Multiple bipolaron formation in oligothiophenes

Semi-empirical calculations on large oligomers of thiophene containing up t o 24 monomer units are carried out. The geometry of neutral as well as high ly oxidized structures with charges of +2, +4, and +6 are optimized in orde r to understand the charge localization in the absence of counter ions. The bond length alternation in the polymer backbone is shown to provide a usef ul tool for studying bipolaron formation. The effects of defects such as br anches, alpha-beta 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 im pede the mobility of them. (C) 1999 Elsevier Science S.A. All rights reserv ed.