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We have explored the structural factors on the photophysical properties in two rhenium(I) diimine complexes in acetonitrile solution, cis,trans-[Re(dmb)(CO)2(PPh2Et)2]+ (Et(2,2)) and cis,trans-[Re(dmb)(CO)2(PPh3)2]+ ((3,3)) (dmb = 4,4'-dimethyl-2,2'-bipyridine, Ph = phenyl, Et = ethyl) using the combination method of time-resolved infrared spectroscopy, time-resolved extended X-ray absorption fine structure, and quantum chemical calculations. The difference between these complexes is the number of phenyl groups in the phosphine ligand, and this only indirectly affects the central Re(I). Despite this minor difference, the complexes exhibit large differences in emission wavelength and excited-state lifetime. Upon photoexcitation, the bond length of Re-P and angle of P-Re-P are significantly changed in both complexes, while the phenyl groups are largely rotated by ∼20° only in (3,3). In contrast, there is little change in charge distribution on the phenyl groups when Re to dmb charge transfer occurs upon photoexcitation. We concluded that the instability from steric effects of phenyl groups and diimine leads to a smaller Stokes shift of the lowest excited triplet state (T1) in (3,3). The large structural change between the ground and excited states causes the longer lifetime of T1 in (3,3). more...

Photocatalytic reduction of CO2 using water as a reductant and visible light as the energy source is an important target in artificial photosynthesis. In this study, we report a highly stable photoelectrochemical CO2 reduction involving a new molecular photocathode comprising a NiO electrode and polymerized complexes of the Ru(II) photosensitizer and Ru(II) catalyst (NiO/PRu-poly-Ru-RuCAT1). CO and HCOOH were stably and selectively produced for over 100 h under visible light irradiation and a low bias. The turnover number of CO2 reduction products exceeded 1200, which represents the highest durability reported for photoelectrochemical reactions using molecular photocathodes. Moreover, a connected system of the NiO/PRu-poly-Ru-RuCAT1 molecular photocathode and a CoOx/BiVO4 photoanode suitable for water oxidation facilitated stable CO2 reduction with water as an electron donor and visible light as the energy source, with no bias for over 24 h. An energy conversion efficiency of 1.7 × 10−2% was obtained, which is the highest value reported for visible light driven systems utilizing a molecular photocatalyst for CO2 reduction with water as the reductant. more...

Ruthenium polypyridyl complexes have been widely studied to convert light energy to chemical or electric energy for various applications. To practically use the complexes, it is necessary to suppre... more...

International audience; Hybrid materials constructed from a visible-light-absorbing semiconductor and a functional metal complex have attracted attention as efficient photocatalysts for CO2 reduction with high selectivity to a desired product. In this work, defect fluorite-type Ln-Ta oxynitrides LnTaOxNy (Ln = Nd, Sm, Gd, Tb, Dy and Ho) were examined as the semiconductor component in a hybrid photocatalyst system combined with known Ag nanoparticle promoter and binuclear ruthenium(II) complex (RuRu'). Among the LnTaOxNy examined, TbTaOxNy gave the highest performance for CO2 reduction under visible light (λ > 400 nm), with a RuRu'-based turnover number of 18 and high selectivity to formate (>99%). Physicochemical analyses indicated that high crystallinity and more negative conduction band potential of LnTaOxNy with the absence of Ln-4f states in the band gap structure contributed to higher activity of the hybrid photocatalyst. more...

Supramolecular photocatalysts for CO2 reduction, constituted of redox photosensitizer, catalyst, and bridging ligand, play crucial roles in constructing hybrid systems with solid materials and phot... more...

Sensitization of a wide-gap oxide semiconductor with a visible-light-absorbing dye has been studied for decades as a means of producing H2 from water. However, efficient overall water splitting using a dye-sensitized oxide photocatalyst has remained an unmet challenge. Here we demonstrate visible-light-driven overall water splitting into H2 and O2 using HCa2Nb3O10 nanosheets sensitized by a Ru(II) tris-diimine type photosensitizer, in combination with a WO3-based water oxidation photocatalyst and a triiodide/iodide redox couple. With the use of Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous Al2O3 clusters as the H2 evolution component, the dye-based turnover number and frequency for H2 evolution reached 4580 and 1960 h-1, respectively. The apparent quantum yield for overall water splitting using 420 nm light was 2.4%, by far the highest among dye-sensitized overall water splitting systems reported to date. The present work clearly shows that a carefully designed dye/oxide hybrid has great potential for photocatalytic H2 production, and represents a significant leap forward in the development of solar-driven water splitting systems. more...

Supramolecular photocatalysts, which consist of redox photosensitizer and catalyst units, have recently attracted attention in the field of artificial photosynthesis. Aiming to construct a durable ... more...

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 compri... more...

Dye-sensitized solar cells and photocatalysts that consist of a light-absorbing dye and a wide gap oxide semiconductor substrate have been studied extensively as a means of solar energy conversion.... more...

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 ch... more...

Improvement in the photochemical formation efficiency of one-electron-reduced species (OERS) of a photoredox photosensitizer (a redox catalyst) is directly linked to the improvement in efficiencies of the various photocatalytic reactions themselves. We investigated the primary processes of a photochemical reduction of two series [Ru(diimine)3]2+ and [Os(diimine)3]2+ as frequently used redox photosensitizers (PS2+), by 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a typical reductant in detail using steady-irradiation and time-resolved spectroscopies. The rate constants of all elementary processes of the photochemical reduction of PS2+ by BIH to give the free PS•+ were obtained or estimated. The most important process for determining the formation efficiency of the free PS•+ was the escape yield from the solvated ion pair [PS•+-BIH•+], which was strongly dependent on both the central metal ion and the ligands. In cases with the same central metal ion, the system with larger -ΔGbet, which is the free energy change in the back-electron transfer from the OERS of PS•+ to BIH•+, tended to lower the escape yield of the free OERS of PS2+. On the other hand, different central metal ions drastically affected the escape yield even in cases with similar -ΔGbet; the escape yield in the case of RuH2+ (-ΔGbet = 1.68 eV) was 5-11 times higher compared to those of OsH2+ (-ΔGbet = 1.60 eV) and OsMe2+ (-ΔGbet = 1.71 eV). The back-electron transfer process from the free PS•+ to the free BIH•+ could not compete against the further reaction of the free BIH•+, which is the deprotonation process giving BI•, in DMA for all examples. The produced BI• gave one electron to PS2+ in the ground state to give another PS•+, quantitatively. Based on these findings and investigations, it is clarified that the photochemical formation efficiency of the free PS•+ should be affected not only by -ΔGbet but also by the heavy-atom effect of the central metal ion, and/or the oxidation power of the excited PS2+, which should determine the distance between the excited PS and BIH at the moment of the electron transfer. more...

Utilization of low concentration CO2 contained in the exhaust gases from various industries and thermal power stations without the need for energy-consuming concentration processes should be an important technology for solving global warming and the shortage of fossil resources. Here we report the direct electrocatalytic reduction of low concentration CO2 by a Re(I)-complex catalyst that possesses CO2-capturing ability in the presence of triethanolamine. The reaction rate and faradaic efficiency of CO2 reduction were almost the same when using Ar gas containing 10% CO2 or when using pure CO2 gas, and the selectivity of CO formation was very high (98% at 10% CO2). At a concentration of 1% CO2, the Re(I) complex still behaved as a good electrocatalyst; 94% selectivity of CO formation and 85% faradaic efficiency were achieved, and the rate of CO formation was 67% compared to that when using pure CO2 gas. The electrocatalysis was due to the efficient insertion of CO2 into the Re(I)–O bond in fac-[Re(dmb)(CO)3{OC2H4N(C2H4OH)2}] (dmb = 4,4′-dimethyl-2,2′-bipyridine). more...

CO2 capture at low concentration by catalysts is potentially useful for developing photocatalytic and electrocatalytic CO2 reduction systems. We investigated the CO2-capturing abilities of two complexes, fac-Mn(X2bpy)(CO)3(OCH2CH2NR2) and fac-Re(X2bpy)(CO)3(OCH2CH2NR2) (X2bpy = 4,4'-X2-2,2-bipyridine and R = -CH2CH2OH), which work as efficient catalysts for CO2 reduction. Both complexes could efficiently capture CO2 even from Ar gas containing only low concentration of CO2 such as 1% to be converted into fac-M(X2bpy)(CO)3(OC(O)OCH2CH2NR2) (M = Mn and Re). These CO2-capturing reactions proceeded reversibly and their equilibrium constants were >1000. The substituents of X2bpy strongly affected the CO2-capturing abilities of both Mn and Re complexes. The density functional theory (DFT) calculation could be used to estimate the CO2-capturing abilities of the metal complexes in the presence of triethanolamine. more...

The development of supramolecular photocatalysts has been an important target in the field of artificial photosynthesis because of their efficient reduction of CO2 and water oxidation and reduction, especially for solid materials, such as semiconductor particles, photoelectrodes, and mesoporous light-harvesting materials. We successfully developed the first supramolecular photocatalyst with a Rh complex as a catalyst unit for CO2 reduction. This new photocatalyst consisting of [Rh(BL)(Cp*)Cl]+ as the catalyst and [Ru(dmb)2(BL)]2+ as the photosensitizer (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl, BL = 1,2-bis(4′-methyl[2,2′-bipyridin]-4-yl)ethane, dmb = 4,4′-dimethyl-2,2′-bipyridine) mainly produced formic acid, even though a mixed system of the corresponding mononuclear complexes photocatalytically produced H2 as the main product. The photocatalytic reaction mechanism was investigated based on in situ UV–vis absorption studies and electrochemical measurements. more...

Trimethylamine N-oxide (Me3NO) could selectively remove only one CO ligand from fac-[Re(N^N)(CO)3(PR2R')]+ (N^N = diimine ligand), whereby only the CO ligand in the trans position to the phosphorus ligand was selectively removed to give cis,trans-[ReI(N^N)(CO)2(PR2R')(L)] n+ in good yields. This decarbonylation reaction using Me3NO was found to be especially useful for synthesizing biscarbonyl Re(I) complexes with electron-withdrawing groups in the diimine ligand, which could not be synthesized or were obtained only in low yields by the photochemical method. Me3NO also selectively removed the carbonyl ligands in the trans position to the phosphorus ligands from the edge Re(I) complex units, which have the fac-[Re(N^N)(CO)3(PR2R')]+ structure, in linear-shaped Re(I) multinuclear complexes. This reaction was successfully applied to synthesize a novel precursor with ring-shaped multinuclear Re complexes (Re-rings) comprising different kinds of Re(I) units. The newly synthesized Re-rings, which consist of one Re unit with a 4,4'-bis(trifluoromethyl)-2,2'-bipyridine (CF3bpy) ligand and one or two Re unit(s) with a 2,2'-bipyridine (bpy) ligand, showed almost quantitative excitation-energy harvesting ability from the Re unit(s) with bpy to that with CF3bpy. more...

The development of highly efficient, selective, and durable photocatalytic CO2 reduction systems that only use earth-abundant elements is key for both solving global warming and tackling the shortage of energy and carbon resources. Here, we successfully developed CO2 reduction photocatalysts using [Cu2(P2bph)2]2+ (CuPS) (P2bph = 4,7-diphenyl-2,9-di(diphenylphosphinotetramethylene)-1,10-phenanthroline) as a redox photosensitizer and fac-Mn(X2bpy)(CO)3Br (Mn(4X)) (X2bpy = 4,4′-X2-2,2′-bipyridine (X = −H and −OMe) or Mn(6mes) (6mes = 6,6′-(mesityl)2-2,2′-bipyridne)) as the catalyst. The most efficient photocatalysis was achieved by Mn(4OMe): The total quantum yield of CO2 reduction products was 57%, the turnover number based on the Mn catalyst was over 1300, and the selectivity of CO2 reduction was 95%. Electronic and steric effects of the substituents (X) in the Mn complexes largely affected both the photocatalytic efficiency and the product selectivity. For example, the highest selectivity of CO formation ... more...

Reduction of CO2 is one of the most important reactions that might solve the problems of global warming and shortage of fossil-fuel resources simultaneously. Metal complex catalysts are sometimes called molecular catalysts because they can be designed and synthesized on the molecular level. Some metal complexes are quite efficient and selective for CO2 reduction. Recently, such complexes have been applied as semiconductor photocatalysts to yield hybrid metal complex/semiconductor systems. Compared to heterogeneous catalysts, metal complexes are more advantageous for elucidating reaction mechanisms. This review summarizes the reaction mechanisms that have been proposed for the photochemical CO2 reduction reaction catalyzed by rhenium and ruthenium complexes. Rhenium complexes efficiently reduce CO2 to selectively produce CO under various reaction conditions. On the other hand, ruthenium complexes yield CO and HCOOH, and the product selectivities are strongly dependent on the reaction conditions. Numerous reaction mechanisms have been proposed; however, no universal mechanism that can completely explain the activities and product selectivities of these catalysts exists. Why are two important intermediates, the η1-CO2 adduct and the hydride complex, proposed? How does the η1-CO2 adduct produce HCOOH? Does the hydride complex yield CO via the formate complex? What is the second electron source for the intermediate that produces CO and HCOOH? This review highlights what is already known about photocatalytic CO2 reduction reaction mechanisms and what remains to be clarified. more...

The Z-scheme CO2 reduction activity of metal complex–semiconductor hybrid photocatalysts was investigated in detail with a focus on the interfacial electron transfer process. Semiconductors of GaN:... more...

Graphitic carbon nitride (g-C3N4) was synthesized by heating urea at different temperatures (773–923 K) in air, and was examined as a photocatalyst for CO2 reduction. With increasing synthesis temperature, the conversion of urea into g-C3N4 was facilitated, as confirmed by X-ray diffraction, FTIR spectroscopy and elemental analysis. The as-synthesized g-C3N4 samples, further modified with Ag nanoparticles, were capable of reducing CO2 into formate under visible light (λ > 400 nm) in the presence of triethanolamine as an electron donor, with the aid of a molecular Ru(II) cocatalyst (RuP). The CO2 reduction activity was improved by increasing the synthesis temperature of g-C3N4, with the maximum activity obtained at 873–923 K. This trend was also consistent with that observed in photocatalytic H2 evolution using Pt-loaded g-C3N4. The photocatalytic activities of RuP/g-C3N4 for CO2 reduction and H2 evolution were thus shown to be strongly associated with the generation of the crystallized g-C3N4 phase. more...

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%. more...

We have previously developed photocatalytic CO2 reduction systems using graphitic carbon nitride (g-C3N4) and a Ru(II) mononuclear complex (e.g., trans(Cl)–[RuII{4,4′-(H2PO3)2bpy}2(CO)2Cl2] bpy = 2,2′-bipyridine, abbreviated as RuP) hybrids and demonstrated its high activities under visible light (λ > 400 nm). To understand the excited-state dynamics of C3N4 and electron-transfer process to RuP, here we examined the photophysical properties of g-C3N4 as well as mesoporous g-C3N4 (mpg-C3N4) by means of time-resolved emission and/or time-resolved infrared absorption (TR-IR) spectroscopy. The emission decay measurements showed that g-C3N4 (as well as mpg-C3N4) has at least three emissive excited states with different lifetimes (g-C3N4; 1.3 ± 0.4, 3.9 ± 0.9, and 15 ± 4 ns at 269 nm photoexcitation) in aqueous suspension. These excited states were not quenched upon addition of a hole scavenger (e.g., disodium dihydrogen ethylenediamine tetraacetate dehydrate) and/or an electron acceptor (RuP), even though phot... more...

Efficient and selective photostimulated CO2-to-CO reduction by a photocatalytic system consisting of an iron-complex catalyst and a mesoporous graphitic carbon nitride (mpg-C3N4) redox photosensitizer is reported for the first time. Irradiation in the visible region (λ ≥ 400 nm) of an CH3CN/triethanolamine (4:1, v/v) solution containing [Fe(qpy)(H2O)2]2+ (qpy = 2,2′:6′,2′′:6′′,2′′-quaterpyridine) and mpg-C3N4 resulted in CO evolution with 97% selectivity, a turnover number of 155, and an apparent quantum yield of ca. 4.2%. This hybrid catalytic system, comprising only earth abundant elements, opens new perspectives for solar fuels production using CO2 as a renewable feedstock. more...

Oxynitrides are promising visible-light-responsive photocatalysts, but their structures are almost confined with three-dimensional (3D) structures such as perovskites. A phase-pure Li2 LaTa2 O6 N with a layered perovskite structure was successfully prepared by thermal ammonolysis of a lithium-rich oxide precursor. Li2 LaTa2 O6 N exhibited high crystallinity and visible-light absorption up to 500 nm. As opposed to well-known 3D oxynitride perovskites, Li2 LaTa2 O6 N supported by a binuclear RuII complex was capable of stably and selectively converting CO2 into formate under visible light (λ>400 nm). Transient absorption spectroscopy indicated that, as compared to 3D oxynitrides, Li2 LaTa2 O6 N possesses a lower density of mid-gap states that work as recombination centers of photogenerated electron/hole pairs, but a higher density of reactive electrons, which is responsible for the higher photocatalytic performance of this layered oxynitride. more...

Photofunctional trinuclear complexes containing three different central metals, i.e. Os(ii), Re(i) and Ru(ii), were synthesised for the first time using stepwise Mizoroki-Heck reactions. The vinylene groups in the bridging ligands of the Os(ii)-Re(i)-Ru(ii) trinuclear complexes were selectively reduced by photochemical hydrogenation in moderate yield, affording novel supramolecular photocatalysts which can absorb a wide range of visible light up to 730 nm and induce CO2 reduction with high selectivity and durability. The turnover numbers of CO formation were over 4300. Details of the photophysical properties of these new trinuclear complexes, especially their intramolecular excitation-energy transfer phenomena, are also reported. more...

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. more...

Hybrid photocatalysts constructed with a mononuclear Ru(II)-complex (RuP), silver nanoparticles, and carbon nitride nanosheets (NS-C3N4) photocatalyze CO2 reduction to selectively form formate under visible light. The structure of the nanoparticulate silver species, which worked as promoters for the reaction, was characterized by X-ray diffraction, UV-VIS diffuse reflectance spectroscopy, high-resolution transmission microscopy, and X-ray absorption fine-structure spectroscopy. The silver promoters were loaded on the surface of NS-C3N4 by an impregnation method from an aqueous solution containing AgNO3 or an in situ photodeposition method. Impregnation of NS-C3N4 with 2.0 wt% Ag followed by reduction with H2 at 473 K (further modified with RuP) resulted in the highest photocatalytic activity, giving a turnover number of 5700 (based on RuP), which was the greatest value among the formate-generating hybrid systems with a mononuclear complex. While the optimized photocatalyst contained highly dispersed Ag2O-like nanoclusters as the major silver species, experimental results suggested that highly dispersed Ag0 species are more important for enhancing CO2 reduction activity, that is, the obtained experimental results led us to conclude that there are two major factors affecting activity: one is the feature size of silver species (smaller is better), and the other is the oxidation state of silver (metallic is better). more...

Reduction of CO2 using water as an electron donor could be deemed one of the most important reactions in artificial photosynthesis. In the current study, electrochemical properties of a water-soluble rhenium(I) tricarbonyl complex with hydroxymethyl groups ([Re(CH2OH)–OH2]+) and its electrocatalysis for CO2 reduction were investigated in aqueous solutions. In comparison with general rhenium(I) carbonyl complexes, [Re(CH2OH)–OH2]+ shows high solubility in water even in the presence of both CO2 and OH–. In an aqueous solution, [Re(CH2OH)–OH2]+ exhibited a catalytic current due to CO2 reduction in a cyclic voltammogram at a positive potential of about 400 mV more than that in a DMF solution. It is suggested that two molecules of a one-electron-reduced Re complex participated in reducing one molecule of CO2 in the aqueous solution. Electrolysis using [Re(CH2OH)–OH2]+ at −1.1 V vs NHE in a CO2-saturated aqueous solution afforded CO and HCOOH with 95 and 4% selectivity, respectively, at pH 6.9; thermodynamicall... more...

The photophysical and photochemical properties of transition metal complexes have attracted considerable attention because of their recent applications as photocatalysts in artificial photosynthesis and organic synthesis, as light emitters in electroluminescent (EL) devices, and as dyes in solar cells. The general control methods cannot be always used to obtain transition metal complexes with photochemical properties that are suitable for the above-mentioned applications. In the fields of solar energy conversion, strong metal-to-ligand charge-transfer (MLCT) absorption of redox photosensitizers and/or photocatalysts in the visible region with long wavelength is essential. However, the usual methods, i.e., introduction of electron-withdrawing groups into the electron-accepting ligand and/or weak-field ligands into the central metal, have several drawbacks, including shorter excited-state lifetime, lower emission efficiency, and lower oxidation and reduction power. Herein we describe a new method to control the photophysical, photochemical, and electrochemical properties of Re(I) diimine carbonyl complexes that have been widely used in various fields such as photocatalysts for CO more...

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... more...

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 CoOx/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. more...

Carbon nitride nanosheets (NS-C3N4) were found to undergo robust binding with a binuclear ruthenium(II) complex (RuRu′) even in basic aqueous solution. A hybrid material consisting of NS-C3N4 (further modified with nanoparticulate Ag) and RuRu′ promoted the photocatalytic reduction of CO2 to formate in aqueous media, in conjunction with high selectivity (approximately 98 %) and a good turnover number (>2000 with respect to the loaded Ru complex). These represent the highest values yet reported for a powder-based photocatalytic system during CO2 reduction under visible light in an aqueous environment. We also assessed the desorption of RuRu′ from the Ag/C3N4 surface, a factor that can contribute to a loss of activity. It was determined that desorption is not induced by salt additives, pH changes, or photoirradiation, which partly explains the high photocatalytic performance of this material. more...

International audience; Achieving visible-light-driven carbon dioxide reduction with high selectivity control and durability while using only earth abundant elements requires new strategies. Hybrid catalytic material was prepared upon covalent grafting a Co–quaterpyridine molecular complex to semiconductive mesoporous graphitic carbon nitride (mpg-C3N4) through an amide linkage. The molecular material was characterized by various spectroscopic techniques, including XPS, IR, and impedance spectroscopy. It proved to be a selective catalyst for CO production in acetonitrile using a solar simulator with a high 98% selectivity, while being remarkably robust since no degradation was observed after 4 days of irradiation (ca. 500 catalytic cycles). This unique combination of a selective molecular catalyst with a simple and robust semiconductive material opens new pathways for CO2 catalytic light-driven reduction. more...

A hybrid photocatalyst constructed with Ta3N5 and a binuclear ruthenium(II) complex had the ability to reduce CO2 into formate with >99% selectivity under visible light (λ > 480 nm), presenting the... more...

Ring-shaped multinuclear Re(I) complexes (Re(I) rings) have the potential to function as useful units in multifarious photofunctional compounds because of their flexibility in molecular design and various photofunctions. The first synthetic example of the coupling reaction using a Re(I) ring as a building block to synthesize a novel spectacle-shaped hexanuclear complex is reported herein. This complex shows an efficient intramolecular energy transfer to accumulate the excitation energy into the central Re(I) units, and has suitable photophysical properties as a photosensitizer. more...

The visible-light responsive Cu(I)-complex photosensitizers were developed by introducing various aromatic substituents at the 4,7-positions of a 2,9-dimethyl-1,10-phenanthroline (dmp) ligand in a heteroleptic CuI(dmp)(DPEphos)+-type complexes (DPEphos = [2-(diphenylphosphino)phenyl]ether) for photocatalytic CO2 reduction. Introducing biphenyl groups (Bp-) on the dmp ligand enhanced the molar extinction coefficient (e) of the metal-to-ligand charge transfer (MLCT) band in the visible region (e = 7,500 M-1cm-1) compared to that of the phenyl (Ph-)-containing analog (e = 5,700 M-1cm-1 at λmax = 388 nm). However, introducing 4-R-Ph- groups (R = the electron-withdrawing groups NC-, or NO2-) led to a red shift in the band to λmax = 390, 400, and 401 nm, respectively. Single-crystal X-ray analysis showed the Ph- groups were twisted because of the steric repulsion between the 2,6-protons of the Ph- groups and 5,6-protons of the dmp ligand. The result strongly indicated that the π-conjugation effect of the 4-R-Ph- groups is so weak that the lowering of the energy of the dmp π* orbitals is small. However, when 4-R-ph- was substituted by a 5-membered heterorings, there was a larger red shift, leading to an increase in the e value of the MLCT absorption band. Thus, the substitution to 2-benzofuranyl- groups resulted in visible-light absorption up to 500 nm and a shoulder peak at around 420 nm (e = 12,300 M-1cm-1) due to the expansion of π-conjugation over the substituted dmp ligand. The photocatalytic reaction for CO2 reduction was tested using the obtained CuI complexes as photosensitizers in the presence of a Fe(dmp)2(NCS)2 catalyst and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole as a sacrificial reductant, which showed improved CO generation. more...

International audience; A push-pull organic dye and a cobaloxime catalyst were successfully co-grafted on NiO and CuGaO$_2$ to form efficient molecular photocathodes for H$_2$ production with >80% Faradaic efficiency. CuGaO$_2$ is emerging as a more effective p-type semiconductor in photoelectrochemical cells and yields a photocathode with four-fold higher photocurrent densities and 400 mV more positive onset photocurrent potential compared to the one based on NiO. Such an optimized CuGaO$_2$ photocathode was combined with a TaONǀCoO$_x$ photoanode in a photoelectrochemical cell. Operated in this Z-scheme configuration, the two photoelectrodes produced H$_2$ and O$_2$ from water with 87% and 88% Faradaic efficiency, respectively, at pH 7 under visible light and in the absence of an applied bias, equating to a solar to hydrogen conversion efficiency of 5.4×10$^{−3}$%. This is, to the best of our knowledge, the highest efficiency reported so far for a molecular-based noble metal-free water splitting Z-scheme. more...

Ruthenium(II) picolinate complex, [Ru(dmb)2(pic)]+ (Ru(pic); dmb = 4,4′-dimethyl-2,2′-bipyridine; Hpic = picolinic acid) was newly synthesized as a potential redox photosensitizer with a wider wavelength range of visible-light absorption compared with [Ru(N∧N)3]2+ (N∧N = diimine ligand), which is the most widely used redox photosensitizer. Based on our investigation of its photophysical and electrochemical properties, Ru(pic) was found to display certain advantageous characteristics of wide-band absorption of visible light (λabs < 670 nm) and stronger reduction ability in a one-electron reduced state (E1/2red = −1.86 V vs. Ag/AgNO3), which should function favorably in photon-absorption and electron transfer to the catalyst, respectively. Performing photocatalysis using Ru(pic) as a redox photosensitizer combined with a Re(I) catalyst reduced CO2 to CO under red-light irradiation (λex > 600 nm). TONCO reached 235 and ΦCO was 8.0%. Under these conditions, [Ru(dmb)3]2+ (Ru(dmb)) is not capable of working as a redox photosensitizer because it does not absorb light at λ > 560 nm. Even in irradiation conditions where both Ru(pic) and Ru(dmb) absorb light (λex > 500 nm), using Ru(pic) demonstrated faster CO formation (TOFCO = 6.7 min−1) and larger TONCO (2347) than Ru(dmb) (TOFCO = 3.6 min−1; TONCO = 2100). These results indicate that Ru(pic) is a superior redox photosensitizer over a wider wavelength range of visible-light absorption. more...

A cyclometalated iridium(III) complex having 2-(pyren-1-yl)-4-methylquinoline ligands (Ir(pyr)) has a strong absorption band in the visible region (e444nm = 67,000 M−1cm−1), but does not act as a photosensitizer for photochemical reduction reactions in the presence of triethylamine as the electron donor. Here, 1,3-dimethyl-2-(o-hydroxyphenyl)-2,3-dihydro-1H-benzo[d]imidazole (BI(OH)H) was used instead of the amine, demonstrating that BI(OH)H efficiently quenched the excited state of Ir(pyr) and can undergo the photochemical carbon dioxide (CO2) reduction catalyzed by trans(Cl)-Ru(dmb)(CO)2Cl2 (dmb = 4,4’-dimethyl-2,2’-bipyridine, Ru) to produce formate as the main product. We also synthesized a binuclear complex combining Ir(pyr) and Ru via an ethylene bridge and investigated its photochemical CO2 reduction activity in the presence of BI(OH)H more...

This account looks comprehensively at the syntheses, photophysical properties and applications of structurally diverse, multinuclear coordination compounds that embed a Re(i)-diimine-carbonyl moiety. Different types of linkages in terms of configuration and proper bridging lead to several types of discrete linear or cyclic macromolecular aggregates with different sizes, which consequently have a significant effect on their properties. The Re-backbone in the multinuclear assembly may have only a structural role and mainly exhibits host-guest behaviour for encapsulation, or it may produce photofunctional properties as a direct result of the Re(i) chromophoric unit, which makes these compounds applicable in light-harvesting systems or photochemical CO2 reduction. In particular, we would like to emphasize bisphosphine-bridged multinuclear complexes that exhibit outstanding photophysical properties. more...

We have successfully demonstrated solar water splitting using a newly fabricated photoelectrochemical system with a Pt-loaded SiC photocathode, a CoOx -loaded BiVO4 photoanode, and a perovskite solar cell. Detection of the evolved H2 and O2 with a 100 % Faradaic efficiency indicates that the observed photocurrent was used for water splitting. The solar-to-hydrogen (STH) efficiency was 0.55 % under no additional bias conditions. more...

Photocatalytic reduction of CO2 into energy-rich compounds utilizing solar light as an energy source is expected to provide a solution to serious problems of the shortage of fossil resources and global warming. In this perspective, we summarize advances in supramolecular photocatalysts for the reduction of CO2, of which photosensitizer and catalyst units are connected via a bridging ligand. The first successful Ru(II)–Re(I) supramolecular photocatalysts reported in 2005 indicated molecular architecture for developing efficient supramolecular photocatalysts for CO2 reduction to CO with high selectivity and durability. On the basis of this architecture, both the bridging ligands and Re(I) catalyst unit were optimized to increase the photocatalytic activity. In addition, the compositional units of supramolecular photocatalytic systems were modified: (1) Ir(III) and Os(II) complexes, free- and metallo-porphyrins, and chlorophyll functioned as alternative or better photosensitizer units in comparison to the Ru... more...

The addition of a tertiary phosphine and O2 to reaction solutions strongly affected the reactivity and selectivity of coupling reactions between transition metal complexes. The Mizoroki–Heck reaction between metal complexes with bromo and those with vinyl groups in the diimine ligand did not proceed using Pd(OAc)2 in the presence of 2-dicyclohexylphosphino-2′,6′-dimethoxybiphenyl (Sphos) under Ar but proceeded selectively after injection of air into the reaction vessel. In the absence of the phosphine ligand, on the other hand, not only the Mizoroki–Heck reaction but also a homo-coupling reaction between the metal complexes with the bromo groups proceeded at the same time. Mechanistic investigation showed that nanoparticles of Pd species were produced in the absence of the phosphine ligand and worked as catalysts for both the Mizoroki–Heck and homo-coupling reactions. On the other hand, larger Pd particles, which were produced in the presence of Sphos but after addition of air for oxidising Sphos, selectively catalysed the Mizoroki–Heck reaction. ‘Molecular’ Pd species that were stabilised in the presence of non-oxidised Sphos could not catalyse both coupling reactions under the reaction conditions. Based on these results, reaction conditions were established for the selective progress of the Mizoroki–Heck and the homo-coupling reactions. more...

New supramolecular photocatalysts comprising an asymmetric bis-tridentate Ru(ii) complex that functions as a photosensitizer and a Ru(ii) carbonyl complex as the catalyst were designed. The complexes photocatalyzed the reduction of CO2 to CO or formic acid with high selectivity. The product distribution depended on the catalyst unit. CO and formic acid were the main products when using [Ru(BL)(Clbpy)(CO)]2+ (BL = bridging ligand, Clbpy = 4,4′-dichloro-2,2′-bipyridine) and Ru(BL)(CO)2Cl2 catalysts, respectively. more...

A visible-light-driven water-splitting system that involves two-step photoexcitation (Z-scheme) was constructed using rutile TiO2 nanorod doped with Ta and N (TiO2:Ta/N) as an O2 evolution photocatalyst. The Ta-doped TiO2 nanorods, prepared by a solvothermal synthesis, underwent nitridation to possess visible-light absorption under mild conditions, even at 623 K under an ammonia flow. The TiO2:Ta/N powders modified with a RuO2 cocatalyst were active under visible light up to 540 nm for water oxidation for producing O2 in the presence of reversible electron acceptors (IO3− or Fe3+), while TiO2:N exhibited negligible activity. The results of time-resolved infrared absorption spectroscopy indicated that co-doping Ta with N into TiO2 prolonged the lifetime of photogenerated free electrons, leading to high photocatalytic activity. Simultaneous H2 and O2 evolution via water splitting was achieved using a combination of RuO2-modified TiO2:Ta/N, Ru-loaded SrTiO3:Rh and an Fe3+/Fe2+ redox couple under visible-light irradiation (λ > 420 nm) and under AM 1.5G simulated sunlight. more...

Electrochemical and photochemical reduction of CO2, or a smart combination of both, are appealing approaches for the storage of renewable, intermittent energies and may lead to the production of fuels and of value-added chemicals. By using only earth-abundant metal (Cu, Ni, Co, Mn, Fe) complexes, cheap electrodes and/or cheap sacrificial electron donors and visible light sensitizers, systems functioning with molecular catalysts have been recently designed, showing promising results, in particular, for the two-electron reduction of the carbon dioxide. By combining experimental and mechanistic studies, key parameters controlling the catalysis efficiency have been deciphered, opening the way to the design of future, more efficient and durable catalysts, as well as to the development of electrochemical or photoelectrochemical cells, all being key steps for the emergence of applied devices. The most recent advances related to these issues are discussed in this review. more...

Vinylene or ethynylene linkers in the bridging ligands of photofunctional multinuclear complexes synthesized by various coupling reactions, such as the Mizoroki-Heck reaction, olefin metathesis, and Sonogashira coupling, were successfully converted to their corresponding saturated carbon chains using photochemical hydrogenation, which proceeded in an MeCN-pyridine-CF3COOH (3:1:0.1 v/v/v) mixed solution containing the starting metal complexes and 1,3-dimethyl-2-phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as a sacrificial electron donor under visible light irradiation in high yields. Hydrogenation of linkers in a Ru2-Re trinuclear complex improved the photocatalytic ability for CO2 reduction. more...

A novel molecular photocatalytic system with not only high reduction ability of CO2 but also high capture ability of CO2 has been developed using a Ru(II)–Re(I) dinuclear complex as a photocatalyst. By using this photocatalytic system, CO2 of 10% concentration could be selectively converted to CO with almost same photocatalysis to that under a pure CO2 atmosphere (TONCO > 1000, ΦCO > 0.4). Even 0.5% concentration of CO2 was reduced with 60% initial efficiency of CO formation by using the same system compared to that using pure CO2 (TONCO > 200). The Re(I) catalyst unit in the photocatalyst can efficiently capture CO2, which proceeds CO2 insertion to the Re–O bond, and then reduce the captured CO2 by using an electron supplied from the photochemically reduced Ru photosensitizer unit. more...