Banana phases achiral liquid crystals

Chiral Liquid Crystals from Achiral Molecules Banana Phases... 

Figure 8.17 Structure and phase sequence of first banana-phase mesogen, reported by Vorlander in 1929, is given. Liquid crystal phase exhibited by this material (actually Vorlander s original sample) was shown by Pelzl et al.36a to have B6 stmeture, illustrated on right, in 2001. Achiral B6 phase does not switch in response to applied fields in way that can be said to be either ferroelectric or antiferroelectric.

It is possible for chiral mesogens to produce essentially achiral mesophases. For instance, in certain ranges of concentration and molecular weight, DNA will form an achiral line hexatic phase. A curious recent observation is of the formation of chiral mesophases from achiral mesogens. Specifically, bent-core molecules (sometimes called banana LCs) have been shown to form liquid crystal phases that are chiral. In any particular sample, various domains will have opposite handedness, but within any given domain, strong chiral ordering will be present. 

There is a third problem for which chirality information is of current interest anisotropic phases are often stabilized by chiral structures. Apart from chiral structures with enantiomorphic crystals of chiral compounds, suprastructural chirality exists in liquid crystal phases built up by chiral molecules as in the cholesteric phases and the smectic C phases. Even liquid crystalline phases with suprastructural chirality originating in achiral, so-called banana-shaped molecules, seem to be possible. Anisotropic polymer films with chiral structures have been found. It can be anticipated that chiroptical spectroscopy with anisotropic chiral systems will lead to new questions and answers. 

Another example is the swollen B4 phase of banana-type (bend-type) liquid crystals. The banana-type molecules form smectic hquid crystals in which the molecules are tilted with respect to the layer normal. This tilt and the orientation of the bow-shaped molecules have been of interest, because even achiral molecules can express a chiral structure. In addition, in the mixed system of pentylcyanobiphenyl (5CB), a typical nematic rodlike mesogen, and (P-8-0-PIMB), one of the banana-type liquid crystals, the specific B4 phase is formed in a wide mixing range of up to 107 wt% 5CB [63]. In addition, there is a new Bx phase in the low-temperature... 

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