Photochemical, Electrochemical, and Photoelectrochemical Water Oxidation Catalyzed by Water-Soluble Mononuclear Ruthenium Complexes

Two mononuclear ruthenium complexes [Ru-(H(2)tcbp)(isoq)(2)] (1) and [Ru(H(2)tcbp)(pic)(2)] (2) (H(4)tcbp = 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 overpoten-tial (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 lightdriven WOC, P1 or P2 as a photosensitizer, and Na2S2O8 or [CoCl(NH3)(5)]Cl-2 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 mu Acm(-2) is obtained by applying 0.3 V bias vs. NHE.