Thin redox-active polymer

Electrochemistry of Thin Redox-Active Polymer films... 

A novel polysiloxane, containing the isocyanide group pendent to the backbone, has been synthesized. It is observed to react with the metal vapors of chromium, iron and nickel to afford binary metal complexes of the type M(CN-[P])n, where n = 6, 5, 4 respectively, in which the polymer-attached isocyanide group provides the stabilization for the metal center. The product obtained from the reaction with Fe was found to be photosensitive yielding the Fe2(CN-[P])q species and extensive cross-linking of the polymer. The Cr and Ni products were able to be oxidized on exposure of thin films to the air, or electrochemically in the presence of an electron relay. The availability of different oxidation states for the metals in these new materials gives hope that novel redox-active polymers may be accessible. 

As outlined above, the electrochemical properties of this redox species are strongly pH-dependent and this behavior can be used to illustrate the supramolecular nature of the interaction between the polymer backbone and the pendent redox center. The cyclic voltammetry data shown in Figure 4.17 are obtained at pH = 0, where the polymer has an open structure and the free pyridine units are protonated (pKa(PVP) = 3.3). The cyclic voltammograms obtained for the same experiment carried out at pH 5.7 are shown in Figure 4.18. At this pH, the polymer backbone is not protonated and upon aquation of the metal center the layer becomes redox-inactive, since protons are involved in this redox process. This interaction between the redox center and the polymer backbone is typical for these types of materials. Such an interaction is of fundamental importance for the electrochemical behavior of these layers and highlights the supramolecular principles which control the chemistry of thin films of these redox-active polymers. Finally, it is important to note that the photophysical properties of polymer films are very similar to those observed in solution. Since the layer thickness is much more than that of a monolayer, deactivation by the solid substrate is not observed. 

As with all supramolecular structures, one of the most important issues is whether a direct relationship between the structure of a material and its function or properties can be established. In the following, some examples of polymer systems which show such a correlation will be discussed. The materials addressed will include block copolymers, polyalkylthiophenes and a multilayer system based on the self-assembly of polyelectrolytes. Detailed studies on the electrochemical properties of redox-active polymers, based on poly(vinyl pyridine) modified with pendent osmium polypyridyl moieties, have shown that electrochemical, neutron reflectivity and electrochemical quartz crystal microbalance measurements can yield detailed information about the structural aspects of thin layers of these materials. 

Two methods, adsorption of redox-active species and synthesis of redox-active polymers, were used to prepare electroactive LPFs. In the following text, each of these methods is discussed in turn. Lowy and Finklea showed that a variety of redox species such as Fe(CN)6 and Cu(EDTA) will adsorb on the surface of polyelectrolyte films [41]. The top layer of LPFs is not intrinsically compensated and thus contains many ion-exchange sites. Redox species adsorbed on the surface of LPFs are electroactive, but the thin film-like cyclic voltammo-grams of these species showed substantial peak splitting (generally 60 mV or more at a scan rate of 100 mV s , even when the redox species was separated from the electrode by just one layer of poly(4-vinyl-methylpyridinium)). Adsorption of the redox couple is likely restricted to the surface of the film thus limiting the amount of electroactive material that can be included. 

In addition to forming continuous organometallic multilayer thin films, we explored the LbL deposition of polyferrocenylsilane polyions onto, for instance, hydrophilically/hydrophobically modified substrates with the aim of building two-dimensionally patterned organometallic multilayers. In general, surfaces modified with microscopically patterned conducting, luminescent, or redox-active polymer films have potential use in microelectronic and optoelectronic devices and microsensor arrays. Area-selective deposition of polyferrocenylsilane polyions can be an attractive method to obtain two-dimensionally patterned redox active films, which may be used as electrochemically modulated diffraction gratings. ... 

In this chapter, the application of this technique to the thin film processing of electroactive polymers is discussed. The fabrication of controlled molecular assemblies of electroactive polymers provides a unique opportunity to investigate the structure-property and structure-function relationships of multilayer thin films containing electrically conductive, optically nonlinear and redox active polymers. These assemblies can also be coupled to appropriate solid supports for application in such molecular electronic devices as ultrathin electrodes and transistors, opticd waveguides and switches, and chemical and biochemical microelectronic sensors. 

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. 

In a step towards the fabrication of prototype sensory devices organisation of redox-active anion receptors on to electrode surfaces is being exploited. Importantly, self-assembled monolayers or thin polymer films of metal-based receptors can generate an amplified response to anion binding akin to the dendritic effect and could potentially become the basis of robust anionsensing devices. 

Figure 3.24 shows the redox behavior of PABA thin films observed at neutral pH in the presence of NADH and NAD" ". The PABA film was redox inactive at neutral pH (Figure 3.24,a) due to deprotonation and loss of dopant as with polyaniline [150,151). However, in the presence of NADH (Figure 3.24, b) and NAD" " (Figure 3.24, c), PABA films became redox active due to complexation of boronic acid with cis-2,3-ribose diols and subsequent formation of self-doped polymer. In the presence of NADH, the cyclic voltammogram of PABA thin film exhibited a single redox couple at pa 0.05 and pc —0.10 V. In contrast, a second redox couple was observed in the presence of NAD+ at pa 0.34 and pc... 

Thin films of polymers are formed by dropping a small amount of a solution of a polymer onto the carrier electrode, and then spinning it at several thousands r.p.m. As in the case of A1 or A2, the redox-active compound is added to the polymer solution or diffused afterwards from a solution. During centrifugal spinning the evaporation of the solution leads to an increase of the concentration of the polymer causing an increased viscosity and the formation of a solid film. Only in the case of Newtonian fluids does the solution of the hydrodynamic equations lead asymptotically to a uniform layer thickness that is independent of the liquid profile at the start of the rotation The thickness of the films depends on the rotation speed, the evaporation rate and the initial viscosity... 

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