Quantum chemical investigation of spin-forbidden transitions in dithiosuccinimide

The vertical and adiabatic electronic spectra of dithiosuccinimide have been investigated by means of multi-reference Moller-Plesset perturbation theory and combined densitiy functional/multi-reference configuration interaction methods. Geometries of the electronic ground state and several low-lying excited states have been optimised at the level of time-dependent density functional theory. We have determined spin-orbit coupling for correlated wavefunctions utilising a non-empirical spin-orbit mean-field approach. Because of the two thiocarbonyl groups present in the molecule, dithiosuccinimide exhibits a dense spectrum of low-lying valence states. The first two excited singlet states (S-1 and S-2) originate from n --> pi* excitations. Nearby, three triplet states are located, two n --> pi* states (T-1 and T-2), and a pi --> pi* triplet excitation (T-3). The experimentally observed strong absorption band with maximum at 3.96 eV arises from the pi --> pi* excited B-1(2) (S-3) state. Computed radiative lifetimes are presented for the experimentally known phosphorescence from the n --> pi* excited T-1 state. Further, we find nearly equal probabilities for the spin-forbidden S-o --> T-2 and the spin-allowed S-o --> S-1 transitions in absorption. On these grounds, we assign band number 3 in the spectra measured by Meskers et al. [J. Phys. Chem. 99 (1995) 1134] to this spin-forbidden transition.