Fullerenes charge delocalization

Based on the depicted equilibrium and the observed lifetime a rate constant for the forward reaction of 10 NT s" was estimated. The slow protonation rate of the one-electron reduced fullerene n-radical anions can be understood in terms of the charge delocalization and also the hybridization of the generated carbanion. Furthermore, the heterogeneous and hydrophobic environments of the host s interior can be assumed to be beneficial for the slow-down of the protonation dynamics. In homogeneous aqueous solutions the protonation rate should be faster, a hypothesis that was substantiated by recent radiolytic experiments with bisfunctionalized fullerene derivatives. The latter compounds are soluble in aqueous solutions without employing a solubiiizer (host) and give rise to protonation rate constants of 3 x 10 M s" (38). 

Some cation-radicals can appear as hydrogen acceptors. Thus, fullerene Cgg is oxidized to the cation-radical at a preparative scale by means of photoinduced electron transfer. As in the case of anion-radical, the fullerene Cgo cation-radical bears the highly delocalized positive charge and shows low electrophilicity. This cation-radical reacts with various donors of atomic hydrogen (alcohols, aldehydes, and ethers) yielding the fullerene 1,2-dihydroderivatives (Siedschlag et al. 2000). 

For an organic compound (Q) in dipolar aprotic solvents, the half-wave potential ( 1/2) of the first reduction step tends to shift to the positive direction with an increase in solvent Lewis acidity (i.e. acceptor number). This is because, for the redox couple Q/Q, the reduced fonn (Q ) is energetically more stabilized than the oxidized fonn (Q) with increasing solvent acidity. The positive shift in E1/2 with solvent acceptor number has been observed with quinones [57 b], benzophenone [57 a, c] and anthracene [57 c], With fullerene (C60), the positive shift in E1/2 with solvent acidity parameter, ET, has been observed for the reductions of C60 to Qo, Qo to Clo, and Cf)0 to Cli, [54c], However, the positive shift in E1/2 is not apparent if the charge in Q is highly delocalized, as in the cases of perylene and fluoren-9-one [57 c]. 

Inspection of the data in Table 7.1 reveals significant differences between the reversible half wave potentials measured for different fullerenes. They cannot be accounted for by solvent and/or background electrolyte effects (see below) and they reflect the unique electronic properties of each fullerene. The first simple explanation of the different electrochemistry of C o and C70 in toluene/dichloromethane mixtures was given by Cox et al. [28]. They found that the first and second reduction potentials were almost identieal for both fullerenes, whereas the third one was around 80 mV more negative for C6o- The difference in the third reduction potential was explained in terms of a simple charge separation, delocalization model. The larger C70 would more easily accommodate three additional electrons than the smaller Ceo- Almost identical first and second reduction potentials for C60 and C70 have... 

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