Ionic Interactions Play a Major Role in Determining the Electrochemical Behavior of Self-Assembling Viologen Monolayers

Electrochemical and quartz crystal microbalance (EQCM) methods are used to probe the redox behavior of self-assembled monolayers of viologen derivatives with long alkyl chains and disulfide or thiol groups which provide for attachment to gold electrodes. The EQCM measurements revealed that (1) anions exit and enter the viologen monolayer during reduction and oxidation, respectively, and (2) water molecules also exit and enter the monolayer during reduction and oxidation, respectively, but in amounts which depend on the identity of the anion. The temperature dependence of the full width at half-height of the cyclic voltammetric peaks (Efwhh) for monolayers with different anions was suggestive of normal thermal broadening for the case of Cl- and NO3- and of some type of attractive interaction between the redox groups, which increased in importance as temperature increased for the CIO4- case. The Etwhh data for the Cl- and NO3_ cases rule out the possibility of hydrophobic interactions between the alkyl chains as the source of these attractions, so the CIO4- data are interpreted as being due to strong ionic interactions between the CIO4-anions and the viologen dications, probably of the ion-pair type.