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

Abstract

A [2+2] cycloaddition cap-to-cap C70 dimer with C 2 h molecular symmetry was synthesized in high yield by pressure treatment of polycrystalline C70 at 1 GPa and 200 °C. It was separated from unreacted monomers by chromatography 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 which have equally low formation energies according to semiempirical modeling calculations. The dimer obtained is the one favored when C70 molecules adopt an ordered packing with parallel D 5 axes. The intercage bonding in 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 UV absorption and fluorescence properties of C140 are not very different from those of C70, suggesting only weak electronic interactions between the two cages of C140. In comparison, the pressure-induced dimerization of C60, under the conditions used for C70, results mainly in C60 oligomers and polymeric chains, but the dimer C120 could be isolated at low yield when short reaction times (≤5 min) were used.

Pressure-Induced Phase Transitions of C70 Nanotubes

“…23,24 Upon polymerization, one of the most characteristic feature is the split of the E ' 2 mode (located at 1567 cm -1 for pristine C 70 ) into two peaks due to the reduced symmetry. 25,26 The Raman spectrum of the pristine sample was shown in Figure 1c, from the insert in it we can observe a single…”

Reversible pressure-induced polymerization of Fe(C5H5)2 doped C70

“…In general, orientational phase transitions in C 70 crystals only cause slope changes in some Raman or IR vibration modes, but in our case we also observe the splitting of some modes in the spectra, which can not be related to the orientational phase transition. Instead, the mode splitting in Raman and IR spectra always appears in C 70 polymerization [16,24]. For example, in pristine C 70 , the chemical bond formation between C 70 molecules results in spectroscopic changes with some new modes appearing at around 1 GPa [23].…”

“…We further compare the Raman and IR spectra of C 70 (Fc) 2 at several selected pressures at around 2-3 GPa with that of the C 70 dimer [16] in Figure S7. All the observed spectroscopic features (transitions) under pressure are quite similar to those of the C 70 dimer [16], which strongly suggests the formation of C 70 dimers in C 70 (Fc) 2 above 2 GPa.…”

“…For both structures, neighboring molecules are linked by four-membered rings in a (2+2) cycloaddition between double bonds close to polar pentagons on the C 70 cages. These structures have also been studied in previous work by different methods HPHT treatment or separation by high performance liquid chromatography) [16,17]. In contrast to the dimer of C 60 , C 70 dimers can have several different structures due to its special elliptical molecular shape.…”

“…This gives strict topological constraints on the formation of long-range ordered polymers of C 70 s. Still, some attempts have been made to produce polymeric C 70 . Several different forms of C 70 dimers have been produced, such as C 2h C 140 , C 2v C 140 and C 1 C 140 [15][16][17], and formation of polymeric zigzag chains in initially hexagonally close packed C 70 single crystals is reported by Soldatov et al [18]. We also notice that when the rare-earth metal Sm is intercalated into C 70 [19], the charge transfer between C 70 s and Sm atoms results in Sm-C 70 bonding in a 2D network structure, with a Sm atom as a bridge.…”