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
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>
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>
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
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
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