Photoresponsive chiral mesogen

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. 

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. 

H. Akiyama, V.A. Mallia, N. Tamaoki, Synthesis, liquid-crystalline properties, and photo-optical studies of photoresponsive oligomeric mesogens as dopants in a chiral glassy liquid crystal. Adv. Funct. Mater. 16, 477-484 (2006)... 

FIGURE 5.2 Classification of photoresponsive CLCs based on chiral mesogens or induced systems. 

The phototuning of BPs can also be fabricated in a pure material system [147]. Das et al. reported a light-induced stable blue phase in photoresponsive diphen-ylbutadiene based mesogen 37. This compound was found to exhibit SmA and N during heating. When the temperature was kept at 118 °C, the photoisomerization induced an isothermal phase transition from SmA to N. Photoirradiation of the SmA film held at a higher temperature (124 °C) for 100 s resulted in transition to a phase with a characteristic classical BP texture showing in Fig. 5.30. The BP was thermodynamically stable and could be maintained at this state for several hours. The characteristic sharp reflection bands compared to the rather broad reflection bands observed for the chiral nematic phase confirmed the formation of BP. The photoinduced formation of the BP exhibited a reflection centered at 510 nm. Subsequent irradiation led to the blue shift to 480 nm in the reflection band. 

Here, we have designed and synthesized multifunctional poly(bithienylene-phenylene)s with either racemic (Poly-9) (M — 14,000) or chiral moieties ((/ )-/ (S)-Poly-lO (Mn = 10,000, 8,000, respectively), which exhibit fluorescence, liquid crystallinity, and photoresponsive properties (Fig. 11.20). The polymers are composed of a 7t-conjugated main chain, poly(bithienylene-phenylene), which acts as a fluorescence moiety and mesogen core, and photochromic DE moieties [71, 72] are linked with racemic or chiral alkyl groups in the side chains. The DE photoresponsive moiety isomerizes between its closed and open forms upon irradiation of UV and visible light, respectively. 

This chapter highlights the photomodulation of CLCs, including light-induced phase transition related to cholesteric mesophase, helix inversion, reflection color control, and creation of mechanical motion in photoresponsive CLCs. The material systems introduced in this chapter are classified into two main groups based on chiral meso-gens and induced CLCs with several subsystems in each. Although the cholesteric phase was initially found with pure mesogenic compounds and the early quest for... 

Finally, it should be mentioned that the chiral additive used to create a chiral nematic phase does not have to be a mesogen itself - it has to be soluble at reasonable concentration levels to produce the desired pitch, but since it is now acting equally as an impurity, the normal phase transition properties of the host material may also be altered. Chiral nematic liquid crystals are highly sensitive in their optical properties, and may therefore be used as contaminant detectors. Equally photoresponsive additives, such as azo-based dyes, may be incorporated into a chiral nematic structure and on photoexcitation they change shape since they then behave as an impurity they can easily alter the pitch or selective reflection properties. Such effects are normally reversible because of the trans-cis and cis-trans back reactions available. This gives a simple imaging device. 

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