Asymmetric fluorinated dimers for dual frequency liquid crystal mixtures : synthesis, mesomorphism & electro-optic properties

Allan, David, Dr.

Chemistry
January 2017

Thesis or dissertation


Rights
© 2017 Dr David Allan. All rights reserved. No part of this publication may be reproduced without the written permission of the copyright holder.
Abstract

Dual frequency liquid crystal mixtures are a technology used to achieve faster response times in electro-optical devices. At low driving frequencies the material acts as a positive dielectric, while at high frequencies the materials switches to a negative dielectric due to dielectric relaxation. This allows for voltage driven reorientation of the liquid crystal in two directions, thereby increasing the device switching speed.

This project aimed to synthesise and investigate the mesomorphic properties of a series of asymmetric liquid crystal dimers targeting dual frequency mixtures. The targeted structures feature two mesogenic arms linked by a flexible methylene spacer. One section of the dimeric mesogen has lateral fluorine substituents and the other section has terminal fluorination, to generate dipoles both perpendicular and parallel to the liquid crystal director. The aim is to synthesise materials with one section of the mesogen contributing positive dielectric anisotropy at low driving frequency and the other section contributing negative dielectric anisotropy at high frequencies.

The synthesis of twelve systems is presented in this thesis, these dimers vary in core length, terminal chain and parity of the spacer. The even lengthed spacers give linear shaped dimers, while the odd lengthed spacers give bent shaped materials. Synthesis proceeded through a number of conventional synthetic procedures including low temperature lithiations and Suzuki cross-couplings. Structure and purity was confirmed using NMR, MS, EA and HPLC, with the targeted materials being of high purity. The mesomorphic behaviour was investigated through a combination of optical polarising microscopy, differential scanning colorimetry and x-ray diffraction techniques. The materials exhibit nematic, smectic A, smectic C and in one case a more highly ordered nematic phase (Nx). Two of the targeted systems, one linear shaped and the other bent shaped both with asymmetric core lengths were further investigated as mixtures with each other and the stability of the Nx phase in mixtures is discussed. For the
same two dimers electro-optic measurements were conducted to measure birefringence and to investigate the voltage and frequency response of the materials and it was found that the linear dimer has relatively low crossover frequency making it viable for dual frequency liquid crystal mixtures.

Publisher
Department of Chemistry, The University of Hull
Supervisor
Hird, Michael; Lewis, Robert A., Dr.
Qualification level
Doctoral
Qualification name
PhD
Language
English
Extent
102 MB
Identifier
hull:14792
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