The instability of a bipolaron versus two polarons: charge localization in cyclo-dodecathiophene

The electronic structure and geometry of dications of polythiophene, using cyclo-dodecathiophene as a model compound, are calculated and their stability is investigated. Two forms, termed ‘bipolaron’ and ‘two separated polarons’, respectively, are considered. In both cases, charge localization accompanied by a geometric distortion of the macrocycle is observed. Two separated polarons are found to be more stable than a bipolaron.