Towards Controlled Crystallisation via Confinement and Epitaxy

Abstract

Crystallisation is crucial across industries, including food and pharmaceutical, and consists of two steps – nucleation and crystal growth. However, the formation mechanisms of sub-critical nuclei remain elusive despite knowledge of the shape and size of critical nuclei for different materials. This thesis aims to gain understanding of the mechanisms of sub-critical nuclei formation by using single crystal X-Ray diffraction to visualise small molecular clusters confined in metal-organic frameworks (MOFs), termed Cluster Confining MOFs (CCMs). To achieve this, previously reported isostructural lanthanide-based frameworks, herein labelled CCM-1–4, exhibiting considerably large pores and high crystallinity were synthesised to be used as confining matrices. This works shows that while the intermolecular interactions of molecules inside the framework pores can mimic those in bulk, the shape of the confined clusters are significantly different to their bulk equivalents. For instance, 1,2,3-triazole crystallises in a 1:1 ratio of its 1H- and 2H-1,2,3-triazole tautomers in bulk and forms similar clusters inside the pores of CCM-1, maintaining hydrogen bonding and π–π interactions. This work also introduces a novel interpenetrated zinc(II)-based MOF CCM-5 with a complex topology and a larger void volume compared to those in CCM-1–4 MOFs, potentially accommodating larger molecular clusters. However, challenges in reproducing the synthesis of CCM-5 precluded these studies. Furthermore, this work explores controlling crystallisation outcomes in coordination complexes through crystal welding, targeting the synthesis of compounds with varied metal centres and halide ligands. While the epitaxial growth attempts were unsuccessful, this method has been reported to be sensitive to specific experimental parameters and thus would require further investigations. Overall, this work highlights the potential of high-crystallinity MOFs to provide novel findings regarding the formation of sub-critical nuclei and uncovering novel polymorphs.