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 redu ctants, the hydrated electron, and water-soluble C60C(COO-)(2), a malo nic acid derivative of C-60 This lack of reactivity is attributed to c lustering of this functionalized fullerene in the aqueous phase, in wh ich it is suggested to form kind of a micellar aggregate with the hydr ophobic C-60 core in the center and the hydrophilic carboxyl groups st icking into the water phase. Reduction of C60C(COO-)2 by e(aq)(-) read ily occurs, however, if clustering is prevented by embedding an isolat ed single fullerene molecule into gamma-cyclodextrin. Further evidence for clustering in polar environment (water and lower alcohols) is pro vided by the ground state absorption spectrum of C60C(COO-)(2), which shows considerable Line broadening in conjunction with lower extinctio n coefficients as compared with C60C(COOH)(2) in less polar, e.g. tolu ene, 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-cyclo dextrin encapsulated (C-3(60))C-(COO-)(2)/gamma-CD (t(1/2) 55 mu s). R eductive quenching of the latter (lambda(max) = 720 nm) by DABCO occur s 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.