Comparison of H-2 photogeneration by [FeFe]-hydrogenase mimics with CdSe QDs and Ru(bpy)(3)Cl-2 in aqueous solution

Two water soluble [FeFe]-hydrogenase ([FeFe]-H(2)ase) mimics, Fe2S2-2SO(3)Na (1) and Fe2S2-SO3Na (2), were designed to compare the performance of photocatalytic H-2 evolution systems using quantum dots (QDs) and the molecular photosensitizer [Ru(bpy)(3)](2+) in water. Under the same conditions, the resulting systems showed that CdSe QDs display much higher activity than [Ru(bpy)(3)](2+), meanwhile, the turnover numbers (TONs) of 2.65 x 10(4) (1) and 1.88 x 10(4) (2) for CdSe QD systems and 178 (1) and 114 (2) for [Ru(bpy)(3)](2+) systems were comparable with the highest reported H-2 evolution systems using the same photosensitizers. Spectroscopic and electrochemical analyses revealed that the excited [Ru(bpy)(3)](2+) undergoes reductive-quenching while the excited CdSe QDs proceed via oxidative-quenching during the catalytic process. The greater free energy change and faster electron transfer rate in the CdSe QD system made the electron transfer between the photosensitizer and [FeFe]-H(2)ase mimics more efficient, which was subsequently corroborated by emission quenching and transient absorption experiments. The reduced [FeFe]-H(2)ase mimics generated from the CdSe QDs were demonstrated to react with protons at a rate of 1.17 x 10(7) M-1 s(-1) for 1 and 8.62 x 10(6) M-1 s(-1) for 2.