Understanding the fast thermal isomerisation of azophenols in glassy and liquid-crystalline polymers

The good solubility of azophenols in low molar mass liquid crystals together with the ability of their related polymers to form homogeneous nematic and glassy thin films make such azoderivatives valuable chromophores to get a great variety of photoactivatable systems with fast switching speeds under ambient conditions. In fact, the final applicability of these systems is mainly determined by the thermal cis-to-trans isomerisation rate of the photoactive azophenol used, in other words, by the intimate mechanism the reaction goes through. The kinetico-mechanistic study reported herein shows that the rate of the thermal back reaction for azophenols is very sensitive to the local environment where the azo chromophore is located, mainly to its capability to establish hydrogen bonding with its surroundings. With a proper design, azophenol-based polymers can exhibit thermal isomerisation rates as fast as those of the monomers in solution even without the presence of any solvent.