Distinct photochemical phase transition behavior of azobenzene liquid crystals evaluated by reflection-mode analysis

Photochemical phase transition behavior of photochromic azobenzene liquid crystals (LCs) was explored by means of reflection-mode analysis. On pulse irradiation at 355 nm, which causes trans-cis isomerization of the azobenzene moiety, these LCs underwent nematic (N) to isotropic (I) phase transition in 100 mu s as probed by change in reflectivity at the interface between the sample and glass substrate. The N-I phase transition was confirmed by calculation of the refractive index of the sample before and after pulse irradiation on the basis of the reflectivity. The decay that represents the recovery of the initial N phase occurred in 2-3 ms and remarkably faster than that observed in the transmission-mode analysis in which transmittance of the probe light through crossed polarizers, with the sample between them, was measured as a function of time. In the reflection-mode analysis, the N phase was assumed to be restored through diffusion of the cis form, followed by reorientation of the trans form that replaced the cis form in the interface region, while in the transmission-mode analysis the N phase recovered through the cis-trans thermal back-isomerization process.