Browsing by Author "Nikolayenko, Varvara I."

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    The dynamics of new self-assembled porous materials
    (Stellenbosch : Stellenbosch University, 2015-12) Nikolayenko, Varvara I.; Barbour, Leonard J.; Stellenbosch University. Faculty of Science. Dept of Chemistry and Polymer Science.
    ENGLISH ABSTRACT: principal goal of this study was to prepare a wide selection of crystalline second- and third-generation metal-organic materials in order to investigate guest sequestration and storage capabilities, as well as guest exchange and sorption-induced dynamics in the solid state. Chapters I and II contain a review of the relevant literature, as well as a description of the experimental techniques employed in this work. Chapter III describes the diversity of metal-organic frameworks (MOFs) obtained from a small selection of pyridyl-functionalised ligands (varying in length and rigidity) combined with a series of carboxylic acids (also varying in length and rigidity) in the presence of one of four transition metals. Twenty one different MOFs varying in metal coordination mode, degree of interpenetration, solvent-accessible space and guest were obtained. Four of these MOFs undergo single-crystal to single-crystal (SC-SC) transformations, including activation, gas sorption and solvent exchange. Chapter IV focuses on two isostructural MOFs. The monochromic zinc-containing framework undergoes SC-SC guest exchange, indicating a mild preference for para-xylene, while gas sorption experiments revealed a clear preference for carbon dioxide. The pleochroic crystals of the cobalt analogue showed no xylene selectivity, but gas sorption experiments revealed preferential selectivity toward methane carbon dioxide and ethane. Crystallisation from solutions containing both metals (and the same ligands) resulted in the formation of trichroic solid solutions, the colour of which could be fine-tuned by varying the ratio of the two metals. To complete the study, a homoepitaxial crystal was grown. Chapter V focuses on a modified version of the ligand described in Chapter IV, with electron-withdrawing fluorine atoms located on the central phenyl ring. Use of this ligand resulted in charaterisation of a twofold interpenetrated porous framework, differing from that described in the previous chapter. Activation, as well as guest exchange, elicited “crank-handle” conformational changes in the ligand. Sorption experiments showed the material to be selective for carbon dioxide over several linear alkanes. Reaction using the hydrochloride salt of this ligand yielded a novel interdigitated MOF. Substitution of cadmium for zinc in the reaction gave a third non-interpenetrated framework where the ligand has undergone [2 + 2] cycloaddition and is present in the anti conformation. SC-SC activation as well as guest exchange experiments revealed this framework to be less flexible than its zinc counterpart. Exposure of the activated material to carbon dioxide indicated substantial uptake of carbon dioxide. A mixture of the DMF and DMSO produced two additional isoskeletal structures differing in paddlewheel construction, ligand conformation and, as a result, in the solvent-accessible void space. Chapter VI describes the response of five MOFs based on diarylethene ligands to light. The structural changes underlying the colour change differ in each case. The first framework is a porous twofold interpenetrated MOF that undergoes change in colour and shape upon UV irradiation. When the same crystal was exposed to white light, the parameters revert back to those of the original form. The activated form of a second fourfold interpenetrated highly porous MOF showed an affinity for carbon dioxide, as well as a change in colour upon irradiation. Incorporation of a more flexible ligand gave a twofold interpenetrated porous material that showed low temperature activation but could not be evacuated without loss of crystallinity. UV irradiation induced a colour change but no structural change. The fourth framework is a twofold interpenetrated structure that collapses to a seemingly non-porous form upon removal of the solvent. UV irradiation of this material results in ring-closure of the diarylethene ligand, along with a colour change. Remarkably, all these processes occur in a SC-SC fashion. This is the first example of photo-induced ring closure of a diarylethene molecule incorporated into a MOF. The fifth framework exhibits a high degree of interpenetration (fourfold) but unlike all previous examples, irradiation with UV light induced no visible colour change and no structural change.