title: 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-G(1)-carbamoyl)-2,2'-bipyridine ((4+)), 5,5'-bis(N-G(1)-carbamoyl)-2,2'-bipyridine ((8+)), and 5,5'-bis(N-G(2)-carbamoyl)-2,2'-bipyridine ((12+)), were prepared, where G(1) = Asp(NHG(3))-NHG(3), G(2) = Asp(NHG(3))-Asp(NHG(3))-NHG(3), and G(3) = -(CH(2))(2)-(+)NC(5)H(4)-C(5)H(4)N(+)-CH(3), 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)()](6+) ((6+)), [Ru(bpy)(2)()](10+) ((10+)), and [Ru(bpy)(2)()](14+) ((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(+) ), 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 , 82.2 ps (79.9%), 1.12 ns (12.4%), and 4.60 ns (7.7%) for , and 43.6 ps (71.6%), 593 ps (20.2%), and 3.75 ns (8.1%) for . 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 , k(ET) = 1.2 x 10(10) s(-1) for , and k(ET) = 2.3 x 10(10) s(-1) for . Further, the lifetimes and quantum yields of charge separated states were determined as tau(CS) = 16 +/- 3 ns and Phi(CS) = 0.81 for , tau(CS) = 20 +/- 3 ns and Phi(CS) = 0.92 for , and tau(CS) = 20 +/- 3 ns and Phi(CS) = 0.64 for . 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 , , and , respectively. From the analysis of electrical conductivity, the major ion-pair adducts in aqueous media were characterized as (PF(6))(5+) (52%) for , (PF(6))(2)(8+) (29%) and (PF(6))(3)(7+) (32%) for , and (PF(6))(3)(11+) (27%) and (PF(6))(4)(10+) (29%) for , 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+)-MV(2+) 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.
authors.affiliation: Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki 6-10-1, Higashi-ku, Fukuoka 812-8581, Japan.
authors.forename: Makoto
authors.initials: M
authors.lastname: Ogawa
authors.forename: Bijitha
authors.initials: B
authors.lastname: Balan
authors.forename: Gopalakrishnan
authors.initials: G
authors.lastname: Ajayakumar
authors.forename: Shigeyuki
authors.initials: S
authors.lastname: Masaoka
authors.forename: Heinz-Bernhard
authors.initials: HB
authors.lastname: Kraatz
authors.forename: Masayasu
authors.initials: M
authors.lastname: Muramatsu
authors.forename: Syoji
authors.initials: S
authors.lastname: Ito
authors.forename: Yutaka
authors.initials: Y
authors.lastname: Nagasawa
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Miyasaka
authors.forename: Ken
authors.initials: K
authors.lastname: Sakai
mesh_terms: D002384:Catalysis; D004579:Electron Transport; D006859:Hydrogen / Q000737:chemistry*; D009942:Organometallic Compounds / Q000737:chemistry*; D010455:Peptides / Q000737:chemistry*; D010782:Photolysis; D010984:Platinum / Q000737:chemistry*; D011789:Quantum Theory; D014755:Viologens / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D010455:Peptides; D014755:Viologens; C415768:tris(2,2'-bipyridyl)ruthenium(II); D010984:Platinum; D006859:Hydrogen