Ultrafast Recombination Dynamics in Dye-Sensitized SnO2/TiO2 Core/Shell Films.

Interfacial dynamics are investigated in SnO2/TiO2 core/shell films derivatized with a Ru(II)-polypyridyl chromophore ([RuII(bpy)2(4,4'-(PO3H2)2bpy)]2+, RuP) using transient absorption methods. Electron injection from the chromophore into the TiO2 shell occurs within a few picoseconds after photoexcitation. Loss of the oxidized dye through recombination occurs across time scales spanning 10 orders of magnitude. The majority (60%) of charge recombination events occur shortly after injection (τ = 220 ps), while a small fraction (≤20%) of the oxidized chromophores persists for milliseconds. The lifetime of long-lived charge-separated states (CSS) depends exponentially on shell thickness, suggesting that the injected electrons reside in the SnO2 core and must tunnel through the TiO2 shell to recombine with oxidized dyes. While the core/shell architecture extends the lifetime in a small fraction of the CSS, making water oxidation possible, the subnanosecond recombination process has profound implications for the overall efficiencies of dye-sensitized photoelectrosynthesis cells (DSPECs).

Ultrafast, Light-Induced Electron Transfer in a Perylene Diimide Chromophore-Donor Assembly on TiO2.

Surface-bound, perylenediimide (PDI)-based molecular assemblies have been synthesized on nanocrystalline TiO2 by reaction of a dianhydride with a surface-bound aniline and succinimide bonding. In a second step, the Fe(II) polypyridyl complex [Fe(II)(tpy-PhNH2)2](2+) was added to the outside of the film, also by succinimide bonding. Ultrafast transient absorption measurements in 0.1 M HClO4 reveal that electron injection into TiO2 by (1)PDI* does not occur, but rather leads to the ultrafast formation of the redox-separated pair PDI(•+),PDI(•-), which decays with complex kinetics (τ1 = 0.8 ps, τ2 = 15 ps, and τ3 = 1500 ps). With the added Fe(II) polypyridyl complex, rapid (<25 ps) oxidation of Fe(II) by the PDI(•+),PDI(•-) redox pair occurs to give Fe(III),PDI(•-) persisting for >400 µs in the film environment.

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.

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.

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.

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.

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.

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.

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].

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.

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.

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.

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.

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.

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).

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+).

Synthesis and oxidation of d6 tungsten pincer complexes: a complete series of tungsten(II) hydridocarbonyl and halocarbonyl pincer complexes.

Reaction of the neutral P(H)NP ligand [HN(SiMe(2)CH(2)PPh(2))(2)] with tungsten hexacarbonyl resulted in coordination of P(H)NP through both phosphorus donor atoms to form the tungsten complex [W(P(HN)P)(CO)(4)] (1). Reaction of P(H)NP with tris(acetonitrile)tricarbonyl tungsten gave both facial and meridional tridentate isomers [W(P(H)NP)(CO)(3)] (2-fac and 3-mer). These three d(6) tungsten complexes could be interconverted under appropriate conditions. The thermodynamically favored isomer 3 was protonated to form seven-coordinate [W(P(H)NP)(CO)(3)H][BF(4)] (4). A related series of cationic tungsten(II) halide complexes was synthesized, [W(P(H)NP)(CO)(3)X](+) (6, X = I; 7, X = Br; 8, X = Cl; 9, X = F), by various routes. All of the tungsten(II) complexes underwent deprotonation at the amine site of the P(H)NP ligand when triethylamine was added, resulting in neutral seven-coordinate complexes. Variable temperature (1)H, (31)P{(1)H}, and (13)C{(1)H} NMR spectroscopy showed fluxional behavior for all the seven-coordinate complexes reported here. Analysis of IR and NMR spectroscopic data showed trends through the series of coordinated halides. Crystal structures of tetracarbonyl 1, meridional tricarbonyl 3, and cationic hydride 4 were determined to confirm the coordination mode of the P(H)NP ligand.

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.