Solvent dependence of the intersystem crossing kinetics of thioxanthone Original Research Article

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 T1–Tn and S1–Sn transitions and to the solvatochromism of the triplet–triplet absorption. The S1–Sn transition, as well as the excited state lifetime has been assigned. Indeed, the population kinetics of the triplet state consists in a fast component k2 presumably higher than 1011 s−1 and a slow component k1 which is dramatically solvent dependent; k1 decreases from 7×1010 s−1 in toluene to 4×108 s−1 in methanol. A strong correlation between 1/k1 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.