Azobenzene-Based CLCs

It is known that the trans isomer of azobenzene has elongated rod shape that is favorable for the stabilization of the LC phases, while the cis isomer in bent form is unfavorable and tends to destabilize the LC phases. The trans-cis photoisomerization will decrease the order parameters, if significantly enough, which could lead to the destruction of the ordered LC phase and formation of isotropic state. The reverse transition can be achieved with either visible light irradiation or thermal relaxation as a result of cis-trans isomerization. This principle was initially used to induce the phase transition from nematic to isotropic in azobenzene LCs [39]. Cholesteric mesophase is intrinsically similar to nematic with additional helical arrangement of nematic layers, thus the photoisomerization of azobenzenes can also bring out the phase transition from the cholesteric state to the photoin-duced isotropic (PHI) state. 

The decrease in order parameter in LCs caused by the photoisomerization of azobenzene molecules are also known to induce phase transition from chiral smectic phase to cholesteric phase. This type of transition is mainly observed in chiral mesogenic compounds. In 1999, My et al. conducted a systematic research on the phase transition behavior of a series of chiral azobenzene LCs [44]. A photo 

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]. 

In CLCs induced by single chiral photoresponsive dopant, the reflection wavelength tuning is mainly determined by the HTP change of the chiral dopant 

Tamaoki et al. reported a series of planar chiral dopant which was employed in commercially available nematic LC to achieve phototunable reflection colors [69, 70]. These compounds were designed based on an azobenzenophane compound having conformational restriction on the free rotation of naphthalene moiety to impose an element of planar chirality. Due to the good solubility, moderately high HTPs, and large changes in HTPs during photoisomerization of the dopant, a fast photon mode reversible color control in induced CLCs was achieved. 

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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... 

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