Chemistry

Side Arms Change Pitch

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Science  22 Mar 2002:
Vol. 295, Issue 5563, pp. 2179
DOI: 10.1126/science.295.5563.2179a

When a nematic liquid crystal is made up of chiral (or handed) molecules, the molecules themselves not only orient with respect to each other but can also adopt a helical pitch. Adding achiral molecules would be expected to decrease (or dilute) the twisting power of the chiral nematic and to increase the helical pitch. Thisayukta et al. examined what happened when banana-shaped molecules, with similar side arms but different central linkers, were added to a linear chiral nematic. Surprisingly, they observed an increase in the twisting power of the chiral nematic and a decrease in the optical pitch. Liquid crystals made of banana-shaped molecules (which lack an asymmetic carbon atom) have been known to form chiral smectic phases. These phases are believed to form as a result of a layering of the molecules, an inherent twisting of the side arms, or a helical twisting of the molecules about the director. In this case, the authors conclude that the chirality does not arise from how the molecules pack but instead is caused by twisting of the side arms. — MSL

J. Am. Chem. Soc., 10.1021/ja0123249.

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