The chemistry of higher fullerenes has been explored little due to the historical low availability and high price. As used herein, “higher fullerenes” refer to fullerenes containing more than 70 carbon atoms. C60 and C70 have been shown to be excellent radical scavengers, but the radical scavenging abilities of fullerenes such as C84, and other fullerenes higher in molecular weight than C70 are little known or totally unknown. The relative radical scavenging efficiencies of different fullerenes may be altered due to different number of graphitic bonds, difference in energy strain (correlated to the degree of flatness which typically increases with increasing fullerene molecular weight), the electron affinities, HOMO-LUMO gaps, etc., any or all of which could contribute to relative differences in the efficiency of different fullerenes to scavenge radicals, and affect the resulting utility of different fullerenes in different applications. Chiang (Chemistry Letters 1998) has shown that different C60 fullerene derivatives which have different strains and/or electron affinities show significantly different radical scavenging efficiencies, and that this relative difference is not predictable a priori from the structure of the derivatives and resulting alterations to the C60 cage. Similarly, differences relative to the C60 cage resulting from changes in the number of carbons and changes to the bond nature and electronic structure of higher fullerenes relative to C60 may give significant differences in radical scavenging efficiency. Therefore, the radical scavenging efficiencies and utility of different higher fullerenes are difficult to predict from physical or chemical theories or considerations.
Current commercial-type production methods such as combustion produce sufficient quantities of such fullerenes to make them interesting for a variety of applications, including but not limited to pharmaceuticals and personal care, where radical scavenging, or antioxidant, capacity can be very beneficial.
Because of the high quantum yield of triplet states, C60 is known to be an efficient producer of singlet O2, a reactive oxygen species (ROS) under irradiation. ROS are known to be detrimental to human health and lead to lipid peroxidation, neural damage, skin damage, and other destructive bio-chemical processes. Use of C60 as, for example, an antioxidant for prevention of skin damage may also lead to production of singlet O2 in the presence of sunlight or other irradiation, which in turn may damage the skin. Similarly, the generation of singlet O2 may be undesirable for other reasons, such as in chemical reactions where radical scavenging is desired without the generation of singlet O2, which could react in undesired side reactions. Further, the formation of triplet states of C60 and C70 results in differences in reduction potential (electron accepting ability), so that in redox reaction systems where electron accepting ability of the fullerene leads to undesired chemical pathways, it would be beneficial to have a fullerene that had lower quantum yields of the triplet states. Further still, the triplet states of C60 and C70 may result in energy transfer processes leading to other singlet or triplet states of molecules other than oxygen, resulting in undesired alterations in reactivity of these molecules. Preserving the radical scavenging benefits demonstrated for the fullerenes C60 and C70, namely their high efficiencies of radical scavenging, while minimizing or preventing effects resulting from the high quantum yield of triplet states of C60 and C70, including but not limited to the production of singlet O2, would be highly beneficial.