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.