The dynamics of new self-assembled porous materials

Nikolayenko, Varvara I. (2015-12)

Thesis (PhD)--Stellenbosch University, 2015.

Thesis

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.

AFRIKAANSE OPSOMMING: Die doel van hierdie studie was om 'n wye verskeidenheid kristallyne tweede- en derde-geslag metaal-organiese materiale voor te berei, om gas sekwestrasie, stoor vermoëns, sowel as gas uitruiling en sorpsie geïnduseerde dinamika in die vaste toestand te bestudeer. Hoofstukke I en II bevat 'n oorsig van die relevante literatuur en 'n beskrywing van die eksperimentele tegnieke in hierdie werk. Hoofstuk III beskryf die verskeidenheid metaal-organiese raamwerke (MOFs) wat gesintetiseer is vanuit 'n klein seleksie piridiel gefunksionaliseerde ligande (met verskillende lengte en rigiditeit) gekombineer met 'n reeks karboksielsure (ook met verskillende lengte en rigiditeit) in die teenwoordigheid van een van vier oorgangsmetale. Een en twintig verskillende MOFs met verskillende metaal koördinasie modusse, graad van interpenetrasie, oplosmiddel toeganklike ruimte en gaste was gesintetiseer. Vier van hierdie MOFs toon enkel-kristal tot enkel-kristal (SC-SC) transformasies, insluitend aktivering, gas sorpsie en die uitruil van hulle oplosmiddel. Hoofstuk IV fokus op twee isostrukturele MOFs. Die monokromiese sink-bevattende raamwerk ondergaan SC-SC gas uitruiling, wat dui op 'n matige voorkeur vir para-xileen, terwyl gas sorpsie eksperimente ‘n duidelike voorkeur vir koolstofdioksied toon. Die pleokroïede kristalle van die kobalt analoog toon geen selektiwiteit teenoor xileen nie, terwyl gas sorpsie eksperimente ‘n voorkeur teenoor metaan etaan en koolstofdioksied vertoon. Kristallisasie van oplossings wat beide metale bevat (asook dieselfde ligande) lei tot die vorming van trikroïede vaste oplossings, en die kleur hiervan kan verander word deur die verhouding van die twee metale te wissel. Om die studie te voltooi, is 'n homoepitaksiale kristal gegroei. Hoofstuk V fokus op 'n aangepaste weergawe van die ligand wat in Hoofstuk IV beskryf is, met elektron-onttrekkende fluooratome wat op die sentrale fenielring geleë is. Gebruik van hierdie ligand het gelei tot die karakterisering van 'n tweevoud geïnterpenetreerde poreuse raamwerk, wat verskil van dit wat in die vorige hoofstuk beskryf is. Aktivering, sowel as gas uitruiling, lok slinger konformasie veranderings in die ligand uit. Sorpsie eksperimente toon aan dat die materiaal koolstofdioksied bo 'n paar lineêre alkane verkies. Reaksie met die chloriedsout van hierdie ligand het ‘n nuwe MOF opgelewer. Vervanging van kadmium met sink in die reaksie lewer 'n derde nie-geïnterpenetreerde raamwerk op, waar die ligand [2 + 2] siklo-addissie ondergaan het en in die anti-konformasie teenwoordig is. SC-SC aktivering, sowel as gas uitruilingseksperimente, het getoon dat hierdie raamwerk minder buigsaam as sy sink eweknie is. Blootstelling van die geaktiveerde materiaal aan koolstofdioksied dui op 'n verstommende opname van koolstofdioksied. Die byvoeging van DMSO tot die DMF oplossing het tot die vorming van nog twee isoskeletale strukture gelei, wat verskil in hul spaanwiel konstruksie, ligand konformasie en, as gevolg hiervan, ook in hul oplosmiddel-toeganklike leë ruimte. Hoofstuk VI beskryf die reaksie van vyf MOFs gebaseer op diarieleteen ligande met lig. Die kleurveranderinge en onderliggende strukturele veranderinge verskil van kristal tot kristal. Die eerste raamwerk is ‘n poreuse tweeledig geïnterpenetreerde MOF, wat 'n dramatiese kleur- en vormverandering tydens UV-bestraling ondergaan. Wanneer dieselfde kristal aan wit lig blootgestel word, keer die selgrense terug na dié van die oorspronklike vorm. Die geaktiveerde vorm van 'n tweede viervoudig geïnterpenetreerde hoogs poreuse MOF het 'n affiniteit vir koolstofdioksied, en toon ook 'n kleurverandering tydens bestraling aan. Die gebruik van 'n meer buigsame ligand het 'n tweevoud geïnterpenetreerde poreuse materiaal opgelewer wat by lae temperatuur geaktiveer word, maar nie ontruim kon word sonder verlies van kristalliniteit nie. Bestraling veroorsaak 'n kleur verandering, maar geen strukturele verandering nie. Die vierde raamwerk is 'n tweeledig geïnterpenetreerde struktuur wat ineenstort en 'n oënskynlik nie-poreuse struktuur vorm wanneer die oplosmiddel verwyder word. Bestraling van hierdie materiaal lei tot ring-sluiting van die diarieleteen ligand, wat met 'n kleurverandering gepaard gaan. Dit is opvallend dat al hierdie prosesse d.m.v. 'n SC-SC transformasie plaasvind. Dit is die eerste voorbeeld van die foto-geïnduseerde ring-sluiting van 'n diarieleteen ligand as deel van 'n MOF. Die vyfde raamwerk toon 'n hoë graad van interpenetrasie (viervoud), maar in teenstelling met al die vorige voorbeelde, veroorsaak bestraling met UV-lig geen sigbare kleurveranderinge en geen strukturele veranderinge nie.

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