Luminescent Liquid Crystalline Polymers: Molecular Fabrication, Structure-Properties and Their Applications

Zhenxing Li; Zhiwang Luo; Ping Wang; Zhenqiang Yu; Erqiang Chen; Helou Xie

doi:10.7536/PC210437

Progress in Chemistry ›› 2022, Vol. 34 ›› Issue (4) : 787-800 DOI: 10.7536/PC210437

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Luminescent Liquid Crystalline Polymers: Molecular Fabrication, Structure-Properties and Their Applications

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Abstract

Luminescent liquid crystalline polymers (LLCPs), combining the ordering, stability, mechanical properties of the liquid crystalline polymer with the luminescent properties of chromophores, show broad applications. In order to obtain high-efficiency LLCPs, various LLCPs with different structures have been successfully designed and synthesized, including main-chain, side-chain, mesogen-jacketed LLCPs and LLCP network, etc. Meanwhile, interaction of the molecular structure, liquid crystalline phase structure and optical physical properties, has also been carefully investigated. In this paper, the latest progress of LLCPs is summarized, including the molecular design and synthesis, structure and properties, and their applications. At last, a brief outlook on the future development in this field is presented.

Contents

1 Introduction

2 Molecular design of luminescent liquid crystalline polymers

2.1 Main-chain luminescent liquid crystalline polymers

2.2 Side-chain luminescent liquid crystalline polymers

2.3 Mesogen-jacketed luminescent liquid crystalline polymers

3 Applications of luminescent liquid crystalline polymers

3.1 Polarized luminescence

3.2 Anti-counterfeiting

3.3 Optical information storage

4 Conclusions and outlook

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Keywords

luminescent liquid crystalline polymer / main-chain / side-chain / mesogen-jacketed / liquid crystal polymer network / intelligent material

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Zhenxing Li, Zhiwang Luo, Ping Wang, Zhenqiang Yu, Erqiang Chen, Helou Xie. Luminescent Liquid Crystalline Polymers: Molecular Fabrication, Structure-Properties and Their Applications. Progress in Chemistry, 2022, 34(4): 787-800 DOI:10.7536/PC210437

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1 Introduction

Luminescent liquid crystals (LLCs) combine the ordering of liquid crystals and the luminescence of chromophore molecules, which can be widely used in emissive liquid crystal displays, organic light-emitting diodes, optical information storage and so on^[1,2]^[3,4]^[5]. Devices made from these materials have deeper color saturation, wider viewing angle, higher contrast, and lower power consumption. Therefore, it has attracted many scientists to study in the past decades. Luminescent liquid crystalline polymers (LLCPs) can synergistically utilize the ordered structure of liquid crystalline polymers and the Luminescent properties of different chromophores to achieve fluorescence or phosphorescence emission, and usually show strong fluorescence quantum yield and optical stability. High molecular weight makes LLCPs have good film forming, mechanical properties and low environmental dependence, which is expected to further broaden the application range of luminescent liquid crystals in frontier fields^{[6⇓⇓⇓⇓⇓~12]}.

In order to obtain efficient luminescent liquid crystal polymers, different luminescent chromophores, such as oligofluorene, oligothiophene, oxadiazole, fluorene derivatives, and metal compounds, have been explored to construct luminescent liquid crystal polymers^{[13⇓⇓~16]}. However, the formation of delocalized excitons can lead to strong intermolecular π-π stacking interaction, and this kind of chromophores usually show strong luminescence behavior in solution, while the luminescence behavior tends to be weakened or quenched in the aggregated state.This Aggregation-induced quenching (Aggregation caused quenching, ACQ) effect is common in most fluorescent conjugated molecular systems, which limits the further development of related research and practical applications of luminescent materials^[17⇓~19]. At the same time, the use of traditional ACQ chromophore in luminescent liquid crystal polymers is contrary to the self-assembly characteristics of liquid crystals, and the luminescent efficiency of the prepared luminescent materials tends to be reduced. In order to solve the contradiction between the luminous efficiency of Aggregation state and the ordered structure of liquid crystal, an effective method is to replace the traditional luminescent group with chromophores with Aggregation induced emission (AIE) effect in luminescent liquid crystal polymers. The luminescence efficiency of AIE chromophores in good solvents is usually very low, or even no luminescence, but in poor solvents or solids, due to the Restriction of intramolecular motion (RIM), the non-radiative transition is weakened, and the luminescence is significantly enhanced. When the liquid crystal polymer containing AIE chromophore forms liquid crystal phase in solid state, the ordered arrangement of molecules can further enhance the RIM degree of AIE chromophore^[20,21]. Compared with traditional luminescent materials with ACQ effect such as perylene diimide derivatives and quantum dots, luminescent liquid crystal polymers constructed by AIE chromophores show higher solid-state luminescent efficiency and photochemical stability, and show unique advantages in the fields of polarized emission systems, liquid crystal displays and the like^[22,23]^[24]. In addition, the in-depth study of luminescent liquid crystal polymers is also helpful to further understand the relationship between structural order and photophysical properties.

In recent years, in order to understand the relationship between the structure and properties of luminescent liquid crystalline polymers, luminescent liquid crystalline polymers with different structures have been successfully designed and synthesized, such as main-chain type, side-chain type, "shell" type luminescent liquid crystal polymers, luminescent liquid crystal polymer networks, etc. In this paper, the research progress of luminescent liquid crystalline polymers in recent years is summarized, including the design and synthesis of molecular structures, structures and properties, and related applications.

2 Molecular Design of Luminescent Liquid Crystal Polymer

2.1 Main chain type luminescent liquid crystal polymer

The traditional main-chain liquid crystal polymer places the mesogen on the main chain, while the main-chain luminescent liquid crystal polymer places both the mesogen and the chromophore on the main chain. It is worth mentioning that some chromophores are mesogens themselves. Therefore, the main-chain luminescent liquid crystalline polymer not only shows the unique luminescent properties of its chromophore, but also has the physical and chemical properties similar to those of the ordinary main-chain liquid crystalline polymer, such as larger chain rigidity, higher melting point, glass transition temperature, good thermal stability, wider liquid crystal interval and so on.

Luminescent liquid crystal polymers can be constructed by polymerization of rigid fluorescent groups or copolymerization of rigid fluorescent groups with other liquid crystal groups. Chen et al. Prepared a series of luminescent polymers with liquid crystal properties using Polyfluorene^[25]. The polymer exhibits a nematic liquid crystal structure and blue luminescence, and its glass transition temperature can be controlled by controlling the number of aliphatic side chains. Yasuda et al. Constructed a series of luminescent liquid crystal polymers based on thiophene system^[26]. The results show that with the increase of the number and length of the side chains, it shows a transition from smectic phase to columnar phase to micellar cubic phase, and the peripheral flexible chains and connecting units can be used to adjust the intermolecular interaction and liquid crystal structure. Marin et al. Reported five new semi-flexible polyazomethines containing rigid chromophores^[27]. Based on the different chromophores connected to the liquid crystal element, the polymer can realize photoluminescence in different wavelength bands. In addition, the heavy metal ions in the polymer system are uniformly distributed through the ordered structure of the liquid crystal phase, and the constructed ionic liquid crystal polymer can not only realize fluorescence emission, but also enhance the mechanical strength of the whole polymer. Zhang et al. Reported a series of main-chain luminescent liquid crystal polymers P1 ~ P6 based on different europium ion contents (Figure 1A). The study showed that these polymers had good thermal stability and formed a nematic liquid crystal structure with a wide temperature range. The introduction of europium ions did not change the overall liquid crystalline and thermal properties of the polymers (Figure 1b)^[28]. With the increase of europium ion content, the fluorescence emission intensity of P1 ~ P6 gradually increased, because the europium ions which can form good coordination with carboxyl groups can be well dispersed in the polymer matrix with ordered liquid crystal structure, thus further improving the emission efficiency of europium ions. The emission intensity of P1-P6 shows a negative correlation with temperature, and the fluorescence intensity of the series of polymers can be quantitatively adjusted by accurately controlling the temperature.

The luminescent liquid crystal polymers constructed with traditional chromophores have inherent ACQ effect, usually show poor solid-state fluorescence emission behavior and low emission efficiency, which limits their application in optical information storage, anti-counterfeiting and other frontier fields^[29,30]. Combining the advantages of ordered packing structure formed by self-assembly of AIE chromophore and mesogen, Tang et al. constructed a series of main-chain luminescent liquid crystal polymers P7 ~ P12 with different spacers by click chemistry reaction of Tetraphenylethylene (TPE) group with AIE properties and biphenyl mesogen, as shown in Figure 2a^[31]. The results show that all the polymers exhibit AIE properties, and the fluorescence quantum yield of P7 reaches 63. 7% in the solid state. However, the longer spacer length in the solid state is not conducive to the interaction between chromophores TPE, which makes the internal rotation of TPE less limited, so the increase of spacer length will lead to the decrease of the fluorescence quantum yield of polymers in the solid-state. The results of X-ray diffraction (XRD) and Polarized light microscope (PLM) show that the polymers have good thermal stability and liquid crystallinity. The polymers with shorter spacer groups, such as P8, show nematic liquid crystalline structure due to the existence of rigid backbone. When the spacer is longer, the biphenyl mesogens of polymer P12 are more orderly stacked into a smectic phase structure (Fig. 2b-e).

2.2 Side chain luminescent liquid crystal polymer

Side-chain luminescent liquid crystal polymers are obtained by connecting liquid crystal elements or chromophores to the main chain of the polymer in a tail-connected or waist-connected manner through a flexible spacer. Compared with the main-chain light-emitting liquid crystalline polymer, the side-chain light-emitting liquid crystalline polymer can be prepared by a variety of polymerization reactions, such as addition polymerization, ring-opening metathesis polymerization, condensation polymerization and so on^[32]. In addition, the mesogens of the side chain can also be bonded to the polymer backbone by post-modification and other methods (such as hydrosilylation). The chemical structure of the side chain liquid crystal polymer is easier to control, and the molecular design is more convenient, thereby being beneficial to regulating the liquid crystal phase structure and the phase transition temperature^[33,34].

Zhang et al. synthesized a series of side chain luminescent liquid crystal polymers P13 ~ p16 with different spacer lengths (Figure 3A) by using α-cyanostilbene chromophore with AIE characteristics as the main body, and all the polymers showed Aggregation induced-enhanced emission (AIEE) characteristics^[35,36]. With the increase of the spacer, the solid-state fluorescence quantum yield of P13 ~ p16 increases from 3.34% to 16.89% (Figure 3B), because the increase of the spacer will further reduce the coupling effect between the main and side chains, which is conducive to the self-assembly of cyanostilbene molecules into J-shaped aggregates to prolong the degree of conjugation between molecules, thus greatly improving the solid-state fluorescence quantum yield. XRD and Differential scanning calorimetry (DSC) showed that P13 ~ P16 all exhibited double-layer smectic A phase structure, and the increase of spacer length contributed to the improvement of the overall molecular motion ability and led to the gradual decrease of the isotropic temperature. However, the solid state fluorescence quantum yield of the above polymers is low. In order to improve the quantum yield, the alkoxy tail chain of the α-cyanostilbene chromophore was replaced by a cyano group, and a series of novel side chain luminescent liquid crystal polymers P17 ~ P20 were prepared (Figure 3A). On the one hand, the cyano group at the end of the side chain in the polymer has strong electron-withdrawing ability, and the alkoxy group between the alpha-cyanostilbene and the main chain has certain electron-donating ability,The resulting D-π-a structure can delocalize the π electrons of the chromophore and reduce the energy difference between the orbitals. On the other hand, α-cyanostilbene can form a larger conjugated system with the terminal — CN group. Under the synergistic effect of these two aspects, the solid-state fluorescence quantum yield of the polymer is greatly increased, and the solid-state quantum yield of the polymer P20 can reach 42.7% (Fig. 3B). The results show that P17 and P18 with shorter spacer length exhibit amorphous structure, while P19 and P20 with longer spacer length can form nematic liquid crystal, because the longer spacer will reduce the coupling effect between the main chain and the side chain of the polymer, thus facilitating the formation of liquid crystal phase.

Chen et al. Prepared a series of side-chain light-emitting liquid crystal polymers P21 ~ P26 with polynorbornene as the main chain and α-cyanostilbene with Z/E configuration as the side chain by living Ring-opening metathesis polymerization (ROMP), as shown in Figure 4A^[37]. The polymers obtained by this polymerization method have high molecular weight and narrow molecular weight distribution, and all of them show AIE characteristics. Irradiation of the polymer solution with a 365 nm ultraviolet lamp can cause the Z configuration of the α-cyanostilbene group on the side chain to transform to the E configuration. It can be seen from the XRD experimental results (fig. 4B) that the polymers P25 and P26 with the side chain of E configuration are amorphous. For the polymer with Z-configured side chains, the liquid crystal phase structure can be transformed from lamellar liquid crystal phase structure to hexagonal columnar liquid crystal structure with the increase of the number of terminal alkoxy flexible tail chains (Fig. 4C). The above results show that the polymer can not form either lamellar or columnar liquid crystal phase when the configuration of α-cyanostilbene changes from Z to E, which proves that the configuration of cyanostilbene plays an important role in the liquid crystal phase of the polymer. In addition, the liquid crystal phase structure of the polymer can be precisely controlled by changing the number of alkoxy chains.

Different from the above light-emitting liquid crystal polymers constructed by connecting the light-emitting liquid crystal unit with the polymer main chain by using the alkyl flexible chain, Tian et al. Constructed a series of light-emitting liquid crystal polymers P27 ~ P31 by connecting the α-cyanodistyryl group to the methacrylate main chain by using the oligoethylene glycol chain with different lengths as the spacer, as shown in fig. 5A^[38]. All polymers were able to self-assemble into a hexagonal columnar liquid crystalline phase structure and exhibited highly efficient fluorescence emission. Experiments show that with the increase of the length of the flexible spacer, the size of the hexagonal columnar liquid crystal phase becomes larger and the degree of order improves (Fig. 5B). The reason may be that the nano-separation between the hydrophilic spacer and the hydrophobic α-cyanostilbene mesogen contributes to the formation of a more ordered columnar phase. At the same time, the maximum emission wavelength and fluorescence lifetime of the polymer system are inversely proportional to the length of the flexible spacer, because the larger the size of the formed hexagonal columnar liquid crystal phase is, the weaker the electron coupling strength of the cyanostilbene liquid crystal unit in the excited state in the polymer is, which leads to the blue shift of the emission wavelength.

The synthesis route of traditional liquid crystal polymer constructed by covalent bonds is relatively complex and limited by the polymerization activity of monomers and reaction conditions. In contrast, liquid crystalline polymers constructed by hydrogen bonding interactions are easy to prepare, and their supramolecular structures can be adjusted by adjusting the ratio of hydrogen bonding donors and acceptors. Chen et al. prepared a series of side chain luminescent liquid crystal polymers P32 based on hydrogen bond construction by using polyvinylpyridine as a hydrogen bond acceptor and benzoic acid derivatives as a hydrogen bond donor (Figure 6a). With the increase of the content of benzoic acid derivatives in the supramolecular liquid crystal polymer, its liquid crystal phase structure will change from smectic phase to hexagonal columnar phase^[39]. The solid state films thus prepared exhibited stronger fluorescence emission than the bulk of the benzoic acid derivative. On the other hand, its solid-state quantum yield decreases with the transition from the smectic phase to the hexagonal columnar phase. Chen et al. Also used polyvinylpyridine as the main chain of the supramolecular polymer, but selected the photoisomerizable α-cyanostilbene derivative as the hydrogen bond donor to prepare the side chain supramolecular polymers P33 and P34 with Z/E configuration of the side chain (Fig. 6a)^[40]. The results show that the polymer P33 with Z configuration exhibits a hexagonal columnar liquid crystal phase structure when 0. 4 ≤ X ≤ 1.0 (X is the molar ratio of the chromophore containing side chain unit to the polymer main chain repeating unit), while the polymer P34 with E configuration exhibits a lamellar liquid crystal phase structure when 0. 3 ≤ X ≤ 0.7. Compared with P34, P33 can induce E/Z isomerization through photothermal effect to achieve reversible control of self-assembled structure, surface morphology and properties, birefringence, fluorescence intensity and color, as shown in Fig. 6B.

Luminescent liquid crystal polymers are usually based on fluorescence emission, while Room temperature phosphorescence (RTP) liquid crystal polymers are rarely studied. Yu et al. Designed a series of side-chain room temperature phosphorescent liquid crystal copolymers P35, P36, P37 (Figure 7A) using phosphorescent molecules bromophthalimide and acrylamide and cholesterol molecules with different contents as raw materials^[41]. The experimental results show that all the copolymers exhibit RTP behavior and that their phosphorescence lifetime and intensity are greatly enhanced by annealing. The DSC results showed that the three kinds of copolymers all showed typical liquid crystallinity. However, only P37 with the highest cholesterol content can observe the nematic liquid crystal texture under polarized light, because the low cholesterol content in P35 and P36 makes it difficult for the liquid crystal domains in the polymer to develop into liquid crystal domains that can be observed under polarized light, as shown in Figure 7B. This experimental result proves that the liquid crystallinity of the copolymer is proportional to its cholesterol content. By further comparing the XRD results of the three types of copolymers before and after annealing, it can be seen that only P37 with the highest cholesterol content has a good ordered arrangement of mesogens after annealing (Fig. 7C), indicating that annealing can enhance the RTP effect of this type of copolymer.

2.3 Carapace type luminescent liquid crystal polymer

The "shell" liquid crystal polymer refers to a bulky pendant group (which may not be a liquid crystal element) connected to the polymer backbone by a covalent bond or a very short spacer,The high density of side groups tightly wraps the main chain like a "carapace", forcing the main chain to assume an extended chain conformation, and then the whole molecule behaves like a rigid rod-like structure, which can be further stacked to form various liquid crystal ordered structures^[42,43]. "Shell" liquid crystalline polymers usually have high glass transition temperature, excellent thermal stability and stable liquid crystalline phase structure.

Xie et al. introduced the AIE active TPE group into the typical "carapace" liquid crystal polymer polyvinyl terephthalate, and constructed a series of "carapace" luminescent liquid crystal polymers P38 ~ P43, as shown in Figure 8A^[44]. The study showed that all the polymers exhibited aggregation-induced enhanced luminescence effect and stable liquid crystalline phase structure (Fig. 8 B). The liquid crystal phase structure of P38, P39 and P40 is smectic A (SmA). P41, P42 and P43 are hexagonal columnar phase structure. This is because as the length of the alkyl linker between the TPE and the side chain center terephthalate increases, the TPE motif becomes more prone to align around the polymer backbone. But at the same time, the solid-state fluorescence quantum yield decreased from 52% to 18% (Fig. 8C), indicating that the short spacer is conducive to enhancing the "carapace" effect, making the overall structure of the polymer more rigid, restricting the movement of the TPE group, thus helping the exciton in the excited state to return to the ground state by radiative transition.

In order to further construct more efficient luminescent liquid crystal polymers, Xie et al. Directly replaced the intermediate flexible spacer with ester groups or carbon-carbon single bonds on the basis of the above work, and connected the modified TPE groups to the 2 and 5 positions of the benzene ring of polystyrene, in which the monomer containing ester groups could be prepared into homopolymer P44 by free radical polymerization under mild conditions^[45]. However, the monomer prepared by carbon-carbon single bond cannot be homopolymerized due to steric hindrance, and the copolymer P45 ~ P47 can be further formed only under harsh reaction conditions, as shown in Figure 9a. The fluorescence emission of P44 was quenched due to the photoelectron transfer effect caused by the ester group in P44, while the copolymers P45 ~ P47 showed strong solid-state emission (the fluorescence quantum efficiency of P45 ~ P47 was 18. 4% ~ 24.3%). The PLM results showed that all the polymers (P44 ~ P47) exhibited birefringence (Fig. 9b). XRD results show that P44 ~ P47 exhibit hexagonal columnar liquid crystalline phase structure. Since the above luminescent liquid crystal polymer is difficult to synthesize, the homopolymerization under mild conditions was achieved by adjusting the position of the TPE group on the styrene to reduce its steric hindrance, and the two polymers obtained were P48 and P49 (Figure 9a)^[46]. The test results show that both homopolymers form a stable bilayer stacking smectic phase structure, and although the volume effect of the side groups is reduced, they still force the entire backbone to exhibit an extended conformation, forming a stable liquid crystal phase structure (Fig. 9b). At the same time, comparing the fluorescence emission of the two polymers before and after annealing (fig. 9c), it can be seen that the degree of order is further improved after annealing, resulting in a significant increase in the fluorescence emission intensity.

Chiral "carapace" luminescent liquid crystal polymers can be obtained by introducing chiral mesogens as side groups into the polymer backbone through reasonable molecular modification. Xie et al. Constructed a class of luminescent liquid crystal polymers P50 by introducing cholesterol molecules into TPE groups (Fig. 10a)^[47]. The obtained polymer molecule not only shows a typical 'carapace' effect, namely higher glass transition temperature and stable liquid crystal phase structure, but also shows AIEE characteristics due to the existence of the 'carapace' effect. The incorporation of a small amount of small molecule liquid crystal 5CB can plasticize this kind of liquid crystal polymer, reduce its glass transition temperature and liquid crystal phase transition temperature, so that its own chiral behavior induces the formation of a chiral SmC^* phase, and the resulting liquid crystal phase shows excellent stability, as shown in Fig. 10B and C.

In addition to the preparation of "carapace" type luminescent liquid crystal polymers with different functions according to the selection of different valence bond modes and liquid crystal elements, the "carapace" type luminescent liquid crystal polymers can also be realized by combining the preparation method of "carapac" type liquid crystal polymers with chromophores with phosphorescent properties. Chen et al. Prepared a room temperature phosphorescent liquid crystalline polymer P51 based on isophthalate derivatives, which is the first reported liquid crystalline polymer with RTP, and its chemical structure is shown in Figure 11A^[48]. Since the side chain isophthalate units are directly attached to the polyethylene backbone, the mesh-type effect of the side chain reduces the motion of the molecule, and the formation of a columnar liquid crystalline phase imparts a nanoconfined environment in which phosphorescence is achieved (Fig. 11 B, C). The importance of polymer P51 is that it fully illustrates that the formation of liquid crystal phase can also achieve RTP in addition to the hydrogen bond crosslinking network or crystallization, and the results show that the reasonable selection of side groups with phosphorescence emission can effectively enhance the RTP characteristics.

Hydrogen bonds can also be used to construct "shell" luminescent liquid crystal polymers. Xie et al. Synthesized a series of "carapace" light-emitting liquid crystal polymers P52-x based on hydrogen bond construction using polyvinylterephthalic acid as a hydrogen bond donor and 4,4 '-dibutoxytetrastyrene-1-pyridine (PTC4) as a hydrogen bond acceptor (Figure 12A)^[49]. The experimental results show that the content of side chain PTC4 has an important effect on the formation and phase structure of polymer liquid crystal. With the decrease of the content of the side chain, the liquid crystalline phase structure of this series of luminescent liquid crystalline polymers undergoes a transition from lamellar phase to columnar nematic phase, while when the content of the side chain PTC4 is further reduced, no liquid crystalline phase structure is formed because the side chain cannot produce a strong enough "carapace" effect (Fig. 12b). At the same time, as the content of small molecular acceptors decreases, the polymer shows a red shift in the emission spectrum and a decrease in the solid-state quantum yield. Due to the difficulty in the synthesis of this hydrogen bond donor poly (vinyl terephthalate), a series of "carapace" luminescent liquid crystal polymers P53-x with poly (vinyl alcohol) (PVA) as the main chain were further designed and synthesized (Figure 12A)^[29]. In this system, when X ≥ 0.6 (X is the molar ratio between PTC4 and PVA), the polymer as a whole changes from a crystalline structure to a lamellar liquid crystalline structure (Fig. 12c). Due to the protonation effect of the pyridine group and the existence of multiple hydrogen bonds in the polymer, multicolor fluorescence switching can be realized and good stability can be shown under the action of proton acids with different strengths, as shown in Figure 12d, thus realizing the construction of a stimuli-responsive luminescent liquid crystal polymer combining hydrogen bonds and stimuli-responsive groups.

2.4 Luminescent liquid crystal polymer network

Luminescent liquid crystal polymer network is a kind of crosslinked network structure formed by appropriate crosslinking between molecular chains in luminescent liquid crystal polymer. Compared with the above three luminescent liquid crystal polymers, the liquid crystal polymer has good film forming property and excellent mechanical property.

Xie et al. Recently synthesized a class of monocyanostilbenyl fluorescent monomer M1 with D-A configuration (Fig. 13A), and the results showed that the molecule exhibited a crystalline structure during heating and cooling, and its fluorescence quantum yield could reach 31.89% in the solid state^[50]. In this system, the liquid crystal monomer M1 was blended with M2 (fig. 13A) as a crosslinking agent monomer to prepare a mixture showing liquid crystal properties, and the luminescent liquid crystal polymer network was constructed by in-situ photopolymerization in a liquid crystal cell with a substrate uniaxial friction orientation. PLM and XRD demonstrated that the polymer network in the liquid crystal cell obtained after photocrosslinking was well oriented along the rubbing direction (fig. 13b, C).

In addition to monocyanostilbene derivatives, monomeric molecules prepared from dicyanostilbene, which also have AIE properties, have recently been used to construct luminescent liquid crystal polymer networks. Guo et al. prepared a monomer molecule M3 based on dicyanostilbene (Fig. 13A), and DSC and PLM proved that the molecule has good liquid crystallinity and shows a typical nematic liquid crystal texture in the liquid crystal region^[51]. Compared with the traditional non-polymerizable dicyanostilbene derivative molecules, the diacrylate bond of M3 endows it with good polymerizability, which can be used for crosslinking reaction to prepare luminescent liquid crystal polymer networks. Subsequently, the luminescent liquid crystal polymer network with the combination of parallel orientation and vertical orientation of liquid crystal units was constructed by further combining photoorientation, and the schematic diagram of the preparation process is shown in Figure 13d.

Yang et al. prepared a luminescent liquid crystal polymer network with AIE characteristics by using a liquid crystal monomer M4 with tautomeric smectic A phase (Fig. 14a) and a crosslinking agent molecule M5 based on TPE chromophore with AIE characteristics (Fig. 14a) through in situ Acyclic diene metathesis (ADMET) polymerization method, and the preparation process is shown in Fig. 14b^[52]. The results show that the luminescent liquid crystal polymer network has good thermal stability, and can realize the transformation from lamellar smectic C phase to nematic phase and then to isotropic phase in the process of heating and cooling. The PLM results prove that the liquid crystal units in the system achieve a highly uniform orientation effect in the process of stretching orientation and further crosslinking (Fig. 14C). At the same time, the fluorescence emission intensity of the crosslinked liquid crystalline polymer network decreased gradually and the emission wavelength shifted to the red due to the enhancement of vibration and rotation in the TPE group and the conformational change of the molecule.

3 Related Applications Based on Luminescent Liquid Crystal Polymer

At present, the application of luminescent liquid crystal polymers is still in the exploratory stage, mainly focusing on polarized luminescence, anti-counterfeiting, optical information storage, etc. This paper focuses on the related applications of luminescent liquid crystal polymers in the above fields.

3.1 Polarized luminescence

Polarized light-emitting materials are generally divided into linearly polarized light-emitting materials and circularly polarized light-emitting materials. Under the action of external force, luminescent liquid crystal molecules can realize various complex orientations and emit different polarized light, which may replace the existing optical polarizing devices. For the linearly polarized luminescent materials prepared by luminescent liquid crystal polymers, it is usually only necessary to modify the liquid crystal units reasonably and realize the uniaxial orientation of the liquid crystal units under the condition of applying an external field. Xie et al. Constructed a self-supporting film with uniaxial orientation by using fluorescent liquid crystal monomer M2 and crosslinking agent molecule M3 (fig. 13A), which showed good linearly polarized emission behavior^[50]. As shown in fig. 15A, when the polarizer axis is parallel to the orientation direction of the film, it shows a maximum emission at a wavelength of 526 nm, and the emission intensity gradually decreases with the rotation of the polarizer axis, and the fluorescence emission intensity reaches a minimum when the orientation direction of the film is perpendicular to the polarizer axis (fig. 15b).

Circularly polarized luminescence (CPL) materials can directly produce Circularly polarized luminescence without complex device preparation and show efficient energy conversion compared with circularly polarized luminescence induced by traditional physical methods. By combining chiral or achiral mesogens with chromophores, CPL liquid crystalline polymers with good properties can be prepared by using the long-range ordered stacking characteristics of liquid crystals. Zhang et al. Reported an undoped chiral liquid crystal polymer CPL material, and the obtained polymer can self-assemble into a helically arranged chiral smectic C(SmC^*) phase.The CPL strength of the polymer decreases with the increase of the spacer length due to the decoupling effect between the chiral polyglutamic acid backbone and the cyanostilbene side group^[53]. Xie et al. Prepared a fluorescent liquid crystal polymer P50 containing a TPE derivative with cholesterol as a chiral group (fig. 10a)^[47]. Plasticized with a certain amount of 5CB, polymer P50 forms a chiral smectic C^*(SmC^*) phase, which can show efficient CPL characteristics in the aggregated state. The experimental results show that when the concentration of P50 in 5CB is 30 wt%, its asymmetry factor g_lum value can reach up to + 0.45 (Fig. 15C), and it shows stable CPL emission (Fig. 15d). The above work shows that luminescent liquid crystal polymers with different polarized emission behaviors can be constructed by inducing the ordered orientation of liquid crystal units and the formation of chiral liquid crystal phases.

3.2 Anti-counterfeiting

The oriented luminescent liquid crystal polymer can show different luminescent behaviors and photoresponsiveness, which makes it have important application value in the field of anti-counterfeiting. Guo et al. Prepared a dual-response film based on the principle of photoinduced orientation and photoisomerization^[51]. When irradiated with ultraviolet light, the pattern prepared by photoisomerization in the polymer film can be directly observed by the naked eye, while the pattern prepared based on the photoinduced orientation part shows alternating light and dark changes under polarizers at different angles. Based on this principle, a class of two-dimensional codes is designed, which can further expand the application in the frontier anti-counterfeiting field (Figure 16A). Xie et al. designed and prepared a fast and stable linear polarization light-emitting liquid crystal polymer film with tunable wavelength by introducing pyridine groups into the liquid crystal polymer network. The protonation or deprotonation of pyridine groups under the action of acid/alkali vapor enables this kind of free-standing film to achieve reversible linear polarization fluorescence switching, as shown in Figure 16b^[50]. Guo et al. also reported for the first time a luminescent liquid crystal polymer film based on doped lanthanide upconversion nanoparticles, and realized the preparation of upconversion fluorescence patterns by using an electric field to control the orientation and photocrosslinking of liquid crystal units, as shown in Figure 16c^[54]. Compared with other methods, because the up-conversion luminescent material has the unique property of converting near-infrared light into short-wavelength light, the luminescent material prepared by the method is not easy to be copied and counterfeited, and the application of the luminescent liquid crystal polymer material in the high-end anti-counterfeiting field is further promoted.

3.3 Optical information storage

The fluorescent liquid crystal copolymer P47 (Fig. 9a) reported by Xie et al. Has high solid-state fluorescence quantum yield and excellent processing performance. They used Poly (dimethylsiloxane) (PDMS) as a template to prepare efficient two-dimensional fluorescence patterns by nanoimprinting technology, as shown in Fig. 17A. This kind of fluorescent liquid crystal polymer material can be applied to optical information storage^[45].

In addition, the change of the ordered structure of the luminescent liquid crystal polymer will lead to the change of its photophysical properties. In addition to the inherent thermal stimulation, mechanical, optical and chemical stimulation are also common means. The combination of these stimuli can be extended to the preparation of more complex stimulus-responsive materials, thus constructing reversible optical information storage materials. Using the Z/E configurational isomerization of the cyanostilbene motif on the side chain, Chen et al. Prepared a photoresponsive polymer P24 based on living ring-opening metathesis polymerization, as shown in Figure 4A.Under the irradiation of 365 nm ultraviolet light, the polymer shows fluorescence emission at different wavelengths due to the change of the Z/E configuration of the cyanostilbene chromophore unit in the polymer, while under the rapid irradiation of 254 nm ultraviolet light, it can achieve good photoresponse erasure characteristics^[37]. By using different masks, different fluorescence patterns can be constructed in the same polymer film, and good reversible repeatability can be achieved (fig. 17b). Xie et al. Prepared an acid-base stimulus-responsive luminescent liquid crystal polymer material P53-x based on hydrogen bonding, as shown in Figure 12A^[29]. Due to the existence of multiple hydrogen bonds and the protonation effect of the pyridine group, the material can realize the switching of multiple fluorescent colors under the stimulation of different protonic acids, and has good sensitivity, rapid response behavior and more convenient trigger conditions for operation. The optical information storage device based on this response mechanism has high contrast and rewritability (fig. 17c). Luminescent materials emitting in different bands can be obtained by combining the existing irritant response liquid crystal polymers and synergistically utilizing different luminescent behaviors, so that more complex optical information storage patterns can be designed.

4 Summary and Prospect

In this paper, the molecular design, synthesis strategies of luminescent liquid crystalline polymers and their functional materials in recent years are summarized. Luminescent liquid crystal polymers can combine chromophores with mesogens through covalent bonds and hydrogen bonds, and then obtain luminescent liquid crystal polymers and liquid crystal polymer networks through different polymerization methods. The diversity of photophysical properties of materials can be effectively expanded by combining liquid crystal elements with different phase behaviors with chromophores with different stimulus response modes and luminescent behaviors,This feature is expected to achieve a variety of functions in practical applications, and promote the innovation and development of anti-counterfeiting materials, optical information storage devices, chiral sensing and other fields.

However, light-emitting liquid crystal polymer materials still have some shortcomings. First of all, in terms of molecular design and synthesis, the preparation methods of main-chain liquid crystal polymers are still relatively single. In the future, it should be combined with other synthetic techniques or new polymerization methods to broaden the research of main-chain luminescent liquid crystal polymers and their structures and properties. Secondly, most of the current work focuses on the multiple photoresponse behavior induced by ultraviolet light, mostly fluorescence emission. Due to the influence of photoionization, the performance of the polymer matrix is easily degraded, and excessive energy loss is caused. Although some studies in recent years have also shown that up-conversion luminescence can be induced by doping lanthanide elements in liquid crystal polymer systems, there is no report on liquid crystal polymers that can emit up-conversion luminescence spontaneously. Finally, at present, the principles of luminescent liquid crystal polymers in polarized emission, anti-counterfeiting, optical information storage and other applications mainly focus on polarized/non-polarized fluorescence emission under different stimulus response modes, and there are few reports on phosphorescent liquid crystal polymers. Further development of liquid crystal polymer optical devices with polarized/non-polarized room temperature phosphorescence emission can broaden the application range of luminescent liquid crystal polymer materials. With systematic and in-depth exploration in the future, the problems of luminescent liquid crystal polymers listed above will be well solved, and more innovative work will emerge, which will enable the further development and application of luminescent liquid crystal polymer materials.

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