Theoretical study on the emeraldine salt - impact of the computational protocol

We report a theoretical study on the geometry characteristics and the electronic properties of bipolaron defects introduced in oligomers (tetramer to hexadecamer) of emeraldine salt. The main goal of the present investigation is to establish a quantum chemical method suitable for description of the target system and for determination of the oligomer segment perturbed by protonation. For the purpose, a series of first principles methods (HF and DFT) combined with different basis sets are tested and the influence of the medium (water) dielectric constant is estimated using PCM. It is shown that the optimized molecular geometry is critically dependent on the theoretical formalism applied. HF yields geometry of the tetramer substantially distorted from planarity and bipolaron, which is highly localized in the quinoid ring. The bipolaron produced by DFT tends to delocalize, thus enhancing the conjugation along the oligomer chain. The reliability of the separate methods is commented on the basis of a list of criteria such as reproducibility of available experimental geometry, spin contamination of wavefunctions and quantification of correlation effects. The BLYP/6-31G∗/PCM method is outlined as the most appropriate. It is found that irrespective of the oligomer chain length and the degree of protonation (partial or full) a bipolaron spreads on three phenyl rings and the nitrogen atoms bound to them.