Electronic states in polypyrrole

doi:10.1016/0038-1098(83)90581-1

Solid State Communications

Volume 46, Issue 6, May 1983, Pages 477-480

https://doi.org/10.1016/0038-1098(83)90581-1 Get rights and content

Abstract

Electron energy loss spectra of valence excitations in polypyrrole doped with boron tetrafluoride, undoped, and subsequently redoped with arsenic pentafluoride were measured. Energy loss peaks associated with intraband and interband excitations which show characteristically different momentum dependence were observed. Intraband peaks exhibit weak negative dispersion while interband excitations show strong positive dispersion. This suggests that interband excitations occur between wide energy bands (∼5 eV) in long polymetric molecules while intraband excitations may be due to intrinsic or extrinsic intermolecular charge transfer transitions in a narrow band (∼0.5 eV).

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At higher energies, inelastic electron scattering or electron energy-loss spectroscopy (EELS) in transmission has been the classical method for investigating the collective excitations of electrons, that is, plasmons. The method has been further improved during the years to provide information on interband transitions between valence bands and conduction bands and on core-level excitations to learn about the unoccupied density-of-states and on the lattice structure. This chapter reviews EELS studies using a dedicated electron energy-loss spectrometer. It introduces theory of EELS and describes the Karlsruhe spectrometer. The chapter also describes work on nearly-free-electron metals, rare gas bubbles in metals, amorphous carbon, conducting polymers, and superconductors. It provides an illustration of the present state of the technique applied to solid-state physics and to material science. One of the serious problems in the application of EELS to solid-state physics and material science is the preparation of the samples. While in some cases preparation can be done easily, preparation in other cases is extremely difficult or not feasible at all.
Ab initio band structure calculations of organic polymers with large unit cells and treatment of aperiodicity caused by dopants
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Highly conducting polypyrrole films are obtained either by electrochemical oxidation or by a multistep redox reaction with electron acceptors like AgBF₄, AgClO₄, molecular oxygen and other substances. Radical cations in the conjugated aromatic macromolecule are formed and neutralized by the present anions or oxygen based anions. The ratio of pyrrole moieties to anions is found, experimentally, to be about 4:1. In this work emphasis has been placed on the study of the electronic and chemical structure of polypyrrole and the oxidizing model agents HBF ₄ and HClO₄ in the preoxidation phase. Energy band structures have been computed using the ab initio Hartree—Fock crystal orbital method for periodic salt-like samples poly(pyrrole⁺ + BF₄⁻) and macromolecular complexes poly(pyrrole + HBF₄). For both kinds of adduct the energetically optimal structure with respect to the intermolecular distance has been determined. The dependence of the positions and widths of the valence and conduction bands on the geometry is analyzed and discussed. The negative factor counting method in the tight-binding approximation is used to determine the effect of doping on the density of states of the valence band of polypyrrole. Various orientations and concentrations of the dopant relative to the aromatic heterocyclic units are studied. Preliminary results on polypyrrole doped with HCl₃ are reported.
The electrodeposition of polypyrrole films from aqueous solutions
1984, Journal of Electroanalytical Chemistry
Cyclic voltammetry, potential step experiments and coulometry have been used to investigate the electrodeposition of polypyrrole films from acidic, neutral and basic aqueous nitrate solutions and neutral media containing other anions. It is shown that the initial step in the formation of all films is a nucleation process and, indeed, all the electrochemical data resembles that for the nucleation and growth of a metal film. On the other hand, the properties of the films and also the detailed mechanism of their formation are a function of the conditions used for the synthesis, e.g. potential, charge, pH, anion. In particular, the oxidation of pyrrole is shown to occur in several steps dependent on the potential. There is a strong correlation between the experimental results and reported measurements of physical properties but the behaviour is much more difficult to explain in terms of the chemical composition of the film and reported structures.
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Electronic states in polypyrrole

Abstract

Syn. Met.

Phys. Rev. Lett.

Angew. Chem. Int. Ed. Engl.

Molecular Metals

The Physics and Chemistry of Low Dimensional Solids

Chemica Scripta

J.C.S. Chem. Commun.