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The mechanism of Ce(IV) water oxidation catalyzed by [Ru(tpy)(bpm)(OH(2))](2+) (tpy = 2,2':6',2''-terpyridine; bpm = 2,2'-bipyrimidine) and related single-site catalysts has been determined by a combination of mixing and stopped-flow experiments with spectrophotometric monitoring. The mechanism features O---O coupling by water attack on Ru(V)=O(3+) and three peroxidic intermediates that have been characterized by a combination of spectroscopy and DFT calculations.

We describe here the preparation of a family of photoanodes for water oxidation that incorporate an electron acceptor–chromophore–catalyst in single molecular assemblies on nano-indium tin oxide (nanoITO) electrodes on fluorine-doped tin oxide (FTO). The assemblies were prepared by using a layer-by-layer, Atomic Layer Deposition (ALD), self-assembly approach. In the procedure, addition of an electron acceptor viologen derivative followed by a RuII(bpy) chromophore and a pyridyl derivative of the water oxidation catalyst [Ru(bda) (L)2] (bda = 2,2′-bipyridine-6,6′-dicarboxylate)2, were linked by ALD by addition of the bridge precursors TiO2, ZrO2, and Al2O3 as the bridging groups giving the assemblies, FTO|nanoITO|–MV2+–ALD MO2–RuP22+–ALD M′O2–WOC. In a series of devices, the most efficient gave water oxidation with an incident photon to current efficiency of 2.2% at 440 nm. Transient nanosecond absorption measurements on the assemblies demonstrated that the slow step in the intra-assembly electron transfer is the electron transfer from the chromophore through the viologen bridge to the nanoITO electrode. [ABSTRACT FROM AUTHOR]

The rate of electrocatalytic water oxidation by the heterogeneous water oxidation catalyst [Ru(bda)(4-O(CH2)3P(O3H2)2-pyr)2], 1, (pyr = pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate) on metal oxide surfaces is greatly enhanced relative to water as the solvent. In these experiments with propylene carbonate (PC) as the nonaqueous solvent, water is the limiting reagent. Mechanistic studies point to atom proton transfer (APT) as the rate limiting step in water oxidation catalysis. [ABSTRACT FROM AUTHOR]

A Ru(ii) polypyridyl-derivatized polypropylacrylate end-capped with a water-oxidation-catalyst (WOC) has been synthesized by using reversible addition–fragmentation chain transfer polymerization (RAFT) followed by click reaction and end-group functionalization. In cyclic voltammograms in propylene carbonate, chromophore oxidation occurs at 1.27 V vs. NHE and the RuIII/II wave for the catalyst at 0.84 V vs. NHE. Upon excitation of the Ru(ii) chromophore, excited-state energy migration occurs by site-to-site, –RuII*–→–RuII–, energy transfer hopping along the polymer chain, in part, reaching the terminal catalyst site where –RuII*–→–RuII–OH22+ energy transfer is favored by ΔGen = −2100 cm−1. Added MV2+ as an electron transfer acceptor oxidizes the –RuII*– excited state on the polymer to Ru(iii), –RuII*– + MV2+→–RuIII– + MV+, and ultimately, the catalyst, by site-to-site electron transfer hopping and oxidation, UGRAPHIC DISPLAY="INLINE" ID="UGT1" SRC="UGT1"/. Oxidation is followed by relatively slow, diffusional back electron transfer from MVṖ+ to Ru(iii) sites on the polymer chain. The mixed chromophore-catalyst polymer is a water oxidation catalyst with potential for enhanced light harvesting and water oxidation. [ABSTRACT FROM AUTHOR]

The use of electropolymerization to prepare electrocatalytically and photocatalytically active electrodes for water oxidation is described. Electropolymerization of the catalyst RuII(bda)(4-vinylpyridine)2 (bda=2,2′-bipyridine-6,6′-dicarboxylate) on planar electrodes results in films containing semirigid polymer networks. In these films there is a change in the water oxidation mechanism compared to the solution analogue from bimolecular to single-site. Electro-assembly construction of a chromophore-catalyst structure on mesoporous, nanoparticle TiO2 films provides the basis for a dye-sensitized photoelectrosynthesis cell (DSPEC) for sustained water splitting in a pH 7 phosphate buffer solution. Photogenerated oxygen was measured in real-time by use of a two-electrode cell design. [ABSTRACT FROM AUTHOR]

Solvent and temperature dependent paramagnetism is reported for the complex [RuII(bpy)2(phendione)]- (PF6)2 (bpy = 2,2'-bipyridine, phendione = 1,10-phenanthroline-5,6-dione), 1. Magnetometry, ¹H-NMR, EPR and substituent effects confirm that the paramagnetic character is localized on the phendione ligand, and arises due to mixing of the MLCT excited state with an open shell triplet state on the phendione moiety, a process that is most likely driven by aromatization. The stabilized open shell phendione structure, in which the triplet lies lower in energy than the singlet, can then be thermally populated from the ground state of the complex. This process is facilitated by admixture of singlet character from the MLCT state. Paramagnetic behavior is absent in the free phendione ligand, and disappears in the presence of Lewis acids and in the presence of water due to hydrogen bonding (or hydrate formation at sufficient water concentrations). Control experiments rule out the presence of monoradicals such as semiquinone anions. Implications for spin filtering, sensing, and other electronic applications using such structures are discussed. [ABSTRACT FROM AUTHOR]

Surface-initiated, oligomeric assemblies of ruthenium(II) vinylpolypyridyl complexes have been grown within the cavities of mesoporous nanoparticle films of TiO2 by electrochemically controlled radical polymerization. Surface growth was monitored by cyclic voltammetry as well as UV/Vis and X-ray photoelectron spectroscopy. Polymerization occurs by a radical chain mechanism following cyclic voltammetry scans to negative potentials where reduction occurs at the π* levels of the polypyridyl ligands. Oligomeric growth within the cavities of the TiO2 films occurs until an average of six repeat units are added to the surface-bound initiator site, which is in agreement with estimates of the internal volumes of the pores in the nanoparticle films. [ABSTRACT FROM AUTHOR]

Ein an nanoITO gebundenes Assoziat aus Rutheniumchromophor und ‐ katalysator (siehe Bild) fungiert als Elektrokatalysator bei der Wasseroxidation, wobei in 0.1 M HClO4 O2 bei einem Überpotential von 230 mV entwickelt wird. Die Potentialantwort der Elektrode spricht dafür, dass 3 e−/2 H+ ‐ oxidiertes [RuaIIIRubIVO]5+ die aktive Form des Assoziats ist. ITO=Indium ‐ Zinn ‐ Oxid. [ABSTRACT FROM AUTHOR]

In 1974, the metal-to-ligand charge transfer (MLCT) excited state, [Ru(bpy)3]2+*, was shown to undergo electron transfer quenching by methylviologen dication (MV2+), inspiring a new approach to artificial photosynthesis based on molecules, molecular-level phenomena, and a "modular approach". In the intervening years, application of synthesis, excited-state measurements, and theory to [Ru(bpy)3]2+* and its relatives has had an outsized impact on photochemistry and photophysics. They have provided a basis for exploring the energy gap law for nonradiative decay and the role of molecular vibrations and solvent and medium effects on excited-state properties. Much has been learned about light absorption, excited-state electronic and molecular structure, and excited-state dynamics on timescales from femtoseconds to milliseconds. Excited-state properties and reactivity have been exploited in the investigation of electron and energy transfer in solution, in molecular assemblies, and in derivatized polymers and oligoprolines. An integrated, hybrid approach to solar fuels, based on dye-sensitized photoelectrosynthesis cells (DSPECs), has emerged and is being actively investigated. [ABSTRACT FROM AUTHOR]

The metal complex [(tpy)(Mebim-py)Ru"(S)]2+ (tpy = 2,2' : 6',2"-terpyridine; Mebim-py = 3-methyl-1-pyridylbenzimidazol-2-ylidene; S = solvent) is a robust, reactive electrocatalyst toward both water oxidation to oxygen and carbon dioxide reduction to carbon monoxide. Here we describe its use as a single electrocatalyst for CO2 splitting, CO2 → CO + 1/2 O2, in a two-compartment electrochemical cell. [ABSTRACT FROM AUTHOR]

Results of CW and lifetime emission studies have been used to demonstrate facile intra-strand energy transfer in the derivatized polystyrene polymer [PS-4-CH2CH2NHC(O)-(RuII(4,4'-(CONEt2)2bpy)2)17(OsII(bpy)2))3](PF6)40 in four rigid media: frozen 5:4 (v:v) propionitrile:butyronitrile solutions at 77 K, polymethyl-methacrylate (PMMA) and polyethylene glycol-dimethacrylate (PEG-DMA) films, and silica xerogel monoliths at room temperature. Continued rapid energy transfer in rigid media is in contrast to electron transfer which is inhibited. This can be explained by energy transfer theory and is due to a decrease in the energy transfer barrier because of the frozen nature of the medium. The abbreviation used for the polymer defines the chemical link to the polystyrene backbone and gives the extent of loading out of 20 available sites. [ABSTRACT FROM AUTHOR]

Results of CW and lifetime emission studies have been used to demonstrate facile intra-strand energy transfer in the derivatized polystyrene polymer [PS-4-CH2CH2NHC(O)-(RuII(4,4′-(CONEt2)2bpy)2)17(OsII(bpy)2))3](PF6)40in four rigid media: frozen 5:4 (v:v) propionitrile:butyronitrile solutions at 77 K, polymethyl-methacrylate (PMMA) and polyethylene glycol-dimethacrylate (PEG-DMA) films, and silica xerogel monoliths at room temperature. Continued rapid energy transfer in rigid media is in contrast to electron transfer which is inhibited. This can be explained by energy transfer theory and is due to a decrease in the energy transfer barrier because of the frozen nature of the medium. The abbreviation used for the polymer defines the chemical link to the polystyrene backbone and gives the extent of loading out of 20 available sites. [ABSTRACT FROM AUTHOR]

Investigates the use of Ru[sup IV]-oxo complex adsorbed on optically transparent TiO[sub 2] films in controlling surface oxidation. Oxidation of hydroquinone to quinone; Oxidation kinetics of cyclohexene, benzyl alcohol, phenol and trans-stilbene by surface-bound Ru[sup IV]=0[sup 2+]; Reduction of Ru[sup IV] to Ru[sup III]; Comparison of rate constants on surface and in solution.

Demonstrates the synthesis of coordinatively asymmetrical ruthenium(II) polypyridine complexes. Novel approach based on the trimethylamine N-oxide-assisted removal of the carbonyl ligand; Preparation of isothiocyanate complexes; Formation of geometrical isomers.

Investigates the reactions between trans-dioxo complexes of ruthenium. Oxidation rates; Solvolysis; Use of stopped-flow/rapid-scan spectrophotometry; Intramolecular isomerization of the oxo group; Transfer of a single proton on the basis of a mole fraction study.

Discusses the construction of nonchromophoric oligonuclear trans complexes of Ru(II) using the complex trans-Ru(CN)2(py)4 as starting point. Selective substitution of the chloride ligands in a crystallographically characterized trimer; 1H nuclear magnetic resonance studies; Mass spectrometric studies; Infrared studies; Ultraviolet-visible studies; Electrochemistry.

Discusses the synthesis and structural studies on trans-tetrapyridine complexes of ruthenium. Products of reaction with nitrogen ligands; Absorption spectra of complexes; Electrochemical studies.

Presents a strategy for the preparation of asymmetrically functionalized trans-[Ru(bpy)2]2+ complexes in which pyridyl ligand bearing 4-substituents are introduced in sequential fashion. Reaction of trans-[Ru(bpy)2(H2O)2](CF3SO3)2 with DMSO; Thermal and photochemical substitution of pyridyl ligands; Chemical methods of replacement; Preparation in four steps.

Electrocatalytic reduction of carbon dioxide to carbon monoxide and of water/weak acids to hydrogen by a class of polypyridyl ruthenium complexes both occur by well-defined mechanisms involving common intermediates. Based on available mechanistic insight, conditions have been found for competitive electrocatalytic reduction of CO2–H2O–H2PO4− mixtures by a single metal complex catalyst to give syngas products with the H2 : CO ratio controllable by controlling the composition of the solutions. [ABSTRACT FROM AUTHOR]

The numerous innovative contributions of Henry Taube to modern inorganic chemistry are briefly reviewed. Highlights include the determination of solvation numbers and lability, elucidation of substitution mechanisms, discovery and documentation of inner-sphere electron transfer, and discovery of the remarkable coordination chemistry of ruthenium and osmium ammine complexes with unsaturated ligands and mixed-valence complexes and their fundamental relationship to intramolecular electron transfer. [ABSTRACT FROM AUTHOR]

Studies dinitrogen formation by oxidative intramolecular nitrogen-nitrogen coupling in an oxo-bridged ruthenium complex. Key issues of interest; Analysis of pertinent topics and relevant issues; Implications on studies of inorganic chemistry.

Examines the interconversion of bound nitrogen from nitrosyl/nitrite to ammonium (NH3) in polypyridyl complexes of ruthenium and osmium. Electrochemical or chemical reduction of the nitrido complexes in acidic aqueous solution; Reactions which appear to proceed through deprotonated osmium intermediates.

The use of sunlight to make chemical fuels (i.e., solar fuels)is an attractive approach in the quest to develop sustainable energysources. Using nature as a guide, assemblies for artificial photosynthesiswill need to perform multiple functions. They will need to be ableto harvest light across a broad region of the solar spectrum, transportexcited-state energy to charge-separation sites, and then transportand store redox equivalents for use in the catalytic reactions thatproduce chemical fuels. This multifunctional behavior will requirethe assimilation of multiple components into a single macromolecularsystem. [ABSTRACT FROM AUTHOR]

Solvent and temperature dependent paramagnetism is reported for the complex [RuII(bpy)2(phendione)]- (PF6)2 (bpy = 2,2'-bipyridine, phendione = 1,10-phenanthroline-5,6-dione), 1. Magnetometry, ¹H-NMR, EPR and substituent effects confirm that the paramagnetic character is localized on the phendione ligand, and arises due to mixing of the MLCT excited state with an open shell triplet state on the phendione moiety, a process that is most likely driven by aromatization. The stabilized open shell phendione structure, in which the triplet lies lower in energy than the singlet, can then be thermally populated from the ground state of the complex. This process is facilitated by admixture of singlet character from the MLCT state. Paramagnetic behavior is absent in the free phendione ligand, and disappears in the presence of Lewis acids and in the presence of water due to hydrogen bonding (or hydrate formation at sufficient water concentrations). Control experiments rule out the presence of monoradicals such as semiquinone anions. Implications for spin filtering, sensing, and other electronic applications using such structures are discussed. [ABSTRACT FROM AUTHOR]

We describe a poly(methyl methacrylate) (PMMA) dip-coating procedure, which results in surface stabilization of phosphonate and carboxylate derivatives of Ru(II)-polypyridyl complexes surface-bound to mesoporous nanoparticle TiO2 and nanoITO films in aqueous solutions. As shown by contact angle and transmission electron microscopy (TEM) measurements, PMMA oligomers conformally coat the metal-oxide nanoparticles changing the mesoporous films from hydrophilic to hydrophobic. The thickness of the PMMA overlayer on TiO2—Ru(II) can be controlled by changing the wt % of PMMA in the dipcoating solution. There are insignificant perturbations in electrochemical or spectral properties at thicknesses of up to 2.1 nm with the Ru(III/Il) couple remaining electrochemically reversible and E1/2 values and current densities nearly unaffected. Surface binding by PMMA overlayers results in stable surface binding even at pH 12 with up to a ~100-fold enhancement in photostability. As shown by transient absorption measurements, the MLCT excited state(s) of phosphonate derivatized [Ru(bpy)2((4,4'-(OH)2PO)2bpy)]2+ undergo efficient injection and back electron transfer with pH independent kinetics characteristic of the local pH in the initial loading solution. [ABSTRACT FROM AUTHOR]

The capture and storage of solar energy requires chromophores that absorb light throughout the solar spectrum. We report here the synthesis, characterization, electrochemical, and photophysical properties of a series of Ru(II) polypyridyl complexes of the type [Ru(bpy)2(N-N)]2+ (bpy = 2,2'-bipyridine; N-N is a bidentate polypyridyl ligand). In this series, the nature of the N-N ligand was altered, either through increased conjugation or incorporation of noncoordinating heteroatoms, as a way to use ligand electronic properties to tune redox potentials, absorption spectra, emission spectra, and excited state energies and lifetimes. Electrochemical measurements show that lowering the π* orbitals on the N-N ligand results in more positive Ru3+/2+ redox potentials and more positive first ligand-based reduction potentials. The metal-to-ligand charge transfer absorptions of all of the new complexes are mostly red-shifted compared to Ru(bpy)32+ (λmax = 449 nm) with the lowest energy MLCT absorption appearing at λmax = 564 nm. Emission energies decrease from λmax = 650 nm to 885 nm across the series. One-mode Franck-Condon analysis of room-temperature emission spectra are used to calculate key excited state properties, including excited state redox potentials. The impacts of ligand changes on visible light absorption, excited state reduction potentials, and Ru3+/2+ potentials are assessed in the context of preparing low energy light absorbers for application in dye-sensitized photoelectrosynthesis cells. [ABSTRACT FROM AUTHOR]

Surface-initiated, oligomeric assemblies of ruthenium(II) vinylpolypyridyl complexes have been grown within the cavities of mesoporous nanoparticle films of TiO2 by electrochemically controlled radical polymerization. Surface growth was monitored by cyclic voltammetry as well as UV/Vis and X-ray photoelectron spectroscopy. Polymerization occurs by a radical chain mechanism following cyclic voltammetry scans to negative potentials where reduction occurs at the π* levels of the polypyridyl ligands. Oligomeric growth within the cavities of the TiO2 films occurs until an average of six repeat units are added to the surface-bound initiator site, which is in agreement with estimates of the internal volumes of the pores in the nanoparticle films. [ABSTRACT FROM AUTHOR]

Water-stable, surface-bound chromophores, catalysts, and assemblies are an essential element in dye-sensitized photoelectrosynthesis cells for the generation of solar fuels by water splitting and CO2 reduction to CO, other oxygenates, or hydrocarbons. Phosphonic acid derivatives provide a basis for stable chemical binding on metal oxide surfaces. We report here the efficient synthesis of 4,4'-bis(diethylphosphonomethyl)-2,2'-bipyridine and 4,4'-bis(diethylphosphonate)-2,2'-bipyridine, as well as the mono-, bis-, and tris-substituted ruthenium complexes, [Ru(bpy)2(Pbpy)]2+, [Ru(bpy)(Pbpy)2]2+, [Ru(Pbpy)3]2+, [Ru(bpy)2(CPbpy)]2+, [Ru(bpy)(CPbpy)2]2+, and [Ru(CPbpy)3]2+ [bpy = 2,2'-bipyridine; Pbpy = 4,4'-bis(phosphonic acid)-2,2'-bipyridine; CPbpy = 4,4'-bis(methylphosphonic acid)-2,2'-bipyridine]. [ABSTRACT FROM AUTHOR]

The phosphonated ruthenium complex, [Ru(tpy-PO3H2)(OH2)3]2+ (1) (tpy-PO3H2 = 4′-phosphonato-2,2′:6′,2″-terpyridine), was synthesized and attached to glass∣ITO or glass∣ITO∣TiO2 electrodes. After attachment to the metal oxide surface through the phosphonate linkage, 1 can be oxidized (either chemically or electrochemically) to the reactive Ru(Vl)-dioxo complex, glass∣ITO∣[((HO)2OP)tpy)RuVI(O)2(OH2)]2+, which remains attached to the surface. The attached Ru(VI) complex reacts with benzyl alcohol through mechanisms similar to those proposed for the solution analog. More specifically, Ru(VI) is reduced in a stepwise fashion to Ru(IV) and then finally to Ru(II). The reduction of Ru(VI) is accompanied by a rate-limiting insertion to the C-H bond of benzyl alcohol, followed by solvolysis of the aldehyde hydrate. In addition, the surface-bound Ru(VI) acts as an electrooxidation catalyst which carries out ~130 (2e-) turnovers before deactivation. [ABSTRACT FROM AUTHOR]

We report temperature-dependent excited-state lifetime measurements on [Ru(bpy)[sub 2]dppz][sup 2+] in both protic and aprotic solvents. These experiments yield a unifying picture of the excited-state photophysics that accounts for observations in both types of solvent. Our measurements support the notion of bpy-like and phz-like states associated with the dppz ligand and show that the ligand orbital associated with the bright state is similar in size to the corresponding orbital in the ³MLCT state of [Ru(bpy)[sub 3]][sup 2+]. In contrast to the current thinking, the experiments presented here indicate that the light-switch effect is not driven by a state reversal. Rather, they suggest that the dark state is always lowest in energy, even in aprotic solvents, and that the light-switch behavior is the result of a competition between energetic factors that favor the dark state and entropic factors that favor the bright (bpy) state. [ABSTRACT FROM AUTHOR]

The article presents information on a new method for the sensitization of Ru-Polypyridine complexes by using semiconductor nanocrystal quantum dots (NQDs). This new approach shows that the mixing of Rupolypyridine complexes and NQDs results in the formation of electronically coupled assemblies. Oxidation of the adsorbed complex via efficient hole transfer from the NQD is resulted due to photoexcitation of these assemblies.

Studies the excited state electronic structures of ruthenium(II) polypyridine complexes obtained by a step-scan Frourier transform spectroscopy. Comparison of transient spectra with metal-to-ligand charge transfer states of the region; Time resolution of data collected.