Liquid Crystalline AIE Luminogens: Properties and Applications - ACS

Chapter 6

Liquid Crystalline AIE Luminogens: Properties and Applications Downloaded by CORNELL UNIV on October 14, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch006

Dongyu Zhao* Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of the Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, P. R. China *E-mail: [email protected]

Aggregation-Induced Emission (AIE)-active liquid crystals (LCs) is a charming research area. Melding the advantages of AIE and LC, AIE-active LCs not only solve the annoying fluorescence quench naturally accompanying the formation of a mesophase but also exhibit a lot of distinguishing features. Despite the promising prospects of AIE-active LCs, the relative research is still rarely reported since the requirements for the LC and AIE characteristic are hard to fulfill simultaneously in one molecule. In this chapter, we summarized the state-of-the-art development of AIE LCs based on different chromphore cores, including silole, tetraphenylethylene and other AIEgen. The relationship between phase behavior and luminescence properties, as well as the aggregate state of the mesophase and luminescence properties are emphatically discussed. The versatile applications of this composite luminescent liquid crystal material are also introduced.

© 2016 American Chemical Society Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

Downloaded by CORNELL UNIV on October 14, 2016 | http://pubs.acs.org Publication Date (Web): September 27, 2016 | doi: 10.1021/bk-2016-1227.ch006

1. Introduction Luminescent liquid crystals (LE-LCs) has drawn growing attention because of the basic phenomenological interest and attractive technological applications such as anisotropic light-emitting diodes (1), polarized organic lasers (2), light-emitting liquid-crystal displays (LCDs) (3, 4), information storage (5), sensors (6) and one-dimensional semiconductors (7). The combination of intrinsic light-emitting properties with its unique self-organizing features endows the LE-LCs plenty of novel advantages. For example, LE-LCs can be used to manufacture stimuli-responsive luminescent materials owing to the sensitive response of LCs to external force stimuli, such as grinding, smearing and pressing. Among the variety of applications of LE-LCs, the linearly or circularly polarized luminescence is of especial importance for construction of highly efficient emissive LCDs. As an emissive display, the luminescent LCD fabricated from LE-LCs may enjoy quite a few advantageous features. For instance, the power-consuming extra backlight, polarizers and color filters would be obviated and thus enhance the power efficiency remarkably. Moreover, the LE-LCs allows simpler device design and hence device brightness, efficiency, contrast ratio and viewing angle of LCDs would be substantially increased (8). Despite these thrilling prospects of LE-LCs, achieving LCs with light emission characteristic remains a big challenge due to the aggregation caused emission quenching (ACQ) for traditional organic luminophores. Similarly, chromophoric mesogens are usually composed of rigid conjugated segments to self-assemble into LC phases. However, the dense packing of these rigid units probably suffer the ACQ, leading to the quench of fluorescence. Therefore, avoidance of the quench of the LC materials in aggregate state becomes a primary issue. Moreover, the efficiency of luminescence in the liquid crystalline phase remains in question, since aggregation or self-organization is a natural process in forming a mesophase which will also suffer the fluorescence quench. Aggregation-induced emission (AIE) is a fantastic photophysical phenomenon (9, 10) proposed by Tang et al. in 2001. In sharp contrast to annoying ACQ effect (11), AIE molecules usually exhibit brightly emission in the aggregate or solid state, which can be explained by the restriction of intramolecular motion (RIM) (12, 13) mechanism. Therefore, the AIEgens are expected to be promising candidates for developing LE-LCs. Up to now, a number of efforts have been made to develop AIE-based LE-LCs. Scientists incorporated different AIE cores with mesogens, leading to new AIE molecules with preservation of their mesomorphic properties (14–26). Conventional AIE luminogens (Scheme 1) such as silole, tetraphenylethylene (TPE), and cyanostilbene are used to construct the AIE-LCs. Some unusual AIE core are also explored to develop the AIE-LCs. The peripheral flexible side chains including alkyl and alkoxy chain are often introduced into the AIE-LC molecules as pendants. In this chapter, the state-of-the-art development of AIE-LCs is introduced, including their phase types, structure-property relationship, and their technological applications.

152 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Scheme 1. The molecule structure of representative AIE molecules.

2. Materials and Methods 2.1. Silole-Based AIE-LCs Since the rigid group and flexible group are two indispensable parts of a LC molecule, we try to summarize the LC from the point of rigid group, which is often also the luminescent unit in the molecule. Usually, the molecules delivering pronounced AIE effect possess propeller-shaped structures. As a well-known AIE core, silole with the non-coplanar geometry was used to construct the AIE-LC molecules. Wan et al. (14) described the preparation of two new AIE-LCs based on silole containing amide units and alkoxy chains. As homologues, these two silole-based compounds could be prepared in the same way with three procedures through a key intermediate compound, 2,5-bis(4-amidophenyl)-3,4,-diphenylsilole, accompanied with a coupling reaction (Figure 1A). Although containing a non-planar structure, the twisted tetraphenylsiole cores self-assembled into liquid-crystalline phases over a wide temperature range with the aid of the amide units and the alkoxy chains. In the DSC curves, both 1a and 1b showed two phase transitions during the heating and cooling processes, indicating undoubtedly these are two enantiotropic LCs. Moreover, increasing the aliphatic side-chain length of the tetraphenylsilole derivatives from dodecyloxy to hexadecyloxy lowered the melting point and clearing point, and the temperature range of the LC phase also decreased (146 °C for 1a, 122 °C for 1b). POM of 1a and 1b also showed anisotropic textures. As shown in Figure 1B, the textures of compound 1a obtained at 112.4 °C and 60.6 °C belong to smectic mesophase; however, the texture of 1b at 127.2 C and 67.6°C showed that compound 1b is an atypical LC molecule, indicating that the mesomorphic packing arrangements of LCs are sensitive to the length of the substituted alkoxy chains. In addition to the mesospheric properties, the compound 1a can thermo-reversibly form stable organogel in cyclohexane. And the emission of gel was much stronger than that of the solution, almost 100 times as much as which (Figure 1C, up). The drastic enhancement of fluorescence intensity after gelation could be observed by the naked eye (Figure 1C, down) and be explained by the 153 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


RIM mechanism. After aggregation, the intramolecular rotation of the phenyl substituents was hindered which blocked the non-radiative decay and resulted in the stronger PL intensity in the gel state. This gelation will induce diverse ordered aggregates which may have potential applications in explosive and ion detections.

Figure 1. (A) The molecule structure of LE-LC compounds 1a and 1b. (B) Optical micrographs of compounds 1a and 1b between crossed polarizers: liquid crystalline texture for 1a at 112.4 °C and 60.6 °C and 1b at 127.2 °C and 67.6 °C. (C, top) Fluorescence spectra of 1a in gel state and CH2Cl2 solution with the same concentration (14 mg/mL); photographic images of corresponding gel and solution observed upon UV irradiation at 365 nm (C, bottom). (Reprinted with permission from ref. (14). Copyright 2010 Royal Society of Chemistry.) 2.2. TPE-Based AIE-LCs TPE is another typical AIE luminogen. The advantages of TPE such as facile synthesis, high solid-sate efficiency, and versatile functionalization approaches make it an ideal candidate for constructing novel functional AIEgens. To date, several research groups have reported a few LC examples carrying TPE units. In general, four mesogenic pendants are usually needed to link with TPE to form the LC molecule. Yuan W Z et al. (15) reported the synthesis and characteristic of TPE4Me. As shown in Scheme 2, TPE4Me was prepared through a nucleophilic substitution and followed by a McMurry coupling, affording AIEgen-mesogens with both luminogenic and mesogenic properties (Scheme 2). In the THF/water mixture, TPE4Me demonstrated typical AIE-active features. At water fractions (fw) ~20% level, weak PL signals were recorded while in 90% aqueous mixture, the PL intensities at 380 and 450 nm increased by about 16 and 42 times (Figure 2A, 2B), respectively, attributed to the aggregation of luminogenic TPE4Me molecules. In the solid state, the absolute ΦF was as high as 67.4 ±5.0%. Upon cooling form the isotropic state, two anisotropic 154 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


mesomorphic textures of fan-shaped texture and focal conic texture emerged at 190°C and 100°C, respectively, as recorded by POM (Figure 2C, 2D). Combined with the DSC trace, it can be concluded that TPE4Me exhibited two mesomphoric phases and the phase transitions at 194.2°C and 140.8°C were associated to the isotropic–LC and LC–LC transitions, respectively. As suggested by WXRD analysis, although the disc-like TPE cores are twisted in conformation, the central TPE cores are still capable of packing together to generate face-on aligned columns. In the meantime, the peripheral mesogens self-assemble into a tetragonal smectic building blocks which were orthogonal to the TPE columns, resulting in a unique biaxially oriented mesomorphic structure, as shown in Figure 2E-2G.

Scheme 2. Synthetic route of TPE4Me. (Reprinted with permission from ref. (15). Copyright 2012 Royal Society of Chemistry.) Another kind of propeller-shaped TPE derivatives were prepared (16), which contain a TPE core and multiple long alkoxyl chains, linked by four 1,2,3-triazolyl groups between the TPE unit and substituted phenyl rings through click reaction (Figure 3A). As molecular design, the triazolyl group with polar character was expected to reinforce the microphase separation and stable LC phases would hence be produced. Since the space-filling of alkyl chains could affect the packing of propeller-shaped mesogens, the number of peripheral dodecyl chains was altered from eight in TPE-LC1 to 12 in TPE-LC2 to manipulate the packing structure of the propeller-like mesogens. The thermal properties of TPE-LC1 and TPE-LC2 were studied with the conventional methods of DSC analysis and POM observation. In the DSC curve of TPE-LC1, two reversible first-order transitions were observed during both the heating and the cooling scan, suggesting it was an entropic liquid crystal. Such a thermal behavior of TPE-LC1 was also verified by POM photographs obtained in the cooling process. Upon cooling from the isotropic state to 181 °C, the LC phase displayed a typical fan-like texture associated with the columnar phase of discotic LCs at 181 °C (Figure 3B). When it was continuously cooled, the second LC phase was attained at 159 °C and the fanlike texture remained unchanged till to room temperature except the texture color due to the variation of the sample thickness by temperature change (Figure 3C) , implying that there was significant structural change with the LC-to-LC transition. In comparison with TPE-LC1 possessing eight dodecyl chains, TPE-LC2 with twelve dodecyl chains showed only one LC phase with a fanlike texture that is an indicator of the formation of a columnar phase (Figure 3D). 155 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 2. (A) PL spectra of TPE4Mes in THF and THF/water mixtures; (B) Plots of the emission intensities at 450 and 380 nm versus water fractions (fw) for TPE4Mes in THF and THF/water mixtures. Concentration =4 mM. Excitation wavelength=310 nm. The inset graph is TPE4Mes in THF and 10/90 THF/water mixture taken under UV illumination. (C) and (D): POM images recorded on cooling TPE4Me to (C) 190 and (D) 100 °C from its isotropic state. (E)-(G): Schematic illustration of the biaxially oriented packing model of TPE4Me in the low temperature phase. (E) side view perpendicular to the side-chain mesogens, (F) top view, and (G) side view along the side- chain mesogens of the model. (Reprinted with permission from ref. (15). Copyright 2012 Royal Society of Chemistry.)

Owing to intrinsic AIE features, the propeller-like aromatic mesogens TPE-LC1 and TPE-LC2 exhibited their respective thermochromic behaviors upon heating and the emission color changes were associated with the conformational variations. Under 365 nm UV light irradiation, the emission color of TPE-LC1 was sky blue in the rectangular columnar (Colrec) phase, at a temperature of 170 °C, the color change to emerald which should be correlated with the hexagonal columnar (Colhex) phase (Figure 4A). Further rising the temperature, the emission turned to be dimmer. For TPE-LC2, with the temperature increasing, the fluorescence color exhibited a continuous change from sky blue to green in the Colhex phase, accompanied with a fluorescence decrease. The temperature-variable small and wide angle X-ray scattering methods were employed to study the microstructures of the propeller-like mesogens. The results revealed that unusual intercolumnar transformation and the packing dependency of the fluorescence property occurred in TPE-LC1 (Figure 4B-4D). The transition from Colrec to Colhex phase was conformed to first-order transition rather than the “second-order” transitions that usually occurs in discotic or polycatenar LCs based on a common tilt mechanism (Figure 4B). At the same time, TPE-LC1 adopted an unprecedented zigzag stacking in the Colrec phase. The fluorescence color change from sky blue to green was thus reversibly observed during the LC transition to Colhex in TPE-LC1, due to the planarization of the propeller mesogen. Taking full advantage of this reversible color change, some stimuli-responsive materials could be rationally prepared. 156 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


From AIE behaviors and mesophoric properties of luminescent compounds TPE-LC1 and TPE-LC2, we can make a conclusion that the peripheral flexible side chains play an important role on the phase transition and the type of mesophase, which will in turn lead to the different emission wavelength due to different stacking fashions of mesophases. Tang group also developed another TPE-functionalized molecule, named TPE-PPE that behaved as an AIE-LC (17). Conjugating four mesogenic units to the TPE core (Figure 5A), TPE-PPE was simply prepared through the Sonogashira coupling reaction between a brominated TPE and 4-ethynylpropylbenzene. From POM observation, an atypical anisotropic texture emerged during the heating scan (Figure 5B) and the mesophase appeared within the range from 218 to 228 °C was identified as sematic LC by XRD analysis. Attributed to the TPE unit, TPE-PPE exhibited evident AIE activity in the THF/water mixture: it is almost nonluminescent in pure THF while addition of water into its THF solution intensively enhanced its light emission, exhibiting a bright emission with its maximum at 530 nm.

Figure 3. (A) Molecular structures of TPE-LC1 and TPE-LC2 with the propeller-like TPE unit. The double-ended arrow indicates the torsional variation by the rotation of the phenyl rotor. Temperatures are given in °C. The values in parentheses are the enthalpy change (kJmol-1) of each transition. Optical textures of (B) TPE-LC1 at 181 °C, (C) TPE-LC1 at 159 °C, and (D) TPE-LC2 at 131 °C. Colrec, rectangular columnar; Colhex, hexagonal columnar; I, isotropic liquid phase. (Reprinted with permission from ref. (16). Copyright 2014 John Wiley and Sons.)



By dissolving 0.1 wt% of TPE-PPE into the nematic LC host, the linearly polarized emission was obtained on the unidirectional orientated LC cell. Firstly, for the nonpolarized emission, it was found that photoluminescence of the LC cell with 1.0 wt% concentration TPE-PPE was stronger than those of 0.5 wt% and 0.1 wt%, showing the evident AIE behavior (18). Second, the polarized fluorescence did not depend on the concentration of the luminescent dye. The photoluminescence polarization ratio of the LC cell reached to 4.16 in the directions perpendicular and parallel to the azimuthal direction. Utilizing the emissive anisotropy of TPE-PPE, two kinds of photoluminescent liquid crystal displays (PL-LCD) were fabricated with patterned ITO glass substrates and only one polarizer. For the first device, at the electric field-off state, the device emitted no light under UV illumination. However, when an electric field was applied on the device, the letters of ‘CDR HKUST’ with yellowish-green color were clearly seen. The off/on performance of this LE-LC devices could be easily switched by an electric field (Figure 6A). Moreover, the second PL-LCD based on the photoalignment technology has potential application in anti-counterfeiting (Figure 6B). This approach simplified device design, lowered the energy consumption and increased brightness of the LCD.

Figure 4. (A) Emission color change of TPE-LC1 as a function of temperature under 365 nm UV light at a heating rate of 10 °C min−1. (B) Models of “tilt” and “zigzag” stackings. (C) The molecular organization in the zigzag stacking of the Colrec phase of TPE-LC1 and (D) schematics for the variation in the degree of interdigitation between propeller-like mesogens at the intercolumnar transition. (Reprinted with permission from ref. (16). Copyright 2014 John Wiley and Sons.) 158 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


In addition to the linearly polarized emission, circularly polarized luminescence based on AIE-active TPE-PPE was attained (19). By dissolving TPE-PPE into chiral nematic LCs (N*LC), luminescent N*LC containing selective light reflection and circularly polarized luminescence could be attained. Like the TPE-PPE/N-LC system, the concentration of TPE-PPE had no influence on the CPL degree of the LC composites but was essential for the emission intensity, owing to the AIE effect in the system. Based on the selective reflection and CPL behaviors of the TPE-PPE/N*LC composite, a reflective-luminescent N*LC display device was constructed which could work under both sunlight and total darkness conditions (Figure 7). Such a work has paved the way for high efficiency LE-LC devices with low power consumption.

Figure 5. (A) Chemical structure of TPE-PPE. (B) POM photograph recorded on heating TPE-PPE to 222 °C. (Reprinted with permission from ref. (17). Copyright 2015 John Wiley and Sons.)

Besides the symmetric TPE derivatives, the dissymmetric TPE-based molecules also could form mesomphoric phase (20). Derived from tetraphenylethylene and gallic acid, Miao Luo et al. reported synthesis of two single substituted compounds of TPE, luminescent LCs P4 and P5, and the synthetic route is shown in Scheme 3. POM and DSC measurements indicated that compounds P4 and P5 exhibited enantiotropic LC behaviors under heating and cooling processes. The temperatures of melting point and clearing point of P5 were higher than that of P4, owing to the additional phenyl unit. Both the POM (Figures 8A, 8B) and WXRD revealed that P4 and P5 probably belongs to smectic LC phase. Attributed to their twisted structure, P4 and P5 exhibited strong AIE activity. In Comparison with P4, P5 had bigger molecular size and steric hindrance (Figure 8C), and thus showed a stronger AIE activity with an I/I0 ratio of 101 than the 44 of P4 (Figure 8D). Moreover, P4 and P5 possessed different gelation behavior in organic solvents. Also, the gelation would induce fluorescence enhancement for both P4 and P5. In the case of the same concentration, P5 was almost non-luminescent in n-hexane but became much more emissive (the fluorescence intensity increased by almost 120 times) in the gel state, due to the formation of self-assembly aggregates (Figure 8E). The fluorescence of P5 can reversibly modulated between on and off states by the gel-solution transition via alternate 159 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


cooling and heating cycles. This thermo-responsive AIEgen-based organogels have potential applications in information storage, optical devices and biological applications.

Figure 6. (A) Photograph of the LE-LCD device in the electric field-off and field-on states using the light-emitting LC mixture. The LC mixture = Nematic LC PA0182 + 0.1 wt% TPE-PPE. (B) The structure and the photographs of the LE-LCD with photo-patterned alignment using light-emitting LC mixture. There are two regions in the device that are aligned orthogonally. In the electric field-off state, the regions with and without figures in the device will be alternately bright and dark, viewing through the rotatable polarizer under UV irradiation. In the electric field-on state, both the two regions will be light-emitting and the figures disappeared under UV irradiation. (Reprinted with permission from ref. (17). Copyright 2015 John Wiley and Sons.) 160 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 7. Photographs of the reflective-luminescent display device with an applied DC electric field (60 V) under sunlight and under UV-light irradiation using the light-emitting N*LC mixture. The LC mixture = N*LC-3+0.5 wt% TPE-PPE. The wavelength of the excitation light was 365 nm. (Reprinted with permission from ref. (19). Copyright 2016 John Wiley and Sons.)

Scheme 3. Synthetic routs for LE-LCs P4 and P5. (Reprinted with permission from ref. (20). Copyright 2014 Elsevier.) In addition to the AIE behaviors and gelation ability, the piezofluorochromic properties were also recorded. For compound P4, there was no obvious piezofluorochromic property; However, compound P5 exhibited the clear emission color change from blue to blue-green upon pressing, and the fluorescence could be recovered through heating or solvent fuming. In consideration of molecular structure and molecular aggregation, since P5 was more flexible than P4 due to the one more phenyl ring in the linkage, the crystalline-amorphous phase transformation turned to be easily happened and thus the piezofluorochromic properties were easy to be induced. 161 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 8. Optical micrographs of compounds P4 and P5 between crossed polarizers: (A) liquid crystalline texture for P4 at 120 °C; (B) liquid crystalline texture for P5 at 128 °C. (C) PL spectra of P5 in THF/water mixtures with different water fractions (inset: the images of P5 were taken at room temperature under 365 nm UV light in THF and 90% water); (D) Change in PL peak intensities of P4 and P5 with different water fractions in THF/water mixtures (concentration 10 mM, excitation wavelength 365 nm). (E) Fluorescence spectra of P5 in gel state and solution state in n-hexane with the same concentration (20 mg/mL), inset: Fluorescence images of P5 for various temperatures. (Reprinted with permission from ref. (20). Copyright 2014 Elsevier.) 2.3. Cyanostilbene-Based AIE-LCs Besides the usual AIE luminescent cores including sisole and TPE, there are still unusual light-emissive cores that were utilized in constructing AIE LCs. For example, cyanostilbene is a special AIEgen, it could form both smectic and columnar luminescent liquid crystals, depending on the substituent groups on the luminescent core. Lu H et al. (21) has reported a cyanostilbene-based AIEgen, (Z)-CN-APHP, with a dodecyloxy tail directly linking with the cyanostilbene core (Figure 9A). The introduction of electron-donating -NH2 group was aimed to enhance compatibility with LC molecules and made CN-APHP form a gel in the liquid crystalline phase. The result of POM and DSC suggested that (Z)-CN-APHP was an enantiotropic LC (Figure 9B). Under heating, (Z)-CN-APHP entered the liquid crystalline phase with the melting point at ~80 °C and clearing point at ~112 °C, respectively. The focal conic fan-shaped texture obtained at 75°C during the cooling process illustrated that (Z)-CN-APHP has a smectic mesophase. As a specific AIEgen, cyanostilbene unit in (Z)-CN-APHP underwent the E−Z isomerization process after irradiation at 365 nm in the LC phase (90 °C), accompanied by the transition from intensive green luminescence with a ΦF of 19.5% to the light bluet fluorescence with a radical low ΦF of 9.2% (Figure 9C). As revealed by the XRD analysis, before irradiation of the thin film of (Z)-CN-APHP, rod-like shaped Z-isomer showed a well-ordered liquid crystalline phase and turned to an obvious amorphous structure after 2 hours UV 162 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


irradiation, leading to the E-isomer. The strongly twisted two rings of bent-shaped E-isomer prevented the ordered packing of the molecules in the aggregation state. Moreover, the transition from Z-isomer to E-isomer reduced its effective π-conjugation length, resulting in the hypsochromic shift of ~24 nm, the decrease of photoluminescence intensity and the loss of LC phase (Figure 9D). As predicted, this E−Z isomerization process of AIE-LC molecule between LC state and amorphous state will induce many optical applications. The fluorescent molecule-dispersed liquid crystals was prepared by mixing 16.7% (Z)-CN-APHP with 83.3% positive liquid crystals E7. After filling into the unidirectional rubbed LC cell, the Z- E-isomerization process occurred upon UV irradiation and led to phase separation as a result of the poor compatibility of E-isomers with liquid crystals, forming the so-called fluorescent-molecule dispersed liquid crystals (FMDLC) (Figure 9E, 9F). Between the applied electric field-on (30v) and electric field-off states, the photoluminescence intensity of FMDLC could be switched repeatedly, owing to the different internal scattering of the excitation light (Figure 9G). Take advantages of the internal scattering-based mechanism, this modulation could also be utilized to develop other light-emissive liquid crystal systems, such as fluorescent polymer dispersed liquid crystals and fluorescent polymer stabilized liquid crystals.

Figure 9. (A) Schematic illustration of Z−E isomerization of (Z)-CN-APHP upon UV irradiation in the liquid crystal phase. (B) DSC curves of (Z)-CN-APHP at a heating/cooling rate of 10°C per minute. (C) Photo images of (left) unirradiated and (right) irradiated films of 521a on the quartz cells. (D) POM images of the (left) unirradiated film and (right) irradiated film of (Z)-CN-APHP. (E) Fluorescence emission spectra of FMDLC at field-off (0 V) and field-on (30 V) states. (F) Repeated switching of photoluminescence between the field-off (0 V) and field-on (30 V) states. (G) Schematic illustration of the mechanism of electrically switched photoluminescence of FMDLC. (Reprinted with permission from ref. (21). Copyright 2014 Royal Society of Chemistry.) 163 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Another cyanostilbene-based AIE-LC compound, namely, (Z)-2,3bis(3,4,5-tris (dodecyloxy)phenyl) acrylonitrile (GCS, Scheme 4), was also developed to explore the potential use of cyanostilbene compounds by virtue of the photoisomerization property (22). GCS was synthesized via the Knoevenagel reaction of 3,4,5-tris(dodecyloxy)benzaldehyde and 2-(3,4,5-tris(dodecyloxy)phenyl)acetonitrile. The molecular structure of GCS consists of a cyanostilbene moiety and the tris-(dodecyloxy) groups as peripheral flexible side chains which were directly attached to the mesogenic core to facilitate mesomorphic organization.

Scheme 4. Molecule structure of LE-LC GCS. (Reprinted with permission from ref. (22). Copyright 2014 John Wiley and Sons.) In studying the liquid crystalline properties of GCS, a typical focal-conic fan-shaped texture which is an indicator of a columnar hexagonal (Colh) phase was observed in the cooling process with POM (Figure 10A, 10C). The formation of the Colh phase was further confirmed by XRD patterns of the GCS in LC phase temperature, demonstrating a two-dimensional hexagonal lattice with p 6 mm symmetry. Near room temperature (RT), the isothermal crystallization of GCS was observed, which transformed the columnar hexagonal LC phase to the crystalline phase with a tetragonal arrangement under prolonged natural cooling (Figure 10B, 10D), implying the limited mesophase stability of GCS induced by intermolecular forces. Usually, azo materials were utilized to fabricate SRG (surface relief grating) through photo-isomerization of azobenzene resulting from the photoinduced mass migration; however, fluorescent SRG is scarce because the azobenzenes are generally non-fluorescent while fluorescent dyes that doped into azobenzene systems are prone to suffer from quenching by the non-fluorescent azobenzene components. Taking advantages of the E-Z photo-isomerization of GCS, fluorescence patterning was obtained based on photoinduced mass transfer. It was found that, under UV light irradiated and at 39 (±1) °C the crystalline phase totally disappeared, resulting in the dramatic decrease in the fluorescence intensity (Figure 10E, 10F). Thin film of GCS was thus prepared by spin-coating and then irradiated with a non-polarized Hg lamp at 365 nm through a micropatterned-photomask to fabricate the SRG. The maximum diffraction efficiency reached to almost 30%. Figure 10G shows the AFM image of the SRG. Regular sinusoidal undulation was observed clearly, consistent with the spacing of the photomask (Figure 10H). Because of the intrinsic AIE feature of GCS, strongly fluorescent micropattern with high contrast was generated (Figure 10I), thus avoided the concentration quenching. 164 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


Figure 10. (A) POM image of the Colh phase of GCS obtained at 30 °C during cooling. (B) POM image of the crystal phase of GCS obtained at room temperature during cooling. (C) XRD patterns of the GCS in LC phase at 28 °C and (D) in the crystal phase. The insets in (C) show the magnified diffraction peaks and the 2D pattern image. (E) POM image and (F) Fluorescence optical microscopic image of the crystalline film. (G) Topographical AFM image and (H) height profile along the white line. (I) Fluorescence optical microscopy image. (Reprinted with permission from ref. (22). Copyright 2014 John Wiley and Sons.) Besides GCS containing single cyanogroup, Park et al. also synthesized another AIEgen (GDCS) which is a phasmidic molecule comprising dicyanostilbene moieties and the terminal trisdodecyloxy fragments (23). Different from GCS, GDCS contains two cyanostilbene in its molecular structure (Figure 11A). Similarly, GDCS also exhibited a luminescent columnar hexagonal phase at room temperature. As revealed by the POM observation, a pseudo focal-conic fan-shaped texture of a hexagonal columnar (Colh) phase emerged in both heating and cooling cycles (Figure 11B), illustrating it is an enantiotropic LC. DSC trace also confirmed the enantiotropic characteristic with the phase-transitions of the heating and cooling processes (Figure 11C). From the calculated optimized molecular structure of GDCS based on the XRD data, a pair of GDCS molecules self-assembled into a molecular disk in a side-by-side disposition, driven by the secondary bonding interactions of the lateral polar 165 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


cyano group. And thus, the hexagonal structure of columnar LC phase was organized from the supramolecular disks. Owing to the AIE feature of GDCS, the fluorescence efficiency (ΦF) in THF solution, LC phase and crystalline (ΦF = 0.45) state was 1.1 × 10−2, 0.25 and 0.45, respectively, demonstrating an obvious increasing tendency. During the cooling process from 200 °C to RT, the PL spectra of GDCS in condensed state under 365 nm UV light underwent a bathochromic shift, followed by a gradually increased PL intensity, from liquid state to liquid crystal state and to the crystalline state (Figure 12A-12C). Above the clearing point, the molecule entered the isotropic state and complete absence of light emission was observed. In addition, the LC and crystalline state emitted intense green and yellow lights, respectively. The thermochromic fluorescence of GDCS were attributed to the peculiar intra- and intermolecular interactions of its dipolar cyanostilbene units. The hierarchical mesomorphic organization of the GDCS was comprehensively presented in Figure 12D, explaining the PL behaviors of the GDCS material in different phases.

Figure 11. (A) Molecular structures of GDCS. (B) Pseudo focal-conic fan-shaped texture of GDCS observed by POM in the Colh phase at 40 °C on the cooling process. (C) DSC trace of GDCS on heating/cooling rate of 10 °C per minute. (Reprinted with permission from ref. (23). Copyright 2012 John Wiley and Sons.) The specific molecular stacking processes in the LC and crystalline phases lead to the restriction of the molecular disks’ rotational motions, making it present an AIE effect. This deep insight of the structure-property relationship would provide guidance for the design of the LE-LC with efficient luminescence in the LC state. Moreover, since the different stacking fashion will result in the variety of excited-state molecular coupling and thus the different emission colors. This 166 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


provides the possibility to develop a probe that can distinguish the phases of thermotropic liquid crystals.

Figure 12. (A) Photos of GDCS in condensed state during the cooling process from 200 °C to RT under 365 nm UV light. (B) PL wavelength maximum shift behavior and (C) PL intensity change behavior depending on the temperature at the cooling process. (D) Schematic representation of the hierarchical mesomorphic organization of GDCS molecule concurrent with the intra- and intermolecular actions related to the emission characteristics. (Reprinted with permission from ref. (23). Copyright 2012 John Wiley and Sons.) 2.4. AIE-Active Liquid Crystalline Polymers (AIE-LCPs) In addition to AIE-LC small molecule, AIE active liquid crystalline polymers (AIE-LCPs) can also be achieved by incorporating AIE mesogens into the polymer backbones. Yuan et al. reported liquid crystalline polytriazoles with high solidstate emission efficiencies by click polymerization (24), affording polymers LCP1 and LCP2 et al. characterized by different spacer lengths.

Scheme 5. Molecular structures of aggregation-Induced emission active liquid crystalline polymers. (Reprinted with permission from ref. (24). Copyright 2011 American Chemical Society.) 167 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.


As shown in scheme 5, the long alkyl chains and the TPE group endowed these polymers with good solubility, high regioregularity and obvious AIE activity. The photophysical properties of these polytriazoles showed obvious structural dependence, and those having longer spacer length showing lower solid-state ΦF. Besides, the mesomorphic properties was closely relative to the spacer length. LCP1 that possesses rigid main chains belong to nematic LC phase, while those polytriazoles with longer flexible spacers formed smectic phases through better mesogenic packing (LCP2a and LCP2b; Figure 13B, 13E). It seems the polymers with longer spacer lengths prefer to exhibit nematicity due to its better mesogenic packing.

Figure 13. Mesomorphic textures observed on cooling (A) LCP1a to 159.9 °C, (B) LCP2a to 89.9 °C, (C) LCP2c to 94.9 °C, (D) LCP2b to 250.6 °C, (E) LCP2b to 69.8 °C, and (F) LCP2d to 114.9 °C from their melting states at a cooling rate of 1 °C/min. Photos in parts A, B, D, and E are taken after application of a shearing force. (Reprinted with permission from ref. (24). Copyright 2011 American Chemical Society.) 2.5. Other AIE-LCs In above AIE-LCs, their AIE feature is well preserved no matter whether the core is silole, TPE, or cyanostilbene. In addition to these common AIE cores, some unusual AIEgens were also explored to construct AIE-LCs. Fujisawa K. et al. have synthesized several liquid-crystalline gold complexes that comprised a rigid body and a flexible alkoxy tail, showing smectic phase and AIE characteristics (25). During the phase transitions between LC and isotropic phases as well as crystalline and LC phases, the fluorescence color could be reversibly switched. The stronger emission of LC complexes in solid states demonstrated their potential application in light-emitting devices. Kana Tanabe et al. reported the AIE-active luminescent ionic liquid crystals that have hexagonal or rectangular columnar structures in their LC phase (26). The fluorescent color covering the visible region could be easily tuned by changing electron-donating and electron-accepting moieties of the 168 Fujiki et al.; Aggregation-Induced Emission: Materials and Applications Volume 2 ACS Symposium Series; American Chemical Society: Washington, DC, 2016.

compounds, providing the luminescent materials potential in organic fluorescent sensors and optoelectronic applications.


3. Conclusion In general, the research on AIE-active LCs is an attractive area with thrilling prospects. By integrating the advantages of light emission and liquid crystal, many photofunctional liquid crystal materials were achieved such as stimuli-responsive luminescent LCs and organogelators. More importantly, the polarized fluorescence of the LE-LCs with AIE properties are very useful for fabrication of novel luminescent optoelectronic devices. Despite the achievements, research in this area is still not enough. The mesophase of these luminescent LC materials is very limited, preventing many important applications such as stimuli-responsable luminesence; the fluorescence anisotropy of these LCs is urgently needed to be enhanced. Therefore, the next step of research in this area should be enrichment in the variety of AIE-LCs. In addition, the real-world technological applications of these superior LC materials should be explored further.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant 51203005) and the New Teacher Fund for Doctor Station and the Ministry of Education of China (Grant 20121102120045).

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Liquid Crystalline AIE Luminogens: Properties and Applications - ACS

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