School of Chemistry - Theses
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ItemFunctional conjugated polymers: from design to devices( 2015)Conjugated polymers are among the most promising materials for future technologies. These organic materials combine many of the properties of conventional polymers with the behaviour of inorganic semiconductors. The low-cost and solubility of many conjugated polymers make them attractive materials for use in future electronic and optoelectronic devices, such as field-effect transistors (FETs), polymer light-emitting diodes (PLEDs) and organic solar cells (OSCs). However, both design and synthesis of conjugated polymers with well-defined structures involve many intellectual and technological challenges. Although conjugated polymers of simple aromatic monomers have been studied in great detail, to date there is no sound methodology in rationalised design and synthesis of functional conjugated polymers. Therefore, controlling and fine-tuning the properties of conjugated polymers remains rather difficult to achieve, and thus limits their applications in devices. The purpose of this research is to develop a coherent strategy for the design and synthesis of functional conjugated polymers with well-controlled structures and properties, from simple aromatic molecules. The history, background and applications of conjugated polymers are reviewed before introducing the central concept in this work - modular design and synthesis of new materials. The approach adopted seeks to develop “greener” pathways to target materials, by avoiding many highly toxic solvents, highly corrosive reagents, and expensive organometallic catalysts. Some simple reactions in water or ethanol were utilised to synthesise early building blocks in large quantities, while Horner-Wadsworth-Emmons (HWE) olefination and Kumada coupling reactions were employed to construct carbon-carbon double and single bonds, respectively. A group of three conjugated polymers were first synthesised to investigate the practicality of the modular approach. These polymers were based on the alternating copolymer of phenylenevinylene (PV) with (2-ethylhexyloxy-4-methoxy)phenylenevinylene (MEHPV), or alt-co-MEH-PPV. Each polymer consists of three types of aromatic monomers in an alternating pattern. The properties of the novel polymers were compared to their parent polymers to reveal effects of inserting an additional type of monomer. Subsequent work investigated the synthesis and properties of conjugation confined MEHPPV polymers. The target polymers differ from polymers of similar type studied by other groups in both synthesis and properties. Regioregular, bifunctional oligomers of MEHPV were first synthesised, then polymerised with 4,4’-bis(methylenediethylphosphonate)-1,1’-biphenyl (bis(MDEP)-biphenyl) to yield poly(MEHPV-oligomer-alt-co-bp) polymers. These polymers have a higher density of chromophores, and demonstrated a clear structure-property correlation in their photophysical properties. Furthermore, a simple, empirical quantitative model was developed to describe the basic absorption and emission characteristics of the polymers. Successes in developing an efficient synthesis of bifunctional MEHPV oligomers paved the paths towards conjugated polymers with increased structural complexity. Similar schemes were developed to synthesis bifunctional oligomers of 3,4-dihexylthienylenvinylene (DHTV). Using a combinatorial matrix approach, varying the combination of the MEHVP and DHTV oligomers modules produced 16 conjugated polymers with well-defined structures. Basic photophysical properties of these polymers were studied in some detail and a number of simple quantitative, empirical models were discovered from data analysis, which described their absorption and emission characteristics to a high degree of accuracy. Photovoltaic devices were fabricated with selected polymers as the active layer in blends with PC60BM and an efficiency of 2.0% was obtained. The device performance was superior compared to devices utilising MEH-PPV or PTV. The combined combinatorial-modular approach was then used to synthesise another type of copolymers of interest, those of MEHPV and regioregular 3-hexylthiophene (3HT) oligomers. MEH-PPV and P3HT were chosen as they are among the most intensely investigated conjugated polymers. An efficient, palladium free synthesise of 3HT oligomers was developed, and the bifunctional modules were then combined with MEHPV modules to produce 12 alternating copolymers of MEHPV and 3HT oligomers with well-defined structures. The absorption and emission characteristics showed the limitations of the empirical model developed in the other synthesised polymers, demonstrating the complex role of polymer chain morphology in solutions and the solid state polymer photophysics. At Finally, 4 polymers consisting of DHTV and 3HT oligomers were synthesised and their basic photophysical properties were determined.
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ItemExploring the Petasis reaction through amino acid synthesis( 2015)The Petasis reaction was reviewed and shown to be a versatile and efficient reaction for the synthesis of nitrogen containing compounds and α-amino acids. Many different amines and amine equivalents can be used in the Petasis reaction, in conjunction with a wide variety of aryl and vinyl boronic acids and esters, and a small selection of aldehydes. Chiral reagents can enforce stereochemical control in the reaction. Certain chiral amines and chiral amine equivalents give the highest selectivity. Several limitations remained for the Petasis reaction: yields were low with sterically small amines and the organoborons were largely limited to aryl, heteroaryl and vinyl derivatives. These limitations were addressed to make the Petasis reaction a more well-rounded and useful synthetic method. tert-Butyl sulfinamide was explored as an amine equivalent and the kinetics of the Petasis reaction with this reagent were investigated through the use of in situ FT-IR and 1H NMR spectroscopy analysis. tert-Butyl sulfinamide and glyoxylic acid both had rate orders of one, whereas styrenyl boronic acid had a rate order of two. This accounted for an observed dramatic increase in reaction rate. A mechanism for this reaction system was proposed, in which the boronic acid acts as both a reagent and as a Lewis acid catalyst. Allyl boronic acid pinacol esters were synthesised by palladium catalysed borylation of allyl alcohols, and then reacted with tert-butyl sulfinamide and glyoxylic acid to yield allyl glycine derivatives. Isolated yields of the final amino acids were excellent, but the diastereoselective ratios achieved were low to moderate. The addition of scandium(III) triflate to the allyl-Petasis reaction gave excellent control over the syn/anti configuration of the product, resulting in diastereomeric ratios in the order of >20:1. However, stereochemical control at the α-carbon was still moderate. A mechanism was devised to explain this observation and several supporting reactions were conducted. N-Methyl tert-butyl sulfinamide was synthesised racemically in a single step from the commercially available tert-butyl sulfinyl chloride and methylamine solution. The product was isolated in a pure yield of 98%. Racemic N-methyl tert-butyl sulfinamide was applied to a modified allyl-Petasis reaction, which employed molecular sieves to promote the formation of the initial iminium ion, to yield N-methyl amino acids in a quick and efficient manner. The use of scandium(III) triflate gave excellent control of the syn/anti configurations. Enantiopure N-methyl tert-butyl sulfinamide was also synthesised and applied to the Petasis reaction, resulting in excellent yields and stereochemical control. This work demonstrated the robust and widely applicable nature of the Petasis reaction as a method to synthesise α-amino acids in an efficient manner. The Petasis reaction can therefore be utilised in the chemical total synthesis of more complex natural products containing unusual amino acids residues.