Theoretical study of p- and n-type doping of the leucoemeraldine base form of polyaniline: Evolution of the geometric and electronic structure

We report quantum-chemical studies on the p- and n-type doping of the leucoemeraldine base form of polyaniline. The attention focuses on the characterization of the geometric and electronic structure related to the charge-storage species which are formed upon doping. When considering p-type doping, which leads to the emeraldine salt form, we find in agreement with previous studies that two polaron- or bipolaron-related bands are induced within the original bandgap. Only the lower of these is deep in the gap and is susceptible to be observed in (photo-)absorption experiments below 3.5 eV. Comparison is made between ring- and nitrogen-centered positive polarons which possess nearly identical stabilities; at low doping level, the two types of polarons induce nearly the same electronic structure; at high doping level, the band structure evolves differently but leads to similar calculated values for the intragap electronic transitions. The geometric and electronic structure obtained for the ring-centered polaron lattice on a single emeraldine salt chain indicates that every other ring adopts a semiquinoid-type geometry and carries most of the unpaired electron density. The characteristics of the positive bipolaron are also investigated; the geometrical results are in close agreement with experiment. In the case of n-doping, the negative defects do not correspond to the formation of localized polarons or bipolarons; the geometry modifications are significantly weaker and more extended than in the p-doping case. The corresponding band structures are calculated to differ very little from that of the neutral case.