This thesis presents a spectroscopic and computational study of a variety of substituted oligothiophenes. The purpose of this study was to elucidate the structural properties of their neutral, oxidised and excited states. This was accomplished using a number of spectroscopic methods, including electronic absorption, resonance Raman and fluorescence spectroscopy. Analysis of the data provided by these methods was aided by the application of density functional theory (DFT) to the neutral and charged species, allowing conclusions regarding bond length changes and molecular orbitals to be drawn. Advancing the understanding of the structural properties of conductive materials such as oligothiophenes is important due to their potential application in plastic electronic devices (organic solar cells, field effect transistors and light emitting diodes, for instance) and the need to clarify the charge transport mechanism. The compounds examined in this thesis are primarily based on 3� styryl-substituted terthiophene. Substituted derivatives of this molecule include those with groups on the para position of the phenyl ring or methyl α,α� end caps. Larger ethenyl-aryl groups were also investigated, as were analogous thiophenes and sexithiophenes. Indeed, the 3�-substituted sexithiophenes formed an integral part of this investigation since they rapidly form in the head-to-head orientation from the oxidation and consequent [sigma]-dimerisation of the substituted terthiophenes. DFT calculations on the sexithiophene charged species have indicated the presence of a full polaronic or bipolaronic structural defect in the centre of the thiophene backbone, as defined by the reversal of the CC bond length alternation to create a domain of quinoidal bond sequence. However, the structural defect of the styryl sexithiophenes is more strongly localised than that observed for unsubstituted sexithiophene; indeed, the charged defect appears to be confined by the positions of the styryl substituents. This defect confinement along the thiophene backbone is particularly apparent in the styryl sexithiophene dications. Electronic absorption and resonance Raman spectroscopy have been applied to the oxidation products of the styryl terthiophenes. Only sexithiophene charged species are observed when the α termini are unsubstituted; these include sexithiophene radical cations, [pi]-dimers (a stabilising face-to-face interaction between two radical cations) and dications. The resonance Raman spectra of these charged species are characterised by a very intense symmetrical CC stretching mode of the thiophene inter-ring bonds, which is also evident in the theoretical spectra. The extensive differences between the styryl and unsubstituted sexithiophene dication Raman spectra have been attributed to defect confinement: the strongest Raman band of the unsubstituted sexithiophene dication is due to a vibrational mode localised over the entire sexithiophene backbone while that of the styryl sexithiophenes is localised over the central two thiophene rings - the same area occupied by the confined defect. A combined steady-state and picosecond time-resolved emission study on the styryl terthiophenes revealed that by judicious choice of the para R group in styryl terthiophenes it is possible to tune the nature of the first excited singlet state from [pi],[pi]* to charge transfer character. Charge transfer states may offer possible strategies for solar cells in which charge separation is a key step.

Report (M.S.)--Virginia Polytechnic Institute and State University. M.S. 1991. / Vita. Abstract. Includes bibliographical references (leaves 99-101). Also available via the Internet.

M.Sc. / The development of synthetic lubricants was the logical result of high demands being made by modern technology, the exhaustion of natural mineral oil sources and the search for environmentally friendly substances. Alpha olefins are products of the Sasol Fischer-Tropsch process and are currently part of the wider petrol pool. Substantial value can be added to the alpha olefins by the production of high value polyalphaolefins (PADs). This project was aimed at finding explanations for previous unsuccessful attempts to produce acceptable PADs from Sasol feedstreams. High priority was given to the modification of experimental procedures to produce a synthetic oil that conforms to specific standards. Oxygenates, aromatics and "other than alpha" olefins (branched, internal and cyclic olefins) were identified as problem components in the Sasol feedstream. These compounds led to early termination of oligomerisation and in the presence of BF3 , unwanted compounds were even incorporated into the PAD products. The feedstream was thus purified by distillation and a MeDH/H2D extraction procedure. As a result, better reaction control (with regard to oligomer distribution) was gained and the product quality improved. Reaction conditions were optimised and basic reaction models (to predict oligomer distributions) were developed. It became clear however, that product quality did not only depend on oligomer distribution, but also on the specific structures present in the various oligomers. GC analyses showed that each oligomer consists of a great number of isomers - even more than can be explained in terms of the classic cationic mechanism for oligomerisation. GC-MS analyses could not successfully distinguish between different branched isomers, because of the complexity of the mixture. Summary The question was asked whether the complex mixture has already been formed during the primary oligomerisation process, or at a later stage by skeletal rearrangements. Research work indicated that one can distinguish between a primary (normal oligomerisation) process, and a secondary process (skeletal rearrangement). It became clear that the secondary process is favoured by specific reaction conditions (e.g. long reaction times and high co-catalyst concentrations). The formation of isomers increased under these conditions and product quality was adversely affected. The possibility to produce PAD products from Sasol feedstreams identical to commercial products still exists, and necessitate further research work.

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published_or_final_version / abstract / Chemistry / Doctoral / Doctor of Philosophy

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PhD, Faculty of Science, University of the Witwatersrand

by Chi-wai Wan. / Thesis (Ph.D.)--Chinese University of Hong Kong, 1999. / Includes bibliographical references (p. 129-136). / Electronic reproduction. Hong Kong : Chinese University of Hong Kong, [2012] System requirements: Adobe Acrobat Reader. Available via World Wide Web. / Mode of access: World Wide Web. / Abstracts in English and Chinese.