Azobenzene cis-trans Photoisomerization Mechanism: Characterization of the Decay Ways from the Lowest pipi* Absorbing Singlet State

In this paper, we analyze the photoisomerization processes of azobenzene after its excitation in the bright S(ππ*). By state of the art/ ab initio / Complete Active Space calculations followed by perturbative corrections (CASPT2//CASSCF) we have identified the critical structures, the Minimum Energy Paths originating on the bright S(ππ*) and on other relevant excited states including the state S1(nπ*). The seams of conical intersections that are important in guiding the photoreaction are determined. We aim at establishing the mechanism of decay and of photoisomerization for the S(ππ*) state and at explaining the difference between the quantum yields found for the two lowest energy S1(nπ*) and S(ππ*) excited states. We found that an excited state based on the πN = NπN = N→π*N = Nπ*N = N configuration is a photoreaction intermediate that plays a very important role in the decay the bright S(ππ*). This doubly excited state, by driving the photoisomerization along the torsion path and by inducing a fast internal conversion to the S1(nπ*) occurring in a variety of geometries, explains all the most important features of the S(ππ*) azobenzene photoisomerization.