Stability of the antiferroelectric phase in dimeric liquid crystals having two chiral centres with CF3 or CH3 groups; evaluation of conformational and electric interactions

Abstract

The synthesis of a novel series of dimeric liquid crystals with two chiral centres with CF₃ or CH₃ groups connected by an alkylene spacer is described. The two mesogenic units in the dimer tend to align in anticlinic zigzag ordering or in parallel ordering depending on whether the alkylene spacer has an odd or even number of – CH₂– groups, n, stabilizing the antiferroelectric S_CA* phase or the ferroelectric S_C* phase, respectively. This odd-even rule in the conformational effect is no longer valid when the length of the alkylene spacer is increased to over n= 10: the S_CA* phase emerges instead of the S_C* phase for n= 12, as already reported for the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 dimers 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), although 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 dimer with CH₃ groups, indicating that the dimer with CF₃ groups shows more stable antiferroelectricity, the same as in the monomers. These facts suggest the importance of an electric interaction between mesogens for the emergence of the S_CA* phase.