Light-Driven Handedness Inversion of Cholesteric Liquid Crystals

Xiaojun Liu; Lang Qin; Yanlei Yu

doi:10.7536/PC220806

Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (2) : 247-262 DOI: 10.7536/PC220806

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Light-Driven Handedness Inversion of Cholesteric Liquid Crystals

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Abstract

Light-driven cholesteric liquid crystals are soft intelligent photonic crystal materials, which change their optical properties upon irradiation of light. The molecules are organized into helical superstructures to selectively reflect the circularly polarized light with the same handedness as the light-driven cholesteric liquid crystals. By modulating the helical superstructures with light stimuli, the wavelength or polarization of the selective reflection is tuned. Light-driven handedness inversion of helical superstructures in cholesteric liquid crystals is currently in the limelight. The inversion of handedness alters the chirality of the circularly polarized light, which has wide potential for applications in tunable filters, anti-counterfeiting technologies, circularly polarized lasers, and 3D displays. However, inducing the handedness inversion of cholesteric liquid crystal still remains a challenge because the energy barrier between the opposite twist sense is difficult to overcome. It is necessary to figure out the universal strategies for designing light-driven cholesteric liquid crystal systems with reversible handedness inversion. This review mainly focuses on the development of the light-driven cholesteric liquid crystals with handedness inversion. The reported strategies for controlling the handedness are summarized, including the handedness inversion induced by reverse molecular chirality upon photoirradiation and introduction of chiral conflict. The reverse molecular chirality in different chiral molecular switches with tetrahedral, planar, or axial chirality induced by azobenzene, dithienylethene, overcrowded alkene, or cyano-functionalized diarylethene is concluded. During the photoisomerization process, the changes of conjugation, geometry, and dipole moment are analyzed. The strategies to introduce chiral conflict in cholesteric liquid crystals are demonstrated, and the mechanism of chiral conflict to facilitate handedness inversion is explained. Most importantly, the existing challenges and opportunities toward the systems are discussed.

Contents

1 Introduction

2 Handedness inversion in light-driven cholesteric liquid crystals

3 Handedness inversion induced by helicity change of light-driven chiral switches

4 Handedness inversion controlled by chiral conflict of light-driven chiral switches and chiral dopants

5 Conclusion and outlook

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Keywords

cholesteric liquid crystals / handedness inversion / light-driven chiral switches / helical superstructures / photoswitches

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Xiaojun Liu, Lang Qin, Yanlei Yu. Light-Driven Handedness Inversion of Cholesteric Liquid Crystals. Progress in Chemistry, 2023, 35(2): 247-262 DOI:10.7536/PC220806

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1 Introduction

Helical structure is ubiquitous in nature, which is an important basis for the formation of life and its special functions, such as DNA, RNA, and collagen, which constitute human organs^[1,2]. Among them, the spiral structure in some animals and plants can interact with light to show unique optical phenomena, which has attracted wide attention of researchers^[3⇓⇓~6]. For example, the exoskeleton of Chrysina gloriosa (Plusiotis boucardi) has a periodic spiral structure formed by the rotation of chitin nanofiber layers along the normal direction, which is reflected only when there is left-handed polarized light in the incident light, showing a green metallic luster, so as to achieve communication between companions^[7⇓~9]. The fruit of Pollia condensata, which grows in the rainforest of Africa, also has a similar spiral structure. According to the direction of the spiral structure in the fruit, it reflects left-handed or right-handed circularly polarized light, showing brilliant colors to attract disseminators to multiply^[10,11]. Inspired by nature, researchers have developed artificial microstructure materials with periodically changing refractive index, which are named photonic crystals^[12,13]. Photonic crystals have a Photonic bandgap, so that light of a specific wavelength cannot pass through and is selectively reflected. Among many photonic crystals, the materials with periodic spiral structure are the most unique, which are expected to expand the application of photonic crystal materials in the field of polarization optical devices because of their circular polarization selectivity of reflection wavelength^[14⇓~16].

Cholesteric liquid crystal constructed by "bottom-up" method has periodic spiral structure, which is an ideal material to achieve the above goal^[17⇓~19]. Molecules in cholesteric liquid crystals are self-assembled into layers, and the molecules in a layer are arranged in parallel along a certain direction, and the molecules between two adjacent layers rotate at a certain angle (about 15 °) along the layer normal direction to form a periodic spiral structure^[20]. Pitch (p) and Handedness are two important parameters to characterize the helical structure of cholesteric liquid crystals, which determine the wavelength and circular polarization of the reflected light, respectively. The pitch is the interlayer spacing at which the molecular orientation rotates 360 ° along the screw axis, and according to the Bragg reflection law λ = np (n is the average refractive index of the liquid crystal matrix), the wavelength of the selective reflection of the cholesteric liquid crystal can be adjusted by changing the pitch. The helix direction is the direction of molecular rotation between layers, which is divided into left-handed helix and right-handed helix, and determines the circular polarization of reflected light. That is, cholesteric liquid crystals with right-handed helices reflect only right-handed circularly polarized light and vice versa.

So far, the study of using external stimuli to regulate the pitch of cholesteric liquid crystals has been widely reported, but there are still great challenges in regulating the reversal of helical structure^{[21⇓⇓⇓~25]}. This is because the process of helical inversion in cholesteric liquid crystals requires significant changes in molecular arrangement, not only changing the spatial configuration of molecules, but also regulating the phase state of cholesteric liquid crystals. If the spiral direction of the cholesteric liquid crystal can be dynamically controlled, the circular polarization of the reflected light is expected to be changed, and the application of the material in the fields of adjustable filters, anti-counterfeiting and encryption technologies, circularly polarized lasers, circularly polarized light detection, optical data storage, three-dimensional display and the like is expanded^{[26⇓⇓⇓⇓⇓⇓~33]}. Compared with the use of external stimuli such as electricity, heat, magnetism and humidity to regulate the spiral structure of cholesteric liquid crystals, light has the advantages of remote non-contact and precise manipulation of local space^[34]. Therefore, the use of light to control the helical direction of cholesteric liquid crystal helical structure and change the circular polarization of reflected light has become a hot topic in this field^[35]. In this review, we focus on the research work on the helical inversion of photoresponsive cholesteric liquid crystals, summarize the strategies for regulating the helical structure of cholesteric liquid crystals, analyze the influence of molecular structure on the degree of helical inversion, and discuss the challenges faced by material systems and possible development directions.

2 Helical Inversion of Photoresponsive Cholesteric Liquid Crystal

Introduction of a Chiral molecular switch into a nematic liquid crystal is a common method for the preparation of photoresponsive cholesteric liquid crystals^[36,37]. On the one hand, the chiral center of the chiral molecular switch can induce the liquid crystal molecules to arrange into a helical structure, and on the other hand, the isomerization reaction of the optical switch under illumination can control the helical pitch and even the helical direction. The ability of the chiral molecular switch to induce the nematic liquid crystal to form a Helical structure is called the helical twisting force (Helical twisting power, β), and its relationship with the pitch follows the formula β = 1/pc (C is the doping concentration of the chiral molecular switch). In addition, the positive and negative signs of the helix twisting force represent right-handed and left-handed helices, respectively. Helical inversion of cholesteric liquid crystals controlled by light mainly includes two processes (Fig. 1A): First, light causes the isomerization reaction of chiral molecular switches, which reduces the helical twisting force and increases the pitch of cholesteric liquid crystals. When the helical twisting force approaches zero, the helical pitch approaches infinity, and the number of left-handed helices and right-handed helices in the liquid crystal mixture is equal, which shows the properties of nematic liquid crystal macroscopically, and is called "Compensated nematic phase". Subsequently, the helical twisting force increases in the opposite direction across 0, the pitch gradually decreases, and the liquid crystal transforms from the nematic phase to the cholesteric phase with the opposite helical direction.

There are two main strategies to induce helical inversion of cholesteric liquid crystals by light stimulation (Fig. 1b): (1) The chiral molecular switch undergoes an isomerization reaction under light to reverse its inherent "molecular helicity", change the interaction with the liquid crystal matrix molecules, and make the helical structure of cholesteric liquid crystals helical inversion; (2) a chiral molecular switch with opposite helicity and a non-responsive chiral dopant are simultaneously introduced into a nematic matrix, and the two helicities compete with each other in the illumination process to realize the regulation of the helix direction.

In the above strategy, the chiral molecular switch should have the following two characteristics: first, the chiral molecular switch should show significant spatial configuration changes before and after illumination; Secondly, the chiral molecular switch should have good solubility in the liquid crystal matrix. So far, the photoswitches used to regulate the helical superstructure of cholesteric liquid crystals include azobenzene, diarylethene, molecular motors, cyanovinyl, spiropyran, etc. Among them, the photoresponsive chiral molecular switches that have been used to regulate the helical inversion of cholesteric liquid crystals mainly include azobenzene, diarylethene and molecular motors (Fig. 2)^{[38⇓⇓⇓~42]}^[43]. Usually, azobenzene chiral molecules undergo π-π * and n-π * electronic transitions from the rod-like trans isomer to the excited state under the action of ultraviolet light, and then undergo inversion or rotational isomerization to the bent cis isomer, which changes the spatial configuration of the chiral molecular switch and regulates the helical structure of cholesteric liquid crystals^[44⇓~46]. The resulting cis isomer can be reversibly converted back to the trans isomer under visible light or heat. The azobenzene chiral molecular switch has simple synthesis, good compatibility with liquid crystal matrix, and large molecular configuration change between trans and cis isomers,Azobenzene chiral molecular switches are still the most widely used photoresponsive groups to regulate the helical structure of cholesteric liquid crystals so far, because they are conducive to inducing large changes in the helical structure of cholesteric liquid crystals to achieve helical inversion.

Diarylethene compounds have two configurations of ring-opening and ring-closing, wherein the ring-opening isomer has an absorption band at a short wave, and is converted into the ring-closing isomer through a ring-closing reaction after being irradiated by ultraviolet light,Due to the conjugated structure of the ring-closed isomer, a new absorption band appears in the long wavelength range, so that the ring-closed isomer can be changed into a ring-opened configuration again under the irradiation of visible light. Because the ring-opening reaction of diarylethene can only be driven by visible light, this kind of chiral molecular switch has good thermal stability^[47⇓~49]. Compared with the above two photoresponsive groups, the isomerization reaction of molecular motor chiral molecular switches (double-bond olefin compounds) under illumination is more complex, and the difference of molecular spatial configuration changes is more significant^[50,51]. The molecular motor is mainly composed of a "stator" and a "rotor", and the two parts are connected by an olefinic bond as a rotation axis. Under illumination, the "rotor" rotates unidirectionally relative to the "stator", and light-induced cis-trans isomerization and irreversible thermal isomerization occur to form different isomers. Using these photoresponsive groups, the researchers designed chiral molecular switches that induce helical inversion in the helical structure of cholesteric liquid crystals. The following is an introduction to the related research work based on the two mechanisms of helical inversion of cholesteric liquid crystals.

3 Helicity change of chiral molecular switch induce helix inversion

Chiral molecular switches that induce helical flip in cholesteric liquid crystals mainly include tetrahedral chirality, axial chirality and molecular motors. Among them, the spatial configuration change of tetrahedral chiral molecular switch is limited, which mainly depends on the interaction between chiral molecules and liquid crystal matrix molecules to regulate the helix inversion, and the design of molecular structure and the selection of liquid crystal matrix are more stringent. Axially chiral molecular switches vary greatly in space and can change their chirality to induce helical inversion according to certain molecular design principles. The spatial configuration of molecular motors changes the most, and almost all molecular motors can induce helix inversion.

In 1998, Feringa et al. First doped diarylethene molecules with tetrahedral chirality, 1(β_initial=13μm^-1,wt%,LC:ZLI-389), into the nematic liquid crystal matrix ZLI-389 to regulate the helical structure (Figure 3A)^[52]. Because the helical twisting force of the closed-loop structure of chiral molecular switch 1 is almost zero, it can not maintain the cholesteric phase, so this work can only regulate the reversible change of liquid crystal between cholesteric phase and nematic phase. Subsequently, Li et al. Attempted to connect the cholesterol group to the diarylethene through a flexible spacer to prepare chiral molecular switches 2a ~ 2C (Fig. 3B)^[53]. These chiral molecular switches are liquid crystalline and can self-organize to form a cholesteric phase, and the helical structure formed by their twisted nematic liquid crystal matrix 5CB can reversibly transform between cholesteric and nematic phases under 310 nm ultraviolet light and 670 nm visible light (Figure 3C).

From the above work, it can be found that the spatial structure of the two configurations of diarylethenes has little change, which is difficult to cause significant changes in the helical twisting force of the chiral molecular switch and induce the helical structure of cholesteric liquid crystals to flip. Ichimura et al. Used the azobenzene group to replace the diarylethene, and realized the switch of 3(β_initial=0.81μm^-1,β_UV=-4.3μm^-1,β_vis=0.47μm^-1,wt%,LC:MLA-2039 by using tetrahedral chiral molecules. Fig. 4A) Isomerization induced helical inversion in cholesteric liquid crystals^[54]. They used a polarizing microscope to observe and demonstrate the process of helical inversion in cholesteric liquid crystals (Figure 4 B). Under the continuous irradiation of ultraviolet light, the liquid crystal mixture undergoes a photochemical phase transition from cholesteric to nematic and then to cholesteric, and the whole process is dynamically reversible. It is worth mentioning that they found that this helical flip phenomenon mainly depends on the interaction between the liquid crystal matrix and the chiral molecular switch. When the nematic liquid crystal matrix contains a vinyl group at the end of the molecule, the chiral molecular switch 3 triggers the helix flip. This is due to the large steric hindrance of the methyl group attached to the ortho position of azobenzene in molecular switch 3. Under the influence of the special liquid crystal matrix molecules, the end of the molecular switch is easier to rotate, and the isomers formed have different helical twisting forces. The inability of azobenzene without methyl substitution at the ortho position to cause helical flip also verifies the importance of the interaction between the chiral molecular switch and the liquid crystal matrix.

Due to the small difference in the spatial configuration before and after the isomerization reaction of the tetrahedral chiral molecular switch, there are few systems that can induce the helical inversion of cholesteric liquid crystals. Comparatively speaking, the axially chiral molecular switch has a higher helical twisting force and a larger degree of spatial configuration change, which is easier to induce helical inversion. The typical axially chiral molecular structure is binaphthyl and its derivatives, and its chiral configuration (R or S) and dihedral configuration (θ = 90 ° for transcoid, θ < 90 ° for cisoid) are two important parameters determining the helical direction of cholesteric liquid crystals. As shown in fig. 5, when the chiral configuration is R and the dihedral angle θ is less than 90 °, the binaphthyl shows M helicity and induces the liquid crystal to align to form a cholesteric liquid crystal with a left-handed helical direction; When the dihedral angle θ = 90 °, binaphthyl shows P helicity and induces the formation of cholesteric liquid crystal with right-handed helix direction; If the chiral configuration is S, the result is opposite to the above case^[55].

Based on this molecular design principle, Akagi et al. Designed and synthesized a series of diarylethene chiral molecular switches 4A ~ 4E connecting R-configured non-bridged binaphthyls (Figure 6a)^[56]. By changing the length of the terminal alkyl chain of the binaphthyl group, the helical structure of cholesteric liquid crystals was regulated, and the phenomenon of light-driven helical inversion of cholesteric liquid crystals induced by a diarylethene chiral molecular switch was reported for the first time (Fig. 6B). The chiral molecular switch 4a(β_initial=6.6μm^-1,β_UV=-8.3μm^-1,mol%,LC:PCH) underwent a ring-closure reaction under UV irradiation at 254 nm, and the dihedral angle of the binaphthyl was changed from above 90 ° to below 90 °, and the helicity characteristics were changed, resulting in the inversion of cholesteric liquid crystal from right-handed to left-handed. With the extension of the alkyl chain, the helical twisting force of the chiral molecular switch 4c(β_initial=12.7μm^-1,β_UV=±1.0μm^-1,mol%,LC:PCH) decreases under UV irradiation, but it cannot increase reversely, and it can only induce the liquid crystal to change from cholesteric phase to nematic phase. Under the irradiation of ultraviolet light, the chiral molecular switch 4e(β_initial=24.3μm^-1,β_UV=21.9μm^-1,mol%,LC:PCH) can only increase the pitch of cholesteric liquid crystal, but can not observe the change of liquid crystal phase.

Li et al. Attached azobenzene groups to both ends of the S-binaphthyl molecule to prepare molecular switches 5A and 5B that induce helical flip in cholesteric liquid crystals (Fig. 7A)^[57]. The solubility of the molecule is enhanced by the ability of the halogen atom at the end of the molecule to form a halogen bond with the cyano group at the end of the commercial nematic liquid crystal 5CB. Furthermore, by introducing a fluorine atom with strong electron-withdrawing effect at the ortho position of azobenzene, the n orbital energy of the cis isomer is reduced, and the n-π * absorption peak of the cis isomer is separated from the n-π * absorption peak of the trans isomer, so that the trans-cis isomerization of azobenzene can be induced by visible light^[58]. In the initial state, the dihedral angles of the molecular switches 5A and 5B are greater than 90 °, which induces the cholesteric liquid crystal to form a left-handed helical structure. Under the irradiation of 530 nm green light, the helical twisting force of the molecular switch gradually decreases, in which the isomerization transition of 5a(β_initial=-26.9μm^-1,β_{440 nm}=-12.8μm^-1,β_{530 nm}≈0μm^-1,mol%,LC:5CB) is low and can only induce the cholesteric phase to nematic phase, while 5b(β_initial=-23.2μm^-1,

β 440 n m

=-7.3μm^-1,β_{530 nm}=13.4μm^-1,mol%,LC:5CB) can induce the helical structure of cholesteric liquid crystal to flip. 440 nm(2.5 mW·cm^-2) The opposite process occurs under blue light. Furthermore, they used the chiral molecular switch 5B to construct a visible-light-driven grating and showed the diffraction patterns of a red laser (632.8 nm) passing through the left-handed spiral and right-handed spiral cholesteric liquid crystal gratings, respectively (Fig. 7B).

Compared with the above non-bridged structures, the dihedral angle of the bridged binaphthyl is usually less than 90 °, the helical twisting force is higher, and the appropriate increase in the length of the bridged alkyl chain is helpful to improve the degree of molecular isomerization^[59]. Li et al. Designed and synthesized a series of P-helicity molecular switches by directly connecting the bridged S-configurational axial chiral center binaphthyl group with diarylethene, and the initial state of the cholesteric liquid crystal induced by the switches was right-handed helix^[48,60]. Among them, chiral molecular switch 6 (Fig. 8A) can induce helical inversion in cholesteric liquid crystals^[60]. 0.8 mol of 6 was doped into the liquid crystal matrix 5CB (β_initial=54μm^-1,β_UV=-75μm^-1,mol%,LC:5CB) and poured into the wedge-shaped cell and the vertically aligned liquid crystal cell, respectively, and the change process was observed by a polarizing microscope (Fig. 8B). Under the irradiation of ultraviolet light, the distance between adjacent staggered lines in the wedge-shaped cell gradually increases, and the fingerprint texture in the vertically aligned liquid crystal cell gradually becomes sparse, which indicates that the pitch in the cholesteric liquid crystal is increasing. When the pitch is infinite, the liquid crystal changes into nematic phase, and the disclination line and fingerprint texture disappear, and the cross extinction phenomenon is observed in the conoscopic mode. After continuous irradiation, the liquid crystal was transformed into cholesteric phase with opposite spiral direction, and the dislocation line and fingerprint texture were regenerated. After molecular simulation, it was found that the dihedral angle of the binaphthyl group in chiral molecular switch 6 changed from 76 ° to 93 ° after the ring-closing reaction, which changed the helicity characteristics of the binaphthyl group and the cholesteric liquid crystal from right-handed helix to left-handed helix. In addition, a reversible tunable laser emitter driven by both electrical and optical stimuli can be constructed by co-doping chiral molecular switch 6 and fluorescent dye into nematic matrix E7. The emission wavelength of the laser emitter can be controlled over a wide range of about 120 nm, and the polarization state is reversibly switched between left and right circular polarization^[61].

Subsequently, Sun, Yan and Li added upconversion nanoparticles to the system and developed cholesteric liquid crystal materials that can undergo helical inversion under near-infrared irradiation, avoiding the use of high-energy ultraviolet light, which is harmful to organisms and has weak penetration ability^[62]. Under the excitation of 808 nm and 980 nm near-infrared light, the upconversion nanoparticles emit ultraviolet and visible light, respectively, which induce the ring-closing and ring-opening reactions of chiral molecular switch 6, and reversibly control the helical direction of cholesteric liquid crystal.

Furthermore, three-dimensional optical control of the helical axis of cholesteric liquid crystals can be achieved by using the chiral molecular switch 6^[63]. 1.2 mol% of chiral molecular switch 6 was doped into liquid crystal matrix E7 and filled into a 3.7 μm thick parallel alignment liquid crystal cell. When 6 is a closed loop structure, the cholesteric liquid crystal is a left-handed helix, and the helix axis lies flat in the liquid crystal cell; Under the irradiation of visible light, the screw shaft begins to rotate clockwise, and then the screw shaft stands in the liquid crystal cell; When the content of the ring-opening configuration increases to a certain extent, the cholesteric phase is transformed into a nematic phase, and the helical structure disappears; The continuous irradiation of visible light makes the liquid crystal change into the cholesteric phase with the opposite spiral direction, and then the spiral axis changes from standing to lying in the liquid crystal cell and continues to rotate clockwise. Finally, the cholesteric liquid crystal returns to the state in which the helical axis lies flat in the liquid crystal cell, but the helical direction is different from the initial state. The reverse process of three-dimensional regulation can also be accomplished under the irradiation of ultraviolet light. They explored in detail the conditions for three-dimensional control of the helical structure of cholesteric liquid crystals, and found that the process depends on the interaction of many factors, such as the thickness of the liquid crystal cell, surface anchoring force, pitch and external stimuli. The ratio of the thickness of the liquid crystal cell to the pitch is the key parameter for the flat arrangement of the screw axis, which represents the coupling result of the surface anchoring force and the liquid crystal twist elastic energy. When the ratio is close to an integer multiple of 0.5, the spiral axis is regularly arranged parallel to the surface of the liquid crystal cell, and with the increase of the ratio, the arrangement regularity decreases. Under light irradiation, the chiral molecular switch undergoes an isomerization reaction, and the molecular interaction between the chiral molecular switch and the liquid crystal matrix and the elastic energy of the liquid crystal itself synergistically regulate the tilt arrangement of the liquid crystal molecules. The tilt angle is determined by the azimuthal angle of the director of the LC molecules in the middle layer, and changing the azimuthal angle induces an in-plane rotation of the screw axis. Using the property of three-dimensional light control screw axis, the researchers demonstrated the diffraction pattern change of bilayer cholesteric liquid crystals on a two-dimensional scale. This unique control method of cholesteric liquid crystal helical axis and helical direction has a wide range of applications in bottom-up micro-nano scale self-assembly systems.

In addition, the cyclic axial chiral molecular switches 7A and 7B are also used to control the helical inversion of the cholesteric liquid crystal superstructure^[64]. Chiral molecular switches 7 a and 7 B have good solubility in liquid crystal matrices E7, K15 and ZLI-1132, and not only have high helical twisting force, but also have a large change in helical twisting force before and after illumination. Similar to the tetrahedral chiral molecular switch, the control process of cholesteric liquid crystal helical structure is closely related to the choice of liquid crystal matrix. When 10 wt% of chiral molecular switch 7 a is doped into liquid crystal matrix E7, ultraviolet light only controls the reflection wavelength of cholesteric liquid crystal. When 10 wt% of the chiral molecular switch 7a(β_initial=8μm^-1,β_UV=-26μm^-1,β_vis=6μm^-1,wt%,LC:ZLI-1132) was doped into the liquid crystal matrix ZLI-1132, the helical structure would flip under illumination (Figure 10). When the cholesteric liquid crystal is poured into a parallel-aligned liquid crystal cell, an oil-like stripe texture can be observed in the initial state; After UV irradiation for 5 s, the oil-like stripe texture disappeared, and the liquid crystal cell was observed to change from light to dark when rotated under a polarizing microscope, indicating that the liquid crystal changed into nematic liquid crystal. After 10 s of UV irradiation, the oily stripe texture reappeared, and the spiral inversion of the liquid crystal occurred, forming a cholesteric phase with the opposite spiral direction. When the chiral molecular switch 7b(β_initial=32μm^-1,β_UV=-10μm^-1,β_vis=26μm^-1,wt%,LC:E7;β_initial=32μm^-1,β_UV=-16μm^-1,β_vis=24μm^-1,wt%,LC:ZLI-1132) is doped into the liquid crystal matrix E7 and ZLI-1132, the helical structure of the liquid crystal matrix E7 and ZLI-1132 is reversed under illumination.

In addition to the design of photoresponsive chiral molecular switches by combining photoresponsive groups with commonly used axially chiral binaphthyl structures, there is also a unique class of axially chiral molecular switches, molecular motors. This kind of chiral molecular switch is photoresponsive, and its high helical twisting force and large degree of spatial configuration change ensure that it can induce helical inversion of cholesteric liquid crystals under illumination^{[65⇓⇓⇓~69]}. For example, Feringa et al. Designed a molecular motor 8(β_initial=90μm^-1,β_UV=-59μm^-1,wt%,LC:E7, fig. 11 a), in which the structure of the lower end of the carbon-carbon double bond is similar to the rigid element of the liquid crystal molecule, which greatly enhances the compatibility between the molecular motor and the liquid crystal matrix; The upper end of the carbon-carbon double bond contains a right-handed helical structure and a steric center^[66]. When 1 wt% of molecular motor 8 was doped into the liquid crystal matrix MDA-1444, the characteristic fingerprint texture of cholesteric liquid crystal was observed.The rotation of the cholesteric liquid crystal spiral structure can also drive the micron-sized rods to move together (Fig. 11b), which is a classic case of using nanoscale molecular motors to convert light energy into mechanical energy to do work. White and Bunning et al. Studied the change of helical structure of cholesteric liquid crystals in detail^[69]. The cholesteric liquid crystal in the initial state is a right-handed helix, and under ultraviolet irradiation, the cholesteric liquid crystal first transforms into a nematic phase, and then transforms into a cholesteric phase with the opposite helix direction. When the ultraviolet light is removed, the molecular motor 8 undergoes thermal recovery, and the cholesteric liquid crystal is reconverted into a right-handed helix.

Subsequently, Lacaze, Katsonis and Pablo prepared cholesteric liquid crystal by doping molecular motor 9(β_initial=44.3μm^-1,β_UV=-17.3μm^-1,wt%,LC:E7) into liquid crystal matrix E7, and constructed emulsion microspheres with liquid crystal molecules aligned parallel to the surface, and explored the change of topological defect structure in microspheres with the helical inversion of cholesteric liquid crystal under illumination^[70]. The results show that the higher the content of molecular motors, the more obvious the structural changes of topological defects caused by helical flip. When the initial pitch of the cholesteric liquid crystal is 1. 95 μm, the topology of the microspheres (diameter R = 13. 0 μm) changes from the Radial spherical structure of the cholesteric phase to the Bipolar structure of the compensated nematic phase, and finally to the Radial spherical structure with opposite handedness. Recently, Yang et al. Realized the polarization modulation of circularly polarized luminescence by using the property that the isomerization of molecular motors changes the helical direction of cholesteric liquid crystals^[71]. Circularly polarized light emitting materials were prepared by doping molecular motor 10(β_initial=-20.73μm^-1,β_UV=12.29μm^-1,wt%,LC:SLC1717) and fluorescent molecules into liquid crystal matrix SLC1717. The circular polarization of the luminescence is determined by the helical direction of the cholesteric liquid crystal, and the molecular motor induces the cholesteric liquid crystal to form a right-handed helix in the initial state. When the emission wavelength of the fluorescent molecule falls within the photonic band gap, the right-handed circularly polarized light is reflected inside the cholesteric liquid crystal, so that the left-handed circularly polarized light is emitted as a whole. The change of the helicity of the molecular motor after UV irradiation makes the polarization of the circularly polarized luminescence change from left-handed to right-handed.

Katsonis, Feringa and Chen et al. Designed and prepared molecular motors 11 a ~ 11 C with three stable molecular configurations under illumination, and the shapes of each molecular configuration are quite different (as shown in Figure 12 a).Therefore, the molecular motor can not only induce the helical inversion of the cholesteric liquid crystal, but also reflect the left-handed and right-handed circularly polarized light in the visible light, which is the cholesteric liquid crystal system that can induce the helical inversion and reflect the left-handed and right-handed circularly polarized light in the visible light range so far^[72]. Among them, the molecular motor (R, P) -trans isomer is long rod-shaped, while the (R, M) -cis-unstable and (R, P) -cis-stable isomers show "V-shaped" with opposite helicity. Under 312 nm UV irradiation, the (R, P) -trans isomer of molecular motor 11 a ~ 11c was isomerized to (R, M) -cis-unstable isomer, followed by thermal helix inversion to (R, P) -cis-stable isomer. Since the process of thermal helix flipping is irreversible, the rotation of the molecular motor is unidirectional. When the cis isomer is irradiated with 312 nm UV light, the molecular motor is converted to the unstable trans isomer and thermally returned to the initial state. Compared with 11 a and 11 C, the substituents at the 3 ′ and 3 ′ positions of the molecular motor 11 B extend the rigid structure, which is beneficial to enhance the helical shape, and the obtained cis isomer has higher helical twisting force (β_initial=4.8μm^-1,β_cis-unstable=-88.9μm^-1,β_cis-stable=75.5μm^-1,wt%,LC:ZLI1132), and the helical twisting range exceeds the 160μm^-1, which is a molecular switch with high positive and negative helical twisting force while changing the helicity so far. Cholesteric liquid crystal was prepared by doping 4 wt% molecular motor 11b into liquid crystal matrix ZLI1132. In the initial state, the cholesteric liquid crystal selectively reflects green right-handed circularly polarized light, and the reflection wavelength is red-shifted when the helix is gradually unfolded under 300 nm ultraviolet illumination, and the reflection wavelength is blue-shifted when the helix is reversed. Using this property, an optical pattern reflecting circularly polarized light in a different direction from the background was prepared with the help of a photomask (Fig. 12b), and only a colorful flower pattern could be seen under the illumination of left-handed circularly polarized light, while only a colorful background could be seen under the illumination of right-handed circularly polar-ized light.

4 Chirality (helicity) competition induced helix flip

The strategy of inducing helical inversion in cholesteric liquid crystals by using the spatial configuration change of chiral molecular switches requires that the chiral molecular switches have great differences in different configurations, and even can reverse their helicity under light drive. Although there have been successful cases of using axially chiral molecular switches to control the handedness of helical structures, the degree of molecular configuration transition is still difficult to predict, which limits the range of reflection wavelength control before and after helical inversion of cholesteric liquid crystals. In addition, the influence of the liquid crystal matrix on the molecular configuration transition is difficult to control, and it is impossible to ensure that a certain chiral molecular switch induces helical inversion in a variety of cholesteric liquid crystals.

In order to solve the above problems, Li et al. Proposed a new strategy of using chiral competition to induce helical inversion in cholesteric liquid crystals (Fig. 13A)^[73]. Chiral competition, as its name implies, is the use of two structures with opposite helicity to control the helical direction of cholesteric liquid crystals, mainly including photoresponsive chiral molecular switches and non-responsive chiral dopants with opposite helicity. In the initial state, the helical direction induced by the molecular switch is dominant; Under light irradiation, the chiral molecular switch undergoes an isomerization reaction, and the helical twisting force decreases, and the final helical direction is dominated by the chiral dopant. Based on this principle, they designed a series of (S, R, S) -type chiral molecular switches 12a~12c(β_initial=111,39,57μm^-1,β_UV=-33,-33,-26μm^-1,β_vis=55,36,44μm^-1,mol%,LC:E7 by introducing multiple axial chiral centers with opposite helicities into one chiral molecular switch, Fig. 13 B)^[73]. The helix twisting force of the binaphthyl group with S chirality at the left and right ends of the chiral molecular switch 12A ~ 12c is a fixed value, while the helix twisting force of the binaphthyl group with R chirality in the middle is high in the initial state and is in a dominant position, and the cholesteric liquid crystal is a right-handed helix on the whole. Under the irradiation of ultraviolet light, the helical twisting force of the binaphthyl chiral center with R chirality in the middle decreases, while the helical twisting force of the binaphthyl group with S chirality at both ends dominates, so the cholesteric liquid crystal behaves as a left-handed helix as a whole.

Similarly, Guo et al. Reported the design of (R, S, R) -type molecular switches 13 a ~ 13 d by connecting molecules with different chiral centers through hydrogen bonds, in which the S-chiral binaphthyl azobenzene molecule was used as the proton acceptor and the R-chiral binaphthyl acid at both ends was used as the donor^[74]. Furthermore, they explored the effect of the length of the terminal alkyl chain of binaphthoic acid on the control of the pitch and helix direction of cholesteric liquid crystals. With the increase of the alkyl chain length of binaphthoic acid, the helical twisting force of the proton donor increases. Under UV irradiation, the isomerization of binaphthyl azobenzene with middle S chirality occurs, and the helix twisting force decreases. At this time, the longer the alkyl chain, the more dominant the R-chiral binaphthyl acid plays, which causes the cholesteric liquid crystal to gradually change from left-handed helix to opposite handedness. The chiral molecular switch 13b(β_initial=-2.78μm^-1,β_UV≈0μm^-1,wt%,LC:SLC1717) can change the cholesteric phase into nematic phase, while 13c(β_initial=-0.89μm^-1,β_UV=2.89μm^-1,wt%,LC:SLC1717) can reverse the helical structure of cholesteric liquid crystal.

Recently, Li and Guo et al. Also used hydrogen bonds to design fluorescent chiral molecular switches 14A and 14B (Fig. 14A), which can change their helicity under illumination. Cholesteric liquid crystals based on them have circularly polarized luminescence characteristics, and can reversibly control the handedness of circularly polarized luminescence under illumination^[75]. Similar to photoresponsive groups such as azobenzene and diarylethene, cyanoethylene groups with fluorescent properties undergo isomerization under illumination^[71⇓~73]. The (R, S, R) type chiral molecular switch was prepared by using it as an optical switch to connect with the S chiral binaphthyl center to form a hydrogen bond acceptor, and the R chiral molecule at both ends as a hydrogen bond donor. The 14a(β_initial=9.35μm^-1,β₃₆₅≈0μm^-1,β₄₅₀=-7.59μm^-1,wt%,LC:SLC1717) can not only induce the liquid crystal phase to change into nematic phase under illumination, but also further reverse the helical structure. On the contrary, the isomerization reaction of 14b only changes the pitch of cholesteric liquid crystal. In addition, unlike previous chiral molecular switches, the helical twisting force of molecular switch 14 a increases under 365 nm ultraviolet light or 450 nm blue light. In the initial state, the R chirality is dominant, and the cholesteric liquid crystal is induced to form a right-handed helix and is stimulated to emit left-handed circularly polarized light. After 365 nm UV irradiation to the photosteady state, the helical twisting force of the S chiral photoresponse part in the chiral molecular switch 14 a increases, and the two chiralities cancel each other to induce the liquid crystal to form a nematic phase, and the fluorescence emitted at this time has no polarization selectivity. Under the irradiation of 450 nm blue light, the helical twisting force of the S chiral part is further increased, and the cholesteric liquid crystal undergoes helical inversion into a left-handed helix, which is stimulated to emit right-handed circularly polarized light. Using the properties of adjusting the on/off of circularly polarized light emitted by cholesteric liquid crystals and the adjustable polarization direction, they designed a "binary code for circularly polarized light emission" combined with photomask technology. Bright unpolarized fluorescence is emitted at 365 nm, marked as "0"; right-handed circularly polarized light is emitted at 450 nm, marked as "1" due to its low luminous intensity; Left-handed circularly polarized light is emitted from the rest of the unilluminated area. The encoded information can be read by reading the binary code composed of different luminous States (Fig. 14b, C).

Compared with the method that structures with opposite helicities are connected on the same molecular switch, the method that the molecular switch chirality and the non-response chiral dopant with opposite helicities are doped at the same time to induce the helical inversion of the cholesteric liquid crystal is simpler,Usually, the variation trend and wavelength variation range of cholesteric liquid crystal helical structure can be predicted according to the helical twist force of two chiral molecular switches to control the doping ratio. Each chiral molecule and its concentration obey the following relation p=1/(c₁·β₁+c₂·β₂), where c₁ and c₂ are the concentrations of the two chiral molecules, respectively^[76⇓~78].

Kurihara et al. Doped a left-handed photoresponsive chiral molecular switch 15 (fig. 15A) and a non-responsive chiral dopant R811 into a nematic liquid crystal matrix E44, and observed that the liquid crystal phase could change from a cholesteric phase to a compensated nematic phase^[79]. Furthermore, they increased the content of photoresponsive chiral molecular switch 15 and realized the transformation of cholesteric liquid crystal helix structure from left-handed helix to right-handed helix by using light^[80]. Ryabchun et al. Prepared a chiral molecular switch 16(β_initial=61.5μm^-1,β_UV=-46.7μm^-1,β_vis=27.1μm^-1,wt%,LC:ZLI1132 in which twisted liquid crystal molecules form a right-handed helical structure, Fig. 15 B), and cholesteric liquid crystals with helical inversion under ultraviolet irradiation could be obtained by adjusting the doping ratio of it and left-handed chiral dopant LM36(β=-10.3μm^-1,wt%,LC:ZLI1132)^[81]. It is worth noting that the Z isomer of the chiral molecular switch is thermally stable and reverts to the E isomer after 100 H at 29 ℃ in the dark. By using this property, the regulation of local helical structure reversal is realized.

Li and Hu et al. Doped a left-handed commercial azobenzene chiral molecular switch and a right-handed chiral dopant R5011 in a nematic liquid crystal matrix to prepare a cholesteric liquid crystal with a left-handed helix in the initial state^[82]. The cholesteric liquid crystal can change from reflecting left-handed orange circularly polarized light (618 nm) to reflecting right-handed green circularly polarized light (555 nm) when illuminated by 405 nm violet light (Figure 16A). The opposite process occurs under green light. Taking advantage of the material's ability to control cholesteric liquid crystals to reflect circularly polarized light in different directions over a wide range of wavelengths, they programmed the arrangement of helical structures with the help of photoalignment technology.Optically driven geometric phase elements such as deflectors, lenses, Airy beams (Fig. 16b) and orbital angular momentum generators are constructed, which greatly expands the application of cholesteric liquid crystal spiral structure inversion in the field of optical devices.

Recently, Lu, Hu, and Chen extended the same material system to dynamic multiplexed holography^[83]. They used photoalignment technology combined with photomask to make the upper and lower surfaces of the liquid crystal cell show two different centrosymmetric structural color holographic patterns under illumination. In the initial state, the cholesteric liquid crystal is a left-handed helix, and only left-handed circularly polarized light is reflected to display the holographic image. Under the irradiation of 405 nm light (8.8 mW·cm^-2), the cholesteric liquid crystal is gradually transformed into a right-handed helix, and the holographic image is displayed by reflecting only right-handed circularly polarized light. The opposite process occurs under green light. It is worth noting that the images presented under left and right circularly polarized light are mirror-symmetric to each other, for example, the pattern "2018" under left circularly polarized light becomes "8102" under right circular polarized light. This is caused by the conjugate phase between the left and right circularly polarized light in the holographic imaging process. This new holographic technology with tunable light and selectable spiral direction provides a design strategy for the development of information technology with high capacity and high security.

In addition to using the chiral competition strategy to realize the helical inversion of two-dimensional in-plane cholesteric liquid crystals, researchers have also tried to use this strategy to regulate the helical structure in three-dimensional space. Li et al. Designed a left-handed helical chiral molecular switch 17(β_initial=64.0μm^-1,β_UV=18.6μm^-1,β_vis=52.1μm^-1,wt%,LC:SLC1717 with high helical twisting force, Fig. 17A), doped it with the R5011(β=106.6μm^-1,wt%,LC:SLC1717) of right-handed helix in the liquid crystal matrix at the same time, prepared monodisperse emulsion microspheres with regular molecular orientation by means of capillary microfluidic technology, and observed the change of internal optical phenomena of microspheres with the reversal of helix^[84]. The liquid crystal molecules in the microsphere are arranged in parallel along the surface, that is, the screw axis of the cholesteric liquid crystal is arranged in a radial direction. In the initial state, the double helix pattern within the microsphere can be seen using a polarizing microscope (fig. 17b). The counterclockwise rotating helix verifies that the cholesteric liquid crystal is a left-handed helix. Under UV irradiation, the chiral molecular switch 17 isomerizes, the helical twisting force decreases, and the period of the helical structure of the cholesteric liquid crystal increases gradually, resulting in the formation of point defects at both ends of the microsphere and the Bipolar nematic configuration. With the continuous irradiation of ultraviolet light, a clockwise rotating helix was formed in the microsphere, indicating that the cholesteric liquid crystal was transformed into a right-handed helix, and the period of the helical structure was gradually increased to show a concentric circle pattern. When the photosteady state is reached, the conformation in the microsphere changes to a Diametrical configuration with ring defects. The opposite process occurs under visible light. By increasing the doping amount of the two chiral molecular switches in the liquid crystal matrix, the initial state of the selectively reflective red (700 nm) left-handed cholesteric liquid crystal was finally transformed into a reflective near-infrared (750 nm) right-handed cholesteric liquid crystal.

5 Conclusion and prospect

Helical inversion of photoresponsive cholesteric liquid crystals refers to the use of external light stimulation to control the helical direction of cholesteric liquid crystals, so that it can be reversibly changed between the left and right handed helices. The change of the spiral direction causes the change of the circular polarization of the reflected light, and this unique optical property has attracted the attention of researchers. In this paper, we summarize the research progress of helix flip in photoresponsive cholesteric liquid crystals, and discuss two main strategies for regulating helix flip: photoresponsive chiral molecular switch directly introducing reversible helicity transition, and chiral competition between molecular switch with opposite helicity and chiral dopant. The key to the use of regulatory strategies lies in the degree of spatial configuration changes of molecular switches. In the process of photoisomerization, the first strategy is preferred when the spatial configuration changes enough to induce the change of the helicity of the chiral switch itself, that is, to use a single molecule to control the helix direction of cholesteric liquid crystals. However, this strategy is often difficult to design, because most molecular switches are difficult to cause helicity changes after isomerization. The second strategy is to induce helical inversion of cholesteric liquid crystals by introducing chiral dopants with opposite helicity to the molecular switch and regulating the ratio of the two helicities in the binary doping system. This strategy has a wider range of material systems and provides a more flexible design idea for regulating the helical inversion of cholesteric liquid crystals. In addition, the wavelength variation range of the left and right circularly polarized reflected light can be predicted according to the mathematical relationship.

Photoresponsive chiral molecular switches that have been reported to control the helical flip of cholesteric liquid crystals mainly include azobenzene, diarylethene, and molecular motors, which are summarized in Table 1. In general, the molecular switch designed by connecting the optical switch with the axial chirality has a large change in the spatial configuration, which is easier to regulate the helical direction of cholesteric liquid crystals. In particular, molecular motor chiral molecules can undergo intramolecular rotation under light stimulation to form a variety of isomers and improve spatial configuration changes. Cholesteric liquid crystal materials with the widest range of left and right circular polarized light wavelengths have been constructed by using the isomerization reaction of molecular motor-like chiral molecular switches^[72].

Although the helical inversion of cholesteric liquid crystals is expected to be used in circularly polarized luminescence, anti-counterfeiting and information storage, optically driven geometric phase elements and other fields, most of the material systems are limited by the degree of isomerization of chiral molecular switches, and the regulation of helical structure can not meet the practical application. This requires not only researchers to constantly innovate molecular design and regulation strategies, but also a deep understanding of the relationship between molecular structure changes and performance regulation. In addition, in order to endow cholesteric liquid crystals with more abundant functions, there is still an urgent need to develop and develop new optical switches, such as cyanovinyl optical switches with unique luminescence characteristics, which will also shine brilliantly in the construction of cholesteric liquid crystal materials with tunable helical structure^[85]. Furthermore, in order to expand the optical applications of this kind of materials, the regulation of the helical structure should meet higher requirements-while the helical direction spans the left-handed and right-handed helices, the pitch should be regulated so that the reflected light covers or even exceeds the visible light range in both directions. In addition to the above control of the helical superstructure of cholesteric liquid crystal mixtures, the recent control of the helical structure in cholesteric liquid crystal polymer systems also provides new ideas for expanding optical applications^[86]. In the future, the control of the helical direction of cholesteric liquid crystals will not even be limited to the two-dimensional plane, and the control of the helical structure in the three-dimensional space will bring new opportunities for this kind of materials. Photonic crystal material systems such as chiral smectic and blue phases, which have helical structures similar to cholesteric liquid crystals, will also become a booming research field.

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