Dynamics of photoinduced isomerization of azobenzene moieties in liquid-crystalline polymers

Forced Rayleigh scattering was used to investigate the dynamics of photoinduced isomerization of azobenzene involved in reversible holographic information storage. The studies were performed with statistical liquid-crystalline side-group copolymers having different concentrations of the dye, in a copolymer forming no mesophase and in a physical mixture of the dye in a liquid-crystalline side-group homopolymer. Two relaxation processes were found in the glassy state of these polymers, the faster one being distributed over a wide range of relaxation times ascribed mainly to reorientational relaxation of the dye as well as the glassy matrix. The second is a single exponential, ascribed to the relaxation of azobenzene back to its trans conformation. The two processes could be assigned through a control experiment using an irreversibly isomerizing dye. The results of a calculation of the angular dependence of azobenzene isomerization was compared with the experiment, in order to judge the influence of the anisotropic matrix on the relaxation process.