School of Chemistry - Theses
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ItemTowards the synthesis of the emestrin family of natural products( 2018)A Cope rearrangement of a vinyl pyrrole epoxide (397) was utilised to form the dihydrooxepino[4,3-b]pyrrole core (398) of the emestrin family of natural products which involved the first examples of the dearomatisation of pyrrole in this type of rearrangement. It was found that an electron withdrawing ester substituent on the C2 position of the epoxide was essential for the [3,3]-rearrangement to occur. The vinyl pyrrole epoxides were synthesised in an efficient manner by a vinylogous Darzens reaction. Density functional calculations showed lower transition state energies for Cope rearrangements of epoxides with C2 esters when compared to the unsubstituted substrates which agreed with the observed experimental results. Silyl substituted vinyl bromide esters also participated in the Darzens reactions to give the desired vinyl pyrrole epoxides in good to excellent yields. Only the triethoxysilyl vinyl epoxide 313c underwent Cope rearrangement to provide the fully substituted emestrin core dihydrooxepine. The anion derived from an aryl bromosulfone did not give the Darzens product but underwent a previously unobserved stereoselective trimerization to afford the cyclohexene 343 as a single diastereoisomer. A mechanistic rationale involving SN2’ additions, [3,3]-Cope rearrangements and a stereoselective intramolecular conjugate addition was proposed and this was supported by density functional theory (DFT) calculations. A four-step total synthesis of biaryl ether natural product violaceic acid (11) is described. The steps include an SNAr reaction to afford the biaryl ether 136, tin chloride-mediated chemoselective reduction of the nitro group to amine 135. A Cu-mediated Sandmeyer reaction of 135 gave violaceic acid methyl ester 374 which is hydrolysed to give pure violaceic acid 11. An improved synthesis of the known biaryl iodide 119 is also described via a Sandmeyer reaction of amine 135.
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ItemConfined conjugation MEH-PPV derivatives: synthesis, photophysics and energy transfer( 2012)Conjugated polymers based on poly(2-methoxy-5-(2’-ethylhexyloxy)-1,4- phenylenevinylene) (MEH-PPV) have received considerable attention owing to their interesting luminescent and semi-conducting properties. The performance of optoelectronic devices based on conjugated polymers, such as organic light-emitting diodes (OLEDs) and organic photovoltaic devices (OPVs), is predicated by an understanding of energy transfer processes occurring within and between polymer chains. Despite the large body of knowledge accumulated over the last twenty years, there remains a need to develop an energy transfer model capable of describing the observed fluorescence properties of MEH-PPV. The purpose of this thesis is to explore the photophysics and energy transfer dynamics of well-defined MEH-PPV oligomers and pendant polymers. These materials allow for the establishment of structure-property relationships, and serve as models for understanding the fluorescence properties of MEH-PPV. A series of MEH-PPV oligomers, ranging from 3 to 5 repeat units, was prepared using Horner-Wadsworth-Emmons chemistry from readily synthesised precursors. The oligomers displayed the expected red-shift in absorption and emission maxima as conjugation length increased. Fluorescence quantum yields and fluorescence lifetimes decreased in the order trimer > tetramer > pentamer, owing to increases in the rates of both radiative and non-radiative processes. The non-mirror image relationship between absorption and emission spectra, also observed in MEH-PPV, was investigated using a number of techniques. X-ray crystallography on n-hexyl derivatives, synthesised via a similar route to the MEH-PPV oligomers, showed that the oligomers adopted twisted helical conformations in the ground state, with dihedral angles of ~7° measured for the MEH-PPV trimer. MM2 calculations on a methoxy substituted trimer indicated that a family of conformers was likely to exist in room temperature solution. Temperature dependent absorption and fluorescence measurements provided additional evidence for the presence of non-isoenergetic torsional isomers in the ground state. The rate of conformational relaxation in the excited state was measured using ultrafast transient absorption spectroscopy on the MEH-PPV tetramer and pentamer, with relaxation times of 4 and 5 ps obtained, respectively. Luminescence measurements in neat films and in PMMA matrices demonstrated aggregation occurred in the solid state. Pendant polymers consisting of short MEH-PPV segments (from 3 to 5 units) attached to a non-conjugated poly(styrene) backbone were prepared using a post-polymerisation functionalisation approach. This method involved RAFT polymerisation of vinylbenzyl chloride, followed by a series of post-polymerisation modifications. The final polymers possessed narrow polydispersities (1.08 for each), and molecular weights ranging from 10100 to 17500 g/mol. The absorption and fluorescence profiles of the polymers redshifted as side-chain conjugation length increased. The non-mirror image relationship between absorption and emission spectra was attributed to absorption by a population of side-chain torsional isomers, followed by geometry relaxation to more planar structures prior to emission. Fluorescence decays were modelled using a double-exponential function, although this most likely approximated more complex decay behaviour. Thin film measurements provided evidence for aggregation in the series, with red-shifting of the [0,0] band observed in the neat film emission spectra. Energy transfer in the series was investigated using fluorescence polarisation measurements, and was found to be highly efficient. A Förster resonance energy transfer model was applied for describing energy transfer dynamics between adjacent chromophores, giving rates in the order of 1012 s-1. Pendant polymers labelled with low band-gap acceptors were also investigated. A statistical donor-acceptor polymer consisting of 15 trimer donor chromophores for every pentamer acceptor was prepared by a post-polymerisation functionalisation approach. Photophysical measurements indicated that energy transfer occurred with an average efficiency of 85%. A pendant polymer bearing a terminal perylene diimide (PDI) end-group was also prepared. This synthesis required the use of a PDIfunctionalised RAFT agent, followed by post-polymerisation functionalisation to attach the tetramer donor chromophores. This system was found to undergo electron transfer following selective excitation of the tetramer donors. The relative HOMO/LUMO levels were determined for the model poly(tetramer), and found to be conducive for electron transfer to PDI. The synthesis and solution photophysics of a water-soluble polymer were also explored. Water solubility was conferred by functionalisation of each pendant trimer chromophore with a PEG-3 group. This polymer adopts an aggregate structure in aqueous solution and intermolecular energy transfer between this polymer and an encapsulated MEH-PPV pentamer acceptor was demonstrated