Factors Affecting Photoalignment of Liquid Crystals Induced by Polymethacrylates with Coumarin Side Chains

It was confirmed that 313 nm light irradiation of thin films of polymethacrylates with coumarin side chains results in photodimerization without any side reaction. When the light was linearly polarized, the dichroism of coumarin chromophores was generated after prolonged irradiation as a result of axis-selective photodimerization. It was found that the photoalignment of liquid crystals was critically influenced by the polymer structure and the nature of liquid crystals when cells were assembled by using substrate plates covered with photoirradiated polymer thin films. The photoalignment direction was parallel to the electric vector of linearly polarized 313 nm light when polymethacrylates with coumarin moieties through ethylene and hexamethylene spacers were used. In contrast, a polymethacrylate having coumarin side chains without spacer induced the reversion of photoalignment. In this case, the parallel orientation at the early stage of polarized light irradiation was reversed into perpendicular orientation. This orientational reversion of the photoalignment was interpreted in terms of the balance between molecular interactions of liquid-crystal molecules with photogenerated cycloadduct(s) and those with unreacted coumarin moieties.