Evolution of the electronic structure of polyacetylene and polythiophene as a function of doping level and lattice conformation

Band-structure calculations are presented for different lattice conformations of doped polyacetylene and polythiophene. At intermediate doping levels, the intraband transition energies are found to be in good agreement with experimental optical-absorption data for a soliton lattice conformation in polyacetylene and a bipolaron lattice conformation in polythiophene. At high doping levels, where both polymers exhibit a metalliclike behavior, we find the best agreement with observed optical-absorption and magnetic data to occur for a polaron lattice conformation. Important qualitative differences are found between the polaron lattice conformations obtained in this work and those reported previously. The polaron lattice band structures for polyacetylene and polythiophene are calculated to be very similar to one another, which is also consistent with the experimental trends. The evolutions of the π-${\pi }^{\mathrm{*}}$ and the π-to-soliton band energy gaps for polyacetylene are studied and compared with results of the Takayama, Lin-Liu, and Maki (TLM) model. We find that, in comparison with optical data probing these transitions, the valence effective Hamiltonian results are superior to the results of the TLM model. We stress that our results do not constitute a proof of the existence of polaron lattice conformation at high doping levels but indicate that the presence of such a conformation is in agreement with a large number of experimental (ir, optical conductivity, magnetic, electron-energy-loss spectroscopy) data.