In Situ Infrared Spectroscopic Studies of Redox Active Self-Assembled Monolayers on Gold Electrode Surfaces

Monolayers of N-ethyl-N′-octadecyl-4,4′-bipyridinium (EOB) have been prepared on smooth gold surfaces by spontaneous self-asembly of the electrochemically reduced form of the amphiphile and have been examined with in situ Fourier transform infrared reflection absorption spectroscopy (FTIRRAS). Prominent positive-going peaks with hardly any detectable negative-going counterparts were observed in the normalized potential difference spectra of the layer in the reduced state (using as a reference the spectra for the layer in the fully oxidized state) in the region between 1100 and 1700 cm⁻¹. In analogy with a model put forward by other workers, it is proposed that the spectral bands originate from vibronically activated totally symmetric modes of EOB^•+ in monomeric and/or dimeric form. The large intensity of these vibrationally based charge oscillations suggests that at least in the reduced state the plane of the bipyridinium moiety is close to parallel to the electrode surface. An alternate explanation based on a molecular reorientation induced by changes in the redox state was also considered. Such a model, however, was discarded as it fails to account for the large differences in the intensities of the positive- and negative-going peaks observed for solution phase FTIR spectra of the closely related N,N′-dimethyl-4,4′-bipyridyl species. The spectra also displayed two strong negative-going peaks in the region between 2800 and 3000 cm⁻¹. This can be explained on the basis that the alkyl chains in the dication are in a collapsed (or unstretched) form adopting a more stretched configuration (with the skeletal axis pointing away from the surface) when in the reduced state, leading to a net decrease in the C-H stretching contribution to the spectra.