The different photoisomerization efficiency of Azobenzene in the lowest nπ* and ππ* singlets: the role of a phantom state

Azobenzene EZ photoisomerization, following excitation to the bright S(ππ*) state, is investigated by means of ab initio CASSCF optimizations and perturbative CASPT2 corrections. Specifically, by elucidating the S(ππ*) deactivation paths, we explain the mechanism responsible for azobenzene photoisomerization, the lower isomerization quantum yields observed for the S(ππ*) excitation than for the S1(nπ*) excitation in the isolated molecule, and the recovery of the Kasha rule observed in sterically hindered azobenzenes. We find that a doubly excited state is a photoreaction intermediate that plays a very important role in the decay of the bright S(ππ*). We show that this doubly excited state, which is immediately populated by molecules excited to S(ππ*), drives the photoisomerization along the torsion path and also induces a fast internal conversion to the S1(nπ*) at a variety of geometries, thus shaping (all the most important features of) the S(ππ*) decay pathway and photoreactivity. We reach this conclusion by determining the critical structures, the Minimum Energy Paths originating on the bright S(ππ*) state and on other relevant excited states including S1(nπ*) and by characterizing the conical intersections seams that are important in deciding the photochemical outcome. The model is consistent with the most recent time-resolved spectroscopic and photochemical data.