A visible-light harvesting system for CO2 reduction using a Ru(II) -Re(I) photocatalyst adsorbed in mesoporous organosilica

Self-assembled supramolecular artificial light-harvesting nanosystems: construction, modulation, and applications

Artificial light-harvesting systems, an elegant way to capture, transfer and utilize solar energy, have attracted great attention in recent years. As the primary step of natural photosynthesis, the principle of light-harvesting systems has been intensively investigated, which is further employed for artificial construction of such systems. Supramolecular self-assembly is one of the feasible methods for building artificial light-harvesting systems, which also offers an advantageous pathway for improving light-harvesting efficiency. Many artificial light-harvesting systems based on supramolecular self-assembly have been successfully constructed at the nanoscale with extremely high donor/acceptor ratios, energy transfer efficiency and the antenna effect, which manifests that self-assembled supramolecular nanosystems are indeed a viable way for constructing efficient light-harvesting systems. Non-covalent interactions of supramolecular self-assembly provide diverse approaches to improve the efficiency of artificial light-harvesting systems. In this review, we summarize the recent advances in artificial light-harvesting systems based on self-assembled supramolecular nanosystems. The construction, modulation, and applications of self-assembled supramolecular light-harvesting systems are presented, and the corresponding mechanisms, research prospects and challenges are also briefly highlighted and discussed.

Solar driven CO2 reduction with a molecularly engineered periodic mesoporous organosilica containing cobalt phthalocyanine

A molecular cobalt phthalocyanine (CoPc) catalyst has been integrated in an ethylene-bridged periodic mesoporous organosilica (PMO) to fabricate a hybrid material, CoPc-PMO, that catalyses CO₂ reduction to CO in a photocatalytic system using [Ru(bpy)₃]²⁺ (bpy = 2,2'-bipyridine) as a photosensitizer and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as an electron donor. CoPc-PMO displays a Co-based turnover number (TON_CO) of >6000 for CO evolution with >70% CO-selectivity after 4 h irradiation with UV-filtered simulated solar light, and a quantum yield of 1.95% at 467 nm towards CO. This system demonstrates a benchmark TON_CO for immobilised CoPc-based catalysts towards visible light-driven CO₂ reduction.

Photocatalytic Systems for CO2 Reduction: Metal-Complex Photocatalysts and Their Hybrids with Photofunctional Solid Materials

Photocatalytic CO₂ reduction is a critical objective in the field of artificial photosynthesis because it can potentially make a total solution for global warming and shortage of energy and carbon resources. We have successfully developed various highly efficient, stable, and selective photocatalytic systems for CO₂ reduction using transition metal complexes as both photosensitizers and catalysts. The molecular architectures for constructing selective and efficient photocatalytic systems for CO₂ reduction are discussed herein. As a typical example, a mixed system of a ring-shaped Re(I) trinuclear complex as a photosensitizer and fac-[Re(bpy)(CO)₃{OC₂H₄N(C₂H₄OH)₂}] as a catalyst selectively photocatalyzed CO₂ reduction to CO with the highest quantum yield of 82% and a turnover number (TON) of over 600. Not only rare and noble metals but also earth abundant ones, such as Mn(I), Cu(I), and Fe(II) can be used as central metal cations. In the case using a Cu(I) dinuclear complex as a photosensitizer and fac-Mn(bpy)(CO)₃Br as a catalyst, the total formation quantum yield of CO and HCOOH from CO₂ was 57% and TON_CO+HCOOH exceeded 1300.

Efficient supramolecular photocatalysts for CO₂ reduction, in which photosensitizer and catalyst units are connected through a bridging ligand, were developed for removing a diffusion control on collisions between a photosensitizer and a catalyst. Supramolecular photocatalysts, in which [Ru(N^∧N)₃]²⁺-type photosensitizer and Re(I) or Ru(II) catalyst units are connected to each other with an alkyl chain, efficiently and selectively photocatalyzed CO₂ reduction in solutions. Mechanistic studies using time-resolved IR and electrochemical measurements provided molecular architecture for constructing efficient supramolecular photocatalysts. A Ru(II)–Re(I) supramolecular photocatalyst constructed according to this molecular architecture efficiently photocatalyzed CO₂ reduction even when it was fixed on solid materials. Harnessing this property of the supramolecular photocatalysts, two types of hybrid photocatalytic systems were developed, namely, photocatalysts with light-harvesting capabilities and photoelectrochemical systems for CO₂ reduction.

Introduction of light-harvesting capabilities into molecular photocatalytic systems should be important because the intensity of solar light shone on the earth’s surface is relatively low. Periodic mesoporous organosilica, in which methyl acridone groups are embedded in the silica framework as light harvesters, was combined with a Ru(II)–Re(I) supramolecular photocatalyst with phosphonic acid anchoring groups. In this hybrid, the photons absorbed by approximately 40 methyl acridone groups were transferred to one Ru(II) photosensitizer unit, and then, the photocatalytic CO₂ reduction commenced.

To use water as an abundant electron donor, we developed hybrid photocatalytic systems combining metal-complex photocatalysts with semiconductor photocatalysts that display high photooxidation powers, in which two photons are sequentially absorbed by the metal-complex photosensitizer and the semiconductor, resulting in both high oxidation and reduction power. Various types of dye-sensitized molecular photocathodes comprising the p-type semiconductor electrodes and the supramolecular photocatalysts were developed. Full photoelectrochemical cells combining these dye-sensitized molecular photocathodes and n-type semiconductor photoanodes achieved CO₂ reduction using only visible light as the energy source and water as the reductant. Drastic improvement of dye-sensitized molecular photocathodes is reported.

The results presented in this Account clearly indicate that we can construct very efficient, selective, and durable photocatalytic systems constructed with the metal-complex photosensitizers and catalysts. The supramolecular-photocatalyst architecture in which the photosensitizer and the catalyst are connected to each other is useful especially on the surface of solid owing to rapid electron transfer from the photosensitizer to the catalyst. On basis of these findings, we successfully constructed hybrid systems of the supramolecular photocatalysts with photoactive solid materials. These hybridizations can add new functions to the metal-complex photocatalytic systems, such as water oxidation and light harvesting.

Ruthenium Assemblies for CO2 Reduction and H2 Generation: Time Resolved Infrared Spectroscopy, Spectroelectrochemistry and a Photocatalysis Study in Solution and on NiO

Two novel supramolecular complexes RuRe ([Ru(dceb)₂(bpt)Re(CO)₃Cl](PF₆)) and RuPt ([Ru(dceb)₂(bpt)PtI(H₂O)](PF₆)₂) [dceb = diethyl(2,2′-bipyridine)-4,4′-dicarboxylate, bpt = 3,5-di(pyridine-2-yl)-1,2,4-triazolate] were synthesized as new catalysts for photocatalytic CO₂ reduction and H₂ evolution, respectively. The influence of the catalytic metal for successful catalysis in solution and on a NiO semiconductor was examined. IR-active handles in the form of carbonyl groups on the peripheral ligand on the photosensitiser were used to study the excited states populated, as well as the one-electron reduced intermediate species using infrared and UV-Vis spectroelectrochemistry, and time resolved infrared spectroscopy. Inclusion of ethyl-ester moieties led to a reduction in the LUMO energies on the peripheral bipyridine ligand, resulting in localization of the ³MLCT excited state on these peripheral ligands following excitation. RuPt generated hydrogen in solution and when immobilized on NiO in a photoelectrochemical (PEC) cell. RuRe was inactive as a CO₂ reduction catalyst in solution, and produced only trace amounts of CO when the photocatalyst was immobilized on NiO in a PEC cell saturated with CO₂.

Reticular-Chemistry-Inspired Supramolecule Design as a Tool to Achieve Efficient Photocatalysts for CO2 Reduction

Photocatalytic CO₂ reduction into C1 products is one of the most trending research subjects of current times as sustainable energy generation is the utmost need of the hour. In this review, we have tried to comprehensively summarize the potential of supramolecule-based photocatalysts for CO₂ reduction into C1 compounds. At the outset, we have thrown light on the inert nature of gaseous CO₂ and the various challenges researchers are facing in its reduction. The evolution of photocatalysts used for CO₂ reduction, from heterogeneous catalysis to supramolecule-based molecular catalysis, and subsequent semiconductor-supramolecule hybrid catalysis has been thoroughly discussed. Since CO₂ is thermodynamically a very stable molecule, a huge reduction potential is required to undergo its one- or multielectron reduction. For this reason, various supramolecule photocatalysts were designed involving a photosensitizer unit and a catalyst unit connected by a linker. Later on, solid semiconductor support was also introduced in this supramolecule system to achieve enhanced durability, structural compactness, enhanced charge mobility, and extra overpotential for CO₂ reduction. Reticular chemistry is seen to play a pivotal role as it allows bringing all of the positive features together from various components of this hybrid semiconductor-supramolecule photocatalyst system. Thus, here in this review, we have discussed the selection and role of various components, viz. the photosensitizer component, the catalyst component, the linker, the semiconductor support, the anchoring ligands, and the peripheral ligands for the design of highly performing CO₂ reduction photocatalysts. The selection and role of various sacrificial electron donors have also been highlighted. This review is aimed to help researchers reach an understanding that may translate into the development of excellent CO₂ reduction photocatalysts that are operational under visible light and possess superior activity, efficiency, and selectivity.

Effect of Micropores of a Porous Coordination Polymer on the Product Selectivity in RuII Complex-catalyzed CO2 Reduction

To develop an efficient CO2 reduction catalyst, hybridizing a molecular catalyst and a porous coordination polymer (PCP) is a promising strategy because it can combine both advantages of the precise reactivity control of the former and the CO2 adsorption property of the latter. Although several PCP hybrid catalysts have been reported to date, the CO2 sorption behavior and the CO2 reduction reactivity have been investigated separately, and the CO2 enrichment during the catalysis is still unclear. We report CO2 photoreduction under different temperatures and pressures using a PCP-RuII complex hybrid catalyst. The product selectivity (CO or HCOOH) varied depending on the reaction conditions. The altered selectivity could be interpreted in terms of the CO2 capture in the micropores of a PCP.

Mechanistic study of photocatalytic CO2 reduction using a Ru(ii)-Re(i) supramolecular photocatalyst

Supramolecular photocatalysts comprising [Ru(diimine)₃]²⁺ photosensitiser and fac-[Re(diimine)(CO)₃{OC(O)OC₂H₄NR₂}] catalyst units can be used to reduce CO₂ to CO with high selectivity, durability and efficiency. In the presence of triethanolamine, the Re catalyst unit efficiently takes up CO₂ to form a carbonate ester complex, and then direct photocatalytic reduction of a low concentration of CO₂, e.g., 10% CO₂, can be achieved using this type of supramolecular photocatalyst. In this work, the mechanism of the photocatalytic reduction of CO₂ was investigated applying such a supramolecular photocatalyst, RuC2Re with a carbonate ester ligand, using time-resolved visible and infrared spectroscopies and electrochemical methods. Using time-resolved spectroscopic measurements, the kinetics of the photochemical formation processes of the one-electron-reduced species RuC2(Re)−, which is an essential intermediate in the photocatalytic reaction, were clarified in detail and its electronic structure was elucidated. These studies also showed that RuC2(Re)− is stable for 10 ms in the reaction solution. Cyclic voltammograms measured at various scan rates besides temperature and kinetic analyses of RuC2(Re)− produced by steady-state irradiation indicated that the subsequent reaction of RuC2(Re)− proceeds with an observed first-order rate constant of approximately 1.8 s⁻¹ at 298 K and is a unimolecular reaction, independent of the concentrations of both CO₂ and RuC2(Re)−.

Formation processes and reactivity of an important intermediate of photocatalytic CO₂ reduction, one-electron reduced species of a Ru(ii)–Re(i) supramolecular photocatalyst with a carbonate ester ligand, were investigated in detail.

Photocatalysis of a Dinuclear Ru(II)-Re(I) Complex for CO2 Reduction on a Solid Surface

The development of CO₂-reduction photocatalysts is one of the main targets in the field of artificial photosynthesis. Recently, numerous hybrid systems in which supramolecular photocatalysts comprised of a photosensitizer and catalytic-metal-complex units are immobilized on inorganic solid materials, such as semiconductors or mesoporous organosilica, have been reported as CO₂-reduction photocatalysts for various functions, including water oxidation and light harvesting. In the present study, we investigated the photocatalytic properties of supramolecular photocatalysts comprised of a Ru(II)-complex photosensitizer and a Re(I)-complex catalyst fixed on the surface of insulating Al₂O₃ particles: the distance among the supramolecular photocatalyst molecules should be fixed. Visible-light irradiation of the photocatalyst in the presence of an electron donor under a CO₂ atmosphere produced CO selectively. Although CO formation was also observed for a 1:1 mixture of mononuclear Ru(II) and Re(I) complexes attached to an Al₂O₃ surface, the photocatalytic activity was much lower. The activity of the Al₂O₃-supported photocatalyst was strongly dependent on the adsorption density of the supramolecular moiety, where the initial rate of photocatalytic CO formation was faster at lower density and higher photocatalyst durability was achieved at higher density. One of the main reasons for the former phenomenon is the decreased quenching fraction of the excited state of the photosensitizer unit by the reductant dissolved in the solution phase in the case of higher density. This is due to the self-quenching of the excited photosensitizer unit and steric hindrance between the condensed supramolecular photocatalyst molecules attached to the surface. The higher durability of the more condensed system is caused by intermolecular electron transfer between reduced supramolecular photocatalyst molecules, which accelerates the formation of CO in the photocatalytic CO₂ reduction. Coadsorption of a Ru(II) mononuclear complex as a redox photosensitizer could drastically reinforce the photocatalysis of the supramolecular photocatalyst on the surface of the Al₂O₃ particles: more than 10 times higher turnover number and about 3.4 times higher turnover frequency of CO formation. These investigations provide new architectures for the construction of efficient and durable hybrid photocatalytic systems for CO₂ reduction, which are composed of metal-complex photocatalysts and solid materials.

Designing and understanding light-harvesting devices with machine learning

Understanding the fundamental processes of light-harvesting is crucial to the development of clean energy materials and devices. Biological organisms have evolved complex metabolic mechanisms to efficiently convert sunlight into chemical energy. Unraveling the secrets of this conversion has inspired the design of clean energy technologies, including solar cells and photocatalytic water splitting. Describing the emergence of macroscopic properties from microscopic processes poses the challenge to bridge length and time scales of several orders of magnitude. Machine learning experiences increased popularity as a tool to bridge the gap between multi-level theoretical models and Edisonian trial-and-error approaches. Machine learning offers opportunities to gain detailed scientific insights into the underlying principles governing light-harvesting phenomena and can accelerate the fabrication of light-harvesting devices.

Improving Photocatalysis for the Reduction of CO2 through Non-covalent Supramolecular Assembly

We report the enhancement of photocatalytic performance by introduction of hydrogen-bonding interactions to a Re bipyridine catalyst and Ru photosensitizer system (ReDAC/RuDAC) by the addition of amide substituents, with carbon monoxide (CO) and carbonate/bicarbonate as products. This system demonstrates a more-than-3-fold increase in turnover number (TON_CO = 100 ± 4) and quantum yield (Φ_CO = 23.3 ± 0.8%) for CO formation compared to the control system using unsubstituted Ru photosensitizer (RuBPY) and ReDAC (TON_CO = 28 ± 4 and Φ_CO = 7 ± 1%) in acetonitrile (MeCN) with 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial reductant. In dimethylformamide (DMF), a solvent that disrupts hydrogen bonds, the ReDAC/RuDAC system showed a decrease in catalytic performance while the control system exhibited an increase, indicating the role of hydrogen bonding in enhancing the photocatalysis for CO₂ reduction through supramolecular assembly. The similar properties of RuDAC and RuBPY demonstrated in lifetime measurements, spectroscopic analysis, and electrochemical and spectroelectrochemical studies revealed that the enhancement in photocatalysis is due not to differences in intrinsic properties of the catalyst or photosensitizer, but to hydrogen-bonding interactions between them.

Kinetics and Mechanism of Intramolecular Electron Transfer in Ru(II)-Re(I) Supramolecular CO2-Reduction Photocatalysts: Effects of Bridging Ligands

The supramolecular photocatalysts in which a Ru(II) complex as a molecular redox photosensitizer unit and a Re(I) complex as a molecular catalyst unit are connected with a various alkyl or ether chain have attracted attention because they can efficiently photocatalyze CO₂ reduction with high durability and high selectivity of CO formation, especially on various solid materials such as semiconductor electrodes and mesoporous organosilica. The intramolecular electron transfer from the one-electron reduced photosensitizer unit to the catalyst unit, which follows excitation of the photosensitizer unit and subsequent reductive quenching of the excited photosensitizer unit by a reductant, is one of the most important processes in the photocatalytic reduction of CO₂. We succeeded in determining the rate constants of this intramolecular electron transfer process by using subnanosecond time-resolved IR spectroscopy. The logarithm of rate constants shows a linear relationship with the lengths of the bridging chain in the supramolecular photocatalysts with one bridging alkyl or ether chain. In conformity with the exponential decay of the wave function and the coupling element in the long-distance electron transfer, the apparent decay coefficient factor (β) in the supramolecular photocatalysts with one bridging chain was determined to be 0.74 Å^-1. In the supramolecular photocatalyst with two ethylene chains connecting between the photosensitizer and catalyst units, on the other hand, the intramolecular electron transfer rate is much faster than that with only one ethylene chain. These results strongly indicate that the intramolecular electron transfer from the one-electron reduced species of the Ru photosensitizer unit to the Re catalyst unit proceeds by the through-bond mechanism.

Relaxation dynamics of [Re(CO)2(bpy){P(OEt)3}2](PF6) in TEOA solvent measured by time-resolved attenuated total reflection terahertz spectroscopy

To reveal highly efficient photocatalytic properties of an artificial photosynthesis material [Re(CO)₂(bpy){P(OEt)₃}₂](PF₆), we have directly observed the photo-induced relaxation dynamics and reductive quenching process of the photo-excited state on a photosynthesis material in Triethanolamine (TEOA) solvent as an electron donor by time-resolved attenuated total reflection spectroscopy in the terahertz (THz) region. The spectrum of the complex in TEOA has an intermolecular vibrational mode between the complex and TEOA molecules, which reflects the precursor of the reductive quenching process. The intermolecular vibrational mode has three-step relaxation process in a picosecond timescale after photo-excitation, where firstly the triplet metal-to-ligand charge transfer excited state is vibrationally cooled down, secondly the distance between Re and TEOA is reduced by the rotation of TEOA molecules due to dipole-dipole interaction accelerated by heat transfer, and finally electrons transfer from TEOA to Re. These observations provide us the detailed information of the electron transfer process of photocatalytic properties of [Re(CO)₂(bpy){P(OEt)₃}₂](PF₆) in TEOA solvent.

Photoelectrochemical CO2 Reduction Using a Ru(II)-Re(I) Supramolecular Photocatalyst Connected to a Vinyl Polymer on a NiO Electrode

A Ru(II)-Re(I) supramolecular photocatalyst and a Ru(II) redox photosensitizer were both deposited successfully on a NiO electrode by using methyl phosphonic acid anchoring groups and the electrochemical polymerization of the ligand vinyl groups of the complexes. This new molecular photocathode, poly-RuRe/NiO, adsorbed a larger amount of the metal complexes compared to one using only methyl phosphonic acid anchor groups, and the stability of the complexes on the NiO electrode were much improved. The poly-RuRe/NiO acted as a photocathode for the photocatalytic reduction of CO₂ at E = -0.7 V vs Ag/AgCl under visible-light irradiation in an aqueous solution. The poly-RuRe/NiO produced approximately 2.5 times more CO, and its total Faradaic efficiency of the reduction products improved from 57 to 85%.

Synthesis and Optical Applications of Periodic Mesoporous Organosilicas

Periodic mesoporous organosilicas (PMOs), synthesized via surfactant-directed self-assembly of a polysilylated organic precursor (R[Si(OR')₃]_n; n≥2, R: organic group), are promising candidates such as catalysts and adsorbents, and for use in optical and electrical devices, owing to their high surface area, well-defined nanoporous structure, and highly functional organosilica framework. Their framework functionality can be widely tuned by selecting appropriate organic groups and controlling their arrangement. This chapter describes the synthesis and structure of PMOs with simple organic groups such as ethane and benzene, and the unique properties and optical applications of functional PMOs. Special light-harvesting properties and their exploitation in photocatalysis, highly emissive PMOs and their application to color-tunable transparent films, hole-transporting PMOs and their use in organic solar cells, and PMOs containing chelating ligands and their use as solid supports for heterogeneous metal complex catalysis are described.

Investigation of excited state, reductive quenching, and intramolecular electron transfer of Ru(ii)-Re(i) supramolecular photocatalysts for CO2 reduction using time-resolved IR measurements

Supramolecular photocatalysts in which Ru(ii) photosensitizer and Re(i) catalyst units are connected to each other by an ethylene linker are among the best known, most effective and durable photocatalytic systems for CO2 reduction. In this paper we report, for the first time, time-resolved infrared (TRIR) spectra of three of these binuclear complexes to uncover why the catalysts function so efficiently. Selective excitation of the Ru unit with a 532 nm laser pulse induces slow intramolecular electron transfer from the 3MLCT excited state of the Ru unit to the Re unit, with rate constants of (1.0-1.1) × 104 s-1 as a major component and (3.5-4.3) × 106 s-1 as a minor component, in acetonitrile. The produced charge-separated state has a long lifetime, with charge recombination rate constants of only (6.5-8.4) × 104 s-1. Thus, although it has a large driving force (-ΔG0CR ∼ 2.6 eV), this process is in the Marcus inverted region. On the other hand, in the presence of 1-benzyl-1,4-dihydronicotinamide (BNAH), reductive quenching of the excited Ru unit proceeds much faster (kq[BNAH (0.2 M)] = (3.5-3.8) × 106 s-1) than the abovementioned intramolecular oxidative quenching, producing the one-electron-reduced species (OERS) of the Ru unit. Nanosecond TRIR data clearly show that intramolecular electron transfer from the OERS of the Ru unit to the Re unit (kET > 2 × 107 s-1) is much faster than from the excited state of the Ru unit, and that it is also faster than the reductive quenching process of the excited Ru unit by BNAH. To measure the exact value of kET, picosecond TRIR spectroscopy and a stronger reductant were used. Thus, in the case of the binuclear complex with tri(p-fluorophenyl)phosphine ligands (RuRe(FPh)), for which intramolecular electron transfer is expected to be the fastest among the three binuclear complexes, in the presence of 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH), kET was measured as kET = (1.4 ± 0.1) × 109 s-1. This clearly shows that intramolecular electron transfer in these RuRe binuclear supramolecular photocatalysts is not the rate-determining process in the photocatalytic reduction of CO2, which is one of the main reasons why they work so efficiently.

Molecular engineering for efficient and selective iron porphyrin catalysts for electrochemical reduction of CO2 to CO

Iron porphyrins Fe-pE, Fe-mE, and Fe-oE were synthesized and their electrochemical behavior for CO₂ reduction to CO has been investigated. The controlled potential electrolysis of Fe-mE gave exclusive 65% Faradaic efficiency (FE) whereas Fe-oE achieved quasi-quantitative 98% FE (2% H₂) for CO production.

Heterogeneous catalysis based on supramolecular association

Non-covalent assembly of individual components can develop a material with activity to promote the transformation of substrates into products.

Interfacial Manipulation by Rutile TiO2 Nanoparticles to Boost CO2 Reduction into CO on a Metal-Complex/Semiconductor Hybrid Photocatalyst

Metal-complex/semiconductor hybrids have attracted attention as photocatalysts for visible-light CO₂ reduction, and electron transfer from the metal complex to the semiconductor is critically important to improve the performance. Here rutile TiO₂ nanoparticles having 5-10 nm in size were employed as modifiers to improve interfacial charge transfer between semiconducting carbon nitride nanosheets (NS-C₃N₄) and a supramolecular Ru(II)-Re(I) binuclear complex (RuRe). The RuRe/TiO₂/NS-C₃N₄ hybrid was capable of photocatalyzing CO₂ reduction into CO with high selectivity under visible light (λ > 400 nm), outperforming an analogue without TiO₂ by a factor of 4, in terms of both CO formation rate and turnover number (TON). The enhanced photocatalytic activity was attributed primarily to prolonged lifetime of free and/or shallowly trapped electrons generated in TiO₂/NS-C₃N₄ under visible-light irradiation, as revealed by transient absorption spectroscopy. Experimental results also indicated that the TiO₂ modifier served as a good adsorption site for RuRe, which resulted in the suppression of undesirable desorption of the complex, thereby contributing to the improved photocatalytic performance. This study presents the first successful example of interfacial manipulation in a metal-complex/semiconductor hybrid photocatalyst for improved visible-light CO₂ reduction to produce CO.

Insights into the Mechanism of a Covalently Linked Organic Dye-Cobaloxime Catalyst System for Dye-Sensitized Solar Fuel Devices

A covalently linked organic dye-cobaloxime catalyst system based on mesoporous NiO is synthesized by a facile click reaction for mechanistic studies and application in a dye-sensitized solar fuel device. The system is systematically investigated by photoelectrochemical measurements, density functional theory, time-resolved fluorescence, transient absorption spectroscopy, and photoelectron spectroscopy. The results show that irradiation of the dye-catalyst on NiO leads to ultrafast hole injection into NiO from the excited dye, followed by a fast electron transfer process to reduce the catalyst. Moreover, the dye adopts different structures with different excited state energies, and excitation energy transfer occurs between neighboring molecules on the semiconductor surface. The photoelectrochemical experiments also show hydrogen production by this system. The axial chloride ligands of the catalyst are released during photocatalysis to create the active sites for proton reduction. A working mechanism of the dye-catalyst system on the photocathode is proposed on the basis of this study.

Hybrid photocathode consisting of a CuGaO2 p-type semiconductor and a Ru(ii)-Re(i) supramolecular photocatalyst: non-biased visible-light-driven CO2 reduction with water oxidation

A CuGaO2 p-type semiconductor electrode was successfully employed for constructing a new hybrid photocathode with a Ru(ii)-Re(i) supramolecular photocatalyst (RuRe/CuGaO2). The RuRe/CuGaO2 photocathode displayed photoelectrochemical activity for the conversion of CO2 to CO in an aqueous electrolyte solution with a positive onset potential of +0.3 V vs. Ag/AgCl, which is 0.4 V more positive in comparison to a previously reported hybrid photocathode that used a NiO electrode instead of CuGaO2. A photoelectrochemical cell comprising this RuRe/CuGaO2 photocathode and a CoO x /TaON photoanode enabled the visible-light-driven catalytic reduction of CO2 using water as a reductant to give CO and O2 without applying any external bias. This is the first self-driven photoelectrochemical cell constructed with the molecular photocatalyst to achieve the reduction of CO2 by only using visible light as the energy source and water as a reductant.

Activation of the Carbon Nitride Surface by Silica in a CO-Evolving Hybrid Photocatalyst

Photocatalytic reduction of CO₂ to CO proceeded by visible light (λ>400 nm) using mesoporous graphitic carbon nitride (C₃ N₄ ) coupled with a Ru^II -Re^I binuclear complex (RuRe) containing a photosensitizer and catalytic units. The selectivity to CO exceeded 90 % during the initial stage. Photocatalytic reactions (including isotope tracer experiments) and electrochemical measurements revealed that the reaction proceeded according to a two-step photoexcitation of C₃ N₄ and the Ru^II photosensitizer unit, that is, it followed the Z-Scheme mechanism. Modification of C₃ N₄ with highly dispersed silica was found to improve the ability of C₃ N₄ to accommodate RuRe, which enhanced the photocatalytic activity for CO production.

A review of solar and visible light active oxo-bridged materials for energy and environment

Oxo-bridged systems are versatile photo-redox catalysts for environmental decontamination and artificial photosynthesis.

Visible-light-driven CO2 reduction on a hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON in aqueous solutions

A hybrid photocatalyst consisting of a Ru(ii) binuclear complex and a Ag-loaded TaON reduced CO2 by visible light even in aqueous solution. The distribution of the reduction products was strongly affected by the pH of the reaction solution. HCOOH was selectively produced in neutral conditions, whereas the formation of HCOOH competed with H2 evolution in acidic conditions. Detailed mechanistic studies revealed that the photocatalytic CO2 reduction proceeded via 'Z-schematic' electron transfer with step-by-step photoexcitation of TaON and the photosensitizer unit in the Ru(ii) binuclear complex. The maximum turnover number for HCOOH formation was 750 based on the Ru(ii) binuclear complex under visible-light irradiation, and the optimum external quantum efficiency of the HCOOH formation was 0.48% using 400 nm monochromic light with ethylenediaminetetraacetic acid disodium salt as a sacrificial reductant. Even in aqueous solution, the hybrid could also convert visible-light energy into chemical energy (ΔG0 = +83 kJ mol-1) by the reduction of CO2 to HCOOH with methanol oxidation.

Iridium(III) 1-Phenylisoquinoline Complexes as a Photosensitizer for Photocatalytic CO2 Reduction: A Mixed System with a Re(I) Catalyst and a Supramolecular Photocatalyst

An Ir(III) complex with 1-phenylisoquinoline (piq) ligands [Ir(piq)2(dmb)](+) (Ir, dmb = 4,4'-dimethyl-2,2'-bipyridine) exhibited strong absorption in the visible region, and the lifetime of its excited state was very long (τ = 2.8 μs). Photochemical reduction of Ir efficiently proceeded with 1-benzyl-1,4-dihydronicotinamide (BNAH) and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as reductants, giving the one-electron-reduced species (OERS), which was stable in solution at ambient temperature. The OERS of the Ir complex possessed strong reductive power, sufficient to supply an electron to fac-Re(dmb)(CO)3Br (Re). The photocatalytic reduction of CO2 proceeded efficiently using a mixed system constructed with Ir as a redox photosensitizer and Re as a catalyst, selectively giving CO (ΦCO = 0.16 using BNAH at λex = 480 nm). Ir was a more suitable photosensitizer for evaluating the activity of the Re catalyst in the photocatalytic reaction compared to [Ru(dmb)3](2+) (Ru) because the Ir complex was more stable in the photocatalytic reaction, and its decomposition products did not function as catalysts for CO2 reduction while the decomposition products of the Ru complex functioned as catalysts for the reduction of CO2 to HCOOH, inducing a drastic perturbation of the product distribution. A supramolecular photocatalyst (Ir-Re), in which the Ir(III) photosensitizer and the Re(I) catalyst were connected by a bridging ligand, was newly synthesized. When using BNAH, Ir-Re possessed a greater photocatalytic ability (ΦCO = 0.21, TONCO = 130) than the corresponding mixed system of the Ir and Re mononuclear complexes. Using BIH as the reductant, both Ir-Re and the mixed system showed very high photocatalytic activity (ΦCO = 0.40-0.41, TONCO = 1700).