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° 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 ₁ minimum, were found on the torsion pathway for CNNC angles in the range 95-90°. The lowest CI 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°. 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 S ₁ 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 (∼10 ps) and that thermal isomerization can proceed via the nonadiabatic torsion route involving the S₀-T ₁-S₀ crossing with preexponential factor and activation energy in agreement with the values obtained from kinetic measures.