A study of unsymmetrical thioindigo dopants for ferroelectric liquid crystal photoswitches.

摘要

Chiral smectic C (SmC*) liquid crystals are ferroelectric and exhibit a permanent electric polarization known as the spontaneous polarization ( PS). The PS is a macroscopic manifestation of molecular chirality and, as such, depends on the nature of the chiral component of the SmC* phase. The orientation of the molecules and the direction of PS can be switched by the application of an electric field, creating a ferroelectric liquid crystal spatial light modulator (FLC-SLM) device when positioned between crossed polarizers. To develop FLC switches triggered by light instead of an electric field, much interest has focused on developing photoactive FLC materials. Chiral photochromic thioindigo dopants can be dissolved in SmC hosts, and used to modulate bulk properties such as P S upon trans-cis photoisomerization. This photomodulation of PS has been used to create an optically-addressed SLM (OASLM) device based on a combination of thioindigo and non-photoactive dopants inducing PS of opposite.signs. One disadvantage of using thioindigo dopants is their low miscibility in SmC hosts. This is addressed in the first part of this thesis by assessing the miscibility of a series of unsymmetrical thioindigo dopants using polarized microscopy in conjunction with 2H NMR spectroscopy. Some of these new thioindigo dopants form enantiotropic SmA* phases which exhibit electroclinic properties, and are the first examples of thioindigo-containing liquid crystals.; The effect of structural variations on the degree of P S photomodulation is investigated by comparing thioindigo dopants with different cores and side-chains, and correlating these changes to calculated changes in transverse dipole moment upon trans-cis photoisomerization. Based on these findings, the design of the first thioindigo dopant that photoinverts the sign of PS in SmC hosts is reported, and this principle is used to create a novel OASLM device based on the photoisomerization of a single thioindigo dopant.; The degree of thioindigo trans-cis photoisomerization in the SmC* phase is also investigated using visible absorption spectroscopy, and related to the degree of PS photomodulation, which is shown to be dependent on both dopant concentration and the nature of the SmC host.