Photodriven Electron Transport within the Columnar Perylenediimide Nanostructures Self-Assembled with Sulfonated Porphyrins in Water

Columnar stacks of <i>N</i>,<i>N</i>′-di­(2-(trimethylammoniumiodide)­ethylene) perylenediimide (TAIPDI)_<i>n</i> can host <i>meso</i>-tetrakis­(4-sulfonatophenyl)­porphyrin zinc tetrapotassium salt (ZnTPPSK₄) molecules at different ratios through the ionic and π–π interactions prompted by an aqueous environment. Photoexcitation of this host–guest complex generates very fast charge separation (1.4 × 10¹² s^–1). Charge recombination is markedly decelerated by a probable electron delocalization mechanism along the long-range of tightly stacked TAIPDIs (4.6 × 10⁸ s^–1), giving an exceptional <i>k</i>_CS/<i>k</i>_CR ratio of 3000 as determined by using time-resolved transient absorption techniques

Efficient Catalytic Interconversion between NADH and NAD⁺ Accompanied by Generation and Consumption of Hydrogen with a Water-Soluble Iridium Complex at Ambient Pressure and Temperature

Regioselective hydrogenation of the oxidized form of β-nicotinamide adenine dinucleotide (NAD⁺) to the reduced form (NADH) with hydrogen (H₂) has successfully been achieved in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex [Ir^III(Cp*)­(4-(1<i>H</i>-pyrazol-1-yl-κ<i>N</i>²)­benzoic acid-κ<i>C</i>³)­(H₂O)]₂ SO₄ [<b>1</b>]₂·SO₄ under an atmospheric pressure of H₂ at room temperature in weakly basic water. The structure of the corresponding benzoate complex Ir^III(Cp*)­(4-(1<i>H</i>-pyrazol-1-yl-κ<i>N</i>²)-benzoate-κ<i>C</i>³)­(H₂O) <b>2</b> has been revealed by X-ray single-crystal structure analysis. The corresponding iridium hydride complex formed under an atmospheric pressure of H₂ undergoes the 1,4-selective hydrogenation of NAD⁺ to form 1,4-NADH. On the other hand, in weakly acidic water the complex <b>1</b> was found to catalyze the hydrogen evolution from NADH to produce NAD⁺ without photoirradiation at room temperature. NAD⁺ exhibited an inhibitory behavior in both catalytic hydrogenation of NAD⁺ with H₂ and H₂ evolution from NADH due to the binding of NAD⁺ to the catalyst. The overall catalytic mechanism of interconversion between NADH and NAD⁺ accompanied by generation and consumption of H₂ was revealed on the basis of the kinetic analysis and detection of the catalytic intermediates

Catalytic Formation of Hydrogen Peroxide from Coenzyme NADH and Dioxygen with a Water-Soluble Iridium Complex and a Ubiquinone Coenzyme Analogue

A ubiquinone coenzyme analogue (Q₀: 2,3-dimethoxy-5-methyl-1,4-benzoquinone) was reduced by coenzyme NADH to yield the corresponding reduced form of Q₀ (Q₀H₂) in the presence of a catalytic amount of a [C,N] cyclometalated organoiridium complex (<b>1</b>: [Ir^III(Cp*)­(4-(1<i>H</i>-pyrazol-1-yl-κ<i>N</i>²)­benzoic acid-κ<i>C</i>³)­(H₂O)]₂SO₄) in water at ambient temperature as observed in the respiratory chain complex I (Complex I). In the catalytic cycle, the reduction of <b>1</b> by NADH produces the corresponding iridium hydride complex that in turn reduces Q₀ to produce Q₀H₂. Q₀H₂ reduced dioxygen to yield hydrogen peroxide (H₂O₂) under slightly basic conditions. Catalytic generation of H₂O₂ was made possible in the reaction of O₂ with NADH as the functional expression of NADH oxidase in white blood cells utilizing the redox cycle of Q₀ as well as <b>1</b> for the first time in a nonenzymatic homogeneous reaction system

Proton-Coupled Electron-Transfer Reduction of Dioxygen Catalyzed by a Saddle-Distorted Cobalt Phthalocyanine

Proton-coupled electron-transfer reduction of dioxygen (O₂) to afford hydrogen peroxide (H₂O₂) was investigated by using ferrocene derivatives as reductants and saddle-distorted (α-octaphenylphthalocyaninato)­cobalt­(II) (Co^II(Ph₈Pc)) as a catalyst under acidic conditions. The selective two-electron reduction of O₂ by dimethylferrocene (Me₂Fc) and decamethylferrocene (Me₁₀Fc) occurs to yield H₂O₂ and the corresponding ferrocenium ions (Me₂Fc⁺ and Me₁₀Fc⁺, respectively). Mechanisms of the catalytic reduction of O₂ are discussed on the basis of detailed kinetics studies on the overall catalytic reactions as well as on each redox reaction in the catalytic cycle. The active species to react with O₂ in the catalytic reaction is switched from Co^II(Ph₈Pc) to protonated Co^I(Ph₈PcH), depending on the reducing ability of ferrocene derivatives employed. The protonation of Co^II(Ph₈Pc) inhibits the direct reduction of O₂; however, the proton-coupled electron transfer from Me₁₀Fc to Co^II(Ph₈Pc) and the protonated [Co^II(Ph₈PcH)]⁺ occurs to produce Co^I(Ph₈PcH) and [Co^I(Ph₈PcH₂)]⁺, respectively, which react immediately with O₂. The rate-determining step is a proton-coupled electron-transfer reduction of O₂ by Co^II(Ph₈Pc) in the Co^II(Ph₈Pc)-catalyzed cycle with Me₂Fc, whereas it is changed to the electron-transfer reduction of [Co^II(Ph₈PcH)]⁺ by Me₁₀Fc in the Co^I(Ph₈PcH)-catalyzed cycle with Me₁₀Fc. A single crystal of monoprotonated [Co^III(Ph₈Pc)]⁺, [Co^IIICl₂(Ph₈PcH)], produced by the proton-coupled electron-transfer reduction of O₂ by Co^II(Ph₈Pc) with HCl, was obtained, and the crystal structure was determined in comparison with that of Co^II(Ph₈Pc)

Small Reorganization Energies of Photoinduced Electron Transfer between Spherical Fullerenes

Rate constants of photoinduced electron transfer between spherical fullerenes were determined using triscandium nitride encapsulated C₈₀ fullerene (Sc₃N@C₈₀) as an electron donor and the triplet excited state of lithium ion-encapsulated C₆₀ fullerene (Li⁺@C₆₀) as an electron acceptor in polar and less polar solvents by laser flash photolysis measurements. Upon nanosecond laser excitation at 355 nm of a benzonitrile (PhCN) solution of Li⁺@C₆₀ and Sc₃N@C₈₀, electron transfer from Sc₃N@C₈₀ to the triplet excited state [³(Li⁺@C₆₀)*] occurred to produce Sc₃N@C₈₀^•+ and Li⁺@C₆₀^•– (λ_max = 1035 nm). The rates of the photoinduced electron transfer were monitored by the decay of absorption at λ_max = 750 nm due to ³(Li⁺@C₆₀)*. The second-order rate constant of electron transfer from Sc₃N@C₈₀ to ³(Li⁺@C₆₀)* was determined to be <i>k</i>_et = 1.5 × 10⁹ M^–1 s^–1 from dependence of decay rate constant of ³(Li⁺@C₆₀)* on the Sc₃N@C₈₀ concentration. The rate constant of back electron transfer from Li⁺@C₆₀^•– to Sc₃N@C₈₀^•+ was also determined to be <i>k</i>_bet = 1.9 × 10⁹ M^–1 s^–1, which is close to be the diffusion limited value in PhCN. Similarly, the rate constants of photoinduced electron transfer from C₆₀ to ³(Li⁺@C₆₀)* and from Sc₃N@C₈₀ to ³C₆₀* were determined together with the back electron-transfer reactions. The driving force dependence of log <i>k</i>_et and log <i>k</i>_bet was well fitted by using the Marcus theory of outer-sphere electron transfer, in which the internal (bond) reorganization energy (λ_i) was estimated by DFT calculations and the solvent reorganization energy (λ_s) was calculated by the Marcus equation. When PhCN was replaced by <i>o</i>-dichlorobenzene (<i>o</i>-DCB), the λ value was decreased because of the smaller solvation changes of highly spherical fullerenes upon electron transfer in a less polar solvent

Efficient Two-Electron Reduction of Dioxygen to Hydrogen Peroxide with One-Electron Reductants with a Small Overpotential Catalyzed by a Cobalt Chlorin Complex

A cobalt chlorin complex (Co^II(Ch)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O₂) by one-electron reductants (ferrocene derivatives) to produce hydrogen peroxide (H₂O₂) in the presence of perchloric acid (HClO₄) in benzonitrile (PhCN) at 298 K. The catalytic reactivity of Co^II(Ch) was much higher than that of a cobalt porphyrin complex (Co^II(OEP), OEP^2– = octaethylporphyrin dianion), which is a typical porphyrinoid complex. The two-electron reduction of O₂ by 1,1′-dibromoferrocene (Br₂Fc) was catalyzed by Co^II(Ch), whereas virtually no reduction of O₂ occurred with Co^II(OEP). In addition, Co^II(Ch) is more stable than Co^II(OEP), where the catalytic turnover number (TON) of the two-electron reduction of O₂ catalyzed by Co^II(Ch) exceeded 30000. The detailed kinetic studies have revealed that the rate-determining step in the catalytic cycle is the proton-coupled electron transfer reduction of O₂ with the protonated Co^II(Ch) ([Co^II(ChH)]⁺) that is produced by facile electron-transfer reduction of [Co^III(ChH)]²⁺ by ferrocene derivative in the presence of HClO₄. The one-electron-reduction potential of [Co^III(Ch)]⁺ was positively shifted from 0.37 V (vs SCE) to 0.48 V by the addition of HClO₄ due to the protonation of [Co^III(Ch)]⁺. Such a positive shift of [Co^III(Ch)]⁺ by protonation resulted in enhancement of the catalytic reactivity of [Co^III(ChH)]²⁺ for the two-electron reduction of O₂ with a lower overpotential as compared with that of [Co^III(OEP)]⁺

Robustness of Ru/SiO₂ as a Hydrogen-Evolution Catalyst in a Photocatalytic System Using an Organic Photocatalyst

Effects of various metal oxide supports (SiO₂, SiO₂–Al₂O₃, TiO₂, CeO₂, and MgO) on the catalytic reactivity of ruthenium nanoparticles (RuNPs) used as a hydrogen-evolution catalyst have been evaluated in photocatalytic hydrogen evolution using 2-phenyl-4-(1-naphthyl)­quinolinium ion (QuPh⁺–NA) and dihydronicotinamide adenine dinucleotide (NADH) as a photocatalyst and an electron donor, respectively. The 3 wt % Ru/SiO₂ catalyst freshly prepared by an impregnation method exhibited the highest catalytic reactivity among RuNPs supported on various metal oxides, which was nearly the same as that of commercially available Pt nanoparticles (PtNPs) with the same metal weight. However, the initial catalytic reactivity of 3 wt % Ru/SiO₂ was lost after repetitive use, whereas the catalytic reactivity of PtNPs was maintained under the same experimental conditions. The recyclability of the 3 wt % Ru/SiO₂ was significantly improved by employing the CVD method for preparation. The initial catalytic reactivity of 0.97 wt % Ru/SiO₂ prepared by the CVD method was higher than that of 2 wt % Ru/SiO₂ prepared by the impregnation method despite the smaller Ru content. The total amount of evolved hydrogen normalized by the weight of Ru in 0.97 wt % Ru/SiO₂ was 1.7 mol g_Ru^–1, which is now close to that normalized by the weight of Pt in PtNPs (2.0 mol g_Pt^–1). Not only the preparation method but also the morphology of SiO₂ supports affected significantly the catalytic activity of Ru/SiO₂. The Ru/SiO₂ catalyst using nanosized SiO₂ with undefined shape exhibited higher catalytic activity than Ru/SiO₂ catalysts using mesoporous SiO₂ or spherical SiO₂. The kinetic study and TEM observation of the Ru/SiO₂ catalysts suggest that the microenvironment of RuNPs on SiO₂ surfaces plays an important role to exhibit the high catalytic performance in the photocatalytic hydrogen production

Visible-Light-Induced Oxygenation of Benzene by the Triplet Excited State of 2,3-Dichloro-5,6-dicyano‑<i>p</i>‑benzoquinone

Photocatalytic oxygenation of benzene to phenol occurs under visible-light irradiation of 2,3-dichloro-5,6-dicyano-<i>p</i>-benzoquinone (DDQ) in an oxygen-saturated acetonitrile solution of benzene and <i>tert</i>-butyl nitrite. The photocatalytic reaction is initiated by photoinduced electron transfer from benzene to the triplet excited state of DDQ

Much Enhanced Catalytic Reactivity of Cobalt Chlorin Derivatives on Two-Electron Reduction of Dioxygen to Produce Hydrogen Peroxide

Effects of changes in the redox potential or configuration of cobalt chlorin derivatives (Co^II(Ch_<i>n</i>) (<i>n</i> = 1–3)) on the catalytic mechanism and the activity of two-electron reduction of dioxygen (O₂) were investigated based on the detailed kinetic study by spectroscopic and electrochemical measurements. Nonsubstituted cobalt chlorin complex (Co^II(Ch_<i>1</i>)) efficiently and selectively catalyzed two-electron reduction of dioxygen (O₂) by a one-electron reductant (1,1′-dimethylferrocene) to produce hydrogen peroxide (H₂O₂) in the presence of perchloric acid (HClO₄) in benzonitrile (PhCN) at 298 K. The detailed kinetic studies have revealed that the rate-determining step in the catalytic cycle is the proton-coupled electron transfer reduction of O₂ with the protonated Co^II(Ch_<i>1</i>) complex ([Co^II(Ch_<i>1</i>H)]⁺), where one-electron reduction potential of [Co^III(Ch_<i>1</i>)]⁺ was changed from 0.37 V (vs SCE) to 0.48 V by the addition of HClO₄ due to the protonation of [Co^III(Ch_<i>1</i>)]⁺. The introduction of electron-withdrawing aldehyde group (position C-3) (Co^II(Ch_<i>3</i>)) and both methoxycarbonyl group (position C-13²) and aldehyde group (position C-3) (Co^II(Ch_<i>2</i>)) on the chlorin ligand resulted in the positive shifts of redox potential for Co­(III/II) from 0.37 V to 0.45 and 0.40 V, respectively, whereas, in the presence of HClO₄, no positive shifts of those redox potentials for [Co^III(Ch_<i>n</i>)]⁺/Co^II(Ch_<i>n</i>) (<i>n</i> = 2, 3) were observed due to lower acceptability of protonation. As a result, such a change in redox property resulted in the enhancement of the catalytic reactivity, where the observed rate constant (<i>k</i>_obs) value of Co^II(Ch_<i>3</i>) was 36-fold larger than that of Co^II(Ch_<i>1</i>)

Enhanced Photoinduced Electron-Transfer Reduction of Li⁺@C₆₀ in Comparison with C₆₀

Kinetics of photoinduced electron transfer from a series of electron donors to the triplet excited state of lithium ion-encapsulated C₆₀ (Li⁺@C₆₀) was investigated in comparison with the corresponding kinetics of the photoinduced electron transfer to the triplet excited state of pristine C₆₀. Femtosecond laser flash photolysis measurements of Li⁺@C₆₀ revealed that singlet excited state of Li⁺@C₆₀ (λ_max = 960 nm) underwent intersystem crossing to the triplet excited state [³(Li⁺@C₆₀)*: λ_max = 750 nm] with a rate constant of 8.9 × 10⁸ s^–1 in deaerated benzonitrile (PhCN). The lifetime of ³(Li⁺@C₆₀)* was determined by nanosecond laser flash photolysis measurements to be 48 μs, which is comparable to that of C₆₀. Efficient photoinduced electron transfer from a series of electron donors to ³(Li⁺@C₆₀)* occurred to produce the radical cations and Li⁺@C₆₀^•–. The rate constants of photoinduced electron transfer of Li⁺@C₆₀^•– are significantly larger than those of C₆₀ when the rate constants are less than the diffusion-limited value in PhCN. The enhanced reactivity of ³(Li⁺@C₆₀)* as compared with ³C₆₀* results from the much higher one-electron reduction potential of Li⁺@C₆₀ (0.14 V vs SCE) than that of C₆₀ (−0.43 V vs SCE). The rate constants of photoinduced electron transfer reactions of Li⁺@C₆₀ and C₆₀ were evaluated in light of the Marcus theory of electron transfer to determine the reorganization energies of electron transfer. The reorganization energy of electron transfer of Li⁺@C₆₀ was determined from the driving force dependence of electron transfer rate to be 1.01 eV, which is by 0.28 eV larger than that of C₆₀ (0.73 eV), probably because of the change in electrostatic interaction of encapsulated Li⁺ upon electron transfer in PhCN