Photoinduced Dissociative Electron Transfer: Is the Quantum Yield Theoretically Predicted to Equal Unity?

An attractive manner of fighting back-electron transfer to the ground state in photoinduced electron transfer reactions is to use a system in which the donor and/or the acceptor in the ion-pair undergoes a rapid fragmentation. Intuitively, it seems that an ideal situation in this respect, leading to a unity quantum yield, should be met when fragmentation and electron transfer are concerted. Accordingly, a quantum yield below 1 would be the signature of a nonconcerted two-step mechanism. It is shown, from first principles, that a purely dissociative photoinduced electron transfer is not necessarily endowed with a unity quantum yield. The reason is that the system partitions between fragmentation and back-electron transfer in the funnel offered by the upper first-order potential energy surface combining the ground state and fragments zero-order surfaces. A semiclassical model is presented, relating the quantum yield to the electronic matrix coupling element, H. Only in the case of a completely nonadiabatic ground-state electron transfer (H = 0) should the quantum yield reach unity. Upon increasing H, the quantum yield rapidly decreases to a distinctly smaller value which can be as low as 0.5

Selective and Efficient Photocatalytic CO₂ Reduction to CO Using Visible Light and an Iron-Based Homogeneous Catalyst

Converting CO₂ into valuable compounds using sunlight as the energy input and an earth-abundant metal complex as the catalyst is an exciting challenge related to contemporary energy issues as well as to climate change. By using an inexpensive organic photosensitizer under visible-light excitation (λ > 400 nm) and a substituted iron(0) tetraphenylporphyrin as a homogeneous catalyst, we have been able to generate carbon monoxide from CO₂ selectively with high turnover numbers. Sustained catalytic activity over a long time period (<i>t</i> > 50 h) did not lead to catalyst or sensitizer deactivation. A catalytic mechanism is proposed

Successive Removal of Chloride Ions from Organic Polychloride Pollutants. Mechanisms of Reductive Electrochemical Elimination in Aliphatic Gem-Polychlorides, α,β-Polychloroalkenes, and α,β-Polychloroalkanes in Mildly Protic Medium

The factors that control the successive reductive expulsion of chloride ions from aliphatic gem-polychlorides are investigated, taking as examples the electrochemical reduction of polychloromethanes and polychloroacetonitriles in N,N-dimethylformamide. At each elimination stage, the reaction involves, as a rate-determining step, the transfer of one electron concerted with the cleavage of the carbon−chloride bond. The second step is an immediate electron transfer to the ensuing radical, taking place at a potential more positive than the potential at which the first electron transfer occurs. The carbanion thus formed is sufficiently basic to be protonated by any trace weak acid present in the reaction medium. The three successive elimination steps require increasingly negative potentials. Application of the “sticky” dissociative electron transfer model allows one to quantitatively unravel the factors that control the energetics of the successive reductive expulsion of chloride ions. The large potential gaps between each stage stem primarily from large differences in the dissociative standard potentials. They are also strongly affected by two cumulative intrinsic activation barrier factors, namely, the bond dissociation energy of the substrate that decreases with the number of chlorine atoms and the interaction between chloride ion and the radical that increases in the same direction. In the case of α,β-polychloroethanes (Cl3CCCl3, Cl2HCCCl3, Cl2HCCHCl2, ClH2CCHCl2) too, the first step is a dissociative electron transfer with sizable ion−radical interactions in the product cluster. Likewise, a second electron transfer immediately leads to the carbanion, which however prefers to expel a second chloride ion, leading to the corresponding olefin, than to be protonated to the hydrogenolysis product. The ion−radical interaction in the product cluster plays a major role in the control of the reduction potential. The reduction of the α,β-polychloroethenes (Cl2CCCl2, ClHCCCl2, ClHCCHCl) follows a similar 2e-−2Cl- reaction sequence, leading then to the corresponding alkynes. However, unlike the polychloroethane case, the expulsion of the first chloride ion follows a stepwise electron transfer/bond cleavage mechanism. The reduction potential is thus essentially governed by the thermodynamics of the anion radical formation

Does Catalysis of Reductive Dechlorination of Tetra- and Trichloroethylenes by Vitamin B12 and Corrinoid-Based Dehalogenases Follow an Electron Transfer Mechanism?

Knowing the mechanism by which dangerous organic chloride pollutants, such as tetra- or trichloroethylene, are reductively cleaved is an important task for the establishment of remediation strategies and for a better comprehension of bacterial dehalorespiration by corrinoid-based dehalogenases. On the basis of electrochemical and thermodynamic data, application of outersphere and dissociative electron transfer theories allows the prediction of the pertinent activation/driving force relationships characterizing the electron transfer mechanism. They are validated by application of the redox catalysis method to the reaction with two typical outersphere electron donors. The kinetic gap is more than 11 and 7 orders of magnitude for the dehalogenase and for cobalamin, respectively, showing that the electron transfer mechanism is not operative. Multistep mechanisms in which the chloroethylene molecule enters the cobalt coordination sphere are preferred

Toward Visible-Light Photochemical CO₂‑to-CH₄ Conversion in Aqueous Solutions Using Sensitized Molecular Catalysis

Solar fuels may be generated upon visible light induced catalytic reduction of carbon dioxide. This appealing approach remains highly challenging, especially when earth abundant catalysts, mild conditions, and water as a solvent were used. Employing an iron tetraphenyl porphyrin complex substituted by positively charged trimethylammonio groups at the para position of each phenyl ring and reduction with three electrons by the excited state of an iridium sensitizer (λ > 420 nm) reduce CO₂ to CO and to CH₄ in both acetonitrile and aqueous solutions (acetonitrile/water 3:7 v:v) with good selectivity. Stability of the catalytic system remains a weakness and the reasons were analyzed

Carboxylates as Proton-Accepting Groups in Concerted Proton−Electron Transfers. Electrochemistry of the 2,5-Dicarboxylate 1,4-Hydrobenzoquinone/2,5-Dicarboxy 1,4-Benzoquinone Couple

Concerted proton and electron transfers (CPET) currently attract considerable theoretical and experimental attention, notably in view of their likely involvement in many enzymatic reactions. The role of carboxylate groups as proton-accepting sites in CPET reactions is explored by means of a cyclic voltammetric investigation of the 2,5-dicarboxy 1,4-benzoquinone/2,5-dicarboxylate 1,4-hydrobenzoquinone couple in a nonaqueous medium. The presence of carboxylate groups ortho to the phenol groups induces the removal of an electron to be coupled with the transfer of the phenolic proton to a carboxylate oxygen. The kinetics of the electrochemical reaction and the observation of a significant hydrogen/deuterium kinetic isotope effect unambiguously indicate that electron transfer and proton transfer are concerted, thus providing an additional demonstration of the role of carboxylate groups as proton-accepting sites in concerted proton−electron transfer reactions

Stabilities of Ion/Radical Adducts in the Liquid Phase as Derived from the Dependence of Electrochemical Cleavage Reactivities upon Solvent

The idea that significant ion/radical interactions should vary with solvent if they do exist in the liquid phase was pursued by an investigation of the dissociative electron-transfer reactivity of carbon tetrachloride and 4-cyanobenzyl chloride in four different solvents, 1,2-dichloroethane, N,N-dimethylformamide, ethanol, and formamide, by means of their cyclic voltammetric responses. Modification of the conventional dissociative electron transfer theory to take account of an interaction between fragments in the ion/radical pair resulting from the dissociative electron reaction allows a satisfactory fitting of the experimental data leading to the determination of the interaction energy. There is an approximate correlation between the interaction energies in the ion/radical pair and the solvation free energies of the leaving anion, Cl-. The interaction is maximal in 1,2-dichloroethane, which is both the least polar and the least able to solvate Cl-. The interaction is smaller in the polar solvents, albeit distinctly measurable. The two protic solvents, ethanol and formamide, which are the most able to solvate Cl-, give rise to similar interaction energies. The interaction is definitely stronger in N,N-dimethylformamide, which has a lesser ability to solvate Cl- than the two other polar solvents. The existence of significant ion/radical interactions in polar media is thus confirmed and a route to their determination opened

Fragmentation of Aryl Halide π Anion Radicals. Bending of the Cleaving Bond and Activation vs Driving Force Relationships

Recent rate data for very fast cleaving of aryl chloride and bromide anion radicals may be accommodated satisfactorily within rate constant versus ArX/ArX•- standard potential existing correlations provided the standard potential is determined experimentally. Cyclic voltammetry is used for this purpose, taking careful account of the electron transfer/fragmentation reaction mixed character of the kinetics. The ensuing activation/driving force relationships allow the determination of the intrinsic barriers, the magnitude of which are discussed in the framework of a new Morse curve model that includes and emphasizes the role of bond bending

Adiabatic and Non-adiabatic Concerted Proton−Electron Transfers. Temperature Effects in the Oxidation of Intramolecularly Hydrogen-Bonded Phenols

The one-electron electrochemical and homogeneous oxidations of two closely similar aminophenols that undergo a concerted proton−electron transfer reaction, in which the phenolic proton is transferred to the nitrogen atom in concert with electron transfer, are taken as examples to test procedures that allow the separate determination of the degree of adiabaticity and the reorganization energy of the reaction. The Marcus (or Marcus−Hush−Levich) formalism is applicable in both cases, but not necessarily in its adiabatic version. Linearization of the activation−driving force laws simplifies the treatment of the kinetic data, notably allowing the use of Arrhenius plots to treat the temperature dependence of the rate constant. A correct estimation of the adiabaticity and reorganization energy requires the determination of the variation of the driving force with temperature. Application of these procedures led to the conclusion that, unlike previous reports, the homogeneous reaction is non-adiabatic, with a transmission coefficient of the order of 0.005, and that the self-exchange reorganization energy is about 1 eV lower than previously estimated. With such systems, the intramolecular reorganization energy, although sizable, is in fact rather modest, being only slightly larger than that for the outer-sphere electron transfer that produced the cation radical. The electrochemical reaction is, in contrast, adiabatic, as revealed by the temperature dependence of its standard rate constant obtained from cyclic voltammetric experiments. This difference in behavior is deemed to derive from the effect of the strong electric field within which the electrochemical reaction takes place, stabilizing a zwitterionic form of the reactant (in which the proton has been transferred from oxygen to nitrogen). Taking this difference in adiabaticity into account, the magnitudes of the reorganization energies of the two reactions appear to be quite compatible with one another, as revealed by an analysis of the solvent and intramolecular contributions in both cases

Electrochemical and Homogeneous Proton-Coupled Electron Transfers: Concerted Pathways in the One-Electron Oxidation of a Phenol Coupled with an Intramolecular Amine-Driven Proton Transfer

Proton-coupled electron transfers currently attract considerable attention in view of their likely involvement in many natural processes. Electrochemistry, through techniques such as cyclic voltammetry, is an efficient way of investigating the reaction mechanism of these reactions, and deciding whether proton and electron transfers are concerted or occur in a stepwise manner. The oxidation of an ortho-substituted 4,6-di (tert-butyl)-phenol in which the phenolic hydrogen atom is transferred during the reaction to the nitrogen atom of a nearby amine is taken as illustrative example. A careful analysis of the cyclic voltammetric responses obtained with this compound and its OD derivative allows, after estimation of the various thermodynamic parameters, ruling out the occurrence of the square scheme mechanism involving the proton−electron and electron−proton sequences. Simulation and comparison of the rate constant and H/D kinetic isotope effect with theoretical predictions show that the experimental value of the preexponential factor is ca. 1 order of magnitude larger than the theoretical value. Detailed calculations suggest that an electric field effect is responsible for this discrepancy