A study of the trans-cis photoisomerization mechanism of azobenzene in liquid crystals

In this work we have studied, by means of Molecular Dynamics simulations, the trans-cis photoisomerization mechanism of azobenzene dissolved in a liquid crystal when excited in the state associated to the npi* transition (first excited singlet S1). A great number of photochemical applications are based on the trans-cis photoisomerization of azobenzene-containing materials, and, in particular, liquid crystals, and even though many studies have been done, the actual conversion mechanism in condensed phases is still not completely known. Herein, with the purpose to continue the work of Tiberio et al., which analyzed the npi* photoisomerization in vacuum and in various organic solvents, we start the study of the same phenomenon taking place in a liquid crystal (8CB) via a modifed molecular dynamics simulation adopting a QM-based class II force field in the ground and excited state, electronic transitions and stochastic decay events to the fundamental state. We describe the procedure used to parameterize the new force field for azobenzene in ground and first excited state, analyze the molecular trajectories, determine the trans-cis photoisomerization quantum yield and decay times and compare our results with experimental ones where available. Quantum mechanics calculations showed that it is fundamental in the decay pathway to reach a molecule conformation in which the central dihedral angle CNNC is about 90° and the two bending angles CNN are asymmetric. With molecular dynamics simulations we see that this mechanism is followed, although with some differences, in vacuum as well as in 8CB.