STRUCTURAL CHANGES OBSERVED IN POLYPYRROLE FILMS WITH IN-SITU RESONANCE RAMAN TECHNIQUES

Abstract: The 488 nm in situ Raman spectra of electrochemically prepared polypyrrole films permit reliable assessment of oxidation state as a function of doping potential. The more complex 785 nm spectra are analysed to give more detailed information about the nature of the charge carriers in the doped polymer. The electronic properties of polypyrrole change markedly with the degree of doping or oxidation. Evidence from electronic spectra and solid-state NMR studies suggests that specific structural changes accompany the electronic changes. Undoped or pristine polypyrrole appears to possess an aromatic ground-state, and the process of p-doping or oxidation introduces so-called polarons and bipolarons, being discrete regions in the polymer backbone where the positive charge is concentrated. Significantly, these regions adopt a quinoid ground-state and have characteristic electronic transitions towards the red end of the visible spectrum, rather than the ca. 450 nm band-gap of the aromatic polymer. As such, manipulation of the resonance conditions by selecting an appropriate excitation wavelength allows analysis of the structure of either aromatic or quinoid segments of the polymer with minimal interference from the other. To this end we have used 488 nm and 785 nm excitation to study polypyrrole films under electrochemical control, and in th