Optical anisotropy and four possible orientations of a nematic liquid crystal on the same film of a photochromic chiral smectic polymer

Spin coated films of a chiral comb-like liquid crystalline copolymer contai ning azobenzene chromophores in its side chains are optically isotropic in their twisted smectic-like glassy state. In contact with a nematic liquid c rystal (5CB, E7, MBBA) they provide a degenerate planar orientation. When i rradiated by unpolarized UV light, they orient the same nematics homcotropi cally. Treated with linearly polarized UV light they orient nematics homoge neously with the director along the electric vector of the exciting light. After a combined irradiation first with unpolarized UV light and then with linearly polarized visible light, the films again provide a homogeneous liq uid crystal orientation, this time with the director perpendicular to the v isible light electric vector. The phenomena observed are related to the lig ht induced optical anisotropy. Two main processes are responsible for the a nisotropy (1) a UV light depletion of trans-isomers of the azobenzene chrom ophores from the chosen direction and (2) a reorientation of the chromophor es by polarized visible light.