Introduction to Liquid Crystals


      Chiral liquid crystal molecules have the potential to form chiral mesophases. In fact, it was quite a surprise to discover that DNA (which is highly chiral) could form achiral line hexatic phases.(1,2) Normally, chiral molecules lead to chiral phases, where the director twists as it progresses through the phase.


Cholesteric mesogen

      If a nematic liquid crystal is inherently chiral, then adjacent mesogens will have a preferential twist with respect to one another, which leads to the larger-scale twisting of the internal order. The chiral nematic phase is also called the cholesteric phase (since the phase was first discovered in cholesterols). The chemical structure is of cholesteryl myristate is shown.

      The structure of the twisting nematic layers is shown below. The different nematic layers have been coloured differently so as to differentiate them. The twisting of the nematic director from layer to layer is clear.

Cholesteric phase structure

Idealized cholesteric phase structure


      Beside is a more idealized view of the nematic twisting of the cholesteric phase (viewed both on-angle and dead-on). Clearly the cholesteric phase is chiral, with a very specific handedness. This chiral structure can be observed using optical techniques (for instance, by viewing the phases through crossed polarizers).


      The chiral analogue of the Smectic C phase is denoted by a star (*). In this phase, the tilt direction of the mesogens rotates as one progresses through the layers. In the figure, the different layers are coloured differently for convenience. The diagram to the far right shows the twisting of the mesogens in a more idealized way, and emphasizes the chiral twisting of the director.

Smectic C* phase structure


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1.Podgornik, R.; Strey, H.H.; Gawrisch, K.; Rau, D.C.; Rupprecht, A.; Parsegian, V.A. Proc. Natl. Acad. Sci. USA 1996, 93, 4261.
2.Strey, H.H.; Wang, J.; Podgornik, R.; Rupprecht, A.; Yu, L.; Parsegian, V.A.; Sirota, E.B. Phys. Rev. Lett. 2000, 84, 3105.


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