Electron Self-exchange Reaction Research Articles

Redox: Organic Robust Radicals and Their Polymers for Energy Conversion/Storage Devices.

Persistent radicals can hold their unpaired electrons even under conditions where they accumulate, leading to the unique characteristics of radical ensembles with open-shell structures and their molecular properties, such as magneticity, radical trapping, catalysis, charge storage, and electrical conductivity. The molecules also display fast, reversible redox reactions, which have attracted particular attention for energy conversion and storage devices. This paper reviews the electrochemical aspects of persistent radicals and the corresponding macromolecules, radical polymers. Radical structures and their redox reactions are introduced, focusing on redox potentials, bistability, and kinetic constants for electrode reactions and electron self-exchange reactions. Unique charge transport and storage properties are also observed with the accumulated form of redox sites in radical polymers. The radical molecules have potential electrochemical applications, including in rechargeable batteries, redox flow cells, photovoltaics, diodes, and transistors, and in catalysts, which are reviewed in the last part of this paper.

Determination Method of Diffusion Coefficient in a Neat Redox-Active Ionic Liquid at a Microdisk Electrode in the Domains Ranging from the Steady-State to Potentiodynamic Near-Steady-State.

In redox-active ionic liquids (RAILs), either or both of the constituent ions are redox-active. Because of the high concentration of the ions, RAILs exhibit not only ion conduction but also electron conduction through the bimolecular electron self-exchange reaction. Because neat RAILs do not contain any supporting electrolyte, migration of the redox active ions results in enhancement or diminishment of the redox current at an electrode. To treat the migration effect for electrochemical analysis, a limiting current correction was theoretically derived by Oldham, Hyk, and Stojek (Oldham, K. J. Electroanal. Chem. 1992, 337, 91-126; Hyk, W.; Stojek, Z. Anal. Chem. 2002, 747, 4805-4813) for the steady-state voltammetry. Although steady-state voltammetry is a robust method in electrochemistry, the actual measurement is time-consuming and cannot be always made because of the instability of the electrochemical system. To overcome the problem, we propose the use of cyclic voltammetry to evaluate the diffusion coefficient of the redox-active ion that constitutes RAIL. The peak currents were analyzed by the purely diffusional framework of the Aoki-Matsuda-Osteryoung equation (Aoki, K.; Akimoto, K.; Tokuda, K.; Matsuda, H.; Osteryoung, J. J. Electroanal. Chem. 1984, 171, 219-230.) in the range from several mV s-1 to several ten mV s-1, and the migration correction to the near-steady-state limiting current was applied on the basis of the Oldham-Hyk-Stojek theory to scale the diffusion coefficient. As an example of RAILs, [FcC6ImC1][TFSI], which exhibits charge increase reaction with the same sign (S+ - e- ⇌ P2+), was used and the cyclic voltammograms were recorded at various sizes of the microdisk electrodes and various scan rates. The peak currents obeyed the Aoki-Matsuda-Osteryoung equation with the scaled diffusion coefficient, which has the same value as determined by the steady-state voltammogram. Our approach can be used to evaluate the diffusion coefficient of redox-active ions that constitute the RAIL with the charge increase reaction with the same sign.

An energy decomposition and extrapolation scheme for evaluating electron transfer rate constants: a case study on electron self-exchange reactions of transition metal complexes.

A simple approach to the analysis of electron transfer (ET) reactions based on energy decomposition and extrapolation schemes is proposed. The present energy decomposition and extrapolation-based electron localization (EDEEL) method represents the diabatic energies for the initial and final states using the adiabatic energies of the donor and acceptor species and their complex. A scheme for the efficient estimation of ET rate constants is also proposed. EDEEL is semi-quantitative by directly evaluating the seam-of-crossing region of two diabatic potentials. In a numerical test, EDEEL successfully provided ET rate constants for electron self-exchange reactions of thirteen transition metal complexes with reasonable accuracy. In addition, its energy decomposition and extrapolation schemes provide all the energy values required for activation-strain model (ASM) analysis. The ASM analysis using EDEEL provided rational interpretations of the variation of the ET rate constants as a function of the transition metal complexes. These results suggest that EDEEL is useful for efficiently evaluating ET rate constants and obtaining a rational understanding of their magnitudes.

Ionic Effect on Electrochemical Behavior of Water-Soluble Radical Polyelectrolytes

Water-soluble redox-active polymers (RAPs) have emerged as attractive electroactive materials for aqueous redox flow batteries because these systems rely on readily available size-exclusion membranes rather than expensive ion-selective membranes. While incorporating ionic units is the most effective strategy for forming water-soluble redox polyelectrolytes, little is known about how charges dictate their electrochemical behavior. Here, we design a series of water-soluble TEMPO (2,2,6,6-tetramethylpiperidyl-1-oxy) radical polyelectrolytes with identical radical distribution but various ionic groups through a sequential postmodification method. Physical and electrochemical characterizations show disparate diffusion coefficients (D) and charge transfer kinetics (k0) of these radical polyelectrolytes at various pH. Particularly, pH exerts a strong impact on k0 of the negatively charged polymer (i.e., with carboxylic acid groups). The bimolecular reaction rate kex determined from redox-induced polymer films shows electrostatic interactions between charged segments can enhance the electron self-exchange reaction rate by ten folds. The results suggest that charge effects are of great importance when designing water-soluble redox polymers for electrochemical applications.

Thermal and photoinduced electron transfer reactions of phthalocyanine complexes of Zn(II) and Cu(II) in acetonitrile.

Phthalocyanine that has four peripheral 2-methoxyphenyl substituents at the α-position and its Zn(II) and Cu(II) complexes were synthesized. Chemical oxidation by the Cu(II) ion and electrochemical oxidation of these metal complexes were investigated spectrophotometrically in acetonitrile. The UV-visible absorption spectra of these metal complexes and their one-electron oxidized π-cation radicals showed no concentration dependence, indicating that these species exist as monomers in solution. Kinetics of the thermal electron transfer reaction from each phthalocyanine complex to Cu2+ and the photoinduced electron transfer reaction of the Zn(II) phthalocyanine complex with V(V) and V(IV) Schiff base complexes were studied using conventional spectrophotometric and transient absorption techniques, and the electron transfer rate constants were analysed using the Marcus cross relationship. The obtained rate constants of the electron self-exchange reaction between the parent phthalocyanine complexes and their π-cation radicals were in the order of 109 to 1011 M-1 s-1 at T = 298.2 K. These large electron self-exchange rate constants are consistent with the phthalocyanine-centred redox reactions where small reorganization energies are required with little structural change during the electron transfer process.

Uncovering the Shuttle Effect in Organic Batteries and Counter-Strategies Thereof: A Case Study of the N,N'-Dimethylphenazine Cathode.

The main drawback of organic electrode materials is their solubility in the electrolyte, leading to the shuttle effect. Using N,N'-dimethylphenazine (DMPZ) as a highly soluble cathode material, and its PF6 - and triflimide salts as models for its first oxidation state, a poor correlation was found between solubility and battery operability. Extensive electrochemical experiments suggest that the shuttle effect is unlikely to be mediated by molecular diffusion as commonly understood, but rather by electron-hopping via the electron self-exchange reaction based on spectroscopic results. These findings led to two counter-strategies to prevent the hopping process: the pre-treatment of the anode to form a solid-electrolyte interface and using DMPZ salt rather than neutral DMPZ as the active material. These strategies improved coulombic efficiency and capacity retention, demonstrating that solubility of organic materials does not necessarily exclude their applications in batteries.

Kinetic Analysis of Electrochemical Redox Reaction of Nitroxyl Radical in Ionic Liquids

Organic radical batteries (ORBs) have been attracted much attention as a promising second battery because of its high-power density, flexibility, and fast charge/discharge properties. A typical ORB is composed of a poly (2,2-6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA) cathode, a carbon anode and an electrolyte consisting of carbonate solvents and LiPF6 which has a wider potential window than the formal redox potential of PTMA (3.6 V vs. Li/Li+) [1]. The PTMA shows high stability of radicals and high durability during repeated redox reaction, allowing reversible charging and discharging as an electrode active material for a second battery. The redox reaction of PTMA takes place on the cathode with PF6- moving to/from the cathode. Besides, the PTMA is changed into gel state by the absorption of organic solvent-based electrolyte. In the PTMA gel, charge transportation occurs by the electron self-exchange reaction between nitroxyl radicals and oxoammonium cations. However, the organic solvents-based electrolytes are at risk of inflammation when the electrolytes leak out from the batteries. To solve this issue, we are focusing on an ionic liquid as an electrolyte for the ORB because of its non-volatility and non-flammability [2]. However, to the best of our knowledge, electrochemical fundamental studies such as stability and reaction rate of 2,2-6,6-tetramethylpiperidinyloxy (TEMPO) in ionic liquids have not been reported yet.In this study, the electrochemical properties of TEMPO were investigated in the ionic liquids with the electrochemical potential windows which are wide enough to be applied for the ORB. The ionic liquids used in this study were as follows: 1-ethyl-3-methylimidazolium tetrafluoroborate (EMI-BF4), 1-butyl-3-methylimidazolium BF4 (BMI-BF4), and N-N-diethyl-N-methyl-N-(2-methoxyethyl)-ammonium BF4 (DEME-BF4). Small amount of water originally absorbed in these ionic liquids were firstly removed by heating at 100 oC for 12 h under vacuum. 0.1 M TEMPO was dissolved in the water-removed ionic liquids, followed by bubbling with argon gas for 30 min to remove dissolved oxygen. A three-electrode system was constructed using a silver-silver ion electrode (Ag/Ag+) for a reference electrode, a glassy carbon electrode (Φ 3.0 mm) for a working electrode, and a platinum wire for a counter electrode. The electrode potential was swept from -2 to + 1 V vs. Ag / Ag + using a potentiostat to induce redox reaction of the TEMPO on the glassy carbon electrode. Figure 1 shows cyclic voltammograms detected in three kinds of ionic liquids at scan rate of 100 mV/s. When the potential was swept between -2 V to 1 V vs. Ag/Ag+, clear peak currents were detected for oxidation and reduction of TEMPO in the ionic liquid. The separations between the electrochemical potentials at which oxidation and reduction peak currents were observed were 218 mV for EMI-BF4, 380 mV for BMI-BF4, and 403 mV for DEME-BF4, respectively. The peak potential separation is one of the parameters to study reversibility of the target redox reaction. These values were larger than that of ideal reversible reaction, 57 mV, suggesting semi-reversible or irreversible reaction of TEMPO in these ionic liquids. The peak current densities of TEMPO detected in these ionic liquids were calculated from Figure 1 as 4.91 mA/cm2 for EMI-BF4, 2.58 mA/cm2 for BMI-BF4, and 1.10 mA/cm2 for DEME-BF4, respectively. The viscosities of EMI-BF4, BMI-BF4, and DEME- BF4 were reported in the catalog to be 53, 180, and 272 cPs at 25 °C, respectively. The peak current density for redox specie changes depending on its diffusion coefficient which is inversely proportional to the viscosity of solvent. This result suggested that viscosity of the ionic liquid is one of the factors governing the current density. We are now trying to calculate the diffusion coefficient and the reaction rate constant of TEMPO in each ionic liquid from dependency of peak potential separation on the scan rate using Nicholson method. [1] K. Nakahara et. al., Chem. Phys. Lett., 359, 351 (2002). [2] Y. Dai et. al., J. Electrochem. Soc., 158, A291 (2011). Figure 1

Are the current theories of electron transfer applicable to reactions in ionic liquids? An ESR-study on the TCNE/TCNE(-)˙ couple.

Chemical reactivity is profoundly affected by solvent properties. Room temperature ionic liquids (RTILs) obtain molecular environments that differ vastly from those established using molecular solvents with comparable macroscopic properties. In particular, charges are expected to be completely shielded in RTILs even though their dielectric constants are typically low. This raises the question whether electron transfer (ET) reactions in RTILs can be described in terms of Marcus' theory, a model that is fundamentally based on continuum dielectric theory. Herein, we elucidate this question by studying a degenerate electron transfer process, which by design, is not affected by ambiguities in the driving force of the reaction and thus allows a clear-cut assessment of the ET activation energy. We report the rate constants and the activation parameters of the electron self-exchange reaction in the TCNE/TCNE˙(-) couple in seven ionic liquids. The exchange rate constants range from 5.4 × 10(7) M(-1) s(-1) to 9.1 × 10(8) M(-1) s(-1) at 330 K and the activation energies vary from 14 kJ mol(-1) to 41 kJ mol(-1). The results are discussed in the framework of Marcus' theory. It is found that the solvent dependence of the rate constants cannot be described by the classical proportionality to the Pekar factor γ = (1/n(2) - 1/εs).

Electron Propagation within Redox-Active Microdomains in Thin Films of Ferrocene-Containing Diblock Copolymers.

This paper reports the electrochemical behavior of redox-active microdomains in thin films of ferrocene-containing diblock copolymers, polystyrene-block-poly(2-(acryloyloxy)ethyl ferrocenecarboxylate) (PS-b-PAEFc). PS-b-PAEFc with different PAEFc volume fractions (PS154-b-PAEFc51, PS154-b-PAEFc26, and PS154-b-PAEFc12, where the subscripts represent the polymerization degree of each block; f(PAEFc) = 0.47, 0.30, and 0.17, respectively) was synthesized by sequential atom transfer radical polymerization. PS-b-PAEFc films of controlled thicknesses (20-160 nm) were prepared on gold substrates via spin-coating and characterized by ellipsometry. Microdomains were observed via atomic force microscopy on the surfaces of PS154-b-PAEFc51 and PS154-b-PAEFc26 thin films but not on the surfaces of PS154-b-PAEFc12 thin films. Electrochemical behavior of films was assessed by cyclic voltammetry and chronocoulometry in acetonitrile solution. The redox potential of ferrocene moieties was similar (ca. + 0.29 V vs Fc(+)/Fc) regardless of fPAEFc and film thickness. For PS154-b-PAEFc51 and PS154-b-PAEFc26, thicker films afforded larger faradaic peak currents and exhibited diffusion-controlled voltammograms at faster sweep rates. PS154-b-PAEFc26 produced voltammograms less influenced by solvent-induced swelling than PS154-b-PAEFc51, reflecting the improved morphological stability of PAEFc microdomains by redox-inert PS frameworks. In contrast, PS154-b-PAEFc12 films yielded similar faradaic peak currents regardless of film thickness and exhibited voltammograms indicative of surface-confined species. These observations suggest that PS154-b-PAEFc51 and PS154-b-PAEFc26 films contain continuous PAEFc microdomains extending from the electrode to the surface, in contrast to the PS154-b-PAEFc12 films which contain isolated PAEFc microdomains buried within the PS matrix. Electron propagation took place only through PAEFc microdomains that could electrically communicate with the underlying electrode. Apparent diffusion coefficients within PAEFc microdomains were similar (≈ 2 × 10(-11) cm(2)/s) for PS154-b-PAEFc51 and PS154-b-PAEFc26. The relatively low efficiency in electron propagation was attributable to ineffective electron self-exchange reaction within the PAEFc microdomains and/or limited counterion migration through the acetonitrile-swollen microdomains. These results provide guidance in design of redox-active metalloblock copolymers for various applications, which include electrocatalysis, electrochemical mediation in enzyme sensors, and redox-controlled molecular deposition.

Structure and Properties of Precursor/Successor Complex and Transition State of the FeCl2+/Fe2+ Electron Self-Exchange Reaction via the Inner-Sphere Pathway

The electron self-exchange reaction FeCl(OH2)5(2+) + Fe(OH2)6(2+) → Fe(OH2)6(2+) + FeCl(OH2)5(2+), proceeding via the inner-sphere pathway, was investigated with quantum chemical methods. Geometry and vibrational frequencies of the precursor/successor complex, (H2O)5Fe(III)ClFe(II)(OH2)5(4+)/(H2O)5Fe(II)ClFe(III)(OH2)5(4+) (P/S), and the transition state, (H2O)5FeClFe(OH2)5(4+⧧) (TS), were computed with the LC-BOP functional and CPCM hydration. Bent and linear structures were computed for the TS and P/S. The electronic coupling matrix element (Hab) and the electronic energies were calculated with multistate extended general multiconfiguration quasi-degenerate second-order perturbation theory (XGMC-QDPT2) and spin-orbit configuration interaction (SO-CI). Since the Fe···Fe distance changes considerably along the electron transfer step, the transformation P → TS → S, equations based on the hypothesis of a fixed donor-acceptor distance cannot be applied. Hence, the rate constant for the electron transfer step (ket) was calculated as described previously (Rotzinger, F. P. Inorg. Chem. 2014, 53, 9923). ket is very fast, ∼9.4 × 10(8)-6.6 × 10(9) s(-1) at 0 °C. The experimental rate constant of the title reaction (k) is much slower and controlled by the formation of the precursor complex. The substitution of a water ligand by FeCl(OH2)5(2+) at Fe(OH2)6(2+) is rate-determining.

Simple (17) O NMR method for studying electron self-exchange reaction between UO2 (2+) and U(4+) aqua ions in acidic solution.

(17) O NMR spectroscopy is proven to be suitable and convenient method for studying the electron exchange by following the decrease of (17) O-enrichment in U(17) OO(2+) ion in the presence of U(4+) ion in aqueous solution. The reactions have been performed at room temperature using I = 5 M ClO4 (-) ionic medium in acidic solutions in order to determine the kinetics of electron exchange between the U(4+) and UO2 (2+) aqua ions. The rate equation is given as R = a[H(+) ](-2) + R', where R' is an acid independent parallel path. R' depends on the concentration of the uranium species according to the following empirical rate equation: R' = k1 [UO(2 +) ](1/2) [U(4 +) ](1/2) + k2 [UO(2 +) ](3/2) [U(4 +) ](1/2) . The mechanism of the inverse H(+) concentration-dependent path is interpreted as equilibrium formation of reactive UO2 (+) species from UO2 (2+) and U(4+) aqua ions and its electron exchange with UO2 (2+) . The determined rate constant of this reaction path is in agreement with the rate constant of UO2 (2+) -UO2 (+) , one electron exchange step calculated by Marcus theory, match the range given experimentally of it in an early study. Our value lies in the same order of magnitude as the recently calculated ones by quantum chemical methods. The acid independent part is attributed to the formation of less hydrolyzed U(V) species, i.e. UO(3+) , which loses enrichment mainly by electron exchange with UO2 (2+) ions. One can also conclude that (17) O NMR spectroscopy, or in general NMR spectroscopy with careful kinetic analysis, is a powerful tool for studying isotope exchange reactions without the use of sophisticated separation processes. Copyright © 2015 John Wiley & Sons, Ltd.

ESR studies on the pressure and temperature dependence of electron self-exchange kinetics between tetrathiafulvalene (TTF) and its radical cation in ionic liquids and organic solvents

The question whether chemical reactions and diffusion processes in ionic liquids are comparable with those taking place in classical organic liquids is a current issue in the literature. Pressure- and temperature-dependent investigations on simple electron self-exchange reactions between the two partners of a redox couple are good tools to get a better understanding of how the solvent influences such reactions. The electron self-exchange reaction between tetrathiafulvalene (TTF) and its radical cation has been investigated in two ionic liquids and two organic solvents using electron spin resonance (ESR) line broadening experiments at variable temperature and pressure. Rate constants are reported for the ionic liquids 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)imide ([emim+][Tf2N−]) and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim+][Tf2N−]) within a temperature range of 298 K ≤ T ≤ 368 K and a pressure range of 0.1 MPa ≤ p ≤ 100 MPa. The self-exchange reaction of the redox couple [TTF/TTF•+] has been found to be diffusion-controlled in the used ionic liquids over the entire temperature range. The observed rate constants in ionic liquids at higher pressures are larger than those predicted by common diffusion, and suggest that the electron transfer takes place within a solvent cage. Also, the self-exchange reaction of the [TTF/TTF•+] redox couple in classical solvents (dimethylphthalate (DMP) and acetonitrile) was investigated and compared to the results with those obtained in ionic liquids. The high viscosity of the ionic liquids makes it difficult to extract the electron transfer rate constants reliably, making interpretation within the framework of the Marcus Theory impossible.

Structure and properties of the precursor/successor complex and transition state of the CrCl²⁺/Cr²⁺ electron self-exchange reaction via the inner-sphere pathway.

The electron self-exchange reaction CrCl(OH2)5(2+) + Cr(OH2)6(2+) → Cr(OH2)6(2+) + CrCl(OH2)5(2+), proceeding via the inner-sphere pathway, was investigated with quantum-chemical methods. Geometry and vibrational frequencies of the precursor/successor (P/S) complex, (H2O)5Cr(III)ClCr(II)(OH2)5(4+)/(H2O)5Cr(II)ClCr(III)(OH2)5(4+), and the transition state (TS), (H2O)5CrClCr(OH2)5(4+‡), were computed with density functional theory (DFT) and conductor polarizable continuum model hydration. Consistent data were obtained solely with long-range-corrected functionals, whereby in this study, LC-BOP was used. Bent and linear structures were computed for the TS and P/S. The electronic coupling matrix element (H(ab)) and the reorganizational energy (λ) were calculated with multistate extended general multiconfiguration quasi-degenerate second-order perturbation theory. The nuclear tunneling factor (Γ(n)), the nuclear frequency factor (ν(n)), the electronic frequency factor (ν(el)), the electron transmission coefficient (κ(el)), and the first-order rate constant (k(et)) for the electron-transfer step (the conversion of the precursor complex into the successor complex) were calculated based on the imaginary frequency (ν(‡)) of the TS, the Gibbs activation energy (ΔG(‡)), H(ab), and λ. The formation of the precursor complex via water substitution at Cr(OH2)6(2+) was also investigated with DFT and found to be very fast. Thus, the electron-transfer step is rate-determining. For the substitution reaction, only a bent TS structure could be obtained. The overall rate constant (k) was estimated as the product K(A)k(et), whereby K(A) is the equilibrium constant for the formation of the ion aggregate of the reactants Cr(OH2)6(2+) and CrCl(OH2)5(2+), Cr(H2O)6·CrCl(OH2)5(4+) (IAR). k calculated for the bent and linear isomers agrees with the experimental value.

Theoretical Calculation of Reorganization Energy for Electron Self-Exchange Reaction by Constrained Density Functional Theory and Constrained Equilibrium Thermodynamics

Within the framework of constrained density functional theory (CDFT), the diabatic or charge localized states of electron transfer (ET) have been constructed. Based on the diabatic states, inner reorganization energy λin has been directly calculated. For solvent reorganization energy λs, a novel and reasonable nonequilibrium solvation model is established by introducing a constrained equilibrium manipulation, and a new expression of λs has been formulated. It is found that λs is actually the cost of maintaining the residual polarization, which equilibrates with the extra electric field. On the basis of diabatic states constructed by CDFT, a numerical algorithm using the new formulations with the dielectric polarizable continuum model (D-PCM) has been implemented. As typical test cases, self-exchange ET reactions between tetracyanoethylene (TCNE) and tetrathiafulvalene (TTF) and their corresponding ionic radicals in acetonitrile are investigated. The calculated reorganization energies λ are 7293 cm(-1) for TCNE/TCNE(-) and 5939 cm(-1) for TTF/TTF(+) reactions, agreeing well with available experimental results of 7250 cm(-1) and 5810 cm(-1), respectively.

Enhanced bimolecular exchange reaction through programmed coordination of a five-coordinate oxovanadium complex for efficient redox mediation in dye-sensitized solar cells

Electrochemical reversibility and fast bimolecular exchange reaction found for VO(salen) gave rise to a highly efficient redox mediation to enhance the photocurrent of a dye-sensitized solar cell, leading to an excellent photovoltaic performance with a conversion efficiency of 5.4%. A heterogeneous electron-transfer rate constant at an electrode (k0) and a second-order rate constant for an electron self-exchange reaction (k(ex)) were proposed as key parameters that dominate the charge transport property, which afforded a novel design concept for the mediators based on their kinetic aspects.

Linker-Based Control of Electron Propagation through Ferrocene Moieties Covalently Anchored onto Insulator-Based Nanopores Derived from a Polystyrene–Poly(methylmethacrylate) Diblock Copolymer

This paper reports the effects of linker length on electron propagation through ferrocene moieties covalently anchored onto insulator-based cylindrical nanopores derived from a cylinder-forming polystyrene-poly(methylmethacrylate) diblock copolymer. These nanopores (24 nm in diameter, 30 nm long) aligned perpendicular to an underlying gold electrode were modified via esterification of their surface COOH groups with OH-terminated ferrocene derivatives having different alkyl linkers (FcCO(CH(2))(n)OH; n = 2, 5, 15). Cyclic voltammograms were measured in 0.1 M NaBF(4) at different scan rates to assess the efficiency of electron propagation through the ferrocene moieties. The redox peaks of the anchored ferrocenes were observed at nanoporous films decorated with FcCO(CH(2))(15)OH and FcCO(CH(2))(5)OH, but not at those with FcCO(CH(2))(2)OH. Importantly, the higher electron propagation efficiency was observed in the use of the longer linker, as shown by the apparent diffusion coefficients (ca. 10(-12) cm(2)/s for n = 15; ca. 10(-13) cm(2)/s for n = 5; no electron propagation for n = 2). The observed electron propagation resulted from electron hopping across relatively large spacing that was controlled by the motion of anchored redox sites (bounded diffusion). The longer linker led to the larger physical displacement range of anchored ferrocene moieties, facilitating the approach of the adjacent ferrocene moieties within a distance required for electron self-exchange reaction. The linker-based control of redox-involved electron propagation on nanostructured, insulating surfaces will provide a means for designing novel molecular electronics and electrochemical sensors.

BODIPY-Sensitized Photocharging of Anthraquinone-Populated Polymer Layers for Organic Photorechargeable Air Battery

Photocharging of anthraquinone (AQ)-populated polymers was accomplished by incorporating chromophores with redox potentials suitable for the 2e− reduction of the AQ pendants near −1 V versus Ag/AgCl in aqueous basic electrolytes. The photoactive polymers were designed to exhibit redox-isolated behaviors and reversible charge–discharge responses, either by employing a polyacetylene main chain with the π–π* absorption bands in a visible region, or with an aliphatic chain incorporating a BODIPY dye in the form of a multilayer. The charge storage density was maximized by minimizing the molecular weight of the compact repeating units, thus reducing the redox site-to-site distance to allow redox gradient-driven electron self-exchange reactions to proceed throughout the slab of the polymer layer. Homogeneous layers of poly(2-ethynylanthraquinone) and poly(2-vinylanthraquinone) were both sufficiently robust for charge storage application, swellable, and yet insoluble in aqueous basic electrolyte solutions. Such properties allowed the accommodation of external cations from the electrolyte to permeate through the layer for electroneutralization of the negative charge produced by the electroreduction, which led to the repeatable charging and discharging cycles of the photoanodes without degradation of the charge storage capabilities. By virtue of the swelling properties of the polymer layers in the basic aqueous electrolyte, exploration of the aqueous electrochemistry and photochemistry of AQ that had been inaccessible by the lack of the solubility in H2O became feasible. A striking feature of the photoanodes was the capability of employing a hydroxide ion (OH−) as the sacrificial reducing agent, which enabled the use of the O2/OH− redox couple for the cathode reaction during discharging. Combination of the photoanodes with the MnO2/carbon composite cathode, sandwiching the electrolyte layer of aqueous KOH, gave rise to photorechargeable battery effect under irradiation and repeatable discharging with the consumption of O2 at the cathode.

High Pressure ESR Studies of Electron Self-Exchange Reactions of Organic Radicals in Solution

Simple electron self-exchange reactions are often used to study the role of the reaction medium on a chemical process, commonly implying the use of various solvents with different physical properties. In principle, similar studies may be conducted using a single solvent, changing its physical properties by application of elevated pressures, but so far only little information is available on pressure dependent exchange reactions. In this work, we have used a recently constructed high pressure apparatus for use with electron spin resonance (ESR) spectroscopy to investigate simple electron self-exchange reactions involving 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and tetracyanoethylene (TCNE) and their respective radical anions as well as TMPPD and its radical cation in three different solvents. The self-exchange was observed by ESR line broadening experiments, yielding rate constants and volumes of activation. The experimental results were compared to theoretical calculations based on Marcus theory and taking into account solvent dynamic effects. The use of elevated pressures has enabled the study of solvent effects without commonly encountered problems like solubility issues or chemical reactions between solvent and solute which sometimes limit the range of useable solvents.

NMR Study of Ligand Exchange and Electron Self-Exchange between Oxo-Centered Trinuclear Clusters [Fe3(μ3-O)(μ-O2CR)6(4-R′py)3]+/0

The syntheses, single crystal X-ray structures, and magnetic properties of the homometallic μ₃-oxo trinuclear clusters [Fe₃(μ₃-O)(μ-O₂CCH₃)₆(4-Phpy)₃](ClO₄) (1) and [Fe₃(μ₃-O)(μ-O₂CAd)₆(4-Mepy)₃](NO₃) (2) are reported (Ad = adamantane). The persistence of the trinuclear structure within 1 and 2 in CD₂Cl₂ and C₂D₂Cl₄ solutions in the temperature range 190-390 K is demonstrated by ¹H NMR. An equilibrium between the mixed pyridine clusters [Fe₃(μ₃-O)(μ-O₂CAd)₆(4-Mepy)(3-x)(4-Phpy)(x)](NO₃) (x = 0, 1, 2, 3) with a close to statistical distribution of these species is observed in CD₂Cl₂ solutions. Variable-temperature NMR line-broadening made it possible to quantify the coordinated/free 4-Rpy exchanges at the iron centers of 1 and 2: k(ex)²⁹⁸ = 6.5 ± 1.3 × 10⁻¹ s⁻¹, ΔH(‡) = 89.47 ± 2 kJ mol⁻¹, and ΔS(‡) = +51.8 ± 6 J K⁻¹ mol⁻¹ for 1 and k(ex)²⁹⁸ = 3.4 ± 0.5 × 10⁻¹ s⁻¹, ΔH(‡) = 91.13 ± 2 kJ mol⁻¹, and ΔS(‡) = +51.9 ± 5 J K⁻¹ mol⁻¹ for 2. A limiting D mechanism is assigned for these ligand exchange reactions on the basis of first-order rate laws and positive and large entropies of activation. The exchange rates are 4 orders of magnitude slower than those observed for the ligand exchange on the reduced heterovalent cluster [Fe(III)₂Fe(II)(μ₃-O)(μ-O₂CCH₃)₆(4-Phpy)₃] (3). In 3, the intramolecular Fe(III)/Fe(II) electron exchange is too fast to be observed. At low temperatures, the 1/3 intermolecular second-order electron self-exchange reaction is faster than the 4-Phpy ligand exchange reactions on these two clusters, suggesting an outer-sphere mechanism: k₂²⁹⁸ = 72.4 ± 1.0 × 103 M⁻¹ s⁻¹, ΔH(‡) = 18.18 ± 0.3 kJ mol⁻¹, and ΔS(‡) = -90.88 ± 1.0 J K⁻¹ mol⁻¹. The [Fe₃(μ₃-O)(μ-O₂CCH₃)₆(4-Phpy)₃](+/0) electron self-exchange reaction is compared with the more than 3 orders of magnitude faster [Ru₃(μ₃-O)(μ-O₂CCH₃)₆(py)₃](+/0) self-exchange reaction (ΔΔG(exptl)(‡298) = 18.2 kJ mol⁻¹). The theoretical estimated self-exchange rate constants for both processes compare reasonably well with the experimental values. The equilibrium constant for the formation of the precursor to the electron-transfer and the free energy of activation contribution for the solvent reorganization to reach the electron transfer step are taken to be the same for both redox couples. The larger ΔG(exptl)(‡298) for the 1/3 iron self-exchange is attributed to the larger (11.1 kJ mol⁻¹) inner-sphere reorganization energy of the 1 and 3 iron clusters in addition to a supplementary energy (6.1 kJ mol⁻¹) which arises as a result of the fact that each encounter is not electron-transfer spin-allowed for the iron redox couple.

Electrochemistry of Ferrocene-Functionalized Phosphonium Ionic Liquids

The synthesis of and voltammetry in undiluted form of several ferrocene-functionalized phosphonium ionic liquids is reported. Electron transport in the mixed valent diffusion layer around an electrode occurs primarily by Fc+/0 electron self-exchange reactions, as opposed to physical diffusion of the ferrocenated phosphonium species. The apparent diffusion coefficients for electron transport and for the counterions of the ionic liquid, and in particular their activation energy barriers, are similar to one another, evidence for the control of rates of electron transport by relaxation of the counterion atmosphere. For a net transport of electronic charge to accompany electron transfer, counterion displacement must occur to relax the charge displacement associated with the electron transfer. The ferrocenated phosphonium ionic liquids thus behave in a manner like that of previously investigated redox ionic liquids based on combinations of redox groupings with imidazolium and with short-chain polyether functionalities.