STABILITY OF THE ANTIFERROELECTRIC PHASE IN DIMERIC LIQUID-CRYSTALS HAVING 2 CHIRAL CENTERS WITH CF3 OR CH3 GROUPS - EVALUATION OF CONFORMATIONAL AND ELECTRIC INTERACTIONS

The synthesis of a novel series of dimeric liquid crystals with two ch iral centres with CF3 or CH3 groups connected by an alkylene spacer is described. The two mesogenic units in the dimer tend to align in anti clinic zigzag ordering or in parallel ordering depending on whether th e alkylene spacer has an odd or even number of -CH2- groups, n, stabil izing the antiferroelectric S-CA phase or the ferroelectric S-C* phas e, respectively. This odd-even rule in the conformational effect is no longer valid when the length of the alkylene spacer is increased to o ver n = 10: the S-CA phase emerges instead of the S-C* phase for n = 12, as already reported for the dimer with CF3 groups. Furthermore, we found that the dimers with n = 10 also show a S-CA phase, if the dim ers with CH3 or CF3 groups are sandwiched between thick cells. In thin cells, on the other hand, the S-C phase emerges instead of the S-CA* phase in the dimers with CH3 (n = 10 and 12) and CF3 (n = 10), althou gh the S-CA phase is stable even in thin cells in the dimer with CF3 (n = 12). Thus, the cell thickness dependence is pronounced in the dim er with CH3 groups, indicating that the dimer with CF3 groups shows mo re stable antiferroelectricity, the same as in the monomers. These fac ts suggest the importance of an electric interaction between mesogens for the emergence of the S-CA phase.