Doped molecules, induced phase transition

INDUCED PHASE TRANSITION BY PHOTOCHROMIC REACTIONS OF DOPED MOLECULES... 

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 phase transitions starting from the smectic phase were also observed in several doped systems [57-60]. The smectic LC host 8CB, doped with chiral molecules and azobenzene compounds, showed the phase transition from smectic to cholesteric due to the photo-induced isomerization of azobenzene molecules, and the prolonged irradiation drove the phase transition further to the isotropic phase. Matsui et al. also reported a chiral smectic C (SmC )-cholesteric (N ) phase sequence in azo-dye-doped ferroelectric LCs [57]. 

It is weU known that E Z photoisomerization of azobenzene-containing LC molecules can lead to a nematic-to-isotropic transition as well as photochromism [167]. It was reported that UV irradiation of a nematic mixture doped by chiral azobenzene bent-core LCs leads to the N -I transition and shift in selective reflection band position of the N phase [177]. These chiral bent-core molecules can induce the N helix upon doping them into a nematic base mixture, and their helical twisting power (HTP) is given by P = UPC where P is the helical pitch length, and C is the concentration of a chiral dopant. The P value for the exclusively E isomer is maximum and decreases with the increase in the ratio of the Z isomer. UV irradiation causes E Z conversion, therefore increasing the helical pitch and shifting the selective reflection band of the N phase. 

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