CHEMISTRY: Coloring Chiral Excess

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Science  28 Sep 2001:
Vol. 293, Issue 5539, pp. 2351a
DOI: 10.1126/science.293.5539.2351a

In nematic liquid crystals, weak interactions between the rod-like molecules cause them to align locally along one “average” direction. Adding a small amount of a chiral nematic (cholesteric) can cause the entire mixture to behave like the cholesteric liquid crystals used in displays, for which the average alignment direction rotates as one traverses the sample. However, most chiral materials have only a weak helical twisting power.

van Delden and Feringa have coupled a chiral amine or alcohol to an achiral liquid crystalline group and show that the helical twisting power can be increased. When the coupled material then is blended with a nematic, the helical pitch approaches visible wavelengths, and a color signal is obtained. In mixtures containing 15 to 20 weight % of the coupled molecule, the color shifted from violet to red as the enantiomeric excess decreased from 100 to 50%. This technique offers promise for the rapid screening of chiral catalysts generated by combinatorial chemistry. — MSL

Angew. Chem. Int. Ed. 40, 3198 (2001).

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