL is to measure emission dissymmetry factor (gem = 2(IL − IR)/(IL + IR)), which provides excited-state emission difference of left (L) and right (R) circularly polarized light. For both CD and CPL, a high g value is an important standard to evaluate the chiroptical property of a material. Until now, most traditional chiral fluorescent materials were tested in solution.10,11 They often suffer from notorious aggregationcaused quenching (ACQ) in the condensed phase,12 which dramatically affects their chiroptical performance. Although
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hirality is a common geometric property of some molecules and ions that are nonsuperimposable to their mirror images. A well-known example is the separation of two isomers of sodium ammonium tartrate by Louis Pasteur in 1848. In nature, all known life-forms show specific chirality at the molecular scale and macroscopic behavior.1 For example, nature uses amino acids, saccharides, and nucleobases as building blocks to create varied biological systems. Investigation
Investigation of chiral phenomenon and potential applications of chiral materials help us better understand our life processes of chiral phenomenon and potential applications of chiral materials help us better understand our life processes.2−4 Taking these into account, enantiomeric characterization is important in pharmaceutical chemistry, material science, food analysis, and life science.5,6 Because of the development of modern spectroscopic techniques, some novel photophysical phenomena observed in chiral luminogens, such as circular dichroism (CD) and circularly-polarized luminescence (CPL), have created increas© 2019 American Chemical Society
Received: April 17, 2019 Accepted: June 6, 2019 Published: June 6, 2019 192
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barrier enhances because of the restriction of intramolecular motion. Thus, a couple of enantiomers can be separated by crystallization induction or chiral high performance liquid chromatography (HPLC). TPE is such a typical chiragen. Because of the strong steric hindrance, the four phenyl rings rotate either in a clockwise or anticlockwise direction. Therefore, its helical chirality can be achieved by crystallization induction.27 The AIE process refers to molecule aggregation, meanwhile, aggregation always relates to self-assembly. Thus, the AIE and chiroptical properties of AIEgens can be readily tuned by the supramolecular morphology. Normally, the driving forces for self-assembly are non-covalent in nature including electrostatic interactions, hydrophobic effects, hydrogen bonding interactions, van der Waals forces. Through precise control on the assembly conditions, diverse well-defined architectures may be formed and transmission of chiral signals from the molecular level to the macroscopic assembly may be achieved. Up to now, a series of supramolecular assembles with various morphologies, such as vesicle, micelles, belts, fibers and so on, have been prepared through supramolecular assembly of AIEgens.28−33 Because of chirality transfer and amplification, helical supramolecular structures have drawn great attention recently.
much effort has been made to address this issue, the problem has still not been solved perfectly. Recently, a new type of propeller-like luminogen, such as hexaphenylsilole (HPS),13 tetraphenylethene (TPE),14−17 and 2-phenylcinnamylnitrile,18,19 were nonemissive or showed only weak emission in solution but fluoresced strongly upon aggregation (Figure 1). This novel photophysical phenomenon
Figure 1. Photos of perylene (A) and hexaphenylsilole (HPS) (B) taken under 365 nm UV light in tetrahydrofuran (THF) and mixed solvent of THF and water with various water fractions ( f w). Concentration: 20 μM. Modified and adapted with permission from ref 24. Copyright 2015 American Chemical Society.
Because of their novel AIE characteristics, propeller-shape, and interesting photophysical properties, silole derivatives have been shown to be promising building blocks for constructing chiral AIEgens
was coined aggregation-induced emission (AIE),13,20 and its mechanism is attributed to restriction of intramolecular motion (RIM).21−24 In dilute solution, for example, the four phenyl rings and double bond of TPE can rotate and vibrate freely in the excited state. This leads to the relaxation of excited state energy to ground state through a nonradiative transition. However, in the aggregate state, such motion is restricted to turn off the nonradiative decay.25 As such, TPE shows intense emission.26 To overcome the ACQ effect and construct ideal chiral fluorescent molecules, incorporation of chiral units in the AIE skeletons seems to be a promising protocol. For example, Tang reported a chiral AIE luminogen (AIEgen) 1 composed of silole and mannose units with high gem value in the solid state. Thus, these AIEgens not only can effectively inhibit fluorescence quenching and serve as “turn-on” chiral sensors but also show enhanced CPL intensity in the aggregate state. In addition to these general design strategies, exploration of latent chiral AIEgens with molecular rotors or vibrators has also drawn increasing attention. Here, we describe such molecules as with the term “chiragens”. They are AIEgens with intrinsic chirality, but this chiral character is not detected in solution because of the rapid molecular motion. Upon aggregation, their flipping energy
Thanks to the great effort made by many groups, lots of excellent work has been reported for chiral AIEgens. In this Review, we will mainly present our work in this area in recent years. We will discuss chiral AIE molecular structures, helical self-assembly of AIEgens, and recent developments and applications of chiral sensors, CD/CPL based on AIE, and so on. Attention is also given to latent chiral AIEgens (chiragens) and circularly polarized organic light-emitting diodes (CPOLEDs).
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STRUCTURE Silole-Containing Chiral AIEgens. 1-Methyl-1,2,3,4,5pentaphenyl-silole is the first reported AIEgen and shows high fluorescence efficiency and photostability.13,34 Afterwards
Figure 2. Molecular structures of chiral silole-containing AIEgens. 193
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Figure 3. Molecular structures of chiral TPE-containing AIEgens.
cast film. Another two polymers 9 and 10 were prepared by Sonogashira coupling reaction and click polymerization of TPEdiyne with appropriate chiral monomers, respectively. Unlike the above-mentioned AIEgens with point chirality, 1,1′-bi-2naphthol (BINOL) not only can serve as a luminophore but also can endow target molecules with axial chirality. Compounds 11−13 with BINOL attached to TPE scaffold show chirality. Tang reported a TPE-BINOL derivative 11, where TPE was selected as a AIE building block to endow 11 with AIE characteristics.46 Molecules 12 and 13 exhibit aggregation− annihilation CD, which is exactly opposite to that most observed in chiral AIEgens.47 Other Chiral AIEgens. In addition to the silole- and TPEbased chiral AIEgens, some BINOL-containing AIEgens 14−19 were also prepared by Tang.48−50 Through intramolecular N−B coordination, we developed a new kind of AIE-active material 14 with multicolor emission in solution and solid state.48 Chiral Au(I) complex 15 bearing a BINOL core featuring AIE property was synthesized recently and showed various morphologies through hierarchical self-assembly processes.49 Enantiomers of 16−19 with AIE and delayed fluorescence properties were designed for fabricating circularly polarized organic lightemitting diodes.50 Because of its facile synthesis and inherent AIE characteristic, 2-phenylcinnamylnitrile derivatives were widely used to construct AIEgens. For example, chiral AIEgens D/L-20 containing tartaric acid moieties were purified through recrystallization in high yield.51 Chiral compounds 21 and 22 bearing aminol and amine groups can be used for enantioselective recognition of carboxylic acids, such as 2,3-dibenzoyltartaric acid and mandelic acid.52,53 On the other hand, a cholesterol-bearing alkyl group shows a typical hydrophobicity
numerous silole derivatives were designed and synthesized for sensors, and optoelectronic devices.35 Because of their novel AIE characteristics, propeller-shape, and interesting photophysical properties, silole derivatives have been shown to be promising building blocks for constructing chiral AIEgens. Recently, Tang reported a series of chiral silole-based AIEgens 1−4. Among them, the first chiral silole derivative 1 was synthesized by attaching mannose units to the tetraphenylsilole core via azide−alkyne “click” reaction.36 Through same synthetic strategy, another two chiral AIEgens 2 and 3 bearing 37,38 L-valine- and L-leucine moieties were prepared. On the basis of these research results, chiral AIEgen 4 was synthesized through combination of tetraphenylsilole and chiral phenylethanamine moieties through thiourea linker.39 These silole derivatives all show typical AIE characteristics. TPE-Containing Chiral AIEgens. Although silole-based AIEgens possess high-emission efficiency, their sophisticated preparation limits its wide applications. Thus, TPE, another star AIEgen, was widely used to construct various AIEgens thanks to its high emission efficiency and simple synthesis. Up to now, thousands of TPE derivatives have been reported, including chiral AIEgens with CPL behaviors. For example, Tang reported a chiral TPE derivative 5 containing an L-leucine methyl ester moiety with typical AIE characteristic.40 Using a similar strategy, another two chiral TPE-based AIEgens 6 and 7 with L-valine methyl ester were synthesized.41,42 AIEgens 6 and 7 show no CD signals in solution. Upon aggregation or in cast film, strong Cotton effects were observed. Our group also synthesized some chiral TPE-cored polymers 8−10.43−45 Polymer 8 was synthesized by combination of alkynes, carbonyl chlorides, and chiral primary amines. It is CD-active in both solution and 194
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Figure 4. Molecular structures of other chiral AIEgens.
amphiphilic tetraphenylpyrazine (TPP)-based cage 26 was facilely synthesized by immobilization of propeller-shaped TPP.58 TPP-Cage 26 showed obvious Cotton effects in the corresponding absorption regions of TPP unit in both solution and nanoparticles. Hexaphenylsilole 27, on the other hand, is a well-known AIEgen, and its six phenyl rings are also arranged either in a clockwise or anticlockwise manner around silole core.59 Applying the same strategy, Tang then synthesized another two chiragens 28 and separated by chiral HPLC. Apart from propeller-like AIEgens, some other molecules with vibrators are also latent chiral AIEgens. For example, 29 and 30 are AIE-active because of the stereo-configuration in the solid state.60 Its enantiomers could be separated by chiral HPLC and showed stable CD signals in solution. As stated above, AIEgens with molecular rotors and vibrators are chiragens, and they actually belong to a kind of atropisomer. Because of huge steric hindrance between neighboring phenyl rings, these propeller-type chiragens adopt either anticlockwise (M) or clockwise (P) arrangement (Figure 6A). Because they
and chirality. It can form helical architectures through supramolecular assembly.54,55 Taking this into account, a butterfly-like chiral pyran-based 23 bearing two cholesterol units was designed and synthesized by Tang.56 Chiragens are a kind of latent chiral AIEgens with molecular rotors or vibrators. In TPE, the four phenyl peripheries rotate in
Chiragens are a kind of latent chiral AIEgens with molecular rotors or vibrators one direction (clockwise or anticlockwise) because of steric hindrance of neighboring phenyl rings. Thus, TPE should show helical chirality.57 However, TPE derivatives 24 and 25 are CDsilent in solution because of the rapid reversible configuration change. Interestingly, the M- or P-helical chirality of TPE-based atropisomer can be characterized by CD and CPL in the crystal state because of mirror symmetry breaking.27 Moreover, 195
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Figure 5. Molecular structures of prochiral AIEgens.
reported; for example, De Rossi found that an achiral benzimidocyanine derivative containing long alkyl chains can self-assemble to nanomaterials showing optical activity.61 Thus, it is possible for propeller-like chiragens to show chiral character in the aggregate state. Similarly, the phenyl rings of shell-like chiragens could dynamically bend or vibrate in the solution state. However, in the aggregate state, they can form Figure 6. Schematic illustration showing the origins of chirality based on chiragens. (A) Propeller-like chiragens adopted either anticlockwise (M) or clockwise (P) arrangement. (B) Shell-like chiragens were vibrated to up or down. These chiragens are generally CD-silent in solution but become CD-active when its molecules pack orderly.
asymmetrical structures if the energy barrier for vibration becomes too high to overcome (Figure 6B).
The (latent)-chiral AIEgens always show CD-activity in the aggregate state, which implies these chiral molecules can induce latent CD signals through self-assembly
lack of microscopic chiral centers, theoretically, chiragens should form racemes of M- and P-typed aggregates in the solid state with no chirality. However, some examples of spontaneous emergence of chiral character or chiral symmetry breaking from achiral constituents or racemes have been 196
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Figure 7. (A) CD spectra of 1 in 90% DCM/hexane mixtures with various concentrations. (B) Changes of fluorescence QY versus f H of DCM/ hexane mixtures of 1. The insets are the images of 1 in 0% and 90% DCM/hexane mixture and its powder under irradiation of 365 nm UV light. (C and D) TEM photos of 1 in a 90% DCM/hexane mixture. Modified and adapted with permission from ref 36. Copyright 2012 Royal Society of Chemistry.
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ASSEMBLY The (latent)-chiral AIEgens always show CD-activity in the aggregate state, which implies these chiral molecules can induce latent CD signals through self-assembly. Recently, our group reported a series of chiral silole- and TPE-based AIEgens 1−7 and investigated their morphological behaviors. These chiral molecules all show obvious AIE property. Then the photophysical properties of molecules 1−7 were studied through CD spectroscopy. Compound 1 shows no CD signals in DCM solution regardless of concentration. When increasing the concentration in DCM/hexane mixture with 90% f H, its CD peaks at 249, 278, and 340 nm become stronger gradually (Figure 7A); meanwhile, its gabs value increases from 1.59 × 10−3 to 2.23 × 10−3. Moreover, 1 emits almost no fluorescence when the hexane fraction is less than 80% in a mixed solvent of DCM/ hexane.36 Upon f H > 80%, it fluoresces strongly. Moreover, its emission efficiency enhanced from 0.6% in DCM to 31.5 % in a mixed solvent of DCM and hexane (1/9, v/v) to 81.3 % in the film state (Figure 7B). Images of transmission electron microscopy (TEM) disclose that nanobelts with right-handed helicity were formed with helical pitch and width of about 120 and 30 nm in the suspension solution (Figure 7C and D). The driving force for such self-assembly may stem from synergistic effect of hydrogen-bonding interactions, hydrophobic interactions, and stereoconfiguration complementarity between sugar-based substituents. In addition to silole-containing AIEgens, TPE-cored chiral AIEgens could also self-assemble into helical architectures.
Their self-assembly manner was explored by using scanning electron microscopy (SEM). For example, 5 is a typical AIEactive molecule, and helical nanobelts in a wide range of width were obtained in a 90% dichloroethane (DCE)/hexane mixture.40 SEM images showed that these helical nanobelts were formed by wrapping of ribbons (Figure 8A and B).
Figure 8. (A and B) SEM images of 5 in a mixed solvent of dichloroethane/hexane with 90% hexane. [5] = 1.0 × 10−4 M. Modified and adapted with permission from ref 40. Copyright 2015 Royal Society of Chemistry.
Above-mentioned examples show the final morphologies of these chiral AIEgens in the aggregate state but afford no detailed information on their formation process. Therefore, exploration of the hierarchical self-assembly processes can help us to understand the underlying mechanism. Then a couple of chiral Au(I) complexes R/S-15 were selected to elucidate this assembly process.49 As shown in Figure 9, time-dependent morphology variations of R-15 was recorded by SEM. Vesicles 197
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FUNCTION
CPL and CPOLEDs. CD spectroscopy has been regarded as a powerful technique for characterization of chiral molecules at the ground state. For most reported chiral AIEgens, aggregationinduced CD were always observed in different aggregate state.62 Compared to CD, CPL always provides important chiral information at the S1 state. These as-prepared chiral AIEgens show strong fluorescence in the solid state, which is beneficial for chemists to fabricate CPL-based organic light-emitting devices (CPOLEDs). In recent years, chiral materials with CPL have drawn increasing research interests in organic optoelectronic materials.34,63,64 To date, many traditional CPL materials were explored in the solution with gem values fall in a scope from 10−5 to 10−2 in solution; however, their CPL performance was dramatically influenced in the film or solid state due to the notorious ACQ effect. Thus, design and preparation of chiral AIEgens is a good approach to address this problem. As-prepared chiral AIEgen 1 exhibits bright fluorescence in the solid state. Thus, it may emit CPL with high possibility.36 The CPL performance of 1 was investigated in various formats, including solution, suspension and PMMA film. Because 1 is non-emissive in solution. Thus, no obvious CPL signal is detected in its DCM solution. When it forms aggregates in suspension or film, strong CPL signals can be obtained. Their gem values vary largely and fall in range from −0.08 to −0.17 in suspension, cast film or 10 wt % in PMMA. Interestingly, a high gem value of −0.32 was attained in Teflon-based microfluidic channels through evaporating DCM/toluene solution of 1 (Figure 11A and B). The ordered stacking of 1 in the confined environment may benefit to generate great emission differential between right- and left-circularly polarized light.
Figure 9. SEM images of R-15 at different formation time in a mixed solvent of THF/water (1/4, v/v). [R-15] = 1 × 10−4 M. Scale bar: 500 nm. Modified and adapted with permission from ref 49. Copyright 2019 American Chemical Society.
were generated when R-15 formed aggregates in the newly prepared THF/water solutions. After 1 h, the vesicles began to adjoin with each other to form the necklace-like nanospheres. Further growth of these structures can result in loosely twisted helical rods or fibers after 6 h. After 3 days, left-handed helical nanobelts were formed. The helical chirality generated by the two enantiomers was completely opposite, which further verified by the CD spectra. As a latent chiral molecule, 27 may assemble to helical structures if the mirror symmetry is broken in the crystalline state.59 To confirm our hypothesis, atomic force microscopy (AFM) was utilized to investigate morphology of 27. Lefthanded helical nanofibers were obtained in pure THF solution (Figure 10). With increasing of water content, helical nanofibers were still observed in an entwined manner.
Figure 11. (A) Fluorescent photograph of 1 in microfluidic channels under 365 nm UV light. (B) CPL gem value of 1 in different forms including DCM solution, cast film, DCM/hexane mixture (1/9, v/ v), PMMA with 10 wt % of 1 and fabricated microfluidic channels. Modified and adapted with permission from ref 36. Copyright 2012 Royal Society of Chemistry.
Development of CPOLEDs with high efficiency has shown great potential in high contrast 3D displays. Recently, a series of binaphthyl-containing chiral AIEgens 16−19 with delayed fluorescence were successfully developed for CPOLEDs by our group.50 These devices exhibited a relatively high external quantum yields of 9.3% and 3.5% in doped and neat film, respectively. The devices based on doped films of S-16, S-17, S18, and S-19 exhibit intense green and yellow electroluminescence (Figure 12A). As shown in Figure 12B, the current−voltage−luminance (J−V−L) characteristics of the CPOLEDs exhibit low turn-on voltages and high luminance (Lmax) of up to 2948 cd m−2. Moreover, obvious CPL signals were recorded in these devices and the calculated gEL value is
Figure 10. AFM images of 27 in THF solution (A) and mixed solvent of THF/water with increasing water fraction from 50% (B) to 80% (C) to 90% (D). [27] = 1 × 10−5 M. Modified and adapted with permission from ref 59. Copyright 2017 Royal Society of Chemistry. 198
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Figure 12. (A) Photographs of devices of doped films of S-16, S-17, S-18, and S-19 and (B) their associated current density−voltage−luminance characteristics. (C and D) gEL value of doped and neat films of molecules 16−19. Modified and adapted with permission from ref 50. Copyright 2018 Wiley-VCH Verlag GmbH & Co. KGaA.
+0.026, +0.025, +0.027, and +0.016 for S-16, S-17, S-18, and S19 and −0.021, −0.030, −0.027, and −0.027 for R-16, R-17, R18, and R-19, respectively (Figure 12C and D). Propeller-like AIEgens are a kind of latent chiral molecules, which always display left- or right-handed chirality in the crystalline state, when the mirror symmetry is broken. Recently, our group prepared several latent chiral AIEgens 24−29.27,57−59 For example, silole-cored 27 without chiral pendant shows no Cotton effect in THF solution. Its cast film exhibits obvious CD signals at the maximum absorption wavelength, indicating that single helical conformation was formed in the aggregate state.59 Moreover, CPL measurements were carried out in THF solution and cast film to further study its latent chirality, which gave results consistent with those of CD spectra. Strong negative CPL signals were detected in the cast film from 475 to 600 nm with a large gem value of −0.01. The above results clearly verify that propeller-like AIEgens are latent chiral molecules, which can be used to analyze solid-state CD and CPL information. Chiral Recognition. Chiral fluorescent sensors could form diastereomers with one of enantiomers through noncovalent interactions to show fluorescence differential; thus, it can be detected by a spectroscopic instrument. Two criteria to evaluate an enantioselective molecular sensor are sensitivity and enantioselectivity. Numerous chiral sensors based on traditional fluorophores have been developed during the past decades. However, their further practical applications are largely limited by notorious ACQ effect. Nevertheless, the successful discovery of AIE materials provides us a totally new idea in exploring functional chiral light-emitting materials. First example of combination of AIE and chiral recognition was reported by Zheng in 2009.51 The chiral compounds D/L20A are nonemissive in solution but exhibit strong fluorescence upon aggregation. By virtue of its AIE and chiral property, D/L-
Figure 13. CD spectra (A) and CPL spectra (B) of 27 in THF solution ([27] = 1 × 10−4 M) and cast film. This figure is extracted from ref 59 with thanks. Modified and adapted with permission from ref 59. Copyright 2017 Royal Society of Chemistry.
20A can be used for discrimination of enantiomers through fluorescence differential. According to acid−base interaction, a series of chiral amines are studied. Then, Zheng and Tang groups designed a set of chiral sensors with high selectivity.48,52,53,65−67 Chiral compound 21 bearing aminol groups can be used for enantioselective recognition of chiral carboxylic acids, for example, 2,3-dibenzoyltartaric acid and mandelic acid. Among them, (1S,2R)-21 exhibits much stronger affinity to Smandelic acid, giving a high fluorescence intensity ratio (IS/IR) of 598.52 Then another chiral amine compound (1R,2R)-22 with AIE characteristic was facilely synthesized by Zheng.53 (1R,2R)-22 shows excellent performance not only in high selectivity to a pair of enantiomers but also in a wide substrate applicability to chiral carboxylic acids. The fluorescence intensity ratios of the two enantiomers fall in the range of 10− 1.6 × 104. Thus, (1R,2R)-22 is suitable for determination of the enantiomeric purity. (Figure 14 A). Such high fluorescence 199
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thanks to their improved selectivity and sensitivity as sensors and high-emission efficiency in CPL 3D display materials.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Hai-Tao Feng: 0000-0003-2388-0955 Haoke Zhang: 0000-0001-7309-2506 Ben Zhong Tang: 0000-0002-0293-964X
Figure 14. (A) PL spectra of a mixture of (1R,2R)-22 and chiral acid 31. Inset: Photograph of mixture of (1R,2R)-22 with respective enantiomer taken under day light. [(1R,2R)-22] = 2 × 10−3 M. Modified and adapted with permission from ref 53. Copyright 2011 Royal Society of Chemistry. (B) Stern−Volmer plots of R-14 with a couple of chiral menthol. Modified and adapted with permission from ref 48. Copyright 2015 Royal Society of Chemistry.
Author Contributions §
H.-T.F. and C.L. contributed equally.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This project was financially supported by the National Science Foundation of China (Grants 21788102 and 21805002), the University Grants Committee of Hong Kong (AoE/P-03/08), the Research Grants Council of Hong Kong (Grants 16308016, 16305518, and C6009-17G), the Innovation and Technology Commission (Grants ITC-CNERC14SC01 and ITS/254/17), and Natural Science Basic Research Plan in Shaanxi Province of China (Program 2018JQ2046).
differences arise from the formation of different aggregate states after complexation. After interaction with two enantiomers, one mixture is clear and nonemissive, and the other one forms a sticky suspension and luminesces intensely. Boron-containing chiral AIEgen R-14 was applied to differentiate a pair of enantiomeric menthol.48 Because of the quenching effect of the complexation of boron atom to Lewisbase, the UV−vis titration was implemented to analyze enantioselectivity of menthol through a Stern−Volmer Plot (Figure 14 B). Compared to (−)-menthol, R-14 shows much higher affinity to (+)-menthol with a binding constant of 0.012 M−1. In summary, we mainly discussed our recent work on (latent)chiral AIEgens, including molecular structure, self-assembly, CPL, and chiral recognition. From the point of molecular design, chiral AIEgens can be synthesized by attaching point or axially chiral pendants to the AIE core. On the other hand, another new term, chiragens, refers to latent chiral AIEgens with molecular rotors or vibrators can display chirality when the mirror symmetry is broken, and such research has increased our interest. As the AIE process relates to self-assembly of AIEgens, through fine tuning their aggregation process, a series of helical architectures with amplified chiral signals can be obtained. CPL activity was discussed at the molecular and macroscopic level. It is found that aggregates with ordered morphology have better performance in emission efficiency and dissymmetry factor. Efficient CPOLEDs were fabricated using chiral AIEgens, which were expected to find wide high technological applications. The discussion on chiral AIEgens in this Review covers only a small part of chiral fluorescent molecules. We believe more and more novel chiral AIEgens will be delivered in the future. Apart from
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Apart from typical open-chain chiral AIEgens, we anticipate that chiral macrocycles with AIE features will draw increasing attention thanks to their improved selectivity and sensitivity as sensors and high-emission efficiency in CPL 3D display materials typical open-chain chiral AIEgens, we anticipate that chiral macrocycles with AIE features will draw increasing attention 200
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