Voltammetric Time Scale Research Articles

Straightforward determination of homogeneous kinetics using Fourier transform alternating current voltammetry (FTACV)

This work describes a novel method by which the homogeneous kinetics for an ErevCirrev system can be deduced using only the forward scan of second harmonic FTACV. We show that ratio of the heights of the two lobes of the 2nd harmonic voltammograms readily provides this information by fitting data to a working curve modelled by the simple equation ilobe,right/ilobe,left=1/(1+kτs), where τs is the characteristic voltammetric timescale. The validity of the relationship was tested by comparison with both simulated and experimental data. This novel approach offers the advantage of quick and easy determination of kf, while increasing accuracy by omitting the ambiguity in determining a baseline and minimising the effects of electrode passivation. We also show that these changes can be explained by considering the change in surface concentration of the [Ox] and [Red] species over the course of a scan, which are more profoundly expressed in the FTACV response compared with CV.

Quinone voltammetry for redox-flow battery applications

The electrochemical behaviour of octafluoro-9,10-anthroquinone is reported, motivated by the use of this lipophilic quinone as the negative electrode couple (anolyte) in redox flow battery applications. At low concentrations, the reduction mechanism is found to follow the classic EE process, expected for quinones in aprotic solution and both anionic species are stable on the voltammetric timescale. The reduction process is, however, strongly influenced by a number of factors, including the identity of the electrolyte cation. The dianion is also unstable on longer timescales, with a disproportionation mechanism affecting the stability of reduced products, even at potentials above the reduction potential of the mono-anion. An insoluble film is shown to form on the electrode surface, which gradually degrades the voltammetric response. Conclusions from the electrochemical data are supported by in situ EPR and ex situ NMR spectroscopy.

Reversible Multielectron Release from Redox-Active Three-Dimensional Molecular Barrels with Ruthenium-Alkenyl Moieties.

Three-dimensional molecular barrels Ru6-4 and Ru6-5 were synthesized in high yields from dinuclear ruthenium-vinyl clamps and tritopic triphenylamine-derived carboxylate linkers and characterized by multinuclear NMR spectroscopy including 1H-1H COSY and 1H DOSY measurements, high-resolution electrospray ionization mass spectrometry, and X-ray crystallography. The metal frameworks of the cages adopt the shape of twisted trigonal prisms, and they crystallize as racemic mixtures of interdigitating Δ- and Λ-enantiomers with a tight columnar packing in Ru6-4. Electrochemical studies and redox titrations revealed that the cages are able to release up to 11 electrons on the voltammetric timescale and that their cage structures persist up to the hexacation level. IR and UV-vis-near-infrared spectroelectrochemical studies confirm substituent-dependent intramolecular electronic communication within the π-conjugated 1,3-divinylphenylene backbone in the tricationic states, where all three divinylphenylene-bridged diruthenium clamps are present in mixed-valent radical cation states. The formation of 1:3 charge-transfer salts with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane as the electron acceptor is also demonstrated.

Voltammetry in sheep's blood: Membrane-free amperometric measurement of O2 concentration

An amperometric method was applied for the electroanalytical measurement of oxygen content in sheep's blood. This method was based on a bare platinum microdisc electrode coupled with the use of chronoamperometry. A linear relationship between the chronoamperometric current and the oxygen concentration was observed in both saline solution and sheep's blood. The developed method was able to measure the oxygen percentage with an error of ca. 1.3% in sheep's blood. In addition, this article presents the first study on direct voltammetry in sheep's blood and a dissociative CE process was proposed to explain the electrochemical behaviour of oxygen reduction in blood on a platinum electrode in which the ‘free’ oxygen was first dissociated from oxyhaemoglobin prior to electron transfer with the magnitude of the observed current controlled by the diffusion of oxyhaemoglobin to the electrode where for sufficiently large electrodes (greater than ca. 1 μm in radius) the dissociation proceeds to completion on the voltammetric timescale allowing quantitative measurements.

Reversible Proton-Coupled Reduction of an Iron Nitrosyl Porphyrin within [DBU-H]+-Based Protic Ionic Liquid Nanodomains.

The reduction of [Fe(OEP)(NO)] has been studied in the presence of aprotic room-temperature ionic liquids (RTIL) and protic (PIL) ionic liquids dissolved within a molecular solvent (MS). The cyclic voltammetric results showed the formation of RTIL nanodomains at low concentrations of the RTIL/PIL solutions. The pKa values of the two PILs studied (i.e., trialkylammonium and [DBU-H]+-based ionic liquids) differed by four units in THF. While voltammetry in solutions containing all three RTILs showed similar potential shifts of the first reduction of [Fe(OEP)(NO)] to [Fe(OEP)(NO)]- at low concentrations, significant differences were observed at higher concentrations for the ammonium PIL. The trialkylammonium cation had previously been shown to protonate the {FeNO}8 species at room temperature. Visible and infrared spectroelectrochemistry revealed that the [DBU-H]+-based PIL formed hydrogen bonds with [Fe(OEP)(NO)]- rather than formally protonating it. Despite these differences, both PILs were able to efficiently reduce the nitrosyl species to the hydroxylamine complex, which could be further reduced to ammonia. On the voltammetric time scale and when the switching potential was positive of the Fe(II)/Fe(I) potential, the hydroxylamine complex was re-oxidized back to the NO complex via direct oxidation of the coordinated hydroxylamine at low scan rates or initial oxidation of the ferrous porphyrin at high scan rates. The results of this work show that, while [DBU-H]+ does not protonate electrochemically generated [Fe(OEP)(NO)]-, it still plays an important role in efficiently reducing the nitroxyl ligand via a series of proton-coupled electron transfer steps to generate hydroxylamine and eventually ammonia. The overall reaction rates were independent of the PIL concentration, consistent with the nanodomain formation being important to the reduction process.

Effect of the 2-R-Allyl and Chloride Ligands on the Cathodic Paths of [Mo(η3-2-R-allyl)(α-diimine)(CO)2Cl] (R = H, CH3; α-diimine = 6,6'-Dimethyl-2,2'-bipyridine, Bis(p-tolylimino)acenaphthene).

The new, formally Mo(II) complexes [Mo(η3-2-R-allyl)(6,6′-dmbipy)(CO)2Cl] (6,6′-dmbipy = 6,6′-dimethyl-2,2′-bipyridine; 2-R-allyl = allyl for R = H, 2-methallyl for R = CH3) and [Mo(η3-2-methallyl)(pTol-bian)(CO)2Cl] (pTol-bian = bis(p-tolylimino)acenaphthene) share, in this rare case, the same structural type. The effect of the anionic π-donor ligand X (Cl– vs NCS–) and the 2-R-allyl substituents on the cathodic behavior was explored. Both ligands play a significant role at all stages of the reduction path. While 2e–-reduced [Mo(η3-allyl)(6,6′-dmbipy)(CO)2]− is inert when it is ECE-generated from [Mo(η3-allyl)(6,6′-dmbipy)(CO)2(NCS)], the Cl– ligand promotes Mo–Mo dimerization by facilitating the nucleophilic attack of [Mo(η3-allyl)(6,6′-dmbipy)(CO)2]− at the parent complex at ambient temperature. The replacement of the allyl ligand by 2-methallyl has a similar effect. The Cl–/2-methallyl ligand assembly destabilizes even primary radical anions of the complex containing the strongly π-accepting pTol-Bian ligand. Under argon, the cathodic paths of [Mo(η3-2-R-allyl)(6,6′-dmbipy)(CO)2Cl] terminate at ambient temperature with 5-coordinate [Mo(6,6′-dmbipy)(CO)3]2– instead of [Mo(η3-2-R-allyl)(6,6′-dmbipy)(CO)2]−, which is stabilized in chilled electrolyte. [Mo(η3-allyl)(6,6′-dmbipy)(CO)2]− catalyzes CO2 reduction only when it is generated at the second cathodic wave of the parent complex, while [Mo(η3-2-methallyl)(6,6′-dmbipy)(CO)2]− is already moderately active at the first cathodic wave. This behavior is fully consistent with absent dimerization under argon on the cyclic voltammetric time scale. The electrocatalytic generation of CO and formate is hampered by the irreversible formation of anionic tricarbonyl complexes replacing reactive [Mo(η3-2-methallyl)(6,6′-dmbipy)(CO)2]2 along the cathodic route.

Mechanism of hydrazine oxidation at Palladium electrodes: Long-lived radical di-cation formation

The mechanism of the catalytic oxidation of hydrazine at Palladium (Pd) electrodes was studied in aqueous solutions between pH 2 and 11. The voltammetry recorded at pH 2 and 11 revealed that both the unprotonated hydrazine N2H4 and the protonated form N2H5+ are electro-active at the Pd surface in contrast to glassy carbon (GC) where N2H4 is the only species which undergoes oxidation in the potential range of 0.2 to 1.0 V (vs the Saturated Calomel Electrode).An unexpected reductive voltammetric wave was observed during the cyclic voltammetry of the oxidation of protonated hydrazine and concluded to originate from the reduction of a radical di-cation N2H5•2+ which is stable on the voltammetric timescale. The di-cation was inferred to result from the loss of one electron from the single lone pair of electrons on N2H5+. It is suggested that, unlike the case of N2H4, the absence of a lone pair on the N adjacent to that being oxidised as a result of protonation leads to the stability of the radical di-cation whereas in the oxidation of N2H4, the available adjacent lone pair facilitates rapid follow up chemical reaction leading to nitrogen formation.

The Electro-Oxidation of Hydrazine: A Self-Inhibiting Reaction.

The electro-oxidation of hydrazine to form dinitrogen is reported over a wide range of both pH and unbuffered conditions at glassy carbon electrodes. It is shown that hydrazine molecules are only electro-active in their unprotonated form, N2H4, whereas the protonated species N2H5+ is electro-inactive. The oxidation of N2H4 releases four protons per molecule which are diffusing away from the electrode to rapidly (on the voltammetric time scale) protonate unreacted N2H4 molecules diffusing to the electrode converting them into the electro-inactive form, N2H5+; the reaction is self-inhibiting, and the currents flowing are significantly reduced compared to those expected for a simple electrolytic conversion to an extent reflecting the pH and buffer content of the solution local to the electrode. The local pH in turn is controlled partly by the quantity of protons released electrolytically. The self-inhibition is modeled by solving the relevant transport equations with coupled homogeneous chemical kinetics, utilizing Marcus-Hush electron transfer, giving predicted reduced currents reflecting the pKa and kinetics of the N2H4/N2H5+ equilibrium in excellent agreement with experimental voltammetric wave shapes.

New perspective on ECE mechanism of monohydroxycinnamic acid oxidation with carbon nanotube electrode

In this study, the electrochemical-chemical-electrochemical (ECE) mechanism of monohydroxycinnamic (ferulic, sinapic, and p-coumaric) acid oxidation is presented from a new perspective. Successful mechanistic interpretation employs a carbon nanotube (CNT)-carboxymethylcellulose (CMC) electrode, which exhibits distinctive ECE behavior in cyclic voltammetry (CV), unlike other electrodes. The proposed mechanism is confirmed by simulations using DigiElch software. Monohydroxycinnamic acids are oxidized by an EECE mechanism (ErErCiEr = A ⇄ B + e−; B ⇄ C + e−; C ⇄ D; D + 2e− ⇄ E). The first electrochemical step (ErEr) comprises two consecutive oxidations (A ⇄ B + e−; B ⇄ C + e−), which appear as separate anodic peaks. The reaction at the less positive potential involves oxidation of the hydroxyl group to a phenoxy radical (B), which exists in three mesomeric forms that are oxidized in the second reaction to cations (C) that undergo hydrolysis to their respective o-quinones (D). The hydrolysis reaction (C → D) exhibits a large rate constant and occurs rapidly on the voltammetric time scale. Thus, the overall ErErCi process appears to be chemically irreversible. Finally, the monohydroxycinnamic acids are converted into dihydroxycinnamic acids (caffeic or 5-hydroxyferulic acids), which condense into a reversible, two-electron catechol/o-quinone redox system (D + 2e− ⇄ E). Electrochemical quantitation is based on the proportionality of the peak currents to the monohydroxycinnamic acid concentration. The voltammetrically determined monohydroxycinnamic acid content in real food samples agrees with the high-performance liquid chromatography results. The CNT-CMC electrode extends fundamental studies of the ECE mechanism to systems that cannot be examined using conventional electrodes, thereby demonstrating its utility in electrochemical studies.

Electrochemical Reduction of [Ni(Mebpy)3]2+: Elucidation of the Redox Mechanism by Cyclic Voltammetry and Steady‐State Voltammetry in Low Ionic Strength Solutions

AbstractBipyridine complexes of Ni are used as catalysts in a variety of reductive transformations. Here, the electroreduction of [Ni(Mebpy)3]2+ (Mebpy=4,4′‐dimethyl‐2,2′‐bipyridine) in dimethylformamide is reported, with the aim of determining the redox mechanism and oxidation states of products formed under well‐controlled electrochemical conditions. Results from cyclic voltammetry, steady‐state voltammetry (SSV) and chronoamperometry demonstrate that [Ni(Mebpy)3]2+ undergoes two sequential 1e reductions at closely separated potentials (E10’=−1.06±0.01 V and E20’=−1.15±0.01 V vs Ag/AgCl (3.4 M KCl)). Homogeneous comproportionation to generate [Ni(Mebpy)3]+ is demonstrated in SSV experiments in low ionic strength solutions. The comproportionation rate constant is determined to be >106 M−1 s−1, consistent with rapid outer‐sphere electron transfer. Consequentially, on voltammetric time scales, the 2e reduction of [Ni(Mebpy)3]2+ results in formation of [Ni(Mebpy)3]+ as the predominant species released into bulk solution. We also demonstrate that [Ni(Mebpy)3]0 slowly loses a Mebpy ligand (∼10 s−1).

Electrochemically Directed Synthesis of Cobalt(II) and Nickel(II) TCNQF21–/2– Coordination Polymers: Solubility and Substituent Effects in the TCNQFn (n=0, 1, 2, 4) Series of Complexes

The reversible diffusion controlled cyclic voltammetry for the reduction of TCNQF­n0/1–/2– (where n=0, 1, 2, 4) changes significantly on addition of Co2+ and Ni2+ transition metal ions (M2+) because the kinetics associated with electrocrystallisation of the resulting coordination polymers [M(TCNQF2)2(H2O)2] and [M(TCNQF2)] are rapid on the voltammetric time scale. The voltammetry of solutions containing M2+ and TCNQF­2 was undertaken in acetonitrile (0.1M Bu4NPF6) at both GC and ITO electrodes. New one electron reduced TCNQF2 materials prepared via electrochemically directed synthesis were shown to have the formula [M(TCNQF2)2(H2O)2], assessed by vibrational (IR and Raman) spectroscopy, elemental analysis and thermogravimetric analysis. The solubility of [Ni(TCNQF2)2(H2O)2] (Ksp=8.29×10−11 M3) was significantly higher than the [Co(TCNQF2)2(H2O)2] (Ksp=1.43×10−11M3). Cyclic voltammetric data suggest the electrocrystallisation of two phases of [Ni(TCNQF2)2(H2O)2] occurs, which is not evident for [Co(TCNQF2)2(H2O)2]. Electrocrystallisation of the highly insoluble [M(TCNQF2)] was achieved at low M2+ and TCNQF2 concentrations. A comparison with published data on the voltammetry of TCNQF­n (n=0, 1, 2 and 4) for the series of TCNQF­n (n=0, 1, 2 and 4) containing M2+ is provided. An assessment of the electronic impact of the fluorine substituent of the underlying redox reactions also is established. Predictions are made for the voltammetric behaviour expected for the other transition metal cations with reduced TCNQFn derivatives.

Unprecedented Formation of a Binuclear Au(II)-Au(II) Complex through Redox State Cycling: Electrochemical Interconversion of Au(I)-Au(I), Au(II)-Au(II), and Au(I)-Au(III) in Binuclear Complexes Containing the Carbanionic Ligand C6F4PPh2.

The rational design of binuclear Au(I)-Au(I), Au(II)-Au(II), and Au(I)-Au(III) complexes requires an understanding of how the redox states interconvert. Herein, the electrochemical interconversion of the three oxidation states I, II, and III is reported on the voltammetric (cyclic and rotating disk electrode) time scales for binuclear gold complexes containing C6F4PPh2 as a ligand, to demonstrate for the first time formation of a binuclear Au(II)-Au(II) from a Au(I)-Au(III) complex. Results are supported by bulk electrolysis and coulometry with reaction products being identified by 31P NMR and UV-vis spectroscopy. All electrochemical processes involve an overall two-electron charge-transfer process with no one-electron intermediate being detected. Importantly, the kinetically rather than thermodynamically favored isomer [Au2IIX2(μ-2-C6F4PPh2)2] is formed on redox cycling of [XAuI(μ-2-C6F4PPh2)(κ2-2-C6F4PPh2)AuIIIX] (X = Cl, ONO2). Finally, a mechanism is proposed to explain the simultaneous change of coordination of the chelating carbanionic ligand to bridging mode and interconversion of oxidation states in binuclear gold complexes.

Aptamer-Modified Microelectrodes for the Electrochemical Impedance Spectroscopy Detection of Neuropeptide Y

The design novel biosensors is a broad field including a wide range of applications including medical diagnostics and environmental monitoring. Small sizes of the electrodes have advantages used for selective identification of a biological target, such as voltammetric time scales, lower background noise and higher spatial resolution. Platinum and Carbon nanofiber has been some promising materials for fabrication of electrodes because of their electrical conductivity, chemical stability and high surface area. The intended application is to the possibility to create point of care diagnostics for monitoring the health of astronauts. Neuropeptides are short amino acid chains that are synthesized in the soma and dendrites, and they are stored in large dense core vesicles until released from nerve endings as well as from dendrites. Also, it’s not recycled and there is no reuptake system, but they are broken by peptidases, which require the cell to always synthesize new molecules to replace them by sending them from the soma to the nerve endings and dendrites. Other small neurotransmitters are normally released together with neuropeptides; therefore, it is essential to develop a technique for their detection as selective as possible and to have a strategy to cancel out the signal exerted by other neurotransmitters. The structural similarity between neuropeptides makes their detection impossible using common electrochemical techniques. Their study is currently done measuring the expression of neuropeptides in-vitro or using microdialysis coupled with mass spectrometry, which gives temporal resolution of tens of minutes. In this project, we have developed a modification protocol that reproducibly adds highly selective aptamers to the microelectrodes and the adsorption of neuropeptide Y differentially detected using microelectrodes. Based on the results, changes in concentration due to the release and absorption of neuropeptide Y can be followed with continuous electrochemical impedance measurements using microelectrodes, with the specificity provided by the used of aptamers in the surface of the microelectrodes.

Impact of sp2 Carbon Edge Effects on the Electron-Transfer Kinetics of the Ferrocene/Ferricenium Process at a Boron-Doped Diamond Electrode in an Ionic Liquid

The electrochemical properties of boron-doped diamond (BDD) electrodes are strongly influenced by the boron doping level and the presence of sp2 carbon impurities. In this study, the impact of highly localized sp2 carbon concentrated at the edge of a BDD electrode, arising from laser cutting during fabrication and exposed during electrode polishing, on the resulting overall electrode kinetics is identified. Fourier transformed large-amplitude alternating current (FTAC) voltammetric data for the usually ideal Fc0/+ (Fc = ferrocene) process in the highly viscous ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate show relatively poor agreement with simulations based on a uniformly active electrode surface using the Butler–Volmer formalism for the electrode kinetics. In this ionic liquid medium, the impact of heterogeneity on the macroscopic electrode activity is enhanced under the conditions of slow mass transport, where sites of disparate activities are spatially decoupled on the voltammetric time scale. Physically blocking this edge region leads to a response that is much more consistent with a uniform electrode and substantially improves the agreement between the FTAC voltammetric experiment and simulation (standard heterogeneous electron-transfer rate constant, k0 = 0.0015 cm s–1). To complement the macroscopic measurements, local voltammetric measurements with an electrochemical droplet cell show directly that the sp2 carbon found in the edge region is able to support much faster electron-transfer kinetics for the Fc0/+ process than the sp3 BDD surface. Overall, this study demonstrates that caution should be taken in reporting electrode kinetic data obtained at BDD or any other heterogeneous electrode materials. Macroscopic electrode kinetic characterization in traditional electrochemical media such as acetonitrile is blind to such heterogeneities in the activity. However, tuning the diffusional time scale through the use of FTAC voltammetry in viscous ionic liquids provides a powerful approach for detecting the spatially nonuniform electrode activity.

Electrochemical Oxidation of the Phenolic Benzotriazoles UV‐234 and UV‐327 in Organic Solvents

AbstractThe electrochemical behavior of selected phenolic benzotriazoles (BZTs), namely 2‐(2H‐benzotriazol‐2‐yl)‐4,6‐bis(1‐methyl‐1‐phenylethyl)phenol and 2,4‐di‐tert‐butyl‐6‐(5‐chlorobenzotriazol‐2‐yl)phenol (commercial names UV234 and UV327, respectively) were examined with cyclic voltammetry (CV) and controlled potential electrolysis (CPE) in acetonitrile and dichloromethane solutions. CV indicated that both phenolic BZTs undergo a chemically irreversible oxidation process at approximately Ep°x=+1.0 V vs. Fc/Fc+ (where Ep°x is the anodic peak potential and Fc=ferrocene) to form compounds that cannot be electrochemically converted back to the starting material on the voltammetric timescale. In basic conditions, cyclic voltammetry experiments indicated that the corresponding phenolates (prepared by reacting the phenols with equiv. mols of n‐Bu4NOH) were oxidized at Ep°x∼−0.2 V vs. Fc/Fc+ via a one‐electron diffusion controlled process with anodic (ip°x) to cathodic (ipred) peak current ratios (ip°x/ipred)≫1, suggesting that the produced phenoxyl radicals decomposed rapidly via a chemical step. However, electron paramagnetic resonance (EPR) experiments performed on the bulk electrolyzed solutions of the phenolates after one‐electron bulk oxidation indicated long lifetimes of the UV234. and UV327. phenoxyl radicals. Therefore, the long timescale CPE and spectroscopic (UV‐vis and EPR) studies provided good evidence of a reversible dimerization mechanism between the phenoxyl radicals, which explained the apparent discrepancy with the short timescale CV experiments.

Use of a Redox-Responsive 4 H-Bond Ureidopyrimidone (UPy) Array to Control Polymerization in a Upy-Based Supramolecular Polymer

Ureidopyrimidinone (UPy) derivatives, introduced by Meijer and co-workers, are capable of forming dimers linked by four strong H-bonds. They have been widely used as reversible crosslinkers in self-healing supramolecular polymers and gels. The goal of the project to be presented is to create a redox-controllable UPy supramolecular polymer system by adding a redox-responsive UPy monomer as a capping agent to a solution of a non-redox-active UPy supramolecular polymer. Specifically, a UPy containing a phenylenediamine redox couple, UPyH, that we have previously studied,1 will be used as the capping agent. In previous work we have shown that UPyH dimerizes in methylene chloride with Kdim > 105 as expected. Oxidation of the phenylenediamine to the radical cation does not break apart the dimer at mM concentrations. However, further oxidation to the quinoidal dication weakens the binding enough to allow an external base to remove the most acidic protons and thereby create a non-self-complementary H-bonding motif that cannot dimerize. As long as the base is not too strong the process is reversible on the cyclic voltammetric time scale, leading to re-formation of the UPyH dimer upon reduction. Previous studies show that 1 equivalent of 4-trifluoromethylpyridine is a good choice for the base. To create the redox-responsive supramolecular polymer system, a known UPy oligomer will be used to prepare a supramolecular polymer solution in methylene chloride electrolyte solution. The degree of polymerization will be assessed by measuring the viscosity of the solution. UPyH plus an equal amount of 4-trifluoromethylpyridine will be then added until there is a significant decrease in viscosity indicating that the UPyH is H-bonding to the UPy’s in the non-redox active UPy oligomer and thereby decreasing the length of the supramolecular chains. The solution will then be oxidized either chemically or electrochemically to convert the UPyH to its fully oxidized, de-protonated form. Since this form is incapable of dimerizing with itself or another UPy, the UPyH will lose its viability as a chain terminator, thus allowing the non-redox active Upy oligomers to reform supramolecular chains. This will be signaled by an increase in the viscosity of the solution. 1L. A. Clare and D. K. Smith, Chemical Communications 2016, 52, 7253-7256.

Effect of Large Electrolyte Anions on the Sequential Oxidations of Bis(fulvalene)diiron Attached to Glassy Carbon by an Ethynyl Linkage.

Two ethynyl-derivatized isomers of bis(fulvalene)diiron (BFD, 1,1'-biferrocenylene) were prepared and covalently attached to glassy carbon electrodes through their ethynyl group by three different electrode modification methods. Cyclic voltammetry and square wave (SW) voltammetry were used to characterize surface coverages of 1.4-5.5 × 10-10 mol cm-2, the higher of these corresponding to roughly a monolayer, based on computation of an idealized close-packing structure for ethynylbis(fulvalene)diiron (E-BFD) on a solid surface. In a dichloromethane solution containing a smaller electrolyte anion such as [PF6]- or [ClO4]-, the E-BFD-modified electrodes exhibited two quasi-Nernstian one-electron oxidations. In contrast, the current for the second oxidation process, [E-BFD]+/2+, was diminished in electrolytes containing one of the large fluoroaryl borate anions, [B(C6F5)4]- or [B(C6H3(CF3)2)4]-. The effect was enhanced for electrodes having higher surface coverages being probed at shorter voltammetric time scales. SW voltammetry showed that the diminished currents for [E-BFD]+/2+ in large-anion electrolytes are not caused by slow electron transfer. Rather, they are attributed to mixed diffusivity of the counter-anions at the electrode/solution interface, as [E-BFD]+ and the anion form the optimum (lowest-energy) configuration of a 1:1 ion pair. The interior transport of the anion required to reach this configuration may be sterically encumbered, accounting for the diminished charge transfer observed with electrolytes containing large anions.

Molecular Au25(SR)18 Nanoclusters: Lifetime of Electrogenerated Species and Dielectric Properties of the Protecting Monolayer

We used a series of molecular Au25(SCnH2n+1)18 clusters (n = 2, 3, 4, 5, 6, 8, 10, 12, 14, 16, 18) to study the effect of n and the presence of electrolytes on their redox potentials. We found that the average dielectric constant of the monolayer, ε m, depends on n and is the result of contributions from ligands, solvent and electrolyte. When n increases, ε m decreases until a virtually constant value is attained for sufficiently long ligands. The electrochemically calculated HOMO-LUMO energy gap, on the other hand, does not depend on n and, therefore, ε m.1 We also studied the effect of n on the lifetime of electrogenerated species. Whereas on the voltammetric timescale the -1, 0, and +1 charge state of Au25 clusters are stable, formation of the -2 and +2 states causes fragmentation of the cluster. Voltammetric analysis indicates that the former entails reactions involving ligands in the Au12(SCnH2n+1)18 protecting shell, whereas the latter is accompanied by a slower but also more substantial fragmentation affecting the integrity of the central Au13 core. 2

Exploring Collective Substituent Effects: Dependence of the Lifetime of Charged States of Au25(SCnH2n+1)18 Nanoclusters on the Length of the Thiolate Ligands

AbstractElectrochemical methods have been often successfully employed to study substituent effects, a field pioneered by Petr Zuman. Here we used cyclic voltammetry to explore a new dimension of the effect of substituents on the lifetime of electrogenerated species. 18 thiolate ligands were collectively changed in molecular Au25(SCnH2n+1)18 nanoclusters by changing the number of carbon atoms forming the alkanethiolate linear chains: 2, 3, 4, 5, 6, 8, 10, and 12. Whereas on the voltammetric timescale the −1, 0, and +1 charge state of Au25 clusters are stable, formation of the −2 and +2 states causes fragmentation of the cluster. Voltammetric analysis indicates that the former entails reactions involving ligands in the Au12(SCnH2n+1)18 protecting shell, whereas the latter is accompanied by a slower but also more substantial fragmentation affecting the integrity of the central Au13 core. Whereas the lifetime of the dianion is ca. 3 ms and virtually unaffected by the length of the thiolate substituent, the lifetime of the dication increases from 0.11 to 5 s as the alkanethiolate chains are made longer. This difference in behavior is explained by the different effect exerted on the two types of fragmentation by the structure of the protecting monolayer.

Polyoxometalates as solution-phase electrocatalytic mediators for reduced electrode fouling and the improved oxidative response of phenols

The electrochemical oxidation of phenols typically passivates electrodes, due to the reactive one-electron oxidation intermediate, a phenoxyl radical cation. Here, we show that the addition of polyoxometalates to the electrolyte can prevent electrode passivation via an EC`E mechanism; the initial oxidation is unchanged, but oxidation of the phenoxyl radical intermediate is mediated. As such the electrochemistry assumes an ideal EE process, as confirmed by cyclic voltammetric simulation, with no electrode fouling on the voltammetric timescale. Bulk electrolysis of phenolic molecules can even be performed to isolate solution-phase products. This is exemplified by the scenario of vanillin (phenolic compound) and phosphotungstic acid (H3[PW12O40], a polyoxometalate).