Surface redox chemistry of adsorbed viologens on Cu(100)

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 DBV(ads)2+ species to the corresponding radical mono-cation DBV(ads).+ causes a phase transition from the pre-existing DBV 2+ ( 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 pi - pi-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.