5 research outputs found

    Post-Assembly Covalent Di- and Tetracapping of a Dinuclear [Fe<sub>2</sub>L<sub>3</sub>]<sup>4+</sup> Triple Helicate and Two [Fe<sub>4</sub>L<sub>6</sub>]<sup>8+</sup> Tetrahedra Using Sequential Reductive Aminations

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    The use of a highly efficient reductive amination procedure for the postsynthetic end-capping of metal-templated helicate and tetrahedral supramolecular structures bearing terminal aldehyde groups is reported. Metal template formation of a [Fe<sub>2</sub>L<sub>3</sub>]<sup>4+</sup> dinuclear helicate and two [Fe<sub>4</sub>L<sub>6</sub>]<sup>8+</sup> tetrahedra (where L is a linear ligand incorporating two bipyridine domains separated by one or two 1,4-(2,5-dimethoxyaryl) linkers and terminated by salicylaldehyde functions is described. Postassembly reaction of each of these “open” di- and tetranuclear species with excess ammonium acetate (as a source of ammonia) and sodium cyanoborohydride results in a remarkable reaction sequence whereby the three aldehyde groups terminating each end of the helicate, or each of the four vertices of the respective tetrahedra, react with ammonia then undergo successive reductive amination to yield corresponding fully tertiary-amine capped cryptate and tetrahedral covalent cages

    Photoinduced Stepwise Oxidative Activation of a Chromophore–Catalyst Assembly on TiO<sub>2</sub>

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    To probe light-induced redox equivalent separation and accumulation, we prepared ruthenium polypyridyl molecular assembly [(dcb)<sub>2</sub>Ru(bpy-Mebim<sub>2</sub>py)Ru(bpy)(OH<sub>2</sub>)]<sup>4+</sup> (Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>II</sup>–OH<sub>2</sub>) with Ru<sub>a</sub> as light-harvesting chromophore and Ru<sub>b</sub> as water oxidation catalyst (dcb = 4,4′-dicarboxylic acid-2,2′-bipyridine; bpy-Mebim<sub>2</sub>py = 2,2′-(4-methyl-[2,2′:4′,4″-terpyridine]-2″,6″-diyl)bis(1-methyl-1H-benzo[<i>d</i>]imidazole); bpy = 2,2′-bipyridine). When bound to TiO<sub>2</sub> in nanoparticle films, it undergoes MLCT excitation, electron injection, and oxidation of the remote −Ru<sub>b</sub><sup>II</sup>–OH<sub>2</sub> site to give TiO<sub>2</sub>(e<sup>–</sup>)–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>III</sup>–OH<sub>2</sub><sup>3+</sup> as a redox-separated transient. The oxidized assembly, TiO<sub>2</sub>–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>III</sup>–OH<sub>2</sub><sup>3+</sup>, similarly undergoes excitation and electron injection to give TiO<sub>2</sub>(e<sup>–</sup>)–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>IV</sup>O<sup>2+</sup>, with Ru<sub>b</sub><sup>IV</sup>O<sup>2+</sup> a known water oxidation catalyst precursor. Injection efficiencies for both forms of the assembly are lower than those for [Ru(bpy)<sub>2</sub>(4,4′-(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>bpy)]<sup>2+</sup> bound to TiO<sub>2</sub> (TiO<sub>2</sub>–Ru<sup>2+</sup>), whereas the rates of back electron transfer, TiO<sub>2</sub>(e<sup>–</sup>) → Ru<sub>b</sub><sup>III</sup>–OH<sub>2</sub><sup>3+</sup> and TiO<sub>2</sub>(e<sup>–</sup>) → Ru<sub>b</sub><sup>IV</sup>O<sup>2+</sup>, are significantly decreased compared with TiO<sub>2</sub>(e<sup>–</sup>) → Ru<sup>3+</sup> back electron transfer

    Photoinduced Electron Transfer in a Chromophore–Catalyst Assembly Anchored to TiO<sub>2</sub>

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    Photoinduced formation, separation, and buildup of multiple redox equivalents are an integral part of cycles for producing solar fuels in dye-sensitized photoelectrosynthesis cells (DSPECs). Excitation wavelength-dependent electron injection, intra-assembly electron transfer, and pH-dependent back electron transfer on TiO<sub>2</sub> were investigated for the molecular assembly [((PO<sub>3</sub>H<sub>2</sub>-CH<sub>2</sub>)-bpy)<sub>2</sub>Ru<sub>a</sub>(bpy-NH-CO-trpy)­Ru<sub>b</sub>(bpy)­(OH<sub>2</sub>)]<sup>4+</sup> ([TiO<sub>2</sub>–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>II</sup>–OH<sub>2</sub>]<sup>4+</sup>; ((PO<sub>3</sub>H<sub>2</sub>-CH<sub>2</sub>)<sub>2</sub>-bpy = ([2,2′-bipyridine]-4,4′-diylbis­(methylene))­diphosphonic acid); bpy-ph-NH-CO-trpy = 4-([2,2′:6′,2″-terpyridin]-4′-yl)-<i>N</i>-((4′-methyl-[2,2′-bipyridin]-4-yl)­methyl) benzamide); bpy = 2,2′-bipyridine). This assembly combines a light-harvesting chromophore and a water oxidation catalyst linked by a synthetically flexible saturated bridge designed to enable long-lived charge-separated states. Following excitation of the chromophore, rapid electron injection into TiO<sub>2</sub> and intra-assembly electron transfer occur on the subnanosecond time scale followed by microsecond–millisecond back electron transfer from the semiconductor to the oxidized catalyst, [TiO<sub>2</sub>(e<sup>–</sup>)–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>III</sup>–OH<sub>2</sub>]<sup>4+</sup>→[TiO<sub>2</sub>–Ru<sub>a</sub><sup>II</sup>–Ru<sub>b</sub><sup>II</sup>–OH<sub>2</sub>]<sup>4+</sup>

    Sensitized Photodecomposition of Organic Bisphosphonates By Singlet Oxygen

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    During efforts to stabilize metal oxide bound chromophores for photoelectrochemical applications, a novel photochemical reaction has been discovered. In the reaction, the bisphosphonate functional groups −C­(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>(OH) in the metal complex [Ru­(bpy)<sub>2</sub>(4,4′-(C­(OH)­(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>bpy)]<sup>2+</sup> are converted into −COOH and H<sub>3</sub>PO<sub>4</sub>. The reaction occurs by sensitized formation of <sup>1</sup>O<sub>2</sub> by the lowest metal-to-ligand charge transfer excited state(s) of [Ru­(bpy)<sub>2</sub>(4,4′-(C­(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>(OH))<sub>2</sub>(bpy))]<sup>2+</sup>* followed by <sup>1</sup>O<sub>2</sub> oxidation of the bisphosphonate substituent. A related reaction occurs for the bisphosphonate-based drug, risedronic acid, in the presence of O<sub>2</sub>, light, and a singlet oxygen sensitizer ([Ru­(bpy)<sub>3</sub>]<sup>2+</sup> or Rose Bengal)

    Structure–Property Relationships in Phosphonate-Derivatized, Ru<sup>II</sup> Polypyridyl Dyes on Metal Oxide Surfaces in an Aqueous Environment

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    The performance of dye-sensitized solar and photoelectrochemical cells is strongly dependent on the light absorption and electron transfer events at the semiconductor–small molecule interface. These processes as well as photo/electrochemical stability are dictated not only by the properties of the chromophore and metal oxide but also by the structure of the dye molecule, the number of surface binding groups, and their mode of binding to the surface. In this article, we report the photophysical and electrochemical properties of a series of six phosphonate-derivatized [Ru­(bpy)<sub>3</sub>]<sup>2+</sup> complexes in aqueous solution and bound to ZrO<sub>2</sub> and TiO<sub>2</sub> surfaces. A decrease in injection yield and cross surface electron-transfer rate with increased number of diphosphonated ligands was observed. Additional phosphonate groups for surface binding did impart increased electrochemical and photostability. All complexes exhibit similar back-electron-transfer kinetics, suggesting an electron-transfer process rate-limited by electron transport through the interior of TiO<sub>2</sub> to the interface. With all results considered, the ruthenium polypyridyl derivatives with one or two 4,4′-(PO<sub>3</sub>H<sub>2</sub>)<sub>2</sub>bpy ligands provide the best balance of electron injection efficiency and stability for application in solar energy conversion devices
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