Excited-State Decay in the Photoisomerisation of Azobenzene: A New Balance between Mechanisms

The mechanism of the photoisomerisation of azobenzene has been studied by means of multiconfigurational ab initio calculations. Our results show that it is necessary to account for the dynamic electron correlation in the location of the critical points (CASPT2 optimizations) to obtain a correct description of the topography of the potential energy surfaces of the low energy singlet excited states. By using this methodology, we have found that the state populated by the initial excitation is the S-2 (pi pi*) state, which decays very efficiently to the S-1 (n pi*) state at a pedal-like non-rotated geometry. In the S-1 state, relaxation leads to a rotated geometry where the system decays to the ground state, in which further relaxation can lead to either the trans or cis geometries. However, the S-1/ S-0 conical intersection seam also extends to planar geometries, so this reaction path is also accessible for rotation-constrained systems. Our results explain the experimental observations satisfactorily.