Density functional theory study of the structure and energetics of negatively charged oligopyrroles

First‐principles calculations are used to investigate the electronic properties of negatively charged n‐pyrrole oligomers with n = 2–18. Chains of neutral oligomers are bent, whereas the negatively charged oligomers become almost planar due to accumulation of negative charge at the end monomers. Isomers of short oligomers (n < 6) display negative electron affinity although the corresponding anions are energetically stable. For longer oligomers with n ≥ 6, the electron affinity is small and positive, slowly increasing with oligopyrrole length. Doping of 12‐pyrrole with lithium atoms shows that negative oxidation states are possible due to electron transfer from dopant to oligomer at locations close to dopant. These 12‐pyrrole regions support extra negative charge and exhibit a local structural change from benzenoid to quinoid structure in the C? C backbone conjugation. Comparison between neutral and doped polypyrrole (PPy) indicates that doped polymers displays a substantial depletion of the band gap energy and the appearance of dopant‐based bands in the gap for a 50% per monomer doping level. It is predicted that Li‐doped PPy is not metallic. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011