Exploring the properties of bent-core liquid crystals

[Nature India Special Issue: Lighting the way in physics]

Anisotropy of molecular shape is essential for the formation of thermotropic liquid crystals (LCs)¹. Rod-like mesogens have been known for well over a century and disc-like mesogens were identified in 1977. They have an essentially uniaxial shape and the nematic phase exhibited by many of them also has uniaxial symmetry with an apolar orientational order along a line.

Several theories suggested the possibility of the formation of the biaxial nematic phase (with three orthogonal directions with apolar order) in mixtures of the two types of molecules¹, though only a coexistence of the two types of nematic was found, with interesting defect structures².

Bent core (BC) molecules ― also popularly known as banana-shaped molecules ― are intrinsically biaxial with shape polarity and they exhibit unusual types of LCs, mainly forming layers with a polar order3³.

An intensive international effort to synthesise and study the physical properties of compounds with BC molecules with different core structures began in the late 1990s in many laboratories, and in particular by Bookinkere Kapanipathaiya Sadashiva at the Raman Research Institute (RRI). Rod-like mesogens usually have aromatic cores with large polarisability, attached with aliphatic chains with low polarisability, leading to a nano-segregation between the two moieties and the formation of smectic LCs with layered structures1. As BC molecules are just bent rods, long chains give rise to the layered B2 phases with tilted molecules³.

Mixing mesogens with the two types of molecules is of obvious interest, but if the dimensions of the cores and chains of the two components match, only a simple intermediate structure is exhibited by the mixture³.

Sadashiva had also synthesised some rod-like mesogens with a chain at only one end of the rod, resulting in a bilayer SmA2 phase. When this was mixed with a compound with BC molecules exhibiting the B2 phase, the resulting phase diagram was quite unexpected⁴. At low concentrations of the rod-like molecules, the latter just fit in with the B2 layers. Beyond ~16 mole%, the rods break up the layers and the B1 phase with a 2D rectangular lattice is found, the rods acting as glue between the broken layers. When the concentration of rod-like molecules exceeds ~62%, the break-up is complete, giving rise to the B6 phase in which the rods stitch the aromatic cores of the BC molecules so that the latter intercalate to form layers with a spacing which is only half the molecular length. Interestingly, the B1 and B6 phases are exhibited as the chain length is decreased in the pure compound with BC molecules.

The most interesting part of the phase diagram occurs when the concentration of rod-like molecules exceeds ~86%. In this rod-rich region, the bilayers of the SmA2 phase have to accommodate the BC molecules and the biphilic nature of both types of molecules forces the long axes of the BC molecules to orient in a direction perpendicular to the layer-normal.

At higher temperatures, the long axes of BC molecules are orientationally disordered in the layers and the SmA2 phase has uniaxial symmetry. As the temperature is lowered to some concentration-dependent value, the long axes develop an orientational order and the medium becomes the biaxial SmA2b phase, as established by the nature of the defects exhibited by the medium⁴.

Later, the compound with rod-like molecules was replaced by another one which also had the chain attached only at one end. This produced an essentially similar phase diagram, and polarised infrared spectroscopy was used to establish the relative orientations between the two types of molecules in different phases⁵. Subsequent simulations on mixtures of hard rods and hard BC rods found only the SmAPAF phase with an antiferroelectric order of the polarisation of successive layers³, bringing out the essential role of biphilicity of the two types of molecules for the experimentally observed mutual orientations, as noted in the perspective by Tom Lubensky⁶. The SmAAF phase has been found in pure BC compounds at the Raman Research Institute⁷ and elsewhere³. The macroscopic symmetry of this structure is also that of SmAb although the low frequency electric field response arises from polar interaction, rather than the apolar interaction as in the SmA2b phase. The rod-rich mixtures also exhibit the uniaxial nematic phase as the SmA2 phase is heated above its transition temperature. The elasticity to resist bend distortion of the orientational field of the nematic was found to decrease as the temperature was lowered in the nematic, even though the orientational order parameter increased⁸. The anomalous trend arises from a stronger coupling of the distortion with the bent shape of the BC molecules at lower temperatures. Later, a similar trend was found in nematics made of pure BC molecules³.

The results reported in 20004 required careful molecular engineering of the two types of molecules and in fact most other studies on other mixtures have not found the occurrence of new phases³. The results also triggered an effort to find the SmAb phase ― mainly of the type SmAPAF ― in pure compounds made of BC molecules³, and to find interesting electro-optic responses. The strong reduction of the bend elastic constant of the nematic with BC molecules also leads to a sharper electro-optic response in such materials8, which can be useful for large liquid crystal display devices.