LUMINESCENCE PROBE AND VOLTAMMETRY STUDY OF ION-TRANSPORT DURING REDOX SWITCHING OF POLY(PYRROLE) THIN-FILMS

The ion transport accompanying the redox switching of poly(pyrrole) thin-film electrodes was studied in situ by a new luminescence probe technique. Counterions such as pyrene sulfonate and naphthalene sulfonate, which are fluorescent yet electrochemically silent over the potential window of interest, were used for this purpose. The anodically synthesized poly(pyrrole)-pyrene sulfonate (PP/PS) and poly(pyrrole)-naphthalene sulfonate (PP/NS) thin films were first characterized by cyclic voltammetry. Distinctly different voltammetric fingerprints with associated memory effects were registered for these electrodes depending on the details of the medium in which they were redox cycled. For the in situ study of pseudocathodic processes involving cation transport, positively charged luminescent probes such as Ru(bpy)3(2+) (bpy = 2,2'-bipyridyl) and acridine orange hydrochloride were employed. It is shown that the luminescence intensity modulations in the solution phase due to anion or cation transport have different polarities for the electrode potential thus mimicking an electronic (logic) device feature. The temporal aspects of the steady-state luminescence growth were analyzed for the reduction of PP/NS thin films. Diffusion coefficients of 10(-10)-10(-11) cm2/s were thus obtained and were a function of the polymer film thickness. Finally, a novel potential-scan technique combining linear sweep voltammetry with in situ fluorescence monitoring was developed for PP/NS. Only a fraction (2-40%) depending on the potential scan rate and the electrolyte cation, of the total cathodic current resulted in the ejection of the NS- ions into the fluorometrically sampled solution phase adjacent to the PP/NS thin-film electrodes. The residual current thus is attributed to the competing influence of cation transport into the polymer phase.