Study of Ternary Complexes of Cucurbit[8]uril and their applications in Self Diagnostic Composites

Abstract

With its ability to form tunable ternary complexes that can affect the optical properties of the corresponding guests, Cucurbit[8]uril is an interesting platform to develop probes for use in smart responsive materials. One of the interesting applications of such probes is the ability to detect early damage, especially in load bearing composite materials used for high performance applications. Micro-damage when left unchecked can grow under subsequent stress, leading to catastrophic structural failure. The sensing of early stage damage via nondestructive methods is therefore critically important for the safe use of these materials. The main scope of my PhD thesis is to address this issue through the design and synthesis of supramolecular host guest systems and their study as cross linkers in polymeric materials to identify early stage damage in carbon fibre reinforced composite materials. We describe a novel system for early stage damage detection in a fibre reinforced polymer composite by the incorporation of supramolecular crosslinks into the matrix facilitated by the host molecule Cucurbit[8]uril (CB[8]). Cucurbit[8]uril, one the larger homologues of the cucurbituril series of macrocyclic hosts, has the ability to form heteroternary complexes of high stability in polar environments. CB[8] encapsulates two pendant molecules in the matrix, a donor and a fluorescent acceptor forming a ternary complex by stabilizing a charge transfer pair within its cavity. Within the ternary complex, the emission of the probe is suppressed via photoinduced electron transfer. Under the application of stress, the weak supramolecular link breaks apart and the fluorescence of the probe is reinstated. Several ternary complexes of the host molecule and fluorescent guest molecules were investigated and a Perylene imide (PMI)-CB[8] based system was found to be best suitable as a strain sensor. In addition to compressive and tensile testing, the specimens were subjected to fatigue testing to assess the performance of the material under similar conditions during actual use. The ability to detect fatigue damage is an important advantage as it is one of the major reasons of in-service failure of materials. Carbon fibre-epoxy 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. 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.Three tubular frameworks of CB[8] were synthesised through chaperone induced methods and metal coordination. Finally 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. In addition 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 are also reported. 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.