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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...

Reversible thermochromism in thin solid films of poly(3-hexylthiophene), or P3HT, has been studied using ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS, respectively). The UPS and XPS spectra, as well as previously published optical absorption spectra, are analyzed using the results of valence effective Hamiltonian (VEH) quantum chemical calculations of the electronic structure of isolated polymer chains. The analysis of the spectra indicates that at elevated temperatures thermally induced electronic localization occurs as a consequence of thermally induced conformational disorder.

We present ab initio Hartree–Fock and valence effective Hamiltonian (VEH) calculations on polyparaphenylene, polypyrrole, and polythiophene dimers and polymer chains. These polymeric materials are among the most studied compounds in the field of conducting polymers. We examine, as a function of the torsion angle between consecutive rings, the evolution of electronic properties such as ionization potential, bandgap and width of the highest occupied bands and of the carbon–carbon bond length between rings. This investigation is motivated by the fact that many derivatives of these compounds have substituents that lead to an increase of the torsion angle between adjacent rings, as a result of steric interactions. As expected, on going from a coplanar to a perpendicular conformation, the ionization potential and bandgap values increase and the width of the highest occupied bands decreases. This makes it more difficult to ionize or reduce the polymer chains and can result in achieving lower maximum conductivities on doping. However, since the evolution of the electronic properties is found to follow a cosine law (related to the decrease of the overlap between the π orbitals on adjacent rings), the modifications up to a ∼40° torsion angle are not very large. For instance, in all three polymers, the ionization potential value for a 40° torsion angle is about 0.4 eV larger than the coplanar conformation value. Therefore, substituents that lead to torsion angles between consecutive rings smaller than 40° are quite acceptable. Finally we discuss the importance, for the substituted compounds, of the possibility of achieving a coplanar conformation upon doping, in order to permit high intrachain mobilities of charge carriers such as bipolarons.

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