Polaron Pair versus Bipolaron on Oligothiophene Chains: A Theoretical Study of the Singlet and Triplet States

Double positive charges on an isolated conjugated chain: do they remain together forming a single bipolaron (2+) entity (shown in picture) stabilized by strong geometry distortion even in the absence of a counterion or do they repel, yielding two separate polarons (+)? The balance is subtle. Our correlated ab initio and semiempirical calculations show that in oligothiophenes a polaron pair is preferred, for chains containing seven or eight rings and longer.