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Your institute does not have access to this article Relevant articles Open Access articles citing this article. Photocathode functionalized with a molecular cobalt catalyst for selective carbon dioxide reduction in water Palas Baran Pati, Ruwen Wang … Marc Robert Nature Communications Open Access 13 July 2020 Access options Access to this article via Universite Libre De Bruxelles is not available. Change institution Subscribe to Journal Get full journal access for 1 year 111,95 € only 9,33 € per issue Subscribe Tax calculation will be finalised during checkout. Rent or Buy article Get time limited or full article access on ReadCube. from$8.99 Rent or Buy All prices are NET prices. Additional access options: Log in Learn about institutional subscriptions Data availability The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request. References 1. Hammarström, L. & Hammes-Schiffer, S. Artificial photosynthesis and solar fuels. Acc. Chem. Res. 42, 1859–1860 (2009). Article Google Scholar 2. Barber, J. Photosynthetic energy conversion: natural and artificial. Chem. Soc. Rev. 38, 185–196 (2009). Article Google Scholar 3. Morris, A. J., Meyer, G. J. & Fujita, E. Molecular approaches to the photocatalytic reduction of carbon dioxide for solar fuels. Acc. Chem. Res. 42, 1983–1994 (2009). Article Google Scholar 4. Chen, S., Takata, T. & Domen, K. Particulate photocatalysts for overall water splitting. Nat. Rev. Mater. 2, 17050 (2017). Article Google Scholar 5. Grätzel, M. Photoelectrochemical cells. Nature 414, 338–344 (2001). Article Google Scholar 6. Hagfeldt, A., Boschloo, G., Sun, L., Kloo, L. & Pettersson, H. Dye-sensitized solar cells. Chem. Rev. 110, 6595–6663 (2010). Article Google Scholar 7. Han, Z. & Eisenberg, R. Fuel from water: the photochemical generation of hydrogen from water. Acc. Chem. Res. 47, 2537–2544 (2014). Article Google Scholar 8. Li, F. et al. Organic dye-sensi

Halide photoredox chemistry is of both practical and fundamental interest. Practical applications have largely focused on solar energy conversion with hydrogen gas, through HX splitting, and electrical power generation, in regenerative photoelectrochemical and photovoltaic cells. On a more fundamental level, halide photoredox chemistry provides a unique means to generate and characterize one electron transfer chemistry that is intimately coupled with X–X bond-breaking and -forming reactivity. This review aims to deliver a background on the solution chemistry of I, Br, and Cl that enables readers to understand and utilize the most recent advances in halide photoredox chemistry research. These include reactions initiated through outer-sphere, halide-to-metal, and metal-to-ligand charge-transfer excited states. Kosower’s salt, 1-methylpyridinium iodide, provides an early outer-sphere charge-transfer excited state that reports on solvent polarity. A plethora of new inner-sphere complexes based on transition and main group metal halide complexes that show promise for HX splitting are described. Long-lived charge-transfer excited states that undergo redox reactions with one or more halogen species are detailed. The review concludes with some key goals for future research that promise to direct the field of halide photoredox chemistry to even greater heights.

Semiconductor-based photocathodes with high light-absorption capability are of interest in the production of solar fuels, but many of them are limited by low efficiencies due to rapid interfacial back electron transfer. We demonstrate here that a nanowire-structured p-type Si (p-Si) electrode, surface-modified with a perylene-diimide derivative (PDI′), can undergo photoreduction of a surface-bound, water reduction catalyst toward efficient H2 evolution under a low applied bias. At the electrode interface, the PDI′ layer converts green light into high-energy holes at its excited state for extraction of photogenerated electrons at the photoexcited p-Si. The photogenerated electrons at the reduced PDI′ are subsequently transferred to the molecular H2-evolution catalyst. Involvement of the photoexcited PDI′ enables effective redox separation between the electrons at the reduced catalyst and the holes at the valence band of p-Si. The heterojunction photocathode was used in a tandem cell by coupling with a dye-sensitized photoanode for solar-driven water splitting into H2 and O2.

The direct, site-selective alkylation of unactivated C(sp3)–H bonds in organic substrates is a long-standing goal in synthetic chemistry. General approaches to the activation of strong C–H bonds include radical-mediated processes involving highly reactive intermediates, such as heteroatom-centered radicals. Herein, we describe a catalytic, intermolecular C–H alkylation that circumvents such reactive species via a new elementary step for C–H cleavage involving multisite-proton-coupled electron transfer (multisite-PCET). Mechanistic studies indicate that the reaction is catalyzed by a noncovalent complex formed between an iridium(III) photocatalyst and a monobasic phosphate base. The C–H alkylation proceeds efficiently using diverse hydrocarbons and complex molecules as the limiting reagent and represents a new approach to the catalytic functionalization of unactivated C(sp3)–H bonds.

Iodide redox chemistry is intimately coupled with the formation and breaking of chemical bonds that are relevant to emerging solar energy technologies. In this Account, recent advances in dye-sensitized iodide oxidation chemistry in organic solutions are described. Here RuII sensitizers with high cationic charge, tuned reduction potentials, and specific iodide receptor site(s) are shown to self-assemble in organic solvents and yield structures that rapidly oxidize iodide and generate I–I bonds when illuminated with visible light. These studies provided new insights into the fascinating behavior of our most polarizable and easily oxidized monatomic anion. Sensitized iodide photo-oxidation in CH3CN solutions consists of two mechanistic steps. In the first step, an excited-state sensitizer oxidizes iodide (I–) to an iodine atom (I•) through diffusional encounters. The second step involves the reaction of I• with I– to form the I–I bond of diiodide, I2•–. The overall reaction converts a green photon into about 1.64 eV of free energy in the form of I2•– and the reduced sensitizer. The free energy is only transiently available, as back-electron transfer to yield ground-state products is quantitative. Interestingly, when the free energy change is near zero, iodide photo-oxidation occurs rapidly with rate constants near the diffusion limit, i.e., >1010 M–1 s–1. Such rapid reactivity is in line with anecdotal knowledge that iodide is an outstanding electron donor and is indicative of adiabatic electron transfer through an inner-sphere mechanism. In low-dielectric-constant solvents, dicationic RuII sensitizers were found to form tight ion pairs with iodide. Diimine ligands with additional cationic charge, or “binding pockets” that recognize halides, have been utilized to position one or more halides at specific locations about the sensitizer before light absorption. Diverse photochemical reactions observed with these supramolecular assemblies range from the photorelease of ha

Ground- and excited-state control of halide supramolecular assembly was achieved through the preparation of a series of ester- and amide-functionalized ruthenium polypyridyl complexes in CH2Cl2. Hydrogen-bonding amide and alcohol groups on the receptor ligand were found to direct interactions with halide, while halide association with the ethyl ester groups was not observed. The various functional groups on the receptor ligands tuned the ground-state equilibrium constants over 2 orders of magnitude (1 × 105 to 1 × 107 M–1), and the fractional contribution of each hydrogen-bond donor to the total equilibrium constant was determined. Pulsed-laser excitation of the complexes resulted in excited-state localization on the ester- or amide-functionalized ligands. In the case where the excited state was oriented toward an associated halide ion (the amide complexes), an 80 ± 10 meV Coulombic repulsion was induced that lowered the excited-state equilibrium constant (K*eq) and resulted in halide photorelease. The rate constants for excited-state halide release (k*21) were determined, and the values varied based on the functional groups present in the receptor ligand. Complexes with more hydrogen-bonding donors had smaller rate constants for halide photorelease. In a complex without a specific receptor ligand, the excited-state dipole was not oriented toward the associated halide, and the excited state was therefore found to have a larger equilibrium constant for halide association than the ground state.

In artificial photosynthesis, the sun drives water splitting into H2 and O2 or converts CO2 into a useful form of carbon. In most schemes, water oxidation is typically the limiting half-reaction. Here, we introduce a molecular approach to the design of a photoanode that incorporates an electron acceptor, a sensitizer, an electron donor, and a water oxidation catalyst in a single molecular assembly. The strategy mimics the key elements in Photosystem II by initiating light-driven water oxidation with integration of a light absorber, an electron acceptor, an electron donor, and a catalyst in a controlled molecular environment on the surface of a conducting oxide electrode. Visible excitation of the assembly results in the appearance of reductive equivalents at the electrode and oxidative equivalents at a catalyst that persist for seconds in aqueous solutions. Steady-state illumination of the assembly with 440 nm light with an applied bias results in photoelectrochemical water oxidation with a per-photon absorbed efficiency of 2.3%. The results are notable in demonstrating that light-driven water oxidation can be carried out at a conductive electrode in a structure with the functional elements of Photosystem II including charge separation and water oxidation.

A thermodynamic analysis of the forward and reverse rate constants for adiabatic and nonadiabatic electron transfer equilibria over an 80 K temperature range is reported. The kinetic data were acquired by a spectroscopic approach that utilized excited state injection into TiO2 by sensitizers with two redox active groups linked through aromatic bridges that allow for intramolecular adiabatic (bridge = phenyl) or nonadiabatic (bridge = xylyl) electron transfer. Two impactful results were garnered from this analysis: (1) entropic barriers controlled the adiabatic electron transfer kinetics, and (2) the free energy barriers were unaffected by the degree of electronic coupling within experimental uncertainty. The second result stands in contrast to the common expectation that enhanced electronic coupling lowers the free energy barrier. This analysis provides new insights into how electronic coupling influences the free energy and barriers for electron transfer reactions.

Three chromophores of the general form [Ru(bpy′)2(4,4′-(PO3H2)2-2,2′-bipyridine)]2+, where bpy′ is 4,4′-(C(CH3)3)2-2,2′-bipyridine (Ru(dtb)2P); 4,4′-(CH3O)2-2,2′-bipyridine (Ru(OMe)2P), and 2,2′-bipyridine (RuP) were anchored to mesoporous thin films of TiO2 nanocrystallites at saturation surface coverages to investigate lateral self-exchange RuIII/II intermolecular hole hopping in 0.1 M LiClO4/CH3CN electrolytes. Hole hopping was initiated by a potential step 500 mV positive of the E1/2 (RuIII/II) potential or by pulsed laser (532 nm, 8 ns fwhm) excitation and monitored by visible absorption chronoabsorptometry and time-resolved absorption anisotropy measurements, respectively. The hole hopping rate constant kR extracted from the potential step data revealed self-exchange rate constants that followed the trend: TiO2|Ru(OMe)2P (ket = 1.4 × 106 s–1) > TiO2|RuP (7.1 × 105 s–1) > TiO2|Ru(dtb)2P (6.5 × 104 s–1). Analysis of the anisotropy data with Monte Carlo simulations provided hole hopping rate constants for TiO2|RuP and TiO2|Ru(dtb)2P that were within experimental error the same as that measured with the potential step. The hole hopping rate constants were found to trend with the TiO2(e–)|RuIII → TiO2|RuII charge recombination rate constants. The atomic layer deposition of an ∼10 Å layer of Al2O3 on top of the dye-sensitized films was found to prevent hole hopping by both initiation methods even though the chromophore surface coverage exceeded the percolation threshold and excited-state injection was efficient. The dramatic hole hopping turnoff was attributed to a larger outer-sphere reorganization energy for self-exchange due to the restricted access of electrolyte to the redox active chromophores. The implications of these findings for solar energy conversion applications are discussed.

Layer-by-Layer Assembly of Molecular Photoelectrodes on Conductive Metal Oxide

Design of Molecular Photoelectrodes of Conductive Metal Oxide

Layer-by-Layer Assembly of Molecular Photoelectrodes on Conductive Metal Oxide for Water Splitting

DecretOANoAutActif, info:eu-repo/semantics/published

info:eu-repo/semantics/published

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/cm2 at pH 7 in 0.1 M [PO4H2]-/[PO4H]2-solution with minimal loss in catalytic behavior over 30 min, and produced an incident photon to current efficiency (IPCE) of 24.8% at 440 nm.

Transparent conductive oxides (TCOs) are widely used commercially available materials for opto-electronic applications, yet they have received very little attention for dye-sensitization applications. Now, mesoporous thin films of conductive indium-doped tin oxide (ITO) nanocrystallites are shown to support long-lived charge separation with first-order recombination kinetics (k=1.5 s−1). A layer-by-layer technique was utilized to spatially arrange redox and/or chromophoric molecular components on ITO. Spectroelectrochemical measurements demonstrated that upon light absorption, each component provided a free-energy gradient to direct electron transfer at the conductive oxide interface. The long-lived nature of the photogenerated charge separated states provide favorable conditions for photocatalytic solar fuel production. Furthermore, the first-order recombination kinetics are most ideal for the fundamental understanding of interfacial charge separation dynamics.

The synthesis of seven ruthenium(II) polypyridyl complexes bearing one dicationic bis-4,4′-(trimethylaminomethyl)-2,2′-bipyridine (tmam) ligand is reported. The ancillary ligands of each complex were 2,2′-bipyrazine (bpz), 2,2′-bipyridine (bpy), 4,4′-tert-butyl-2,2′-bipyridine (dtb), 4,4′-dimethyl-2,2′-bipyridine (4,4′-dmb), 5,5′-dimethyl-2,2′-bipyridine (5,5′-dmb), 4,4′-nonyl-2,2′-bipyridine (nonyl), and 4,4′-methoxy-2,2′-bipyridine (MeO). The metal-to-ligand charge transfer excited state was localized on the tmam ligand in all instances with the exception of [Ru(bpz)2(tmam)]4+, where it was localized on the bpz ligand. All [PF6]− complexes were shown to form strong ion pairs with chloride in a Ru/Cl 1:2 stoichiometry in acetone, as evidenced by 1H NMR and UV–visible titrations. With the exception of [Ru(bpz)2(tmam)]4+, ion pairing with chloride anions resulted in excited states that were ∼25% longer-lived and with an ∼50% increase in the photoluminescence quantum yields compared to the [PF6]− ion pairs. It was shown that the quantum yield enhancements originated from a decreased nonradiative rate constant and an increased radiative rate constant. [Ru(bpy)2(tmam)]4+ showed curious excited state quenching behavior at higher equivalents of chloride, the origin of which is not understood.

Reduction of calix[4]arene-tetradiazonium salts at the vicinity of a surface, notably a carbon surface, leads to the formation of a dense and robust covalently bound monolayer. To expand the versatility of this unique methodology, the efficient synthesis of a calix[4]arene bearing a single diazonium group and thus a single anchoring point was achieved. The introduction of the diazonium group was developed through an original approach involving a copper-catalyzed reaction for the preparation of an intermediate azido-calixarene. The grafting ability of the calix[4]arene-monodiazonium cation was evaluated by investigating its electrochemical reduction at glassy carbon electrodes. Studies of the blocking properties of the modified surfaces toward redox probes evidenced the formation of compact covalently bound layers at the carbon substrates, opening interesting perspectives in the field of surface modification by calixarene-based diazonium salts.

10.1021/acsami.8b09134

The electrochemical and spectroscopic properties of eight bis(tridentate) cyclometalated RuII compounds covalently linked by a phenyl- or xylyl-thiophene bridge to a pendant triphenylamine (TPA) were characterized in fluid solution and immobilized on metal oxide surfaces. Upon surface immobilization, the TPA+/0 reduction potentials of the phenyl-bridged compounds exhibited large changes, ±100 mV, relative to solution-based values, yet those observed for the xylyl-bridged compounds were relatively unchanged. The highest occupied molecular orbital of the surface-immobilized compounds was associated with either TPA or RuII, enabling the study of the electron transfer in opposite directions. Electron transfer in the mixed-valent states of the compounds was found to proceed by different optical pathways for RuII → TPA+ relative to TPA → RuIII. Mulliken–Hush analysis of intervalence charge transfer bands for the phenyl-bridged compounds revealed that the electronic coupling matrix element, HDA, was ∼950 cm–1 for RuII → TPA+, while HDA for TPA → RuIII appeared to be 2500 cm–1. In contrast, the xylyl-bridged compounds were weakly coupled. A superexchange analysis, where unoccupied bridge orbitals were taken directly into account, led to a very different conclusion: HDA did not depend on the charge-transfer direction or path. The results imply that the electron-transfer direction can alter optical charge transfer pathways without influencing the electronic coupling.

Biosensors that can determine protein concentration and structure are highly desired for biomedical applications. For the development of such biosensors, the use of Fourier transform infrared (FTIR) spectroscopy with the attenuated internal total reflection (ATR) configuration is particularly attractive, but it requires appropriate surface functionalization of the ATR optical element. Indeed, the surface has to specifically interact with a target protein in close contact with the optical element and must display antifouling properties to prevent nonspecific adsorption of other proteins. Here, we report robust monolayers of calix[4]arenes bearing oligo(ethylene glycol) (oEG) chains, which were grafted on germanium and gold surfaces via their tetradiazonium salts. The formation of monolayers of oEGylated calix[4]arenes was confirmed by AFM, IR, and contact angle measurements. The antifouling properties of these modified surfaces were studied by ATR-FTIR spectroscopy and fluorescence microscopy, and the nonspecific absorption of bovine serum albumin was found to be reduced by 85% compared to that of unmodified germanium. In other words, the organic coating by oEGylated calix[4]arenes provides remarkable antifouling properties, opening the way for the design of germanium- or gold-based biosensors.

A series of 18 ruthenium(II) polypyridyl complexes were synthesized and evaluated under electrochemically oxidative conditions, which generates the Ru(III) oxidation state and mimics the harsh conditions experienced during the kinetically limited regime that can occur in dye-sensitized solar cells (DSSCs) and dye-sensitized photo-electrosynthesis cells, to further develop fundamental insights into the factors governing molecular sensitizer surface stability in aqueous 0.1 M HClO4. Both desorption and oxidatively induced ligand substitution were observed on planar fluorine-doped tin oxide (FTO) electrodes, with a dependence on the E1/2 Ru(III/II) redox potential dictating the comparative ratios of the processes. Complexes such as RuP4OMe (E1/2 = 0.91 vs Ag/AgCl) displayed virtually only desorption, while complexes such as RuPbpz (E1/2 > 1.62 V vs Ag/AgCl) displayed only chemical decomposition. Comparing isomers of 4,4′- and 5,5′-disubstituted-2,2′-bipyridine ancillary ligands, a dramatic increase in the rate of desorption of the Ru(III) complexes was observed for the 5,5′-ligands. Nanoscopic indium-doped tin oxide thin films (nanoITO) were also sensitized and analyzed with cyclic voltammetry, UV–vis absorption spectroscopy, and X-ray photoelectron spectroscopy, allowing for further distinction of desorption versus ligand-substitution processes. Desorption loss to bulk solution associated with the planar surface of FTO is essentially non-existent on nanoITO, where both desorption and ligand substitution are shut down with RuP4OMe. These results revealed that minimizing time spent in the oxidized form, incorporating electron-donating groups, maximizing hydrophobicity, and minimizing molecular bulk near the adsorbed ligand are critical to optimizing the performance of ruthenium(II) polypyridyl complexes in dye-sensitized devices.

Supramolecular assembly is shown to provide control over excited-state chloride release. Two dicationic chromophores were designed with a ligand that recognizes halide ions in CH2Cl2 and a luminescent excited state whose dipole was directed toward, 12+, or away, 22+, from an associated chloride ion. The dipole orientation had little influence on the ground-state equilibrium constant, Keq ∼ 4 × 106 M–1, but induced a profound change in the excited-state equilibrium. Light excitation of [12+,Cl–]+ resulted in time-dependent shifts in the photoluminescence spectra with the appearance of biexponential kinetics consistent with the photorelease of Cl–. Remarkably, the excited-state equilibrium constant was lowered by a factor of 20 and resulted in nearly 45% dissociation of chloride. In contrast, light excitation of [22+,Cl–]+ revealed a 45-fold increase in the excited-state equilibrium constant. The data show that rational design and supramolecular assembly enables the detection and photorelease of chloride ions with the potential for future applications in biology and chemistry.

Light-driven water splitting was achieved using a dye-sensitized mesoporous oxide film and the oxidation of bromide (Br−) to bromine (Br2) or tribromide (Br3−). The chemical oxidant (Br2 or Br3−) is formed during illumination at the photoanode and used as a sacrificial oxidant to drive a water oxidation catalyst (WOC), here demonstrated using [Ru(bda)(pic)2], (1; pic=picoline, bda=2,2′-bipyridine-6,6′-dicarboxylate). The photochemical oxidation of bromide produces a chemical oxidant with a potential of 1.09 V vs. NHE for the Br2/Br− couple or 1.05 V vs. NHE for the Br3−/Br− couple, which is sufficient to drive water oxidation at 1 (RuV/IV≈1.0 V vs. NHE at pH 5.6). At pH 5.6, using a 0.2 m acetate buffer containing 40 mm LiBr and the [Ru(4,4′-PO3H2-bpy)(bpy)2]2+ (RuP2+, bpy=2,2′-bipyridine) chromophore dye on a SnO2/TiO2 core–shell electrode resulted in a photocurrent density of around 1.2 mA cm−2 under approximately 1 Sun illumination and a Faradaic efficiency upon addition of 1 of 77 % for oxygen evolution.

The electrografting of [Ru(ttt)(tpy-C6H4-N2+)]3+, where “ttt” is 4,4′,4″-tri-tert-butyl-2,2′:6′,2″-terpyridine, was investigated on several wide band gap metal oxide surfaces (TiO2, SnO2, ZrO2, ZnO, In2O3:Sn) and compared to structurally analogous sensitizers that differed only by the anchoring group, i.e., -PO3H2 and -COOH. An optimized procedure for diazonium electrografting to semiconductor metal oxides is presented that allowed surface coverages that ranged between 4.7 × 10-8 and 10.6 × 10-8 mol cm-2 depending on the nature of the metal oxide. FTIR analysis showed the disappearance of the diazonium stretch at 2266 cm-1 after electrografting. XPS analysis revealed a characteristic peak of Ru 3d at 285 eV as well as a peak at 531.6 eV that was attributed to O 1s in Ti-O-C bonds. Photocurrents were measured to assess electron injection efficiency of these modified surfaces. The electrografted sensitizers exhibited excellent stability across a range of pHs spanning from 1 to 14, where classical binding groups such as carboxylic and phosphonic derivatives were hydrolyzed.

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.