Optically resolved zero field splittings in the phosphorescence spectra of aromatic thioketones

Abstract

The phosphorescence spectra of xanthione (XS), thioxanthione (TXS), and thiofluorenone (TF) have been investigated at different temperatures below 4.2 K. Narrow-line spectra are observed in n-alkine matrices, the full width at half maximum intensity being ≈4.5 cm −1 for XS in n-hexane. The emissions of XS and TXS consist of two optically resolved subspectra which are assigned to the (T x , T y ), and T z triplet sublevel manifolds. The intensity of the latter subspectrum vanishes when T is reduced to 1.1 K. The T dependence of the relative intensifies of the subspectra is consistent with the populations of the sublevels existing in thermal equilibrium over the entire temperature range over which T z emission is observable. The lack of T z emission at 1.1 K, the lowest temperature obtained in these measurements, provides strong evidence that intersystem crossing leads to the selective population of T x and T y . We find that D = −5 ± 1 cm −1 for both XS and TXS from measurement of the splitting of the 0,0-bands. Resolved are not observed in the TF phosphorescence, but T-dependent line positions allow an estimate of D = −8 cm −1. The smaller zero field splitting between T x and T y is obtained for each molecule by ODMR. The large value of | D| is attributed to spin-orbit coupling induced by the presence of the S atom as well as the presence of a low-lying 3A 1 state. The latter is predicted to lie within ≈ 10 3 cm −1 of T ⊥ in XS.