ISOLATION AND PROPERTIES OF SMALL-BANDGAP FULLERENES

The diversity of molecular structures exhibited by fullerenes(1) sugge sts a wide range of interesting and useful properties. Several fullere nes are now considered to be well characterized, but only minor variat ions in their chemical and physical properties have been observed(2). Here we show that there are in fact two distinct classes of fullerenes , with some very different chemical properties. Members of the first c lass, typified by C-60 and C-70, have large energy gaps between the hi ghest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO ), and are soluble in many organic solvents: The second, previously un recognized class is represented by C-74 and selected isomers of the hi gher fullerenes, such as that of C-80 with icosahedral symmetry: these are either free radicals or have small HOMO-LUMO gaps. Like radical m etallofullerenes, they are kinetically unstable and react readily to f orm insoluble, polymerized solids. These intermolecular bonds can be b roken by electrochemical reduction. After reducing them to soluble ani ons, we have been able to isolate and characterize these new fullerene s.