Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
David M. Walba
Joseph E. MacLennan
Within the liquid crystal community, the evolution of flat-panel displays towards the next generation of 3-D holographic imaging technologies can only be realized with the synthesis of mesogens that exhibit analog electrooptic switching speeds - on the order of microseconds at room temperature. Liquid crystals phases exhibiting V-shaped (analog) switching, which is characterized as a collective reorientation of the secondary axis of a biaxial medium with a spatially invariant optic axis in response to applied electrical fields, are promising for integration into these futuristic technologies. In this regard, the Boulder liquid crystal group has recently reported the design, synthesis, and characterization of a class of mesogens exhibiting a new liquid crystal phase, the SmAPF, which is lamellar, orthogonal, and spontaneously polar in the layer plane (here referred to as ferroelectric). Furthermore, phase-only electrooptic modulation with optical latching in the SmAPF phase of two isomeric SmAPF mesogens, W586 and W623, has been demonstrated. The electrooptic switching speed of these materials is quite fast (on the order of 10 µs), but only at temperatures above 120°C. In this thesis, synthesis and characterization of several homologous libraries of SmAPF mesogens following on this lead is reported.
The first section describes the general properties and characterization of liquid crystals, and gives an overview of the characterization of SmAP liquid crystal phases through polarized light microscopy and electrooptical measurements. The experimental portion of this thesis is divided into three main sections: The first section reports on the correlation between the occurrence of SmAPFF/SmAPA (lamellar, orthogonal, and "antipolar" in adjacent layers) phases with variations in the number of carbons in the liquid crystal "tails," which exhibits a perfect "odd-even" effect. This was shown by the synthesis of three libraries of W586 homologues with modifications in the aromatic core/tail linkers or variations in ester linkages within the aromatic core units. Experimental trends in phase behavior and layer spacing data were also used towards the development of a very simple computational model that attempts to extrapolate theoretical geometrical molecular orientations within supramolecular assemblies of targeted phases.
The second part of this thesis describes the discovery of two new liquid crystal phases exhibited by these bent-core structures: low-temperature phases occurring below the SmAP phase, and the onset of "spaghetti-like" phases upon heating from this phase. These low-temperature phases can be induced through the application of an electrical field at specific temperature ranges in the liquid crystal phase sequence, and are observable by both polarized light microscopy and X-ray diffraction. The third part of this thesis describes attempts to reduce the onset temperature of the SmAPF phase through eutectic mixtures directed towards development of room-temperature SmAPF materials to demonstrate fast switching at useful temperatures. Finally, future target structures are suggested to further lower the temperature range of the SmAPF phase.
Kolber, Maria Mischa, "Structure/Property Relationships in Homologous Libraries of Bent Core Mesogens Forming Orthogonal, Polar, and Achiral Smectic Liquid Crystal Phases" (2014). Chemistry & Biochemistry Graduate Theses & Dissertations. 207.