| Abstract
| - A [2+2] cycloaddition cap-to-cap C70 dimer with C2h molecular symmetry was synthesized in high yield bypressure treatment of polycrystalline C70 at 1 GPa and 200 °C. It was separated from unreacted monomers bychromatography and characterized by 13C NMR, Raman, and infrared spectroscopy, and other methods.Remarkably, only one isomer was produced out of the five possible [2+2] cycloaddition products whichhave equally low formation energies according to semiempirical modeling calculations. The dimer obtainedis the one favored when C70 molecules adopt an ordered packing with parallel D5 axes. The intercage bondingin C140, its thermal stability, and intercage vibrational modes are similar to those found for the C60 dimer,C120. Both dimers photodissociate to the monomers in solution, probably via excited triplet states. The UVabsorption and fluorescence properties of C140 are not very different from those of C70, suggesting only weakelectronic interactions between the two cages of C140. In comparison, the pressure-induced dimerization ofC60, under the conditions used for C70, results mainly in C60 oligomers and polymeric chains, but the dimerC120 could be isolated at low yield when short reaction times (≤5 min) were used.
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