A Vibrational Spectroscopic Study of the Structure of Electroactive Self-Assembled Monolayers of Viologen Derivatives

Surface-enhanced Raman spectroscopy (SERS) and infrared reflection absorption spectroscopy (IRRAS) are used to determine the orientation of the viologen redox group and the relative degree of ordering of the alkyl chain segments in self-assembled monolayers of various alkylviologen derivatives. The monolayers were assembled and examined on rough Ag electrodes (SERS) and smooth, vapor-deposited Au electrodes (IRRAS). Attachment to the Ag or Au surfaces which were studied occurs by virtue of strong interaction of a disulfide group on the molecule with the surface. In the present case, analysis of the relative intensities of the viologen A_g N-R stretching mode at 1169 cm^-1 and two of the A_g ring modes at 1298 and 1637 cm^-1 reveals that the net orientation of the long axis of the viologen moiety is roughly perpendicular to the plane of the electrode surface, in accordance with the surface coverages at saturation as determined from previous electrochemical data. The relative intensities of the methylene asymmetric (2890 cm^-1) and symmetric (2850 cm^-1) stretches in the SERS spectra on rough Ag and the peak positions of the asymmetric and symmetric methylene C–H stretching modes in the IRRAS on smooth Au (at 2929 and 2857 cm^-1, respectively) reveal a liquid-like environment for the alkyl chains. These results are consistent with the relative sizes of the viologen and alkyl groups and the expected packing arrangement based on these, as well as with previous electrochemically derived surface coverages for saturated self-assembled monolayers of these compounds.