Fluorescence of polyaniline films on platinum surfaces. Influence of redox state and conductive domains

The photoluminescence of polyaniline films onto platinum electrodes has been studied as a function of the polymer oxidation state between the fully reduced (leucoemeraldine) and half oxidized (emeraldine) forms. The fluorescence emission spectra have been corrected for absorption and reflection from the underlying metal surface. From these results the quantum yield relative to the reduced state as a function of electrode potential has been obtained. This quantum yield is maximum for the reduced state and decays between ca. 0.3 and 0.4 V in the standard hydrogen electrode scale, reaching a minimum value. This behavior is interpreted in terms of three oxidation states: leucoemeraldine, intermediately oxidized protoemeraldine and emeraldine, each having different absorption and emission behavior. In the emeraldine form, the presence of conductive, quasi-crystalline domains is considered to fully quench emission from excitons located both inside the domains and within a range equal to the exciton delocalization length. A good agreement with the experimental data is found.