NEW THERMOTROPIC LIQUID-CRYSTALS DERIVED FROM THIOPHENES

Properties of new homologous series of low molecular mass thermotropic liquid crystals containing non-linear mesogenic cores, 2,5-thiophene (T) and 2,2'-bithiophene (BT), are presented for the purpose of refining molecular structural prerequisities for liquid crystallinity. Herein we focus on the effect of core linearity on mesophase stability. The bis(p-alkoxyphenyl) dicarboxylate derivatives containing the bent T and kinked BT central units are found to form enantiotropic liquid crystal phases with nematic and smectic polymorphism, depending on the length of their terminal chains. All members of the non-linear T and BT homologous series have narrower mesomorphic temperature ranges than the corresponding compounds with linear central units (p-phenylene and 4,4'-biphenyl); liquid crystallinity is not found in the analogous m-phenylene derivatives. This finding is consonant with the establsihed principles of prolate mesogen structural requirements, i.e. core linearity is closely related to liquid crystal phase formation and deviation from core linearity lowers mesophase stability. The results of this study indicate that the 2,5-thiophene and 2,2'-bithiophene moieties with non-linear structures are viable mesogenic core units and may be substituted for p-phenylene units in conventional liquid crystal molecules. Moreover, these moieties should be particularly useful for modifying the transition temperatures of liquid-crystalline polymers.