Molecular photo-charge-separators enabling single-pigment-driven multi-electron transfer and storage leading to H₂ evolution from water

Abstract

Novel unimolecular architectures that enable single-pigment-driven multi-electron transfer coupled with double-electron-storage leading to catalytic H2 evolution are reported.

Tris(2,2′‐bipyridine)ruthenium Derivatives with Multiple Viologen Acceptors: Quadratic Dependence of Photocatalytic H2 Evolution Rate on the Local Concentration of the Acceptor Site

“…As part of our studies in this area, we recently succeeded in developing Ru(bpy) 3 2+ derivatives (bpy=2,2′‐bipyridine) covalently linked to twelve pendant viologen acceptors [Ru( x , x ′‐MV4) 3 ] 26+ ( x =4 or 5; x denotes the positions at which the substituents are connected on a 2,2′‐bipyridyl ligand; see Figure ), which we term molecular “photo‐charge separators (PCSs)” . The study showed that (i) multiple electrons can be photochemically pumped out from sacrificial electron donors to be stored over the PCS framework, leading to storage of 7–8 electrons per PCS, (ii) the highly positively charged PCSs form ion‐pair adducts with di‐deprotonated EDTA anions (YH 2 2− , where EDTA=YH 4 ) to give {[Ru( x , x ′‐MV4) 3 ](YH 2 ) n } (26−2 n )+ in aqueous solution in which reductive quenching of the triplet by YH 2 2− is strongly enhanced, (iii) the one‐electron‐reduced acceptors (i.e., MV + .…”

“…The large negative bleach effect around 650 nm arises from the mixing of non‐eliminated 3 MLCT emission from the Ru*(bpy) 3 2+ site (Figure S5). Figure a shows the TA decay profiles at 400 nm observed for the new PCSs, in which the decay feature previously reported for [Ru(4,4′‐MV4) 3 ] 26+ is overlaid for comparison …”

“…The colored lines were obtained by using the least‐squares fitting calculations. For comparison, a calculated line for [Ru(4,4′‐MV4) 3 ] 26+ , reported in reference , is overlaid. b) Plot of Δ A 400 (CS) versus the number of MV 2+ units per PCS, where Δ A 400 (CS) is estimated from the decay at 400 nm by eliminating the triplet decay component (fitting data not shown), and is linear to the quantum yield of MV + .…”

“…For instance, the least‐squares fitting analysis reveals involvement of three decay components; τ 1 =3.62 ns (10.4 %), τ 2 =10.6 ns (77.8 %), and τ 3 =28.2 ns (11.8 %) for [Ru(bpy) 2 (4,4′‐MV4)] 10+ (Table ), where the values in parenthesis are the relative contributions. The time constant for the major FET component (10.6 ns) is comparable to the value of 10.9 ns (71.0 %) previously observed for [Ru(4,4′‐MV4) 3 ] 26+ . Interestingly, these time constants are also consistent with those observed as the major FET components for [Ru(bpy) 2 (4,4′‐MV2)] 6+ and [Ru(bpy)(4,4′‐MV4) 2 ] 18+ (Table ; see also Figure S3 in the Supporting Information).…”

“…The study showed that (i) multiple electrons can be photochemically pumped out from sacrificial electron donors to be stored over the PCS framework, leading to storage of 7–8 electrons per PCS, (ii) the highly positively charged PCSs form ion‐pair adducts with di‐deprotonated EDTA anions (YH 2 2− , where EDTA=YH 4 ) to give {[Ru( x , x ′‐MV4) 3 ](YH 2 ) n } (26−2 n )+ in aqueous solution in which reductive quenching of the triplet by YH 2 2− is strongly enhanced, (iii) the one‐electron‐reduced acceptors (i.e., MV + . sites) exhibit preferential formation of diamagnetic π‐dimer (MV + ) 2 sites, (iv) the thus‐stored multiple electrons over the PCS can be effectively consumed to drive H 2 production in the presence of a colloidal Pt H 2 ‐evolving catalyst, and (v) back electron transfer (BET) events in these PCSs are effectively suppressed as a result of the enhancement of electron migration (EM) within the dodecaviologen frameworks …”