Polypyrrole electronic structure

Figure 3.72 Electronic structure diagrams for a polypyrrole chain containing (a) a polaron and (b) a bipolaron (c) The band structure obtained for highly oxidised (33 mol.0,, ) polypyrrole showing the presence of two broad bipolaron bands in the gap. After Bredas et al. (1984).

The TB MO calculation on the 15N chemical shift of polypyrrole in the solid state allows useful information to be extracted from the observed spectra, namely that the two peaks obtained are correctly assigned to the quinoid and aromatic structures.(l 1,38) ( The quinoid structure is closely to the electric conductivity.) A decrease in the band gap leads to a downfleld shift. These results on conducting polymers demonstrate that the chemical shift behavior provides information about the band gap which, in turn, is a measure of the electric conductivity. It can be said that TB MO calculations offer useful perspectives in interpreting the results of NMR nuclear shieldings in polymers, both in terms of the structure in the solid state and in understanding the effect of intermolecular interactions on nuclear shieldings. The latter are shown to operate through the electronic structures of the polymers considered. 

Polypyrrole and polythienylene (Fig. 16) have the same number of tt electrons as poly(p-phenylene). The electronic structure of polypyrrole has been examined by several authors (Ford et al., 1982 Br6das et al., 1983a). The HO and the LU bands are of tt nature. The orbital pattern of the HOMO accompanied by the top of the valence band has no contribu-... 

By producing PPy films, electrical conductivities up to 150 S/cm can be obtained. Electropolymerized PPy films differ in their molecular structure according to polymerization conditions such as the electrochemical parameters of the polymerization. At low current densities (l.c.d.) below 3 mA/cm one-dimensional polypyrrole chain structures are mainly produced [3]. Higher current densities predominantly lead to two-dimensional molecular polymer structures. The electronic state of such PPy films produced with high current density (h.c.d.) has been investigated by several solid-state spectroscopic methods such as ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS), as well as temperature-dependent electrical conductivity measurements [4-6]. 

Salzner, U., Lagowski, J., Pickup, P, Poirier, R., 1998. Comparison of geometries and electronic structures of polyacetylene, polyborole, polycyclopentadiene, polypyrrole, polyfuran, polysilole, polyphosphole, polythiophene, polyselenophene and polyteUurophene. Synth. Met. 96,177-189. 

The other group of materials, which covers the electronic conductors, includes conjugated polymers whose electronic structure may be significantly modified by electrochemical processes, sometimes designated as doping processes, which involve the oxidation (removal of n electrons) or the reduction (addition of n electrons) of the polymer chain. Typical examples are the heterocyclic polymers, such as polypyrrole, polythiophene and their derivatives, and the polyanilines. 

J. E. Bredas, B. Themans, J. G. Fripiat, J. M. Andre, R. R. Chance, Highly conducting polyparaphenylene, polypyrrole and polythiophene chains - an ah initio study of the geometry and electronic structure modifications upon doping, Physical Review B 1984, 29, 6761. 

Bredas, J. L., Themans, B., and Andre, J. M., Valence effective Hamiltonian technique for nitrogen-containing polymers electronic structure of polypyrrole, pyrolized polyacrylonitrile and derivates, Phys. Rev. B, 27, 7827-7842 (1983). 

Let us suppose an infinite nondegenerate polymer chain (e.g., polythiophene) doped heavily with electron acceptors. At a high dopant content, the polymer-chain structure and electronic structure of the doped polymer are radically different from those of the intact polymer. As typical cases, we will describe two kinds of lattice structures of doped polythiophene (dopant content, 25 mole% per thiophene ring) a polaron lattice and a bipolaron lattice. They are the regular infinite arrays of polarons and bipolarons. The schematic polymer-chain structures are shown in Figure 4-16. Band-structure calculations have been performed for polaron and/or bipolaron lattices of poly(p-phenylene) [124], polypyrrole [124], polyaniline [125], polythiophene [124, 126], and poly( p-phenylenevinylene) [127], with the valence-effective Hamiltonian pseudopotential method on the basis of geometries obtained by MO methods. The schematic electronic band structures shown in Figure 4-17... 

A very powerful method to study the electronic structure of solids is electron energy-loss spectroscopy (EELS). Because electrons transfer not only energy to the solid (as do photons) but also momentum, in addition to the energetic position of the electronic states, the dispersion of the bands can also be studied. Figure 1.58 shows the energy-loss spectrum of polypyrrole doped with the sulphonic acid H—(CH2)4—SO3 [126]. The various curves were recorded at different values of momentum transfer q. The dashed lines connect corresponding peaks. Vertical lines are characteristic of transitions into narrow bands (without dispersion), whereas inclined lines indicate broad bands. The maxima in the spectrum with low momentum transfer (bottom curve) are related to the maxima in the electronic joint density of states, but for... 

P. Pfluger, G. Weiser, J. Campbell Scott, and B. Street, Electronic structure and transport in the organic amorphous semiconductor polypyrrole, in Handbook of Conducting Polymers, Vol, 2 (T. A. Skotheim, ed.), Marcel Dekker, New York, 1986, Chap. 38. 

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