Redox metallopolymers

FIGURE 2.21. Reaction order plot with respect to layer thickness for the reduction of 0.2 mAf Fe (aq) ion in 1.0 M HC104-supporting electrolyte at the Os-loaded redox metallopolymer. The linear regression line and the 98% confidence limits are shown. Note that k ME is independent of layer thickness in this case. 

The analogous osmium polymers have also been studied in great detail. The synthetic procedures required for these metallopolymers are the same as those described above for ruthenium however, the reaction times are longer. The similarity between the analogous mononuclear and polymeric species is further illustrated by the fact that the corresponding osmium polymers have considerably lower redox potentials and are also photostable, as expected on the basis of the behavior observed for osmium polypyridyl complexes. 

The example considered is the redox polymer, [Os(bpy)2(PVP)ioCl]Cl, where PVP is poly(4-vinylpyridine) and 10 signifies the ratio of pyridine monomer units to metal centers. Figure 5.66 illustrates the structure of this metallopolymer. As discussed previously in Chapter 4, thin films of this material on electrode surfaces can be prepared by solvent evaporation or spin-coating. The voltammetric properties of the polymer-modified electrodes made by using this material are well-defined and are consistent with electrochemically reversible processes [90,91]. The redox properties of these polymers are based on the presence of the pendent redox-active groups, typically those associated with the Os(n/m) couple, since the polymer backbone is not redox-active. In sensing applications, the redox-active site, the osmium complex in this present example, acts as a mediator between a redox-active substrate in solution and the electrode. In this way, such redox-active layers can be used as electrocatalysts, thus giving them widespread use in biosensors. 

More recently, there has been growing interest in a new type of redox polymer that is a hybrid of materials from PTs and will be referred to as conjugated metallopolymers. The key feature of this class of material is that the metal is coordinated directly to the conjugated backbone of the polymer, or forms a link in the backbone, such that there is an electronic interaction between the electroactive metal centers and the electroactive polymer backbone. This can enhance electron transport in the polymer, enhance its electrocatalytic activity, and lead to novel electronic and electrochemical properties <1999JMC1641>. 

Strong electronic communication between the metal centers is indicated by the presence of IVCT bands and the M(II)2/M(II)M(III) and Mai)M(m)/M(ffl)2 redox waves with large separations [44-47]. Note that a recent review of conjugated metallopolymers is available [48] and there is also an earlier review by Pickup [49]. 

The nature of coordination around the central atom is of prime importance for the redox potential of the electroactive center. During synthesis of the metallopolymers, the reaction is therefore continuously monitored. This is most conveniently carried out using absorption spectroscopy or cyclic voltammetry. Electronic... 

In Chapter 8, coauthored by Kelly and Vos, the electrochemical behavior of osmium and ruthenium poly(pyridyl) redox polymers is discussed in some detail. Vos has made significant contributions in this area. This chapter ties in well with the more general discussion presented by Lyons in Chapters 1 and 2, in that many of theoretical concepts addressed in the latter chapters are again discussed by Kelly and Vos with specific reference to redox-active metallopolymer materials. 

Pickup, PG. 1999. Conjugated metallopolymers. Redox polymers with interacting metal based redox sites. / Mater Chem 9 1641-1653. 

Holliday, B.J., and T.M. Swager. 2005. Conducting metallopolymers The roles of molecular architecture and redox matching. Chem Commun 23-36. 

In polymers which have an electron-conducting backbone with pendant or built-in redox groups (e.g., conjugated metallopolymers), three electron transfer pathways may be operative [114] (see Fig. 6.13). 

Holliday, B.J. and Swager, TM. (2005) Highly conductive metallopolymers the role of redox matching. Chem. Commun., 23-36. 

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