A NOVEL REACTION that could be generally useful for aromatic synthesis has made possible the assembly of a long-sought family of compounds: the cycloparaphenylenes, which are strings of benzenes joined in a ring-around-the-rosy style (J. Am. Chem. Soc. 2008, 130, 17646). The compounds could prove useful for constructing carbon nanotubes, which hold promise for electronics, advanced biosensors, and other applications.
Postdoc Ramesh Jasti, chemistry professor and Howard Hughes Medical Institute investigator Carolyn R. Bertozzi, and coworkers at Lawrence Berkeley National Laboratory's Molecular Foundry and the University of California, Berkeley, carried out the new synthesis. They succeeded by creating rings of benzenes and cyclohexadienes and then using a new aromatization reaction they developed to convert the cyclohexadienes to benzenes.
The aromatization reaction works under low-temperature conditions and generates high yields of pure products; it is an important achievement in itself. Previously, aromatizations of highly strained compounds led to undesirable rearrangements or formation of complex mixtures.
"It's a landmark synthesis" because of its brevity, elegance, creativity, and high product yields, comments Graham J. Bodwell of Memorial University of Newfoundland, in St. John's, who specializes in conjugated "belt" compound synthesis.
The researchers made -, -, and cycloparaphenylenes, which they call "carbon nanohoops" because they are the fundamental circular building blocks of "armchair" carbon nanotubes, so named because of their conformation.
Cycloparaphenylenes that represent fundamental repeating units of armchair carbon nanotubes have been of long-standing interest to theoreticians and nanotube specialists, but no one could get hold of them before. Such cycloparaphenylenes "are challenging targets, and very important ones," Bodwell says. "People have been thinking about structures like these for decades, and a number of groups have been trying to make them."
Spectral characterization of the compounds revealed some unexpected properties. For example, "We saw that the smallest ring absorbed the lowest energy light, which is opposite to what the textbooks teach us," Bertozzi says. "We believe this is a special consequence of the cyclic structure."
Synthetic cycloparaphenylenes could now make it possible to assemble pure armchair nanotubes under low-temperature conditions. It's hard to control the diameter and structural conformation of carbon nanotubes by available assembly techniques, "but both could conceivably be controlled by using carbon nanohoops as seeds for nanotube growth," Bodwell says. "This is extremely plausible and has a very high chance of success. There are a lot of things that can go right now."