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.