"Endohedral" Fullerenes

The usual way to combine fullerenes with other atoms is to put the other atoms between the balls-- something like pouring marbles into a box filled with bowling balls. But really, the bowling balls aren't solid; they are shells. Why not put the other atoms inside the balls?

Indeed, people have done this. Fullerenes with enclosed atoms are called "endohedral" fullerenes. When the atom(s) trapped inside happens to be metallic, they are also called metallofullerenes. Even though C₆₀ is the most common fullerene, few endohedral materials have a C₆₀ cage because it is fairly small inside. Most of these materials are made out of C₈₂, C₈₄, or even higher fullerenes.

You may recall from the page on alkali-C₆₀ compounds that many fullerene compounds are air-sensitive-- the oxygen can literally pull the extra atoms out of the fullerene lattice. Not surprisingly, that can't happen when the other atoms are safely enclosed in the fullerene cage. So endohedral materials are slightly easier to work with.

One possible application is to shield radioactive "tracer" atoms inside fullerene cages, then inject them into people's blood in order to watch blood flow. Or possibly a special fullerene could be designed that carries a drug inside its cage, then releases the drug (probably by dissolving or being broken down by a natural body chemcial) after a time delay. We're still years away from making this practical, however.

It is not easy to get the atoms inside, and get them to stay there. Only a handful of atoms will form stable endohedral compounds (a few of these are lanthanum, yttrium, scandium, and some of the noble gases); the others tend to break up because the chemical bonds cannot form at the correct angles. Getting the atoms inside is difficult to accomplish once the cage has already formed, so endohedral materials must be formed as the cages are made (ie, the cage must "wrap around" the atom as it comes together). Making endohedral materials with trapped noble gas atoms is done in two usual ways: striking an arc between graphite electrodes or bombarding a graphite target with a powerful laser, both in the presence of noble gas vapor. For metallofullernes, the pure graphite is replaced by metal-impregnanted graphite. Both techniques produce a large amount of carbon soot, some small fraction of which consists of endohedral fullerenes. Then a lot of tricky chemistry is required to separate out the endohedral materials from the carbon junk.

Not many measurements have been done on endohedral materials yet because they are still made in fairly small quantities (mg). It's just not that efficient to produce them. This will probably change in the near future because there is increasing excitement about endohedral fullerenes in the chemistry community. As a physicist, I have been able to make my own small contribution by measuring the specific heat of two of these endohedral materials. Using a sensitive microcalorimeter decribed elsewhere on these pages, I have measured La@C₈₂ and Sc₂@C₈₄.

I have written a paper on my results, which has been published in the Journal of Chemical Physics. Here's the reference: Specific heat of endohedral and higher fullerene thin films, J. Chem. Phys., Volume 11, Number 12, p. 5291 (1999).

(The accepted notation for endohedral materials is to use the "at" symbol-- @ -- to show that the first material is inside the second. The "@" is supposed to look like a shell enclosing a smaller molecule).

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Email: kimall (at symbol) mindspring.com