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

摘要

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)=-(CH2)(2)-N+C5H4-C5H4N+-CH3 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 pH5.0, whereas those with lower K-d values (10(3)-10(4)m(-1)) are superior at pH7.0, where K-d=(MV+)(2)/MV+.(2). The (MV+)(2) site can drive H-2 evolution only at pH5.0 as a result of its 0.15eV lower driving force for H-2 evolution relative to MV+., whereas the PCSs with lower K-d values exhibit higher performance at pH7.0 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 MV2+ units per PCS.