Devil's staircase and racemization in antiferroelectric liquid crystals

We have observed conoscopic figures under an applied electric field in 4-(1-methylheptyloxycarbonyl)phenyl 4′-octylbiphenyl-4-carboxylate (MHPBC) and three other trifluorinated antiferroelectric liquid crystals with varying optical purity. The electric field–temperature (E–T) phase diagrams obtained change with the ratio between the R, and S enantiomers, in a similar way that they vary with mixing ratio in some binary mixtures. In particular, the E–T, diagram of 4-(1-trifluoromethylheptyloxycarbonyl)phenyl 4′-octylbiphenyl-4-carboxylate becomes more complex by slightly reducing the optical purity, although it has been tacitly understood that the purity reduction may always simplify the phase sequence as is actually the case in MHPOBC, the prototype antiferroelectric liquid crystal. The observed similarity and complexity substantiate that it is useful to consider in every individual compound the strength of antiferroelectricity and that reducing the optical purity results in a decrease of the strength in antiferroelectric liquid crystals.