Photochromic liquid crystalline moiety

There have been many papers concerning liquid-crystalline polymers having azobenzenes in their side chains (Figure 14) because of the mesogenic nature of the azo chromophore. Liquid-crystalline polymers incorporating azobenzene moieties in their main chains (30) have also been prepared.102 A family of poly(acrylates) substituted with azobenzenes having chiral alkyl groups (31) showed smectic A as well as cholesteric phases.103 The photochromic behavior... 

S. Ivanov, I. Yakovlev, S. Kostromin, V Shivaev, L. Lasker, J. Stumpe, and D. Kreysig, Laser-induced birefringence in homeotropic films of photochromic comb-shaped liquid-crystalline copolymers with azobenzene moieties at different temperatures, Makromol. Chem. Rapid... 

A PAP may be either amorphous or liquid crystalline and usually comprises suitably functionalized photochromic azobenzene and mesogenic moieties as side groups. The latter play an important cooperative or synergistic role with the photo-chromic group enhancing the overall birefringence (photoinduced anisotropy). 

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. 

In reality, the liquid crystalline-to-isotropic phase transition temperature Tc) of the copolymer or LCP mixture with the cw-form Tec) is lower than that with the trans-isomer (Tct). The presence of the cw-azobenzene moieties depresses the transition temperature of the host liquid crystalline phase based on the concentration. If the temperature of the mixture T) is set between Tct and Tec and the sample is irradiated to cause trans-to-cis photoisomerization of azobenzene molecules, Tc decreases with an accumulation of the number of cw-isomer. When Tc becomes lower than the irradiation temperature, the liquid crystalline-to-isotropic phase transition is induced. The photoinduced phase transitions are interpreted in terms of a change in the phase transition temperature of PLCP systems on accumulation of one isomer of the photochromic molecule. As expected from Figure 7.5, AT (= Tct - Tec) is one of the most important parameters in the phase transition [1], When the temperature of systems is set below Tec, no phase transition is induced. When the temperature is set close to Tct, the amount of cw-azobenzene needed to lower Tc is relatively small, enabling one to effectively induce phase transition upon UV irradiation [10]. 

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