Photophysics and photoreactivity of substituted thioxanthones

The photophysics and the interaction of the excited states of substituted thioxanthones have been studied in order to be able to predict their behaviour as photoinitiators in photopolymerization systems. Electron-donor groups in position 2 in the thioxanthone ring lead to high fluorescence quantum yields, red-shifts in the emission spectra and long singlet lifetimes, explained by the separation of the n pi* and pi pi* excited states induced by the higher electron density of the aromatic system. For some of the derivatives a biexponential decay due to slow relaxation of the solvent has been observed. Rate constants for the quenching of the singlet state by amines are near the diffusional limit and are not modified by substituents. On the other hand, substitution of electron-donor groups on the thioxanthone ring in position 2 decreases the triplet reactivity towards amines and alkenes. The incorporation of double bonds on the side chains of the compounds does not affect the lifetimes and reactivity of the thioxanthones.