Photochromic reaction-induced changes of ordering in liquid crystalline films

Kinetics of photochemically and thermally driven isomerization was studied in a photochromic azobenzene derivative dissolved in three liquid crystalline matrices. Deviations from the Arrhenius law, observed for the thermal cis-trans isomerization reaction in the vicinity of nematic-isotropic (N-I) phase transitions, have been explained by a reduction of the intensity of light measured in the experiments, due to scattering. The isotropic phase was found to appear in all studied systems upon UV irradiation, at temperatures substantially lower than the thermodynamic temperatures of the respective N-I phase transitions. The nematic phase reappears due to the cis-trans isomerization reaction that can be thermally driven or can occur upon irradiation with the visible light. The occurrence of 'photo-induced phase transitions' reported for photochromic dye-doped liquid crystals can be rationalised taking into account different compatibilities (and hence different solubilities) of both forms of photochromic systems. The effects observed should be common to many other photochromic systems dissolved in liquid crystals, in the vicinity of phase transitions of the matrices.