Time resolved calorimetric investigations of photoinduced isomeric molecular changes in the nematic phase of an azobenzene compound

By monitoring the time dependence of the thermal conductivity of a p,p’-diheptylazobenzene sample irradiated with UV light of various intensities, a quantitative study is presented on the effects played by the photo-induced trans to cis isomerization on the degree of the homeotropic alignment in its nematic phase. In particular, the kinetics of the changes eventually leading to the nematic–isotropic phase transformation has been investigated. Unlike the case of the thermal diffusivity analyzed in a recent study, the thermal conductivity analysis has enabled the quantitative evaluation of the trans–cis transformation rates since it is not affected by the specific heat critical behavior and, therefore, it is determined only by changes in the molecular structure induced by the photo-isomerization. The study has been possible thanks to the peculiarity of the adopted photopyroelectric calorimetric setup that enabled both the calorimetric evaluations, as well as the sample texture characterization, to be performed during the UV illumination of the sample.