Fullerenes in heterogeneous guest host structures

The insolubilty of pristine fullerenes in polar solvents provokes a variety of complex questions. A viable concept to overcome this solubility problem involves the incorporation of pristine fullerenes into the hydrophobic cavity of water soluble host structures. Table 3 shows a survey of host structures, which were successfully employed to accommodate [60]fullerene etc. and, in turn, to dissolve fullerenes in aqueous media. The large van-der Waals radius of fullerenes, with typical values of 5 1.5 A, limits the selection of adequate host complexes. This will be discussed in more details in the following. 

The reduction experiments disclose a noticeable slow-down of the Ceo/Y-CD reduction (k]4 = 2.7 x 10 NT s ), initiated by the bulky (CH3)2 COH radicals, relative to a bimolecular reaction in homogeneous systems (k4 = 8.5 x 10 M s ). In conclusion, the fullerene core is not completely shielded from the aqueous phase and, thus, can be efficiently reduced via hydrated electrons and various a-hydroxyl radicals. 

Under anaerobic conditions the singly reduced Cso /y-CD is quite stable and reacts only slowly with molecular oxygen (37). 

Addition of base or acid had, however, a significant impact on the stability and yield of the characteristic 1080 nm absorption band. A semilogarithmic correlation between the proton concentration and the intensity of the fullerene 7t-radical anion band (1080 nm) is observed in anaerobic aqueous solutions. This observation has been ascribed to a reversible protonation of Ceo /Y-CD. Experimental proof for this assumption was brought forward by the fact that the 1080 nm absorption, in an alkaline solution (pH 10), diminished upon acidifying (pH 3) and was completely restored upon addition of base (pH 10). The reversible protonation process gives rise to a pKa of 4.5. 

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). 

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