Consecutive Electron Transfer Research Articles

Electrostatic Modeling of the Tunable Potential Difference between the Two Consecutive Oxidation Steps of Dinickel Bisfulvalene

The tuning by medium effect of the potential difference between the two consecutive electron transfers of dinickel bisfulvalene is qualitatively modeled by the Born solvation model and the Coulomb charge attraction/repulsion law. Calls are made for a better electrostatic understanding of the electrochemical experiment and for accurate modeling with theoretical chemistry.

Half-Wave Potential Splittings ΔE1/2as a Measure of Electronic Coupling in Mixed-Valent Systems: Triumphs and Defeats

This review is a cautionary note against the often purported direct relation between the half-wave potential splitting ΔE1/2 (or ΔE°) for stepwise, consecutive electron transfer from systems featuring two or more identical redox sites and the true electronic coupling HAB and charge or spin distribution in the ground state of intermittently formed mixed-valent (MV) systems. Several examples where these different quantities go in parallel are contrasted with other examples where this is not the case. Different kinds of such “non-conformist” behavior are outlined with the aid of representative examples. These include cases of fairly strong electronic couplings and large degrees of ground-state delocalization despite small values of ΔE1/2—sometimes just above the statistical limit or even below that—as well as examples for just the opposite behavior of no detectable electronic coupling despite appreciable electrochemical half-wave potential splitting. The crucial roles of the nominal bridges that interconnect the individual redox sites and of the environment (solvent, supporting electrolyte) in determining ΔE1/2 and HAB are emphasized. We also seek to provide some guidelines for the practitioner as to how to discriminate between these various types of behaviors and how to determine the strength of the electronic coupling between the redox sites.

Au25 Clusters as Electron-Transfer Catalysts Induced the Intramolecular Cascade Reaction of 2-nitrobenzonitrile

Design of atomically precise metal nanocluster catalysts is of great importance in understanding the essence of the catalytic reactions at the atomic level. Here, for the first time, Au25z nanoslusters were employed as electron transfer catalysts to induce an intramolecular cascade reaction at ambient conditions and gave rise to high conversion (87%) and selectivity (96%). Electron spin-resonance spectra indeed confirmed the consecutive electron transfer process and the formation of N radical. UV-vis absorption spectra also verified Au25z was intact after the catalytic circle. Our research may open up wide opportunities for extensive organic reactions catalyzed by Au25z.

The mechanism and kinetics of Zn2+ electroreduction in the presence of octyltrimethylammonium bromide

Zn2+ electroreduction in the presence of octyltrimethylammonium bromide at a mercury electrode has been studied by voltammetric and impedance measurements in the range of 288–308K. Kinetic data showed a two-step character of the electrode process and an inhibiting influence of octyltrimethylammonium bromide on Zn2+ electroreduction rate. Analysis of Tafel plots showed a change of electroreduction mechanism at a certain octyltrimethylammonium bromide concentration. Four models of electroreduction mechanisms were applied: two consecutive electron transfers (EE), ion transfer step followed by electron transfer (IE), ion transfer followed by adsorption (IA), and chemical step followed by electron transfer (CE). The results of investigations of Zn2+ reduction mechanism were combined with studies concerning activation energy for diffusion, standard reaction entropy, and activation enthalpy.

Investigation of the Consecutive Electron Transfer of Metalloporphyrin Species Containing Different Substituents at the Liquid/Liquid Interface by Thin-Layer Cyclic Voltammetry

The cyclic voltammetric behavior of metalloporphyrin species containing different substituents was investigated by thin-layer electrochemistry. In this paper, the two-step electron transfer between zinc porphyrin species containing different substituents in nitrobenzene and Fe(CN)64– in the aqueous phase was studied. The bimolecular rate constants for the two-step electron transfer reactions were obtained simultaneously by thin-layer theory. When we use the thin-layer theory to calculate the kinetics constants, the process of consecutive electron transfer is considered to be complete. However, this was not the case; by comparing the data that are calculated by Faraday’s law of electrolysis and the theory based on thin-layer cyclic voltammetry for consecutive electron transfer across the interface between two immiscible electrolyte solutions, the correction coefficient was obtained. The kinetics constants of multistep interfacial bimolecular reactions can be further calculated with more precision.

Gold Nanoparticles Located at the Interface of Anatase/Rutile TiO2 Particles as Active Plasmonic Photocatalysts for Aerobic Oxidation

Visible-light irradiation (λ > 450 nm) of gold nanoparticles loaded on a mixture of anatase/rutile TiO(2) particles (Degussa, P25) promotes efficient aerobic oxidation at room temperature. The photocatalytic activity critically depends on the catalyst architecture: Au particles with <5 nm diameter located at the interface of anatase/rutile TiO(2) particles behave as the active sites for reaction. This photocatalysis is promoted via plasmon activation of the Au particles by visible light followed by consecutive electron transfer in the Au/rutile/anatase contact site. The activated Au particles transfer their conduction electrons to rutile and then to adjacent anatase TiO(2). This catalyzes the oxidation of substrates by the positively charged Au particles along with reduction of O(2) by the conduction band electrons on the surface of anatase TiO(2). This plasmonic photocatalysis is successfully promoted by sunlight exposure and enables efficient and selective aerobic oxidation of alcohols at ambient temperature.

Reductive activation of the heme iron–nitrosyl intermediate in the reaction mechanism of cytochrome c nitrite reductase: a theoretical study

Cytochrome c nitrite reductase catalyzes the six-electron, seven-proton reduction of nitrite to ammonia without release of any detectable reaction intermediate. This implies a unique flexibility of the active site combined with a finely tuned proton and electron delivery system. In the present work, we employed density functional theory to study the recharging of the active site with protons and electrons through the series of reaction intermediates based on nitrogen monoxide [Fe(II)-NO(+), Fe(II)-NO·, Fe(II)-NO(-), and Fe(II)-HNO]. The activation barriers for the various proton and electron transfer steps were estimated in the framework of Marcus theory. Using the barriers obtained, we simulated the kinetics of the reduction process. We found that the complex recharging process can be accomplished in two possible ways: either through two consecutive proton-coupled electron transfers (PCETs) or in the form of three consecutive elementary steps involving reduction, PCET, and protonation. Kinetic simulations revealed the recharging through two PCETs to be a means of overcoming the predicted deep energetic minimum that is calculated to occur at the stage of the Fe(II)-NO· intermediate. The radical transfer role for the active-site Tyr(218), as proposed in the literature, cannot be confirmed on the basis of our calculations. The role of the highly conserved calcium located in the direct proximity of the active site in proton delivery has also been studied. It was found to play an important role in the substrate conversion through the facilitation of the proton transfer steps.

A first principles study of water oxidation catalyzed by a tetraruthenium-oxo core embedded in polyoxometalate ligands

We present a computational study addressing the catalytic cycle of a recently-synthesized all-inorganic homogeneous catalyst capable to promote water oxidation with low overpotential and high turnover frequency [Sartorel et al., J. Am. Chem. Soc., 2008, 130, 5006; Geletii et al., Angew. Chem., Int. Ed., 2008, 47, 3896]. This catalyst consists of a tetraruthenium-oxo core [Ru(4)O(4)(OH)(2)·(H(2)O)(4)](6+)capped by two polyoxometalate [SiW(10)O(36)](8-) units. The reaction mechanism underpinning its efficiency is currently under debate. We study a reaction cycle involving four consecutive proton-coupled electron transfer (PCET) processes that successively oxidize the four Ru(IV)-H(2)O units of the initial state (S(0)) to the four Ru(V)-OH centers of the activated intermediate (S(4)). The energetics of these electrochemical processes as well as the structural and electronic properties of the reaction intermediates are studied with ab initio Density Functional Theory (DFT) calculations. After characterizing these reaction intermediates in the gas phase, we show that the solvated tetraruthenate core undergoes a solvent-induced structural distortion that brings the predicted molecular geometry to excellent agreement with the experimental X-ray diffraction data. The calculated electronic properties of the catalyst are instead weakly dependent on the presence of the solvent. The frontier orbitals of the initial state as well as the electronic states involved in the PCET steps are shown to be localized on the tetraruthenium-oxo core. The reaction thermodynamics predicted for the intermediate reaction steps is in good agreement with the available cyclic voltammetry measurements up to S(3), but the calculated free energy difference between the initial and the activated state (S(0)/S(4)) turns out to be significantly lower than the thermodynamic limit for water oxidation. Since the oxidizing power of the S(0)/S(4) couple is not sufficient to split water, we suggest that promoting this reaction would require cycling between higher oxidation states.

Characterization of Nanoporous TiO2 Surface Defects by Temperature Dependent Electron Transport Studies on Dye Sensitized Solar Cells

ABSTRACTTemperature dependent, dark current-voltage characteristics (I-V-T) of dye sensitized solar cells (DSSCs) were used to study the TiO2 surface defects on photovoltaic performance. Three kinds of DSSCs were used for this study. (1) DSSC with no surface treatment, (2) DSSCs with Al2O3 and (3) HfO2 treatments. Activation energy of charge transport, obtained from the I-V-T of the three DSSCs, suggest that Al2O2 and HfO2 surface treatment on the nanoporous TiO2 effectively suppressed the electron capture by the surface states from the conduction band of TiO2 and the consecutive electron transfer to the electrolyte in which the TiO2 is permeated. The reference DSSC showed activation energy of 1.03eV while the DSSCs with Al2O3 and HfO2 surface treatment showed 1.27 and 1.31eV respectively. The higher activation energy, in case of HfO2 surface treatment, suggest that electronically active TiO2 surface states present near to the redox potential of the electrolyte was reduced and hence resulted in improved photovoltaic performance than Al2O3 treated DSSC and the reference DSSC.

Observation of bi-polarons in blends of conjugated copolymers and fullerene derivatives

From a fundamental and application point of view it is of importance to understand how charge carrier generation and transport in a conjugated polymer (CP):fullerene blend are affected by the blend morphology. In this work light-induced electron spin resonance (LESR) spectra and transient ESR response signals are recorded on non-annealed and annealed blend layers consisting of alkyl substituted thieno[3,2-b]thiophene copolymers (pATBT) and the soluble fullerene derivative [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) at temperatures ranging from 10 to 180 K. Annealing of the blend sample leads to a reduction of the steady state concentration of light-induced PCBM anions within the blend at low temperatures (T = 10 K) and continuous illumination. This is explained on the basis of the reducing interfacial area of the blend composite on annealing, and the high activation energy for electron diffusion in PCBM blends leading to trapped electrons near the interface with the CP. As a consequence, these trapped electrons block consecutive electron transfer from an exciton on a CP to the PCBM domain, resulting in a relatively low concentration charge carriers in the annealed blend. Analysis of the transient ESR data allows us to conclude that in annealed samples diamagnetic bi-polaronic states on the CPs are generated at low temperature. The formation of these states is related to the generation and interaction of multiple positive polarons in the large crystalline polymer domains present in the annealed sample.

Heterogeneous Consecutive Electron Transfer at Graphite Electrodes under Steady State

In this report, the theory based on thin-layer cyclic voltammetry (TLCV) for consecutive electron transfer (ET) across the interface between two immiscible electrolyte solutions (ITIES) is well developed and experimentally verified. The voltammetric responses to multistep electron transfer at the ITIES are predicted by numerical simulations. Moreover, the impact of empirical parameters on the shape of the multistep current-voltage curve has been examined. The results obtained not only give information regarding the effect of the concentration ratio of the reactants in two phases and the thin-layer thickness on multistep electron transfer, but also prove the excellent agreement between simulations and experiments. The model system of two-step electron transfer of ZnTPP/[Fe(CN)₆]⁴⁻ was studied, indicating that the Bulter-Volmer (B-V) theory is suitable for the consecutive electron transfer. Thus, TLCV is demonstrated to be a useful means for investigating the kinetics of heterogeneous consecutive ET.

Direct evidence of hydrogen-bonding and/or protonation effect on p-benzoquinone electrochemical reduction by in situ IR spectroelectrochemical study

Cyclic voltammetry (CV), IR spectroelectrochemistry cyclic voltabsorptometry (CVA) and derivative cyclic voltabsorptometry (DCVA) technique were used to investigate the electrochemical character of a representative quinone, p-benzoquinone (BQ), in aprotic media, organic-water mixed solvent, unbuffered acidic aqueous ([H +] < [BQ]), and well-buffered aqueous solution ([H +] > [BQ]). Several well-separated IR absorption peak pairs, 1656, 1316; 1502, 1340; and 1230, 1473 cm −1, corresponding to redox transitions (BQ, BQ −, BQ 2−), are observed and can be used to trace the concentration of BQ, BQ − and BQ 2− independently during CV scan. Based on CVA and DCVA techniques, it is clearly distinguished that each step of the two consecutive electron transfer process of BQ in unbuffered neutral aqueous takes place, although only one CV wave is experimentally observed during CV scan. Direct evidence of hydrogen-bonding (H-bonding) forming between BQ 2− (BQ −) and H 2O in unbuffered neutral water and that between BQ and H 3O + in strong acidic solution are observed. The results of IR CVA and DCVA not only help us understand the fundamental factors of H-bonding and protonation on redox mechanism of BQ, but also draw the entire map of the electrochemical reduction of BQ in different solution.

The reaction electronic flux: A new descriptor of the electronic activity taking place during a chemical reaction. Application to the characterization of the mechanism of the Schiff’s base formation in the Maillard reaction

Within the framework defined by the reaction force analysis, the reaction electronic flux, a new descriptor proposed to characterize the electronic activity that takes place during a chemical reaction, is used to elucidate the mechanism of the Schiff’s base formation in the Maillard reaction. The electronic activity is identified as being due to electronic polarization and transfer effects that show up at specific regions along the reaction coordinate. It is found that the Schiff base formation step of the Maillard reaction proceeds through consecutive electron polarization and transfer processes, the energy involved at each step of the reaction is quantified through the reaction works obtained from the reaction force profile.

Cyclic Square Wave Voltammetry of Single and Consecutive Reversible Electron Transfer Reactions

In this report, theory for cyclic square wave voltammetry for single and consecutive reversible electron transfer reactions is presented and experimentally verified. The impact of empirical parameters on the shape of the current-voltage curve is examined. Diagnostic criteria enabling the use of this waveform as a tool for mechanistic analysis of electrode reaction processes are also presented. Since this waveform effectively discriminates against capacitance currents, cyclic square wave voltammetry will enable acquisition of mechanistic information at analyte concentration levels lower than that possible with cyclic voltammetry.

Non-Markovian theory for the waiting time distributions of single electron transfers

We derive a non-Markovian theory for waiting time distributions of consecutive single electron transfer events. The presented microscopic Pauli rate equation formalism couples the open electrodes to the many-body system, allowing to take finite bias and temperature into consideration. Numerical results reveal transient oscillations of distinct system frequencies due to memory in the waiting time distributions. Memory effects can be approximated by an expansion in non-Markovian corrections. This method is employed to calculate memory landscapes displaying preservation of memory over multiple consecutive electron transfers.

Efficient two-step photogeneration of long-lived charges in ground-state charge-transfer complexes of conjugated polymer doped with fullerene

Polarization-sensitive time-resolved visible-infrared pump-probe experiments demonstrate that one can efficiently generate long-lived charges in donor-acceptor charge transfer complex (CTC) of conjugated polymer doped with fullerene, MEH-PPV/dinitroanthraquinone/C(60). In particular, a strong enhancement of the photoinduced charge generation is observed in the red part of the spectrum, i.e. inside the polymer band gap, which makes the current material attractive for photovoltaic applications. The spectroscopic results indicate that enhanced generation of charges is due to a consecutive photoinduced electron transfer from the polymer to the CTC-acceptor in the first step and then, in the second step, to the fullerene. The LUMO energy difference between the CTC-acceptor and fullerene appears to be a key parameter for efficient charge separation in these ternary systems. The results are also discussed in respect to the charge generation processes in widely used polymer-fullerene blends, where formation of weak CTCs has recently been discovered.

Oxygen activation by cytochrome P450 monooxygenase

Unlike photosystem II (PSII) that catalyzes formation of the O-O bond, the cytochromes P450 (P450), members of a superfamily of hemoproteins, catalyze the scission of the O-O bond of dioxygen molecules and insert a single oxygen atom into unactivated hydrocarbons through a hydrogen abstraction-oxygen rebound mechanism. Hydroxylation of the unactivated hydrocarbons at physiological temperatures is vital for many cellar processes such as the biosynthesis of many endogenous compounds and the detoxification of xenobiotics in humans and plants. Even though it carries out the opposite of the water splitting reaction, P450 may share similarities to PSII in proton delivery networks, oxygen and water access channels, and consecutive electron transfer processes. In this article, we review recent advances in understanding the molecular mechanisms by which P450 activates dioxygen.

Interference effects in the counting statistics of electron transfers through a double quantum dot

We investigate effects of quantum interferences and Coulomb interaction on the counting statistics of electrons crossing a double quantum dot in a parallel geometry by using a generating function technique based on a quantum master equation approach. The skewness and the average residence time of electrons in the dots are shown to be the quantities most sensitive to interferences and Coulomb coupling. The joint probabilities of consecutive electron transfer processes show characteristic temporal oscillations due to interference. The steady-state fluctuation theorem that predicts a universal relation between the number of forward and backward transfer events is shown to hold even in the presence of the Coulomb coupling and interference.

Electronic Wiring of a Multi-Redox Site Membrane Protein in a Biomimetic Surface Architecture

Bioelectronic coupling of multi-redox-site membrane proteins was accomplished with cytochrome c oxidase (C cO) as an example. A biomimetic membrane system was used for the oriented immobilization of the C cO oxidase on a metal electrode. When the protein is immobilized with the C cO binding side directed toward the electrode and reconstituted in situ into a lipid bilayer, it is addressable by direct electron transfer to the redox centers. Electron transfer to the enzyme via the spacer, referred to as electronic wiring, shows an exceptionally high rate constant. This allows a kinetic analysis of all four consecutive electron transfer steps within the enzyme to be carried out. Electron transfer followed by rapid scan cyclic voltametry in combination with surface-enhanced resonance Raman spectroscopy provides mechanistic and structural information about the heme centers. Probing the enzyme under turnover conditions showed mechanistic insights into proton translocation coupled to electron transfer. This bioelectronic approach opens a new field of activity to investigate complex processes in a wide variety of membrane proteins.

Single-Electron Counting Spectroscopy: Simulation Study of Porphyrin in a Molecular Junction

Electron counting of a single porphyrin molecule between two electrodes shows a crossover from sub- to super-Poissonian statistics as the bias voltage is scanned. This is attributed to the simultaneous activation of states with electron transfer rates spanning several orders of magnitude. Time-series analysis of consecutive single-electron transfer events reveals fast and slow transport channels, which are not resolved by the average current alone.