J . Phys. Chem. 1995, 99, 8200-8201
8200
Fullerene Cages Breakdown Induced in Solution by Ultraviolet Radiation: Experimental Support for the “Window” Formation in Fullerenes? Libor Juha* and V k a Hamplovii Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 180 40 Prague 8, Czech Republic
Pavel Engst and Pavel Kubiit J. Heyrovs@ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, DolejSkova 3, 182 23 Prague 8, Czech Republic
Emmanuel Koudoumas and Stelios Couris Institute of Electronic Structure and Laser, Foundation for Research and Technology-Hellas, P. 0. Box 1527, Heraklion 71I 1 0, Crete, Greece Received: January 11, 1995@
Experimental results of excimer laser-induced photolysis of fullerene Cm in solution are presented and are found in good agreement with recent ab-initio calculations, which confirm the relatively low energy demand for opening the fullerene carbon cages in their triplet excited states. Different quantum yields of photolysis show that a single photon of an ArF laser or two photons of KrFKeCl lasers could be sufficient for opening the fullerene cage. The reaction of the opened Cm with solvent impurities or products of previous decompositions seems more likely than its oxidation by singlet oxygen or than multiphoton decomposition.
Introduction In a recent theoretical work,’ Murry and Scuseria have proposed that opening the C-C bond in the fullerene cage to create 9- and 10-membered rings is a relatively low energy process in the triplet potential energy surface. Such formation of “windows” in the fullerene cage is corroborated by an experimentally determined2 extremely low value of threshold energy for helium release from the interior of Cm (-3.5 eV in comparison with the expected -8.7 eV). The threshold energy for He@& formation under single collision conditions was shown to depend strongly on the internal energy of (260.~ The estimated threshold energy of 3 eV in experiments with highly excited c60 also supports the theoretical work of Murry and Scuseria and is in good agreement with Saunder’s results.2 Zhu et al! reported the formation of C1-@C60 endohedral complexes from c6OCc12 ablated by XeCl laser. They4 suggest that this finding gives evidence of the windows formation in the excited fullerene cage,’ but they presented no quantitative arguments. MNDO and ab-initio calculations’ predict a relative low amount of energy -5.2 eV for opening the fullerene cage (1.7 eV being the energy required to reach the lowest triplet excited state, and 3.5 eV, the calculated energy needed for the cage opening). Previous MNDO calculations show a 7.2-eV barrier for rearrangement of C-C bonds5 and 11.8 eV for fragmentation to C;?and c58 species6 In this paper we report experimental findings related to the laser-induced decomposition of fullerene c 6 0 in solutions supporting Murry and Scuseria’s hypothesis. Experimental Section Fullerenes solutions were prepared from a Cm and C70 mixture purchased from Aldrich, the mixture containing a very small amount of C60O and higher fullerenes. Spectrophotometric grade n-hexane (Merck) was used as solvent. The concentration of the solutions for the major component, i.e. Ca, is given for
* To whom correspondence @
should be addressed. Abstract published in Advance ACS Abstracts, April 15, 1995.
each experiment. The ratio of C6dC70 in these solutions was determined by liquid chromatography and was found equal to about 3:l. In all experiments, either 5 or 3.5 mL of solution was irradiated in quartz cuvettes by a given number of either ArF or KrF laser pulses (Lambda Physik LPX200 series). A quartz lens (f = 30 cm) was used to focus the beam into the cuvette, and the power density was measured at the entrance window of the cuvette. The irradiated samples were analyzed by high pressure liquid chromatography on a column with a C18 reverse phase, a mixture of toluene and methanol in a ratio of 1:l as the mobile phase, and photometric detection at 330 nm. The W - v i s absorption spectra of each solution were measured before and after irradiation. Some of the samples were bubbled prior to irradiation with an inert gas (helium or argon) for 1 h to remove the oxygen from the solution.
Results and Discussion Influence of the photolyzing wavelength on the efficiency of decomposition of Cm under similar power density conditions is shown in Figures 1 and 2. It is evident that ArF laser-induced photolysis of c m (Ephot= 6.4 eV) is strikingly more efficient than that induced by the KrF laser (Ephot = 5.0 eV), where the difference in quantum yields is higher by more than an order of magnitude. This can be understood through the MunyScuseria mechanism, where opening of the fullerene cage requires a single photon of ArF laser radiation as opposed to the two photon process in the KIF case. The decomposition efficiency difference therefore arises from the different probability amplitudes of one- and two-photon processes. Furthermore, and in support of the Murry and Scuseria mechanism, in solutions saturated by inert gas (He, Ar), in which the c 6 0 triplet state is not quenched by oxygen, laser photolysis is in general more efficient. In addition, it strongly depends on solvent purity being strikingly more efficient in solvents with a higher content of impurities. This may be understood if the impurity molecules can participate more effectively in the
0022-3654/95/2099-8200$09.00/0 0 1995 American Chemical Society
J. Phys. Chem., Vol. 99, No. 20, 1995 8201
W-Induced Breakdown of Fullerene Cages in Solution 1.5 I
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the singlet oxygen. As the photooxidation of fullerenes irradiated by an Ar+ laser has been observed by Taliani et al.,9 it is very likely that only the somehow modified fullerene cages can enter into reaction with the singlet oxygen. We believe that the idea of Muny and Scuseria’ could provide a plausible explanation. The question is whether the rate of Cm opening can compete with subnanosecond thermalization process. The opening of the fullerene cages in highly excited state is connected with breaking one C-C bond. This last process could be completed within a few vibrational periods. Nevertheless, the probability of window formation seems to be low if the energy is to be distributed statistically among 174 vibrational modes of Cm. Estimated very low quantum yields -5 x for ArF and -2 x for KrF laser-induced decomposition of Cm (Figures 1 and 2) can support this idea. The conclusions from the above experimental observations can be summarized in the following scenario of the fullerene decomposition mechanism in the case of small molecules contained in solution (such as oxygen, impurities present in the solvent and in the initial fullerene sample, fullerene decomposition products, solvent molecules, and noble gas atoms). Such small molecules can attach to the damaged fullerene cage which absorbed one (ArF laser) or two (XeC1 and KrF lasers) photons and prevent the incision from a self-repair and finally lead to a fullerene cage breakdown. The self-repairing process can explain the observed of fullerenes irradiated in the gas phase by intense laser radiation. The mechanism considered up to now,I2 which is based on a multiphoton-induced ejection of C2 from Cm is energetically and probabilistically less likely, and it does not account for the crucial role of small molecules in this process. We cannot exclude that these two processes occur simultaneously under given experimental conditions, but the first one, i.e. the photoinduced window formation of the fullerene cage surface, followed by an attachment of small molecules that may eventually lead to breakdown of the cage, is likely to be the predominant mechanism. 1031’3’3314
Acknowledgment. L.J. gratefully acknowledges support from the “UV Laser Facility” operated in FORTH-IESL within the “Access to Large-scale Facilities ERBGEICT 000023” of the European Union and the program of “Cooperation in Science and Technology with Central and Eastern European Countries”.
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Figure 2. Ultraviolet-visible absorption spectra of unexposed 1.2 x M Cm n-hexane solution (containing a small amount of C ~ O of ), the same solution irradiated by 300, 600, 1000, 1500, 2000, and 3000 pulses of an ArF excimer laser (pulse length, 20 ns; energy, 61 mJ; repetition rate, 10 Hz; beam spot at cell wall, 0.72 cm2; 5 mL of solution irradiated in a 2.0 cm long cylindrical quartz cell; spectra measured in a rectangular quartz cell with the length of 1.0 cm). The power density was about 4.2 M W cm-2.
decomposition of the Cm triplet state than the molecular oxygen. A possible role of impurities in fullerene photolysis in solutions has already been mentioned by Taylor.’ Laser photolysis of fullerenes in solutions shows an induction period; a trend has been observed in all our experiments of Wvis spectrophotometry and HPLC. Such an induction period implies that the products of fullerene decomposition can assist in a more efficient photodecomposition of fullerene cages. Fullerene-solvent clathrate microcrystallites exposed to a chemically generated singlet oxygen (at 15 Pa of 0 2 (‘Ag) for 90 min) show no changes in fullerene cages constitution.8 It means that the ground state fullerene cages are unreactive to
References and Notes (1) Muny, R. L.; Scuseria, G. E. Science 1994, 263, 791. (2) Saunders, M.; Jimknez-VBzquez, H. A,; Cross, R. J.; Poreda, R. J. Science 1993, 259, 1428. (3) Sprang, H.; Mahlkow, A.; Campbell, E. E. B. Chem. Phys. Lett. 1994, 227, 91. (4) Zhu, L.; Wang, S.; Li, Y .; Zhang, Z.; Hou, H.; Qin, Q.Appl. Phys.
Lett. 1994, 65, 702. ( 5 ) Muny, R. L.; Strout, D. L.; Odom, G. K.; Scuseria, G. E. Nature 1993, 366, 665. ( 6 ) Stanton, R. E. J . Phys. Chem. 1992, 96, 111. (7) Taylor, R. Philos. Trans. R. Soc. Londo? 1993, A343, 101. (8) Juha, L.; HamplovB, V.; KodymovB, J.; Spalek, 0. J . Chem. Soc., Chem. Commun. 1994, 2437. (9) Taliani, C.; Ruani, G . ; Zamboni, R.; Danieli, R.; Rossini, S.; Denisov, V. N.; Burlakov, V. M.; Negri, F.; Orlandi, G.; Zerbetto, F. J . Chem. SOC.,Chem. Commun. 1993, 220. (10) Weiss, F. D.; Elkind, J. L.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. J . Am. Chem. SOC.1988, 110, 4464. (11) Curl, R. F.; Smalley, R. E. Science 1988, 242, 1017. (12) Juha, L.; Krisa, J.; Liska, L.; HamplovB, V.; Soukup, L.; Engst, P.; KubBt, P.Appl. Phys. 1993, B57, 83. (1 3) O’Brien, S. C.; Heath, J. R.; Curl, R. F.; Smalley, R. E. J . Chem. Phys. 1988, 88, 220.. (14) Gruen, D. M. Nucl. Instrum. Methods Phys. Res. 1993, B78, 118. JP950132S
