UNUSUAL REDOX BEHAVIOR OF A WATER-SOLUBLE MALONIC-ACID DERIVATIVE OF C-60 - EVIDENCE FOR POSSIBLE CLUSTER FORMATION

Although fullerenes are known to be easily and efficiently reduced, no such reaction could be observed between one of the most powerful reductants, the hydrated electron, and water-soluble C60C(COO-)(2), a malonic acid derivative of C-60 This lack of reactivity is attributed to clustering of this functionalized fullerene in the aqueous phase, in which it is suggested to form kind of a micellar aggregate with the hydrophobic C-60 core in the center and the hydrophilic carboxyl groups sticking into the water phase. Reduction of C60C(COO-)2 by e(aq)(-) readily occurs, however, if clustering is prevented by embedding an isolated single fullerene molecule into gamma-cyclodextrin. Further evidence for clustering in polar environment (water and lower alcohols) is provided by the ground state absorption spectrum of C60C(COO-)(2), which shows considerable Line broadening in conjunction with lower extinction coefficients as compared with C60C(COOH)(2) in less polar, e.g. toluene, solutions. The clustering concept is also supported by the short lifetime of the triplet excited state, (C-3(60))C(COO-)(2), in water (t(1/2) approximate to 0.4 mu s) as compared to that of the gamma-cyclodextrin encapsulated (C-3(60))C-(COO-)(2)/gamma-CD (t(1/2) 55 mu s). Reductive quenching of the latter (lambda(max) = 720 nm) by DABCO occurs with 6.9 x 10(6) M(-1) s(-1) and yields the fullerene radical anion (C-60(.-))C(COO-)2/gamma-CD, identifiable by its 1040-nm IR band.