No description available.

No description available.

<p> One process that has the potential of efficiently harvesting solar energy is singlet fission (SF), a process by which one photon of light can produce two excited states. Investigations of three different series of bistetracene (BT) were used to explore the effect of symmetry on the rate and driving force of SF, as well as the electronic coupling. A dimer and the corresponding monomer were used to build a synthetic infrastructure and explore preliminary photophysics. All of the dimers were connected by one to three norbornyl bridges and exhibit various amounts of electronic coupling.</p><p> The first section of this dissertation will discuss a series of cofacial BT dimers with C_2v symmetry that have one (BT1) to three (BT3) norbornyl bridges linking the two tetracene chromophores. Density functional theory calculations of BT1-BT3 were used to explore the SF driving force and the change in through-space versus through-bond contributions to the electronic coupling. The C_2v symmetry was found to be unprofitable to SF, but vibrations accessible to the ground state would break the C_2v symmetry. The thorough synthetic investigation of the monomeric tetracene-norbornyl bridge was developed to build a synthetic library that aided the synthesis of BT1, the first rigid SF dimer. Preliminary photophysics of BT1 and its monomer will also be described.</p><p> In the second portion of this work, the SF driving force, electronic coupling will be calculated for the second and third series of BT dimers, which are symmetry adaptations of BT1-BT3. In the second series, the orbital overlap of the norbornyl bridge and tetracene arms will be exploited by changing how the bridge and arms are connected to make dimers of C₂ and C_s symmetry. In the final series, a heteroatom substitution of BT1 creates a series of C₂ and C_s dimers that can be built using the synthetic infrastructure developed above. Both symmetry adapted series of BT dimers were found to lead to an increase in electronic coupling, which is expected to be productive for SF.</p>

No description available.

No description available.

This thesis is concerned with the development of physico-chemical techniques to characterise the properties of water soluble non-starch polysaccharides (s-NSP). The first part of this project was to characterise the structure and solution properties of s- NSP extracted from a plant, Detarium senegalense Gmelin. The extracted s-NSP was analysed by GLC and found to be similar in structure to tamarind xyloglucan. This was confirmed by comparing the oligosaccharides released on enzymatic digestion with those obtained from tamarind xyloglucan. Histochernical examination of detariurn seed showed the presence of xyloglucan in highly thickened cell walls. The intrinsic viscosity of detariurn gum was found to be high, indicating that the sample was of high molecular weight. The semi-dilute solution characteristics investigated by steady and dynamic shear rheometry, suggest that detarium gum is a well behaved linear polymer entanglement system. Static light scattering was also successfully applied to examine the molecular weight and architecture of the detarium xyloglucan macromolecules. The physiological behaviour of s-NSP when consumed is critically dependent on their physico-chemicapl roperties,i ncluding the rate and degreeo f hydration. In the second part of the project a method for determining the hydration rate of a powdered form of s- NSP was developed. A logarithmic model for describing the hydration kinetics of guar gum was also establishedT. his model was used to investigatet he effects of polymer concentration (C), molecular weight (M), and particle size on the hydration rate of guar gum. The results showed that there was a significant inverse relationship between hydration rate and M and mean particle size, respectively. The hydration rate increased with increasingC at a low concentrationr ange,b ut decreasedw hen C>1.2% (w/w). The pH in the lumen of the gastrointestinal tract of human is acidic, usually between 2.0 and 1.5 after a meal. The stability of guar gum in acidic conditions was investigated at temperatures 25,37 and 50'C. The results indicated that there was unlikely to be significant acidic degradation of guar gum in the human gastrointes final tract. The pH condition was also found to influence the hydration rate of guar gum. In general, the hydration rate was lower in an acid environment than it was in neutral conditions

In this projeot, it has been demonstrated that ouprous iodide - sulphonium iodide double salts, similar to the silver iodide systems previously studied by other workers, are sui table for use in ambient temperatuI9 batteries for low cuzrerre applications. A number of potential solid electrolytes, in which the mobile speoies is oaloium, have also been investigated, and areas for further work have been defined. The I9lavent seotion of the thesis is confidential as patent proceedings a~ possible. . The project has involved not only a search for new electrolytes, but also the development of a search strategy. In view of the limitations of di rect conductivity measurements on compaoted materials, arising from electrioal contact probl~ms, ConsidllJrable use has been made of Lndirac t methods. A critical ~view of standard electroohemical techniques, often adapted from aqueous methods, has been oarried out. This has highlighted shortcomings in the measurements of such parameters as electronic conductivi ty, where the self-discharge rate, a parameter used by battery technologists, has been found more meaningful than the results obtained fran l-/agners blocking eleotrode teohn1que. X-ray diffraction (XRD), and differential thermal analysiS (MA) have been used to disprove the commonassertion that the effect of addition of organio dopant materials to silver iodide has the effeot of stabilizing the high temperature high conductivity alpha phase. CuI based eleotrolytes have been found to differ significantly from the AgI analogues in several respects. No simple structural cri terion, such as the Group Weighting Coefficient for AgI systems, could be found to prediot the effect of addition of organic sulphonium iodide dopants. Battery cells based on CuI did not give the expected the rmodvnnmi o open circuit voltap:e (OCV), and three electrode meaauremen t a failed to elucidate this anomaly. The possible involvement of the cupric ion was explored in several ways, including the first application of Auger electron spectroscopy for this type of electrolyte system. The battery discharge characteristics showed a fairly low efficiency, but indirect evidence including the inadvertent involvement of a novel solid state aurous system, pointed to the majority of the anode material being unavailable for reaction.

No description available.

Non-aqueous dispersions of colloidal gold and silver clusters have been prepared by the reduction of HAuCl4 and AgN03 solutions with NaBH4 in the presence of protecting agents such as quaternary ammonium salts and alkanethiol. The resulting clusters were then investigated both in solution, and in thin film form when deposited on TEM grids. Thiol-derivatised particles form dark brown residues upon removal of the solvent which can easily be re-dispersed in low dielectric solvents such as toluene or pentane. Such exceptional stability has led to the name cluster compounds. The growing interest in nanophase systems has been motivated both by scientific interest in their unique opto-electronic properties which show a marked deviation from bulk behaviour, but also from their potential applications in the fields of opto-electronic devices. Such applications will inevitably require long range order and control of such nanoparticle assemblies and it is with this in mind that the present work is based. A drop of such a colloidal solution when allowed to evaporate onto a carbon coated grid results in the formation of regular self-assembled structures which were then examined using transmission electron microscopy (TEM). Monodisperse gold and silver particles show a tendency to self-assemble into highly ordered pseudo-hexagonal closepacked rafts, in which the interparticle separation can be varied according to the size of the stabilising ligand attached to the gold or silver surface. It is also shown that gold particles can, in addition to conventional f.c.c., h.c.p. stacking in 2D structures, form unusual non-closed packed ring and line like structures. Ordered superlattices composed of bimodal particle size distributions of either gold or silver, or gold-silver binary mixtures, have also been observed. Such structures represent the first example of superlattice ordering on the nanometre scale which, up to now, has only been observed in micrometre scale colloidal crystallites and in atomic scale intermetallic alloys. Moreover, it is shown how the underlying principles for intermetallic alloy formation can readily be extended to explain the formation nanoparticle superstructures in terms of well known phase rules, thus drawing a comparison between the two ordered states.

<p> Due to high sensitivity of biomolecular systems to the electrostatic environments, coupled treatment of conformational and protonation equilibria is required for an accurate characterization of true ensemble of a given system. The research presented in this dissertation examines the effects of conformational and protonation equilibria of varying extent on diverse aspects of computational biomolecular modeling, as introduced in Chapter 1. The effects of protonation and stereoisomerism of two histidines on virtual screening against the M. tuberculosis enzyme RmlC are presented in Chapter 2. In Chapter 3, conformational flexibility of three M. tuberculosis prenyl synthases is probed using molecular dynamics simulations, with implications for computer-aided drug discovery effort for the new generation antibacterial and antivirulence therapeutics. Chapters 4 and 5 consider the conformational and protonation equilibria simultaneously by utilizing constant pH molecular dynamics, in which fluctuations in both conformation and protonation state are possible. In Chapter 4, a computational protocol utilizing constant pH molecular dynamics to compute pH-dependent binding free energies is presented. The methodology is further applied to protein-ligand complexes in Chapter 5, where the thermodynamic linkage between protonation equilibria, conformational dynamics, and inhibitor binding is illustrated.</p>