Thermodynamic model of phase equilibrium and isomerization in photoactive azobenzene-based materials

Abstract

The capability of azobenzene group to undergo photoinduced trans–cis isomerization is the basis of azobenzene-based photoactive materials. In this work, the thermodynamically consistent theory for photoisomerization of azobenzenes [E. R. Soulé, Chemical Physics Letters, 794 (2022) 139503] is extended to mixtures, considering different cases depending on the intrinsic capability of the azobenzene and the second component to form ordered phases. As in pure azobenzenes, reentrant isotropic phases (induced by photoisomerization) and a low temperature isotropic- nematic transition (corresponding to the cis-saturated system), exist in mixtures, but the temperature range where each phase exists depends on concentration. Phase diagrams with several different coexistence regions (isotropic-isotropic, isotropic-nematic and nematic-nematic, including high temperature, low temperature and reentrant cases) and very atypical shapes are found. Metastability is briefly considered, showing that nematic ordering can persist up to an irradiation intensity that can be several orders of magnitude higher than at the stable transition point.