PHOTOINDUCED DICHROISM AND OPTICAL ANISOTROPY IN A LIQUID-CRYSTALLINEAZOBENZENE SIDE-CHAIN POLYMER CAUSED BY ANISOTROPIC ANGULAR-DISTRIBUTION OF TRANS AND CIS ISOMERS

Photochromism has been studied for two comb-like liquid-crystalline co polymers (I) and (II) containing azobenzene chromophores in their side chains. In a smectic glass phase of both copolymers, upon short-time irradiation by UV light, long-living cis isomers are observed. Both co polymers manifest the photoinduced anisotropy, the physical mechanisms of which seem to be quite different. In spin-coated films of polymer (II), the origin of the anisotropy is a strong stable dichroism, which is due to an enrichment and depletion of the chosen angular direction , correspondingly, with trans and cis isomers of the azobenzene chromo phores. Polymer (I) manifests no dichroism at all, and its induced opt ical anisotropy may be accounted for by a rather slow chromophore reor ientation. In copolymer (II) a considerable reorientation of the mesog enic groups also occurs as a secondary phenomenon at the stage of the cis isomer formation only. This observation shed more light on the gen eral process of the light-induced molecular reorientation in polymers, liquid crystals, and Langmuir-Blodgett films, which is of great impor tance for holographic information recording. (C) 1998 American Institu te of Physics. [S0021-8979(98)03419-7].