The performance of a water-soluble cobalt porphyrin ([{meso-tetra­(4-sulfonatophenyl)­porphyrinato}­cobalt­(III)], CoTPPS) as a catalyst for the photoreduction of CO2 in fully aqueous media has been investigated under visible light irradiation using [Ru­(bpy)3]2+ as a photosensitizer and ascorbate as a sacrificial electron donor. CO is selectively produced (>82%) with high efficiency (926 TONCO; TONCO = turnover number for CO). Upon optimization, selectivities of at least 91% are achieved. Efficiencies up to 4000 TONCO and 2400 h–1 TOFCO (TOFCO = turnover frequency for CO) are reached at low catalyst loadings, albeit with loss in selectivity. This work successfully demonstrates the ability of CoTPPS to perform highly efficient photoreduction of CO2 in water while retaining its high selectivity for CO formation.

Without using precious elements, a highly efficient and selective molecular-based photocatalytic system for CO2-to-CO conversion in fully aqueous media has been developed. Our copper­(I)-based water-soluble photosensitizer (CuPS) preserves its highly luminescent and long-lived excited state even in aqueous media. The CuPS-driven CO2 reduction catalyzed by a water-soluble cobalt porphyrin possessing four N-methylpyridinium acceptors at the meso positions (CoTMPyP) achieves the highest catalytic activity among those reported for aqueous systems: TONCO = 2680 and TOFCO max = 1600–2600 h–1 with SelCO2 = 77–90% (selectivity for CO vs H2). The observed photocatalytic enhancement is discussed in terms of the 6-electron chargeable character of CoTMPyP, permitting its rapid release of CO via reduction of CoII to CoI by intramolecular electron transfer from the reducing equivalent stored at one of the acceptors.

The Ru(V)==O species and other intermediates in O(2) evolution from water catalyzed by [Ru(terpy)(bpy)(OH(2))](2+) were spectrophotometrically characterized, and the spectral components observed were identified based on the TD-DFT calculations. Moreover, important insights into the rapid paths after the RDS were given by the DFT studies.

The one-electron-reduced form of methylviologen (MV(+*)), generated in situ by bulk electrolysis of methylviologen (MV(2+)), was for the first time reacted with various Pt(II) complexes in aqueous media without light irradiation to reveal that thermal reduction of water into molecular hydrogen is indeed highly promoted by the Pt(II)-based molecular catalysts.

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