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

分子的表面自组装是一种非常有前途的构建具有可控性能的纳米结构的策略。自然界中的许多生长过程都是这方面的榜样，因为复杂的功能单元通常是“一个分子一个分子”地组合在一起的。不幸的是，控制结构生长的分子识别机制尚未被很好地理解，这阻碍了具有新性能的新材料的设计。在过去的几年中，表征表面上单个吸附分子的可能性已经大大提高。利用协同功能化的原子力显微镜针尖，可以直观地观察到单个分子的化学结构，为研究分子识别过程提供了一种新的工具。在这个项目中，这种所谓的键成像方法将用于分析分子间的卤素键。与有机分子共价结合的卤素在键轴的外端有一个亲电区，即σ-空穴。这种各向异性电荷分布导致卤素键具有显著的方向性，其强度可以通过选择卤素或分子静止来调节。由于这些独特的性质，卤素键非常适合在晶体工程、超分子化学、药物设计等领域的应用。表面上的卤素键可以通过另一个控制旋钮来调节：表面材料。本课题将系统研究不同表面材料对不同卤素的σ-空穴的影响。因此，键成像技术将用于确定各种模型化合物在相对惰性和相对活性底物上的结合选择性、键长、键角和吸附构象。在另一个步骤中，我们将研究发生在不同衬底材料上的结合类型，以及它们的结合几何形状如何通过衬底的选择来主动控制。实验结果将得到DFT计算的补充。这将使我们能够了解底物和分子之间的电荷转移、分子内部的电荷分布以及结合能的不同部分（例如分子-分子vs分子-底物）。该项目将有助于更好地了解卤素键的性质，以便能够在未来控制自组装过程，并将其用于设计具有独特性能的新分子结构。

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.