Study of ternary complexes of cucurbit[8]uril and their applications in self diagnostic composites

Das, Anjali Devi

Study of ternary complexes of cucurbit[8]uril and their applications in self diagnostic composites

Doctoral Thesis

2020

Abstract

Self-assembly in supramolecular systems has profound implications on creating the next generation of smart materials, with potential applications in self healing, energy storage, stimuli responsive hydrogels and sensors. Macrocyclic receptors that form host guest complexes with organic molecules, provide a unique platform for the formation of tailor made materials with programmable properties for specific applications. The cucurbiturils are one such class of receptors that have garnered increasing interest in the last decade due to their unique guest binding abilities. Cucurbit[8]uril, as one of the larger homologues, is a particularly appealing host molecule for functional materials, capable of forming ternary complexes with suitable guests. The main scope of this thesis is to design and employ host guest systems based on the macrocyclic host cucurbit[8]uril as supramolecular probes to identify early stage damage in carbon fibre reinforced composite materials. Chapter 2 outlines the main project, the development of a novel system for early stage damage detection in a fibre reinforced polymer composite. CB[8] has the ability to form heteroternary complexes of high stability of two different guests in polar environments. In the reported system, CB[8] encapsulates two pendant molecules in the matrix, a donor and a fluorescent acceptor forming a complex by stabilising a charge transfer pair within its cavity. The emission of the probe is suppressed within the ternary complex via photo induced electron transfer. The application of stress causes the weak supramolecular link to break apart, and in turn the fluorescence of the probe is reinstated. Several ternary complexes of the host molecule and fluorescent guest molecules were investigated and a Perylene imide (PER) - Cucurbit[8]uril (CB[8]) based system was found to be best suitable as a strain sensor. Studying the photophysical behaviour of the complex showed that the fluorescence of the PER - CB[8] complex was effectively switched off upon the complexation of a second guest molecule such as dibenzofuran or azobenzene. The supramolecular complex cross-linking the polymer chains was seen to provide a fluorescence response induced by strain even if present in a very low amount of 10−6 mol kg−1 , preserving the mechanical characteristics of the matrix. In addition to uniaxial compressive and tensile testing, the specimens were subjected to fatigue to assess the performance of the material under similar conditions during actual use. The ability to detect fatigue damage is especially important, due to it being one of the major causes of in-service failure of materials. Carbon fibreepoxy composite materials are widely used in the structures of aircraft, robots and other machines because of their high specific strength. Self-diagnosis is potentially an important tool for Non-Destructive Evaluation (NDE) of such composite materials used for purposes where structural integrity is absolutely essential. This work has been recently published in ACS Applied Polymer Materials in 2019 and was recognised by ACS as Editor's Choice. While studied as a versatile host molecule for molecular recognition, cucurbit[8]uril also offers a platform for the development of supramolecular organic frameworks and the formation of porous materials. In this context, the fundamental study of the stoichiometry and geometry of CB[8], its complexes in the solid state and the exploration of new binding motifs is pivotal. Indeed, due to their differential solubility, CB[8] complexes have proved to be difficult to characterise in the solid state, and limited examples are present in literature. Chapter 3 outlines the synthesis of nanotubular CB[8] assemblies. Organic molecular porous materials, in particular materials with a one-dimensional nanochannel structure have found a diverse range of applications in separation, energy storage, and adsorption. Three tubular frameworks of CB[8] were synthesised through chaperone induced methods and metal coordination. Finally, in a bid to develop better probes for self diagnostic composites, different CB[8] complexes with varying stoichiometry were studied in solution and in the solid state. Several CB[8] host guest systems were extensively studied with the aim to improve upon the damage reporting system reported in Chapter 2, by two main approaches. Firstly, there is an interest to move to near IR dyes due to their improved depth-penetration of light, allowing visualisation of damage deeper within the composite material. Secondly, moving from a 1:1:1 heteroternary complex by accommodating two different guests, to a 2:1 homoternary complex by accommodating two identical guests should provide an advantage by reducing the complexity of the system. Chapter 4 describes the study of the interaction of a series of potential dye molecules with CB[8] that fulfil these categories with the aim of developing improved supramolecular probes for self diagnostic composites. In addition, this chapter also reports three novel crystal structures of CB[8] complexes of varying stochiometry, from a 2:1, 1:1 to an unusual 1:2 complex with respect to the host. Overall, this thesis deals with the design and characterisation of CB[8] based host guest complexes, the study of their photo-physical behaviour and binding in solution, and exploits this behaviour in the design and implementation of a novel approach to the technologically relevant field of damage detection in composites.

Keywords

Chemistry

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PhD / Doctoral

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