Excited states and electron transfer reactions of C₆₀(OH)₁₈ in aqueous solution

Abstract

Dynamic light scattering of fullerenol solutions [C₆₀(OH)₁₈] reveals evidence for the formation of fullerene aggregates at high solute concentration (up to 3.85×10^-2 mol dm^-3). This hydrophilic fullerene derivative emits very weak fluorescence regardless of its concentration. Photolysis (35 ps; λ_ex=355 nm) of C₆₀(OH)₁₈ in aqueous solution yields the immediate formation of a transient singlet excited state with broad absorption in the 550–800 nm region with ε_670nm=2130 d mol^-1 cm^-1. The energetically higher-lying singlet excited state transforms via intersystem crossing (i.e., with τ_1/2=500 ps) to the also broadly absorbing (550-800 nm), triplet excited state. In contrast, at low solute concentration, the features of the (^∗T₁→^∗T_n) absorption differ significantly exhibiting an absorption maximum at 650 nm concomitant to a shoulder at 570 nm. The π-radical anion of fullerenol, [C₆₀(OH)₁₈]^·−, generated by electron transfer from hydrated electrons and (CH₃)₂C(OH) radicals, absorbs with λ_max at 870, 980 and 1050 nm. Based on electron transfer studies with suitable electron donor/acceptor substrates, the reduction potential of the C₆₀(OH)₁₈/[C₆₀(OH)₁₈]^·− couple was estimated to be in the range between -0.358 and -0.465 V vs. NHE.