A peculiar LC system was discovered recently, which, contrary to predictions, displayed ferroelectric switching (which usually only occurs in chiral LC systems).(1) Banana-shaped mesogens (shown above), first synthesized by Matsunaga and coworkers(2,3), were found to form macroscopically chiral domains. Clearly some form of spontaneous symmetry breaking was responsible for the formation of chiral domains, since the constituent molecules have no intrinsic chirality. The handedness of the domains was random (the left and right-handed versions were equally likely), but one handedness could be formed preferentially with the addition of a small amount of chiral dopant.(4)

      The banana mesogens form a number of phases, shown to the left.(4-7) At low temperature, the banana liquid crystals display the usual crystalline (Cr) phase, and at high temperature melt, as expected, into isotropic liquids (ISO). The B-phases (as they have come to be called) are unlike what is found in other LC systems, since they exhibit many chiral properties, although the banana mesogens are not, themselves, chiral (in stark contradiction to conventional LC wisdom).

      How can an achiral mesogen form a chiral phase? The exact structure of the phase took some time to elucidate. Eventually, however, it was understood how a chiral coordinate system could be defined from polar banana-shaped molecules (the optimized molecular structure, along with orientation of dipole, is shown to the right). The mesogens form a layered structure similar to the smectic C phase, with mesogens tilted with respect to the layer normal. The layer normal, dipole vector, and tilt direction (or equivalently, the layer plane, tilt plane and dipole plane) form a coordinate system (since they are represent three non-coplanar vectors, hence define a basis). The handedness of the coordinate system is based upon the sign of the tilt. Thus, each smectic-like layer of bent polar molecules is chiral.

      The B2 phase is formed from alternating layers of such chiral smectic layers. Two textures are found in the B2 phase, termed Racemic and Homogeneous. The racemic texture has layers which alternate in handedness (subsequent layers have opposite dipole orientation). The homogeneous phase is completely chiral: subsequent layers have opposite dipole orientation and tilt, hence same overall handedness. Each domain of this texture is thus a homogeneously chiral region.

      The polar molecules can be oriented in an electric field, with the layers maintaining their handedness during electrical switching. The B2 phase is thus antiferroelectric in the ground state, and can be ordered to the ferroelectric state with an external electric field (refer to the diagram which follows).