Synthesis, characterization and postsynthetic modification of a novel two-dimensional Zr-based metal-organic framework

Abstract

The focus in many fields of research is the generation of materials with adjustable properties for specific applications. Among porous materials, metal-organic frameworks (MOFs) have developed over the past 20 years to a material class with enormous potential. Due to the modular design of MOFs from metal cations and organic linkers, it is possible to tailor the properties of the resulting extended frameworks by selecting individual components for the needs of specific applications. Careful selection of metal ions and organic molecules with functional groups makes it possible to adjust and influence the pore system, the type of linkage and the physical and chemical properties of the framework. In the present work, the main aspect was the preparation of a novel MOF with high adjustability of its properties, based on light-induced postsynthetic reactions on the organic linker molecules. The framework is composed of Zr4+ ion-based oxo clusters and benzophenone-4,4'-dicarboxylate anions (bzpdc2–) as linker molecules. The benzophenone unit is able to react with any molecule that contains C–H bonds after excitation with photons, resulting in a covalent bond. This reaction opens the possibility of changing the properties of the framework post-synthetically. The novel Zr-bzpdc MOF is chemically and thermally stable and has a two-dimensional structure, which opens up the possibility of obtaining nanometer-thin layers through delamination methods. Further studies have shown that the adaptation of the surface chemistry by the linkage of molecules has a huge impact on the dispersibility in polar and nonpolar solvents. Furthermore, the direct polymerization of a conductive polymer, starting from the surface of the MOF, yields electrically conductive composite materials. Interestingly, in the course of this study it was found that the unmodified MOF is also electrically conductive. Furthermore, a systematic study concerned with the postsynthetic modification of Zr-bzpdc-MOF with alkanes and alcohols of different chain lengths was undertaken. This showed that small hydrophilic molecules react with all linker molecules throughout the crystal, whereas molecules with longer chain lengths and thus more hydrophobic properties only modify the surface of the MOF crystals. The so far largely unaccessed approach to employ specific photochemical modification reactions on the linker molecule the modification of MOFs has been developed here to upgrade the novel Zr-bzpdc-MOF to an unexpectedly versatile compound which is further augmented by the possibility to obtain nanosheets of this substance. Various pathways to adapt this material to specific applications have thus been opened up.