| Ferroelectric liquid crystals (FLCs) are formed by chiral smectic C (S C*) liquid crystals, and exhibit a permanent electric polarization known as the spontaneous polarization (PS). This spontaneous polarization is a macroscopic manifestation of molecular chirality. The direction of P S can be switched using an electric field, causing a reorientation of the molecules. When placed between two crossed polarizers, a FLC light valve is produced, by virtue of the molecular reorientation and the birefringence of the material.; Considerable research has focused on the development of FLC light valves that are triggered by light, rather than an electric field. These optically addressed FLC light valves typically incorporate photochromic dopants dissolved in the FLC in order to achieve optically triggered modulation of properties such as PS. This has been achieved using azobenzene and thioindigo dopants in FLCs. These photochromic systems, however, undergo a thermal reverse reaction and are therefore unsuitable for the development of a bistable FLC photoswitch. Furthermore, thioindigo dopants exhibit low solubility in smectic C hosts.; In the first part of this thesis, the solubilities of a series of selectively deuterated thioindigo dopants in smectic liquid crystal hosts are assessed using a combination of polarized microscopy and 2H NMR spectroscopy. Systematic variation of structural features such as substituents on the aromatic ring and branching of alkoxy side chains shows that two branched alkoxy side chains compromise the solubility of the dopant significantly, and that the nature of the substituents also affects the solubility properties of these compounds. 2H NMR spectroscopic studies indicate that some compounds are partitioned between smectic and isotropic domains, even though the isotropic domains cannot be observed by polarized microscopy.; The second part of this thesis focuses on the design, synthesis, and characterization of novel photochromic dithienylethene dopants for ferroelectric liquid crystals. The novel chiral dithienylethenes prepared in this study were doped into smectic C (SC) hosts at low concentration. The dopants showed very low solubility in the available SC hosts, usually forming biphasic mixtures consisting of smectic and isotropic domains. Although ferroelectric switching was observed in these mixtures, no spontaneous polarization could be measured. An achiral dithienylethene derivative, however, showed moderate solubility in SC hosts, and was doped into hosts containing a non-photoactive chiral dopant. Photomodulation of the spontaneous polarization was achieved by irradiation of these mixtures with UV or visible light, and is attributed to a photomechanical destabilization of the SC* phase upon ring closure of the dithienylethene dopant. This photomodulation of P S was reversible, fatigue resistant, and bistable. |
