Photocontrol of Liquid Crystal Alignment by Polymethacrylates with Diphenylacetylene Side Chains

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 photo-cross-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.