Two mononuclear ruthenium complexes [Ru(H2tcbp)(isoq)2] (1) and [Ru(H2tcbp)(pic)2] (2) (H4tcbp=4,4′,6,6′-tetracarboxy-2,2′-bipyridine, isoq=isoquinoline, pic=4-picoline) are synthesized and fully characterized. Two spare carboxyl groups on the 4,4′-positions are introduced to enhance the solubility of 1 and 2 in water and to simultaneously allow them to tether to the electrode surface by an ester linkage. The photochemical, electrochemical, and photoelectrochemical water oxidation performance of 1 in neutral aqueous solution is investigated. Under electrochemical conditions, water oxidation is conducted on the deposited indium-tin-oxide anode, and a turnover number higher than 15,000 per water oxidation catalyst (WOC) 1 is obtained during 10 h of electrolysis under 1.42 V vs. NHE, corresponding to a turnover frequency of 0.41 s−1. The low overpotential (0.17 V) of electrochemical water oxidation for 1 in the homogeneous solution enables water oxidation under visible light by using [Ru(bpy)3]2+ (P1) (bpy=2,2′-bipyridine) or [Ru(bpy)2(4,4′-(COOEt)2-bpy)]2+ (P2) as a photosensitizer. In a three-component system containing 1 or 2 as a light-driven WOC, P1 or P2 as a photosensitizer, and Na2S2O8 or [CoCl(NH3)5]Cl2 as a sacrificial electron acceptor, a high turnover frequency of 0.81 s−1 and a turnover number of up to 600 for 1 under different catalytic conditions are achieved. In a photoelectrochemical system, the WOC 1 and photosensitizer are immobilized together on the photoanode. The electrons efficiently transfer from the WOC to the photogenerated oxidizing photosensitizer, and a high photocurrent density of 85 μA cm−2 is obtained by applying 0.3 V bias vs. NHE. Chemistry - A European Journal, 2014 Synthesis of the ligand and the complexes. |
