Electron Relay Research Articles

Formation and decay of N, N, N′, N′‐tetraethyl‐p‐phenylenediamine radical cation in aqueous solution. A kinetic study by stopped‐flow technique

AbstractA kinetic study has been carried out on the oxidation of N, N, N′, N′,‐tetraethyl‐p‐phenylenediamine (TEPD) by metal ion like Ce4+, oxoanions viz., MnO4− and Cr2O72−; peroxides such as peroxomonosulphate (PMS), peroxodisulphate (PDS), and H2O2; and halogens namely Cl2, Br2, and I2. The fast kinetics of the formation and decay of the radical cation TEPD˙+ have been analyzed at 565 nm by the stopped‐flow technique under pseudo‐first‐order conditions. From the kinetic data, it has been inferred that the reactions were found to be of first‐order with respect to [TEPD] and [oxidant] but over all it has been of second‐order. The observed second‐order rate constants in both the formation and decay of TEPD˙+ has been correlated with the oxidation potentials of the various oxidants employed in this study. The effect of pH on the oxidation has been investigated in the formation and decay of TEPD˙+ as well as reduction studies have also been carried out using dithionite which has been found to regenerate the TEPD from the TEPD˙+ and the corresponding rate constant has also been determined. Besides these, this article also explains how the TEPD, which forms TEPD˙+ acts as a better electron relay than TMPD(N, N, N′, N′‐tetramethyl‐p‐phenylenediamine) which forms TMPD˙+, even though both of them undergo one‐electron oxidation and are used in the chemical routes to solar energy conversions. The observed rate constants for electron transfer were correlated theoretically using Marcus theory. The observed and calculated rate constants have good correlation. © 1995 John Wiley & Sons, Inc.

The effect of structure on poly(quinone) systems for amperometric glucose sensors

2,5-Etheramino- p-benzoquinone I, poly(2,5-etheramino- p-benzoquinone)s IIa–c ( x = 3, 6 and 33), poly(2,5-ω,ω′-hexamethylenediamino- p-benzoquinone) IV and poly(3,6-etheramino- o-benzoquinone) V ( x = 6) ( x is the number of propylene oxide repeating units) were synthesized and their efficiencies as electron relay systems for amperometric glucose sensors were investigated and compared. Cyclic voltammetry and steady state potential measurements showed that the quinone monomer and polymers efficiently mediated electron transfer from reduced glucose oxidase to a conventional carbon paste electrode. Sensors constructed with 2,5-etheramino- p-benzoquinone showed higher efficiency than those constructed with polymeric systems. Among the poly(2,5-etheramino- p-benzoquinone)s, the length of the propylene oxide chain between quinone moieties was found to be critical for electron relay efficiency. Sensors constructed from polymers with shorter propylene oxide chains between the quinone moieties demonstrated greater efficiency than those constructed from polymers with longer propylene oxide chains ( x = 3 > 6 ⪢ 33). The sensor stability was investigated using sensors containing monomeric quinone compound I, non-crosslinked polymer IIb and crosslinked polymer VI. The sensors with crosslinked polymer were the most stable and maintained their efficiency over 50 days.

Zeolite sensitized photoelectron transfer: Modulation of reduction potentials

The remarkable efficiency of conversion of light to chemical energy observed in green plant photosynthesis arises due to the spatial positioning of the sensitizers and the molecules involved in the electron relay. The architecture of zeolite cages also allows for positioning of molecules spatially, and in a primitive sense mimics photosynthesis. Using entrapped Ru(bpy) 2+ 3 as sensitizer, directional electron transfer to propylviologen sulfonate (PVS) in solution is possible if the mediating viologen in the zeolite has the appropriate reduction potential e.g. N,N′-tetramethylene-2,2′ bipyridinium (DQ 2+). The reduction potentials of viologens is controlled by introducing steric effects in the molecule such that planarity of the two bipyridyl rings upon one-electron reduction is difficult to achieve. We show that by using viologen molecules that span several supercages of a zeolite e.g. benzylviologen (BV2 +), we can also alter the reduction potential, but because of the steric constraints imposed by the zeolite framework. Thus, in this example, not only does the zeolite provide spatial orientation, but also modulates the reduction potential of the encapsulated viologen.

Visible light-induced hydrogen production from water with Pt/Bi2O3/RuO2 in presence of electron relay and photosensitizer

Abstract An attempt is made to prepare Pt/Bi 2 O 3 /RuO 2 (PhC) catalyst and to use it for photocatalytic production of hydrogen from water in the presence of: (i) methyl viologen (MV 2+ ) as an electron relay, (ii) Ru(bpy) 3 2+ as a photosensitizer, or (iii) both (i) and (ii) with visible light of λ ⩾ 400 nm. Compared to the undoped γ-Bi 2 O 3 , the above catalyst is more efficient in hydrogen production. The efficiencies of the different systems constituted by the catalyst with MV 2+ or Ru(bbpy) 3 2+ or both, in hydrogen production are in the order PhC  MV 2+ PhC  Ru ( bpy ) 2+ 3 PhC  MV 2+  Ru ( bpy ) 2+ 3 . Suitable explanations and plausible mechanisms and schemes have been provided to account for the observations in each category of the experiment.

1,4-Bis(1,2,6-triphenyl-4-pyridyl)benzene as a novel hydrophobic electron relay for dihydrogen evolution in photocatalytic systems based on lipid vesicles

Abstract 1,4-Bis(1,2,6-triphenyl-4-pyridyl)benzene (“benzoviologen”) is an efficient lipophilic reversible electron relay, which can be used to build structurally organized molecular systems based on lipid vesicles for photocatalytic water cleavage. Benzoviologen has an advantage over the widely used conventional lipophilic viologens, since its reduced forms possess more negative reduction potentials. This promotes photostimulated transmembrane electron transfer and catalytic water reduction at neutral or even slightly basic pH. The kinetic properties of benzoviologen and its capacity to produce one- and two-electron-reduced forms were studied. The ability to couple vectorial transmembrane electron phototransfer with dihydrogen evolution from water over a dispersed Pd catalyst was proven experimentally.

Supramolecular assemblies for light-induced electron-transfer reactions

In this paper, we present a summary of our work on highly photostable supramolecular ruthenium complexes, which may be incorporated into more complex systems for artificial solar energy conversion. We have used supramolecular chemistry and photochemistry to synthesize highly photostable ruthenium bipyridine cnronates and a bipyridazine podate complex and to enhance photoelectron-transfer reactions in physical model systems for artificial photosynthesis. The recent progress of covalent and non-covalent sensitizer-relay assemblies for highly efficient photoelectron transfer is described. A detailed mechanistic investigation of the binding behavior of cationic species to crown-ether-modified bipyridine derivatives is presented as an example of supramolecular binding in systems for photoelectron transfer. The host properties of the free ligands and the derived bis-heteroleptic ruthenium complexes are compared using UV-visible, luminescence quenching and proton nuclear magnetic resonance titrations. The combination of these three methods confirms that supramolecular binding of cations and the electron relay methykiologen (MF+) to the complexes can be observed. The binding constants determined are of the order of (16) X lo4 1 mol-’ for the crown-ether ruthenium complexes and 1 X @-4X ld I mol-’ for the crown-ether ligands. Single-photon-counting (SK) investigations give strong indications for the coexistence of different binding mechanisms. The kinetic scheme of Yekta et al. has been adapted to interpret the binding mechanism.

Photocatalytic hydrogen evolution from aqueous Pt/TiO2 suspension in the presence of oxalic acid

Accumulation of titanium (III) oxalate was found to accompany hydrogen evolution upon UV irradiation of aqueous suspensions containing Pt/TiO2 and oxalic acid. The former appears to take part in the process of hydrogen evolution as an electron relay.

Photoelectrochemical and quenching studies of tris(2,2′-bipyridine)ruthenium(II) bound to Nafion- and clay-coated electrodes

The photosensitizer [Ru(bipy)3]2+(bipy = 2,2′-bipyridine) was adsorbed on Nafion(nf)- and bentonite clay(bt)-coated electrodes. Photoelectrochemical cells were constructed using Pt–nf–[Ru(bipy)3]2+ and Pt–bt–[Ru(bipy)3]2+ electrodes with cobalt(III) complexes and Fe3+ ion as electron relays. At appropriate applied potentials, anodic and cathodic photocurrents were induced when the electrodes were irradiated with visible light in solutions containing a mixture of a cobalt(III) complex and HClO4. The photoresponse could be switched between anodic and cathodic by controlling the applied electrode potential. The origin of the photocurrent is explained by an oxidative quenching by the cobalt(III) complex of the photoexcited *[Ru(bipy)3]2+ complex confined at the electrode, followed by electrochemical reduction or oxidation of the resulting [Ru(bipy)3]3+ and cobalt(II) complex respectively. The quenching rate constants, kq, for the reaction of *[Ru(bipy)3]2+ with three different cobalt(III) complexes and Fe3+ion were determined by Photoelectrochemical methods.

Functionalized viologens for novel electron relay systems

Molecular electron relay systems based on viologens substituted with, for example, crown ethers offer the opportunity to combine molecular recognition/complexation with electron transfer. Recent progress in the synthesis of such materials is reported, together with examples of their application as electron‐transfer mediators in biomimetic electron transfer systems, and the controlled reduction of organic compounds such as aromatic aldehydes and α‐ketoesters in organic solvents in which the reducing agent (e.g. Na2S2O4) is usually insoluble.

Light-induced hydrogen production using waste compounds as sacrifical electron donors

Systems for the conversion and chemical storage of solar energy are usually based on photo-induced electron transfer reactions from an excited sensitizer to an electron acceptor (i.e. an electron relay compound). We have investigated the photo- and electrochemical properties of two novel electron relay compounds: l,l″,r″′-trimethyl[4,2′; 4′,4″; 6′,4″′]quaterpyridinium trichloride (1) and 1,1′,1″,1″′-tetramethyl[4,2′; 4′,4′’; 6′,4″] quaterpyridinium tetrachloride (2). When solutions containing Ru(bpy)3Cl2 or Zn-porphyrin (as photosensitizer), compound1 or2 and EDTA (as sacrificial electron donor) are irradiated by visible light (λ > 400 nm), only the formation of reduced relay compound 1 occurs. Addition of platinum catalyst to such solutions with compound 1 leads to the formation of hydrogen. Quantum yields are in the range of 5%, calculated for absorbed light. Using other sacrificial electron donors such as alcohols or glucose (or waste compounds like 4-chlorophenol) did not result in hydrogen evolution. However,1 and2 are reduced by these sacrificial electron donors in the absence of an additional photosensitizer, when near UV irradiation light (λ > 280 nm) is used. Quantum yields for hydrogen production with compound1 are about 2%, calculated for absorbed light out of GC-measurements from H2 in the gas phase above the irradiated solution. The photo- and electrochemical properties of compounds1 and2 are discussed.

Electrochemical Conversion of Carbon Dioxide to Methanol with Use of Enzymes as Biocatalysts

Abstract Electrolysis of carbon dioxide-saturated phosphate buffer solution (pH 7) in the presence of formate dehydrogenase and methanol dehydrogenase as electrocatalysts together with methylviologen as an electron relay yielded formaldehyde and methanol as the reduction products of carbon dioxide under suitable electrolysis conditions, and the use of pyrrolo-quinoline quinone in place of methylviologen as the electron relay afforded selective reduction of carbon dioxide to methanol.

Biomimetic approaches to the construction of supramolecular catalysts: titanium dioxide—platinum antenna catalysts to reduce water using visible light

Thermally and photochemically stable catalysts in systems that convert the energy of visible light into the chemical energy of dihydrogen gas have yet to be fully developed and all proposed systems suffer, to varying extents, from loss of activity with use. We have attempted to model nature's photochemical reaction center through the use of TiO 2 antenna catalysts and very small amounts of Pt that form active centers on the TiO 2 semiconductor surface. This system is relatively inexpensive, reproducible, extremely stable, efficient in conversion of light to dihydrogen in aqueous solutions and displays higher linearity in dihydrogen evolution with greater efficiency in recycling than do other previously reported catalysts. Here we report the self-hydrogenation properties of our TiO 2Pt catalysts in comparison with well known Pt, Ru and Os catalysts as well as their catalytic efficiency in systems for the sacrificial photoreduction of water. The sensitizers employed in this study were a recently synthesized new class of bis-heteroleptic complexes [Ru(bpy) 2(PP)] 2+, [Ru(bpy) 2(PPB)] 2+, [Ru(bpy) 2(PPB-pCl)] 2+ containing one pyridyl—pyrimidine ligand and [Ru(bpy) 3] 2+ and [Ru(bpz) 3] 2+ as well known standard sensitizers. In order to obtain maximum information about the TiO 2Pt catalysts we varied the components and conditions in our sacrificial systems for water reduction: MV 2+, PVS MPVS were used as electron relays, EDTA and TEOA as sacrificial electron donors. Furthermore, the dependence of the dihydrogen evolution rates on the pH of the photolysis solution and its ionic strength were investigated. The techniques used in this study are cyclic voltammetry, steady state and time resolved luminescence spectroscopy, Stern—Volmer quenching, measurements of the quantum yields for PVS reduction, and volumetric measurements of dihydrogen photoproduction. A linear dependence was found between the dihydrogen production rate of the TiO 2Pt catalysts and the quantum yield for the MV 2+ and PVS photoreduction at low ionic strength, showing that the new catalysts are reduced by the electron relays and convert protons into dihydrogen in a highly efficient manner

Size effect in the photochemical generation of hydrogen from water by colloidal Fe 2O 3 particles

Colloidal Fe 2O 3 particles of various dimensions were prepared. Their crystal structure and optical properties were studied. These particles were used as photosensitizers for steady state water photolysis. The yield of H 2 generated is dependent on the particle size and the concentration of the colloids. The pH dependence is different for different particle sizes. The effects of methylviologen (MV 2+) as an electron relay and CH 3OH as a hole scavenger on the H 2 yield were also studied.

New sensitizing relay compounds for H2 production

Abstract Hydrogen producing systems, which consist of a photosensitizer (like Ru(bipy)3Cl2), an electron relay (e.g. methyl viologen), a colloidal catalyst (platinum) and a sacrificial electron donor (EDTA) are well known. We have investigated new electron relay substances (1,1″, 1′″-trialkyl [4,2′;4′,4″;6′,4″′] quaterpyridinium trichloride and related compounds), which combine the features of the sensitizer and the electron relay. The maxima of light absorption of these compounds were in the u.v. range, at about 320 nm. The fluorescence of the relay compounds decreased with increasing relay concentration (self-quenching). We have determined the quantum yields of the systems (containing the electron relay, EDTA, platinum catalyst and acetate buffer) by exciting these solutions with near-u.v. light (high-pressure Hg lamp, λ > 280 nm, for the calculations the number of incident photons with λ > 280 nm, but only hydrogen in the gas phase ( 1 2 H 2 ) was taken into account). The quantum yields were in the region of about 2%. Furthermore, we found that only electron relay compounds which have a pyridinium ring with a free electron pair were capable of producing hydrogen without dye stuff. The reaction mechanism is discussed in detail.

Homogeneous catalyses of mixed-valent octanuclear platinum complexes in photochemical hydrogen production from water

An acetamidate-bridged mixed-valent octanuclear platinum complex, [Pt(2.25+) 8 (NH 3) 16(CH 3CONH) 8](NO 3) 10·4H 2O, has been employed as a hydrogen-producing catalyst in a photochemical model system, containing EDTA as a sacrificial electron donor, [Ru (II)(2,2′-bpy) 3] 2+ as a photosensitizer and methyl viologen as an electron relay. The catalytic activity of the octanuclear compound was higher than those previously reported for the mixed-valent tetranuclear platinum complexes with α-pyridonate or α-pyrrolidonate bridging ligands, and is also higher than that observed for another mixed-valent octanuclear platinum complex with 2-fluoroacetamidate bridging ligand, [Pt(2.08+) 8(NH 3) 16(CH 2FCONH) 8] (NO 3) 8.66·4H 2O. Photochemical decomposition of the present system, caused by both hydrogenation of methyl viologen and photodecomposition of the catalyst complex, has been studied by means of HPLC. 1H NMR studies have revealed that cleavage of the acetamidate-bridged Pt (II) 6Pt(III) 2 octamer into Pt(II) 2 and Pt(III) 2 dimers takes place rapidly after dissolution in aqueous media. The dinuclear Pt(II) 2 complex seems to be the major species in the solution under the photolysis conditions and would be the active species as the hydrogen-producing catalyst.

Visible light induced hydrogen production with Cu(II)/Bi2O3 and Pt/Bi2O3/RuO2 from aqueous methyl viologen solution

Abstract γ-Bi 2 O 3 , a rarely studied oxide semiconductor, was doped individually with Cu(II) in different concentrations, Pt and RuO 2 (0.5 wt%) and used as a fine powder for photocatalytic production of hydrogen from water in the presence of methyl viologen (MV 2+ ) as an electron relay. The diffuse absorption spectroscopy of the samples revealed that the doping process improved the absorption of γ-Bi 2 O 3 in the visible region while the XRD spectra indicated no change in crystal structure, but an enhancement in crystallinity. For 90 min irradiation ( λ ≥ 400 nm) of the aqueous slurry of the catalyst in presence of MV 2+ , 0.79 and 0.66 ml of hydrogen were generated by 4 at% Cu(II)/Bi 2 O 3 and Pt/Bi 2 O 3 /RuO 2 respectively. A suitable mechanism involving the reactions of photogenerated e − and h + has been proposed for the photodecomposition of water. The effects of dopant concentration and the catalyst amount on hydrogen production have also been explained.

Ultrasmall Metal Oxide Particles: Preparation, Photophysical Characterization, and Photocatalytic Properties

AbstractThis paper gives an overview of the author's activities in the research of extremely small metal oxide particles in recent years. In particular, the synthesis of transparent colloidal solutions of extremely small zinc oxide, titanium dioxide, hematite, and titanium/iron mixed oxide particles (2 nm < d 20 nm) in water, ethanol, and 2‐propanol is described. Quantum (Q)‐size effects are observed during particle growth and at me final stages of synthesis. A simple molecular orbital (MO) picture is presented for the qualitative interpretation of these effects, while quantitative calculations have been carried out using a quantum mechanical model developed by Brus. The photophysical properties of the particles have also been investigated extensively. Fluorescence spectra of the ZnO sols suggest that adsorbed electron relays are necessary to shuttle electrons from the conduction band to lower‐lying traps. Excess negative charge on the particles, resulting from either deprotonated surface hydroxyl groups or from photogenerated or externally injected charge carriers, causes a blue‐shift in the electronic absorption spectrum, which is explained by electrostatic and MO models. The zero point of charge (pHzpc) of the aqueous colloidal suspensions has been determined by several independent methods. While zinc oxide, titanium dioxide, and titanium/iron mixed oxide particles exhibit considerable photocatalytic activity (as illustrated for the reduction of molecular oxygen and the oxidation of various halogenated carboxyl acids), hematite particles are only found to oxidize S(IV) under bandgap illumination to a reasonable extent (ϕ < 0.3). A mechanism involving surface‐bound molecules and free radical intermediates is presented to explain these differences in reactivity.

Electron relay in polymer systems: Effective electron transfer from dihydronicotinamide to viologen moieties in their copolymers

The first observation of effective electron-transfer from the dihydronicotinamide side groups to the viologen moieties without irradiation in a polymeric system is described

Photosensitized water oxidation to dioxygen in artificial biomimetic systems: achievements and prospects

There are two reasons for the keen interest in the creation of artificial systems for water oxidation to dioxygen. The first reason is connected with the problem of natural photosynthetic oxygen evolution. Photosystem II (PSII) of green plants provides efficient light-driven electron transport from water, which is oxidized to dioxygen, to the pool of plastoquinones. Whereas the sequence of electron relays ensuring rapid and irreversible electron transfer from an oxygen evolving complex (OEC) to the pool of plastoquinones is relatively well established, the detailed structure of OEC and the mechanism of OEC action are rather unclear [l-3]. The second reason is connected with attempts to create artificial photosynthetic units as practical devices for the conversion of solar energy into chemical energy. Among various photochemical reactions that could be used for the purpose, water cleavage to elements attracts the greatest attention [4-71:

Supramolecular sensitizer–relay assemblies. An evaluation of binding constants from non‐linear stern–volmer plots

AbstractA novel technique for determining the binding constants of host–guest complexes, viz. the bis‐cationic electron relays methylviologen and octylviologen in the macrocyclic host of novel (bisheteroleptic) crown ether‐ruthenium sensitizers, is described. This technique allows the separation of the quenching effects of bound and free electron relays. It can be used to calculate binding constants in non‐covalently linked supramolecular systems evaluating the electron transfer process between guest and host. High binding constants comparable to those in many natural enzymes and the photosynthetic reaction centre were found. As an explanation for the behaviour of the sensitizers bound to the bipyridinium cations it is suggested that photoelectron transfer affects the binding between the excited sensitizer and the bound acceptor.