Multielectron redox processes

When the metal complexes constitute the peripheral units (Fig. lb) and/or belong to the branches (Fig. 1 c) of a dendrimer, a number of equivalent metal-based centers are present since dendrimers are usually highly symmetric species by their own nature. The metal-based centers may or may not interact, depending on distance and nature of the connector units. Multielectron redox processes can therefore be observed, whose specific patterns are related to the degree of interaction among the various units. 

So far, except for the iron(III)/iron(II) couple [reaction (6) in Table 14.2], we have considered reduction potentials of half reactions with an overall transfer of an even number of electrons (i.e., 2, 4, 6, etc.). However, in many abiotic multielectron redox processes, particularly if organic compounds are involved, the actual electron transfer frequently occurs by a sequence of one-electron transfer steps (Eberson, 1987). The resulting intermediates formed are often very reactive, and they are not stable under environmental conditions. In our benzoquinone example, BQ is first reduced to the corresponding semiquinone (SQ), which is then reduced to HQ ... 

Electroluminescence, (EL), is a versatile probe for studying such carrier injection processes. Thus, hole injection into the VB of a n-type semiconductor leads to cathodic EL, whereas electron injection into the GB of a p-type semiconductor leads to anodic EL [67]. Examples of studies of cathodic EL are commonplace [68-70] however, anodic EL is not very common because the energy requirement for the redox couple has a very negative redox potential. Nonetheless, anodic EL has been reported for the p-InP-[Cr(CN)6] interface [71]. Radical intermediates can also cause EL as discussed later for multielectron redox processes. EL is treated in more depth in another chapter. 

This section has thus far considered redox electrolytes comprising one electron oxidizing or reducing agents. Multielectron redox processes, however, are important in a variety of scenarios. Consider the reduction of protons to H2 (HER) - a... 

Native semiconductor surfaces are fairly inactive from a catalysis perspective. Thus, noble metal or metal oxide islands have been implanted on photoelectrode surfaces as electron storage centers to drive multielectron redox processes such... 

A further, drastic step towards increasing structural complexity can be made by going from mononuclear to polynuclear complexes. These are systems in which two (or more) metal complex subunits are connected by one (or more) bridging ligand(s) (Fig. 1). Because of this additional complexity, the photophysics of polynuclear complexes is expected to be different from, and more interesting than, that of simple mononuclear species. This is true, in principle, also from the photocatalytic point of view, where the additional possibilities of polynuclear systems, in particular with respect to multielectron redox processes and inter-component energy or electron transfer, could find applications. 

Some of the more interesting and valuable redox processes are multielectron in nature, suggesting the utility of coupling a two- or many-electron event into an excited state process. The study of the excited state photochemistry and photophysics of binuclear and polynuclear (cluster) molecules is thus becoming of importance, and two-electron reactions are being identified. 

Although first-order kinetics in both and are commonly observed, the majority of electrochemical reactions occur in more than one step -. Thus, e.g., for metal electrodeposition, electron and phase transfer occur in two distinct steps. As for homogeneous redox processes, various chemical steps (e.g., ligand substitution) either preceding or following the electron-transfer step often are encountered. Also, there is evidence that multielectron transfer occurs in microscopically separable one-electron steps. Even for one-electron transfer where both Yq and Y, are solution species, a separate adsorption step often precedes electron transfer. 

The presence of two metal centres opens up the possibility to smdy electronic coupling, and for multielectron photoredox processes to take place, whilst relatively long lifetimes of the excited states (see below) makes possible bimolecular reactions in solution. Accordingly, quadruply-bonded di-Re complexes have been reported to engage in bimolecular electron-transfer reactions, whereas the di-Mo and di-W complexes participate in oxidative addition and two-electron redox reactions. For example, UV irradiation of phosphate-supported M2 dinuclear complexes under acidic conditions leads to one- or two-electron oxidation of the metal-metal core accompanied by production of H2 gas by reduction of two protons. 

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