Kinetico-mechanistic study of the thermal cis-to-trans isomerization of 4,4'-dialkoxyazoderivatives in nematic liquid crystals.

4,4'-Dialkoxy-substituted azobenzenes are usually required for technical applications. Here, we study the mechanism through which the azo-dye thermally isomerizes from the unstable cis isomer to the more stable trans isomer when it is incorporated in the nematic liquid-crystalline state. We have determined the kinetic and thermal activation parameters for this process in different nematic environments. Their comparison with those values obtained in isotropic media demonstrates that the mechanism through which the thermal cis-to-trans isomerization takes place is the inversion pathway in all the physical states studied. The nematic order increases the rate of the thermal cis-to-trans isomerization process. This fact is related to a cooperative interaction established between the mesogen molecules and the azo-dye. This effect is not present in the isotropic state.