TRIPLET-STATE DECAY OF SOME THIOKETONES IN SOLUTION

The lifetimes of the triplet states of xanthione (XT), 4H-1-benzopyran -4-thione (BPT) and dimethyl-4H-1-benzothiopyran-4-thione (DMBTPT) hav e been measured as a function of concentration in the temperature rang e 77-296 K. These measurements have demonstrated that the lowest-excit ed triplet state of thioketones is quenched by ground-state molecules of the same kind and this dependence on thioketone concentration, toge ther with intramolecular processes, are responsible for the changes in the measured lifetime of the triplet state observed at different temp eratures. Model calculations including an intramolecular decay path vi a a (fast) thermal equilibrium between the S-1 and the T-1 states, whi ch is feasible given the very small energy gap between these states, i n combination with the intermolecular quenching by ground-state molecu les are shown to be in agreement with these experimental values. Emiss ion spectra have also been measured and interpreted in the temperature range 290-368 K in n-heptane, where spectral contributions due to pho sphorescence and delayed fluorescence are apparent.