Fullerene molecule

A second doping method is the substitution of an impurity atom with a different valence state for a carbon atom on the surface of a fullerene molecule. Because of the small carbon-carbon distance in fullerenes (1.44A), the only species that can be expected to substitute for a carbon atom in the cage is boron. There has also been some discussion of the possibility of nitrogen doping, which might be facilitated by the curvature of the fullerene shell. However, substitutional doping has not been widely used in practice [21]. 

Since the structure and properties of fullerene solids are strongly dependent on the structure and properties of the constituent fullerene molecules, we first review the structure of the molecules, which is followed by a review of the structure of the molecular solids formed from Ceo, C70 and higher mass fullerenes, and finally the structure of Cgo crystals. 

Fig. 2. By rolling up a graphene sheet (a single layer of ear-bon atoms from a 3D graphite erystal) as a cylinder and capping each end of the eyiinder with half of a fullerene molecule, a fullerene-derived tubule, one layer in thickness, is formed. Shown here is a schematic theoretical model for a single-wall carbon tubule with the tubule axis OB (see Fig. 1) normal to (a) the 6 = 30° direction (an armchair tubule), (b) the 6 = 0° direction (a zigzag tubule), and (c) a general direction B with 0 < 6 < 30° (a chiral tubule). The actual tubules shown in the figure correspond to (n,m) values of (a) (5,5), (b) (9,0), and (c) (10,5).

When building clusters by coating the fullerenes with metal, features similar to the electronic and geometric shells found in pure metal clusters[9] are observed in the mass spectra. In the case of fullerene molecules coated with alkaline earth metals (section 3), we find that a particularly stable structure is formed... 

Fig. 3. Mass spectra of photoionized QgCa (top) and C7oCa) (bottom) the lower axis is labeled by the number of metal atoms on the fullerene molecule. The peaks at x = 32 for C Ca and x = 37 for C7oCa , correspond to a first metal layer around the fullerenes with one atom located at each of the rings. The edges at x = 104 and x = 114, respectively, signal the completion of a second metal layer.

If it is possible to put one layer of metal around a fullerene molecule, it is tempting to look for the completion of additional layers also. In the spectra in Fig. 3, the sharp edges at CgoCa o and C7oCaJ i4 would be likely candidates for signaling the completion of a second layer. As we will see below, there is, in fact, a very reasonable way of constructing such a second layer with precisely the number of metal atoms observed in the spectrum. 

The structures observed in the mass spectra of fullerene molecules covered with alkaline earth metals, as described in the previous section, all seem to have a geometric origin, resulting in particularly stable cluster configurations every time a highly symmetrical layer of metal atoms around a central fullerene molecule was completed. When replacing the alkaline earth metals by an alkali metal (i.e., Li, Na, K, Rb, or Cs), a quite different situation arises. 

In general, nanotechnology MBBs are distinguished for their unique properties. They include, for example, graphite, fullerene molecules made of various numbers of carbon atoms (C60, C70, C76, C240, etc.), carbon nanotubes, nanowires, nanocrystals, amino acids, and diamondoids [97]. All these molecular building blocks are candidates for various applications in nanotechnology. 

The nature of the electronic states for fullerene molecules depends sensitively on the number of 7r-electrons in the fullerene. The number of 7r-electrons on the Cgo molecule is 60 (i.e., one w electron per carbon atom), which is exactly the correct number to fully occupy the highest occupied molecular orbital (HOMO) level with hu icosahedral symmetry. In relating the levels of an icosahedral molecule to those of a free electron on a thin spherical shell (full rotational symmetry), 50 electrons fully occupy the angular momentum states of the shell through l = 4, and the remaining 10 electrons are available... 

The most convenient method for the evaluation of fullerene molecules aggregation is the comparison of UV-VIS absorption spectra in various solvents and for Langmuir-Blodgett films (Bensasson et al., 1994). Some preliminary conclusions about the state of aggregation can be made basing only the absorption maximum of fullerene C60 in the region of 3 30-340 nm concern. 

Thus, if mentioned above is to be taken into consideration that absorption band in the region of 330-340 nm reflects to a certain extent the degree of fullerene molecules association in solution, we can come to the conclusion that the less the PVP molecular mass and the less the fullerene contents is the more fullerene molecules are in low associated state. It is quite probable that the increase of the fullerene contents in the complex brings to the formation of adducts, where not single fullerene C60 molecules are bonded with PVP but their associates. It can be one of the reasons of the observed fact of the difference of UV-VIS spectra of C60/PVP complex with PVP of different molecular mass (up to spectra crossover, which is a singular evidence that these compounds are nonidentical). 

In order to prepare the complexes with low association of fullerene molecules it is necessary to start with diluted solutions of C60 (in toluene) and PVP (in CHCy, but the goal can be achieved only at relatively low content of fullerene in the complex. Thus, the degree of association of C60 molecules depends also on the molecular mass of PVP, and low associated complexes can be obtained only with PVP 10,000 and fullerene low concentration. It must be mentioned however that such complexes are relatively unstable - during their storage for about several months the association of fullerene molecules changes what can be seen from the shift of the band in UV-VIS spectra between 330 and 340 ran (bathochromic and hypochromic shifts). 

The most reasonable explanation for the observed differences in fullerene behavior as a part of complex C60/PVP in chemical and biological systems is the fact that the complex itself is stable in the pure water media. Dissolution in the saline causes the formation of fullerene precipitate, which, naturally reveals the photodynamic properties. But a basic difference between water-soluble C60 complexes with organic compounds (PVP, y-CD, etc.) from other forms used for biological investigations is the low degree of fullerene molecules association... 

In order to control the quantity of fullerene, contacting biological objects, FoS were obtained by evaporation of saturated solution of C60 in hexane introduced in the wells of 96-well culture plates ( Sarstedt ). Twenty-five microliters of solution was applied to each well and evaporated at 20-25 °C, after which the procedure was repeated several times to obtain a desirable concentration of fullerene (10, 20, and 30pg/cm2). Application of such volume allows obtaining a surface, covered with fullerene on the bottom and partly on the walls of a well at a high less than 2 mm. Microscopic investigations (optical and electronic microscopy) have shown that the surface was covered irregularly fullerene formed the isolated clusters, so that obtained fullerene films were not the real films, but rather isolated clusters of fullerene molecules (data not shown). However, it should be noted that their dimensions were smaller than those of cells and each cell covered several such clusters. 

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