Dimers biaxial nematics

The existence of the thermotropic biaxial nematic phase was theoretically predicted almost thirty years ago [119] but such a phase has yet to be unambiguously identified. Indeed, as we saw in the previous section, a biaxial nematic phase has been claimed for a class of discotic dimers [111], Theory predicts that mixtures of rod-like and disc-like molecules should exhibit the biaxial nematic phase [120] but experimentally such systems phase separate... 

This experimental difficulty was overcome by simulating the behaviour of a model system and a biaxial nematic phase was indeed obtained [122], 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 flexible spacer yielding a non-symmetric discotic-calamitic liquid crystal dimer [123] ... 

Now the biaxial nematic phase has two order parameters Q and and, in general, three different phases can be distinguished, namely, isotropic (Q = Qi = 0), uniaxial nematic Q, Q i = 0) and biaxial nematic (01,02) phases. Note that biaxial molecules may form both biaxial and uniaxial phases the latter appear due, for instance, to free rotation of biaxial molecules around their long molecular axes. As to the uniaxial molecules, they may also form either uniaxial (as a rule) or biaxial phases the latter may be formed by biaxial dimers or other building blocks formed by uniaxial molecules. 

Hexacatenar compounds derived from cinnamic acid exist as dimeric molecules (Table 41) [213]. 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],... 

In order to investigate the gap between dis-cotic and calamitic liquid crystals and in an attempt to obtain biaxial nematics, Fletcher and Luckhurst [235] synthesized hybrids of rod-like and disk-like mesogens, by linking two units to dimers. Among the several compounds of this type is one example that shows nematic behaviour (21) however, the... 

Y. Wang, H. Yoon, H.K. Bisoyi, S. Kumar, Q. Li, Hybrid rod-like and bent-core liquid crystal dimers exhibiting biaxial smectic-A and nematic phases. J. Mater. Chem. 22, 20363-20367 (2012)... 

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