Electrocatalytic properties of [FeFe]-hydrogenases models and visible-light-driven hydrogen evolution efficiency promotion with porphyrin functionalized graphene nanocomposite

Five tolyl-functionalized monophosphine substituted diiron complexes [Fe2(CO)5(L){μ-SC6H3(CH3)S}] (L = PPh3, 2; Ph2PCH2Ph, 3; Ph2PCH2CH2CH3, 4; Ph2P(2-C5H4N), 5; P(4-C6H4OCH3)3, 6), which could be regarded as the active site models of [FeFe]-hydrogenases, have been successfully synthesized from parent complex [Fe2(CO)6{μ-SC6H3(CH3)S}] (1). The influence of the different phosphine ligands on the reduction potentials of the models has been evaluated by cyclic voltammetry (CV). According to the result of the electrochemical analysis, complexes 1, 2 and 5 were chosen to carry out the photocatalytic experiments. Meanwhile, tetraphenylporphyrin (TPP) covalently functionalized graphene oxide (GO) nanocomposite (TPP-GO) and cystine were applied as the photosensitizer and the electron-donor respectively in the [FeFe]-hydrogenases bionic photocatalytic system. The result of light-driven hydrogen (H2) evolution in aqueous ethanol solution demonstrated that the efficiency of H2 production is affected by following factors: the participation of GO or cystine, the covalent functionalization of GO by TPP and the hetero-atom on the phosphine ligand substituent of [FeFe]-hydrogenases models. Ultraviolet-visible (UV-vis) absorption, fluorescence emission and time-resolved fluorescence were used to analyze and compare the efficiency of electron transferring in the process of photocatalytic H2 production. And a possible mechanism for the light-driven H2 evolution of the photocatalytic system containing TPP-GO was proposed.