Fullerene reduction

In contrast to the reduction, fullerenes are difficult to oxidize [163-165], First, the energy levels of the HOMO andLUMO orbitals are very low lying [122], Second, the structural demand of the cation cations are normally planar, therefore oxida-... 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

In contrast to the relative ease of reduction, oxidation of fullerenes requires more severe conditions [113, 114]. Not only does the resonance stabilization raise the level of the corresponding oxidation potential (1.26 V versus Fc/Fc ), but also the reversibility of the underlying redox process is affected [115]. 

This behaviour also stands for functionalized [60]fullerene derivatives, with, however, a few striking differences. The most obvious parameter is the negative shift of the reduction potentials, which typically amounts to -100 mV. Secondly, the separation of the corresponding reduction potentials is clearly different. Wlrile the first two reduction steps follow closely the trend noted for pristine [60]fullerene, the remaining four steps display an enlianced separation. This has, again, a good resemblance to the ITOMO-LUMO calculations, namely, a cancellation of the degeneration for functionalized [60]fullerenes [31, 116, 117]. 

The electrochemical features of the next higher fullerene, namely, [70]fullerene, resemble the prediction of a doubly degenerate LUMO and a LUMO + 1 which are separated by a small energy gap. Specifically, six reversible one-electron reduction steps are noticed with, however, a larger splitting between the fourth and fifth reduction waves. It is important to note that the first reduction potential is less negative than that of [60]fullerene [31]. 

Parallel to the shift that the reduction of higher fullerenes shows, oxidation of the latter is also made easier... 

In aerated or oxygen saturated solutions, the fullerene triplet lifetime suffers a marked reduction [147, 148]. 

Reduction of fullerenes to fullerides — Reversible electrochemical reduction of Ceo in anhydrous dimethylformamide/toluene mixtures at low temperatures leads to the air-sensitive coloured anions Qo" , ( = 1-6). The successive mid-point reduction potentials, 1/2, at -60°C are -0.82, -1.26, -1.82, -2.33, —2.89 and —3.34 V, respectively. Liquid NH3 solutions can also be used. " Ceo is thus a very strong oxidizing agent, its first reduction potential being at least 1 V more positive than those of polycyclic aromatic hydrocarbons. C70 can also be reversibly reduced and various ions up to... 

For multielectron-transfer (reversible) processes, the cyclic voltammogram consists of several distinct peaks if the E° values for the individual steps are successively higher and are well separated. An example of such a mechanism is the six-step reduction of the fullerenes C60 and C70 to yield the hexaanion products and C7q. Such six successive reduction peaks are observed in Figure 2-4. 

Nakanishi, T, Ohwaki, H., Tanaka, H., Murakami, H., Sagara, T. and Nakashima, N. (2004) Electrochemical and chemical reduction of fullerenes C o and C70 embedded in cast films of artificial lipids in aqueous media, f Phys. Chem. B, 108, 7754-7762. 

The synthesis of C60-based dyads in which the Ccm core is covalently attached to a strong electron acceptor moiety, has been carried out by 1,3-dipolar cycloaddition of in situ generated nitrile oxides with C(,o- As expected, the obtained adducts show reduction waves of the fullerene core that are anodically shifted in comparison with the parent Cr>o. This indicates that they are remarkably stronger acceptors than Ceo-The electron acceptor organic addend also undergoes an anodic shift due to the electronic interaction with the C(,o moiety (545). 

Metal-free initiators, 14 258-259 Metal fullerenes, 2 718-719 Metal-halogen exchange, in pyridine chemistry, 22 107-108 Metal hydrazides, 23 567 Metal hydrides, 23 611-613 amines by reduction, 2 493 hydrogen storage and, 23 851 nitriding, 27 206-207 storage of, 23 786... 

For evaluation of influence of the samples of fullerenes C60 on total metabolic state of thymocytes, EAC, and L1210 cells we have used MTT test based on the reduction of MTT by reductase system that consists, in particular, of mitochondrial succinatedehydrogenase. 

As one may see from the data presented in Fig. 6.5a, upon the presence of photoexcited samples of fullerenes in incubation medium of thymocytes, no significant changes in the rate of MTT reduction were observed during 5 h. Upon the presence of photoexcited fullerene C60 in incubation medium of suspension of L1210 cells,... 

Fig. 6.5 Dependence of the level of MTT reduction (% from control) in thymocytes (a), L1210 (b), and EAC cells (c) incubated in the presence of photoexcited samples of fullerenes (1 - fuller-ene C60 2 - C60-composite-l 3 - C60-composite-2). P < 0.05 compared to the control...

However, fullerene C60-modified surface is adequate for the adhesion and normal growth of cells in culture. Cells of the line MA-104 in the Eagle-MEM medium formed on fullerene film a normal monolayer. Cellular viability was assessed with the resazurin (Alamar Blue) reduction test. The dye resazurin is reduced by mitochondrial dehydrogenases of viable cells into the fluorescent product resorufin (maximum X = 530 nm, max X = 590 nm). The intensity of... 

Table 7.2 Cellular viability of MA-104 culture grown on fullerene on the surfaces (FoS) during 72 h. (resazurin reduction test)...

Irradiation of C70 solution in peanut oil under the same conditions just described for C60 fullerene in linseed oil after 70 minutes of irradiation causes the partial reduction of the intensity of the band at 464 nm and of a shoulder at 398 nm. 

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