Photoresponsive chiral LCs

As chiral LCs can be formed by chiral mesogens or induced by chiral guest molecules, photoresponsive chiral LCs can be derived from these two systems by photosensitization. One strategy is to photosensitize the existing constituent molecules in chiral LCs, i.e. LC host or chiral dopant. The other strategy is to dope additional photoresponsive agents to the existing chiral LC systems. These two 

Photoresponsive molecules are required for the fabrication and photomodulation of photoresponsive CLC materials regardless of being employed as chiral mesogens, achiral LC host, or chiral/achiral dopants. The photoisomerization of molecules leads to change in molecular shape (geometry/conformation) and alter the bulk properties of LC material, which constitutes the basis for the photomodulation in chiral LCs [28, 29]. There are many types of photoresponsive molecules and some examples are shown in Fig. 5.4. 

Azobenzenes are a well-known family of photochromic compounds that can isomerize from its tmns form to cis form upon UV irradiation (Fig. 5.4a). The cis isomer can be switched back to trans form either by visible light or thermal relaxation. The rod shape of the trans form can stabilize calamitic LCs, while the cis form is bent and normally decreases the order parameters of LC phases. Owing to the dramatic molecular shape and property change between the trans and cis isomers, azobenzenes are the most investigated photochromic molecules to function as either mesogens or dopants in chiral LCs. 

Overcrowded alkenes are a special class of olefins with an inherently helical shape, resulting from extensive steric crowding around the central carbon-carbon double bond (Fig. 5.4b) [30]. Related molecules were first reported by Feringa and coworkers, who used the chiral molecules as chiral dopant in CLC materials [31]. The merit of these molecules as chiral dopants is that the photoisomerization between the two isomers with pseudoenantiomeric relationship often results in the handedness inversion of the cholesteric LCs. 

In addition to the above three types of molecules, photoresponsive chiral LCs have also been fabricated with other photochromic molecules such as spirooxazines [34], fulgides [35], butadienes [36], bicyclic ketones [37], and thioindigos [38]. 

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The reports on photoresponsive chiral LCs can be traced back to the 1960s and 1970s, during which a number of examples on photomning of pitch length in cholesteric phases were demonstrated [5-7]. To date, numerous reports have been found to be related to their physics, chemistry and applications. This chapter mainly deals with the recent progress in this area with a survey of three major classes of chiral LC phases, i.e. cholesteric (N ), chiral smectic C (SmC ) and blue phase (BP) from their stmctures to properties and applications. 

The photoisomerization of chiral azobenzene dopants often leads to the dramatic change in their HTPs. When it is used in combination with another non-photoresponsive chiral dopant with opposite chirality to form a compensated system, the change in HTP can be utilized to reversibly switch the LC phase between compensated nematic and cholesteric [49]. 

Fig. 5.9 Top Molecular structure of chiral azobenzene 10 and non-photoresponsive chiral dopant 811 Bottom Polarized optical micrographs of 11.6 wt% (5)-10 and 8.4 wt% (/J)-811 in E44 upon UV and visible light irradiation at 30 °C. The LC mixture was in a 5 pm glass cell without any alignment treatment Reproduced with permission from [59]. Copyright 2001 American Chemical Society...

There are two ways to generate photoresponsive chiral SmC LCs. The simple one is to add a photochromic molecule to SmC LCs and the other is based on chirality amplification, i.e. using a photoresponsive chiral dopant in SmC LCs to induce a phototunable SmC phase. Some examples of mesogens that showing SmC or SmC are presented in Fig. 5.22. 

In recent years, scientists have developed various kinds of powerful photore-sponsive chiral dopants, and the stfategy using nematic LC hosts doped with photoresponsive chiral dopants currently is widely adopted for the investigation of photoresponsive CLCs [15-17],... 

N-N transition can also be achieved by the HTP variation of the photoresponsive chiral dopants in induced CLCs. In these systems, the HTPs of the chiral dopants have to exhibit dramatic variation between its two states. At a certain doping concentration, the isomer with higher HTP can efficiently induce a cholesteric phase, while the isomer with lower HTP only generates an apparent nematic phase. The light-induced isomerization between these two forms can reversibly switch the LC phase between the cholesteric and the nematic. 

The other two subsystems are based on the light-insensitive LC hosts, which are doped with multiple dopants or single photoresponsive chiral dopant. These two types of materials are more efficient for the phototuning of reflection wavelength than the first one. 

Optically active bis-imine-functionalized diarylethene (2-4 %) (Scheme 13) was used as a chiral, photoresponsive dopant in the nematic LC materials K15 and ZLI-389, resulting in stable cholesteric phases. For the open form of 26a, [5m values of 11 [tm-1 (K15) and 13 xm 1 (ZLI-389) were measured, while the closed form 26b did not show any helical twisting power. Irradiation at 300 nm (30-50 s) resulted in the closed form and disappearance of the cholesteric phase. Irradiation with visible light restored the cholesteric phase. The gradual decrease in pitch, representing a multi-... 

This strategy of using multiple dopants to obtain the cholesteric phase was demonstrated by several groups [82-84]. In 2001, Kurihara et al. reported a photo-controlled handedness switching of the photoresponsive CLCs consisting of chiral azobenzene (5)-ll and non-photoresponsive (R)-26 [82] (Fig. 5.19). Chiral azobenzene (5)-ll induced a left-handed helix in achiral nematic E44 whereas (R)-26 induced a right-handed helix. Based on the evaluation of the HTPs of these two dopants, a CLC mixture was formulated by doping 8.4% of (5)-ll and 11.6% of (R)-26 into the LC host E44. At the initial state, the dopant (5)-11 played a dominant role and thus... 

Photoresponsive CLCs based on induced CLCs may include three different subsystems, which have been illustrated in Section 5.2.1. One subsystem is based on the photoresponsive LC host and chiral dopant, which appears to have limited applications in reflection wavelength tuning. An example was reported by Chanishvili et al. in 2004 where the azo-NLC was used as photoresponsive LC hosts [31]. Upon UV irradiation, the reflection wavelength of the CLCs composed of azo-NLC and chiral... 

Li et al. reported two light-driven chiral molecular switches 36 and 37 with tetrahedral and axial chirality [116]. When chiral switch 36 was doped in nematic LC host E31 at 15 wt% concentration, phototuning the reflection color over the entire visible region was observed. An amazing feature of this photoresponsive CLC system is the quick relaxation. After 1 min of exposure to bright white light, it returned to... 

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