Environmental Effects on Redox Potentials of Viologen Groups Embedded in Electroactive Self-Assembled Monolayers

Several new compounds capable of spontaneous self-assembly at Au electrodes have been prepared. These are all of the type CH₃(CH₂)_nV²⁺(CH₂)_mSH, were V²⁺ is N,N′-dialkylbipyridinium (i.e. a viologen group) and the following n and m combinations were employed: n = 0, m = 12; = 9, m = 3; = 9, m = 10. These compounds are referred to as 1V12SH, 10 V3SH, and 10V10SH, respectively. These compounds were allowed to self-assemble at Au electrodes from aqueous solutions of various supporting electrolytes. This process produces stable, electroactive self-assembled monolayers which manifest the well-behaved electrochemical responses of the viologen moiety. Both electrochemical and quartz crystal microbalance (EQCM) techniques are used to investigate the redox behavior of these monolayers. The EQCM results suggest that the degree of solvation of the viologen moiety depends on its proximity to the monolayer/solution interface, with the largest degree of solvation occurring for 1V12SH. The first reduction potential of the viologen moiety (i.e. for V²⁺/V⁺) is also the most negative for this derivative, which is consistent with its being more strongly solvated than the other derivatives, again due to proximity to the monolayer/solution interface. This effect depends strongly on the identity of the anion which is present within the monolayer. The effect is most pronounced for Cl^- and much less pronounced for ClO₄^-. The possible implications of hydration and interactions between redox groups and their counterions within monolayer systems are discussed.