Vibrational Study of the FeCl3-Doped Dimer of Polyaniline; A Good Model Compound of Emeraldine Salt

In this paper, optical and vibrational properties of the FeCl3-doped dimer of polyaniline, a model compound of emeraldine salt are presented. This oligomer is synthesized according to the method reported by Zaghal et al.. A complete assignment of the fundamental in-plane Raman and infrared modes (1700−600 cm-1) of this oligoaniline is proposed. The observed frequencies of the dark blue microcrystals are compared with those acquired by vibrational calculations based on a valence force field model. To show, by a most efficient way, the evolution of the electronic configuration of the dimer upon doping, the experimental vibrational modes of the doped dimer and its related force constants are compared with those of its fully reduced and oxidized forms. The strongly aromatic and semiquinone-like characters of the rings present in the doped dimer is put in evidence by Resonance Raman Scattering and infrared absorption and confirmed by vibrational calculations. Considering the numerous similarities between the vibrational characteristics of the polymeric chain and its well-defined oligomer, this study also provides useful information about the electronic configuration of the protonated emeraldine, which remains under debate.