Structure and Properties of the Fullerene Dimer C140 Produced by Pressure Treatment of C70

A [2+2] cycloaddition cap-to-cap C 70 dimer with C 2 h molecular symmetry was synthesized in high yield by pressure treatment of polycrystalline C 70 at 1 GPa and 200 °C. It was separated from unreacted monomers by chromatography and characterized by 13 C NMR, Raman, and infrared spectroscopy, and other methods. Remarkably, only one isomer was produced out of the five possible [2+2] cycloaddition products which have equally low formation energies according to semiempirical modeling calculations. The dimer obtained is the one favored when C 70 molecules adopt an ordered packing with parallel D 5 axes. The intercage bonding in C 140, its thermal stability, and intercage vibrational modes are similar to those found for the C 60 dimer, C 120 . Both dimers photodissociate to the monomers in solution, probably via excited triplet states. The UV absorption and fluorescence properties of C 140 are not very different from those of C 70, suggesting only weak electronic interactions between the two cages of C 140 . In comparison, the pressure-induced dimerization of C 60, under the conditions used for C 70, results mainly in C 60 oligomers and polymeric chains, but the dimer C 120 could be isolated at low yield when short reaction times (≤5 min) were used.