In this third lecture, we analyzed the history of fullerenes, then described and discussed their chemical properties, the structures obtained in the molecular or solid state, and their physical properties. Some superb fullerene-based ultrastructures obtained by controlled precipitation-aggregation processes were also presented.
Fullerenes are carbonaceous oligomers in which the carbons are sp2-hybridized like those forming graphitic planes, forming a convex polyhedron of generally spherical or ovoidal shape. Spectroscopic studies have shown that the gas clouds of red giant stars contain carbonaceous molecules, oligomers, analogous to chains of formula Cn. In order to identify these Cn molecules, it was necessary to find laboratory conditions simulating the formation of these oligomers. In 1985, Harold W. Kroto, Robert F. Curl and Richard Smalley carried out laboratory simulations of the physical conditions required for the formation of red giant stars, and made an astonishing discovery: the vaporization of a graphite target by a laser in a helium-filled chamber led to the formation of carbonaceous oligomers, as revealed by mass spectrometry. The presence of carbonaceous molecules with a molecular weight of 720 was observed, but in quantities too small for complete and correct characterization. On the basis of this data and the atomic mass of carbon (12), the formula C60 is proposed (12 × 60 = 720). Using this simple datum, and inspired by the structure of the biosphere (former American pavilion) built by architect Buckminster Fuller for the 1967 Montreal Exposition, Harry Kroto proposed a spherical structure for the C60 molecule, called fullerene C60. The proposed structure corresponds to a polyhedron (truncated Icosahedron). Each fullerene forms a unique class of spherical molecules containing a conjugated π-electron system. It is composed of a closed network of fused hexagons and pentagons. This construction principle is a consequence of Euler's theorem, which states that, for the closure of each spherical lattice by n hexagons, twelve pentagons are required, with the exception of n = 1. The presence of pentagons allows the introduction of curvature and requires the carbon atoms to be pyramided, which will lead to different reactivities for these spheroidal compounds compared with conventional aromatic molecules. In simple terms, the structure of C60 is similar to that of a football made up of twenty hexagonal and twelve pentagonal panels. The work and processes developed by Wolfgang Krätschmer and Donald Huffman have enabled the structure of these new carbon allotropes to be characterized in greater detail, and thus unambiguously established. Indeed, the production of large quantities of soot containing fullerenes by a helium-based process using the creation of an electric arc between two graphite electrodes enables sufficient quantities of fullerenes to be produced and thus purified and characterized by mass spectrometry, nuclear magnetic resonance and X-ray diffraction. Not only was the C60 structure confirmed, but the fullerene family was gradually extended by a handful of new polyhedral compounds (C60, C70, C76, C78, C86, C90, C100... C540). The discovery of fullerenes ushered in a new era in chemistry, overturning the traditional assumption that polyaromatic organic chemistry must be based essentially on planar molecules. The functionalization of these new molecules is an absolutely necessary step in order to integrate them into devices in the form of functional materials. This demand has generated a very rich organic chemistry, which we have described through numerous examples.
