Photoinduced electron transfer in tris(2,2′-bipyridine)ruthenium(II)-viologen dyads with peptide backbones leading to long-lived charge separation and hydrogen evolution

Three 5,5'-disubstituted-2,2'-bipyridine ligands tethered to L-Asp-based peptide backbones having pendant viologen-modified branches, i.e., 5-ethoxycarbonyl-5'-(N-G(1)-carbamoyl)-2,2'-bipyridine (MV2(4+)), 5,5'-bis(N-G(1)-carbamoyl)-2,2'-bipyridine (MV4(8+)), and 5,5'-bis(N-G(2)-carbamoyl)-2,2' bipyridine (MV6(12+)), were prepared, where G(1) = Asp(NHG(3))-NHG(3), G(2) = Asp(NHG(3))-Asp(NHG(3))-NHG(3), and G(3) = -(CH2)(2-)(+) NC5H4-C5H4N+-CH3, i.e., 2-(1'-methyl-4,4'-bipyridinediium-1-yl)ethyl. These were reacted with cis-Ru(bpy)(2)Cl-2 to give three new dyads [Ru(bpy)(2)(MV2)](6+) (RuMV2(6+)), [Ru(bpy)(2)(MV4)](10+) (RuMV4(10+)), and [Ru(bpy)(2)(MV6)](14+) (RuMV6(14+)), respectively, where bpy = 2,2'-bipyridine. All these dyads undergo extremely efficient intramolecular quenching leading to the formation of charge separated (CS) states (Ru-III-MV+center dot), and display a triple exponential decay due to the presence of three classes of conformers with each exhibiting the individual rate of electron transfer. The lifetimes (contributions) were determined as 12.5 ps (94.2%), 791 ps (4.5%), and 18.3 ns (1.2%) for RuMV2, 82.2 ps (79.9%), 1.12 ns (12.4%), and 4.60 ns (7.7%) for RuMV4, and 43.6 ps (71.6%), 593 ps (20.2%), and 3.75 ns (8.1%) for RuMV6. The forward electron transfer rate constants (k(ET)) for the major components were calculated as k(ET) = 8.3 x 10(10) s(-1) for RuMV2, k(ET) = 1.2 x 10(10) s(-1) for RuMV4, and k(ET) = 2.3 10(10) s(-1) for RuMV6. Further, the lifetimes and quantum yields of charge separated states were determined as tau(CS) = 16 +/- 3 ns and Phi(CS) = 0.81 for RuMV2, tau(CS) = 20 +/- 3 ns and Phi(CS) = 0.92 for RuMV4, and tau(CS) = 20 +/- 3 ns and Phi(CS) = 0.64 for RuMV6. The backward electron transfer rate constants (k(BET)) were also determined as 6.3 x 10(7), 5.0 x 10(7), and 5.0 x 10(7) s(-1) for RuMV2, RuMV4, and RuMV6, respectively. From the analysis of electrical conductivity, the major ion-pair adducts in aqueous media were characterized as RuMV2(PF6)(5+) (52%) for RuMV2, RuMV4(PF6)(2)(8+) (29%) and RuMV4(PF6)(3)(7+) (32%) for RuMV4, and RuMV6(PF6)(3)(11+) (27%) and RuMV6(PF6)(4)(10+) (29%) for RuMV6, at a total complex concentration of 0.04 mM. The present family is found to be the first example of a Ru(bpy)(3)(2+)-MV2+ system in which three orders of magnitude of difference is achieved between the forward and backward electron transfer rate constants. These dyads were finally combined with a Pt(II)-based H-2-evolving catalyst, i.e., cis-diamminedichloroplatinum(II), to ascertain the applicability of the system towards the visible light-induced water splitting processes.