On the Mechanism of the cis-trans Isomerization in the Lowest Electronic States of Azobenzene: S0, S1, and T1

In this paper, we identify the most efficient decay and isomerization route of the S1, T1, and S0 states of azobenzene. By use of quantum chemical methods, we have searched for the transition states (TS) on the S1 potential energy surface and for the S0/S1 conical intersections (CIs) that are closer to the minimum energy path on the S1. We found only one TS, at 60 degrees of CNNC torsion from the E isomer, which requires an activation energy of only 2 kcal/mol. The lowest energy CIs, lying also 2 kcal/mol above the S1 minimum, were found on the torsion pathway for CNNC angles in the range 95-90degrees. The lowest Cl along the inversion path was found ca. 25 kcal/mol higher than the S1 minimum and was characterized by a highly asymmetric molecular structure with one NNC angle of 174 degrees. These results indicate that the S1 state decay involves mainly the torsion route and that the inversion mechanism may play a role only if the molecule is excited with an excess energy of at least 25 kcal/mol with respect to the S1 minimum of the E isomer. We have calculated the spin-orbit couplings between S0 and T1, at several geometries along the CNNC torsion coordinate. These spin-orbit couplings were about 20-30 cm-1 for all the geometries considered. Since the potential energy curves of S0 and T1 cross in the region of twisted CNNC angle, these couplings are large enough to ensure that the T1 lifetime is very short (about 10 ps) and that thermal isomerization can proceed via the nonadiabatic torsion route involving the S0-T1-S0 crossing with preexponential factor and activation energy in agreement with the values obtained from kinetic measures.