Role of the local order on the thermal cis-to-trans isomerisation kinetics of azo-dyes in nematic liquid crystals.

Azoderivatives show faster thermal cis-to-trans isomerisation kinetics when they are under the influence of the nematic mean field than dissolved in isotropic solution. The local order parameter induced in the azo-dye by the host mesogen determines the rate of its thermal cis-to-trans isomerisation process. In this way, AZO6-LC mixtures with the same order parameter exhibit identical isomerisation rates independently of the mesogen chemical nature. The temperature dependence of the local order parameter of the chromophore is related with the thermal activation parameters of the isomerisation process and they are characteristic for each AZO6-LC nematic mixture.