Probe beam deflection spectroscopy as a tool for mechanistic investigations of modified electrodes

Probe beam deflection spectroscopy together with cyclic voltammetry was used to analyse reaction mechanisms at redox polymer-coated electrodes. With this method, the direction of the concentration gradient at the electrode surface can be probed while a cyclic voltammogram is obtained. If a simple binary electrolyte like HClO4 is used in such an experiment, the direction of the beam deflection can be used to determine whether perchlorate ions or protons are used to compensate the charge of the electron exchange process in a characteristic part of the cyclic voltammogram, and may help to elucidate the reaction mechanism of an electroactive electrode coating.Results obtained from electrodes coated with [Ru(bpy)2Clpoly(4-vinyl-pyridine)]Cl (I) and poly-(1-hydroxyphenazine)(II) are discussed. The deflection signal clearly indicated that the electron transfer in the polymer (I) was accompanied by an anion movement whereas polymer (II) showed a mixed mechanism which uses protons and anions to compensate the charge exchanged during the oxidation and reduction of the polymer.In order to fully understand the results, a numerical simulation of the interfacial process at the polymer coated electrode was necessary. The system of non-linear differential equations which describes the physical and chemical processes at the polymer coated electrode is presented. Using appropriate numerical methods and boundary conditions, it should be possible to calculate the concentration profile for different potentials of the voltammogram. A direct correlation between the magnitude of the slope of the calculated concentration profile and the beam deflection is expected.