News on Nematic-Biaxial Liquid Crystals¹

Liquid Crystal Dimers

“…One way by which the problem of phase separation could be solved experimentally was suggested by Fletcher and Luckhurst and involved covalently linking a rod-like and disc-like unit via a¯exible spacer yielding a non-symmetric discotic-calamitic liquid crystal dimer [123]: 18 These materials did not exhibit liquid crystalline behaviour with just one exception, the hexyl member, which exhibited a strongly monotropic nematic phase. Furthermore, the clearing temperatures of these non-symmetric dimers are considerably lower than the average values of the clearing temperatures for the discotic [112] and calamitic [18] parent symmetric dimers. The authors suggest that this re¯ects the extreme dif®culty in packing the disc-like and rod-like units simultaneously and by increasing the spacer length this problem may, at least in part, be relieved.…”

Chemical Structure and Mesogenic Properties

“…This fact and the occurrence of the columnar (phasmidic) phase are clearly due to the high concentration of aliphatic chains in the molecules. Hexacatenar compounds derived from cinnamic acid exist as dimeric molecules (Table 41) [213]. The compounds represent the seldom occurring biaxial nematic phase.…”

“…The compounds represent the seldom occurring biaxial nematic phase. Some compounds with four rings in the core are also nematic; however, their biaxiality has not been proven [213].…”

A Low‐Molar‐Mass, Monodispersive, Bent‐Rod Dimer Exhibiting Biaxial Nematic and Smectic A Phases

“…[4] Designing a molecular structure for realizing a N b phase is not a trivial task, necessitating careful molecular engineering. Nonetheless, several different classes of LCs have been realized [4][5][6][7][8] in which the shape biaxiality [9] of the rodlike or disclike entity is optimized by breaking the shape symmetry. Another phase, the smectic A (SmA) phase, generally has a uniaxial character (Figure 1 c), although the possibility of a biaxial smectic A (SmA b ) phase in which the molecules are along the layer normal, but have an additional director m in the plane of the layers, was pointed out.…”