On the Adsorption of Extended Viologens at the Electrode|Electrolyte Interface

Extended viologens represent a group of organic molecules intended to be used as molecular wires in molecular electronic devices. Adsorption properties of a novel series of extended viologen molecules were studied at the mercury electrode vertical bar electrolyte interface. These compounds form compact monolayers around the potential of zero charge with a constant differential capacitance value of 2.5 +/- 0.2 mu F cm(-2) independent of temperature, length of the molecule, and its bulk concentration. At more negative potentials their reduction in the adsorbed state takes place. We showed that the adsorption process is diffusion controlled and time needed to fully cover the electrode surface is independent of the electrode potential. A modified Koryta equation was employed for the calculation of the surface concentration of the adsorbates leading to the value of 5.3 x 10(-11) mol cm(-2) for the shortest wire and to 1.6 x 10(-11) mot cm(-2) for the longest one. Based on the space idling model and the differential capacitance value in the compact film region. it was postulated that these molecules lay flat on the electrode surface.