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

Abstract

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-G₁-carbamoyl)-2,2′-bipyridine (MV2⁴⁺), 5,5′-bis(N-G₁-carbamoyl)-2,2′-bipyridine (MV4⁸⁺), and 5,5′-bis(N-G₂-carbamoyl)-2,2′-bipyridine (MV6¹²⁺), were prepared, where G₁ = Asp(NHG₃)-NHG₃, G₂ = Asp(NHG₃)-Asp(NHG₃)-NHG₃, and G₃ = -(CH₂)₂-⁺NC₅H₄–C₅H₄N⁺-CH₃, i.e., 2-(1′-methyl-4,4′-bipyridinediium-1-yl)ethyl. These were reacted with cis-Ru(bpy)₂Cl₂ to give three new dyads [Ru(bpy)₂(MV2)]⁶⁺ (RuMV2⁶⁺), [Ru(bpy)₂(MV4)]¹⁰⁺ (RuMV4¹⁰⁺), and [Ru(bpy)₂(MV6)]¹⁴⁺ (RuMV6¹⁴⁺), 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⁺˙), 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 × 10¹⁰ s⁻¹ for RuMV2, k_ET = 1.2 × 10¹⁰ s⁻¹ for RuMV4, and k_ET = 2.3 × 10¹⁰ s⁻¹ for RuMV6. Further, the lifetimes and quantum yields of charge separated states were determined as τ_CS = 16 ± 3 ns and Φ_CS = 0.81 for RuMV2, τ_CS = 20 ± 3 ns and Φ_CS = 0.92 for RuMV4, and τ_CS = 20 ± 3 ns and Φ_CS = 0.64 for RuMV6. The backward electron transfer rate constants (k_BET) were also determined as 6.3 × 10⁷, 5.0 × 10⁷, and 5.0 × 10⁷ 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)₂⁸⁺ (29%) and RuMV4(PF₆)₃⁷⁺ (32%) for RuMV4, and RuMV6(PF₆)₃¹¹⁺ (27%) and RuMV6(PF₆)₄¹⁰⁺ (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)₃²⁺-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 H₂-evolving catalyst, i.e., cis-diamminedichloroplatinum(II), to ascertain the applicability of the system towards the visible light-induced water splitting processes.