Fluorescence of polyaniline films on electrode surfaces: Thickness dependence and surface influence

Abstract

The dependence of the photoluminescence of polyaniline films, supported onto platinum or fluorine-doped SnO2 (FTO) electrodes, on the film thickness (as measured by the voltammetric charge) has been studied for different polymer oxidation states. The fluorescence emission spectra have been corrected for absorption and reflection from the underlying metal surface. For platinum electrodes, the absolute quantum yield for films of different thicknesses, and at several electrode potentials, has been obtained using as reference anthracene in acrilic films. Also, the corrected emission for growing films at different charges was recorded. The potential dependence of the emission intensity is not affected by the film thickness. The corrected emission intensity on FTO surfaces increases linearly with thickness due to the increase in the number of fluorophores. In the case of Pt, the intensity is null for low thicknesses, quenched by the metallic surface; as the thickness increases beyond a critical value, the emission increases sharply and then reduces its slope increasing more slowly. The quantum yield on Pt surfaces is accordingly zero for sufficiently thin films, and then increases in a nearly linear way. The observed behaviour on metal surfaces is found to be due to the interference between the field of the emitting dipole and the surface reflected field, causing vanishing emission for dipoles sufficiently near to the surface, and a complex distance dependence afterwards. The theoretical model is found to agree with the experiments if a distribution of parallel and perpendicular dipoles is introduced, with dipoles near the surface being predominantly parallel. © 2009 Elsevier B.V. All rights reserved.