Synthesis and characterization of metal–organic frameworks for sorption and luminescence studies

Abstract

Metal–organic frameworks (MOFs) are crystalline hybrid materials formed by the coordination of metal ions with multidentate ligands. They exhibit well-defined architectures, high structural robustness, and permanent porosity, arising from the periodic arrangement of metal nodes interconnected by organic linkers. MOFs offer exceptional potential for gas capture and molecular separations due to their modular architectures and tunable pore environments. However, establishing clear structure–property relationships that link framework flexibility, metal identity, and adsorption behaviour under ambient conditions remains a significant challenge. This thesis addresses this challenge through the rational design, synthesis, structural characterization, and comparative investigation of mixed-ligand MOFs constructed from diimide-based N-donor ligands and aromatic carboxylate linkers, with a central focus on isostructurality, framework dynamics, and post-synthetic metal exchange. Three diimide-based N-donor ligands and two carboxylate donor ligands were successfully synthesized and used together with commercially available carboxylate ligands in solvothermal reactions to yield thirteen crystalline materials. These include nine new and four known MOFs. SCXRD confirmed the formation of extended three-dimensional (3D) architectures dominated by paddle-wheel secondary building units and one-dimensional (1D) channels, while PXRD confirmed bulk-phase purity. To decouple metal identity from framework topology, post-synthetic metal exchange (PSMet) was employed as a modular design strategy. Application of PSMet to selected parent MOFs yielded 28 metal-exchanged derivatives, including 6 complete and 22 partial exchanges, yielding mixed-metal frameworks inaccessible via direct solvothermal synthesis. Structural integrity following metal exchange was confirmed by PXRD, SEM–EDX, and FTIR analyses. Controlled metal substitution enabled systematic tuning of framework flexibility, adsorption reversibility, and guest–framework interactions. In xylene inclusion studies, PSMet reversed adsorption selectivity relative to the parent frameworks, demonstrating the sensitivity of aromatic hydrocarbon discrimination to subtle changes in metal coordination environments. A key contribution of this work is the deliberate construction of isostructural MOF families to enable controlled, comparative studies. In Chapter 3, a four-member isostructural MOF series synthesised via a direct solvothermal method, together with additional PSMet-derived analogues, was designed to isolate the effects of metal identity and ligand environment on framework flexibility and ambient-temperature CO₂ sorption, while maintaining an invariant topology. VT-PXRD and TGA revealed framework-dependent flexibility and thermal stability across these systems, spanning rigid lattices to dynamic materials that undergo reversible structural transformations. Correspondingly, CO₂ adsorption studies showed pronounced metal-dependent behaviour. Rigid frameworks exhibited Type I uptake profiles, whereas flexible analogues displayed cooperative Type IV gate-opening responses, highlighting a direct correlation between metal substitution, structural dynamics, and adsorption mechanism. Beyond these isostructural platforms, Chapter 4 expands the chemical scope of the thesis by synthesizing 3D MOFs, including an isostructural pair, enabling exploration of ligand-centred luminescence and broader structure–function relationships. In Chapter 5, a two-member isostructural family was employed to investigate metal-dependent xylene isomer inclusion and selectivity, demonstrating that substitution of the metal centre alone can modulate host–guest interactions and adsorption behaviour. The findings confirm that selective uptake and release profiles arise from subtle differences in metal–framework electronic environment rather than changes in topology, underscoring metal identity as a decisive parameter for tuning aromatic isomer recognition within structurally conserved MOFs. This thesis establishes diimide-based mixed-ligand systems as robust and tunable platforms for the systematic investigation of framework flexibility and metal-dependent functionality. By combining designed isostructural families with post-synthetic metal exchange, this work provides insights into structure–property relationships relevant to gas adsorption, aromatic hydrocarbon separation, and the rational design of responsive crystalline materials.