Molecular Networks Through Surface-Mediated Reactions - From Hydrogen Bonds to Covalent Links

This thesis deals with adsorption, self-assembly, and surface reactions of organic molecules on solid substrates, with the aim to fabricate higher hierarchical twodimensional (2D) structures. It is of genuine interest in materials science to develop strategies and methods for reproducible growth of extended molecular assemblies with specific and desired chemical, physical and functional properties. The experimental technique used was Scanning Tunneling Microscopy (STM) - an outstanding method to gain real space information of the atomic-scale realm of adsorbates on crystalline surfaces. The investigated systems are characterized by a complex interplay between adsorbateadsorbate interactions and adsorbate-substrate interactions. In one series of experiments this could be illustrated through self-assembly of hydrogen bonded heteromeric molecular networks on a chemically relatively inert graphite substrate. In this case, van-der-Waals forces between adsorbate and substrate have to be balanced with intermolecular hydrogen bonds in concert with weaker van-der-Waals forces. Since the magnitude of van-der-Waals forces between adsorbates and substrates correlates with the contact area, this type of interaction becomes more dominant for larger molecules. By stronger interactions which do not depend on molecule size, it was also possible to grow isotopological molecular networks, i.e. networks following a similar building plan. By varying for instance the length of aliphatic spacers, supramolecular structures with tuneable lattice parameter could be formed. Studies of organic molecules on chemically more active metal substrates show that more complex processes can be involved. In particular the concept of reactivity and surface-catalyzed reactions are discussed and illustrated by an intuitive example. It is demonstrated that strong molecule-substrate interaction can induce unimolecular reactions such as deprotonation of molecules or more generally dissociation of intramolecular bonds. This interaction strength, thus substrate reactivity is highly influenced by a variety of factors which include material, crystallographic surface orientation, and temperature. Further more the importance of so-called active sites on crystal surfaces, i.e. special sites with significantly increased interaction strength, is taken into account and exemplified with experimental results. Exploiting these fundamental principles, C-Br bond scission of brominated aromatic compounds was demonstrated upon adsorption on reactive substrates and followed by successful incorporation in covalently bonded networks. However, irreversibility of covalent bonds prevents similar control and error correction mechanisms over the system as compared to hydrogen bonded networks. A high defect density and a low degree of ordering is the consequence for the resulting 2D structures. In a final set of experiments aromatic thiol molecules could be assembled into highly or dered structures via metal-coordination bonds. The 2D gas of freely diffusing adatoms of a copper surface was thermally excited to finally transform a trithiolate precursor structure into metal-coordination networks via Cu-S metal coordination bonds. Two different coordination geometries were observed giving rise to the formation of two morphologically distinct phases. These studies revealed the impact of the adatom gas for surface reactivity and chemistry of metals.