  The pi-stacking of oxidized thiophene oligomers is investigated using
ab-initio quantum chemistry methods. Dimers of singly-charged oligothiophenes
are found to be unstable in the gas phase, but can be stabilized as bound
dications in the singlet state by a polarizable solvent such as acetonitrile.
Our calculations provide a detailed description of the mechanisms and the
energetics involved in the dimerization phenomenon, and highlight the role and
importance of the environment in the stabilization of the stacks. The need for
accurate treatments of electronic correlations and of solvation effects for a
realistic description of these materials is underscored.
