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Title: Synthesis and characterisation of novel nematic discotic liquid crystals based on twinned triphenylene and coronene
Author: Zhang, Liang
ISNI:       0000 0004 2710 5398
Awarding Body: University of East Anglia
Current Institution: University of East Anglia
Date of Award: 2011
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To date, the study and application of liquid crystals (LCs) have been spread over the world. Compared to the study of calamitic nematic and discotic columnar LCs, the reports on nematic discotic (No) LCs are limited. In this thesis, we designed and synthesised a series of novel, disc-shaped twinned structures that display No mesophases. In chapter 2 we report the synthesis and mesophase behaviour of a series of triphenylene fused annulenes. Initial studies focused on the synthesis of triphenylene twins, linked through their 2,3-positions via diacetylene spacers. The resulting twin is formally antiaromatic and was found to be too unstable to isolate. Attention was then focused on the intriguing isomeric twins linked via the triphenylene 3,6-positions. Synthesis of the precursor triphenylene diacetylenes was achieved by Sonogashira coupling. Direct coupling was achieved by the catalysis of Cu(I)/Cu(II) in dry pyridine and these optimized conditions also led to isolation of higher oligomers. Stepwise syntheses gave model, open dimeric structures. Analysis of the initial twin, in which the other triphenylene beta-positions held hexyloxy-substituents, revealed the formation of a nematic discotic mesophase. XRD showed the twin adopts a nearly flat conformation and a zig-zag packing style in their crystalline state. Higher oligomers were not meso genic. In chapter 3 we extended our research on twins linked through the triphenylene 3,6-positions, now inserting aromatic groups. A 1,3-diethynylbenzene bridged twin was designed first as it was expected to lead to an unstrained twin. The closed system, which is not conjugated because of the 1,3-phenyl linkers, was achieved by the catalysis of Cu(I)/PPh3/K2C03 in dry DMF, which was applied for the subsequent coupling reactions. Nematic discotic mesophase behaviour was again observed. Formation of the conjugated, strained twin linked through 1,4-diethynylbenzene spacers was attempted but the closure of the system was not successful. Considering the importance of thiophene and its polymers, as well as the stability and angle issue, we employed 2,5-diethynylthiophene as a spacer and the resulting twin was successfully synthesized and gave a stable No mesophase. Bithiophene and 2,2' -bithienylacetylene were also introduced to the twin, and a formally aromatic, unsymmetrical twin was designed and prepared. However, the melting. temperatures of these compounds were above 300°C. Chain extension was investigated to lower the transition temperatures and finally an No mesophase was observed for some derivatives. Chapter 4 describes attempts to prepare related twins based on hexabenzocoronenes (HBCs). The precursor 2,5-diacetylene substituted HBC was synthesized via Sonogashira coupling followed by normal deprotection. The coupling of the diacetylene precursor, however, proved impossible without competing decomposition of the product.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available