THE PHOTOPHYSICS OF THIOFLAVONE IN SOLUTION

The absorption, emission and emission excitation spectra, S-2 --> S-0 fluorescence and T-1 --> S-0 phosphorescence quantum yields and S-2 an d T-1 excited state lifetimes of thioflavone (TF) were measured in per fluoro-1,3-dimethylcyclohexane (PFDMCH) and 3-methylpentane (3-MP) at room temperature. The results were analysed to provide a quantitative description of the decay processes of excited TF, with emphasis on the effect of the phenyl rotor on the radiationless decay rates. In the i nert perfluoroalkane solvent, both St and T, relax intramolecularly vi a an S-2 --> S-1 --> T-1 --> S-0 path. S-2 --> S-0 fluorescence and T- 1 --> S-0 phosphorescence also account for a small fraction of the exc ited state decay events, but no thermally activated delayed fluorescen ce, S-1 --> S-0 + h nu(df), is observed. In 3-MP, the decay mechanism is dominated by intermolecular interactions between excited TF and the solvent. The rates of intramolecular radiationless decay are larger i n TF than in rigid thiones having the same electronic energy gaps owin g to the influence of the torsional motion of the phenyl group. By com parison of the observed rate constants for intramolecular radiationles s decay of excited TF with those of a hypothetical rigid thione having the same electronic energy spacings, contributions to the radiationle ss decay rates of S-2 and T-1 due to torsional motion of the phenyl gr oup were quantified. The second-order rate constants for quenching of triplet thione by molecular oxygen and by ground state thione were als o measured. Excitation of TF to singlet excited states higher in energ y than S-2 results in decay to S-1 and T-1 which partially bypasses S- 2.