The first exhaustive purification and characterization of fullerenol samples, prepared by reaction of C60 in toluene with NaOH solution using tetrabutylammonium hydroxide as a phase transfer catalyst, have been performed. The resulting fullerenol was determined to have a molecular formula of Na+n[C60Ox(OH) y]n- (where n = 2--3, x = 7--9, and y = 12--15 for three different, but identical, preparations). The Na +-fullerenol was found to contain both hemiketal moieties (6--8 per fullerenol) and a carbonyl moiety (1 per fullerenol) by an X-ray photoelectron spectroscopy study. In addition, Na+-fullerenol was found to be anionic with an average negative charge of 2--3 and a molar conductance in water in the range of 174--297 cm-1mol -1O-1 at 25°C.
Surprisingly, the Na+-fullerenol is also paramagnetic exhibiting mueffective values in aqueous solution of 1.9--2.1 B.M. at 0.5 T and 300 K and R1 proton relaxivities of 0.55--0.77 mM-1s-1 at 20 MHz and 40°C, values both slightly higher than those expected for a pure S = ½ spin system. In solid state SQUID measurements, the Na+-fullerenols had mueffective values of ca. 1.3 B.M. (300K). ESR studies of frozen aqueous solutions at 1.5 and 5.0 K established that Na+-fullerenol is mainly S = ½ with a minor, but significant, concentration of S = 1. Thus, this is the first report to characterize the widely-studied, water-soluble fullerenols as radical anion species. The Na+-fullerenol paramagnetism was partially quenched by the addition of acid, but was reversed upon addition of base. This acid/base chemistry can be used to partition a Na+-fullerenol sample into less paramagnetic and more paramagnetic fullerenol species, with the more paramagnetic species having more S = 1 character. It is hypothesized that the paramagnetic character of Na+-fullerenol results from the formation of cyclopentadienyl radical centers on the fullerene cage, formed by an oxidation reaction with atmospheric oxygen.
Sodium ions in Na+-fullerenol can be ion exchanged for other cations (e.g. Ba2+, H+, tetrabutylammonium cation, and protonated meglumine) producing fullerenols with varying magnetic and spectroscopic properties.
Finally, paramagnetic Na+-fullerenol (110 mg/kg) has been shown to serve in vivo as a magnetic resonance imaging (MRI) contrast agent in a rat model, although, the rat did not survive the experiment.