The photophysics of pyranthione: a theoretical investigation focussing on spin-forbidden transitions

In this paper we present the results of a theoretical study on the electronic structure and the photophysics of low-lying singlet and triplet states of 4H-pyran-4-thione In a first step, Russel-Saunders coupled wavefunctions and spin-independent properties were determined by means of a recently proposed combination of density functional theory and multi-reference configuration interaction methods. Spin-orbit (SO) coupling was subsequently included at the level of quasi-degenerate perturbation theory. For the evaluation of spin-dependent properties a non-empirical effective one-electron SO mean-field Hamiltonian was employed. Calculated excitation energies, dipole moments, fine-structure splittings, and transition moments compare very well with experimental data. For the T-1 state we find that phosphorescence and non-radiative decay via intersystem crossing to the S-0 state are concurrent processes occurring at approximately equal rates of the order of 10(4) s(-1). The T-2 --> S-0 radiative transition gains its intensity from two sources: (1) Direct SO coupling of the S-0 and T-1 levels combined with the large dipole moment difference between these states and (2) the strong S-2 --> S-0 spin-allowed transition. The computed SO splitting in the T-1 state of D = -18 cm(-1) is mainly due to interaction with the closeby T-2 state. A rapid depletion of the S-1 state via intersystem crossing to the T-1 state can be mediated by the T-2 state, if spin relaxation is fast within the triplet levels. (C) 2001 Elsevier Science B.V. All rights reserved.