Optical Activity in Antiferroelectric Liquid Crystals

Molecular orientational structures in chiral smectic phases of a family of antiferroelectric liquid crystals (AFLCs) with different alkyl chain lengths were investigated by means of optical rotatory power and circular dichroism measurements. In the ferroelectric chiral smectic C phase, SmC*, molecules of rectus configuration are arranged in a left-handed helix, and the pitch of the helix becomes longer and saturates to a constant value with decreasing temperature. In the antiferroelectric chiral smectic C A phase, SmC A *, the helical handedness is opposite to that of SmC*, and the helical pitch is shorter than that of SmC* and independent of temperature. In some AFLCs with shorter alkyl chain, however, a divergence of the pitch and a simultaneous change in the handedness were observed in SmC A *. Alkyl chains in AFLCs play an important role in stabilizing the helical structures, and the molecules with long alkyl chains tend to form a tight helix. The experimental results for subphases between SmC* and SmC A * were discussed on the basis of the competition between helical twisting powers of the phases.