A theoretical study of the lowest electronic states of azobenzene: The role of torsion coordinate in the cis-trans photoisomerization

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 S ₂(ππ*) and S₁(nπ*) states. We have found that there is a singlet state, S₃ at the trans geometry, on the basis of the doubly excited configuration n²π*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(nπ*) 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 T ₁(nπ*) at the density functional theory B3LYP level, which indicates that in this state the isomerization occurs along the twisting coordinate.