UC Berkeley

Metal-organic frameworks (MOFs) are crystalline porous materials, in which the pores represent most of the materials’ volume. Characterization of guest molecules within the pores and the interactions between MOFs and the guests are important research topics in this field for various industrial applications and for deepening our fundamental understanding of the bonding nature that takes place within MOFs. Although the crystalline nature of MOFs allows for atomic level characterization by x-ray diffraction techniques, heavily disordered guest molecules often limit the full characterization of the framework materials. In this dissertation, I present a strategy to make the guests ordered across the unit cells of MOFs in order to determine their structures, including absolute configurations. A chiral metal-organic framework (MOF-520) was used to covalently bond and align disordered molecules. The reduced motional degrees of freedom obtained with this covalent alignment method allowed the structures of molecules to be determined by single-crystal x-ray diffraction techniques. The chirality of the MOF backbone also served as a reference in the structure solution for an unambiguous assignment of the absolute configuration of bound molecules.

An additional problem that the disordered guests cause is a reduction of MOFs’ diffraction intensities, which prevents atomic resolution characterization. The disordered guests interact with the frameworks making them disordered, thus reducing the scattering power of MOFs at high angles. Although there are many reports concerned with guest induced phase transitions - such as breathing effects, unusual thermal expansions, and symmetry changes, where the interactions and changes of the structures are ordered across the unit cells - the disordered interactions have not been the main focus of study in this field. To emphasize the significance of the disordered interactions, UiO-66, which is known for its architecturally robust induced mechanical stability, was chosen and its diffraction qualities with and without guests were compared. UiO-66 without guests exhibited considerably increased mean I/σ values around the resolution limit when compared with UiO-66 with guests. This finding indicated that the disordered interactions significantly impacted the diffraction intensity of UiO-66. To glean additional insight into the nature of the interaction, the unusual phase transition and twinning of MOF-5 was also studied as a function of temperature variation. A single crystal of MOF-5 with guests displayed merohedral twinning accompanied by a phase transition upon temperature decrease, whereas the evacuated MOF-5 did not show such a transition nor twinning.