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Stabilization is a critical issue in the long term operation of dye-sensitized photoelectrosynthesis cells (DSPECs) for water splitting or CO2 reduction. The cells require a stable binding of the robust molecular chromophores, catalysts, and chromophore/catalyst assemblies on metal oxide semiconductor electrodes under the corresponding (photoelectro)chemical conditions. Here, an efficient stabilization strategy is presented based on functionalization of FTO|nanoTiO2 (mesoporous, nanostructured TiO2 deposited on fluorine-doped tin oxide (FTO) glass) electrodes with a vinylsilane followed by surface reductive electropolymerization of a vinyl-derivatized Ru(II) polypyridyl chromophore. The surface electropolymerization was dominated by a grafting-through mechanism, and rapidly completed within minutes. Chromophore surface coverages were controlled up to three equivalent monolayers by the number of electropolymerization cycles. The silane immobilization and cross-linked polymer network produced highly (photo)stabilized chromophore-grafted FTO|nanoTiO2 electrodes. The electrodes showed significant improvements over structures based on atomic layer deposition and polymer dip-coating stabilization methods in a wide pH range from pH ≈ 1 to pH ≈ 12.5 under both dark and light conditions. Under illumination, with hydroquinone added as a sacrificial electron transfer donor, a photoresponse for sustained electron transfer mediation occurred for at least ∼20 h in a pH ≈ 7.5 phosphate buffer (0.1 M NaH2PO4/Na2HPO4, with 0.5 M NaClO4). The overall procedure provides an efficient way to fabricate highly stabilized molecular assemblies on electrode surfaces with potential applications for DSPECs in solar fuels., Here, we introduce a stabilization strategy for Ru(II) polypyridyl chromophores on mesoporous metal oxide electrodes based on silane immobilization and surface reductive electropolymerization.

The development and investigation of materials that leverage unique interfacial effects on electronic structure and redox chemistry are likely to play an outstanding role in advanced technologies for wastewater treatment. Here, the use of surface functionalization of metal oxides with a RuII poly(pyridyl) complex was reported as a way to create hybrid assemblies with optimized electrochemical performance for water remediation, superior to those that could be achieved with the molecular catalyst or metal-oxide electrodes used individually. Mechanistic analysis demonstrated that the molecularly functionalized electrodes could suppress the formation of hydroxyl radicals (i. e., the dominant remediation pathway for bare metal-oxide electrodes), allowing the water remediation to proceed through the highly oxidizing Ru3+ ions in the surface-bound complexes. Furthermore, the underlying metal-oxide substrates played a crucial role in altering the electronic structure and electrochemical properties of the surface-bound catalyst, such that the competing side reaction (i. e., water splitting) was largely inhibited.

Photoanodes in dye-sensitized photoelectrosynthesis cells integrate molecular chromophore/catalyst assemblies on mesoporous n-type metal oxide electrodes for light-driven water oxidation. One limitation for sustainable photoanodes is the stability of chromophore/catalyst assembly on electrode surfaces for long periods. Progress has been made in stabilizing chromophores based on atomic layer deposition, polymer dip coating, C-C cross-coupling by electropolymerization, and silane surface binding, but little progress has been made on catalyst stabilization. We report here the silane-derivatized catalyst, Ru(bda)(L)2 (bda = 2,2'-bipyridine-6,6'-dicarboxylate, L = 4-(6-(triethoxysilyl)hexyl)pyridine), catalyst 1, which is stabilized on metal oxide electrode surfaces over an extended pH range. A surface stabilization study shows that it maintains its reactivity on the electrode surface toward electrochemical oxidation over a wide range of conditions. Its electrochemical stability on electrode surfaces has been systematically evaluated, and its role as a catalyst for water oxidation has been explored. On surfaces of mesoporous nanostructured core/shell SnO2/TiO2, with a TiO2 stabilized inner layer of the Ru(II) polypyridyl chromophore, [Ru(4,4'-(PO3H2)2bpy)(bpy)2]2+ (RuP2+; bpy = 2,2'-bipyridine), highly efficient photoelectrochemical water oxidation catalysis occurs to produce O2 with a maximum efficiency of ∼1.25 mA/cm2. Long-term loss of catalytic activity occurs with time owing to catalyst loss from the electrode surface by axial ligand dissociation in the high oxidation states of the catalyst.

Significance Dye-sensitized photoelectrosynthesis cells (DSPECs) are a very promising approach to convert solar energy into chemical fuels as H 2 or reduced CO 2 species from water or CO 2 . Water oxidation occurring in the photoanode, involving 4e − process, is a critical reaction for the artificial photosynthesis. Here, we inserted an electron mediator between light harvester and water oxidation catalyst. In this role it acts as a mimic for the tyrosine (Yz) between the chromophore and catalyst in PSII. The resulting assembly structures were stable for extended periods (3 h) toward water oxidation to O 2 , and they also extended the chromophores used in the DSPECs for water oxidation to a phosphonated porphyrin dye.

Water oxidation is a critical step in artificial photosynthesis and provides the protons and electrons used in reduction reactions to make solar fuels. Significant advances have been made in the area of molecular water oxidation catalysts with a notable breakthrough in the development of Ru(II) complexes that use a planar "bda" ligand (bda is 2,2'-bipyridine-6,6'-dicarboxylate). These Ru(II)(bda) complexes show lower overpotentials for driving water oxidation making them ideal for light-driven applications with a suitable chromophore. Nevertheless, synthesis of heterogeneous Ru(II)(bda) complexes remains challenging. We discuss here a new "bottom-up" synthetic method for immobilizing these catalysts at the surface of a photoanode for use in a dye-sensitized photoelectrosynthesis cell (DSPEC). The procedure provides a basis for rapidly screening the role of ligand variations at the catalyst in order to understand the impact on device performance. The best results of a water-oxidation DSPEC photoanode based on this procedure reached 1.4 mA/cm

In dye-sensitized photoelectrosynthesis cells, multi-step photoinduced electron transfer takes place to generate redox-separated (RS) states that activate catalysts for water splitting or carbon dioxide reduction. From photoexcitation of the chromophores to formation of the RS states, the solar energy initially stored at the chromophore excited states is reduced stepwise in a sequence of photoinduced electron transfer steps. We report here a water-reduction photocathode based on a supramolecular chromophore, an ethyne-bridged (porphyrinato)zincII and bis(terpyridyl)rutheniumII complex, which is surface-bound to a mesoporous nickel oxide electrode, with an over-layer of nickel sulfide derivative as a water reduction catalyst. Visible light excitation of the chromophore generates a long-lived RS state that forms directly at its excited state with the electron delocalized at the terpyridyl ligands for transferring to the nickel sulfide catalyst, and the hole at the zinc porphyrin moiety for injecting into the nickel oxide electrode. The resulting photocathode shows enhanced photoelectrocatalytic performances relative to the previously reported NiO-based photocathodes. A key element lies in the efficient, direct activation of the catalyst by the long-lived, RS excited state that minimizes the energy loss along the photoinduced electron transfer steps towards water reduction.

Organic chromophores can be synthesized by established methods and offer an opportunity to expand overall solar spectrum utilization for dye-sensitized photoelectrosynthesis cells. However, there are complications in the use of organic chromophores arising from the instability of their oxidized forms, the inability of their oxidized forms to activate a water oxidation catalyst, or the absence of a sufficiently reducing excited state for electron injection into appropriate semiconductors. Three new triarylamine donor–acceptor organic dyes have been investigated here for visible-light-driven water oxidation. They offer highly oxidizing potentials (>1 V vs NHE in aqueous solution) that are sufficient to drive a water oxidation catalyst and excited-state potentials (∼−1.2 V vs NHE) sufficient to inject into TiO2. The oxidized form of one of the chromophores is sufficiently stable to exhibit reversible electrochemistry in aqueous solution. The chromophores also have favorable photophysics. Visible-light-driven...

Transfer of the first electron from (Ph3P)6Cu6H6 to Cp*2Fe+ is fast (k > 106 L·mol–1·s–1). Transfer of a second electron to the same oxidant has a much lower thermodynamic driving force and is considerably slower, with k = 9.29(4) × 103 L·mol–1·s–1. The second oxidation leads to the formation of [(Ph3P)6Cu6H5]+. The structure of [(Ph3P)6Cu6H5]+ has been confirmed by its conversion back to (Ph3P)6Cu6H6 and by microanalysis; X-ray diffraction shows that the complex is a bitetrahedron in the solid state. [(Ph3P)6Cu6H5]+ can also be prepared by treating (Ph3P)6Cu6H6 with MeOTf. With less than 1 equiv of Cp*2Fe+ as oxidant, (Ph3P)6Cu6H6 gives [(Ph3P)7Cu7H6]+ as the major product; X-ray diffraction shows a Cu6 octahedron with one face capped by an additional Cu. [(Ph3P)7Cu7H6]+ can also be prepared by treating (Ph3P)6Cu6H6 with [Cu(CH3CN)4]+ (along with 1 equiv of Ph3P), and can be converted back to (Ph3P)6Cu6H6 with base/H2.

Here, the application of the fluorinated polymer [Dupont AF, a copolymer of 4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole and tetrafluoroethylene] is described in stabilizing phosphonate-derivatized molecular assemblies on oxide electrodes. In the procedure, the polymer was dip-coated onto the surfaces of oxide electrodes with pre-bound, phosphonate-derivatized chromophores and assemblies, including assemblies for water oxidation. The results of the experiments showed a high degree of stabilization by the added polymer and a demonstration of its use in stabilizing surface-bound assemblies for water-oxidation catalysis.

A mesoporous atomic layer deposition (ALD) double-shell electrode, Al2O3 (insulating core)//ALD ZnO|ALD TiO2, on a fluorine-doped tin oxide (FTO) conducting substrate was explored for a photoanode assembly, FTO//Al2O3 (insulating core)//ALD ZnO|ALD TiO2|–chromophore–catalyst, for light-driven water oxidation. Photocurrent densities at photoanodes based on mesoporous ALD double-shell (ALD ZnO|ALD TiO2|) and ALD single-shell (ALD ZnO|, ALD TiO2|) electrodes were investigated for O2 evaluation by a generator–collector dual working electrode configuration. The high photocurrent densities obtained based on the mesoporous ALD ZnO|ALD TiO2 photoanode for O2 evolution arise from a significant barrier to back electron transfer (BET) by the optimized tunneling barrier in the structure with the built-in electric field at the ALD ZnO|ALD TiO2 interface. The charge recombination is thus largely decreased. In the films, BET following injection has been investigated through kinetic nanosecond transient absorption spectra, and the results of energy band analysis are used to derive insight into the internal electronic structure of the electrodes.

A ruthenium polypyridyl chromophore with electronically isolated triarylamine substituents has been synthesized that models the role of tyrosine in the electron transport chain in photosystem II. When bound to the surface of a TiO2 electrode, electron injection from a Ru(II) Metal-to-Ligand Charge Transfer (MLCT) excited state occurs from the complex to the electrode to give Ru(III). Subsequent rapid electron transfer from the pendant triarylamine to Ru(III) occurs with an observed rate constant of ∼1010 s-1, which is limited by the rate of electron injection into the semiconductor. Transfer of the oxidative equivalent away from the semiconductor surface results in dramatically reduced rates of back electron transfer, and a long-lived (τ = ∼165 μs) triarylamine radical cation that has been used to oxidize hydroquinone to quinone in solution.

Integration of photoresponsive chromophores that initiate multistep catalysis is essential in dye-sensitized photoelectrosynthesis cells and related devices. We describe here an approach that incorporates a chromophore assembly surface-bound to metal oxide electrodes for light absorption with an overlayer of catalysts for driving the half-reactions of water splitting. The assembly is a combination of a core-twisted perylene diimide and a ruthenium polypyridyl complex. By altering the connection sequence of the two subunits in the assembly, in their surface-binding to either TiO2 or NiO, the assembly can be tuned to convert visible light into strongly oxidizing equivalents for activation of an electrodeposited water oxidation catalyst (NiCo2Ox) at the photoanode, or reducing equivalents for activation of an electrodeposited water reduction catalyst (NiMo0.05Sx) at the photocathode. A key element in the design of the photoelectrodes comes from the synergistic roles of the vertical (interlayer) charge transf...

Transfer of the first electron from (Ph

Significance Dye-sensitized photoelectrosynthesis cells (DSPECs) provide a basis for artificial photosynthesis and solar fuels production. By combining molecular chromophores and catalysts with high surface area, transparent semiconductor electrodes, a DSPEC provides the basis for light-driven conversion of water to O 2 and H 2 or for reduction of CO 2 to carbon-based fuels. The incorporation of plasmonic cubic silver nanoparticles, with a strongly localized surface plasmon absorbance near 450 nm, to a DSPEC photoanode induces a great increase in the efficiency of water oxidation to O 2 at a DSPEC photoanode. The improvement in performance by the molecular components in the photoanode highlights a remarkable advantage for the plasmonic effect in driving the 4e - /4H + oxidation of water to O 2 in the photoanode.

The octahedral core of 84-electron LCuH hexamers does not dissociate appreciably in solution, although their hydride ligands undergo rapid intramolecular rearrangement. The single-electron transfer proposed as an initial step in the reaction of these hexamers with certain substrates has been observed by stopped-flow techniques when [(Ph3P)CuH]6 is treated with a pyridinium cation. The same radical cation has been prepared by the oxidation of [(Ph3P)CuH]6 with Cp*2Fe(+) and its reversible formation observed by cyclic voltammetry; its UV-vis spectrum has been confirmed by spectroelectrochemistry. The 48-electron trimer [(dppbz)CuH]3 has been prepared by use of the chelating ligand 1,2-bis(diphenylphosphino)benzene (dppbz).