Date of Award

Spring 1-1-2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Chemistry & Biochemistry

First Advisor

David M. Walba

Second Advisor

Douglas L. Gin

Third Advisor

Tarek Sammakia

Fourth Advisor

Tad H. Koch

Fifth Advisor

Joseph Maclennan

Abstract

Two separate projects on thermotropic liquid crystals are described: an odd-even behavior giving ferroelectric or antiferroelectric phases in a tricarbosilane-terminated liquid crystal homologous series, and an electric-field-responsive discotic columnar liquid-crystal formed from an hexa-peri-hexabenzocoronene/oligothiophene hybrid.

A new class of polyphilic mesogens composed of a tolanphenyl carboxylate core, a chiral alkoxy tail, and a tricarbosilane terminated alkoxy tail were synthesized and shown to self-assemble into nanophase-segregated smectic liquid-crystalline (LC) phases. It was found that the number of carbons in the alkoxy spacer between the tricarbosilane and the core determined the phase observed with perfect fidelity: homologues with odd-carbon spacers exhibit antiferroelectric phases, while those with even-carbon spacers exhibit ferroelectric phases. Even more interestingly, homologues with an odd carbon number show the rare and useful chiral orthoconic antiferroelectric SmCA* phase, in which the optic axis tilt alternates layer to layer between +45° and -45°, making the molecular directors in adjacent layers orthogonal. The even-carbon homologues exhibit the SmC* phase with a 45° tilt angle. These findings demonstrate that the supramolecular self-assembly of polyphilic mesogens may in some cases be controlled by the length of the hydrocarbon spacers.

A novel hexa-peri-hexabenzocoronene (HBC) derivative with six covalently tethered alkyl-trizaole-quadra-3-hexylthiophene units was synthesized via “click” chemistry and shown to self-assemble into a hexagonal columnar liquid-crystal (LC) phase. Compared to other HBC-based LC materials, this mesogen shows unprecedented responses to applied electric fields in cells with thickness of several microns, resulting in uniform homeotropic or parallel alignment depending upon the electrode structure. Furthermore, the columnar orientation, once developed by an applied electric field, can be maintained even after removal of the electric field unless the material is heated above its clearing temperature.

Included in

Chemistry Commons

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