Self-Assembly Processes of Organic Molecules on Surfaces: Analysis of Intermolecular Halogen Bonds by High Resolution Low Temperature Atomic Force Microscopy

• Applicant: Dr. Daniel Ebeling
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 448547917

On-surface self-assembly of molecules represents a very promising strategy for building nano structures with controllable properties. Many growth processes in nature serve as a role model for this, since complex functional units are often put together “molecule by molecule”. Unfortunately, the molecular recognition mechanisms that control the structural growth are not yet well understood, which hampers the design of new materials with new properties. In the last few years, the possibilities for characterizing single adsorbed molecules on surface have been significantly improved. By using CO-functionalized AFM tips, the chemical structure of single molecules can be directly visualized, which represents a new toolset for studying molecular recognition processes. In this project, this so-called bond-imaging-method will be employed for analyzing intermolecular halogen bonds. Halogens that are covalently bound to an organic molecule have a electrophilic region at the outer end of their bonding axis, the σ-hole. This anisotropic charge distribution within the halogen leads to a remarkable directionality of halogen bonds, whose strength can be tuned by the choice of halogen or molecular rest. Due to these unique properties halogen bonds are ideally suited for applications in the fields of crystal engineering, supramolecular chemistry, drug design, etc. Halogen bonds on surfaces can be tuned by another control knob: the surface material. In this project, we will systematically study the influence of different surface materials on the σ-hole of different halogens. Therefore, the bond-imaging technique will be applied to determine binding selectivities, bond lengths, bond angles, and adsorption conformations of various model compounds on both relatively inert and relatively reactive substrates. In another step, we will study the binding types that occur on different substrate materials and how their binding geometries can be actively controlled by the choice of substrate. The experimental results will be complemented by DFT computations. These will allow gaining knowledge about the charge transfer between the substrate and the molecule, the charge distribution inside the molecules and the different parts of binding energy (e.g. molecule-molecule vs. molecule-substrate). The project should help to obtain a better understanding about the nature of the halogen bond to be able to control self-assembly processes in the future and use these for the design of new molecular structures with unique properties.

• DFG Programme: Research Grants
• International Connection: Spain
• Cooperation Partner: Professor Dr. Rubén Pérez