THE EFFECT OF THE POSITION OF LATERAL FLUORO-SUBSTITUENTS ON THE STABILITY OF THE S-STAR(C-ALPHA) AND S-STAR(CA) PHASES

Four new chiral series with benzoate cores have been synthesized and characterized. The mesomorphic properties have been analysed by optical microscopy on the pure enantiomeric and the racemic compounds and on mixtures, by D.S.C. and by electro-optic measurements. Three of the series (hydrogenous, monofluoro-substituted ortho to the alkoxy chain, and difluoro-substituted) display a very rich polymorphism including S-A, S-C alpha* , S-C*, S-CFI* and S-CA*, phases, whereas the series monofluoro-substituted in the meta-position does not exhibit S-CFI*, and S-CA*, phases any more except for the dodecyloxy derivative, but gives a large enantiotropic S-C alpha*, phase. A comparison of these four series leads to a discussion about the effect of the transverse dipole moment on the existence of the S-CA*, phase.