Evaluating brominated thioxanthones as organo-photocatalysts

Bromination of the widely used triplet sensitizer thioxanthone extends the absorption spectrum into the visible range with only minor loss of lowest triplet state energy (3 kcal/mol for di-bromination). Because of bromine substitution, a slight increase in triplet quantum yield was observed. The di-brominated derivative was effective as organo-photocatalyst in performing [2 + 2] cross-photocycloaddition of acrylimide-based compounds under visible light irradiation.