Dimerized π-complexes in self-assembled monolayers containing viologens: An origin of unusual wave shapes in the voltammetry of monolayers

A direct comparison of the self-assembly on Au and Ag of thiol and disulfide derivatives of viologens bearing long n-alkyl chains was made in order to ascertain the relative efficiency of monolayer formation for each type of functionality. The structures of the two derivatives that were studied can be written as CH₃V²⁺(CH₂)₁₂SH and [CH₃V²⁺(CH₂)₁₂S]₂ for the thiol and disulfide, respectively, where V²⁺ represents the viologen (i.e. N,N′-dialkyl-4,4′-bipyridinium) redox group. In contrast to the behavior of n-alkane thiols and di-n-alkyl disulfides, which adsorb to give very nearly the same surface coverage and interfacial properties, these two viologen derivatives exhibit different saturation surface coverages of 1.8 × 10^-10 mol cm^-2 for the disulfide and 4.5 × 10^-10 mol cm^-2 for the thiol, as determined from the charge for exhaustive reduction and reoxidation of the viologen redox groups. In addition, monolayers of the thiol derivative that had very high surface coverages exhibited very sharply peaked cyclic voltammetric responses that are attributed to very strong interactions between the one-electron-reduced cation radicals in the monolayer, a phenomenon that does not occur in monolayers prepared from pure samples of the disulfide derivative. Vibrational spectroscopic examination of these monolayers under conditions in which this unique voltammetric response is observed revealed the presence of vibrational spectroscopic signatures of viologen dimer formation. Specifically, surface Raman spectroscopy (including surface-enhanced Raman, surface resonance Raman, and surface-enhanced resonance Raman) was used to demonstrate that the lateral interaction of the cation radical viologen redox groups in these monolayers results in the formation of π complex dimers. The presence of these dimers is correlated with the very sharply peaked cyclic voltammetric responses. The Raman bands due exclusively to the dimer are assigned to the out-of-phase coupling combination of the totally symmetric ring modes of the component cation radicals in the dimer.