Dynamics of photoinduced changes in birefringence and dichroism in a nematic phase containing an azobenzene as dopant.

Changes in birefringence and dichroism induced by light were monitored after pulse excitation of a nematic phase consisting of a mixture of a low molecular weight liquid crystal (ZLI-1132) and a dopant 4-(N,N-dimethylamino)-azobenzene. After excitation in the UV, large changes in the light transmitted by the sample, placed between polarizers at various angles or in the polarized absorption spectra, could be detected and were monitored as a function of time and temperature in samples with 1-10% w/w of dopant. Birefringence experiments, performed with different monitoring wavelengths and with dopants that degrade all the absorbed energy as heat, demonstrate that all the changes observed are due to the photoisomerization process of the dopant. The recovery of the optical delay in the sample and of the polarized absorption follows the same monoexponential kinetics, indicating that E-Z isomerization of the dopant governs the order of the nematic phase. When samples are irradiated near the clearing point, a photoinduced nematic-isotropic phase change is observed.