Reversion of Photoalignment Direction of Liquid Crystals Induced by Cinnamate Polymer Films

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Abstract
The difference in the surface-assisted in-plane photoalignment of a nematic liquid crystal between poly[methyl (p-methacryloyloxy)cinnamate] (pCin0) and poly[methyl (p-methacryloyloxyethoxy)cinnamate] (pCin2) in a film state is described. Irradiation of a film of pCin2 with linearly polarized 313 nm light generated homogeneous alignment of the liquid crystal in the direction parallel to the electric vector of the light. On the other hand, the direction of photoalignment induced by a film of pCin0 was perpendicular to the electric vector of the light in the early stage of photoirradiation, with subsequent reversion to parallel alignment. The mechanism of photoalignment reversion was discussed on the basis of the spectral analysis of product distribution of the cinnamate chromophores in the film state. The perpendicular orientation of the liquid crystal is due to axis-selective E-to-Z photoisomerization, whereas the consequent alignment reversion stems from the predominant contribution of axis-selective formation of photodimer(s).