Quantum Chemical Investigation of Spin-Forbidden Transitions in Dithiosuccinimide

Abstract

Abstract The vertical and adiabatic electronic spectra of dithiosuccinimide have been investigated by means of multi-reference Møller–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 → π* excitations. Nearby, three triplet states are located, two n → π* states (T 1 and T 2), and a π → π* triplet excitation (T 3). The experimentally observed strong absorption band with maximum at 3.96 eV arises from the π → π* excited 1 B 2 (S 3) state. Computed radiative lifetimes are presented for the experimentally known phosphorescence from the n → π* excited T 1 state. Further, we find nearly equal probabilities for the spin-forbidden S 0 → T 2 and the spin-allowed S 0 → 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.