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Title: Conformationally-fixed conjugated polymers via aldol polymerisation for organic semiconducting applications
Author: Onwubiko, Nkechinyerem Adannaya Uzoamaka
ISNI:       0000 0004 9350 9809
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2020
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There is copious potential for carbon-based semiconductors to compliment inorganic materials and open-up a range of applications to technology, as well as offer economic and environmental advantages for both manufacturers and consumers. These organic semiconducting materials are inexpensive and flexible with tunable properties due to covalent bonding within the molecules and intermolecular Van der Waals interactions. Through property-structure led strategic design these lightweight materials can be solution processed at ambient temperatures and highthroughput volumes to minimise energy consumption, expand their compatibility with plastic and flexible substrates and lower manufacturing costs. The organic semiconducting field includes materials such as organic photovoltaic materials, organic light emitting diodes, photodetectors, radio-frequency identification tags and transistors, the focus of this thesis. Polymeric materials for organic field-effect transistors are generally prepared by transition metal mediated coupling reactions that often involve highly reactive, functional group sensitive or toxic reagents. These coupling reactions generate polymers where aromatic units are connected by carbon-carbon single bonds and are therefore subject to arbitrary rotation along the polymer spine. Torsional strain along the polymer spine increases conformational and energetic disorder in the polymer. In addition to this, defects from miscoupling reactions during polymerisation results in higher degrees of disorder in the polymers. Disorder along the polymer backbone has been shown to hinder efficient charge transport and the formation of closely stacked polymer chains and extended crystalline domains for charge mobility. To elicit coplanar, conjugated polymers and facilitate close π − π stacking between polymer chains, we synthesised conformationally-fixed polymers by linking monomer units by carboncarbon double bonds. Our polymers were prepared via metal-free, acid catalysed aldol polymerisation, an environmentally benign synthesis compared to traditional polymerisation reactions. The monomer structures used generated electron deficient polymers with extended delocalised frontier orbitals, near infrared absorption and good ambient stable electron charge transport. We also explored common property-design strategies of elongating the aromatic centres, side-chain engineering and reducing torsional strain to optimise the charge transport properties.
Supervisor: McCulloch, Iain Alexander Sponsor: Engineering and Physical Sciences Research Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral