Theoretical investigation of the geometric and optical properties of neutral and charged oligophenylenes

We have investigated the geometry and the nature of optically allowed transitions in neutral and charged phenylene-based oligomers by means of Hartree-Fock calculations. Geometry optimizations are performed using the semiempirical Austin Model 1 (AM1) method for oligomers containing from two $\mathrm{}\left(2P\right)\mathrm{}$ to twelve $\mathrm{}\left(12P\right)\mathrm{}$ benzene rings. The transition energies and related intensities of the optical-absorption spectra are calculated by means of the intermediate neglect of differential overlap Hamiltonian that is combined with a single configuration interaction technique in order to include electron correlation effects in the description of the excited states. The calculations show that two subgap absorption features appear in short oligomers carrying a single charge (polaron), whereas a single intense peak is observed in the presence of two charges (bipolaron). These results are consistent with a wide range of experimental and theoretical data obtained for various conjugated oligomers. Interestingly, the appearance of a second subgap feature is predicted in the spectra of long doubly oxidized chains as well as for chains supporting interacting bipolarons.