Electroactive groups

A Structural characteristic of conducting organic polymers is the conjugation of the chain-linked electroactive monomeric units, i.e. the monomers interact via a 7t-electron system. In this respect they are fundamentally different from redox polymers. Although redox polymers also contain electroactive groups, the polymer backbone is not conjugated. Consequently, and irrespective of their charge state, redox polymers are nonconductors. Their importance for electrochemistry lies mainly in their use as materials for modified el trodes. Redox polymers have been discussed in depth in the literature and will not be included in this review. 

Since model compounds reveal well-defined cyclic voltammograms for the Cr(CNR)g and Ni(CNR)g complexes (21) the origin of the electroinactivity of the polymers is not obvious. A possible explanation (12) is that the ohmic resistance across the interface between the electrode and polymer, due to the absence of ions within the polymer, renders the potentially electroactive groups electrochemically inert, assuming the absence of an electronic conduction path. It is also important to consider that the nature of the electrode surface may influence the type of polymer film obtained. A recent observation which bears on these points is that when one starts with the chromium polymer in the [Cr(CN-[P])6] + state, an electroactive polymer film may be obtained on a glassy carbon electrode. This will constitute the subject of a future paper. 

Attempts to develop a model for the digital simulation of the cyclic voltammetric behaviour of PVF films on platinum62 electrodes required inclusion of the following features (a) environmentally distinct oxidized and reduced sites within the film (b) interconversion of the above sites and interaction between them (c) rate of electrochemical reactions to depend on the rate of interconversion of redox sites, the rate of heterogeneous electron transfer between film and substrate, intrafilm electron transfer and the rate of diffusion of counter ions and (d) dependence on the nature of the supporting electrolyte and the spacing of electroactive groups within the film. 

There are two factors which limit the application of this treatment. For many of the compounds studied (e.g. carbonyl compounds, nitrocompounds etc.), the conjugate acid is so strong that the pK value lies beyond the accessible acidity range. The other limitation is inherent in the assumption that the electrode process (26 b) does not involve any further proton transfer. For numerous systems, in addition to a rapidly established protonation in an acidic grouping placed at some distance from the electroactive group, a further proton transfer to the electroactive group itself occurs at the electrode surface. It is therefore desirable... 

For heterocyclic substances which carry the electroactive group either directly attached to the heterocyclic ring or in the side-chain,... 

Similarly, it is possible to classify the reactions in polycyclic hetero-cyclics. The situation is then complicated by the possibility that the substituent on the electroactive grouping can be attached either to the heterocyclic or to the benzenoid ring. 

The condition for quantitative comparisons of half-wave potentials is that the electrode process follow the same mechanism for all systems to be compared. This condition is rarely fulfilled when substances bearing different electroactive groups are compared. Any comparison of the reactivity based on comparison of half-wave potentials of systems that are not reduced by the same or an analogous mechanism is at best a rough approximation. 

Another possibility for a rigorous comparison of the reactivities of the individual electroactive groups is the comparison (7,110) of potential ranges (based on the application of the Hammett equations, cf. section 3.1) of half-wave potentials of benzene derivatives bearing an electroactive group either in the side-chain or directly attached to the benzene ring, and further one single substituent in the m- or -position (Fig. 30). 

However, the reduction is not made easier by the presence of the double bond in the vicinity of the electroactive group in all cases. For... 

If the course of the electrode process remains the same for the group R1 when it is bound on various molecular frames A, but the half-wave potentials of the compounds A — R1 depend on the kind of the molecular frame A, it is possible (7) to compare the half-wave potentials with an analogous reaction series of compounds A — R1, which carry a second electroactive group R2 on the varying frame A. If a correlation between... 

Redox polymers are electroactive polymers for which the redox centers are localized on pendent, covalently attached redox centers. The electrochemical properties of such materials depend not only on both the loading and the nature of the redox-active center but also on the type of polymer backbone. The electroactive groups are typically metal complexes, which are covalently attached to a polymer... 

If the two reducible groups are in an 1,2- or 1,4-position, the strong interaction between the two groups can result in a change in mechanism which results in a reversal of the sequence of the reductions, that would be expected based on the behavior of compounds bearing a single electroactive group. 

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