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sphere [5.83], since many [email protected]„c metallofullerenes (where M denotes a metal species) may be sensitive to oxygen/air [5.83,97,98]. With this technique, a sufficient quantity of material has been isolated for EPR, structural, and physical characterization measurements on selected endohedral fullerenes.

To produce macroscopic amounts of endohedral fullerenes by laser vaporization, the target for the laser vaporization is placed in an oven at 1200°C [5.68]. A slow flow of Ar or He gas is passed over the hot La-containing target, and the inert gas then flows down a 50 cm long x 2.5 cm diameter quartz tube centered in a resistively heated furnace. A frequency-doubled YAG laser (green light) operating at 300 mJ and 10 pulses per second was focused through a window on the end of the tube onto a rotating, composite target to vaporize both metal and carbon atoms. Fullerenes and metallofullerenes produced in the process at one end of the tube were entrained in the inert gas, then flowed down the tube, and finally condensed on the quartz tube wall near the opposite end of the furnace. Various mixtures of carbon, carbon-based binder and metal or metal-oxide powders have been consolidated into mechanically stable targets for laser vaporization, similar to the material used in the anode of the arc generator.

Although much progress has been made, no efficient method has yet been found for the production of metallofullerenes ([email protected]„ ) either by laser ablation of M-impregnated targets or by arc vaporization of M-impregnated rods. The weight fraction of [email protected] endohedral fullerenes in the carbon soot produced by arc vaporization is generally found to be no more than ~ 10~3 (or 0.1%). For low concentrations of metal in the M-impregnated rod, the monometal species is favored, but with increasing metal concentration, the dimetal species become more prevalent [5.99].

The extraction of metallofullerenes is also made difficult by their low solubility in solvents for many of the [email protected]„c species (i.e., the solubility is comparable to that for C„c where nc > 70). For example, the following solubility sequence has been reported [5.84] for hexane (room temperature), toluene (room temperature or boiling), or CS2 (room temperature): C60, C70 > C76> C78, C84, [email protected], [email protected] > C86_g6 C98_200.

The separation and isolation of [email protected]„c species remain a challenging aspect of fullerene synthesis research, although steady progress is being made. The first such separation has now been achieved for [email protected]„c (for nc = 74,82,84) [5.83,98] and for [email protected] [5.88], Soot from vaporizing composite rods (M/C ~1 atomic %) was treated in CS2 to extract the soluble fullerenes and [email protected]„c species. The extraction is followed by a two-step chromatography process [5.88,98]. In step I the relative concentration of endohedral fullerenes is enhanced by eliminating C60 and C70, the most common fullerene species. This step involves preparative-scale HPLC, introducing the crude extract from the arc soot into a polystyrene-filled column and using CS2 as the elutant. For example, in the separation of [email protected], four fractions, I, II, III, IV [Fig. 5.11(a)], were observed to elute

Fig. 5.11. High-performance liquid chromatography (HPLC) time profiles taken for (a) the crude extract from carbon soot containing [email protected] and (b) fraction II, which contains mostly C76 and C78 in addition to the small peak labeled M, which is mostly associated with metallofullerenes [5.88].

Retention Time/min Retention Time /mm

Fig. 5.11. High-performance liquid chromatography (HPLC) time profiles taken for (a) the crude extract from carbon soot containing [email protected] and (b) fraction II, which contains mostly C76 and C78 in addition to the small peak labeled M, which is mostly associated with metallofullerenes [5.88].

Fig. 5.12. Time-of-flight mass spectra taken from (a) the crude extract shown in Fig. 5.11(a), (b) fraction II in Fig. 5.11(a), and (c) fraction M in Fig. 5.11(b). Negative ions directly produced by 355 nm laser irradiation were detected as a function of flight time [5.88], from the polystyrene column. Fraction II was found to contain C76, C78, and [email protected], as shown in the time-of-flight mass spectrum in Fig. 5.12(b), which should be compared with the mass spectrum of the crude extract shown in Fig. 5.12(a). Step II of the separation process involves placing fraction II into the Bucky Clutcher I column (Regis Chemical Co.) in which toluene was used as the elutant. The HPLC time profile for the Bucky Clutcher I column is shown in Fig. 5.11(b), where the fraction M was found to contain primarily [email protected], as shown in the mass spectrum of this fraction [Fig. 5.12(c)],

The HPLC separation of a typical [email protected]„c endohedral fullerene is a very time-consuming process. It appears that the principal area requiring improvement would be the development of a more time-efficient precon-centration step for the metallofullerenes. Automated high-performance liq-

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