  The functionality of conjugated polymer systems often relies on oxidations or
reductions, in most cases mediated by the presence of counterions. The effect
that the common counterion hexafluorophosphate (PF6-) has on the intermolecular
interactions between charged oligothiophenes is investigated here using ab
initio quantum chemistry methods. Counterions are explicitly included in the
simulations of oxidized oligothiophenes and in the dimerization process. Our
calculations provide quantitative and qualitative insight into the
intermolecular interactions in oligothiophene-counterion systems and show that
the intermolecular pi-stacking of oligothiophenes is not adversely affected by
the presence of counterions, and that in fact oligothiophene dimerization is
further stabilized by their presence.
