Bipolarons in polypyrrole chains

Abstract

First-principles calculations on the geometric and electronic structures of undoped and doped polypyrrole chains indicate the appearance upon high doping of bipolaron states in the gap. A picture of the band-structure evolution upon doping is presented, showing the presence of bipolaron bands in the gap for experimentally achieved doping levels. A conductivity mechanism based on motion of spinless bipolarons is consistent with the absence of ESR signal in electrochemically cycled highly conducting polypyrrole.Doped organic polymers' constitute a new fascinating area of condensed matter physics, where nonlinear phenomena play an important role and which has several aspects in common with relativistic field theories. 'It has been demonstrated that many peculiar phenomena that occur in trans-polyacetylene, by far the most experimentally and theoretically studied polymer up to now, ' can be explained by the presence of soliton defects on the chain. ' Solitons are possible in trans polyacetyl-ene (PA) as a result of the existence of a degenerate ground state, a consequence of a Peierls dimerization. The observation between l-at. % and -6-at.% doping levels of high conductivity without significant Pauli susceptibility has been interpreted as due to a conductivity mechanism involving the motion of charged solitons carrying no spin.The other polymers usually do not possess a degenerate ground state; they have, so far, received less theoretical attention. ' ' It must be stressed that these polymers nevertheless display transport properties which are very similar to those of trans-PA. A remarkable example has been recently reported in the case of polypyrrole (PPy); it has been observed that in the electrochemically cycled (highly doped) material, high conductivity is obtained, whereas no ESR signal is present. 'In order, on one hand, to interpret that observation and, on the other hand, to propose a coherent theoretical description for a11 doped organic polymers, we report in this paper an ab initio restricted Hartree-Fock (RHF) study of the electronic and geometric structures of undoped and highly sodiumdoped quaterpyrrole (QPy), a model chain for polypyrroie.The key feature of our results is to demonstrate for the first time in the framework of first-principles calculations that high doping induces strong geometric modifications on the QPy chain and that these modifications very naturally lead to the appearance of two states in the gap. These states can be referred to as bipolaron states and, interestingly, are not symmetrically located with respect to the gap center. A picture of the PPy band-structure evolution upon doping is worked out, showing the presence of bipolaron bands in the gap at doping levels comparable to those experimentally achieved.Calculations are performed in the framework of the RHF self-consistent-fieldlinear combination of atomic orbitalsmolecular orbitals (SCF-LCAO-MO) ab initio technique, at minimal Slater-type orbitalthree-Gaussians (STO-36) basis-set level. For computational reasons, sodium has...

Electrochemical preparation and in situ characterization of poly(3-methylpyrrole) and poly(3-methylpyrrole-cyclodextrin) films on gold electrodes

“…As previously pointed out based on theoretical calculations, the respective differences between the LUMO and the lower, binding bipolaron (dication) state and the HOMO as well as between the upper, antibonding bipolaron state are fairly similar to the respective differences found for the polaron (cation radical) state [39,40]. Experimental observations were in slight disagreement.…”

The electronic structure of polyaniline and doped phases studied by soft x-ray absorption and emission spectroscopies

“…Figures 5 and 6 show nonresonant and resonant C K x-ray emission spectra of undoped PANi and the doped phases measured with the excitation energies of 302.2 and 285.0 eV. By comparing the observed emission spectra with molecular orbital calculations 21 and band-structure calculations, 27 three major bands ͑labeled A-C͒ can be identified in the spectra. Peak A corresponds to -like states, peak B is due to both and contributions, with dominating, while peak C is due only to electronic states.…”

First-principles calculation of the conformation and electronic structure of polyparaphenylene

“…Based on a comparison of their work with experimental results, they concluded that the most probable structure of PPP is nearly planar with an inter-ring torsional angle of less than 20°. Some of the earliest band structure calculations for PPP were performed by Brédas et al [27][28][29][30] They used the experimental values of the geometrical parameters and the torsional angle as input for HF calculations on an alternately tilted chain. Bakhshi and Ladik 31 made an HF calculation of the ionization energy, electron affinity, and bandwidth values for both a planar and a helical PPP chain.…”