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  "Endohedral" Fullerenes

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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 C_60 is
the most common fullerene, few endohedral materials have a C_60 cage
because it is fairly small inside. Most of these materials are made out
of C_82 , C_84 , or even higher fullerenes.

You may recall from the page on alkali-C_60 compounds <ac60.html> 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
<../Microcal/index.html> decribed elsewhere on these pages, I have
measured La@C_82 and Sc_2 @C_84 .

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)
<http://www-physics.ucsd.edu/~thinfilm/papers/kallen1.pdf>.

(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).

/Return to the Main Fullerene Page <index.html> /

Copyright © 1997-present Kim Allen

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Email: kimall (at symbol) mindspring.com