Development of synthetic methodologies toward five-membered S-heterocycles and their fluorinated derivatives: Applications to liquid crystal synthesis.

摘要

This dissertation describes the development of synthetic approaches toward a variety of novel liquid crystals containing five-membered S-heterocycles and their fluorinated derivatives. It was anticipated that these S-heterocycles would have several beneficial physical characteristics when compared to their phenyl counterparts, such as lower viscosity, fast switching times and high dielectric biaxiality, and might be useful in surface-stabilized ferroelectric liquid crystal (SSFLC) displays.; Chapter II deals with the synthesis of potential achiral host materials for SSFLC displays. A series of studies toward the synthesis of the liquid crystalline thienylbiphenyl and terphenyl compounds containing 1,1-difluoropentyl terminal chain are delineated, including fluorodeoxygenation (using DAST, MorphDAST and DeoxoFluor®) and fluorodesulfurization ([DBH, NBS, NIS or NOBF₄]/Pyr-xHF) approaches. These 1,1-difluoropentyl-based systems displayed significantly different mesomorphic behavior from the other more conventional alkyl, alkoxy and alkanoyl-based compounds and were characterized by high melting points and a tendency toward orthogonal phase behavior (S _A and S_B phases). Only in a single case was a disordered tilted phase (S_F) observed in such systems. A number of the analogous alkoxythienyl and alkylthienyl mesogens display remarkably high S_C mesophase stabilities for ‘bent’ compounds and warrant further investigation as potential achiral ferroelectric host materials.; Chapters III and IV outline our progress toward chiral non-racemic S-heterocyclic ferroelectric dopants with low melting points and high dielectric biaxiality. Specifically, in Chapter III, the synthesis of several liquid crystals containing 5-alkoxy-1,3-thiazole and 2-alkyl-1,3,4-thiadiazole moieties via microwave-assisted Lawesson's reagent-mediated cyclization of the corresponding 1,4-dicarbonyl precursors is described. These liquid crystals were found to have high clearing points and supported the desired smectic C mesophases.; In Chapter IV, an effort was made to synthesize the fluorinated thiophene moiety via a Balz-Schiemann fluorination approach and introduce it into liquid crystalline materials using regioselective electrophilic bromination and regioselective Suzuki coupling chemistry. One of the target fluorinated mesogens supported tilted smectic C mesophases at substantially lower temperatures than those of the corresponding non-fluorinated derivative. A detailed study of the physical properties of this compound showed that this new material is of exceptional promise for the use in SSFLC displays and other applications.