J. Phys. Chem. B 2006, 110, 16979-16981
16979
Determination of the Thermal Stability of the Fullerene Dimers C120, C120O, and C120O2 Jinying Zhang,*,† Kyriakos Porfyrakis,*,† Mark R. Sambrook,† Arzhang Ardavan,†,‡ and G. Andrew D. Briggs† Department of Materials, Oxford UniVersity, Oxford OX1 3PH, United Kingdom, and Clarendon Laboratory, Department of Physics, Oxford UniVersity, Oxford OX1 3PU, United Kingdom ReceiVed: April 24, 2006; In Final Form: June 19, 2006
We have produced the fullerene dimers C120, C120O, and C120O2 by a high-speed vibration milling technique. The thermal stability of C120, C120O, and C120O2 has been studied in the temperature range 150-350 °C for up to 4 h under vacuum. The bridging oxygen atoms were found to substantially increase the stability of the fullerene dimer molecules.
1. Introduction Endohedral fullerene dimers offer a model system for investigating the interaction between electron spins in molecules with potential for quantum nanotechnologies and quantum information processing (QIP).1 Empty fullerene dimers present a first step toward making endohedral fullerene dimers. To realize a one-dimensional array, for example, fullerene dimers can be inserted into single-walled nanotubes by heating the mixture of dimers and open-end nanotubes at 350 °C for several hours.2 A knowledge of the thermal stability of these fullerene dimers is therefore essential for the application of this methodology. High-speed vibration milling (HSVM) has been used to synthesize several types of fullerene dimers, such as C60C60 and C60-C70 directly bonded [2+2] dimers,3,4 the siliconbridged dimer C60-SiPh2-C60,5 and the germanium-bridged dimer C60-GePh2-C60.6 Several different experimental techniques can be employed for studying the thermal stability of fullerene dimers. Raman scattering,7 differential scanning calorimetry (DSC),3 thermal treatment of fullerene dimer solutions,3 HSVM,8 photochemical dissociation,8 and infrared (IR) spectroscopy9 have all been used to study the thermal stability of the C120 directly bonded dimer. Solid-state thermal treatment10 and IR spectroscopy9 studies of the thermal stability of the C120O furan-linked dimer have also been reported. Each of these methods was used to study the thermal stability of only one specific dimer, and, generally, at only one temperature. The materials investigated were either polymeric or in solution, and the methods employed had limitations; DSC, for example, depends on the scanning rate, and HSVM measures the equilibrium composition. The thermal stability of the fullerene dimer C120O2 has not previously been measured. Here, we present the synthesis of the oxygencontaining dimers C120O and C120O2 by HSVM. The thermal stability of these, and of the directly bonded C120 dimer, in the solid state has been investigated. 2. Experimental Methods A mixture of C60 (MER corporation, 99.5+%) (72 mg, 0.1 mmol) and K2CO3 (Aldrich, 99.995%) (27.6 mg, 0.2 mmol) * To whom correspondence should be addressed. Phone: 00441865273719 (J.Z.); 00441865273724 (K.P.). E-mail:
[email protected] (J.Z.);
[email protected] (K.P.). † Department of Materials. ‡ Department of Physics.
Figure 1. HPLC chromatograms of the fullerene dimers C120, C120O, and C120O2 (Buckyprep M 20×250 mm, toluene eluent, 18 mL/min).
was placed in a HSVM reaction capsule. The reaction was carried out at a speed of 2500 cycle/min for 1 h under an air atmosphere. High performance liquid chromatography (HPLC) (5PBB, 20×250 mm, toluene eluent, 18 mL/min) was used to separate the product mixture. The dimers were characterized by MALDI mass spectrometry (DCTB matrix), ultravioletvisible (UV-vis) spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. About 50 µg of each purified dimer was placed in an electron spin resonance (ESR) quartz tube (3 mm diameter), dried and sealed under vacuum (10-3 mbar). Four (C120O and C120O2) and five (C120) samples were prepared and heated separately in an oven at 150 (C120 only), 200, 250, 300, and 350 °C for 4 h. The samples were collected, extracted with o-DCB, filtered, dried, dissolved in toluene, and then injected into HPLC to determine the composition of the resulting compound mixture. The integrated areas of the absorption peaks from the HPLC chromatogram were used to analyze the composition of the thermally treated carbon materials.
10.1021/jp062506v CCC: $33.50 © 2006 American Chemical Society Published on Web 08/10/2006
16980 J. Phys. Chem. B, Vol. 110, No. 34, 2006
Zhang et al. TABLE 1: Fullerene Dimer Thermal Stability Dataa C120
C120O
C120O2
temperature (°C)
C60
C120
C60
C120O
C120O2
C60
C120O2
150 200 250 300 350
57 65 77 85 93
43 35 23 15 7
4 26 23 66
96 74 43 1
0 0 34 33
5 3 8 13
95 97 92 87
a The numbers show the composition of the mixture after thermal treatment data, expressed in percentage moles of C120: 2 mol C60 ) 1 mol C120, 1 mol C120 ) 1 mol C120, 1 mol C120O ) 1 mol C120, 1 mol C120O2 ) 1 mol C120.
Figure 2. HPLC chromatograms of pure C120, C120O, and C120O2. The insets show the corresponding mass spectra and structures. For more detailed mass spectra, please refer to the Supporting Information.
3. Results and Discussion The HPLC chromatograms of the HSVM product mixture (Figure 1) indicate three dimer products eluting at around 2335 min (C60, 7.2 min; C60O, 7.8 min), as expected from literature data,8,12-14 to be C120, C120O, and C120O2 sequentially. All three dimer products were additionally characterized by UV-vis and FTIR spectroscopies and mass spectrometry and found to be in
agreement with previous reports.3,12 HPLC chromatograms, mass spectra, and the structures of the C120, C120O, and C120O2 dimers are shown in Figure 2. FTIR spectra (KBr disk) of C120, C120O, and C120O2 show four intense transitions at 800, 1018, 1091, and 1261 cm-1, which are not observed in pristine C60. Previously, only C120 has been observed as an HSVM product when the reaction is carried out under nitrogen. Other products were found in addition to C120 when the reaction was carried out under air, but they were not identified.8 In our experiment, C120O and C120O2 dimers were also produced and identified. We deduce that atmospheric oxygen reacts with C60 to form oxygen-bridged dimers under the thermal treatment conditions. The degree of decomposition of all three fullerene dimers was found, as expected, to increase with increasing temperature (Table 1, Figure 3). The experimental results indicate that C120 begins to decompose at temperatures below 150 °C and that it undergoes complete decomposition into C60 at temperatures greater than 350 °C (Table 1). The decomposition temperature of C120 detected in our experiment is slightly lower than that previously reported in the literature by DSC (162 °C)3 and Raman spectroscopy (>190 °C)7 and higher than the temperature determined by IR spectroscopy (fully decomposed at 200 °C for 30 min).9 The C120O dimer was found to be more stable than the C120 dimer, remaining intact at 200 °C. During the thermal treatment, the C120O dimer not only decomposed into C60 but also gave rise to the formation of C120O2 at temperatures greater than 300 °C. The decomposition temperature measured in our experiment is somewhat lower than that reported in the literature by IR spectroscopy measurement (no decomposition at 250 °C and significant decomposition at 280 °C).9 We detected no C120O2 after thermal treatment below 250 °C, but more than 30% of C120O reacted to give C120O2 above 300 °C (Table 1, Figure 3). This is in agreement with previously reported C120O thermolysis results10 with the exception that no C60O was detected in the decomposition products. The C120O2 dimer is very stable, with very little decomposition detected even after heating for 4 h at 350 °C (Table 1, Figure 3). No C60O was detected by HPLC after the thermal treatment. Both C120O and C120O2 could be expected to decompose, initially, into C60 and C60O. We observed that C60 is the only decomposition product, confirming that C60O is unstable across the temperature range 200-350 °C when heated for 4 h, as in agreement with the reported literature.15,16 Figure 3 (1) also shows that C120 is the least stable and C120O2 is the most stable of the three dimers, which is consistent with the previous IR experimental and semiempirical (PM3) and DFTB modeling results. The IR spectroscopy showed that the decomposition temperature of C120 is lower than the decomposition temperature of C120O.9 The PM3 modeling showed that the dissociation energies of C120, C120O, and C120O2 were calculated to be 154.5, 224, and 436 kJ/mol.12,17 DFTB modeling
Thermal Stability of C120, C120O, and C120O2
J. Phys. Chem. B, Vol. 110, No. 34, 2006 16981
Figure 3. Thermal stability of fullerene dimers: (1) the percentage of dimers that survived after thermal treatment; (2) C120O thermal stability results.
gives smaller binding energies, with 29, 198, and 296 kJ/mol respectively.18,19 According to Baeyer strain theory, the C120 dimer connected by a cyclobutane ring, with a bond-angle compression of about 109.5° - 90° ) 19.5°, should be more active than the C120O dimer connected by a furan-like ring, with a bond-angle compression of about 109.5° - 108° ) 1.5°, which is also consistent with the experimental results. 4. Conclusion The directly bonded dimer C120 and oxygen-bridged dimers C120O and C120O2 can be successfully produced when a solid mixture of C60 and catalyst react in HSVM. Under thermal treatment in the solid state, the C120 is found to be the least stable and C120O2 the most stable of these three dimers. In the solid state, the C120 dimer begins to decompose at a temperature below 150 °C and fully decomposes to C60 above 350 °C in a vacuum when heated for 4 h. The C120O dimer decomposes into C60 and reacts to give C120O2 above 300 °C. The C120O2 dimer is stable even at 350 °C. There is little or no C60O in either C120O or C120O2 decomposition compounds. Acknowledgment. We thank Dr A. N. Khlobystov for help and advice and Dr S. M. Lee and Professor D. G. Pettifor for discussion about the modeling of these materials. This research is part of the QIP IRC www.qipirc.org (GR/S82176/01). J.Z. is supported by a Clarendon Scholarship, Overseas Research Student Scholarship and a Graduate Scholarship from The Queen’s College. A.A. thanks the Royal Society for a University Research Fellowship. G.A.D.B. thanks EPSRC for a Professorial Research Fellowship (GR/S15808/01). Supporting Information Available: Mass spectrometry (MALDI, DCTB matrix): C120 (m/z: 720), C120O (m/z: 720, 736, 1457), C120O2 (m/z: 720, 736, 1473). UV-vis spectroscopy: C120 (328 nm, 434 nm, 700 nm), C120O (326 nm, 416 nm, 542 nm, 696 nm), C120O2 (324 nm, 426 nm, 632 nm, 698 nm). FTIR spectroscopy (KBr): C120 (798 cm-1, 1019 cm-1,
1093 cm-1, 1261 cm-1, 1431 cm-1, 1613 cm-1), C120O (799 cm-1, 1019 cm-1, 1090 cm-1, 1261 cm-1, 1376 cm-1, 1453 cm-1, 1633 cm-1, 1712 cm-1), C120O2 (799 cm-1, 1018 cm-1, 1091 cm-1, 1261 cm-1, 1448 cm-1, 1725 cm-1). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Benjamin, S. C.; Ardavan, A.; Briggs, G. A. D.; Britz, D. A.; Gunlycke, D.; Jefferson, J.; Jones, M. A. G.; Leigh, D. F.; Lovett, B. W.; Khlobystov, A. N.; Lyon, S.; Morton, J. J. L.; Porfyrakis, K.; Sambrook, M. R.; Tyryshkin, A. M. J. Phys.: Condens. Matter 2006, 18, S867. (2) Porfyrakis, K.; Khlobystov, A. N.; Britz, D. A.; Morton, J. J. L.; Ardavan, A.; Kanai, M.; Dennis, T. J. S.; Briggs, G. A. D. AIP Conf. Proc. 2004, 723, 255. (3) Wang, G.; Komatsu, K.; Murata, Y.; Shiro, M. Nature 1997, 387, 583. (4) Komatsu, K.; Fujiwara, K.; Murata, Y. Chem. Commun. 2000, 1583. (5) Fujiwara, K.; Komatsu, K. Org. Lett. 2002, 4, 1039. (6) Murata, Y.; Han, A.; Komatsu, K. Tetrahedron Lett. 2003, 44, 8199. (7) Wang, Y.; Holden, J. M.; Bi, X.; Eklund, P. C. Chem. Phys. Lett. 1994, 217, 413. (8) Komatsu, K.; Wang, G.; Murata, Y.; Tanaka, T.; Fujiwara, K. J. Org. Chem. 1998, 63, 9358. (9) Garkusha, O. G.; Solodovnikov, S. P.; Lokshin, B. V. Russ. Chem. Bull., Int. Ed. 2003, 52, 1688. (10) Gromov, A.; Lebedkin, S.; Ballenweg, S.; Avent, A. G.; Taylor, R.; Kratschmer, W. Chem. Commun. 1997, 209. (11) Tagmatarchis, N.; Forman, G. S.; Taninaka, A.; Shinohara, H. Synlett 2002, 235. (12) Eisler, H.; Hennrich, F. H.; Werner, E.; Hertwig, A.; Stoermer, C.; Kappes, M. M. J. Phys. Chem. A 1998, 102, 3889. (13) Lebedkin, S.; Ballenweg, S.; Gross, J.; Taylor, R.; Kratschmer, W. Tetrahedron Lett. 1995, 36, 4971. (14) Tsyboulski, D.; Heymann, D.; Bachilo, S. M.; Alemany, L. B.; Weisman, R. B. J. Am. Chem. Soc. 2004, 126, 7350. (15) Chibante, L. P. F.; Heymann, D. Geochim. Cosmochim. Acta 1993, 57, 1879. (16) Taylor, R.; Penicaud, A.; Tower, N. J. Chem. Phys. Lett. 1998, 295, 481. (17) Matsuzawa, N.; Ata, M.; Dixon, D. A.; Fitzgerald, G. J. Phys. Chem. 1994, 98, 2555. (18) Porezag, D.; Jungnickel, G.; Frauenheim, Th.; Seifert, G.; Ayuela, A.; Pederson, M. R. Appl. Phys. A 1997, 64, 321. (19) Fowler, P. W.; Mitchell, D.; Taylor, R.; Seifert, G. J. Chem. Soc., Perkin Trans. 2 1997, 1901.
