Three new [PtCl(2)(bpy)] derivatives tethered to 2, 4, and 6 dicationic viologen moieties, [PtCl(2)(MV2)](4+) (1), [PtCl(2)(MV4)](8+) (2), and [PtCl(2)(MV6)](12+) (3), have been synthesized (MV2(4+)=5-ethoxycarbamoyl-5'-(N-R(1)-carbamoyl)-2,2'-bipyridine, MV4(8+)=5,5'-bis(N-R(1)-carbamoyl)-2,2'-bipyridine, and MV6(12+)=5,5'-bis(N-R(2)-carbamoyl)-2,2'-bipyridine, in which R(1)=Asp(NH-VG)-NH-VG, R(2)=Asp(NH-VG)-Asp(NH-VG)-NH-VG, and VG=-(CH(2))(2)-(+)NC(5)H(4)-C(5)H(4)N(+)-CH(3)). In spite of the higher charge storage capacity of 2 and 3 due to the higher number of acceptor groups (VG groups), compound 1 with the lowest number of VG tethers has turned out to exhibit an outstanding catalytic performance towards the hydrogen evolution from water. Quantitative analysis of UV/Vis-NIR absorption spectral changes during the photolysis for 2 and 3 reveal that approximately 2 electrons per molecule are stored over the acceptor groups during the photolysis, and the storage events saturate after 20 min. As for 1, the total number of electrons stored per molecule increases once during the initial 10 min and then abruptly decreases down to around 0.1 electrons per molecule at 20 min, during which the storage is maximized at 10-20 min with 0.6-0.7 electrons stored per molecule, thereby indicating that the rates of radical formation and consumption are balanced during the photochemical hydrogen evolution reaction. The electrical conductivity measurements reveal that ion-pair adducts (adducts with PF(6)(-) ions in solution) are formed by 2 and 3 but are not given by 1 under the catalysis conditions. These, together with the results of molecular mechanics calculations, reveal that stack of two [PtCl(2)(bpy)] units becomes unfavorable as the number of sterically bulky and highly charged VG units per molecule increases. We have therefore concluded that dimerization that leads to the formation of a Pt-Pt association is a key step in the effective catalytic enhancement with [PtCl(2)(bpy)]-type catalysts.

Novel unimolecular architectures that enable single-pigment-driven multi-electron transfer coupled with double-electron-storage leading to catalytic H2 evolution are reported.

The design of a highly selective "turn-ON" luminescence chemodosimeter for Cu(2+) is reported. The design strategy made use of the ability of Cu(2+) ions to oxidize aromatic amines in acetonitrile solution. The aromatic amine employed here is a phenothiazine moiety which is covalently linked to one of the bipyridine units of Ru(bpy)(3)(2+). Excitation of the Ru(bpy)(3)(2+) leads to electron transfer from the phenothiazine moiety to the MLCT excited state of Ru(bpy)(3)(2+) which resulted in efficient quenching of the luminescence. In the presence of excess Cu(2+), phenothiazine moiety is oxidized to a stable entity which is incapable of electron donation to the MLCT excited state of Ru(bpy)(3)(2+). The emission of the Ru(bpy)(3)(2+) moiety is thus restored and we show that this strategy can be used as the basis for sensing micromolar amounts of Cu(2+). Only Cu(2+) is capable of this reaction, making this an interesting, hitherto unexplored strategy for the selective detection of micromolar amounts of Cu(2+).

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.