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.

The difference in the surface-assisted in-plane photoalignment of a nematic liquid crystal between poly[methyl ( pmethacryloyloxy)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).

Whereas diphenylacetylene (DPA) exhibits no photoisomerization because of the involvement of a triple bond in the chromophore, prolonged irradiation of the films with 313-nm light resulted in the reduction of absorption, leading to insolubilization as a result of photocross-linking. When the actinic light is linearly polarized, polarized UV absorbances perpendicular to the electric vector of the light decreased more preferentially than those in parallel with that to generate dichroism. Homogeneous alignment of LCs was induced, when cells were fabricated with substrate plates covered with thin films of DPA polymers, followed by linearly polarized light irradiation at temperatures above the transition temperature of LC. Irradiation of thin films of DPA polymers before cell assembly was more convenient because much less exposure doses were required for the LC photoalignment control. The photogenerated LC alignment was highly thermally stable and was not altered even after heating at 100 °C for 1 week. Discussion was made on the mechanism of the photocontrol of LC alignment on the basis of the photochemical behavior of the DPA polymers.

No abstract

2-[2-(2-Pyrrolyl)ethenyl]pyridine (3) forms an intramolecular hydrogen bond between the pyridine and pyrrole ring and undergoes one-way trans→cis isomerization on direct irradiation. Furthermore, cis isomer undergoes intramolecular hydrogen atom transfer to give a tautomer in the excited singlet state. Based on the experimental findings, effects of intramolecular hydrogen bond and the solvent on the photochemical behaviors of 3 were discussed.