Varying the electronic structure of surface-bound ruthenium(II) polypyridyl complexes.

In the design of light-harvesting chromophores for use in dye-sensitized photoelectrosynthesis cells (DSPECs), surface binding to metal oxides in aqueous solutions is often inhibited by synthetic difficulties. We report here a systematic synthesis approach for preparing a family of Ru(II) polypyridyl complexes of the type [Ru(4,4'-R2-bpy)2(4,4'-(PO3H2)2-bpy)](2+) (4,4'(PO3H2)2-bpy = [2,2'-bipyridine]-4,4'-diylbis(phosphonic acid); 4,4'-R2-bpy = 4,4'-R2-2,2'-bipyridine; and R = OCH3, CH3, H, or Br). In this series, the nature of the 4,4'-R2-bpy ligand is modified through the incorporation of electron-donating (R = OCH3 or CH3) or electron-withdrawing (R = Br) functionalities to tune redox potentials and excited-state energies. Electrochemical measurements show that the ground-state potentials, E(o')(Ru(3+/2+)), vary from 1.08 to 1.45 V (vs NHE) when the complexes are immobilized on TiO2 electrodes in aqueous HClO4 (0.1 M) as a result of increased Ru dπ-π* back-bonding caused by the lowering of the π* orbitals on the 4,4'-R2-bpy ligand. The same ligand variations cause a negligible shift in the metal-to-ligand charge-transfer absorption energies. Emission energies decrease from λmax = 644 to 708 nm across the series. Excited-state redox potentials are derived from single-mode Franck-Condon analyses of room-temperature emission spectra and are discussed in the context of DSPEC applications.

Electro-assembly of a chromophore-catalyst bilayer for water oxidation and photocatalytic water splitting.

The use of electropolymerization to prepare electrocatalytically and photocatalytically active electrodes for water oxidation is described. Electropolymerization of the catalyst Ru(II)(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.

Oxygen atom transfer to a half-sandwich iridium complex: clean oxidation yielding a molecular product.

The oxidation of [Ir(Cp*)(phpy)(NCAr(F))][B(Ar(F))4] (1; Cp* = η(5)-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC(1')-pyridine-κN, NCAr(F) = 3,5-bis(trifluoromethyl)benzonitrile, B(Ar(F))4 = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate) with the oxygen atom transfer (OAT) reagent 2-tert-butylsulfonyliodosobenzene (sPhIO) yielded a single, molecular product at -40 °C. New Ir(Cp*) complexes with bidentate ligands derived by oxidation of phpy were synthesized to model possible products resulting from oxygen atom insertion into the iridium-carbon and/or iridium-nitrogen bonds of phpy. These new ligands were either cleaved from iridium by water or formed unreactive, phenoxide-bridged iridium dimers. The reactivity of these molecules suggested possible decomposition pathways of Ir(Cp*)-based water oxidation catalysts with bidentate ligands that are susceptible to oxidation. Monitoring the [Ir(Cp*)(phpy)(NCAr(F))](+) oxidation reaction by low-temperature NMR techniques revealed that the reaction involved two separate OAT events. An intermediate was detected, synthesized independently with trapping ligands, and characterized. The first oxidation step involves direct attack of the sPhIO oxidant on the carbon of the coordinated nitrile ligand. Oxygen atom transfer to carbon, followed by insertion into the iridium-carbon bond of phpy, formed a coordinated organic amide. A second oxygen atom transfer generated an unidentified iridium species (the "oxidized complex"). In the presence of triphenylphosphine, the "oxidized complex" proved capable of transferring one oxygen atom to phosphine, generating phosphine oxide and forming an Ir-PPh3 adduct in 92% yield. The final Ir-PPh3 product was fully characterized.

Water oxidation by an electropolymerized catalyst on derivatized mesoporous metal oxide electrodes.

A general electropolymerization/electro-oligomerization strategy is described for preparing spatially controlled, multicomponent films and surface assemblies having both light harvesting chromophores and water oxidation catalysts on metal oxide electrodes for applications in dye-sensitized photoelectrosynthesis cells (DSPECs). The chromophore/catalyst ratio is controlled by the number of reductive electrochemical cycles. Catalytic rate constants for water oxidation by the polymer films are similar to those for the phosphonated molecular catalyst on metal oxide electrodes, indicating that the physical properties of the catalysts are not significantly altered in the polymer films. Controlled potential electrolysis shows sustained water oxidation over multiple hours with no decrease in the catalytic current.

Controlling ground and excited state properties through ligand changes in ruthenium polypyridyl complexes.

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 Ru(3+/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)3(2+) (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 Ru(3+/2+) potentials are assessed in the context of preparing low energy light absorbers for application in dye-sensitized photoelectrosynthesis cells.

Stabilization of a ruthenium(II) polypyridyl dye on nanocrystalline TiO2 by an electropolymerized overlayer.

The long-term performance of dye-sensitized solar and photoelectrochemical cells is strongly dependent on the stability of surface-bound chromophores and chromophore-catalyst assemblies at metal oxide interfaces. We report here electropolymerization as a strategy for increasing interfacial stability and as a simple synthetic route for preparing spatially controlled, multicomponent films at an interface. We demonstrate that [Fe(v-tpy)2](2+) (v-tpy = 4'-vinyl-2,2':6',2″-terpyridine) can be reductively electropolymerized on nanocrystalline TiO2 functionalized with a phosphonate-derivatized Ru(II) polypyridyl chromophore. The outer:inner Fe:Ru ratio can be controlled by the number of reductive electrochemical scan cycles as shown by UV-visible absorption and energy dispersive X-ray spectroscopy measurements. Overlayer electropolymerization results in up to 30-fold enhancements in photostability compared to the surface-bound dye alone. Transient absorbance measurements have been used to demonstrate that photoexcitation and electron injection by the MLCT excited state(s) of the surface-bound Ru(II) complex is followed by directional, outside-to-inside, Fe(II) → Ru(III) electron transfer. This strategy is appealing in opening a new approach for synthesizing surface-stabilized chromophore-catalyst assemblies on nanocrystalline metal oxide films.

Redox mediator effect on water oxidation in a ruthenium-based chromophore-catalyst assembly.

The synthesis, characterization, and redox properties are described for a new ruthenium-based chromophore-catalyst assembly, [(bpy)(2)Ru(4-Mebpy-4'-bimpy)Ru(tpy)(OH(2))](4+) (1, [Ru(a)(II)-Ru(b)(II)-OH(2)](4+); bpy = 2,2'-bipyridine; 4-Mebpy-4'-bimpy = 4-(methylbipyridin-4'-yl)-N-benzimid-N'-pyridine; tpy = 2,2':6',2"-terpyridine), as its chloride salt. The assembly incorporates both a visible light absorber and a catalyst for water oxidation. With added ceric ammonium nitrate (Ce(IV), or CAN), both 1 and 2, [Ru(tpy)(Mebim-py)(OH(2))](2+) (Mebim-py = 2-pyridyl-N-methylbenzimidazole), catalyze water oxidation. Time-dependent UV/vis spectral monitoring following addition of 30 equiv of Ce(IV) reveals that the rate of Ce(IV) consumption is first order both in Ce(IV) and in an oxidized form of the assembly. The rate-limiting step appears to arise from slow oxidation of this intermediate followed by rapid release of O(2). This is similar to isolated catalyst 2, with redox potentials comparable to the [-Ru(b)-OH(2)](2+) site in 1, but 1 is more reactive than 2 by a factor of 8 due to a redox mediator effect.

Probing the oxidation chemistry of half-sandwich iridium complexes with oxygen atom transfer reagents.

The new complexes [Ir(Cp*)(phpy)3,5-bis(trifluoromethyl)benzonitrile](+) (1-NCAr(+)) and [Ir(Cp*)(phpy)(styrene)](+) (1-Sty(+), Cp* = η(5)-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC(1')-pyridine-κN) were prepared as analogues of reported iridium water oxidation catalysts, to study their reactions with oxygen atom transfer (OAT) reagents at low temperatures. In no case was the desired product, an Ir(V)oxo complex, observed by spectroscopy. Instead, ligand oxidation was implicated. Oxidation of 1-NCAr(+) with the OAT reagent dimethyldioxirane (DMDO) yielded dioxygen when analyzed by GC, but formation of a heterogeneous or paramagnetic species was simultaneously observed. This amplifies uncertainty over the actual identity of iridium catalysts in the harsh oxidizing conditions required for water oxidation. Catalyst stability was then assessed for a reported styrene epoxidation mediated by [Ir(Cp*)(phpy)(OH2)](+) (1-OH2(+)). It was found that the OAT reagent iodosobenzene (PhIO) extensively oxidized the organic ligands of 1-OH2(+). Acetic acid was detected as a decomposition product. In addition, both the molecular structure and the aqueous electrochemistry of 1-OH2(+) are described for the first time. Oxidative scans revealed rapid decomposition of the complex. All of the above experiments indicate that degradation of the organic ligands in catalysts built with the Ir(Cp*)(phpy) framework are facile under oxidizing conditions. In separate experiments designed to promote ligand substitution, an unexpected silver-bridged, dinuclear Ir(III) species with terminal hydrides, [{Ir(Cp*)(phpy)H}2Ag](+) (2), was discovered. The source of Ag(+) for complex 2 was identified as AgCl.

Synthesis, structure, and reactivity of iridium(III) complexes containing a 4,6-dimethyl-1,3-benzenediphenylimine pincer ligand.

A nonheterocyclic bis(imino)aryl ligand with blocking methyl substituents, 4,6-dimethyl-1,3-benzenediphenylimine (NCHN), has been synthesized. Metalation via oxidative addition proceeds under mild conditions with the Ir(I) reagent [Ir(CH(2)═CH(2))(2)(Cl)](2) to produce the Ir(III) product (NCN)Ir(CH(2)CH(3))(Cl). Neutral nucleophiles such as water or triphenylphosphine add readily to the vacant sixth coordination site. Protonation of the ethyl group results in loss of ethane and formation of a dicationic chloride-bridged (NCN)Ir dimer. Alternatively, the chloride ligand can be abstracted from (NCN)Ir(CH(2)CH(3))(Cl) to provide access to various neutral and cationic species, including (NCN)Ir(CH(2)CH(3))(OAc) (OAc = acetate), [(NCN)Ir(CH(2)CH(3))(bpy)][BF(4)] (bpy = 4,4'-bipyridine), [(NCN)Ir(CH(2)CH(3))(NCCH(3))(2)][BF(4)], and [(NCN)Ir(CH(2)CH(3))(OH(2))(2)][BF(4)], which is water soluble.

Synthesis of phosphonic acid derivatized bipyridine ligands and their ruthenium complexes.

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].

Coordination chemistry of single-site catalyst precursors in reductively electropolymerized vinylbipyridine films.

Reductive electropolymerization of [Ru(II)(PhTpy)(5,5'-dvbpy)(Cl)](PF6) and [Ru(II)(PhTpy)(5,5'-dvbpy)(MeCN)](PF6)2 (PhTpy is 4'-phenyl-2,2':6',2″-terpyridine; 5,5'-dvbpy is 5,5'-divinyl-2,2'-bipyridine) on glassy carbon electrodes gives well-defined films of poly{[Ru(II)(PhTpy)(5,5'-dvbpy)(Cl)](PF6)} (poly-1) or poly{[Ru(II)(PhTpy)(5,5'-dvbpy)(MeCN)](PF6)2} (poly-2). Oxidative cycling of poly-2 with added NO3(-) results in the replacement of coordinated MeCN by NO3(-) to give poly{[Ru(II)(PhTpy)(5,5'-dvbpy)(NO3)](+)}, and with 0.1 M HClO4, replacement by H2O occurs to give poly{[Ru(II)(PhTpy)(5,5'-dvbpy)(OH2)](2+)} (poly-OH2). Although analogous aqua complexes (e.g., [Ru(tpy)(bpy)(OH2)](2+)) undergo rapid loss of H2O to MeCN in solution, poly-OH2 and poly-OH2(+) are substitutionally inert in MeCN. The substitution chemistry is reversible, with reductive scans of poly-1 or poly-OH2 in MeCN resulting in poly-2, although with some loss of Faradaic response.

Low-overpotential water oxidation by a surface-bound ruthenium-chromophore-ruthenium-catalyst assembly.

When anchored to nanoITO (indium tin oxide), the ruthenium chromophore-catalyst assembly shown acts as an electrocatalyst for water oxidation, with O2 evolution occurring at an overpotential of 230 mV in 0.1 M HClO4 . The potential response of the electrode points to 3 e(-) /2 H(+) oxidized [Rua (III) Rub (IV) O](5+) as the active form of the assembly.

Seeking a mechanistic analogue of the water-gas shift reaction: carboxamido ligand formation and isocyanate elimination from complexes containing the Tp'PtMe fragment.

A series of stable, isolable Tp'Pt(IV) carboxamido complexes of the type Tp'PtMe(2)(C(O)NHR) (R = Et, (n)Pr, (i)Pr, (t)Bu, Bn, Ph) has been synthesized by addition of amide nucleophiles to the carbonyl ligand in Tp'Pt(Me)(CO) followed by trapping of the Pt(II) intermediate with methyl iodide as the methylating reagent. These compounds mimic elusive intermediates resulting from hydroxide addition to platinum-bound CO in the Water-Gas Shift Reaction (WGSR). Seeking parallels to WGSR chemistry, we find that deprotonation of the carboxamido NH initiates elimination and the isocyanate-derived products form; the resulting platinum fragment can be protonated to reoxidize the metal center and generate Tp'PtMe(2)H, the synthetic precursor to Tp'Pt(Me)(CO). Mechanistic studies on the formation of and elimination from Tp'PtMe(2)(C(O)NHR) suggest a stepwise process with deprotonation from a Pt(IV) species as the key step prompting elimination.

Photoinduced electron transfer in a chromophore-catalyst assembly anchored to TiO2.

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(2) were investigated for the molecular assembly [((PO(3)H(2)-CH(2))-bpy)(2)Ru(a)(bpy-NH-CO-trpy)Ru(b)(bpy)(OH(2))](4+) ([TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+); ((PO(3)H(2)-CH(2))(2)-bpy = ([2,2'-bipyridine]-4,4'-diylbis(methylene))diphosphonic acid); bpy-ph-NH-CO-trpy = 4-([2,2':6',2″-terpyridin]-4'-yl)-N-((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(2) 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(2)(e(-))-Ru(a)(II)-Ru(b)(III)-OH(2)](4+)→[TiO(2)-Ru(a)(II)-Ru(b)(II)-OH(2)](4+).

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(3)H(2))(2)(OH) in the metal complex [Ru(bpy)(2)(4,4'-(C(OH)(PO(3)H(2))(2)bpy)](2+) are converted into -COOH and H(3)PO(4). The reaction occurs by sensitized formation of (1)O(2) by the lowest metal-to-ligand charge transfer excited state(s) of [Ru(bpy)(2)(4,4'-(C(PO(3)H(2))(2)(OH))(2)(bpy))](2+)* followed by (1)O(2) oxidation of the bisphosphonate substituent. A related reaction occurs for the bisphosphonate-based drug, risedronic acid, in the presence of O(2), light, and a singlet oxygen sensitizer ([Ru(bpy)(3)](2+) or Rose Bengal).

An amide-linked chromophore-catalyst assembly for water oxidation.

The synthesis and analysis of a new amide-linked, dinuclear [Ru(bpy)(2)(bpy-ph-NH-CO-trpy)Ru(bpy)(OH(2))](4+) (bpy = 2,2'-bipyridine; bpy-ph-NH-CO-trpy = 4-(2,2':6',2"-terpyridin-4'-yl)-N-[(4'-methyl-2,2'-bipyridin-4-yl)methyl]benzamide) assembly that incorporates both a light-harvesting chromophore and a water oxidation catalyst are described. With the saturated methylene linker present, the individual properties of both the chromophore and catalyst are retained including water oxidation catalysis and relatively slow energy transfer from the chromophore excited state to the catalyst.

Electronic structure of the water oxidation catalyst cis,cis-[(bpy)2(H2O)Ru(III)ORu(III)(OH2)(bpy)2]4+, the blue dimer.

The first designed molecular catalyst for water oxidation is the "blue dimer", cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+). Although there is experimental evidence for extensive electronic coupling across the µ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled Ru(III) ions across the µ-oxo bridge. We present the results of a comprehensive experimental investigation, combined with DFT calculations. The experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound. Experimental evidence has been obtained from molecular structures and key bond distances by XRD, electrochemically measured comproportionation constants for mixed-valence equilibria, temperature-dependent magnetism, chemical properties (solvent exchange, redox potentials, and pK(a) values), XPS binding energies, analysis of excitation-dependent resonance Raman profiles, and DFT analysis of electronic absorption spectra. The spectrum can be assigned based on a singlet ground state with specific hydrogen-bonding interactions with solvent molecules included. The results are in good agreement with available experimental data. The DFT analysis provides assignments for characteristic absorption bands in the near-IR and visible regions. Bridge-based dπ → dπ* and interconfiguration transitions at Ru(III) appear in the near-IR and MLCT and LMCT transitions in the visible. Reasonable values are also provided by DFT analysis for experimentally observed bond distances and redox potentials. The observed temperature-dependent magnetism of the blue dimer is consistent with a delocalized, diamagnetic singlet state (dπ(1)*)(2) with a low-lying, paramagnetic triplet state (dπ(1)*)(1)(dπ(2)*)(1). Systematic structural-magnetic-IR correlations are observed between ν(sym)(RuORu) and ν(asym)(RuORu) vibrational energies and magnetic properties in a series of ruthenium-based, µ-oxo-bridged complexes. Consistent with the DFT electronic structure model, bending along the Ru-O-Ru axis arises from a Jahn-Teller distortion with ∠Ru-O-Ru dictated by the distortion and electron-electron repulsion.

Mediator-assisted water oxidation by the ruthenium "blue dimer" cis,cis-[(bpy)2(H2O)RuORu(OH2)(bpy)2]4+.

Light-driven water oxidation occurs in oxygenic photosynthesis in photosystem II and provides redox equivalents directed to photosystem I, in which carbon dioxide is reduced. Water oxidation is also essential in artificial photosynthesis and solar fuel-forming reactions, such as water splitting into hydrogen and oxygen (2 H(2)O + 4 h nu --> O(2) + 2 H(2)) or water reduction of CO(2) to methanol (2 H(2)O + CO(2) + 6 h nu --> CH(3)OH + 3/2 O(2)), or hydrocarbons, which could provide clean, renewable energy. The "blue ruthenium dimer," cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+), was the first well characterized molecule to catalyze water oxidation. On the basis of recent insight into the mechanism, we have devised a strategy for enhancing catalytic rates by using kinetically facile electron-transfer mediators. Rate enhancements by factors of up to approximately 30 have been obtained, and preliminary electrochemical experiments have demonstrated that mediator-assisted electrocatalytic water oxidation is also attainable.

Making oxygen with ruthenium complexes.

Mastering the production of solar fuels by artificial photosynthesis would be a considerable feat, either by water splitting into hydrogen and oxygen or reduction of CO(2) to methanol or hydrocarbons: 2H(2)O + 4hnu --> O(2) + 2H(2); 2H(2)O + CO(2) + 8hnu --> 2O(2) + CH(4). It is notable that water oxidation to dioxygen is a key half-reaction in both. In principle, these solar fuel reactions can be coupled to light absorption in molecular assemblies, nanostructured arrays, or photoelectrochemical cells (PECs) by a modular approach. The modular approach uses light absorption, electron transfer in excited states, directed long range electron transfer and proton transfer, both driven by free energy gradients, combined with proton coupled electron transfer (PCET) and single electron activation of multielectron catalysis. Until recently, a lack of molecular catalysts, especially for water oxidation, has limited progress in this area. Analysis of water oxidation mechanism for the "blue" Ru dimer cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+) (bpy is 2,2'-bipyridine) has opened a new, general approach to single site catalysts both in solution and on electrode surfaces. As a catalyst, the blue dimer is limited by competitive side reactions involving anation, but we have shown that its rate of water oxidation can be greatly enhanced by electron transfer mediators such as Ru(bpy)(2)(bpz)(2+) (bpz is 2,2'-bipyrazine) in solution or Ru(4,4'-((HO)(2)P(O)CH(2))(2)bpy)(2)(bpy)(2+) on ITO (ITO/Sn) or FTO (SnO(2)/F) electrodes. In this Account, we describe a general reactivity toward water oxidation in a class of molecules whose properties can be "tuned" systematically by synthetic variations based on mechanistic insight. These molecules catalyze water oxidation driven either electrochemically or by Ce(IV). The first two were in the series Ru(tpy)(bpm)(OH(2))(2+) and Ru(tpy)(bpz)(OH(2))(2+) (bpm is 2,2'- bipyrimidine; tpy is 2,2':6',2''-terpyridine), which undergo hundreds of turnovers without decomposition with Ce(IV) as oxidant. Detailed mechanistic studies and DFT calculations have revealed a stepwise mechanism: initial 2e(-)/2H(+) oxidation, to Ru(IV)=O(2+), 1e(-) oxidation to Ru(V)=(3+), nucleophilic H(2)O attack to give Ru(III)-OOH(2+), further oxidation to Ru(IV)(O(2))(2+), and, finally, oxygen loss, which is in competition with further oxidation of Ru(IV)(O(2))(2+) to Ru(V)(O(2))(3+), which loses O(2) rapidly. An extended family of 10-15 catalysts based on Mebimpy (Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine), tpy, and heterocyclic carbene ligands all appear to share a common mechanism. The osmium complex Os(tpy)(bpy)(OH(2))(2+) also functions as a water oxidation catalyst. Mechanistic experiments have revealed additional pathways for water oxidation one involving Cl(-) catalysis and another, rate enhancement of O-O bond formation by concerted atom-proton transfer (APT). Surface-bound [(4,4'-((HO)(2)P(O)CH(2))(2)bpy)(2)Ru(II)(bpm)Ru(II)(Mebimpy)(OH(2))](4+) and its tpy analog are impressive electrocatalysts for water oxidation, undergoing thousands of turnovers without loss of catalytic activity. These catalysts were designed for use in dye-sensitized solar cell configurations on TiO(2) to provide oxidative equivalents by molecular excitation and excited-state electron injection. Transient absorption measurements on TiO(2)-[(4,4'((HO)(2)P(O)CH(2))(2)bpy)(2)Ru(II)(bpm)Ru(II)(Mebimpy)(OH(2))](4+), (TiO(2)-Ru(II)-Ru(II)OH(2)) and its tpy analog have provided direct insight into the interfacial and intramolecular electron transfer events that occur following excitation. With added hydroquinone in a PEC configuration, APCE (absorbed-photon-to-current-efficiency) values of 4-5% are obtained for dehydrogenation of hydroquinone, H(2)Q + 2hnu --> Q + H(2). In more recent experiments, we are using the same PEC configuration to investigate water splitting.