Investigation into liquid crystalline smectic-C* subphase stability using chiral and achiral dopants

In this paper we investigate the influence of chiral and achiral dopants on a chiral smectic liquid crystal material, which exhibits a rich phase sequence, including the antiferroelectric phase and the three-layer intermediate smectic (SmC*FI1) phase. Using polarized optical microscopy, differential scanning calorimetry, and x-ray diffraction we find that small amounts of achiral dopant have the ability to significantly broaden the SmC*FI1 phase, whereas an oppositely-handed dopant (otherwise identical to the host material) destabilizes the phase. This work clearly indicates that bulk chirality strongly influences SmC*FI1 phase formation and that steric effects also play an important role. Interestingly, addition of the shorter achiral molecule (8CB) was observed to increase the smectic layer spacing, most likely by suppressing interdigitation of alkyl chains between adjacent smectic layers. Control of the SmC*FI1 phase width using mixtures in this way is clearly important for effective phase characterization, and could potentially lead to commercially viable materials with a stable SmC*FI1 phase over a large temperature range.