Anion-dominated Redox Reaction of a SAM of an Alkylthiolated Viologen Bearing a Covalently-attached Intramolecular Sulfonate Group on a Gold Electrode

Abstract

The redox processes of the self-assembled monolayers (SAMs) of alkylthiolated viologens on Au electrodes are largely determined by oxidation-state-dependent binding of electrolyte anions to the viologen moiety in an aqueous solution. In this paper, we give an answer to a question: what if the SAM-forming molecule has a covalently-attached intermolecular anionic site? We used the results of the voltammetric measurements for a SAM of a viologen with a covalently-attached sulfonate group, 1-(10-mercaptodecyl)-1′-(3-sulfonatopropyl)-4, 4′-bipyridinium bromide (HS-C10-V-C3-SO3− bromide salt), and examined the effect of intramolecular ion binding on the redox behavior. We found the coexistence of two distinct types of redox couples: one with a viologen moiety, which binds always to the sulfonate group, and the other with a moiety, which catches and releases an electrolyte anion from the solution upon redox. We deciphered rather complicated behavior and discussed the implications.