Tris(2,2′-bipyridine)ruthenium Derivatives with Multiple Viologen Acceptors: Quadratic Dependence of Photocatalytic H₂Evolution Rate on the Local Concentration of the Acceptor Site

Three Ru(bpy)₃²⁺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-G₁-carbamoyl)-2,2′-bipyridine, and 4,4′-MV4=4,4′-bis(N-G₁-carbamoyl)-2,2′-bipyridine, where G₁=Asp(NHG₂)-NHG₂and G₂=-(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 H₂evolution from water in the presence of a colloidal Pt H₂-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=10⁵–10⁶m⁻¹) are superior in driving H₂evolution at pH 5.0, whereas those with lower K_dvalues (10³–10⁴m⁻¹) are superior at pH 7.0, where K_d=[(MV⁺)₂]/[MV^+.]². The (MV⁺)₂site can drive H₂evolution only at pH 5.0 as a result of its 0.15 eV lower driving force for H₂evolution relative to MV^+., whereas the PCSs with lower K_dvalues 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 H₂evolution rate, and shows an intriguing quadratic dependence on the number of MV²⁺units per PCS.