MESOMORPHIC PROPERTIES OF PHENYL 4-(5-ALKYL-1,3,2-DIOXABORIN-2-YL)BENZOATES - INFLUENCE OF TERMINAL AND LATERAL SUBSTITUTION

Thirty-eight non-substituted and laterally fluoro-substituted 4-nitro-, 4-isothiocyanato-, 4-fluoro-, 4-trifluoromethyl- and 4-trifluoromethoxyphenyl 4-(5-alkyl-1,3,2-dioxaborin-2-yl) benzoates have been synthesized and their phase transition temperatures and enthalpies of transition measured. The influences of both terminal and lateral substituents in both benzene rings (in the acidic and phenolic moieties) on the phase transition temperatures in general and on the stability of the smectic A phase in particular are compared. The clearing points of the compounds (a), the thermal stability of the smectic A phase (b) and the tendency to form a smectic A phase (c) decrease in the following way: (a) NCS > NO2 > OCF3 > CF3 > F, (b) NO2 > NCS > OCF3 > CF3 > F, (c) CF3 greater than or equal to OCF3 greater than or equal to NO2 > NCS > F. Substitution of the central benzene ring by a fluorine atom effectively depresses the smectic A phase. The influence of 4-(5-alkyl-1,3,2-dioxaborin-2-yl)benzoates as components of nematic mixtures on their viscosity, optical and dielectric anisotropy and electro-optical properties is discussed.