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Issue 18, 2010
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Photoinduced electron transfer in tris(2,2′-bipyridine)ruthenium(ii)-viologen dyads with peptide backbones leading to long-lived charge separation and hydrogen evolution

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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 (RuIII-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 (kET) for the major components were calculated as kET = 8.3 × 1010 s−1 for RuMV2, kET = 1.2 × 1010 s−1 for RuMV4, and kET = 2.3 × 1010 s−1 for RuMV6. Further, the lifetimes and quantum yields of charge separated states were determined as τCS = 16 ± 3 ns and ΦCS = 0.81 for RuMV2, τCS = 20 ± 3 ns and ΦCS = 0.92 for RuMV4, and τCS = 20 ± 3 ns and ΦCS = 0.64 for RuMV6. The backward electron transfer rate constants (kBET) 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.

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

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Supplementary files

Article information


Submitted
30 Oct 2009
Accepted
11 Mar 2010
First published
30 Mar 2010

Dalton Trans., 2010,39, 4421-4434
Article type
Paper

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

M. Ogawa, B. Balan, G. Ajayakumar, S. Masaoka, H. Kraatz, M. Muramatsu, S. Ito, Y. Nagasawa, H. Miyasaka and K. Sakai, Dalton Trans., 2010, 39, 4421
DOI: 10.1039/B922689C

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