Optical study of poly-β, β′-dimethylpyrrole Perchlorate: Evidence for bipolarons

The optical conductivity spectra of the conducting and insulating forms of the poly-β, β′-dimethylpyrrole Perchlorate (Pβ DMP-ClO ₄) films have been determined by the Kramers-Kronig transformation of the transmittance spectra from 0.074 to 6.25 eV. The optical spectrum of the conducting form is not consistent with Drude-like free carriers. Absorption spectra have been measured on Pβ DMP-ClO₄ films with various concentrations of Perchlorate anions obtained using an electrochemical technique. Compared to those of polypyrrole Perchlorate, the absorption spectra of Pβ DMP-ClO₄ show some significant differences, especially in the low energy region of the almost neutral forms. Evolution of the Pβ DMP-ClO₄ absorption spectra as a function of doping level is inconsistent with a rigid band model and is interpreted in terms of bipolaren formation on the chains.