Thin Film Polymorphism of Cu-based Metal-Organic Frameworks

Thin films of metal organic frameworks (MOF) are highly relevant for technological applications, however, frequently unknown thin film polymorphs are formed during the crystallization process at surfaces. This project will develop a methodology to solve crystal structures from MOF thin films by a combined experimental / theoretical approach. The main difficulty is that only a limited number of diffraction peaks will be available by X-ray diffraction techniques so that established methods cannot be used for this specific class of materials. The internal structure of MOFs (metal nodes are connected by organic linkers) will allow to use Patterson functions to locate the position of the metal nodes within the crystallographic unit cell. Specific algorithms have to be developed to identify the internal framework structure formed by the linker molecules between the metal nodes. The MOFs of choice will use copper atoms as metal nodes together with linkers based on molecules with carboxylate groups, isonicotiate, benzenehexathiole and hexahydroxytriphenylene. Precursors for the MOF synthesis will be prepared by the use of sputtered CuO and of Cu(OH)2 surfaces. The MOF thin films will be grown by a variation of the thin film growth parameters; e.g. by different preparation techniques like liquid processing and vapor phase conversion. Strong preferred orientation of the MOF crystallites and even epitaxial growth is expected; characteristic features about polymorph formation will be obtained. In a subsequent step, Raman spectroscopy in a wide range of wavenumbers from 5 cm-1 up to 2000 cm-1 will be experimentally determined and compared with theoretical calculations based on the solved crystal structures. This approach will serve as a quality control of the crystal structure solution. The innovation of the project is a new methodology of crystal structure solution from thin films together with the crystallographic study of thin film polymorphism of application relevant MOF thin films.

Metal-organic frameworks (MOFs) are innovative new materials due to its high internal porosity. MOFs open new possibilities in applications, and as a consequence new technological and scientific requirements come into play. Of fundamental interest for any application based research is the internal geometry within the MOFs, this means the arrangements of the metal nodes and of the connecting linkers which form the framework structure. The focus of the project is the determination of the internal structure of MOFs from thin films. It is shown that a combined experimental / theoretical approach is a successful approach. This combination allows to overcome the difficulties in the structure determination procedure for MOFs dealing with the lack of knowledge on the exact chemical composition as well as of pore filling due to chemical processing. A number of prominent copper based MOFs are investigated like the using isonicotinate, benzene-1,4-dicarboxylate and 1,4-diazabicyclo[2.2.2]octane (and combination of them) as linker molecules. Not in all cases porous structures evolve during the thin film preparation processes, structures can collapse so that densely packed coordination networks are sometimes present. The work shows that standardized methodology for crystal structure determination is not sufficient to get insights into the structure of MOF thin films. Combination of theoretical and experimental techniques are required.