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 c haracterized. 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 se ries (hydrogenous, monofluoro-substituted ortho to the alkoxy chain, a nd difluoro-substituted) display a very rich polymorphism including S- A, S-C alpha , S-C*, S-CFI* and S-CA*, phases, whereas the series mon ofluoro-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 s eries leads to a discussion about the effect of the transverse dipole moment on the existence of the S-CA, phase.