Abstract
Porous materials have sparked the interest of many researchers because of their extensive surface area and absorptive properties. Depending on their pore size and atomic composition, applications for such materials are vast, ranging from catalysis and wastewater treatment to gas storage and drug delivery.
Reticular chemistry is the widely followed blueprint for synthesis of porous framework materials, which involves combining well-defined nodes with linker units. The various methods for assembly of these building blocks are often separated into distinct research areas, such as metal-coordinate bonding, covalent bonds or non-covalent interactions.
Supramolecular-organic frameworks (SOFs) is the least explored branch and is predominantly comprised of frameworks that utilise hydrogen bonding to drive framework assembly (HOFs). This work strives to synthesise a porous material that self-assembles through ordered π- stacking interactions in three dimensions. Tetrahedral phosphonium nodes bearing four tert- butyl-functionalised hexa-peri-hexabenzocoronene (HBC) arms were predicted to assemble into face-to-face dimers via intermolecular π-interactions. Many HBCs form dense, extended columnar structures in the solid state due to strong π-stacking, but HBCs substituted with peripheral tert-butyl groups adopt a more limited π-stacking dimer motif. The dimerised π- stacking at four corners of the large phosphonium tetrahedron could drive the formation of a porous, diamondoid framework.
Synthetic approaches toward the target molecule were first tested by forming a phosphonium salt with one HBC arm to develop the best route, while simultaneously assessing the stability of the phosphonium core unit. The initial synthetic pathway was focussed on divergent expansion out from the phosphonium core. However, the testing revealed that in order to prevent degradation of the tetrahedral core under some reaction conditions, an integrated divergent/convergent approach was most suited to form the target molecule. The best synthetic results were produced from tests which incorporated a phenylene spacer unit between the phosphonium cationic core and the HBC.
Phenylene and biphenylene spacer-containing tetra-HBC phosphonium salts were successfully synthesised and subjected to a range of recrystallisation conditions in order to obtain a single crystal structure for X-ray diffraction analysis. Unfortunately, the biphenylene spacer-containing tetra-HBC compound exhibits poor solubility in common organic solvents, which makes forming crystals challenging. The derivative with phenylene spacers is soluble in common organic solvents, but despite numerous recrystallisation attempts, crystals big enough for analysis were not forthcoming.