Chromophore-catalyst Assembly Research Articles

A ruthenium catalyst linked to a redox-active ruthenium polypyridine for water oxidation.

Chromophore-catalyst assemblies are interesting benchmark molecules for photocatalysis. We have prepared two examples of these assemblies and characterized their behaviour as catalysts for the water oxidation reaction. In the bimetallic complexes [Ru(tpy)(4,4'-X2-bpy)(μ-CN)Ru(bda)(DMSO)](PF6) (X = -H (1), -OCH3 (2), tpy = 2,2':6',2''-terpyridine, bpy = 2,2'-bipyridine, H2bda = 2,2'-bipyridine-6,6'-dicarboxylic acid and DMSO = (CH3)2SO), a Ru(II)-polypiridine chromophore {Ru(tpy)(4,4'-X2-bpy)} is linked by a cyanide bridge to a {Ru(bda)} water oxidation catalyst. Both complexes catalyze water oxidation both chemically, using Ce(IV) as the terminal oxidant, and electrochemically. Electrochemical and spectroelectrochemical studies, aided with (TD)DFT calculations, allowed us to study the different species involved in the catalytic cycle. For the RuIIRuIII, an intense MMCT (metal-to-metal charge transfer) band suggests a geometrical reorganization during the oxidation process. The wave associated with oxidation to the RuIIIRuIV species shows an increased current that suggests that it may be a catalyst for the water oxidation reaction. Further oxidation results in the RuIIIRuV redox state which is an active catalyst for the water oxidation reaction as demonstrated by the evolution of oxygen.

Influence of Surface and Structural Variations in Donor-Acceptor-Donor Sensitizers on Photoelectrocatalytic Water Splitting.

Conjugated organic chromophores composed of linked donor (D) and acceptor (A) moieties have attracted considerable attention for photoelectrochemical applications. In this work, we compare the optoelectronic properties and photoelectrochemical performance of two D-A-D structural isomers with thiophene-X-carboxylic acid (X denotes 3 and 2 positions) derivatives and 2,1,3-benzothiadiazole as the D and A moieties, respectively. 5,5'-(Benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-3-carboxylic acid), BTD1, and 5,5'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)bis(thiophene-2-carboxylic acid), BTD2, were employed in the study to understand how structural isomers affect surface attachments within chromophore-catalyst assemblies and their influence on charge-transfer dynamics. Crystal structures revealed that varying the position of the -COOH anchoring group causes the molecules to either contort out of a plane (BTD1) or adopt a near-perfect planar conformation (BTD2). BTD1 and BTD2 were co-loaded with either a water oxidation catalyst, [Ru(2,6-bis(1-methylbenzimidazol-2-yl)pyridine)-(4,4'-((HO)2OPCH2)2-2,2'-bipyridine)(OH2)]2, RuCt2+, or proton reduction catalyst [Ni(P2PhN2C6H4CH2PO3H2)2]2+, NiCt2+, on oxide electrodes to facilitate photodriven water splitting reactions. Emission quenching measurements indicate that both BTD1 and BTD2 inject electrons into n-type SnO2|TiO2 electrodes and holes into p-type NiO semiconductors from their respective excited states at high efficiencies >60%. Photocurrent densities of chromophore-catalyst assemblies obtained using linear sweep voltammetry (LSV) show that BTD2-sensitized photoanodes generate significantly more photocurrent than BTD1-sensitized electrodes; however, both exhibit similar performances at the photocathode. Photoelectrocatyltic measurements demonstrate that both BTD1 and BTD2 performed similarly, generating Faradaic efficiencies of 39 and 38% at the anode or 61 and 79% at the cathode. Transient absorption measurements suggest that the differences between the LSV and photoelectrocatalytic measurements result from the differences in quantum yields of the photogenerated redox equivalents, which is also a reflection of the varying metal oxide surface conformation. Our findings suggest that BTD2 should be investigated further in photocathodic studies since it has the structural advantage of being incorporated into diverse types of chromophore-catalyst assemblies.

Polymer Chromophore–Catalyst Assembly for Photocatalytic CO2 Reduction

A polystyrene-based chromophore–catalyst assembly (PS-RuC/RuCat) was synthesized and applied for photocatalytic CO2 reduction in an organic solution in the presence of a sacrificial donor. The polymer assembly contains Ru(bpy)32+ (RuC, bpy = 2,2′-bipyridine) as the chromophore and a derivative of [Ru(tpy)(6-mbpy)(CH3CN)]2+ (RuCat, tpy = 4′-phenyl-2,2′:6′,2″-terpyridine and 6-mbpy = 6-methyl-2,2′-dipyridyl) as the CO2 reduction catalyst. The photophysical and electrochemical properties of the polymer assembly were investigated. Picosecond transient absorption and time-resolved emission reveal that the RuC chromophores are quenched by energy transfer to the RuCat units in the polymer assembly. Cyclic voltammetry shows that PS-RuC/RuCat effects electrocatalytic reduction of CO2 with an onset of approximately −1.25 V versus Ag/AgCl. Photocatalytic studies were conducted separately for the PS-RuC/RuCat polymer assembly and a mixture of monomers RuC and RuCat in a mixture of dimethylformamide and triethanolamine with 1-benzyl-1,4-dihydronicotinamide as a sacrificial reductant. The primary CO2 reduction products CO and H2 were detected in both photocatalytic systems. Formic acid was detected at low levels by nuclear magnetic resonance and is a very minor product. The polymer assembly system shows a lower photocatalytic CO2 reduction quantum yield (6.7%) but exhibits improved stability compared with a mixed molecular chromophore/catalyst system.

Ruthenium Dyes, Charge Transfer, and the Sun

The performance of dye-sensitized solar cells (DSSC) and dye-sensitized photoelectrosynthesis cells (DSPEC)—and their constituent chromophores, catalysts, and substrates—is commonly evaluated by photocurrents, product fluxes, and time-resolved spectroscopies. Kinetic models of solar harvesting systems are often built from the data using phenomenological methods (i.e. sum-of-exponential or global fit analyses). Such models cannot be predictive, and therefore offer limited fundamental insights to the efficiencies of charge injection and photocatalysis. We describe an approach to modeling the molecular photophysics, interfacial electron transfer, and charge separation that yields a comprehensive kinetic framework for these processes. Simulations of femtosecond to steady-state dynamics using this framework provide a detailed picture of dye cycling under both pulsed-monochromatic and continuous broadband illumination at 1 sun intensity. The competition between molecular transitions and charge injection will be discussed, including the potential implications for the design of chromophore-catalyst assemblies.

Photophysics of graphene quantum dot assemblies with axially coordinated cobaloxime catalysts.

We report a study of chromophore-catalyst assemblies composed of light harvesting hexabenzocoronene (HBC) chromophores axially coordinated to two cobaloxime complexes. The chromophore-catalyst assemblies were prepared using bottom-up synthetic methodology and characterized using solid-state NMR, IR, and x-ray absorption spectroscopy. Detailed steady-state and time-resolved laser spectroscopy was utilized to identify the photophysical properties of the assemblies, coupled with time-dependent DFT calculations to characterize the relevant excited states. The HBC chromophores tend to assemble into aggregates that exhibit high exciton diffusion length (D = 18.5 molecule2/ps), indicating that over 50 chromophores can be sampled within their excited state lifetime. We find that the axial coordination of cobaloximes leads to a significant reduction in the excited state lifetime of the HBC moiety, and this finding was discussed in terms of possible electron and energy transfer pathways. By comparing the experimental quenching rate constant (1.0 × 109 s-1) with the rate constant estimates for Marcus electron transfer (5.7 × 108 s-1) and Förster/Dexter energy transfers (8.1 × 106 s-1 and 1.0 × 1010 s-1), we conclude that both Dexter energy and Marcus electron transfer process are possible deactivation pathways in CoQD-A. No charge transfer or energy transfer intermediate was detected in transient absorption spectroscopy, indicating fast, subpicosecond return to the ground state. These results provide important insights into the factors that control the photophysical properties of photocatalytic chromophore-catalyst assemblies.

Self-Assembled Bilayers as an Anchoring Strategy: Catalysts, Chromophores, and Chromophore-Catalyst Assemblies.

Anchoring strategies for immobilization of molecular catalysts, chromophores, and chromophore-catalyst assemblies on electrode surfaces play an important role in solar energy conversion devices such as dye-sensitized solar cells and dye-sensitized photoelectrosynthesis cells. They are also important in interfacial studies with surface-bound molecules including electron-transfer dynamics and mechanistic studies related to small molecule activation catalysis. Significant progress has been made in this area, but many challenges remain in terms of stability, synthetic complexity, and versatility. We report here a new anchoring strategy based on self-assembled bilayers. This strategy takes advantage of noncovalent interactions between long alkyl chains chemically bound to a metal-oxide electrode surface and long alkyl chains on the molecule being anchored. The new methodology is applicable to the heterogenization of both catalysts and chromophores as well as to the in situ "synthesis" of chromophore-catalyst assemblies on the electrode surface.

Stable Molecular Photocathode for Solar-Driven CO2 Reduction in Aqueous Solutions

The performance of dye-sensitized photoelectrodes for artificial photosynthesis is typically limited by instability in aqueous solutions. We describe here a new molecular-based photocathode that integrates functional chromophore–catalyst assemblies for long-term solar-driven CO2 reduction in stabilized polymeric film structures. The assemblies include a silane surface-anchoring bridge, a ruthenium polypyridyl chromophore, and a rhenium-based molecular catalyst. They were prepared on nanocrystalline oxide films by silanization of the oxide and two-step electropolymerization of vinyl-derivatized precursors. The integrated photocathode was stable toward CO2 reduction for over 10 h with a Faradaic efficiency of ∼65%. The long-term stability arises from the silane surface-anchoring groups and the carbon–carbon bonds formed by electropolymerization between the three components. Transient absorption measurements on a nano-to-microsecond time scale show that the assemblies undergo rapid hole injection into the ox...

Pathways Following Electron Injection: Medium Effects and Cross-Surface Electron Transfer in a Ruthenium-Based, Chromophore–Catalyst Assembly on TiO2

Interfacial dynamics following photoexcitation of the water oxidation assembly [((PO3H2)2bpy)2RuII(bpy-bimpy)RuII(tpy)(OH2)]4+, −[RuaII–RubII–OH2]4+, on nanocrystalline TiO2 electrodes, starting from either −[RuaII–RubII–OH2]4+ or −[RuaII–RubIII–OH2]5+, have been investigated. Transient absorption measurements for TiO2–[RuaII–RubII–OH2]4+ in 0.1 M HPF6 or neat trifluoroethanol reveal that electron injection occurs with high efficiency but that hole transfer to the catalyst, which occurs on the electrochemical time scale, is inhibited by local environmental effects. Back electron transfer occurs to the oxidized chromophore on the microsecond time scale. Photoexcitation of the once-oxidized assembly, TiO2–[RuaII–RubIII–OH2]5+, in a variety of media, generates −[RuaIII–RubIII–OH2]6+. The injected electron randomly migrates through the surface oxide structure reducing an unreacted −[RuaII–RubIII–OH2]5+ assembly to −[RuaII–RubII–OH2]4+. In a parallel reaction, −[RuaIII–RubIII–OH2]6+ formed by electron injectio...

Synthesis and Photophysical Properties of a Covalently Linked Porphyrin Chromophore–Ru(II) Water Oxidation Catalyst Assembly on SnO2 Electrodes

We describe here the preparation and surface photophysical properties of a covalently linked, chromophore-catalyst assembly between a phenyl phosphonate-derivatized pentafluorophenyl-substituted porphyrin and the water oxidation catalyst, [RuII(terpyridine)(2-benzimidazolylpyridine)(OH2)]2+, in a derivatized assembly of porph-RuII–OH22+. The results of nanosecond transient absorption measurements on nanoparticle SnO2 electrodes in aqueous acetate buffer at pH 4.7 are consistent with rapid electron injection into SnO2 with transfer of oxidative equivalents to the assembly. Electron transfer from the singlet excited state of the porphyrin to the conduction band of the electrode, SnO2(e–)|-porph+-RuII–OH22+, is favored as the porphyrin singlet excited state lies 0.44 eV above the SnO2 conduction band edge. Electron injection is rapid (⟨τinj⟩ < 10–8 s), and occurs with high efficiency. Based on measured redox potentials, following excitation and injection, intra-assembly oxidation of the catalyst, -porph+-RuII–OH22+ → -porph-RuIII–OH2+ + H+, is favored in the transient equilibrium state by 0.62 eV at pH 4.7. However, immediately after the flash, a distribution exists at the surface between isomers with SnO2(e–)|-porph+-RuII–OH22+ undergoing back electron transfer to the surface with an average lifetime of ⟨τ1⟩ ∼ 10–7 s and a slower component for back electron transfer from SnO2(e–)|-porph-RuIII–OH2+ with ⟨τ2⟩ ∼ 4 × 10–5 s.

Photocathode Chromophore–Catalyst Assembly via Layer-By-Layer Deposition of a Low Band-Gap Isoindigo Conjugated Polyelectrolyte

Low band-gap conjugated polyelectrolytes (CPEs) can serve as efficient chromophores for use on photoelectrodes for dye-sensitized photoelectrochemical cells. Herein is reported a novel CPE based on poly(isoindigo-co-thiophene) with pendant sodium butylsulfonate groups (PiIT) and its use in construction of layer-by-layer (LbL) chromophore–catalyst assemblies with a Pt-based H+ reduction catalyst (PAA-Pt) for water reduction. A novel Stille polymerization/postpolymerization ion-exchange strategy was used to convert an organic-soluble CPE to the water-soluble poly(isoindigo-co-thiophene). The anionic PiIT polyelectrolyte- and polyacrylate-stabilized Pt-nanoparticles (PAA-Pt) were codeposited with cationic poly(diallyldimethylammonium) chloride (PDDA) onto inverse opal (IO), nanostructured indium tin oxide film (nITO) (IO nITO) atop fluorine doped tin oxide (FTO), by using LbL self-assembly. To evaluate the performance of novel conjugated PiIT//PAA-Pt chromphore–catalyst assemblies, interassembly hole transfe...

Chromophore-Catalyst Assembly for Water Oxidation Prepared by Atomic Layer Deposition

Visible-light-driven water splitting was investigated in a dye sensitized photoelectrosynthesis cell (DSPEC) based on a photoanode with a phosphonic acid-derivatized donor-π-acceptor (D-π-A) organic chromophore, 1, and the water oxidation catalyst [Ru(bda)(4-O(CH2)3P(O3H2)2-pyr)2], 2, (pyr = pyridine; bda = 2,2'-bipyridine-6,6'-dicarboxylate). The photoanode was prepared by using a layering strategy beginning with the organic dye anchored to an FTO|core/shell electrode, atomic layer deposition (ALD) of a thin layer (<1 nm) of TiO2, and catalyst binding through phosphonate linkage to the TiO2 layer. Device performance was evaluated by photocurrent measurements for core/shell photoanodes, with either SnO2 or nanoITO core materials, in acetate-buffered, aqueous solutions at pH 4.6 or 5.7. The absolute magnitudes of photocurrent changes with the core material, TiO2 spacer layer thickness, or pH, observed photocurrents were 2.5-fold higher in the presence of catalyst. The results of transient absorption measurements and DFT calculations show that electron injection by the photoexcited organic dye is ultrafast promoted by electronic interactions enabled by orientation of the dye's molecular orbitals on the electrode surface. Rapid injection is followed by recombination with the oxidized dye which is 95% complete by 1.5 ns. Although chromophore decomposition limits the efficiency of the DSPEC devices toward O2 production, the flexibility of the strategy presented here offers a new approach to photoanode design.

Layer-by-Layer Molecular Assemblies for Dye-Sensitized Photoelectrosynthesis Cells Prepared by Atomic Layer Deposition.

In a dye sensitized photoelectrosynthesis cell (DSPEC), the relative orientation of the catalyst and chromophore plays an important role in determining the device efficiency. Here we introduce a new, robust atomic layer deposition (ALD) procedure for the preparation of molecular chromophore-catalyst assemblies on wide bandgap semiconductors. In this procedure, solution deposited, phosphonate derivatized metal complexes on metal oxide surfaces are treated with reactive metal reagents in the gas phase by ALD to form an outer metal ion bridging group, which can bind a second phosphonate containing species from solution to establish a R1-PO2-O-M-O-PO2-R2 type surface assembly. With the ALD procedure, assemblies bridged by Al(III), Sn(IV), Ti(IV), or Zr(IV) metal oxide units have been prepared. To evaluate the performance of this new type of surface assembly, intra-assembly electron transfer was investigated by transient absorption spectroscopy, and light-driven water splitting experiments under steady-state illumination were conducted. A SnO2 bridged assembly on SnO2/TiO2 core/shell electrodes undergoes light-driven water oxidation with an incident photon to current efficiency (IPCE) of 17.1% at 440 nm. Light-driven water reduction with a ruthenium trisbipyridine chromophore and molecular Ni(II) catalyst on NiO films was also used to produce H2. Compared to conventional solution-based procedures, the ALD approach offers significant advantages in scope and flexibility for the preparation of stable surface structures.

Robust Cooperative Photo-oxidation of Sulfides without Sacrificial Reagent under Air Using a Dinuclear RuII -CuII Assembly.

A molecular chromophore-catalyst assembly containing a chromophore ruthenium(II) center (RuIIchro ) and a catalytic copper(II) center (CuIIcat ) has been prepared easily. The assembly was employed for photocatalytic oxidation of sulfides without sacrificial reagent in the presence of dioxygen under blue light irradiation. Unprecedented turnover number (TON) up to 32 000 was achieved. It was elucidated that an electron transferred from excited state of chromophore RuII*chro to CuIIcat along with generation of CuIcat that was further activated by O2 . These results demonstrate a promising strategy for efficient cooperative photocatalytic reactions under air using the chromophore-catalyst assembly.

Polymer Chromophore-Catalyst Assembly for Solar Fuel Generation.

A polystyrene-based chromophore-catalyst assembly (poly-2) has been synthesized and assembled at a mesoporous metal oxide photoanode. The assembly contains water oxidation catalyst centers based on [Ru(trpy) (phenq)]2+ (Ru-Cat) and [Ru(bpy)3]2+ derivatives (Ru-C) as chromophores (trpy= 2,2';6,2″- terpyridine, phenq = 2-(quinol-8'-yl)-1,10-phenanthroline and bpy = 2,2'-bipyridine). The photophysical and electrochemical properties of the polychromophore-oxidation catalyst assembly were investigated in solution and at the surface of mesoporous metal oxide films. The layer-by-layer (LbL) method was utilized to construct multilayer films with cationic poly-2 and anionic poly(acrylic acid) (PAA) for light-driven photochemical oxidations. Photocurrent measurements of (PAA/poly-2)10 LbL films on mesoporous TiO2 demonstrate light-driven oxidation of phenol and benzyl alcohol in aqueous solution. Interestingly, illumination of (PAA/poly-2)5 LbL films on a fluorine doped SnO2/TiO2 core/shell photoanode in aqueous solution gives rise to an initial photocurrent (∼18.5 μA·cm-2) that is in part ascribed to light driven water oxidation.

Fluoropolymer-Stabilized Chromophore-Catalyst Assemblies in Aqueous Buffer Solutions for Water-Oxidation Catalysis.

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.

Polymer chromophore-catalyst assembly for solar fuel generation

Polymer chromophore-catalyst assembly for solar fuel generation

Light-Driven Water Splitting by a Covalently Linked Ruthenium-Based Chromophore–Catalyst Assembly

The preparation and characterization of new Ru(II) polypyridyl-based chromophore–catalyst assemblies, [(4,4′-PO3H2-bpy)2Ru(4-Mebpy-4′-epic)Ru(bda)(pic)]2+ (1, bpy = 2,2′-bipyridine; 4-Mebpy-4′-epic = 4-(4-methylbipyridin-4′-yl-ethyl)-pyridine; bda = 2,2′-bipyridine-6,6′-dicarboxylate; pic = 4-picoline), and [(bpy)2Ru(4-Mebpy-4′-epic)Ru(bda)(pic)]2+ (1′) are described, as is the application of 1 in a dye-sensitized photoelectrosynthesis cell (DSPEC) for solar water splitting. On SnO2/TiO2 core–shell electrodes in a DSPEC configuration with a Pt cathode, the chromophore–catalyst assembly undergoes light-driven water oxidation at pH 5.7 in a 0.1 M acetate buffer, 0.5 M in NaClO4. With illumination by a 100 mW cm–2 white light source, photocurrents of ∼0.85 mA cm–2 were observed after 30 s under a 0.1 V vs Ag/AgCl applied bias with a faradaic efficiency for O2 production of 74% measured over a 5 min illumination period.

ChemInform Abstract: Finding the Way to Solar Fuels with Dye‐Sensitized Photoelectrosynthesis Cells

AbstractReview: 418 refs.

Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells.

The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO2 by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO2 reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO2 reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

Light-Driven Water Oxidation Using Polyelectrolyte Layer-by-Layer Chromophore–Catalyst Assemblies

Layer-by-Layer (LbL) polyelectrolyte self-assembly occurs by the alternate exposure of a substrate to solutions of oppositely charged polyelectrolytes or polyions. Here, we report the application of LbL to construct chromophore–catalyst assemblies consisting of a cationic polystyrene-based Ru polychromophore (PS-Ru) and a [Ru(tpy)(2-pyridyl-N-methylbenzimidazole) (OH2)]2+ water oxidation catalyst (RuC), codeposited with poly(acrylic acid) (PAA) as an inert polyanion. These assemblies are deposited onto planar indium tin oxide (ITO, Sn:In2O3) substrates for electrochemical characterization and onto mesoporous substrates consisting of a SnO2/TiO2 core/shell structure atop fluorine doped tin oxide (FTO) for application to light-driven water oxidation in a dye-sensitized photoelectrosynthesis cell. Cyclic voltammetry and ultraviolet–visible absorption spectroscopy reveal that multilayer deposition progressively increases the film thickness on ITO glass substrates. Under an applied bias, photocurrent measureme...