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

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

Performance of low-cost radio-over-fibre systems

The research presented in this thesis has focused on the use of radio-over-fibre (RoF) technology for improving the quality of mobile/wireless coverage within buildings. The primary aim was to minimise overall system costs by employing commercially available components. For this purpose, a distributed antenna system using low-cost vertical-cavity surface-emitting lasers (VCSELs) operating at 850 nm and multimode fibre (OM1/OM2) has been designed and implemented. A detailed link budget analysis has been performed which allows for the prediction of maximum achievable ranges for the transmission of different wireless systems over the RoF link, while taking into account practical restrictions that are important for bidirectional link operation (e.g. crosstalk and noise emissions). The analysis indicates that when optimised component parameter values are utilised, reasonable cell sizes may be achieved for systems such as GSM, UMTS and WLAN. The link budget predictions were verified for the transmission of ‘real’ WLAN signals over the designed RoF link and complete coverage of a standard office room was demonstrated. The majority of previous research into low-cost RoF links has primarily involved characterisation of the optical path. In this investigation, signal strength and throughput measurements were conducted for the combined optical and wireless paths in order to verify the operation of the complete fibre-fed WLAN system. Throughput values close to 5 Mbps for IEEE 802.11b and 20 Mbps for IEEE 802.1 lg were recorded. Additionally, the transmission of different combinations of emulated mobile/wireless systems in a dual-band configuration over another radio-over-fibre link (also employing 850 nm VCSELs and MMF) has been successfully demonstrated. Experimental investigations have been carried out for the first time to analyse the performance of WLAN-over-fibre networks using different MAC mechanisms such as fragmentation and the use of RTS/CTS in the presence of hidden nodes. Finally, scenarios involving multiple clients accessing a single remote antenna unit and multiple remote antenna units being fed by a single access point have been demonstrated

Probing the hot circumgalactic medium of external galaxies in X-ray absorption II: a luminous spiral galaxy at z≈0.225z\approx 0.225

The circumgalactic medium (CGM) is the most massive baryonic component of a spiral galaxy, shock heated to about $10^6$K for an $\rm L^{\star}$ galaxy. The CGM of the Milky Way has been well-characterized through X-ray absorption line spectroscopy. However, the paucity of bright background sources makes it challenging to probe the CGM of external galaxies. Previously, using broad OVI absorption as a signpost, we successfully detected the CGM of one galaxy in X-rays. Here we report on the detection of the OVII $K\alpha$ absorption line at the redshift of a spiral galaxy at $z\approx0.225$ using 1.2 Ms of Chandra observations. This is a robust detection, clearly showing the presence of the hot gas. The mass in the hot phase is at least an order of magnitude larger than that in the cooler phases detected in the UV. The presence of hot gas $116h^{-1}$kpc from the center of this galaxy provides credence to the existence of the extended CGM of the Milky Way. There has been a report of the detection of OVII absorption from the warm-hot intergalactic medium in this sightline using stacking analysis on an older dataset. We argue that the absorption line is from the CGM of the $z\approx0.225$ galaxy instead.Comment: To be published in MNRAS Letter

Towards continuous-time MPC: a novel trajectory optimization algorithm

This article introduces a numerical algorithm that serves as a preliminary step toward solving continuous-time model predictive control (MPC) problems directly without explicit time-discretization. The chief ingredients of the underlying optimal control problem (OCP) are a linear time-invariant system, quadratic instantaneous and terminal cost functions, and convex path constraints. The thrust of the method involves finitely parameterizing the admissible space of control trajectories and solving the OCP satisfying the given constraints at every time instant in a tractable manner without explicit time-discretization. The ensuing OCP turns out to be a convex semi-infinite program (SIP), and some recently developed results are employed to obtain an optimal solution to this convex SIP. Numerical illustrations on some benchmark models are included to show the efficacy of the algorithm.Comment: Accepted in IEEE Conference on Decision and Control (CDC), 202

The hot circumgalactic medium of the Milky-Way: new insights from XMM-Newton observations

We present XMM-Newton observations around the sightline of Mrk 421. The emission spectrum of the Milky Way circumgalactic medium (CGM) shows that a two phase model is a better fit to the data compared to a single phase model; in addition to the warm-hot virial phase at log ($T/$K) = $6.33_{-0.02}^{+0.03}$ , a hot super-virial phase at log ($T/$K) = $6.88_{-0.07}^{+0.08}$ is required. Furthermore, we present observations of five fields within $5$ degrees of the primary field. Their spectra also require a two-phase model at warm-hot and hot temperatures. The hot phase, first discovered in Das et al. 2019, appears to be widespread. By chemical tagging we show that emission from the supevirial phase comes from the L-shell transitions of Fe XVIII-FeXXII, and that the range of temperatures probed in emission is distinct from that in absorption. We detect scatter in temperature and emission measure (EM) in both the phases, and deduce that there is small-scale density inhomogeneity in the MW CGM. The emitting gas likely has higher density, possibly from regions close to the disk of the MW, while the absorption in the virial phase may arise from low-density gas extended out to the virial radius of the MW. The presence of the super-virial phase far from the regions around the Galactic center implicates physical processes unrelated to the activity at the Galactic center. Hot outflows resulting from star-formation activity throughout the Galactic disk are likely responsible for producing this phase.Comment: 18 pages, 10 figures, 4 tables, submitted to the Ap

Super-virial Hot Phase in Milky Way Circumgalactic Medium: Further Evidences

Recent discoveries of a super-virial hot phase of the Milky Way circumgalactic medium (CGM) has launched new questions regarding the multi-phase structure of the CGM around the Galaxy. We use 1.05 Ms of archival Chandra/HETG observations to characterize highly ionized metal absorption at z=0 along the line of sight of the quasar NGC 3783. We detect two distinct temperature phases with T$_1 = 5.83^{+0.15}_{-0.07}$ K, warm-hot virial temperature, and T$_2=6.61^{+0.12}_{-0.06}$ K, hot super-virial temperature. The super-virial hot phase coexisting with the warm-hot virial phase has been detected in absorption along only two other sightlines and in one stacking analysis. There is scatter in temperature of the hot as well as warm-hot gas. Similar to previous observations, we detect super-solar abundance ratios of metals in the hot phase, with a Ne/O ratio 2$\sigma$ above solar mixtures. These new detections continue the mystery of the mechanism behind the super-virial hot phase, but provide evidence that this is a true property of the CGM rather than an isolated observation. The super-virial CGM could hold the key to understanding the physical and chemical history of the Milky Way.Comment: 10 pages, 6 figures, 3 tables, accepted for publication in MNRA