▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

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Proton-Coupled Electron Transfer

Artificial photosynthesis: molecular systems for catalytic water oxidation.

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.

Chemical approaches to artificial photosynthesis. 2.

Molecular Catalysts for Water Oxidation

6.2. Photosystem II as a Source of Inspiration 2364 6.3. Ruthenium Complexes 2364 6.4. Manganese Complexes 2365 6.5. Concluding Remarks 2366 7. Reduction of Carbon Dioxide 2366 7.

Molecular Catalysis of Electrochemical Reactions. Mechanistic Aspects

The cis,cis-[(bpy)2RuIII(OH2)]2O4+ μ-oxo dimeric coordination complex is an efficient catalyst for water oxidation by strong oxidants that proceeds via intermediary formation of cis,cis-[(bpy)2RuV(O)]2O4+ (hereafter, {5,5}). Repetitive mass spectrometric measurement of the isotopic distribution of O2 formed in reactions catalyzed by 18O-labeled catalyst established the existence of two reaction pathways characterized by products containing either one atom each from a ruthenyl O and solvent H2O or both O atoms from solvent molecules. The apparent activation parameters for μ-oxo ion-catalyzed water oxidation by Ce4+ and for {5,5} decay were nearly identical, with ΔH⧧ = 7.6 (±1.2) kcal/mol, ΔS⧧ = −43 (±4) cal/deg mol (23 °C) and ΔH⧧ = 7.9 (±1.1) kcal/mol, ΔS⧧ = −44 (±4) cal/deg mol, respectively, in 0.5 M CF3SO3H. An apparent solvent deuterium kinetic isotope effect (KIE) of 1.7 was measured for O2 evolution at 23 °C; the corresponding KIE for {5,5} decay was 1.6. The 32O2/34O2 isotope distribution was also ...

Mechanisms of water oxidation catalyzed by the cis,cis-[(bpy)2Ru(OH2)]2O4+ ion.

Scanning electrochemical microscopy (SECM) was used to microfabricate and quantify diaphorase-patterned glass spaces. Deactivated circular and linear micropatterns were produced at diaphorase-immobilized substrates by a localized surface reaction. The oxidation of Br - and Cl - at a microelectrode generated a reactive species which deactivated the localized enzyme molecules at the substrate. The diaphorase-patterned surfaces were characteriied by SECM on the basis of detection of catalytic current of ferrocenylmethanol coupled with oxidation of NADH. The concentration of the immobilized diaphorase was mapped from the quantitative analysis of the catalytic current

Microfabrication and characterization of diaphorase-patterned surfaces by scanning electrochemical microscopy

Scanning electrochemical microscopy (SECM) was employed to quantitatively characterize the oxygen permeation behaviors of poly(dimethylsiloxane) (PDMS) and surface-modified PDMS The mass-transfer 

Oxygen Permeability of Surface-modified Poly(dimethylsiloxane) Characterized by Scanning Electrochemical Microscopy

We developed a high-resolution scanning electrochemical microscope (SECM) for the characterization of various biological materials. Electrode probes were fabricated by Ti/Pt sputtering followed by parylene C-vapor deposition polymerization on the pulled optical fiber or glass capillary. The effective electrode radius estimated from the cyclic voltammogram of ferrocyanide was found to be 35 nm. The optical aperture size was less than 170 nm, which was confirmed from the cross section of the near-field scanning optical microscope (NSOM) image of the quantum dot (QD) particles with diameters in the range of 10−15 nm. The feedback mechanism controlling the probe−sample distance was improved by vertically moving the probe by 0.1−3 μm to reduce the damage to the samples. This feedback mode, defined as “standing approach (STA) mode” (Yamada, H.; Fukumoto, H.; Yokoyama, T.; Koike, T. Anal. Chem. 2005, 77, 1785−1790), has allowed the simultaneous electrochemical and topographic imaging of the axons and cell body o...

Hiroshi Yamada

Papers

Mechanisms of water oxidation catalyzed by the cis,cis-[(bpy)2Ru(OH2)]2O4+ ion.

Microfabrication and characterization of diaphorase-patterned surfaces by scanning electrochemical microscopy

Oxygen Permeability of Surface-modified Poly(dimethylsiloxane) Characterized by Scanning Electrochemical Microscopy

Topographic, electrochemical, and optical images captured using standing approach mode scanning electrochemical/optical microscopy.

Electron-transfer from NADH dehydrogenase to polypyrrole and its applicability to electrochemical oxidation of NADH