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

[Display omitted] •A thermodynamically-consistent model for photoisomerization, where reaction rate is expressed as a function of thermodynamic activity, is used to predict phase behavior in photoactive solutions of azobenzene forming nematic phases.•In addition to the regular, high-temperature nematic phase, reentrant isotropic phases and a low-temperature nematic phase can exist under irradiation.•The range of existence of each phase depends on concentration. Ordered phases can exist in a metastable state in a range spanning orders of magnitude of irradiation intensity. The capability of azobenzene group to undergo photoinducedtrans–cisisomerizationis the basis of azobenzene-based photoactive materials. In thiswork, thethermodynamically consistent theoryfor 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 formordered phases. As in pure azobenzenes, reentrant isotropic phases (induced byphotoisomerization) and a low temperature isotropic- nematic transition (corresponding to thecis-saturated system), existin 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.