Adsorption-desorption processes of redox-active osmium thiol monolayers

The adsorption and desorption processes of a new series of redox-active transition metal complexes of the type [Os(bpy)(2)Cl(Py-(CH2)(n)-SH)](PF6), with n = 4, 6, or 9, capable of forming adsorbed monolayers on gold and platinum electrodes have been studied using cyclic voltammetry. The adsorption behavior of these osmium thiol complexes appears to be consistent with the Freundlich model. Although more than one equivalent layer of the osmium complex was deposited, the multilayer was found to be porous. Variations in the pH of the deposition solutions from pH 1 to pH 5, as well as variations in the ionic strength of the supporting electrolyte, had no effect on the amount of complex deposited. However, variations in the ionic strength of the supporting electrolyte gave rise to changes in the formal potential of the osmium redox couple which were ascribed to charge screening and Donnan effects. The desorption reaction was described by a simple exponential expression. From the temperature dependence of the desorption rate and the Arrhenius relationship, the activation energies for the desorption processes were determined. The activation energies were found to be dependent on both the length of the alkyl chain and the solvent used during desorption. In addition, it was determined that after an 'annealing' time of 24 to 48 h there was a reorganization of the deposited multilayer to a lower energy form with a corresponding 20% increase in the activation energy for desorption, indicative of the multilayer reorganizing into a lower energy, more stable configuration.