Quantifying Triplet State Formation in Zinc Dipyrrin Complexes

Photocatalysis is a promising method to harness solar energy and use it to form fuels and other high value chemicals, but most sensitizers used in photocatalytic reactions are complexes of rare and expensive metals such as ruthenium and iridium. Zinc dipyrromethene complexes have potential to be a more earth-abundant alternative, but their photophysical properties are largely unexplored. In this study, triplet state formation was quantified in two zinc dipyrromethene complexes, with a and without heavy atoms, by transient absorption spectroscopy. Without heavy atoms the triplet quantum yield was 16% in toluene and 27% in THF. With the addition of heavy I atoms the triplet quantum yield is increased to 62-63% and is insensitive to solvent polarity. These triplet yields meet or exceed those of successful organic photosensitizers, illustrating the potential for zinc dipyrromethene complexes as photosensitizers