Subpicosecond population decay time of the first excited singlet state of thioxanthione in fluid solution

The population decay time of the first excited singlet state of an aromatic thione, thioxanthione, has been measured for the first time in fluid solution at room temperature, using a laser pump-probe method. The average value of τ(S1) is 0.53±0.12 ps in three solvents, acetonitrile, acetone and 1-octanol giving a nonradiative S1 decay constant of (1.9 ± 0.4) × 1012 s−1. Using these data and a calculated value of the radiative lifetime of S1, a quantum yield of S1 → S0 fluorescence, φf < 5 × 10−8, is estimated, a value which is consistent with the failure to observe prompt fluorescence in these compounds. The short S1 lifetime of thioxanthione is attributed to fast S1(1A2)−Tn(3A1) intersystem crossing, which can occur by a first order spin-orbit coupling mechanism because Tn(3A1) (n = 1 or 2) has a lower energy than S1(1A2) in the polar solvents employed.