The main factor that determines the formation-efficiencies of photochemically derived one-electron-reduced species.

While the quantum yields of photosensitiser-derived one-electron-reduced species (OERSs) significantly impact the overall efficiencies of various redox-photosensitised photocatalytic reactions, the primary factors that influence them remain unclear. In this study, we systematically compared the photochemical formation quantum yields for OERSs associated with Ru(ii) and Os(ii) tris-diimine, , -[Re(diimine)(CO)(PR)], and cyclometalated Ir(iii) complexes in the presence of the same 1,3-dimethyl-2-phenyl-2,3-dihydro-1-benzo[]imidazole (BIH) reductant. The reduction potentials of the excited metal complexes, the heavy-atom effects of the central metal ions, and the oxidation potentials and charges of their OERSs were examined, which reveals that the driving force for photoinduced electron-transfer is the most important factor that determines the quantum yields associated with photochemical OERS formation. For complexes with higher oxidation power in their excited states, the formation quantum yield of OERSs divided by the quenching efficiency of the excited state by BIH is greater. This finding suggests that a higher photoinduced electron-transfer exergonicity promotes electron transfer over larger excited-complex/BIH distances, which in turn enables more-efficient separation of the resulting OERSs and one-electron-oxidised BIH species.