Comparative Studies on Isomerization Behavior and Photocontrol of Nematic Liquid Crystals Using Polymethacrylates with 3,3‘- and 4,4‘-Dihexyloxyazobenzenes in Side Chains

Abstract

Azobenzene moieties with 3,3‘-dihexyloxy-2,2‘-dimethyl and 4,4‘-dihexyloxy substituents were attached to backbones of polymethacrylates at low contents (7 mol %) to investigate the effect of positional isomerism of the substituents on photochemical and thermal isomerization behavior and the ability to perform the surface-assisted alignment control of liquid crystals photochemically. Photoisomerization in thin films in a glassy state through π−π* excitation deviates markedly from those in solutions, but that through n−π* excitation takes the behavior of solution photoisomerization. This can be explained in terms of the difference in sweep volumes (critical volumes needed for isomerization) of the azobenzenes in relation to the mechanisms of the isomerization. Polarized light-induced anisotropy of the polymer thin films gave rise to the photocontrollability of alignment of a nematic liquid crystal, despite low azo-chromophore contents. Photoorientation and photoreorientation of the nematic liquid crystal depend significantly on sweep volumes for the photoisomerization which is controlled by the selection of the excitation wavelength and the molecular conformation of the azo-chromophores.