Poly pyrrole

Fig. 6. Thermal stability of epoxy-encapsulated poly(pyrrole tosylate) film,, 0.5 Q/sq, and polypyrrole-coated textiles, D, 20 Q/sq, with exposure to...

Most conducting polymers, such as doped poly(acetylene), poly(p-pheny-lene), and poly(/ -phenylene sulfide), are not stable in air. Their electrical conductivity degrades rapidly, apparently due to reaction with oxygen and/or water. Poly(pyrrole) by contrast appears to be stable in the doped conductive state. 

Murakoshi K, Kogure R, Wada Y, Yanagida S (1998) Eabrication of solid-state dye-sensitized Ti02 solar cells combined with poly pyrrole. Sol Energy Mater Sol Cells 55 113-125... 

Figure J.73 Cyclic voltammograms ofa 20nm-(hick poly pyrrole on Pt in (etracthylammonium tetrafluoroboralc/CHjCN solulion. The scan rates arc as shown. From Diaz t>r at. (1981).

The broad nature of the current peaks in the voltammogram of conducting polymers such as poly pyrrole has been interpreted in a number of w one of which was to attribute it to the movement of anions across the polymei, electrolyte interface, a vital process if the overall charge neutrality of the film is to be maintained. The participation of the electrolyte in the electrochemistry of the polymer film is easily seen by comparing the response of polypyrrole in a variety of different electrolytes (see Figure 3.74). 

Figure 3.77 (a) The cycloaliphatic monomer subunits employed by Wegner and Riihe (1989). n varied between 3 and 10. (b) Schematic description of the poly pyrrole backbone wrapped in a layer of poorly conducting methylene moieties of the alkylcne chains fused to the pyrrole units in the 3,4 positions. The minimum separation distance, R, between adjacent chains can be estimated from molecular models. From Wegner and Riihe (1989). 

On the basis of their results the authors concluded that the crossover from the neutral to oxidised form of poly pyrrole, and vice versa, requires the number of spins to pass through a maximum, i.e. the creation and anihilation of bipolarons involves the passage through the polaron state. They expressed this as a two-step redox reaction ... 

Clearly, the in situ studies such as those discussed above resulted in a cohesive picture emerging of the electronic conduction in poly pyrrole. Thus,... 

Figure 3.81 Typical cyclic voltammograms of a poly pyrrole film on Pt in Nrsatu rated 1 M NaClOj. The voltammograms were collected immediately after holding the film at -0.6 V vs, SCE for 5 min and after cycling for 5 min. The scan rate was 100 mV s "1 and the film thickness 84 nm. Reprinted from Electrochimica Acta, 36, P.A, Christensen and A. Hamnett, In situ Spectroscopic Investigations of the Growth, Electrochemical Cycling and Overoxidation of Polypyrrole in Aqueous Solution , pp. 1263-1286(1991), with kind permission from Pergamon Press Ltd., Headington Hill Hall, Oxford OX3 0BW, UK.

T. Konry, A. Novoa, Y. Shemer-Avni, N. Hanuka, S. Cosnier, A. Lepellec, and R.S. Marks, Optical fiber immunosensor based on a poly(pyrrole-benzophenone) film for the detection of antibodies to viral antigen. Anal. Chem. 77, 1771-1779 (2005). 

Nickel, Ni2+ (d8) 4, square planar S-Thiolate, thioether, V-i midazole, poly pyrrole Hydrogenases, hydrolases... 

A poly(pyrrole) film was deposited on a Pt electrode from potentiostatic conditions at 0.8 V vs Ag/AgCl. The film was colorless, its presence was verified by oxidation and reduction of the film in plain electrolyte solution. The infrared spectrum of the electrochemically prepared poly(pyrrole) is similar to the catalytically prepared films indicating the two films are structurally similar. 

R. Hernandez, L. Richter, S. Semancik, S. Stranick, and T. E. Mallouk, Template fabrication of protein-functionaUzed gold-poly(pyrrole)-gold segmented nanowires, Chem. Mater. 16, 3431— 3438 (2004). 

The HOPG (highly oriented pyrolytic graphite) carbon electrode chemically modified with (5[-phenylalanine at the basal surface led to 2% ee in the reduction of 4-acetylpyridine [377]. A cathode modified with a chiral poly(pyrrole) reduced 4-methylbenzophenone or acetophenone in DMF/LiBr and phenol as proton donor to 1-phenylethanol with up to 17% ee [382]. Alkyl aryl ketones have been reduced to the corresponding alcohols at a Hg cathode in DMF/water in the presence of (1R,2S)-A,A-dimethylephedrinium tetrafluorobo-rate (DET), producing (5 )-l-phenylethanol with 55% ee from acetophenone. Cyclovoltammetry supports an enantioselective protonation of the intermediate (PhCOH(CH3)) [383]. 

The use of nitroxyls that are amenable to immobilisation in a polymer layer around the anode would be an ideal way of constraining the catalyst where it can be regenerated electrochemically. Attempts to utilise a poly(pyrrole) film formed by anodic oxidation of the monomer 5 led to a system that will oxidise alkanols but which is unstable in continuous use [40], A more satisfactory polymer layer is... 

As described in Section 3 of Chapter 4, the stabilization of n-Si electrode by coating with poly(pyrrole) has attracted much attention. The stabilization of a small bandgap n-semiconductor electrode against oxidation is of great value not only to convert visible light into chemical energy, but also to construct liquid-junction solar cells operated under visible irradiation. The poly(pyrrole) film is usually electropolymerized on the semiconductor electrode dipped in the aqueous solution of pyrrole. The remarkable stabilizing effect of poly(pyrrole) film on polycrystalline n-Si is shown in Fig. 22 67). The photocurrent obtained under irradiation in the aqueous solution of... 

Fe2+/Fe3+ at the naked n-Si electrode drops to zero in less than 30 s, whereas the poly(pyrrole)coated n-Si gives a stable photocurrent for 5 days. The power conversion efficiency was 3 % corresponding to Isc of 3.2 mA/cm2, Voc of 0.39 V, and a fill factor of... 

One problem for the coated system is that the film is peeled off after prolonged irradiation. In order to have a more adhesive film, the surface of n-Si was modified with N-(3-trimethoxysilyIpropyl)pyrrole (22). Pyrrole was then electrodeposited on this modified electrode as shown in Eq. (24) 85). The durability of the coated poly(pyrrole) was improved by such a treatment of n-Si surface. The n-Si electrode coated only with poly(pyrrole) gave a declined photocurrent from 6.5 to 1.8 mA cm-2 in less than 18 h, while the poly(pyrrole) coated n-Si treated at first with 22 as Eq. (24) gave a stable photocurrent of 7.6 mA cm-2 for 25 h. When an n-Si electrode was coated with Pt layer before the deposition of poly(pyrrole), the stability of the semiconductor was improved remarkably (ca. 19 days)85b). A power conversion efficiency of 5.5% was obtained with iodide/iodine redox electrolytes. 

Returning to the debate over the identity of the carriers in poly pyrrole, another major contribution was made by the epr studies of Genoud and colleagues (1985 Devreux el ai, 1987 Nechtschein et al., 1986) who, in contrast to Scott and colleagues (1983), performed the measurements in situ. 

Figure 3.79 (a) Spin vs. charge concentration in polypyrrole, (b) Charge ( ) and spin ( x ) coneemra I ion vs. potential in poly pyrrole. From Devreux el al. (1987). 

While dicarboxylic acid-functional pyrroles have received only cursory attention in condensation polymerizations, other derivatives have been studied extensively. Pyrrole itself has been electrooxidatively polymerized (81CS145) to give a flexible conductive film, presumably containing poly(2,5-pyrrolediyl) units (23) as the main structural feature. The blue-black polymer obviously contains other functionality, as evidenced by elemental analysis and by the fact that it carries a partial positive charge, and it exhibits p-type conductivities approaching the metallic range (e.g. 100 fi-1 cm-1). The main utility of poly(pyrrole) (23) has been for the modification of electrode surfaces, although numerous other applications can be envisioned. 

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