Electron self-exchange

In the present case, the electron hopping chemistry in the polymeric porphyrins is an especially rich topic because we can manipulate the axial coordination of the porphyrin, to learn how electron self exchange rates respond to axial coordination, and because we can compare the self exchange rates of the different redox couples of a given metallotetraphenylporphyrin polymer. To measure these chemical effects, and avoid potentially competing kinetic phenomena associated with mobilities of the electroneutrality-required counterions in the polymers, we chose a steady state measurement technique based on the sandwich electrode microstructure (19). 

Fig. 5. Plot of apparent electron self exchange rate constants kf P, derived from polymer De values for films containing the indicated metals, mixed valent states, and ligands, all in acetonitrile, using Equation 2, vs. literature heterogeneous electron transfer rate constants k° for the corresponding monomers in nitrile solvents. See Ref. 6 for details. (Reproduced from Ref. 6. Copyright 1987 American Chemical Society.)...

Esr spectroscopy has also been used to study pure solvent dynamics in electron self-exchange reactions (Grampp et al., 1990a Grampp and Jaenicke, 1984a,b). When the systems are not linked by a spacer (i.e. TCNQ- /TCNQ (TCNQ = tetracyanoquinodimethane), the homogeneous bimolecular rate constants /chom are given by (10), with fcA the association constant and kET... 

The electron self-exchange rate constants evaluated by the Marcus expressions (using cross-reaction data) and those determined experimentally differ in the following cases. Give possible reasons for these differences. 

The electron self-exchange rate constants (At) have been assessed for the following couples at 25 °C and x = 0.1 M ... 

Despite the lack of theoretical models for interfacial recombination processes in excitonic solar cells, it is obvious empirically that those cells which function efficiently must have a very slow rate of recombination. In DSSCs, this can be explained simply by the slow electron self-exchange rate of the I /I2 redox couple and the absence of field-driven recombination. However, in the case of solid-state, high-surface-area OPV cells, such as the conducting polymer/C60-derivative cells [36,39], the slow rate of interfacial recombination is an important problem that is not yet understood. 

The reversibility of the [Os(bpy)3]3+/2+ couple makes it useful for the determination of the electron self-exchange rates of other couples by application of the Marcus cross-reaction equation. Recently, this has been applied to the oxidation of S032- to S042- (622). The new rate constant for this reaction of 1.63 x 107 M-1 sec-1 is consistent with the... 

Initially, all the Fc sites further removed from the electrode surface are still in the unoxidized Fc state. Electrons can, however, hop from these distant Fc sites to the Fc+ sites at the electrode surface (Fig. 13.3C). Electron hopping occurs via a well-known chemical process called electron self-exchange whereby the reduced half of a redox couple (Fc) simply gives its electron to an oxidized counterpart (Fc+). This reaction can be written as... 

The Marcus therory provides an appropriate formalism for calculating the rate constant of an outer-sphere redox reaction from a set of nonkinetic parameters1139"1425. The simplest possible process is a self-exchange reaction, where AG = 0. In an outer-sphere electron self-exchange reaction the electron is transferred within the precursor complex (Eq. 10.4). 

Table 10.2. Observed and calculated electron self-exchange rates of hexaamine cobalt(III/II) complexes11321.

Fig. 13.5 Arrhenius plots of electron self-exchange reaction between ZnT(t-Bu)PP+ and ZnT(t-Bu)PP in different solvents [16].

Three processes can control the rate of homogeneous charge transport through a redox-active polymer film, i.e. electron self-exchange between redox-active centers,... 

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