Filling and fathoming fullerene molecules.

Just as jelly-filled doughnuts have extra appeal, so do metal-filled fullerenes. Several research groups report that they have inserted up to four metal atoms into these cage-like, all-carbon molecules, paving the way for a new class of fullerene materials.

Other new successes in understanding and manipulating fullerenes include isolating large fullerenes, making fullerene polymers and chemically modifying the 60-carbon version called the buckyball. Chemists described these results at last week's Materials Research Society meeting in Boston.

Mark M. Ross and his colleagues at the Naval Research Laboratory in Washington, D.C., made fullerenes with one or more yttrium atoms inside and studied them with two kinds of mass spectroscopy. They created the material by using a laser to vaporize yttrium powder in a chamber with graphite and fullerenes. The researchers suspect that the laser causes a fullerene molecule to open up and trap a metal atom, explains Ross. Sometimes, two or more of the opened, metal-filled fullerenes collide to form a larger molecule with multiple atoms inside.

At the University of California, Los Angeles, a group led by Robert L. Whetten has created numerous metalfilled fullerenes - predominantly 80-carbon molecules containing two lanthanum atoms each. The team finds that while high-speed fullerenes bounce back when they collide with silicon surfaces, those with metal atoms inside do not rebound as readily.

Constantino S. Yannoni and his colleagues at IBM's Almaden Research Center in San Jose, Calif., made microgram quantities of various fullerenes containing lanthanum, scandium or yttrium atoms. When they heated the resulting materials, they discovered that one form of the 82-carbon fullerene had converted into another form, with the metal still inside. The IBM group used electron spin resonance spectroscopy to analyze the distribution of electrons in these substances and found that the metal atoms inside give up a total of three electrons to the fullerene, Yannoni says.

"These [new reports] are the first independent confirmations that metal-containing fullerenes have been made," notes Richard E. Smalley, whose group at Rice University in Houston first reported putting atoms inside a buckyball (SN: 8/24/91, p.120).

Researchers get a variety of sizes when they create metal-fulurene complexes. The Rice team's most common product is the 82-carbon molecule with one lanthanum or yttrium inside, says Smalley. He and his co-workers believe they have also produced molecules with four lanthanum atoms lined up like peas in a long fullerene pod. He notes, however, that no one yet has successfully separated filled from empty fullerenes for further study.

Organic chemist Fred Wudl says his group at the University of California, Santa Barbara, can add up to six carbon atoms, one at a time, to a buckyball without destroying the molecule's soccerball shape or properties.

In addition, Wudl says he and his colleagues are working on two kinds of fullerene polymers. One strings the fullerene molecules like pearls in a necklace; the other dangles them from a central backbone like charms from a bracelet. Douglas A. Loy at Sandia National Laboratories in Albuquerque, N.M., also reports making a buckyball polymer.

Paul J. Krusic, a materials scientist at the Du Pont Central Research and Development Experimental Station in Wilmington, Del., says his team finds buckyballs "extraordinarily reactive to free radicals." This ability to soak up charged molecules suggests that fullerenes may prove useful as catalysts.