Macrocycle for Nuclear Waste

Using a molecule with just 96 carbon atoms, researchers in Japan have created a new type of self-assembled structure: helical, graphitelike nanotubes up to 10 µm long [Science, 304, 1481 (2004)]. When oxidized, the nanotubes have conductivity properties that suggest potential electronics applications for the new structures.

The work was carried out by University of Tokyo chemistry professor Takuzo Aida, Takanori Fukushima, and colleagues as part of the Aida Nanospace Project. The researchers report that they first designed an asymmetrically substituted hexa-peri-hexabenzocoronene (HBC) molecule as a building block for supramolecular self-assembly. The platelike systems of 13 fused benzene rings that make up the HBC moieties are akin to small graphite fragments. Consequently, they tend to stack via -electronic interactions.

By adding hydrophobic dodecyl chains to one side of the HBC core and hydrophilic triethylene glycol (TEG) chains to the other, the researchers imagined that, in a relatively polar solvent like tetrahydrofuran, the amphiphilic molecules would self-assemble into a two-dimensional bilayer ribbon. The researchers indeed found that the nonpolar dodecyl chains interdigitate, like sticky fingers, bringing the two HBC layers of the bilayer ribbon together. On the ribbon’s surface, the polar TEG chains suppress the buildup of additional layers.

This ribbon then curls into a helix, driven by steric repulsion among the phenylene substituents that anchor the TEG chains, Aida says. And when the helix curls tightly, it forms a tube.

Using this strategy, Aida and Fukushima’s group was able to prepare nanotubes in quantitative yield—a great advantage over the synthesis of other types of graphitic nanotubes, according to Aida. The tubes exhibit remarkable uniformity, with inner and outer diameters measuring 14 nm and 20 nm, respectively. -Stacking interactions occur along the length of the tube between the small graphitic building blocks, which are stacked into columns. The nanotubes are thermally stable, maintaining their integrity even at 100 °C.

In an accompanying commentary, Werner J. Blau and Alexander J. Fleming of Trinity College, Dublin, write that the new route “demonstrates that precise control of intermolecular and environmental forces can lead to graphitic nanotubes with defined dimensions, helicity, and electronic properties—exactly as one frequently needs in molecular electronics and most other applications of carbon nanotubes.”

Motivated by the nanotubes’ potential in molecular electronics, the researchers investigated the electroconductive properties of the structures. As prepared, the tubes are insulating. But when oxidized, their conductivity increases with decreasing temperature—indicating that the oxidized tube is semiconducting.

This conductivity is interesting, the researchers say, “because it is realized by a long-range intermolecular electronic communication through graphitelike molecular arrays.”