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Dienstmaier, Jürgen (2013): From supramolecular self-assembly to two-dimensional covalent organic frameworks. Dissertation, LMU München: Faculty of Geosciences



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.