Dependence of the thioxanthone triplet-triplet absorption spectrum with solvent polarity and aromatic ring substitution

The triplet-triplet (TT) and transient absorptions of non-substituted and substituted thioxanthones has been studied in different solvents in order to ascertain the effect of the solvent, as well as the substituents on the aromatic ring. Spectra taken after a couple of mu s after the flash show three main transient absorptions due to the triplet state (600-650 nm), the thioxanthone ketyl radical (similar to 450 nm) and an overlap of both (similar to 300 mn). The amount of radicals formed in non hydroxylic solvents is much lower than in alcohols. The maxima of the TT absorption peaks show a good correlation with the E-T (30) solvent parameter.