On the structure of polaronic defects in thiophene oligomers: a combined Hartree–Fock and Density Functional Theory study

Thiophene oligomer radical cations were studied as model for the polaronic defects in doped polythiophenes. Oligomers of increasing size (2 to 10 rings) were characterized by means of ab initio calculations. Optimized geometries and charge distributions were computed at both the Hartree–Fock (HF) and Density Functional Theory (DFT) levels. When electron correlation is taken into account the whole shape of the polaronic defect changes so that the self-localization of the positive charge and the corresponding quinoid structural distortion are greatly reduced. The energetics of the aromatic vs. quinoid geometry is investigated using substituted bithiophenes as model compounds. The discrepancy between HF and DFT results is confirmed. Traditional post-HF approach is found to agree with DFT.