Controlling intermolecular interactions at surfaces through chemical ligands : supramolecular aggregation, covalent coupling and chirality at reduced dimensions

Abstract

The development of scanning probe methods enabled the investigation of molecules adsorbed on surfaces with impressive resolution. A delicate balance between molecule-substrate and intermolecular interactions such as van der Waals interactions, H-bonding or dipolar coupling guides the arrangement of molecules in well-ordered patterns. A very appealing concept is to profit from the order of these pre-organized structures and to interlink the molecular building blocks to macromolecules, which is the main goal of the first part of this thesis. By making use of a new concept, we are able to control both, the molecular self-assembly and the subsequent intermolecular coupling reactivity by protecting group chemistry. We describe heat induced formation of polymeric structures from biphenyl derivatives adsorbed on both Cu(111) and Ag(111) surfaces. Moreover, we studied how to control the arrangement and the size of the resulting polymeric structures by modification of the end groups of the biphenyl units. For all biphenyl derivatives, well-ordered supramolecular networks are transformed into covalently bound dimers through annealing at elevated temperatures. Further annealing of such structures results in the formation of interlinked cross-like or trimeric structures and long chains. In the second part, the effect of the chirality on the resulting surface assemblies is studied for the case of a helicene derivative deposited on a Cu(111) surface. The helicene derivative used in this work consists of a helicene core to which two opposed cyano side groups are attached. Herein, it is shown how the cyano groups influence the adsorption geometry of the molecules upon deposition on the surface and the formation of the self-assembled structures. Adsorption of both racemic and enantiopure forms of the molecule are analyzed in detail and a chirality transfer of the molecular building blocks into the extended molecular domains is confirmed from the comparison of the experimental results of both cases. Finally, behaviour of the enantiopure form of the molecule at room temperature is studied, which reveals a metastable nanoporous network. In this section, a time induced phase transition of the nanoporous network into a stable dimeric arrangement is shown and possible reasons for this conversion are discussed.