Triplet Photosensitizers Showing Strong Absorption of Visible Light and Long-Lived Triplet Excited States and Application in Photocatalysis: A Mini Review

Triplet photosensitizers (PSs) have been studied as photocatalysts in photocatalytic H2 evolution by water splitting and photoredox catalytic synthetic organic reactions, etc. The applications share common features that the photocatalysts (triplet PSs) harvest the photoexcitation energy, then undergo intersystem crossing (ISC), and finally initiate the electron transfer or triplet energy transfer, which are intermolecular processes in most cases. Thus, triplet PSs showing a strong visible-light-harvesting ability and long triplet excited state lifetimes are desired because they can enhance the above intermolecular processes. The conventional transition metal complexes (i.e., Ru(bpy)3X2 or Ir(ppy)3) have been widely used; however, these complexes are not satisfactory due to their weak absorption of visible light and short triplet excited state lifetimes. To a large extent, the photophysical property of these complexes is due to the low-lying metal-to-ligand charge transfer (MLCT) state, the S0 → 1MLCT transition is weakly allowed, and a strong heavy atom effect exists for the 3MLCT state, which results in weak visible light absorption and a short triplet excited state lifetime, respectively. This mini review introduces the recent progress of the development of the transition metal complexes as well as the organic triplet PSs that show a strong visible-light-harvesting ability and long triplet excited state lifetimes. The application of these new triplet PSs in photocatalytic H2 evolution and photoredox catalytic synthetic organic reactions is summarized.