Tris(2,2′‐bipyridine)ruthenium Derivatives with Multiple Viologen Acceptors: Quadratic Dependence of Photocatalytic H 2 Evolution Rate on the Local Concentration of the Acceptor Site

Three Ru(bpy) 3 2+ derivatives tethered to multiple viologen acceptors, [Ru(bpy) 2 (4,4′‐MV2)] 6+ , [Ru(bpy) 2 (4,4′‐MV4)] 10+ , and [Ru(bpy)(4,4′‐MV4) 2 ] 18+ [bpy=2,2′‐bipyridine, 4,4′‐MV2=4‐ethoxycarbonyl‐4′‐( N ‐G 1 ‐carbamoyl)‐2,2′‐bipyridine, and 4,4′‐MV4=4,4′‐bis( N ‐G 1 ‐carbamoyl)‐2,2′‐bipyridine, where G 1 =Asp(NHG 2 )‐NHG 2 and G 2 =‐(CH 2 ) 2 ‐N + C 5 H 4 ‐C 5 H 4 N + ‐CH 3 ] were prepared as “photo‐charge separators (PCSs)”. Photoirradiation of these complexes in the presence of a sacrificial electron donor (EDTA) results in storage of electrons per PCS values of 1.3, 2.7, and 4.6, respectively. Their applications in the photochemical H 2 evolution from water in the presence of a colloidal Pt H 2 ‐evolving catalyst were investigated, and are discussed along with those reported for [Ru(bpy) 2 (5,5′‐MV4)] 10+ , [Ru(4,4′‐MV4) 3 ] 26+ , and [Ru(5,5′‐MV4) 3 ] 26+ ( Inorg. Chem. Front . 2016 , 3 , 671–680). The PCSs with high dimerization constants ( K d =10 5 –10 6   m −1 ) are superior in driving H 2 evolution at pH 5.0, whereas those with lower K d values (10 3 –10 4   m −1 ) are superior at pH 7.0, where K d =[(MV + ) 2 ]/[MV + . ] 2 . The (MV + ) 2 site can drive H 2 evolution only at pH 5.0 as a result of its 0.15 eV lower driving force for H 2 evolution relative to MV + . , whereas the PCSs with lower K d values exhibit higher performance at pH 7.0 owing to the higher population of free MV + . . Importantly, the rate of electron charging over the PCSs is linear to the apparent H 2 evolution rate, and shows an intriguing quadratic dependence on the number of MV 2+ units per PCS.