Electronic structure of polypyrrole and oligomers of pyrrole

Abstract

The ultraviolet photoemission and optical absorption spectra of pyrrole and polypyrrole are analyzed using the spectroscopically parametrized CNDO/S3 model. Calculated charge densities, bond orders, and orbital eigenvalues for pyrrole are compared with the extensive literature on this molecule. The densities of valence states are computed for pyrrole, oligomers of pyrrole, and an infinite planar polypyrrole chain. They provide a good description of ultraviolet valence electron photoemission spectra measured for gas-phase pyrrole, condensed thin films of pyrrole, and thin-film polypyrrole. The photogenerated radical cation states in polypyrrole seem to extend over four or more pyrrole units for the higher binding energy σ-electron states but are localized on fewer units for the lowest binding energy π-electron states. The relationship between the density of valence states and the energy band structure of polypyrrole is established. The energies of the first few dipole allowed optical absorption transitions are calculated for pyrrole and its oligomers via a limited configuration interaction analysis. Examination of the trends in these transition-state energies as a function of the size of the oligomer suggests that the lowest-energy optically excited states in polypyrrole are excitons which extend over at least four pyrrole units. Comparison of the model predictions for ring and linear-chain molecular conformations indicates that both are compatible with existing spectroscopic measurements.