Triplet state decay of some thioketones in solution

Abstract

The lifetimes of the triplet states of xanthione (XT), 4H-1-benzopyran-4-thione (BPT) and dimethyl-4H-1-benzothiopyran-4-thione (DMBTPT) have been measured as a function of concentration in the temperature range 77–296 K. These measurements have demonstrated that the lowest-excited triplet state of thioketones is quenched by ground-state molecules of the same kind and this dependence on thioketone concentration, together with intramolecular processes, are responsible for the changes in the measured lifetime of the triplet state observed at different temperatures. Model calculations including an intramolecular decay path via a (fast) thermal equilibrium between the S₁ and the T₁ states, which is feasible given the very small energy gap between these states, in combination with the intermolecular quenching by ground-state molecules are shown to be in agreement with these experimental values. Emission spectra have also been measured and interpreted in the temperature range 290–368 K in n-heptane, where spectral contributions due to phosphorescence and delayed fluorescence are apparent.