Dynamics of the isotropic to nematic phase conversion induced by the cis to trans isomerization in a liquid crystalline azobenzene

We report on the kinetics of the molecular orientation recovery over the isotropic to nematic phase transition induced by the thermally driven cis to trans isomerization in p,p’-diheptylazobenzene (7AB) liquid crystal. In this study the photopyroelectric calorimetric technique has been specifically employed to probe the time dependence of the thermal conductivity, being such a parameter in monodomain aligned samples directly related to the degree of molecular orientational order. Unlike previous results obtained in the same liquid crystal in the photoinduced nematic to isotropic phase transition, the present investigations indicate that the thermal conductivity variation over time during the nematic phase recovery is remarkably different from the one observed for the trans isomers concentration change. In addition, such a variation has been found to be significantly affected by the initial value of the photoinduced cis isomers concentration. In this regard, a threshold process has been proposed as a tentative explanation for such results. We believe that such studies may enable to gather complementary information to those provided by other widely used techniques, which can play a crucial role in the design of optical devices based on azobenzene nematic liquid crystals.