Surface redox chemistry of adsorbed viologens on Cu(100)

Abstract

The surface redox-chemistry of adsorbed viologens is studied by means of cyclic voltammetry (CV) in combination with in situ scanning tunneling microscopy (STM). 1,1′-Dibenzyl-4,4′-bipyridinium molecules (DBV²⁺) adsorb on a chloride modified Cu(100) electrode surface under formation of a laterally well ordered 2D array of supramolecular cavitand ensembles. Each cavitand consists at least of 4 individual DBV²⁺ sub-units which are arranged in a certain circular manner making this supramolecular cavitand chiral. Both possible enantiomeric forms are found in two mirror domains at the surface. Reducing the di-cationic DBV²⁺_(ads) species to the corresponding radical mono-cation DBV^•+_(ads) causes a phase transition from the pre-existing DBV²⁺_(ads) cavitand phase to a stripe pattern following a nucleation and growth mechanism. DBV^•+_(ads) species are adsorbed with their main molecular axis parallel to the surface in a side-on adsorption geometry. Enhanced intermolecular π–π-interactions are identified as the main driving force for the formation of 1D oligomer and polymer chains as the characteristic structural motif of the DBV^•+_(ads) phase. These structural motifs are generally independent of the electronic and structural substrate properties. Chloride desorption through the viologen film is discussed as the reason for an order–disorder transition within the viologen film at even more negative potentials.