| AVS 56th International Symposium & Exhibition | |
| Surface Science | Thursday Sessions |
| Session SS2-ThM |
| Session: | Electron Activated Molecular Interfaces |
| Presenter: | K. Wandelt, University of Bonn, Germany |
| Authors: | D.-T. Pham, University of Bonn, Germany C. Safarowsky, University of Bonn, Germany S. Huemann, University of Bonn, Germany S. Breuer, University of Bonn, Germany R. Hunger, Technical University of Darmstadt, Germany P. Broekmann, University of Berne, Switzerland K. Wandelt, University of Bonn, Germany |
| Correspondent: | Click to Email |
The reactive and non-reactive adsorption of redox-active viologens (1,1`-disubstituted-4,4`-bipyridinium molecules) on a chloride modified copper electrode has been studied using a combination of cyclic voltammetry (CV), in-situ scanning tunneling microscopy (STM) and ex-situ photoemission techniques. Two prototypes of viologens, 1,1`-dibenzyl-4,4`-bipyridinium molecules and 1,1`-diphenyl-4,4`-bipyridinium molecules (abbreviated as DBV and DPV respectively), are studied here with respect to their redox behaviour upon adsorption on a chloride modified copper surface. DBV molecules can be adsorbed and stabilized on a chloride modified Cu(100) electrode surface in their di-cationic state at potentials above the main reduction wave in the cyclic voltammogram. Electrostatic attraction between the solvated viologen di-cations and the anionic chloride layer is the main driving force for the DBV adsorption onto the electrode surface. By reducing the adsorbed dicationic DBV2+ species to the corresponding radical mono-cation DBV•+, a quasi-reversible phase transition is initiated from a “cavitand” to a “stripe pattern” phase on the chloride layer. Analysis of the N1s and O1s core level shifts of the adsorbed DBV molecules points to a non-reactive DBV adsorption leaving the DBV2+ads solvation shell partly intact. The laterally ordered DBV2+ads monolayer is highly hydrophilic with at least 8 water molecules per viologen present within this cationic organic film. The analysis of the Cl2p core level shift reveals that no other chloride species is present on the surface than the one underneath the organic molecules in direct contact with the metallic copper surface.
DPV2+ molecules are much more reactive upon adsorption and cannot be stabilized on the electrode surface in di-cationic state, at least within the narrow potential window of copper. The N1s core level shift points to DPV2+ molecules which are upon adsorption instantaneously reduced to the corresponding mono-reduced DBV.+ads species even at potentials above the main redox wave in the voltammogram. This process leads to the formation of a highly hydrophobic monolayer film with polymeric DBV.+ads stacking chains as the characteristic structural motif .