From supramolecular self-assembly to two-dimensional covalent organic frameworks

The two main subjects of this thesis are the realization of
supramolecular self-assembled monolayers at surfaces and the
formation of surface-supported two-dimensional covalent organic
frameworks. Both topics, albeit different, yield long-range ordered
open-pore networks with quite different stabilities, depending on
the strength and type of bonds holding them together. The surface
of choice is mainly graphite, which is considered an inert
substrate. Graphite yields pristine clean, very large and flat
surfaces when cleaved, facilitating the observation in real space
of the molecular networks adsorbed on these surfaces by means of
the Scanning Tunneling Microscope (STM). STM is the main
experimental technique used here. It was used to image mostly at
the liquid-solid interface under ambient conditions. Using a large
tricarboxylic acid adsorbate, long-range order supramolecular
self-assembled monolayers were obtained. These monolayers are
formed via a delicate interaction balance between adsorbates,
substrate, and solvent molecules. Weak van der Waal forces mediate
the adsorbate-substrate interaction; hydrogen bonds, the
adsorbate-adsorbate interaction. Also, depending on the solvent
used and the concentration of adsorbates dissolved in it, different
polymorphs are found on the substrate. To understand the nucleation
and growth mechanism that give rise to the different self-assembled
monolayers, thermodynamical considerations are used. Enthalpic and
entropic contributions are evaluated for several of the polymorphs
found, explaining their occurance on the basis of the Gibbs free
energy per unit area. However, even if this work sheds some light
on supramolecular self-assembly, adding also that much research has
been done in this field, it is still very difficult to know a
priori how adsorbates will behave on a substrate. Thus predictions
of which patterns will ultimately arise are hampered. To realize
structures that are more stable than those formed via
supramolecular self-assembly, several strategies have been
proposed. Covalent bond formation is one of them, yielding strong
and lightweight structures by using organic molecules composed
primarily of light elements. The strength of covalent bonds ranges
from strong to very strong, when compared to van der Waals and
hydrogen bonds. This characteristic makes correction of possible
structural errors difficult to almost impossible. However, when
molecules with suitable functional groups are allowed to react
under reversible conditions, error correction of covalent bonds
becomes feasible, yielding regular structures with the
energetically most favorable configurations. In this thesis, this
is exemplified with the small 1,4-benzenediboronic acid molecules,
yielding monolayers composed of very regular, long-range ordered
covalent organic frameworks on graphite. Thermal stability is
probed by exposing the structures to relatively high temperatures
for prolonged times under atmospheric conditions. Further
experiments with larger para-diboronic acids, under similar
reversible conditions, yield the expected isotopological regular
frameworks with larger unit cell parameters. This demonstrates the
proof of principle for the formation of two-dimensional covalent
organic frameworks. These two main topics, supramolecular
self-assembly and covalent bond formation on surfaces, constitute
the basis of this thesis. It is organized as follows: A first part
deals with the theoretical background of the main analytical
instruments used in this work. Then, the thermodynamics of
supramolecular self-assembly is presented, along with the studies
of the different polymorphs found using a large tricarboxylic acid
as building block. The final part deals with the formation of
two-dimensional, long-range ordered covalent organic frameworks,
made from organic molecules composed only of light elements. This
work show that these last mentioned networks exhibit higher thermal
stabilities when compared to self-assembled monolayers held
together mainly by strong hydrogen bonds. The viability of larger
heteromeric isotopological networks is also explored.