Angewandte Chemie International Edition
Copyright © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
For full article and contact information, see Angew. Chem. Int. Ed. 1999, 38 (17), 2603 - 2606
Versatile Liquid Crystals
A "hospitable" molecule lines up
in long chains when "visited"
by the right guest
Liquid crystals are commonly found in digital alarm clocks, whose displays contain rod-shaped molecules that are normally disordered - in this state they are transparent. If an electric voltage is applied, however, the molecules arrange themselves parallel to each other, like sardines in a can, so densely that light can no longer pass through them. Liquid crystals are capable of much more; Julius Rebek, Jr. and his co-workers at The Scripps Research Institute in La Jolla, California have, for example, developed a molecule that turns into a liquid crystal only after an exactly fitting guest molecule settles into a preformed molecular cavity. On top of this, the liquid crystal can then be pulled into threads reminiscent of Kevlar or spider silk.
Rebek calls these interesting molecules "polycaps". They consist of two hollow, hemispherical pieces that are bound to each other by a short, rigid beam attached to their convex side. They look like dumbbells whose weights have been cut through the center The main attraction: if these dumbbells are dissolved in a liquid such as chloroform, two of the hemispheres arrange themselves around a chloroform molecule like protective hands. This not only encloses the solvent molecules in a sort of hollow sphere (made of the two hemispheres), the capsules are also lined up in long chains. These chains then act like those in many liquid crystals - they change the nature of light just like the well-known examples. Under specific conditions, Rebek's co-workers were even able to make fibers of this material.
Most plastics are made of similar molecular chains (polymers), with the difference that Rebek's polycaps only form in the presence of molecules that fit exactly into the two hollow hemispheres. If the polycaps are placed in certain solvents, the sensitive dumbbell chains are destroyed immediately. Rebek is confident, however, that his hospitable molecules are good for all sorts of promising jobs. For example, if the hemispherical portions are tailored such that only a very specific type of molecule, such as pollutants, can be enclosed, a display made of these liquid crystals would darken as soon as it came in contact with these substances.