Bent-Core Liquid Crystals

In the year 1996, an unusual ferroelectric behavior of liquid crystals composed of achiral bent-core (or banana-shaped) molecules was reported. Since then, the field of bent-core liquid crystals (BCLCs) has emerged as one of the most exciting novel areas of investigation. The sterically induced packing of bent-core molecules forms unique smectic phases, which have no counterpart in conventional calamitic liquid crystals. These mesophases of non-chiral molecules have been found to have fluid layers that show two spontaneous symmetries—breaking instabilities: polar molecular orientational ordering about the layer normal and molecular tilt. A chiral layer structure with handedness depending on the sign of tilt results due to combination of these instabilities. The mesomorphic behavior has been found to depend upon the type of bent unit, the number of aromatic units in the calamitic substituents, the nature of linking groups, the lateral substituents, and the length and type of terminal flexible chains. A brief overview of the developments in the field of BCLCs is given in this chapter. The main experimental results and concepts concerning the structure of bent-core molecules, polar order and superstructural chirality, structure–property relationship and physical properties namely electro-optic properties, light scattering properties, birefringence, optical properties, refractive indices, elastic properties, effects of different units of bent-core molecules on the behavior of mesophases, factors influencing the mesomorphic properties, phase transitions properties, etc., are discussed. The important effects which have been observed in bent-core mesogens and few other important aspects related to BCLCs are also presented.