RESONANCE RAMAN-SCATTERING FROM THE POLARON STATE IN POLYANILINE

The charge storing mechanisms of polyaniline during electrochemical doping is analyzed to proceed via the formation of single polarons in a first step followed by the creation of a polaron lattice structure. An in-situ study of resonance Raman scattering identifies the polaronic state and locates it on the benzene ring next to an oxidized nitrogen. A quantum mechanical calculation of the Raman excitation profile yields a polaron transition energy of 2.75 eV in excellent agreement with recent band structure calculations. We are able for the first time to explicitely correlate Raman cross-section of the benzene ring vibrational mode and polaron defect states with the doping concentration. The temperature dependence of the optical spectra of protonated polyemeraldine between 10 K and 300 K leads to the assumption of a transition around 180 K from a polaron lattice at high temperatures to bipolarons at low temperatures.