Azobenzenes nematic-isotropic transition temperatures

A new system has been developed in which every mesogen in the LC or LC polymer is photosensitive (Ikeda, 2001 Tsutsumi et al., 1998b Tsutsumi et al., 1997 Ikeda and Tsutsumi, 1995). For example, the azobenzene moiety could play roles as both a mesogen and a photosensitive moiety (Fig. 3.20) in azobenzene derivatives that form LC phases. These azobenzene LCs show a stable LC phase only when the azobenzene moiety is in the trans form, but do not show an LC phase when all the azobenzene moieties are in the cis form. Examination of these azobenzene LCs revealed that a nematic-isotropic phase transition is induced in the azobenzene LC polymers within 200 ns over a wide temperature range imder optimized conditions (Ikeda, 2001). 

Similar to the phase transition from nematic to isotropic phase induced by azobenzene molecules, the trans-cis isomerization also destabilize the SmC phase composed of calamitic mesogens and lower the Curie point, which is a transition temperature where the SmC will transform from ferroelectric to non-ferroelectric. Some examples have been reported with an early demonstration by Ikeda et al. [130-135]. For example, a photoresponsive SmC was formulated by doping 3 mol % of 4,4 -disubstituted azobenzene 29 into a FLC host 27 and the UV irradiation at 260 nm resulted in the lowering of Curie point and the coercive force Ec required to switch the SSFLC due to the destabilization of bent shape cis isomers [130] (Fig. 5.23). When the electric field was close to Ec before irradiation, the flip of polarization of SmC was achieved. It is noteworthy that its response time 500 ps is much faster than normally observed for photochemical N-I phase transitions [131]. 

The shape of rran -azobenzene is rod-like and tends to stabilize the phase structure of liquid crystals (LCs), whereas cw-azobenzene is bent and tends to destabilize the phase structure of LCs. This property can be used to control the phase of LCs photochemically [20-29], For instance, when a mixture of azobenzene and nematic LC in which the concentration of the azobenzene ranges from 0.1 to 5 mol% is irradiated with UV light (—350 nm) to bring about the /rans-cw-photoisomerization of the azobenzene moiety, the nematic (N) to isotropic (I) phase transition temperature (T i) of the mixture is lowered on accumulation of the cis-form, because this form acts as an impurity to the system. When T , falls below the irradiation temperature, N to I phase transition occurs isothermally. This process is reversible and, when the isotropic mixture is irradiated with visible light (—450 nm) to cause the cis-trans back isomerization, T i is again raised and isothermal I to N phase transition is observed. The irradiation temperature is crucial to induce the photochemical phase transition it must be set between T , of the /ran5-azobenzene-nematic host mixture [ T i)trans] and the T , of cis-azobenzene-nematic host mixture [(r i)cw]. When the irradiation temperature is lower than T,)cis, no phase transition can be induced by the photochemical reaction of the azobenzene guest molecule. The photochemical... 

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