Empty fullerenes

In other recent reports concerning enzymatic inhibition and receptor binding of organofullerene materials, C60 monomalonate adducts were found to selectively inactivate the neuronal nitric oxide synthase isoform in a manner completely preventable by the concurrent presence of superoxide dismutase and catalase [295]. Also two fullerene-steroid hybrids were synthesized and found to decrease both the ATP hydrolysis and Ca2+ uptake activity of SR Ca2+-ATPase while the inhibitions were concentration dependent [296]. 

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While there are no studies of the toxicity of endohedral fullerenes in general, or gadofullerenes in particular, there are several preliminary studies regarding empty fullerenes and their derivatives. In initial steps toward characterizing the safety of water-soluble fullerene derivatives, a cytotoxicity study of various fullerene materials, including underivatized C60, C60[C(COOH)2]3, and two C.0(OH)r species, was conducted (Sayes et al., 2004). The results clearly showed that water-soluble derivatives were significantly less toxic than underivatized C60, and the trend was one of... 

The production of endohedral fullerene complexes in visible amounts was first accomplished by a pulsed laser vaporization of a lanthanum oxide-graphite composite rod in a flow of argon gas at 1200 °C [66]. In this procedure, the newly formed endohedrals, together with empty fullerenes, sublime readily and are carried away in the flowing gas, depositing on the cool surfaces of the apparatus. This sublimate contains the complexes La CgQ, La C74 and La Cg2 (Figure 1.9). Among these, the endohedral molecule La Cg2 exhibits an extra stability. It can... 

TABLE 8.1 Electrochemical Potentials (in V Versus Fc +/Fc) of Empty Fullerenes Obtained in Various Compounds... 

TABLE 8.4 Half-Wave Redox Potential Unless Otherwise Stated (in V Versus Fc + /Fc) of the Yb C2 , 2 Compounds and Some Empty Fullerenes Obtained in Acetonitrile/Toluene (1/4) + 0.1 M (n-Bu NCKL Unless Otherwise Stated... 

In conclusion, carbon-based nanomaterials generally exhibit a good electron-accepting ability. Empty fullerenes are capable of undergoing multiple, distinct one electron... 

The quantum structure and properties of empty fullerenes C are extensively studied with the help of the powerful photoelectron (photoabsorption) spectroscopy technique [2,11]. As for the spectroscopy of gas phase doped... 

The differences between the endohedral and empty fullerene are much greater for binding energies below 5 eV, i.e. for the 7t-MO s. The most striking alteration is the population of MOs below 1 eV which were formerly unoccupied in the empty fullerene. This points to a transfer of charge from the Gd ion to low-lying... 

Figure 3a shows photoemission data from a Gd C82 [7] film measured with Al Ka x-rays compared to the empty fullerene C82. Even at a glance it is easy to see that the x-ray photoemission spectrum of Fig. 3a differs strongly from that shown in Fig. 2 now the C 2s/2p-derived crand 7tMO s are much less distinct and the spectrum is dominated by a large structure centred at a binding energy of about 11 eV. The C82 spectrum can be taken as a good approximation for the C 2s/2p contribution to the photoemission spectrum, and thus can be used to subtract out the non-Gd related emission, resulting in the spectrum shown in Fig. 3b. This pure Gd emission can then be simulated using a simple atomic multiplet...

EMF.V were isolated from soot using extraction with boiling o-dichlorobenzene (DCB) (>99 %, Aldrich) in argon atmosphere. Then the solution of EMF.v and empty fullerenes was thoroughly filtered off, and the solvent was removed in a rotor evaporator. The resulting solution was filtered off, the solvent was evaporated, and the residue was dried at 90 °C for 1 h in vacuum. 

Metallofullerenes can be synthesized typically in two ways similar to the s)mthesis of empty fullerenes, which involves the generation of a carbon-rich vapor or plasma in He or Ar gas atmosphere. The two methods have been routinely used to date for preparing macroscopic amounts of metallofullerenes the high-temperature laser vaporization or "laser-furnace method (Chai et al., 1991 Haufler et al., 1991 Ying et al., 1994) and the standard direct current (DC) arc discharge method (Haufler et al., 1990). Both methods simultaneously generate a mixture of hollow fullerenes (Ceo, C70, C76, C78, Cs4,...) together with metallofullerenes. The production of metallofullerenes can be followed by procedures to extract from soot and to separate/purify the metallofullerenes from the hollow fullerenes (see Sections 2.2, 3.1, and 3.2). 

However, in the solid state, the evidence for the endohedral nature of the contradictory results were reported for extended X-ray absorption fine structure (EXAFS) experiments on an impurified extract of Y Cs2 (i.e., a mixture of Y Cs2 and empty fullerenes). Soderholm et al. (1992) reported that the yttrium atom is exohedrally attached from the outside to the Cs2 cage, whereas Park et al. (1993) reported an endohedral nature of Y Cs2 the nearest-neighbor C-Y distances obtained were 2.53 0.02 and 2.4 A, respectively. Kikuchi et al. (1994a) performed an EXAFS experiment on a purified La Cs2 powder material and reported that the nearest-and next-to-nearest-neighbor C-La distances are 2.47 0.02 and 2.94 0.07 A, respectively. 

Absorption spectra of endohedral mefallofullerenes in fhe ulfraviolet-visible-near IR (UV-Vis-NIR) region are unique as compared with those of empty fullerenes. Normally, the absorption spectra of mefallofullerenes have long fails fo fhe red down fo 1,500 nm or more. The absorption spectra of fhe major isomers of mono-mefallofullerenes M Cs2 (M = Y, La, Ce, Pr, Nd, Gd, Tb, Dy, Ho, Er, Lu) are similar to each other and well represented by a sharp peak around 1,000 nm and a broad peak around 1,400 nm. These absorption peaks may be related to the intrafullerene electron transfers from the encaged metal atom to the carbon cage. 

It had been expected, before the first macroscopic production and extraction of La Cs2 (Chai et al., 1991), that metallofullerenes based on the Cgo cage would be the most abundant metallofullerenes that were prepared in macroscopic amoimts, as was the case in empty fullerenes. This is simply because that Ceo is the most abundant fullerene which can be easily produced by either the arc-discharge or the laser furnace method (cf. Section 2.1). In fact, an earlier gas phase experiment on the production of carbon clusters containing La via the laser-vaporization cluster-beam technique (Heath et al., 1985) indicated that La Cgo is a prominent "magic number" species among various La C (44 < n < 80) clusters (Figure 1). 

Studies of endohedral metallofullerenes directed to application have been reported since the metallofullerenes are structurally and electronically much more unique than the normal empty fullerenes. 

Extracts of different organic solvents normally contain the same soluble species empty fullerenes such as Ceo, C70, and Cs4 are dominant, whereas M Cs2 is always the most abundant EMF species and M2 Cso is normally the most abundant di-metallofullerene. Extraction of TNT cluster metallofullerenes is easier because of both the high production yield and the high solubility in common solvents. The general extraction process for TNT EMFs is the following the soot is first washed by acetone to eliminate hydrocarbon byproducts then the residue is subjected to solvent extraction with CS2 or xylene [68, 69]. 

As indicated, empty fullerenes contain such species as Ceo, C70, Cga, and Cge, whereas Gd-EMFs are dominated by GdCg2 with lower abundances of GdCgo and GdCge. 

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