Photochemical behavior and the ability to control liquid crystal alignment of polymethacrylates with styrylpyridine side chains

Photoproduct distribution of styrylpyridine (SP) side chains tethered to polymethacrylate backbones upon irradiation with 313 nm light was analyzed UV-spectroscopically in solutions and films of a homopolymer and copolymers of 2-[2-(4-methacryloxyphenyl)ethenyl]pyridine. Whereas E-to-Z photoisomerization of SP moieties proceeds in solution, followed by the slow oxidative photocyclization to give an azaphenanthrene, the predominant photochemistry in films consists of photodimerization and E-to-Z photoisomerization. Irradiation of films of the SP-polymers with linearly polarized 313 nm light generated dichroism of the chromophores due to the axis-selective consumption of the E-isomer. Exposing films of SP-polymers to relatively lower doses of linearly polarized light induced homogeneous alignment of a nematic liquid crystal with a perpendicular orientation with respect to the polarization plane of the light. The photoalignment was thermally stabilized by efficient photodimerization leading to crosslinked structures. However, prolonged irradiation with the polarized light deteriorated the photoalignment. On the basis of these results, it is concluded that the homogeneous photoalignment is determined by remaining E-isomers whereas the photodimerization yielding crosslinks enhances the thermal stability of the alignment.