Redox potentials, fullerenes

The two effects can add or compensate each other. In this way, the measurement of the shift in redox potentials through electrochemical techniques offers a simple, but powerful tool to evaluate the overall electronic factors governing the interaction of the metal fragments with the fullerene ligands. 

Recently, a rather stable Cgo salt was isolated and the redox potentials of the fullerene... 

The C90 cage has 46 possible constitutional isomers, out of which only five can be isolated. The electrochemistry of C90 shows two oxidations and six reductions. The redox potentials for C90 are given in Table 8.1. The first reduction potential appears at 0.49 V versus ferrocene/ferrocenium, thus making C90 the easiest to reduce among the empty cage fullerenes. 

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... 

Reversible step transfer of up to 6 electrons per one molecule to form anion radicals is observed in the cathodic polarization of C6o fullerene solutions [6], Anion particles are stable in aprotic media, however in the general case anion stability decreases with increasing charge [7]. The number of observed steps depends on the medium and experimental conditions. The literature analysis [8-10] has revealed that the use of different solvent systems, base electrolytes and electrodes results in considerable variations in redox potentials for the most extensively studied pairs of C(JC , C(,fC(f2, C(,fCeo"3. 

Table 5.2. Redox potentials (V vs. NHE) of Ceo and fullerene derivatives. Experimental conditions reference electrode quasi-Ag/AgCl wire (calibrated with ferrocene), working and counter electrodes Pt foils, supporting electrolyte Bu4NPF6 (0.1 M), solvent ODCB/MeCN (4/1), scan rate 100 mV/s, room temperature...

Table 2 Values for the redox potentials (V) of the fullerenes Cgo, C70, C76, and Cg4 in solution...

Table 1. Redox potentials, rate constants for the radiolytic reduction and r-radical anion absorption maxima of fullerenes [11, 65, 66, 68-70],...

Table 2. Redox potentials and rate constants for the radiolytic reduction of some water-soluble fullerene derivatives [95].

Fullerene derivatives functionalized with pyridine or imidazole substituents can coordinate with several metal porphyrins, Pc, and Nc in solution by means of self-assembly. Such electron-rich macrocyclic dyes are good electron donors as they absorb light in the visible near-infrared (IR) region and exhibit a usable redox potential. 

Electrochemistry holds a central place in the study of fullerene anions. After several experiments, the optimized electrochemical conditions made it possible to observe the fifth and sixth reduction waves on the cyclic voltammetry (CV) time scale (Fig. 13.6) [34]. It was found that the medium highly influences the redox potential [133, 134]. The counter cations influence the kinetics, whereas the solvent affects the redox potentials substantially. For a number of fullerenes, a good correlation was found between the calculated HOMO-LUMO gap and the difference between the first reduction potential and the first oxidation potential in the CV [135]. 

Redox potentials of fullerenes. 3.1 Electrochemistry of thin films 384... 

Redox potentials of fullerenes. Experiments and comparison with theory... 

In their first study [78], Fawcett et al. determined apparent rate constants at the formal redox potentials for the first reduction of Cgo and C70 in benzonitrile and dichloromethane. The rate constants obtained for both fullerenes were rather similar. Higher rates were observed for dichloromethane (Table 7.5). Fawcett et al. [78] pointed out that the rate constants at the formal potential are underestimated with respect to the rate constants at the standard potential, since the ion association, which follows the electron transfer reaction, shifts the equilibrium potential in a positive direction. This effect was only estimated in the first study [78], but in a subsequent report [39] on the kinetics of the C6o/C6o and systems, a... 

Reported LUMO levels of PCBM vary between 3.7 eV and -4.3 eV, because of different measurement methods, either in solution or solid, and different equations used to estimate the values. One of the most convenient teeh-niques to determine the LUMO level of fullerenes is cyclic voltammetry. The LUMO levels of fullerene derivatives can be estimated from the first redue-tion potential. Because reduction potentials depend on the choice of the reference electrode, and vary slightly as a result of different electrochemical cell settings and choice of solvent, the use of an internal reference compound is required for accurate determination of the redox potentials of fullerenes, and any other molecule for that matter. Ferrocene is the most widely used... 

---