How Does the Trans-Cis Photoisomerization of Azobenzene Take Place in Organic Solvents?

The trans-cis photoisomerization of azobenzene-containing materials is key to a number of photomechanical applications, but the actual conversion mechanism in condensed phases is still largely unknown. Herein, we study the n,pi* isomerization in a vacuum and in various solvents via a modified molecular dynamics simulation adopting an ab initio torsion-inversion force field in the ground and excited states, while allowing for electronic transitions and a stochastic decay to the fundamental state. We determine the trans-cis photoisomerisation quantum yield and decay times in various solvent's (n-hexane, anisole, toluene, ethanol, and ethylene glycol), and Obtain results comparable with experimental ones where available. A pro-found difference between the isomerization mechanism in vacuum and in solution is found, with the often neglected mixed torsional-inversion pathway being the most important in solvents.