Multiple bipolaron formation in oligothiophenes

Semi-empirical calculations on large oligomers of thiophene containing up to 24 monomer units are carried out. The geometry of neutral as well as highly oxidized structures with charges of +2, +4, and +6 are optimized in order to understand the charge localization in the absence of counter ions. The bond length alternation in the polymer backbone is shown to provide a useful tool for studying bipolaron formation. The effects of defects such as branches, α–β bondings or deviations from planarity on the structures of oxidized forms are investigated. The bipolarons are found to spread over six monomer units. Defects disturb the bipolaron formation and possibly impede the mobility of them.