Formation of Supramolecular Cavitands on Copper Electrode Surfaces

The adsorption of 1,1‘-dibenzyl-4,4‘-bipyridinium molecules (dibenzyl-viologen or DBV2+ for the sake of simplicity) on chloride precovered Cu(100) has been studied in an electrochemical environment by means of cyclic voltammetry and in situ scanning tunneling microscopy. DBV2+ spontaneously forms a highly ordered phase on the chloride c(2 × 2) adlayer at potentials close to the onset of the copper dissolution reaction when the pure supporting electrolyte (10 mM HCl/5 mM KCl) is exchanged by one also containing DBV2+. This ordered phase can be described by a (√53 × √53)R15.9° unit cell relating the organic adlayer to the chloride c(2 × 2) structure underneath or alternatively by a (√106 × √106)R29.05° unit cell relating the organic layer to the Cu(1 × 1) substrate structure. Thus, the negatively charged chloride layer acts as a template for the adsorption and phase formation of DBV2+. Compared to the copper−chloride interaction, the DBV2+−chloride interaction appears to be weaker since the organic layer can be easily removed from the surface by the tunneling tip when drastic tunneling conditions (low bias voltage, high tunneling current) are applied. A key structural element of the DBV2+ adlayer is an assembly of four individual DBV2+ molecules forming square-shaped supramolecular units with pronounced cavities in their center. Characteristically, the supramolecular assemblies reveal a preferential rotational orientation resulting in the appearance of two chiral forms of these assemblies. Furthermore, these two chiral supramolecular assemblies occur in two mirrored domains of the (√53 × √53)R15.9° structure. It can be assumed that these viologen-based supramolecular architectures can be used as potential host cavitands for the inclusion of smaller organic molecules.