Can N-ethyl- N′-octadecyl viologen dimerize on octadecanethiol-coated gold electrode ?

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Electrochemical and ex situ FTIR spectral techniques have been used to study the insertion of asymmetric alkyl viologen, N-ethyl- N′-octadecyl viologen (C 18EV 2+) into the self-assembled monolayers (SAMs) of octadecanethiol (ODT) on Au electrode surface. In the IR reflection absorption (IRRA) spectroscopic measurements, the relative magnitude of the CH 2 and CH 3 stretching modes of ODT and their peak positions indicated that ODT forms a densely packed crystalline-like assembly on polycrystalline Au electrode. Furthermore, a considerable increase in the CH 2 stretching mode intensity relative to that of the CH 3 stretching mode in the C 18EV 2+-inserted ODT monolayer suggested a greater disorder in the alkyl chains of the C 18EV 2+-intercalated ODT monolayer than in the pure ODT monolayer. In the cyclic voltammetry studies, C 18EV 2+ showed a neat broad redox wave at low surface coverage of C 18EV 2+ on the ODT-coated electrode and a sharp reduction and multiple oxidation peaks at high surface coverage. The potential difference subtractively normalized interfacial FTIR (SNIFTIR) spectral studies showed that at high surface coverage of C 18EV 2+ inserted on the ODT-coated electrode, the electrogenerated radical cations of C 18EV 2+ are close and in an orientation proper enough to form the dimer; the spectral features characteristic of the dimer were observed. On the other hand, at similar surface coverage of C 18EV 2+, such spectral features were not observed for the SAMs of C 18EV 2+ on bare Au electrode because of the less ordered orientation of C 18EV 2+.