Photoinduced Electron Transfer In Tris(2,2′-bipyridine)ruthenium(ii ...

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

Makoto Ogawa,a Bijitha Balan,a Gopalakrishnan Ajayakumar,a Shigeyuki Masaoka,ab Heinz-Bernhard Kraatz,c Masayasu Muramatsu,d Syoji Ito,d Yutaka Nagasawa,d Hiroshi Miyasakad and Ken Sakai*a Author affiliations

* Corresponding authors

a Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka, Japan E-mail: [email protected]

b PRESTO, Japan Science and Technology Agency (JST), Honcho 4-1-8, Kawaguchi, Saitama, Japan

c Department of Chemistry, The University of Western Ontario, 1151 Richmond Street, London, Ontario, Canada

d Division of Frontier Materials Science, Graduate School of Engineering Science, Osaka University, Toyonaka, Osaka, Japan

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.

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