Evolution of the electronic structure of cyclic polythiophene upon bipolaron doping

Abstract

The evolution of the electronic structure of cyclic polythiophene (PT) upon bipolaron doping is studied to explore the possibility of uniform charge density ground state in the metallic regime. The ground state geometry of the neutral PT, the structure of a single bipolaron, and the experimentally observed optical transitions due to it are reproduced. Since the cyclic PT doped with an odd number of bipolarons (the O configuration) creates an aromatic polyene backbone containing (4n+2)π electrons, it is driven toward the quinoid form. Consequently, we find an insulator-metal transition for dopant concentration ≥14 mol % and an ∼0.8 eV redshift in Fermi energy with respect to the neutral system at 30 mol % which agree very well with experimental findings. For an even number of bipolarons, there are two possible configurations, namely (i) the (1,1) or the T configuration and (ii) the (2,0)/(0,2) or the S configuration. The T configuration and the O configuration behave similarly and merge to a single asymptotic configuration. The energy difference per unit cell between the S and the T configurations decreases as the ring size increases. So we propose that in the sufficiently large cyclic PT the T as well as the O configurations will be the stable entities if their stability is further augmented by additional interactions.