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 S-1, T-1, and S-0 states of azobenzene. By use of quantum chemical methods, we have searched for the transition states (TS) on the S-1 potential energy surface and for the S-0/S-1 conical intersections (CIS) that are closer to the minimum energy path on the S-1. We found only one TS, at 60degrees 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 S, 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 S, minimum and was characterized by a highly asymmetric molecular structure with one NNC angle of 174degrees. These results indicate that the S-1 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 S, minimum of the E isomer. We have calculated the spin-orbit couplings between So and T, 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 S-0 and T-1 cross in the region of twisted CNNC angle, these couplings are large enough to ensure that the T-1 lifetime is very short (similar to10 ps) and that thermal isomerization can proceed via the nonadiabatic torsion route involving the S-0-T-1-S-0 crossing with preexponential factor and activation energy in agreement with the values obtained from kinetic measures.