The quantitative excited energy transfer reaction between cationic porphyrins on an anionic clay surface was successfully achieved. The efficiency reached up to ca. 100% owing to the “Size-Matching ...Rule” as described in the text. It was revealed that the important factors for the efficient energy transfer reaction are (i) suppression of the self-quenching between adjacent dyes, and (ii) suppression of the segregated adsorption structure of two kinds of dyes on the clay surface. By examining many different kinds of porphyrins, we found that tetrakis(1-methylpyridinium-3-yl) porphyrin (m-TMPyP) and tetrakis(1-methylpyridinium-4-yl) porphyrin (p-TMPyP) are the suitable porphyrins to accomplish a quantitative energy transfer reaction. These findings indicate that the clay/porphyrin complexes are promising and prospective candidates to be used for construction of an efficient artificial light-harvesting system.
Near-Infrared Plasmon-Assisted Water Oxidation Nishijima, Yoshiaki; Ueno, Kosei; Kotake, Yuki ...
The journal of physical chemistry letters,
05/2012, Volume:
3, Issue:
10
Journal Article
Peer reviewed
We report the stoichiometric evolution of oxygen via water oxidation by irradiating a plasmon-enhanced photocurrent generation system with near-infrared light (λ: 1000 nm), in which gold ...nanostructures were arrayed on the surface of TiO2 electrode. It is considered that multiple electron holes generated by plasmon-induced charge excitation led to the effective recovery of water oxidation after the electron transfer from gold to TiO2. The proposed system containing a gold nanostructured TiO2 electrode may be a promising artificial photosynthetic system using near-infrared light.
Two‐electron water oxidation initiated by one‐electron oxidation of aluminum porphyrins (AlTMPyP) is an alternative water oxidation to the conventional four‐electron pathway and could help to avoid ...the bottleneck subject of photon‐flux density in artificial photosynthesis. Here, a dramatic enhancement of the reactivity by bicarbonate ion in the two‐electron water oxidation to form H2O2 is reported. An addition of sodium carbonate (Na2CO3) controlled both catalytic current and product selectivity of the two‐electron water oxidation to enhance the activity of AlTMPyP at pH≈10–11. Controlled potential electrolysis experiments at different concentrations of Na2CO3 (10–100 mm) showed that peroxide selectivity was improved up to approximately 73 % by the increase of Na2CO3 added to the system. The promotion of the reaction cycle was induced by an enhanced dynamic capturing of H2O2 from the hydroperoxy complex of AlTMPyP through an attack of a bicarbonate ion. The detailed electrochemical studies and product selectivity indicated that the bicarbonate ion served as a good cofactor for producing H2O2 from water. At stronger alkaline conditions (pH 12.5), however, a retardative effect of the addition of Na2CO3 on the catalytic reactivity was observed.
Two is better than four: A dramatic enhancement of the reactivity and peroxide selectivity by bicarbonate ion in the one‐electron‐initiated two‐electron oxidation of water catalyzed by aluminum porphyrins is studied. The incorporation of the carbonate cofactor results in approximately 73 % product selectivity with a lowest overpotential of 55–100 mV for two‐electron water oxidation.
A plasmon‐induced water splitting system that operates under irradiation by visible light was successfully developed; the system is based on the use of both sides of the same strontium titanate ...(SrTiO3) single‐crystal substrate. The water splitting system contains two solution chambers to separate hydrogen (H2) and oxygen (O2). To promote water splitting, a chemical bias was applied by regulating the pH values of the chambers. The quantity of H2 evolved from the surface of platinum, which was used as a reduction co‐catalyst, was twice the quantity of O2 evolved from an Au‐nanostructured surface. Thus, the stoichiometric evolution of H2 and O2 was clearly demonstrated. The hydrogen‐evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon‐induced charge separation at the Au/SrTiO3 interface promotes water oxidation and the subsequent reduction of a proton on the backside of the SrTiO3 substrate. The chemical bias is significantly reduced by plasmonic effects, which indicates the possibility of constructing an artificial photosynthesis system with low energy consumption.
Photochemistry: A plasmon‐induced water splitting system that operates under irradiation by visible light using both sides of the same SrTiO3 substrate is reported (see picture). The hydrogen‐evolution action spectrum closely corresponds to the plasmon resonance spectrum, indicating that the plasmon‐induced charge separation at the Au/SrTiO3 interface promotes water oxidation and the reduction of protons.
The reaction mechanism of photocatalytic CO2 reduction using rhenium(I) complexes has been investigated by means of a detailed comparison of the photocatalyses of three rhenium(I) complexes, ...fac-Re(bpy)(CO)3L (L = SCN- (1-NCS), Cl- (1-Cl), and CN- (1-CN)). The corresponding one-electron-reduced species (OER) of the complexes play two important roles in the reaction: (a) capturing CO2 after loss of the monodentate ligand (L) and (b) donation of the second electron to CO2 by another OER without loss of L. In the case of 1-NCS, the corresponding OER has both of the capabilities in the photocatalytic reaction, resulting in more efficient CO formation (with a quantum yield of 0.30) than that of 1-Cl (quantum yield of 0.16), for which OER species have too short a lifetime to accumulate during the photocatalytic reaction. On the other hand, 1-CN showed no photocatalytic ability, because the corresponding OER species does not dissociate the CN- ligand. Based on this mechanistic information, the most efficient photocatalytic system was successfully developed using a mixed system with fac-Re(bpy)(CO)3(CH3CN)+ and fac-Re{4,4‘-(MeO)2bpy}(CO)3{P(OEt)3}+, for which the optimized quantum yield for CO formation was 0.59.
We have been investigating complexes composed of nanolayered materials with anionic charges such as clay nanosheets and dye molecules such as cationic porphyrins. It was found that the structure of ...dye assembly on the layered materials can be effectively controlled by the use of electrostatic host–guest interaction. The intermolecular distance, the molecular orientation angle, the segregation/integration behavior, and the immobilization strength of the dyes can be controlled in the clay–dye complexes. The mechanism to control these structural factors has been discussed and was established as a size-matching effect. Unique photochemical reactions such as energy transfer through the use of this methodology have been examined. Almost 100% efficiency of the energy-transfer reaction was achieved in the clay–porphyrin complexes as a typical example for an artificial light-harvesting system. Control of the molecular orientation angle is found to be useful in regulating the energy-transfer efficiency and in preparing photofunctional materials exhibiting solvatochromic behavior. Through our study, clay minerals turned out to serve as protein-like media to control the molecular position, modify the properties of the molecule, and provide a unique environment for chemical reactions.
Porphyrin derivatives are known as useful functional dyes. Porphyrin derivatives exhibit various properties in complexes with inorganic host materials that are much different from those in ...homogeneous solutions. In this paper, the structure and photochemical properties of porphyrins in inorganic host materials such as clays, layered semiconductors, nanotubes, and mesoporous materials are described. The photochemical properties, including the absorption properties and excited lifetimes, are much affected by the complex formation with inorganic materials. Aggregation phenomena, structural perturbations, and selected chemical reactions such as metalation and protonation affect the photochemical properties of porphyrins accommodated in inorganic host materials. The combination of porphyrin derivatives and inorganic materials should be promising for the construction of novel hybrid materials. Inorganic materials can act as novel environments for photochemical reactions. The utilization of inorganic materials for photochemical reactions is also described.
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•Concept of photon-flux-density problem of sun light radiation is introduced.•Concept of renewable energy factor (REF) is introduced.•Two-electron oxidation of water to form hydrogen ...peroxide by metalloporphyrins was developed.
Recent development on alternative route to bypass the bottle-neck of water oxidation, two-electron oxidation of water catalyzed by earth-abundant metalloporphyrins to form hydrogen peroxide, has been briefly reviewed. The three pioneering reports by Fujishima (Honda-Fujishima effect), Meyer (chemical oxidation of water), and Lehn (photochemical reduction of CO2) triggered the extensive studies on artificial photosynthesis to have been forming the modern history of artificial photosynthesis. The current situation, especially for artificial photosynthesis by molecular catalysts, however, has faced with bottleneck subject of photon-flux-density problem of sun light radiation. In this review article, three milestones in the modern history of artificial photosynthesis, photon-flux-density problem as the bottleneck subject in water oxidation, crucial viewpoint of renewable energy factor (REF) in designing artificial photosynthesis even in fundamental stage of studies are explained. Several approaches to minimize the energy cost in REF for molecular catalysts such as utilization of earth-abundant elements and synthesis of molecular catalyst in water at ambient temperature were also introduced. Recent development of two-electron oxidation of water to form hydrogen peroxide, which would uncover the blind point of the bottleneck subject, would induce a game change in the methodologies of artificial photosynthesis.
In a mixed solvent of water and dimethylformamide (DMF), porphyrin molecules have two types of orientation, tilted and parallel, toward a surface of silicate nanosheet. In the solvent, tilted species ...have lower energy. The T n ← T1 absorption of porphyrin molecules adsorbed at the surface of the nanosheet in the mixed solvent was observed at five different temperatures. The decay curve was analyzed with an equation for transient absorption difference, describing the behavior of parallel and tilted adsorbed species in the ground state and excited state to determine the rate constants for the orientation change and the radiationless deactivation. The rate constants of the orientation change increased with the temperature. The activation energy and energy gap between parallel and tilted species were estimated by analyzing the temperature dependence of the rate constants. The energy gap obtained in this kinetic study was consistent with our thermodynamically obtained value previously reported.
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•Water-soluble Zn(II)TMPyP was synthesized in water at ambient temperature.•Zn(II)TMPyP has two water molecules as axial ligands to exhibit protolysis equilibria among five ...species.•Two-electron oxidation of water to form hydrogen peroxide is initiated by one-electron oxidation of Zn(II)TMPyP.
Two-electron water oxidation is an energy-efficient way for solar energy conversion and one of the promising candidates to get through the bottleneck of artificial photosynthesis, photon- flux density problem. In light of renewable energy factor (REF) that is defined by Energy output/Energy input, for the realization of practical systems, the energy input for catalyst preparation have much importance. Herein, we report a facile, cost-effective and environment benign synthesis of zinc (II) 5, 10, 15, 20-tetrakis(N-methylpyridinium-4′-yl)porphyrin (ZnTMPyP) in water at room temperature. We observed coordination of water molecules to the central Zn(II) ion of ZnTMPyP and four-step protolytic equilibria among five axially ligated species. Electrochemical and controlled potential electrolysis experiments, as well as theoretical DFT calculation, showed that ZnTMPyP(OH)(O−) exhibits the two-electron water oxidation to form hydrogen peroxide as the primary product initiated by one-electron oxidation process of the catalyst with a moderate turnover frequency (96.4 s-1) and a small overpotential (∼60 mV). Isotope-labeled studies clearly showed that water molecule served as an oxygen atom source in the formation of hydrogen peroxide.
