In the present paper we report the results of a multiconfigurational computational study on potential-energy curves of azobenzene along the NN twisting to clarify the role of this coordinate in the decay of the S₂(ππ*) and S₁(nÏ€*) states. We have found that there is a singlet state, S₃ at the trans geometry, on the basis of the doubly excited configuration n²Ï€*², that has a deep minimum at about 90° of twisting, where it is the lowest excited singlet state. The existence of this state provides an explanation for the short lifetime of S₂(ππ*) and for the wavelength-dependence of azobenzene photochemistry. We have characterized the S₁(nÏ€*) state by calculating its vibrational frequencies, which are found to correspond to the recently observed transient Raman spectrum. We have also computed the potential-energy curve for the triplet T₁(nÏ€*) at the density functional theory B3LYP level, which indicates that in this state the isomerization occurs along the twisting coordinate.

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 S₁ 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 character₁ 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 (~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.