Comparison of H2 photogeneration by [FeFe]-hydrogenase mimics with CdSe QDs and Ru(bpy)3Cl2 in aqueous solution†
Two water soluble [FeFe]-hydrogenase ([FeFe]-H2ase) mimics, Fe2S2-2SO3Na (1) and Fe2S2-SO3Na (2), were designed to compare the performance of photocatalytic H2 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 × 104 (1) and 1.88 × 104 (2) for CdSe QD systems and 178 (1) and 114 (2) for [Ru(bpy)3]2+ systems were comparable with the highest reported H2 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]-H2ase mimics more efficient, which was subsequently corroborated by emission quenching and transient absorption experiments. The reduced [FeFe]-H2ase mimics generated from the CdSe QDs were demonstrated to react with protons at a rate of 1.17 × 107 M−1 s−1 for 1 and 8.62 × 106 M−1 s−1 for 2.