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

Abstract

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).

Electrode reactions in slowly relaxing media

“…Ionic liquids are obviously different from dipolar molecular solvents, 86,87 but still match in many respects dielectric properties of molecular liquids. 80,81,88,89 Polar re-sponse of an ionic liquid is mostly due to translational motions of the ions. Rotations of molecular dipoles m (mostly belonging to cations 90 ) were considered as an alternative mechanism of polar response, but are now viewed as a minor contribution to solvation as a result of recent computer simulations.…”

“…80,81,88 However, since the relaxation of RTILs is much slower, fast electrode reactions in RTILs have puzzled many researchers. 89,121 Our resolution of this puzzle is in terms of nonergodic effects on the solvent reorganization energy: the nonergodic character of the electrode reaction, due to complex dynamics of RTILs, leads to a significantly lower reorganization energy (Fig. 3) and a correspondingly lower activation barrier.…”

Dynamic Interplay between Transport and Reaction Kinetics of Luminophores on the Operation of AC-Driven Electrochemiluminescence Devices

“…For the surface redox reactions 1, 2-1, and 2-2, which are first-order reactions, the rate constants at varying operation frequencies were experimentally determined. The second-order rate constant for the annihilation reaction (eq 3) was determined to be 2 × 10 5 M −1 s −1 , 23,24 which yielded good agreement between the experimental and simulated transient ECL profiles. The rate constant for the light-emission reaction (eq 4) was chosen to be 5.4 × 10 6 s −1 .…”

“…where k a (=2 × 10 5 M −1 s −1 ) 23,24 and k l (=5.4 × 10 6 s −1 ) 25 are the rate constants for the annihilation reaction (eq 3) and light emission (eq 4), respectively. At the electrodes, the oxidation (eq 1) and reduction (eq 2-1) reactions were considered in period 1 and period 2, respectively.…”

A Binary Mixture of Ionic Liquids as a New Approach for an Experimental Diffusion Correction in the Study of Activated Processes

“…[8] In recenty ears, we successfully used this experimental approacht oa ddress different current scientific questions on how electrons are transferred in ILs. [9] The measured rates usually have to be corrected for diffusion to eliminate the temperature effect of viscosity.W ec hose this system because it has already been established as totally diffusion controlled.…”