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

摘要

In this paper, we analyze the photoisomerization processes of azobenzene after its excitation in the bright S(ππ~*). By state of the art/ ab initiol 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 S_1(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 S_1(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(mt*). This doubly excited state, by driving the photoisomerization along the torsion path and by inducing a fast internal conversion to the Sl(nπ~*) occurring in a variety of geometries, explains all the most important features of the S(ππ~*) azobenzene photoisomerization.