A [2+2] cycloaddition cap-to-cap C-70 dimer with C-2h molecular symmetry was synthesized in high yield by pressure treatment of polycrystalline C-70 ar I Cpa and 200 degrees C. It was separated from unreacted monomers by chromatography and characterized by C-13 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 inter-cage 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 tripler 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 fur 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 (less than or equal to 5 min) were used.