Heterogeneous electron transfer dynamics

The heterogeneous electron transfer dynamics of a diverse range of organic and inorganic species and also the dynamics and energetics of ultrafast heterogeneous electron transfer dynamics of immobilized electroactive species on an electrode surface have been investigated with ultrafast voltammetry under a wide variety of experimental conditions of timescale, temperature, solvent, and electrolyte (see for example Fig. 5.16, obtained from [54]). 

R. J. Forster, Hopping Across Interfaces Heterogeneous Electron Transfer Dynamics, Interface 9(4) 24, 2000. 

Despite the many elegant investigations that have been conducted into the heterogeneous electron transfer dynamics of solution phase reactants, the magnitude of the diffusion-controlled current at short times ultimately places a lower limit on the accessible timescale. As described by Eq. (10), the thickness of the diffusion layer... 

An attractive approach to solving the mass transfer limitations of these investigations is to immobilize the electroactive species at the electrode surface within a monomolecular film. Clearly, if the electroactive species is immobilized at the electrode surface, diffusion of the species to the electrode does not need to occur prior to electron transfer. In addition, immobilization at an electrode surface can preconcentrate the species of interest, resulting in higher currents that are easier to detect. Electroactive adsorbed monolayers have been developed that exhibit close to ideal reversible electrochemical behavior under a wide variety of experimental conditions of timescale, temperature, solvent, and electrolyte. These studies have elucidated the effects of electron transfer distance, tunneling medium, molecular structure, electric fields, and ion pairing on heterogeneous electron transfer dynamics. 

Despite the many elegant investigations that have been conducted on heterogeneous electron transfer dynamics of solution phase reactants, the magnitude of the diffusion-controlled current at short times ultimately places a lower limit on the accessible timescale. For diffusive species, the thickness of the diffusion layer, 8, is defined as = (nDty (equation (6.1.5.1)) and is, therefore, proportional to the square root of the polarization time, t. One can estimate that the diffusion layer thickness is approximately 50 A if the diffusion coefficient is 1 X 10 cm sec and the polarization time is 10 nsec. Given a typical bulk concentration of the electroactive species of 1 mM, this analysis reveals that only 10,000 molecules would be oxidized or reduced at a 1-pm radius microdisk under these conditions. The average current for this experiment is only 170 nA, which is too small to be detected with low nanosecond time resolution. 

Dynamic Aspects of Heterogeneous Electron-Transfer Reactions at Liquid-Liquid Interfaces... 

Dynamic electroanalytical measurements at a solid electrode involve heterogeneous electron transfer. Electrons are transferred across the solution electrode interface during the electrode reaction. In fact, the term electrode reaction implies that such an electron-transfer process occurs. 

The electron transfer dynamics of monolayers based on osmium polypyridyl complexes linked to an electrode surface through conjugated and non-conjugated bridges, e.g. frans-l,2-bis(4-pyridyl)ethylene (bpe) and 1,2-bis(4-pyridyl)ethane (p2p), respectively, have been explored [18]. The standard heterogeneous electron transfer rate constant, k°, depends on both a frequency factor and a Franck-Condon barrier, as follows [19-21] ... 

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The heterogeneous rate constant [236] of electrochemical reduction of [Co "(bpy)3] + to [Co"(bpy)3] + is relatively slow, about 0.1 cm s in CH3CN or CH2CI2. Detailed studies of solvent and pressure effects [236, 237] have revealed that the rate of heterogeneous electron transfer is controlled by solvent dynamics. This implies that the electron transfer is adiabatic. 

The rate constant for the heterogeneous electron transfer to aromatic compounds is usually somewhat higher in MeCN than in DMF, which indicates that not only the dielectric but also the dynamic properties of the solvents influence the rate of radical anion formation [270]. 

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