Phenylene-bridged bis(benzimidazolium) (BBIm): a dicationic organic photoredox catalyst.

A dicationic photoredox catalyst composed of phenylene-bridged bis(benzimidazolium) (BBIm) was designed, synthesised and demonstrated to promote the photochemical decarboxylative hydroxylation and dimerisation of carboxylic acids. The catalytic activity of BBIm was higher than that for a monocation analogue, suggesting that the dicationic nature of BBIm plays a key role in these decarboxylative reactions. The rate constant for the decay of the triplet-triplet absorption of the excited BBIm increased with increasing concentration of the carboxylate anion with a saturated dependence, suggesting that photoinduced electron transfer occurs within the ion pair complex composed of the triplet excited state of BBIm and a carboxylate anion.