N. T. M. Hai

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 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.

Electrochemical reactions at a porphyrin–copper interface

The structure and reactivity of a Cu(100) single crystal electrode surface covered with free base meso-tetra (N-methyl-4-pyridinium) porphyrin (abbreviated as H(2)TMPyP) as a function of electrode potential have been investigated with cyclic voltammetry (CV), electrochemical scanning tunneling microscopy (ECSTM), and UV-Vis and Raman spectroscopy. The well-ordered self-assembled layer of the porphyrin is consistent with the adsorption of the reduced porphyrin species after the first two-electron reduction step. The copper dissolution reaction in the presence of the stable self-assembled porphyrin layer starts at step edges on both upper and lower terraces and coincides with the preferential oxidation of reduced porphyrin species at step sites. The dissolved copper cations are incorporated into the free base porphyrin molecules leading to the formation of CuTMPyP. As a consequence this new species accumulates in the solution with time and a copper redeposition in the cathodic potential scan is lacking.