Redox centres

Fig. 6. Theoretical cyclic voltammogram for a redox-active polymer film with noninteracting redox centres...

Intensive research on the electrocatalytic properties of polymer-modified electrodes has been going on for many years Until recently, most known coatings were redox polymers. Combining redox polymers with conducting polymers should, in principle, further improve the electrocatalytic activity of such systems, as the conducting polymers are, in addition, electron carriers and reservoirs. One possibility of intercalating electroactive redox centres in the conducting polymer is to incorporate redoxactive anions — which act as dopants — into the polymer. Most research has been done on PPy, doped with inter alia Co 96) RyQ- 297) (--q. and Fe-phthalocyanines 298,299) Co-porphyrines Evidently, in these... 

Chapman SK, Daff S, Munro AW (1997) Heme The Most Versatile Redox Centre in Biology 88 39-70... 

Here we mention as an example that in the coordination-chemistry field optical MMCT transitions between weakly coupled species are usually evaluated using the Hush theory [10,11]. The energy of the MMCT transition is given by = AE + x- Here AE is the difference between the potentials of both redox couples involved in the CT process. The reorganizational energy x is the sum of inner-sphere and outer-sphere contributions. The former depends on structural changes after the MMCT excitation transition, the latter depends on solvent polarity and the distance between the redox centres. However, similar approaches are also known in the solid state field since long [12]. 

A discussion of the charge transfer reaction on the polymer-modified electrode should consider not only the interaction of the mediator with the electrode and a solution species (as with chemically modified electrodes), but also the transport processes across the film. Let us assume that a solution species S reacts with the mediator Red/Ox couple as depicted in Fig. 5.32. Besides the simple charge transfer reaction with the mediator at the interface film/solution, we have also to include diffusion of species S in the polymer film (the diffusion coefficient DSp, which is usually much lower than in solution), and also charge propagation via immobilized redox centres in the film. This can formally be described by a diffusion coefficient Dp which is dependent on the concentration of the redox sites and their mutual distance (cf. Eq. (2.6.33). 

Figure 2.9 Schematic structures of Fe-S redox centres, their electron-transfer reactions and approximate range of potentials. See text for details. Individual iron atom oxidation states are not specified for delocalized cluster spin states are indicated. Reprinted with permission from Holm et al., 1996. Copyright (1996) American Chemical Society.

Figure 5a schematizes LAJs based on two Hg electrodes. By bringing in contact the two Hg-drops inside an electrolyte solution and connecting them to a macroscopic reference electrode by a potentiostat (Figs. 15a and 16a), an electrochemical junction is created this junction allows for independent control of the potentials applied to the two Hg electrodes, so that the cathode can act as electron donor (source) and the anode as electron acceptor (drain) with respect to the redox centre. 

These results show that significant anodic shifts in the ferrocene oxidation wave result if cations are added to the conjugated receptor systems where the 7r-electron system links the heteroatoms of the ionophore to the redox centre. 

Scheme 5 Conformational change in the bis-crown receptor results in a change in the environment of the redox centres.

Scheme 7 A direct coordination coupling pathway is not possible between the redox centre of [49] and the binding site.

Direct coordination between the redox centre and guest leads to generally large perturbations in the electrochemical behaviour of the host. A particularly elegant example of this type redox/coordination coupling is given by the quinone crown ether species synthesized by Cooper and co-workers (Wolf and Cooper, 1984 Delgado et al., 1992) (Fig. 24). 

A more detailed discussion on this topic will be considered in Chapter 4, Section 1.3. Structurally, to a first approximation, a criterium to determine the electronic interaction between redox active sites is to evaluate the distance between the sites the greater the distance between redox centres, the smaller their interaction. 

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