A monomer–dimer equilibrium in self-assembled monolayers of N-ethyl-N′-octadecylviologen on the electrode surface. The influence of water content and hexafluorophosphate ion

This paper describes the dimerization of self-assembled monolayers (SAMs) of N-ethyl-N′-octadecylviologen (1) on GC and Au electrode surfaces in the presence of 0.1 M NH₄PF₆ aqueous solutions. The ‘wet’ and ‘dry’ SAMs of 1 showed multiple redox peaks for the first reduction of 1 in the presence of NH₄PF₆, in contrast to the case of other supporting electrolytes (typically KCl, NaNO₃, Na₂SO₄ and NaClO₄) where both wet and dry SAMs of 1 exhibited a single redox wave for the first reduction. The dry SAM showed a well defined reduction peak at ?0.57 V along with a shoulder reduction peak at ?0.50 V and two oxidation peaks at ?0.50 and ?0.42 V. On the contrary, the wet SAM gave a very sharp reduction peak at ?0.50 V and a small shoulder peak at ?0.57 V in addition to two oxidation peaks like those observed for the dry SAM. The reduction peak of ?0.50 V was ascribed to the reduction of strongly hydrated dications of 1, while the reduction peak at more negative potential (?0.57 V) was attributed to the reduction of the dehydrated dications of 1. The two oxidation peaks at ?0.50 and ?0.42 V were ascribable to the oxidation of the usual radical cation monomer and the radical cation dimer, respectively. In the case of the wet SAM, upon continuous potential cycling, the sharp reduction peak of ?0.50 V clearly decreased, whereas the more negative reduction peak of ?0.57 V was highly stable. In this case, in the oxidation process, the monomer peak of ?0.50 V increased, while the dimer peak of ?0.42 V decreased. Thus it is reasonably assumed that in the wet SAM, initially the radical cations of 1 feel an aqueous environment in the monolayer where the dimerization is highly favored and at subsequent potential cycles, due to the entry of hydrophobic anions of PF₆^? into the monolayer, the pre-existent water molecules are expelled from the monolayer and under this circumstance the radical cations of 1 may feel the environment very similar to non-aqueous media where the dimerization is totally suppressed. The adsorption tendency of 1 on the electrode surface was also studied using the SAMs prepared by dissolving 1 in water+ethanol mixtures of different ratios. The appearance of multiple peaks was found to depend significantly on the alkyl chain length of asymmetric viologen. The inclusion/expulsion of solvents and anions into/from the SAM during the redox reaction were studied by the electrochemical quartz crystal microbalance (EQCM). It was found that in the presence of SO₄^2? ions ca. 17 water molecules per one SO₄^2? ion were transported to the SAM of 1 during the oxidation, whereas ca. five water molecules were transported in the presence of PF₆^? ions.