Dm. Guldi et al., UNUSUAL REDOX BEHAVIOR OF A WATER-SOLUBLE MALONIC-ACID DERIVATIVE OF C-60 - EVIDENCE FOR POSSIBLE CLUSTER FORMATION, Journal of physical chemistry, 99(36), 1995, pp. 13487-13493
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.