OPTICAL-ACTIVITY IN ANTIFERROELECTRIC LIQUID-CRYSTALS

Molecular orientational structures in chiral smectic phases of a famil y of antiferroelectric liquid crystals (AFLCs) with different alkyl ch ain lengths were investigated by means of optical rotatory power and c ircular 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 antiferroelect ric chiral smectic C-A phase, SmCA, the helical handedness is opposit e 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 SmCA. Alkyl chains in AFLCs play an impor tant role in stabilizing the helical structures, and the molecules wit h long alkyl chains tend to form a tight helix. The experimental resul ts for subphases between SmC and SmCA* were discussed on the basis of the competition between helical twisting powers of the phases.