Single- and Multi-electron Transfer Processes

Skourtis, S. S. Beratan, D. N. Single- and Multi-electron Transfer Processes. In Principles and Theories-, Piotrowiak, P.,... 

The photocatalytic properties and electron/photon-induced processes related to natural systems treated in Chapters 13 and 14 have been researched in depth. Different single fundamental multi-electron catalytic processes and photoexcited state electron-transfer reactions, both in polymer matrixes, are described in relation to photosynthesis (Section 13.2). It is now necessary to combine these reactions step by step to produce artificial photosynthetic systems. Some photoinduced energy-transfer processes (photooxidations) have now reached the level of practical application for wastewater cleaning (Section 13.4) and should be extended to other reactions induced by irradiation with visible light. 

The design of such artificial photosynthetic systems suffers from some basic limitations a) The recombination of the photoproducts A and S+ or D+ is a thermodynamically favoured process. These degra-dative pathways prevent effective utilization of the photoproducts in chemical routes, b) The processes outlined in eq. 2-4 are multi electron transfer reactions, while the photochemical reactions are single electron transformations. Thus, the design of catalysts acting as charge relays is crucial for the accomplishment of subsequent chemical fixation processes. 

In this section, the current-potential curves of multi-electron transfer electrode reactions (with special emphasis on the case of a two-electron transfer process or EE mechanism) are analyzed for CSCV and CV. As in the case of single and double pulse potential techniques (discussed in Sects. 3.3 and 4.4, respectively), the equidiffusivity of all electro-active species is assumed, which avoids the consideration of the influence of comproportionation/disproportionation kinetics on the current corresponding to reversible electron transfers. A general treatment is presented and particular situations corresponding to planar and nonplanar diffusion and microelectrodes are discussed later. 

The rate of energy and electron transfer in any of these systems is an ensemble average of all sites within the layered assembly, and thus there is no single rate for any of the processes in the assemblies. Nonetheless, in addition to the increased quantum yield, assemblies constructed with the HTiNbOs spacer exhibit a long-lived charge-separated state component not observed in the a-ZrP spaced assemblies, of x = 900 ps. While there is still much to be learned from the study of multi-chromophore arrays, this synthetic approach appears to hold much promise for the creation of organic-inorganic hybrid solid state devices. 

Many of the models for electron-transfer (ET) reactions discussed in this work assume the following 1) just one electron is transferred, 2) the transfer occurs from donor to acceptor in a single step, and 3) the bridge is rigid during the process. Recent experimental and theoretical advances indicate that these assumptions are insufficient in many circumstances. Indeed, multi-electron, multistate, and dynamic bridge effects enrich the subject substantially. In this chapter we shall examine the influence of these effects on chemical and biological ET reactions. 

We begin with a summary of the standard single-electron rigid-bridge model for electron transport [1,2], and then describe effects that arise from bridge dynamics. We next examine issues in multistep multi-center electron transfer. The closely related problem of two-electron transfer is then discussed. Multi-center and multielectron processes are of great relevance for ET in DNA, proteins, and catalytic reactions. 

An example of the simplest (in the sense of the number of kinetic parameters) electrochemical reaction is reduction of silver ions (Ag+) from a dilute aqueous solution of a well soluble silver salt (e.g., nitrate) in the presence of excess of an indifferent salt (e.g., potassium nitrate) on a liquid silver-mercury alloy (also called amalgam) electrode. Besides the transfer of a single electron, only diffusion steps are involved in this process. The entire reaction can be very well modeled and the kinetic parameters are determined experimentally with high level of accuracy. The information gleaned while analyzing the mechanism of silver ion reduction can be used in elucidating more complex, multi-step, multiphase processes, such as the electrochemical reaction in a lithium-ion cell. 

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