The electron transfer reactions involving quinones, hydroquinones, and catechols are very important in many areas of chemistry, especially in biological systems. The therapeutic efficiency as well as toxicity of anthracycline anticancer drugs, a class of anthraquinones, is governed by their electrochemical properties. Other quinones serve as important functional moiety in various biological systems like electron-proton carriers in the respiratory chain and their involvement in photosynthetic electron flow systems. The present paper summarizes literatures on the reduction of quinones in different solvents under various conditions using different electrochemical methods. The influence of different reaction conditions including pH of the media, nature of supporting electrolytes, nature of other additives, intramolecular or intermolecular hydrogen bonding, ion pair formation, polarity of the solvents, stabilization of the semiquinone and quinone dianion, catalytic property, and adsorption at the electrode surface, are discussed and relationships between reaction conditions and products formed have been presented.

https://downloads.hindawi.com/journals/ijelc/2011/816202.pdf

Electrochemical Reduction of Quinones in Different Media: A Review

3.3. Influence of the Bridge Redox States 5159 4. Multidimensional Organic Mixed Valence 5163 5. Intervalence Charge Transfer across Noncovalent Pathways 5167 6. Use of EPR Spectroscopy as a Direct Test for Hush Theory 5170 7. Solvent and Ion Pairing Effects 5171 8. Crystallographic Studies 5173 9. Concluding Remarks 5175 Author Information 5175 Biographies 5175 Acknowledgment 5176 References 5176

Organic mixed valence.

Certainly one of the first topics to come to mind on seeing the term “molecular electrochemistry” is the subject of electron-transfer reactions. In most cases, a multistep electrode reaction will be initiated by an electron-transfer step, either gain or loss of an electron. The product of that reaction will subsequently participate in chemical and electrochemical reactions leading finally to the end product of the reaction. Thus, an understanding of electron transfer is basic to the entire field of molecular electrochemistry. For a majority of electrode reactions of organic and organometallic species, only a single oxidation or reduction is encountered within the accessible potential range of the solvent/electrolyte medium that is chosen. This is often because that single electron transfer is situated fairly close to the solvent breakdown potential so that second or subsequent electron transfers are masked. However, in many instances two separate electron-transfer reactions can be detected with the normal pattern being that addition or removal of the second electron occurs with greater difficulty than the first. In many of these cases, but not all, the molecule contains two identical electroactive groups, an example being the reduction of dinitroaromatics such as 1,4-dinitrobenzene.

One-electron and two-electron transfers in electrochemistry and homogeneous solution reactions.

A monolayer of a phosphonated triarylamine adsorbed on nanocrystalline TiO2, ZrO2, or Al2O3 film deposited on conducting glass displays reversible electrochemical and electrochromic behavior although the redox potential of the electroactive molecules (0.80 V vs NHE) lies in the forbidden band of the semiconducting or insulating oxides. The mechanism of charge transport was found to involve hole injection from the conducting support followed by lateral electron hopping within the monolayer. The apparent diffusion coefficient ranged from 2.8 × 10-12 m2 s-1 in the neat 1-ethyl-2-methylimidazolium bis(trifluoromethylsulfonyl)imide (EtMeIm+Tf2N-) to 1.1 × 10-11 m2 s-1 in acetonitrile + 2 M EtMeIm+Tf2N-. A percolation threshold for electronic conductivity was found at a surface coverage corresponding to 50% of a full monolayer.

Efficient lateral electron transport inside a monolayer of aromatic amines anchored on nanocrystalline metal oxide films

The light-sensitive elements of conventional photographic materials are crystallites principally of the bromide or chloride salts of silver. Certain chemical modifications employing sulphur and gold compounds are used to maximize the response, and organic dyes are adsorbed to provide sensitivity throughout the visible spectrum. Absorption of a few photons results in the reductive formation of one or more clusters of silver or gold atoms on the surface. These clusters act as catalysts to promote further reduction of the host crystallites during photographic development. The success of the system depends upon the particular physical properties of the silver halides, such as their bonding and electron structure, the dielectric properties, the point defect structure, and the electronic transport characteristics. These are all reviewed, and the ways in which they interact to lead to the favourable photodecomposition are followed in some detail. A nucleation-and-growth mechanism operates to provide sev...

The silver halide photographic process

The rate constants ket of electron self exchange between neutral molecules and the corresponding radical ions, measured by ESR line broadening, are discussed for seven organic ϕ-systems in 18 aprotic solvents of a wide range of relaxation times, τL (0.2 < τL/ps < 10), and polarities, γ(0.05 < γ 0.53) in terms of the theoretical approaches of Marcus and Sumi and of Jortner and Bixon, both valid for comparable magnitudes of inner and outer reorganization energies. (γ = < γ = 1/n2-1/e; n = refractive index; e = dielectric constant.) The electron transfer is diabatic for four substituted p-phenylenediamines, it is adiabatic relaxation overdamped for tetracyanoquinodimethane (TCNQ), tetrathiafulvalene (TTF) and tetracyano-ethylene (TCNE). By changing the solvents no transition between both mechanisms is found. To explain the results, the reactants are represented by ellipsoids. Their centre to centre distance in the transition state is derived from slopes of lnket τL γ−1/2 vs. γ (for adiabatic reactions) or of Infketγ1/2 vs. γ (for diabatic reactions). The mutual orientation of the reactants follows the minimum of electrostatic energy. The resonance splitting V for this orientation, calculated from the overlap integrals agrees well for all systems with the observed diabatic or adiabatic behaviour of the reaction, except for TCNE. The resonance splitting and the equilibrium constant of the precursor complex are derived experimentally from the temperature dependence of the electron transfer rate constant ket. Negative association energies of the complex are due to ion-dipole interactions of the reactants. An additional reorganization term is observed for the aliphatic TCNE system in ethereal solvents. To describe the experiments quantitatively, both theories used are nearly equivalent.

Kinetics of Diabatic and Adiabatic Electron Exchange in Organic Systems Comparison of Theory and Experiment

The rate constants k1, and k2 of the self exchange reactions between the half oxidized and the totally oxidized or the reduced forms of some p-phenylenediamine derivatives are investigated at 293 K by ESR line-broadening in various aprotic solvents (dielectric constants ϵ between 4.7 and 49). The dimerization constants of the radicals at 183 K and the enthalpies of dimerization are measured, too. The results are discussed in terms of Marcus' theory. In k1, is found to depend linearly on γ = 1/n2 - 1/ϵ (n = refractive index), the covered range being 0.27 < γ < 0.53. The distance parameter g is obtained from the slope and compared with values derived from different molecular models. The results are represented satisfactorily by ellipsoidal molecules with half axes obtained from crystallographic data, the distance in the activated complex being approximately the same as in the radical dimers. The inner reorganization energy is calculated by Huckel methods and compared with experimental values obtained from the intercept of the straight line ln k1 (γ). The deviations are considerable. The calculated Marcus activation free enthalpies are smaller then the measured ones. — The slow rate constant k2 of the second step can be computed, if the Coulomb forces between the radical cation and the dication of the quinone diimine are considered. The inner reorganization energy of the transformation between aromatic and quinoidic structures is negligible. The rate constants of different cross reactions with p-phenylene diamines are calculated with Marcus' formula which, however, has to be extended by work terms. The symproportionation rate is nearly diffusion controlled. With some other reactions the conformity of measured and calculated rate constants is excellent.

ESR‐Spectroscopic Investigation of the Homogeneous Electron Transfer Reactions between Substituted p‐Phenylenediamines and Quinonediimines, and the Validity of Marcus' Theory. I. Measurements at 293 K

Heterogeneous electron transfer to quinones in aprotic solvents: Part II. The dependence on solvent and supporting electrolyte☆

The homogeneous electron self-exchange reaction of N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPPD/TMPPD˙+) shows the classical solvent dependence according to Marcus' theory. The rate constants are a linear function of γ=(1/n2–1/Iµ)(n= refractive index, Iµ= dielectric constant of the solvent). For 0.27 ⩽γ⩽ 0.53, log kex= const. ×γ. However, the reactions of tetracyanoquinonedimethane (TCNQ/TCNQ˙–) and tetracyanoethylene (TCNE/TCNE˙–) are dominated by the solvent dynamics, including the longitudinal relaxation time τL of the solvent. This is expressed by log (kexτLγ–1/2)= const. ×γ, for 0.05 ⩽γ⩽ 0.53. The role of solvent reorganization dynamics is discussed together with the decision equation of Ovchinnikova.

The role of solvent reorganization dynamics in homogeneous electron self-exchange reactions

The homogeneous electron exchange of the N,N,N′N′-tetramethyl-p-phenylenediamine radical (S˙+) with the reduced (R) and the totally oxidized (T2+) forms has been measured in aprotic solvents by means of e.s.r. line-broadening. Solvent polarity and temperature were varied. For each temperature the logarithm of the rate constant k1(S˙+⇌ R) is a linear function of the solvent parameter γ= 1/n2–1/Iµ(where n is the refractive index and Iµ the dielectric constant) and at constant γ(0.27 ⩽γ⩽ 0.53) a linear function of 1/T. The outer reorganization energy can be calculated using an ellipsoidal model of the reactants and a reaction distance equal to the molecular distance in the radical dimer. The inner reorganization energy is calculated from i.r. and crystallographic data. Nuclear tunnelling is considered since the intramolecular frequencies are high. The experimental function ln k1(γ, T–1) cannot be reproduced by the theory unless additional interaction energies such as ion association S˙+⋯ClO–4 and especially ion-molecule interactions S˙+⋯R are allowed for. The preexponential factors of the theoretical expressions are considerably greater than the experimental values, especially if a precursor equilibrium is assumed. The rate constant k2(S˙+⇌ T2+) is small compared with k1. The difference is mainly caused by the Coulomb repulsion term.

Walther Jaenicke

Papers

Kinetics of Diabatic and Adiabatic Electron Exchange in Organic Systems Comparison of Theory and Experiment

ESR‐Spectroscopic Investigation of the Homogeneous Electron Transfer Reactions between Substituted p‐Phenylenediamines and Quinonediimines, and the Validity of Marcus' Theory. I. Measurements at 293 K

Heterogeneous electron transfer to quinones in aprotic solvents: Part II. The dependence on solvent and supporting electrolyte☆

The role of solvent reorganization dynamics in homogeneous electron self-exchange reactions

Kinetics of electron exchange in N,N,N′,N′-tetramethyl-p-phenylenediamine in aprotic solvents and the semiclassical theory of electron transfer