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MATERIALS SCIENCE
Japanese researchers have unveiled a new design strategy for polar liquid-crystalline materials that they expect will be applicable to optoelectronic devices.
In the mid-1990s, Nakamura's group discovered a simple procedure for attaching rodlike aromatic groups to the five carbons encircling a single pentagon of the C60 cage. He realized that by carefully choosing these groups, the molecules could be induced to stack because the aromatic "feathers" form a conical cavity that has a chemical affinity for buckyballs. This stacking could be further promoted by adorning the aromatic rods with long aliphatic side chains, which have a low affinity for C60 and hence prevent slippage of the fullerene apex out of the cone. The resulting conical molecules--reminiscent of the shuttlecock used in badminton--stack to form polar columnar assemblies in which each buckyball is buried in the conical tail of the molecule in front of it [Nature, 419, 702 (2002)]. These head-to-tail columns organize parallel to each other in a regular manner, according to the researchers, forming a hexagonal two-dimensional lattice in which each column is oriented in a direction opposite to the neighboring columns. Nakamura notes that these new materials form liquid-crystal (LC) phases that are dependent on temperature as well as on their concentration in a liquid mixture. In the fullerene-containing liquid crystals that have been reported previously, Nakamura points out, the phase behavior depended only on temperature, and the fullerene moiety did not play a pivotal role in the formation of the liquid-crystalline phase. "This design strategy should be applicable to a range of other molecules and materials," the Tokyo researchers note. Nakamura anticipates that the new liquid crystals will have optical and electronic properties that could be exploited in devices. In an accompanying commentary, chemistry professor Carsten Tschierske of Martin Luther University in Halle, Germany, writes, "Many of the advances in LC research have been stimulated by fresh designs of molecules that form new LC phases." The shuttlecock molecules "have undoubtedly provided a new design principle for research teams to play with." |
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