Homogeneous Electron Transfer Reactions Research Articles

Theory of normal-pulse voltammetric current of electron transfer reaction at liquid/liquid interface

Theory of normal-pulse voltammetric current of electron transfer (ET) reaction at organic solvent(O)/water(W) interface between a redox couple of A species, A RED and A OX , in O phase and another redox couple of B species, B OX and B RED , in W phase has been discussed. A RED is supposed to be a neutral molecule and A OX a cation, whereas both B RED and B OX are supposed to be highly hydrophilic. Then, two kinds of the reaction mechanism, that is, (1) the heterogeneous ET reaction: A RED (O) + B OX (W) = A OX (O) + B RED (W) at the O/W interface producing the voltammetric current, and (2) the transfer of A RED (O) molecule from O to W phase across the interface, then the homogeneous ET reaction: A RED (W) + B OX (W) = A OX (W) + B RED (W) in W phase, followed by the transfer of A OX (W) cation from W to O phase producing the voltammetric current, are considered. Theoretical equations of normal-pulse voltammetric current of the ET reaction at O/W interface are derived under the assumptions (i) that the ET reaction is pseudo-monomolecular with respect to A species, (ii) that the partition of A RED molecule between O and W phases is highly biased to O phase, and (iii) that the rate constants of homogeneous ET reaction in W phase are sufficiently large. Application of the theoretical equations to investigate the mechanism of the ET reaction at O/W interface has been discussed.

The micro-optical ring electrode Part 2: Theory for the transport limited, steady-state photocurrent

The micro-optical ring electrode (MORE) is a photoelectrochemical device based on a ring microelectrode that uses the insulating material interior to the ring electrode as a light guide. In this paper, we derive asymptotic analytical expressions for the steady-state, transport limited photocurrent generated at MOREs with thin microrings ((ring inner radius)/(ring outer radius) values > 0.99) for two general types of photoelectrochemical system (a) the PE (photophysical-electrochemical) system, wherein the photoexcited species itself is directly detected on the ring; and (b) the PCE (photophysical-chemical-electrochemical) system, wherein the photoexcited species undergoes a homogeneous electron transfer reaction prior to electrochemical detection. The expressions are generated by exploiting the properties of discontinuous integrals of Bessel functions to solve the diffusion equation for the photogenerated electroactive species both inside and outside the beam. The resultant solutions are then matched at the beam surface. The expressions themselves are used to design experimental protocols that allow for the complete characterization of the photoelectrochemical kinetics of a system and are tested by using them to interpret the results of a MORE study of the photoelectrochemical behaviour of the Ru(bipy)(3)(2+)/Fe3+ photosensitiser/ quenching agent system. The value of the Stern-Volmer constant for the quenching of photoexcited Ru(bipy)(3)(2+) by Fe obtained (0.36 m(3) mol(-1)) compares favourably with the value obtained from fluorescence measurements (0.9 m(3) mol(-1)). (c) 2006 Elsevier B.V. All rights reserved.

Electrochemical amplification using selective self-assembled alkanethiolate monolayers on gold: A predictive mechanistic model

Electroanalytical current measurements are often challenged by sensitivity limitations. For applications requiring enhanced sensitivity, an electrochemical signal can be amplified using a homogeneous electron-transfer reaction. This report describes the mechanistic details of a redox recycling amplification system based on monomolecular films of alkanethiolates on gold. These interfaces can be prepared to exhibit facile oxidation of functionalized ferrocenes (FcX), while markedly suppressing the electrolysis of ferrocyanide. Thus, ferrocyanide can be used as a sacrificial electron donor to electrogenerated FcX +, providing a pathway for a notable improvement of the detection limit for FcX. Electrochemical methods are used to demonstrate, evaluate, and quantify the amplification mechanism. The dependence of amplification on the rates of heterogeneous and homogeneous electron transfer is qualitatively delineated by digital simulations of cyclic voltammetric (CV) curves. This work also develops a mathematical model for prediction of the maximum amplification observed via CV that is tested by comparisons to experimental data. These results are then used as a guide in performance assessments, which yield an optimal amplification of 225 and a limit of detection of ∼500 nM.

Homogeneous versus Heterogeneous Self-Exchange Electron Transfer Reactions of Metal Complexes: Insights from Pressure Effects

Homogeneous versus Heterogeneous Self-Exchange Electron Transfer Reactions of Metal Complexes: Insights from Pressure Effects

Voltammetric Study of Interfacial Electron Transfer between Bis(cyclopentadienyl)iron in Organic Solvents and Hexacyanoferrate in Water

The electron-transfer reaction between bis(cyclopentadienyl)iron(II) ([Fe(II)(C5H5)2]) in nitrobenzene and a hexacyanoferrate redox couple ([Fe(II/III)(CN)6](4-/3-)) in water at the nitrobenzene / water interface was studied using normal pulse voltammetry. The voltammetric results indicate that the electron-transfer reaction takes place by way of a so-called ion-transfer (IT) mechanism, of which the forward and backward rate constants of the homogeneous electron-transfer reaction between [Fe(II/III)(C5H5)2](0/+) and [Fe(II/III)(CN)6](4-/3-) in the water phase have been determined. The electron-transfer reaction between [Fe(II)(C5H5)2] in 1,2-dichloroethane and [Fe(II/III)(CN)6](4-/3-) in water at the 1,2-dichloroethane / water interface was shown to also take place by the IT-mechanism.

Electron transfer reactions between copper(II) porphyrin complexes and various oxidizing reagents in acetonitrile.

Homogeneous electron transfer reactions of the Cu(II) complexes of 5,10,15,20-tetraphenylporphyrin (TPP) and 2,3,7,8,12,13,17,18-octaethylporphyrin (OEP) with various oxidizing reagents were spectrophotometrically investigated in acetonitrile. The reaction products were confirmed to be the pi-cation radicals of the corresponding Cu(II)-porphyrin complexes on the basis of the electronic spectra and the redox potentials of the complexes. The rate of the electron transfer reaction between the Cu(II)-porphyrin complex and solvated Cu(2+) was determined as a function of the water concentration under the pseudo first-order conditions where Cu(2+) is in large excess over the Cu(II)-porphyrin complex. The decrease in the pseudo first-order rate constant with increasing the water concentration was attributed to the stepwise displacement of acetonitrile in [Cu(AN)(6)](2+)(AN = acetonitrile) by water, and it was concluded that only the Cu(2+) species fully solvated by acetonitrile, [Cu(AN)(6)](2+), possesses sufficiently high redox potential for the oxidation of Cu(ii)-OEP and Cu(ii)-TPP. The reactions of the Cu(II)-porphyrin complexes with other oxidizing reagents such as [Ni(tacn)(2)](3+)(tacn = 1,4,7-triazacyclononane) and [Ru(bpy)(3)](3+)(bpy = 2,2'-bipyridine) were too fast to be followed by a conventional stopped-flow technique. Marcus cross relation for the outer-sphere electron transfer reaction was used to estimate the rate constants of the electron self-exchange reaction between Cu(II)-porphyrin and its pi-cation radical: log(k/M(-1) s(-1))= 9.5 +/- 0.5 for TPP and log(k/M(-1) s(-1))= 11.1 +/- 0.5 for OEP at 25.0 degrees C. Such large electron self-exchange rate constants are typical for the porphyrin-centered redox reactions for which very small inner- and outer-sphere reorganization energies are required.

Electrosynthesis of polyphenylpyrrole coated silver particles at a liquid–liquid interface

This communication reports the production of polyphenylpyrrole coated silver nanoparticles at the liquid/liquid interface by an EC-type mechanism. In the electrochemical step of the reaction N-phenylpyrrole facilitates the transfer of the silver ion from an aqueous to an organic phase. This step is followed by a slow homogeneous electron transfer reaction from the N-phenylpyrrole to the silver ion followed by polymerization and metal cluster growth.

A fluorescence-based imaging-fiber electrode chemical sensor for hydrogen peroxide

The fabrication and characterization of imaging fiber electrodes (IFEs) is presented, and the concept of an electrochemically-modulated, fluorescence-based, imaging-fiber electrode chemical sensor (IFECS) is demonstrated. In brief, an imaging fiber distal tip was sputter-coated with a semi-transparent gold layer to create an IFE, and a fluorescent redox dye was immobilized across the IFE face to create an IFECS. An electroactive analyte was imaged by monitoring the change in the immobilized redox dye’s fluorescence following its homogeneous electron transfer reaction with the electroactive analyte. Fluorescence images were collected through the IFECS and detected by an epi-fluorescence microscope/CCD imaging system. Cyclic voltammetry and fluorescence microscopy were utilized to characterize the electrochemical and fluorescence properties of IFECSs. Reversible voltammetry was observed for the rhodamine B isothiocyanate (RBITC) redox couple at IFEs. IFECSs using RBITC as the fluorescent redox dye and Nafion as the immobilization polymer were fabricated to detect hydrogen peroxide. The IFECS’s RBITC-fluorescence was decreased by ∼27% upon exposure to 250 μM hydrogen peroxide and ∼95% of the original fluorescence was observed following the electrochemical regeneration of the Nafion-immobilized RBITC. These IFECSs represent the first union in which remote fluorescence imaging of an electroactive analyte can be performed through the actual fiber-optic electrode itself.

Electrochemical Electron Transfer Reactions of M(ttcn)2 3+/2+ (M = Co, Pd, Pt, Au; ttcn = 1,4,7-trithiacyclononane): the Relation of Reaction Volumes and Electron Transfer Rate Constants

The reaction volumes (ΔV0 M), corresponding to the volume change for the half-cell reactions for a series of M(ttcn)2 3+/2+ couples were measured by using cyclic voltammetry at elevated pressures. The ΔV0 M valuesreflect the changes in the bond length between M and ligands accompanying the changes in the oxidation state of M from 3+ to 2+. The values of the estimated ΔV0 M are in the order of Co < Au < Pd < Pt. The reaction volume is larger for the slower ET reactions, indicating that the reaction volumes reflect the inner-sphere contribution to the activation free energies. The heterogeneous electron exchange rate constants for the M(ttcn)2 3+/2+ couples, kel, were measured by AC voltammetry, and by cyclic voltammetry. The latter method on the basis of the Kochi-Nicholson’s method was carried out to examine the reliability of this rather simple method by setting the scan rate very fast to make the redox system irreversible. The results obtained by these two methods are in fair agreement with each other, especially when a glassy carbon working electrode was used for the measurements. The relation between the activation free energies for the heterogeneous electron exchange rate constants, ΔG* el, and those for the homogeneous reactions, ΔG* ex, is linear, which indicates that the homogeneous electron transfer reactions of a series of M(ttcn)2 3+/2+ is also governed by the intrinsic structural change. However, the slope of log kel vs. log kex plot is far smaller than 0.5 as predicted by the Marcus theory.

Electrochemical Kinetics of Cyanometalate Complexes in Aqueous Solution at High Pressures.

For the aqueous couples Os(CN)(6)(3-/4-), Mo(CN)(8)(3-/4-), and W(CN)(8)(3-/4-), volumes of reaction DeltaV(Ag/AgCl) relative to Ag/AgCl/4.0 mol L(-)(1) KCl and volumes of activation DeltaV(el)() for the electrode reactions are reported. Values of DeltaV(Ag/AgCl) are consistent with a very small increase in the metal-carbon bond length on reduction in each case, the main component of DeltaV(Ag/AgCl) (other than that of the reference electrode) being electrostrictive solvation change. For media in which the cation is 0.5 mol L(-)(1) K(+) or (for Mo) Na(+), DeltaV(el)() is strongly positive (+9.4 +/- 0.7, +7.3 +/- 0.7, and +10.8 +/- 0.4 cm(3) mol(-)(1) for Os(CN)(6)(3-/4-), Mo(CN)(8)(3-/4-), and W(CN)(8)(3-/4-), respectively, at 25 degrees C), whereas the theoretical expectation for a mechanism involving only the cyanometalate anions is -3 +/- 1 cm(3) mol(-1). For Mo(CN)(8)(3-/4-) in Et(4)NCl, however, DeltaV(el)() is -4.2 +/- 0.2 cm(3) mol(-)(1). In all cases, DeltaV(el)() is just one-half of the corresponding parameter DeltaV(ex)() for the homogeneous (bimolecular) self-exchange reaction of the same couple, giving strong confirmation of the "fifty-percent rule" (Fu, Y.; Swaddle, T. W. J. Am. Chem. Soc. 1997, 119, 7137). These and related results are interpreted in terms of a mechanism for both electrode and homogeneous electron-transfer reactions of cyanometalates in which the counterion mediates the electron-transfer process. For alkali metal cations, partial deaquation to permit this mediation results in positive DeltaV(el)() values, whereas for tetraalkylammonium counterions, there are no aqua ligands to be removed and DeltaV(el)() is "normal".

FT-EPR Study of Spin-correlated Radical Pairs Formed by Electron Transfer Quenching of Porphyrin Triplets in Micellar Solution

The spin-correlated radical pairs (SCRPs) formed by photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin ( ZnTPPS ) to quinones in micelles of the cationic surfactant cetyltrimethylammonium chloride ( CTAC ) were studied by means of Fourier transform EPR (FT-EPR). It is shown that variation of the power of the microwave pulse allows the separation of EPR signals arising from SCRPs and free radicals. The measured kinetics of radical formation can be accounted for in terms of a statistical model taking into account the non-uniform distribution of the solutes over the micelles. The rate constant of electron transfer quenching (kq) of the ZnTPPS triplet state by duroquinone (DQ) is found to be 1.05 × 106 s−1. The FT-EPR measurements gave information also on the kinetics of the homogeneous electron transfer reaction DQ − + DQ → DQ + DQ − in CTAC solution in which the DQ − anion radicals were generated by light-induced electron transfer from ZnTPPS . It is found that the dependence of the rate of this reaction on quinone concentration deviates from the linear relationship found in homogeneous solutions. A statistical model is proposed to account for the data. Based on this model, the rate constant of the self-exchange reaction (k ex ) is 4.1 × 106 s−1. From results obtained with duroquinone and benzoquinone as acceptors, it is concluded that ZnTPPS is located at the micelle/water interface.

Successive electron-transfers in low ionic strength solutions. Migrational flux coupling by homogeneous electron transfer reactions

Analysis of successive heterogeneous electron transfer reactions requires consideration of the coupling of molecular fluxes by homogeneous electron transfer reactions. We demonstrate that bimolecular electron transfer between members of the redox system that differ by +2 in oxidation state (i.e., reproportionation) can strongly influence the magnitude of transport-limited currents when ion migration contributes to molecular transport. A theoretical description of the problem is developed in the limit of infinitely fast homogeneous electron transfer and identical diffusion coefficients. An analytic solution is presented that allows calculation of voltammetric limiting currents at microelectrodes for any stepwise n-electron transfer as a function of the ratio of redox and supporting electrolyte concentrations. The results demonstrate that migrational flux coupling of homogeneous electron transfer reactions must be included in the reaction mechanism to obtain even qualitatively accurate predictions of the current. The theory is found to be in good agreement with values of voltammetric currents measured at an inlaid 11.2-μm radius Pt microdisk for the two- and three-electron reductions of Ru(bpy)32+ in acetonitrile+toluene solutions.

Electrochemical measurement of second-order electron transfer rate constants for the reaction between cytochrome b5 and cytochrome c.

The second-order electron transfer reaction between cytochrome b5 and cytochrome c has been studied by cyclic voltammetry utilizing a gold electrode modified with beta-mercaptopropionate. When cyclic voltammetry is performed on a solution containing a mixture of cytochrome b5, cytochrome c and polylysine, cytochrome b5 undergoes reversible electrochemistry at the electrode surface while cytochrome c discriminates against the electrode surface. The selectivity of the modified electrode for negatively charged proteins makes it possible to selectively reduce a protein possessing a net negative charge and a relatively low reduction potential (outer mitochondrial membrane cytochrome b5, Eo = -102 mV; microsomal cytochrome b5, Eo = 3 mV) in the presence of another protein possessing a net positive charge and a relatively high reduction potential (cytochrome c, Eo = 265 mV). The electrochemical reduction of ferricytochrome b5 at the electrode surface is followed by a second-order electron transfer reaction between ferrocytochrome b5 and ferricytochrome c that yields ferricytochrome b5 and ferrocytochrome c. This fast homogeneous electron transfer reaction which is preceded by a heterogeneous electron transfer reaction results in a characteristic cyclic voltammogram containing a pre-peak to the reduction current. The second-order rate constant for the homogeneous reaction was obtained by invoking the above reaction scheme for digital simulation of a cyclic voltammogram which was subsequently fitted to the experimental data. Second-order rate constants obtained with this method are 2.9 x 10(8) and 8.9 x 10(8) for the homogeneous electron transfer reactions between rat liver outer mitochondrial membrane (OM) ferrocytochrome b5 and beef liver microsomal ferrocytochrome b5, with horse heart ferricytochrome c, respectively. These values are in good agreement with second-order rate constants obtained for the same protein systems by flash photolysis. [Meyer, T. E., Rivera, M., Walker, F. A., Mauk, M. R., Mauk, A. G., Cusanovich, M. A., & Tollin, G. (1993) Biochemistry 32, 622-627].

The effects of uncompensated solution resistance and rate of the homogeneous electron transfer reaction on electrochemiluminescence transients

In double-step electrochemiluminescence experiments, the luminescence emission transients contain interesting information on the electrode reactions involved, the coupled mass transport and the homogeneous annihilation reaction. It was found that the start of luminescence emission, following the second potential step, occurs only after a delay of the order of 100 μs. The effect of Förster-type energy transfer from the electronically excited molecules to the metal electrode is insignificant under the experimental conditions considered. Rather, it is shown that this delay is caused by the IRu drop, which is closely related to the time constant RuCDL, where Ru is the uncompensated electrolyte resistance and CDL is the double-layer capacitance. ECL transients generated with the digital simulation technique using this information agree with the experimental transients. The rate constant of the homogeneous annihilation reaction is derived from such fits.

Coordination, electron transfer and catalytic chemistry of a ruthenium-substituted heteropolytungstate anion as revealed in its electrochemical behavior

The rates of substitution of π-acidic ligands for the aqua ligand in the ruthenium-substituted heteropolytungstate anion (H2O)RuIIPW11O5−39 were measured and compared with those of more familiar complexes of Ru(II). The weakly π-acidic character of the heteropolytungstate anionic ligand produced a decrease in the lability of the aqua ligand. The kinetics of homogeneous electron transfer reactions involving the (H2O)RuIIPW11O5−39 anion were complicated by its high charge and tendency to precipitate with cationic oxidants. However, the specific rate of its reduction of O2 to H2O2 was in good agreement with the value calculated from the Marcus relation. The RuIIIOH2 center in (H2O)RuIIIPW11O4−39 is reversibly oxidizable in two steps to RuIV = O and RuV = O. The RuV = O complex oxidizes benzyl alcohol, methanol and 2-propanol, but the reaction rates are too small to make the complex attractive as an oxidation catalyst.

Heterogeneous and Homogeneous Electron Transfer Reactions of Tetrathiafulvalene in Ambient Temperature Chloroaluminate Molten Salts

The electrochemistry of tetrathiafulvalene (TTF) in an ambient temperature chloroaluminate molten salt composed of 1‐ethyl‐3‐methylimidazolium chloride mixed with has been examined. In both basic and acidic melts TTF is oxidized in two consecutive, one‐electron steps to TTF+ and then to TTF2+. In basic melts, neutral TTF is stable and its oxidations are well‐behaved on the voltammetric time scale. TTF2+ is adsorbed at a glassy carbon working electrode in basic melts. Constant current electrolysis of TTF to TTF+ at a Pt electrode in basic melt produces purple crystals which appear to be electronically conductive. In acidic melts , present as an impurity, oxidizes TTF to TTF+. Diffusion coefficient measurements by pulse voltammetry suggest that TTF forms a slowly diffusing adduct in acidic melts.

Some aspects of the homogeneous (chemical) electron exchange near an interface

The influence of electrode on the homogeneous (chemical) electron transfer (ET) reaction occurring near it has been considered. Using the Langevin dipole (LD) method a model electron exchange reaction between a donor (aluminium atom) and an acceptor (hydroxonium cation) is investigated. The solvent (water) reorganization energies. ET activation energies and their different contributions have been calculated depending on the localization of this reaction pair regarding electrode. A catalytic effect of electrode on the ET homogeneous reaction occurring near it has been studied.

Theoretical aspects of the homogeneous electron transfer reaction kinetics in electrolytes containing chlorohydrogen

A theoretical analysis of the factors conditioning high and independent of electrode potential rate of metal dissolution in dry (without water) chlorohydrogen electrolytes, in tetrahydrofuran and the lowest alcohols has been performed. A relationship between the dissolution rates and the electron affinity values of the oxonium associates stable in the considered electrolytes has been discussed.

Comparison of heterogeneous and homogeneous electron-transfer rates for some nitroalkanes and diketones

Redox catalysis has been used to investigate the homogeneous electron-transfer reactions between the substrates (nitroalkane or diketone) and the reduced form of the catalyst, which was terephthalonitrile. The substrates were RNO 2 (R=Me, Et, i-Pr, and t-Bu) and RC(O)C(O)R (R=Me, Et, and t-Bu). The experiments were conducted in acetonitrile solvent at 298 K with three different electrolytes, R 4 NClO 4 (R=Et, n-Bu, and n-C 7 H 15 )

Self-exchange electron transfer rate constants and reorganization energies for some aromatic compounds in N,N-dimethylformamide determined by elect

The rate constants for homogeneous self-exchange electron transfer reactions of nine aromatic and heteroaromatic compounds in N,N-dimethylformamide have been measured by electron spin resonance line-broadening experiments. The aromatic anion radicals were generated electrochemically in a flow cell. The measured rate constants are compared with literature values when possible and self-exchange reorganization energies λ have been calculated. It is found that for all the aromatic and heteroaromatic compounds, except benzophenone, λ 11 ± 1.5 kcal mol −1. For benzophenone a significantly higher value for λ is found (14.7 kcal mol −1. An electrochemical flow cell connected with the cavity of an ESR spectrometer has been used to measure self-exchange ET rate constants and self-exchange reorganization energies for a number of aromatic and heteroaromatic compounds. The obtained values agree with some of the literature values, but with others the differences observed are unsatisfactory and inexplicable. It seems a general problem with such measurements that although accurate measurements (± 10%) can be made in several laboratories, the results obtained in different laboratories may deviate by more than 100%. When comparing reorganization energies for different compounds it seems safest to compare values obtained in the same investigation.