Browsing by Author "Claassens, Isabella"

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    Structure-property relationships in stimulus-responsive cadmium- and zinc-based metal-organic framework materials
    (Stellenbosch : Stellenbosch University, 2020-03) Claassens, Isabella; Haynes, Delia A.; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept. of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: The study of flexible MOFs or soft porous crystals is an exciting and fast-expanding area of MOF research. These materials respond dynamically to external stimuli, including temperature, mechanical pressure, guest sorption or light. Flexible MOFs are prime candidates for gas separation and storage, catalysis and sensing applications. Thus far the focus has largely been on the creation of new MOFs. However, focusing on fine-tuning and controlling the properties of these materials can produce advanced materials with potential applications in several areas of industry. This study investigates the structure-property relationships of three related flexible MOFs to explore ways in which to tune and control their response to specific stimuli. The effects of guest inclusion, temperature and light on the structure of the material are studied. The first two studies place special focus on photochemical cycloaddition and exploring means with which to control the outcome of the reaction. We describe the use of guest exchange as a means to control the position of photochemical [2+2] cycloaddition between bpeb ligands in a new Cd (II) MOF. The selective synthesis of one of two different isomeric products is demonstrated by exchanging one solvent in the channels of the MOF for another. The nature of the guest controls the conformation of the organic linker (bpeb) within the framework, which has a direct effect on the regioselectivity of the cycloaddition reaction. The study demonstrates how a response to different guests in a flexible MOF can be utilised as a synthetic tool and is the first example of the synthesis of both isomers of the cyclized bpeb dimer in the same framework. In the second study temperature change is employed as a stimulus control regioselectivity of the photochemical cycloaddition reaction in the same Cd(II) MOF. The isomeric product formed depends on the temperature at which irradiation is carried out. A rare temperature-induced phase transition alters the conformation of the bpeb ligand in the MOF, and thereby the position at which cycloaddition occurs. The phase transition is fully described and characterised using variable temperature X-ray diffraction techniques. This work also highlights the multistimulus responsive nature of this Cd(II) MOF, which responds to temperature as well as solvent exchange and light. The final section describes a new highly flexible four-fold interpenetrated fluorinated MOF that undergoes breathing and subnetwork displacement modes of flexibility to give interesting gas sorption behaviour. The effect of fluorinated substituents on the gas sorption behaviour is investigated by comparing it to the non-fluorinated isoreticular analogue. Sorption analysis reveals significant changes in gate-opening pressure as well as the degree of hysteresis observed between the two frameworks. In situ SCXRD techniques and computational analysis are employed to describe and identify the interactions responsible for the change in behaviour. This study shows that fluorination of pillar linkers is a viable pathway to developing frameworks that exhibit favourable sorption profiles for future studies. These studies improve the understanding of how to control or fine-tune MOFs that change their properties in response to particular stimuli.