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Title: The use of microscopy and spectroscopy to probe the dynamic behaviours of metal-organic frameworks and their guests
Author: Tansell, Alexander
ISNI:       0000 0004 8508 8290
Awarding Body: Cardiff University
Current Institution: Cardiff University
Date of Award: 2019
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This thesis describes the stimuli-responsive nature of metal-organic frameworks to light, solvent introduction and solvent loss. Chapter 1 introduces metal-organic frameworks, highlighting the different components of the materials, and reviewing historic work into the development of stimuli-responsive MOFs. Approaches to post-synthetic modification (experimentally undertaken in Chapter 2) are introduced. Specialist experimental techniques and various approaches to the analysis of infrared spectra (used in Chapters 3 and 4) are also defined. Chapter 2 describes how optical spectroscopy and microscopy has been employed to track fluorescent guest molecules through framework pores. As part of this work, size-optimised single crystals of NOTT-100 and Sc2BDC3 were synthesised. In the process of investigating crystal size, damage to crystals upon exposure to the SEM sample chamber vacuum was observed. The effect of common laboratory solvents on crystal quality was qualitatively investigated, and solvent acceptor number and molar volume were found to correlate to crystal damage. Reaction conditions for the post-synthetic modification of frameworks with a monocarboxylate-functionalised dye were established through the use of UV-visible (UV-VIS) absorption spectroscopy and confocal laser scanning microscopy (CLSM). Frameworks were post-synthetically modified with both a spirooxazine-based and a hydrazone-based photoswitch. Solid-state photoswitching of the spirooxazine-based photoswitch was observed during in situ CLSM experiments and on 290 nm irradiation by UV-VIS-diffuse reflectance spectroscopy. The adsorption capabilities of modified materials were measured using N2 sorption isotherms, and modification of materials with dyes was found to hinder the adsorption of guests into frameworks in all cases. Chapter 3 investigates the diffusion of therapeutic agents from MOF single crystals and bulk powders using in situ infrared microscopy and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). A sophisticated experimental technique was devised to establish the kinetics of guest loss from MOFs into a volatile solvent environment. The loss of caffeine from MIL-68(In) into acetonitrile (MeCN) was tracked at six temperatures, and an Ea of +144 kJ mol-1 (calculated error < 1 kJ mol-1) determined for the diffusion process. The loss of carbon monoxide (CO) from fully-desolvated NOTT-100 was tracked at low temperatures. Analysis of spectra confirmed the loss of CO from different Cu-paddlewheel sites to be interconnected processes, and not occurring in isolation from one another. Chapter 4 employs IR microspectroscopy to probe the structural rearrangement of a solvent-switchable, breathing framework, SHF-61. Crystallographic experiments have reported that desolvation from CHCl3 and DMF led to open- and closed-pore structures respectively. In this thesis, examination of the (NH) stretch using IR microspectroscopy was able to track the time-resolved pore-opening process during resolvation with DMF, identifying four states of the pore structure and monitoring their relative abundancies during the resolvation experiment. The open-pore structure was resolvated with and subsequently desolvated from: CHCl3, cyclopentanone and DMF. The rate of resolvation and subsequent desolvation was elucidated, using only (NH) stretches and respective solvent bands. The response of the material to CO2 loading was investigated, and a gas uptake isotherm shape was recreated for one single crystal of material.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available