An ab initio study of the photochemistry of azobenzene

The photochemistry under n -->pi* and pi -->pi* excitation and the thermal isomerization of azobenzene are studied by a theoretical approach. Two mechanisms are considered: torsion around the N=N double bond and the inversion of one N atom. We optimize the most important geometries with CASSCF and we calculate the potential energy curves for the lowest states (4 singlets and 2 triplets) with multireference perturbation theory. Our calculations support the current view that inversion is the preferred pathway for ground state isomerization and probably also by n -->pi* excitation, while torsion occurs by pi -->pi* excitation. We find that two states, 2(1)A and 2(1)B in C-2 symmetry, are important in the pi -->pi* photochemistry.