Role of E/Z Photoisomerization of Cinnamate Side Chains Attached to Polymer Backbones in the Alignment Photoregulation of Nematic Liquid Crystals*1

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Abstract
The mechanism of azimuthal reorientation of nematic liquid crystals induced by linearly polarized light irradiation of thin films of polymers with cinnamate side chains has been discussed. It has been concluded that the photoalignment is induced not by the polarized photodimerization, but by polarized photoisomerization, based on the following results. First, spectral analysis of olefinic chromophores upon UV irradiation has shown that a predominant photochemistry in films is the E/Z photoisomerization which results in the dichroism of the chromophores. Second, the alignment direction can be altered by subsequent irradiation with polarized light with a different electric vector. Photodimerization plays a role in enhancing the thermal stability of photoalignment.