Comparison of H2 photogeneration by [FeFe]-hydrogenase mimics with CdSe QDs and Ru(bpy)3Cl2 in aqueous solution

Abstract

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.