Solvent dependence of the intersystem crossing kinetics of thioxanthone

Intersystem crossing (ISC) of thioxanthone is investigated in a large number of solvents by employing picosecond pump-probe spectroscopy. The measurement of ISC rate constants for this particular molecule is somewhat difficult due to the overlap between the T-1-T-n, and S-1-S-n, transitions and to the solvatochromism of the triplet-triplet absorption. The S-1-S-n, transition, as well as the excited state lifetime has been assigned. Indeed, the population kinetics of the triplet state consists in a fast component k(2) presumably higher than 10(11) s(-1) and a slow component k(1) which is dramatically solvent dependent; k(1) decreases from 7 x 10(10) s(-1) in toluene to 4 x 10(8) s(-1) in methanol. A strong correlation between 1/k(1) and the fluorescence lifetime is observed. A four excited state level diagram is proposed which accounts for the complex kinetics of the S --> T conversions. (C) 2000 Elsevier Science B.V. All rights reserved.