The electronic and geometric structure of dications of oligo-thiophenes

The structure and stability of dications of oligo-thiophenes up to the decamer is calculated by semi-empirical calculations which explicitly take electron-electron interactions into account. The dication of each oligomer is considered as a bipolaron and as two polarons. Contrary to predictions of the widely accepted Su-Schrieffer-Heeger model, yet consistent with experimental data, it is found that the bipolaron is the stable dicationic form only of the shorter oligomers. In longer oligomers the two-polaron state is preferred. Taking both theoretical and reported experimental results into consideration it is suggested that the crossover could occur at the dodecamer.