Concerning apparent intersystem crossing efficiencies in molecules with small S 1T 1 energy gaps

Abstract

Accurate values of the apparent quantum yields φ ST of S 1 ⇝ T 1 intersystem crossing were determined for several aromatic thiones in fluid solutions at room temperature. All values of φ ST are significantly less than unity at 295 K; this can be compared with a value of unity based on limited data previously obtained at 77 K, which has been assumed to hold at room temperature. The importance of triplet decay via back intersystem crossing to S 1 followed by radiative and non-radiative decay to S 0 is assessed. The possible involvement of T 2 in the subpicosecond decay of vibrationally excited S 1 is discussed. The potential importance of back intersystem crossing in other molecules with small S 1T 1 gaps is briefly addressed.