Berlin 2008 – scientific programme

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O: Fachverband Oberflächenphysik

O 59: Molecular Nanostructures

O 59.6: Talk

Thursday, February 28, 2008, 10:45–11:00, MA 041

Aromatic vs. Hydrogen Bonds in Self-Assembled Monolayers of Organic Molecules, a STM-Study — •Rico Gutzler, Sophie Lappe, Wolfgang M. Heckl, and Markus Lackinger — LMU München, Sektion Kristallographie, Theresienstr. 41, 80333 München

The monolayer structure of two distinct organic molecules is investigated by STM. We demonstrate how a small modification of a polyaromatic molecule, which does not affect the functional groups, can have dramatic impact on the structure of the self-assembled monolayer. Both molecules exhibit C3 symmetry, have a central benzene ring in common and three carboxylic groups at the 1,3,5 positions, each separated by a rigid spacer. One compound exhibits methyl groups at the 2,4,6 positions of the central benzene ring, the other remains unsubstituted. This change in molecular structure was found to result in completely different self-assembly behaviour at the liquid-graphite interface. The unsubstituted molecule adsorbs in a row structure, where the molecules are stacked with their planes almost perpendicular to the substrate. The methylated molecule self-assembles into a sixfold chickenwire network where the molecules are adsorbed planar. Based on our experimental finding and supported by force field calculations, we propose that in the first case the intermolecular binding is dominated by aromatic interaction between the extended pi-electron systems, whereas in the latter case the chickenwire structure is driven by twofold intermolecular hydrogen bonds between the peripheral carboxylic groups. This finding is explained by weakening of the pi-pi stacking due to steric hindrance imposed by the methyl groups.