Tomoyoshi Suenobu

Long-Lived Charge Separation and Applications in Artificial Photosynthesis

Researchers have long been interested in replicating the reactivity that occurs in photosynthetic organisms. To mimic the long-lived charge separations characteristic of the reaction center in photosynthesis, researchers have applied the Marcus theory to design synthetic multistep electron-transfer (ET) systems. In this Account, we describe our recent research on the rational design of ET control systems, based on models of the photosynthetic reaction center that rely on the Marcus theory of ET. The key to obtaining a long-lived charge separation is the careful choice of electron donors and acceptors that have small reorganization energies of ET. In these cases, the driving force of back ET is located in the Marcus inverted region, where the lifetime of the charge-separated state lengthens as the driving force of back ET increases. We chose porphyrins as electron donors and fullerenes as electron acceptors, both of which have small ET reorganization energies. By linking electron donor porphyrins and electron acceptor fullerenes at appropriate distances, we achieved charge-separated states with long lifetimes. We could further lengthen the lifetimes of charge-separated states by mixing a variety of components, such as a terminal electron donor, an electron mediator, and an electron acceptor, mimicking both the photosynthetic reaction center and the multistep photoinduced ET that occurs there. However, each step in multistep ET loses a fraction of the initial excitation energy during the long-distance charge separation. To overcome this drawback in multistep ET systems, we used designed new systems where we could finely control the redox potentials and the geometry of simple donor-acceptor dyads. These modifications resulted in a small ET reorganization energy and a high-lying triplet excited state. Our most successful example, 9-mesityl-10-methylacridinium ion (Acr(+)-Mes), can undergo a fast photoinduced ET from the mesityl (Mes) moiety to the singlet excited state of the acridinium ion moiety (Acr(+)) with extremely slow back ET. The high-energy triplet charge-separated state is located deep in the Marcus inverted region, and we have detected the structural changes during the photoinduced ET in this system using X-ray crystallography. To increase the efficiency of both the light-harvesting and photoinduced ET, we assembled the Acr(+)-Mes dyads on gold nanoparticles to bring them in closer proximity to one another. We can also incorporate Acr(+)-Mes molecules within nanosized mesoporous silica-alumina. In contrast to the densely assembled dyads on gold nanoparticles, each Acr(+)-Mes molecule in silica-alumina is isolated in the mesopore, which inhibits the bimolecular back ET and leads to longer lifetimes in solution at room temperature than the natural photosynthetic reaction center. Acr(+)-Mes and related compounds act as excellent organic photocatalysts and facilitate a variety of reactions such as oxygenation, bromination, carbon-carbon bond formation, and hydrogen evolution reactions.

Catalytic Mechanism of Water Oxidation with Single-Site Ruthenium–Heteropolytungstate Complexes

Catalytic water oxidation to generate oxygen was achieved using all-inorganic mononuclear ruthenium complexes bearing Keggin-type lacunary heteropolytungstate, [Ru(III)(H(2)O)SiW(11)O(39)](5-) (1) and [Ru(III)(H(2)O)GeW(11)O(39)](5-) (2), as catalysts with (NH(4))(2)[Ce(IV)(NO(3))(6)] (CAN) as a one-electron oxidant in water. The oxygen atoms of evolved oxygen come from water as confirmed by isotope-labeled experiments. Cyclic voltammetric measurements of 1 and 2 at various pHs indicate that both complexes 1 and 2 exhibit three one-electron redox couples based on ruthenium center. The Pourbaix diagrams (plots of E(1/2) vs pH) support that the Ru(III) complexes are oxidized to the Ru(V)-oxo complexes with CAN. The Ru(V)-oxo complex derived from 1 was detected by UV-visible absorption, EPR, and resonance Raman measurements in situ as an active species during the water oxidation reaction. This indicates that the Ru(V)-oxo complex is involved in the rate-determining step of the catalytic cycle of water oxidation. The overall catalytic mechanism of water oxidation was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates. Complex 2 exhibited a higher catalytic reactivity for the water oxidation with CAN than did complex 1.

Water-soluble mononuclear cobalt complexes with organic ligands acting as precatalysts for efficient photocatalytic water oxidation

The photocatalytic water oxidation to evolve O2 was performed by photoirradiation (λ > 420 nm) of an aqueous solution containing [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine), Na2S2O8 and water-soluble cobalt complexes with various organic ligands as precatalysts in the pH range of 6.0–10. The turnover numbers (TONs) based on the amount of Co for the photocatalytic O2 evolution with [CoII(Me6tren)(OH2)]2+ (1) and [CoIII(Cp*)(bpy)(OH2)]2+ (2) [Me6tren = tris(N,N′-dimethylaminoethyl)amine, Cp* = η5-pentamethylcyclopentadienyl] at pH 9.0 reached 420 and 320, respectively. The evolved O2 yield increased in proportion to concentrations of precatalysts 1 and 2 up to 0.10 mM. However, the O2 yield dramatically decreased when the concentration of precatalysts 1 and 2 exceeded 0.10 mM. When the concentration of Na2S2O8 was increased from 10 mM to 50 mM, CO2 evolution was observed during the photocatalytic water oxidation. These results indicate that a part of the organic ligands of 1 and 2 were oxidized to evolve CO2 during the photocatalytic reaction. The degradation of complex 2 under photocatalytic conditions and the oxidation of Me6tren ligand of 1 by [Ru(bpy)3]3+ were confirmed by 1H NMR measurements. Dynamic light scattering (DLS) experiments indicate the formation of particles with diameters of around 20 ± 10 nm and 200 ± 100 nm during the photocatalytic water oxidation with 1 and 2, respectively. The particle sizes determined by DLS agreed with those of the secondary particles observed by TEM. The XPS measurements of the formed particles suggest that the surface of the particles is covered with cobalt hydroxides, which could be converted to active species containing high-valent cobalt ions during the photocatalytic water oxidation. The recovered nanoparticles produced from 1 act as a robust catalyst for the photocatalytic water oxidation.

Cupric superoxo-mediated intermolecular C-H activation chemistry.

The new cupric superoxo complex [LCu(II)(O(2)(•-))](+), which possesses particularly strong O-O and Cu-O bonding, is capable of intermolecular C-H activation of the NADH analogue 1-benzyl-1,4-dihydronicotinamide (BNAH). Kinetic studies indicated a first-order dependence on both the Cu complex and BNAH with a deuterium kinetic isotope effect (KIE) of 12.1, similar to that observed for certain copper monooxygenases.

Photocatalytic Production of Hydrogen by Disproportionation of One‐Electron‐Reduced Rhodium and Iridium–Ruthenium Complexes in Water

Photocatalytic production of hydrogen (H2) has attracted increasing attention, because H2 is a clean energy source for the future to reduce dependence on fossil fuels and emissions of greenhouse gases in the long term. 2] Extensive efforts have explored the development of photocatalytic H2 evolution systems, which consist of an electron donor, a photosensitizer, an electron mediator such as methyl viologen (MV), and a hydrogen-evolution catalyst. Both heterogeneous catalysts such as platinum nanoclusters and molecular homogeneous catalysts have been used for the photocatalytic hydrogen evolution. Organorhodium complexes were found to be useful in place of colloidal platinum for TiO2 photosensitized hydrogen evolution. However, the detailed photocatalytic mechanism of one-electron transfer among components has yet to be elucidated, because the twoelectron reduction of protons is required for H2 production. In particular, it should be clarified how photoinduced electron transfer of a photosensitizer (a one-electron process) leads to H2 production (a two-electron process) to improve the photocatalytic efficiency of H2 production. We report herein that water-soluble transition-metal complexes [Rh(Cp*)(bpy)(H2O)](SO4) (1; Cp* = h C5Me5, bpy = 2,2’-bipyridine) [16] and [Ir(Cp*)(H2O)(bpm)Ru(bpy)2](SO4)2 (2 ; bpm = 2,2’-bipyrimidine) [17] act as efficient catalysts for photocatalytic hydrogen evolution in water at room temperature. The detailed kinetic analysis and detection of the intermediates, such as a one-electronreduced complex, provide valuable insight into the mechanism of the photocatalytic hydrogen evolution, in which the disproportionation of the one-electron-reduced complexes is the key step for the two-electron reduction of protons to H2. Photocatalytic hydrogen evolution experiments were performed with [Ru(bpy)3] 2+ (2.0 10 m), a catalyst (1: 1.0 10 m, 2 : 1.0 10 m), ascorbic acid (H2A, 0.8m), and sodium ascorbate (NaHA, 0.3m) at various pH values under irradiation with visible light (l> 430 nm). No hydrogen evolution was detected in the absence of [Ru(bpy)3] 2+ under otherwise identical experimental conditions. The formed hydrogen was identified and quantified by gas chromatography. The amount of evolved H2 with 2 (1.0 10 m) at pH 3.6 increased with irradiation time, and the turnover number (TON) per molecule of 2 reached 410 in 100 min (Figure S1 in the Supporting Information). The quantum yield F of the photocatalytic hydrogen evolution was determined from the number of absorbed photons and the maximum hydrogen production rate using monochromatized light at l = 450 nm (see the Experimental Section). Under the present reaction conditions, in which the concentration of [Ru(bpy)3] 2+ (2.0 10 m) is 20 times larger than the concentration of 2 (1.0 10 m), light is mainly absorbed by [Ru(bpy)3] 2+ (lmax = 450 nm, e450nm = 1.4 10m 1 cm ) rather than 2 (lmax = 414 nm, 575 nm; e450nm = 5.2 10m 1 cm ; see Figure S2 in the Supporting Information for the absorption spectra of [Ru(bpy)3] 2+ and 2). The pH dependence of F is shown in Figure 1, where the maximum F value (1.5 10 ) is achieved at pH 3.6 for catalyst 2. The decrease of the F value when the pH value is decreased below 3.6 suggests that ascorbate ion (HA ) rather than H2A acts as an electron donor, because the pKa value of H2A is 4.0. [18] The sharp decline in the F value when the pH value is increased above 3.6 may be explained by the formation of the low-valent iridium complex [Ir(Cp*)(H2O)(bpm)Ru(bpy)2] , which has no catalytic activity for hydrogen evolution, because the pKa value of the iridium hydride complex derived from 2 ([Ir(Cp*)(H)(bpm)Ru(bpy)2] ) is 3.9. The lower F value at pH 3.6 for catalyst 1 (Figure 1) is consistent with the lower catalytic activity of 1 for H2 evolution in the decomposition of formic acid. The decrease of the F value for complex 1 beyond pH 3.6 can be ascribed to the decrease of the concentration of protons, which react with the corresponding hydride [Rh(Cp*)(bpy)(H)]. It should be noted that no H2 was produced when 2 was replaced by [Ir(Cp*)(bpy)(H2O)](SO4) without the {Ru(bpy)2} unit, because the hydride complex [Ir (Cp*)(bpy)(H)] is stable at pH 3.6. It is well known that the emission of the excited state of [Ru(bpy)3] 2+ ([Ru(bpy)3] *) at l = 600 nm is efficiently [*] Prof. Dr. S. Fukuzumi, T. Kobayashi, Dr. T. Suenobu Department of Material and Life Science Graduate School of Engineering, Osaka University 2-1 Yamada-oka, Suita, Osaka 565-0871 (Japan) Fax: (+ 81)6879-7370 E-mail: fukuzumi@chem.eng.osaka-u.ac.jp Homepage: http://www-etchem.mls.eng.osaka-u.ac.jp/

Mononuclear Copper Complex Catalyzed Four-Electron Reduction of Oxygen

A mononuclear Cu(II) complex acts as an efficient catalyst for four-electron reduction of O(2) to H(2)O. Its reduction by a ferrocene derivative (Fc*) and reaction with O(2) leads to the formation of a peroxodicopper(II) complex; this is subsequently reduced by Fc* in the presence of protons to regenerate the Cu(II) complex.

Assembly and Stepwise Oxidation of Interpenetrated Coordination Cages Based on Phenothiazine

A breath of fresh air is sufficient for the eightfold S-monooxygenation of an interpenetrated double cage based on eight phenothiazine ligands and four square-planar-coordinated Pd(II) cations. Besides these two cages, which were both characterized by X-ray crystallography, an eightfold S-dioxygenated double-cage was obtained under harsher oxidation conditions.

Production of hydrogen peroxide as a sustainable solar fuel from water and dioxygen

Hydrogen peroxide was produced as a solar fuel from water and dioxygen using solar energy by combination of a water oxidation catalyst and a photocatalyst for two-electron reduction of O2 in acidic aqueous solutions. Photocatalytic production of H2O2 occurred under photoirradiation of [RuII(Me2phen)3]2+ (Me2phen = 4,7-dimethyl-1,10-phenanthroline) used as a photocatalyst with visible light in the presence of Ir(OH)3 acting as a water oxidation catalyst in an O2-saturated H2SO4 aqueous solution. Photoinduced electron transfer from the excited state of [RuII(Me2phen)3]2+ to O2 results in the formation of [RuIII(Me2phen)3]3+ and a superoxide radical anion (O2˙−) which is protonated to produce H2O2via disproportionation of HO2˙ in competition with back electron transfer (BET) from O2˙− to [RuIII(Me2phen)3]3+. [RuIII(Me2phen)3]3+ oxidises water with the aid of catalysis of Ir(OH)3 to produce O2. The photocatalytic reactivity of H2O2 production was improved by replacing Ir(OH)3 nanoparticles by [CoIII(Cp*)(bpy)(H2O)]2+ in the presence of Sc(NO3)3 in water. The optimised quantum yield of the photocatalytic H2O2 production at λ = 450 nm was determined using a ferrioxalate actinometer to be 37%. The value of conversion efficiency from solar energy to chemical energy was also determined to be 0.25%.

Formation of a long-lived electron-transfer state in mesoporous silica-alumina composites enhances photocatalytic oxygenation reactivity.

A simple donor-acceptor linked dyad, 9-mesityl-10-methylacridinium ion (Acr+-Mes) was incorporated into nanosized mesoporous silica-alumina to form a composite, which in acetonitrile is highly dispersed. In this medium, upon visible light irradiation, the formation of an extremely long-lived electron-transfer state (Acr•-Mes•+) was confirmed by EPR and laser flash photolysis spectroscopic methods. The composite of Acr+-Mes-incorporated mesoporous silica-alumina with an added copper complex [(tmpa)CuII] (tmpa = tris(2-pyridylmethyl)amine) acts as an efficient and robust photocatalyst for the selective oxygenation of p-xylene by molecular oxygen to produce p-tolualdehyde and hydrogen peroxide. Thus, incorporation of Acr+-Mes into nanosized mesoporous silica-alumina combined with an O2-reduction catalyst ([(tmpa)CuII]2+) provides a promising method in the development of efficient and robust organic photocatalysts for substrate oxygenation by dioxygen, the ultimate environmentally benign oxidant.

Hydrogen Evolution from Aliphatic Alcohols and 1,4-Selective Hydrogenation of NAD+ Catalyzed by a [C,N] and a [C,C] Cyclometalated Organoiridium Complex at Room Temperature in Water

A [C,N] cyclometalated Ir complex, [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))benzoic acid-κC(3))(H(2)O)](2)SO(4) [1](2)·SO(4), was reduced by aliphatic alcohols to produce the corresponding hydride complex [Ir(III)(Cp*)(4-(1H-pyrazol-1-yl-κN(2))-benzoate-κC(3))H](-)4 at room temperature in a basic aqueous solution (pH 13.6). Formation of the hydride complex 4 was confirmed by (1)H and (13)C NMR, ESI MS, and UV-vis spectra. The [C,N] cyclometalated Ir-hydride complex 4 reacts with proton to generate a stoichiometric amount of hydrogen when the pH was decreased to pH 0.8 by the addition of diluted sulfuric acid. Photoirradiation (λ > 330 nm) of an aqueous solution of the [C,N] cyclometalated Ir-hydride complex 4 resulted in the quantitative conversion to a unique [C,C] cyclometalated Ir-hydride complex 5 with no byproduct. The complex 5 catalyzed hydrogen evolution from ethanol in a basic aqueous solution (pH 11.9) under ambient conditions. The 1,4-selective catalytic hydrogenation of β-nicotinamide adenine dinucleotide (NAD(+)) by ethanol was also made possible by the complex 1 to produce 1,4-dihydro-β-nicotinamide adenine dinucleotide (1,4-NADH) at room temperature. The overall catalytic mechanism of hydrogenation of NAD(+), accompanied by the oxidation of ethanol, was revealed on the basis of the kinetic analysis and detection of the reaction intermediates.