Reductive side of water splitting in artificial photosynthesis: New homogeneous photosystems of great activity and mechanistic insight

Rhodamine photosensitizers (PSs) substituting S or Se for O in the xanthene ring give turnover numbers (TONs) as high as 9000 for the generation of hydrogen via the reduction of water using [Co^III(dmgH) ₂(py)Cl] (where dmgH = dimethylglyoximate and py = pyridine) as the catalyst and triethanolamine as the sacrificial electron donor. The turnover frequencies were 0, 1700, and 5500 mol H₂/mol PS/h for O, S, and Se derivatives, respectively (ΦH₂ = 0%, 12.2%, and 32.8%, respectively), which correlates well with relative triplet yields estimated from quantum yields for singlet oxygen generation. Phosphorescence from the excited PS was quenched by the sacrificial electron donor. Fluorescence lifetimes were similar for the O- and S-containing rhodamines (∼2.6 ns) and shorter for the Se analog (∼0.1 ns). These data suggest a reaction pathway involving reductive quenching of the triplet excited state of the PS giving the reduced PS^- that then transfers an electron to the Co catalyst. The longer-lived triplet state is necessary for effective bimolecular electron transfer. While the cobalt/rhodamine/triethanolamine system gives unprecedented yields of hydrogen for the photoreduction of water, mechanistic insights regarding the overall reaction pathway as well as system degradation offer significant guidance to developing even more stable and efficient photocatalytic systems.