PHOTOINDUCED PHASE-TRANSITIONS IN AZOBENZENE-DOPED LIQUID-CRYSTALS

The effect of irradiation (UV-visible light) on a nematic liquid crystal doped with a photoactive azobenzene derivative was investigated. The selective irradiation results in either an E only if Z or Z only if E isomerization of the azobenzene unit. The effect of the isomerization is to cause a reversible depression of the liquid crystal to isotropic (LC only if I) phase transition temperature of the doped mixture, which can be monitored optically as an isothermal phase transition. This depression also results in a biphasic liquid crystal + isotropic region which is discussed. We investigate the cause and magnitude of the phase depression as a function of the amount of doped 4-butyl-4'-methoxyazobenzene (photoactive unit) in 4-cyano-4'-n-pentylbiphenyl (liquid crystal unit), and as a function of the percentage conversion of E only if Z (caused by isomerization) in the azobenzene. The photostationary state of the doped mixtures achieved by Z only if E isomerization is considered and its effect upon the transition temperature of the mixture and response time of the system is discussed. We discuss the implications of the photostationary state with regards to the reversibility of the photo-induced phase transition and hence potential applications.