Berlin 2008 – scientific programme

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O: Fachverband Oberflächenphysik

O 51: Metal Substrates: Adsorption of Organic/Bio Molecules III

O 51.1: Talk

Wednesday, February 27, 2008, 15:15–15:30, MA 041

Molecular 2D Ordering of Viologens Under Non-Equilibrium Condition - an EC-STM Approach — •Duc Thanh Pham, Klaus Wandelt, and Peter Broekmann — Institut f. Physikalische und Theoretische Chemie, Uni Bonn

The structural characterization of 1,1’-dibenzyl-4,4’ bipyridinium molecules (dibenzyl-viologen, DBV), adsorbed on a chloride modified Cu(100) electrode is studied by means of Cyclic Voltammetry (CV) and ElectroChemical Scanning Tunneling Microscopy (EC-STM). All previous studies on this system started with the non-reactive adsorption of DBV²⁺ in the double layer regime resulting in the surface assisted organization of a cationic DBV²⁺ monolayer on top of the c(2x2) anionic chloride adlayer. However, within the electrode potential window ranging from E = -150 mV to E = -250 mV where the first reduction step from the di-cationic to the radical mono-cationic viologen form takes place, a "reactive" adsorption of viologen occurs. Preferred reaction products at the surface are metastable viologen dimer species with a viologen coverage of 0.077 ML. Once formed under kinetic control, this dimer phase exhibits an unusually high stability against changes of the potential. It can only be converted irreversibly into either the oxidized dicationic species or the more compact and thermodynamically favorable "polymeric" stacking phase of radical mono-cations by significantly more anodic or cathodic potentials sweeps, respectively.