Potential-Induced Redox Switching in Viologen Self-Assembled Monolayers: An ATR−SEIRAS Approach

Monolayers of viologens (N-alkyl-N‘-(n-thioalkyl)-4,4‘-bipyridinium bromide, n = 6, 8, 10) have been self-assembled from ethanolic solution onto gold substrates. Their structure and redox functionality were studied by cyclic voltammetry and in situ ATR−SEIRAS. Voltammetric experiments revealed that the first redox process, V2+ ↔ V•+, is reversible, whereas the second redox process, V•+ ↔ V0, is quasi reversible. The V2+ adlayer exhibits a sandwich-like structure. Van der Waals interactions between the alkyl chains lead to two well-ordered hydrophobic layers, with the redox-active bipyridinium unit enclosed in between. The alkyl chains are aligned in an all-trans configuration in a tilted orientation with respect to the surface normal and exhibit a slight conformational disorder. The long axis of the central bipyridinium rings is also tilted. The alignment of V2+ is stabilized by interplanar π−π stacking and electrostatic coulomb interactions. Both monomers and dimers of V•+ coexist in the V•+adlayer. The molecular alignment of the V•+ adlayer is stabilized by the strong vibronic coupling within the dimers and the π−π stacking between the monomers. Increasing length of the alkyl chain results in more tilted bipyridinium units, and favors both the dimerization and the alignment of V•+ within the SAMs. The complex reaction, V2+ → V•+, is described as electron transport from the electrode to the V2+ moiety followed by the dimerization of V•+, the delocalization of the radical electrons, and the migration of coadsorbed ClO4- anions.