The study of fullerene aggregates in solution

The electronic spectra of C{dollar}sb{lcub}60{rcub}{dollar} and C{dollar}sb{lcub}70{rcub}{dollar} exhibit extreme solvatochromism and other environmental effects. These arise, in part, because the fullerenes are good electrophiles and may form both ground and excited state complexes with {dollar}pi{dollar}-electron donating aromatic solvents. However, solute aggregation also profoundly affects the electronic spectra of these species.; In this work, we focus on the phenomenon of fullerene aggregate formation in solution and its effect on the electronic spectra of the fullerenes. Aggregates are distinguished from precipitate by characteristic size and their stability in solution. The aggregates may also have a structure distinct from the crystalline phase and may carry charge. We have used both static and dynamic Rayleigh Scattering to examine the growth kinetics and structure of the aggregates and photoluminescence spectroscopy to study the photophysics of the aggregates. In single solvent systems, aggregation was sensitive to the container surface and was inhibited by the addition of a radical scavenger suggesting a radical mechanism of growth. The aggregates continued growing until precipitating from solution at {dollar}sim{dollar}800 nm in diameter. In polar, binary mixed-solvents, aggregation occurred quickly and was dependent on the specific mixture and the fullerene concentration. The most stable solutions were 90% acetonitrile or methanol and 10% toluene and contained negatively charged aggregates {dollar}sim{dollar}200 nm in diameter. The photoluminescence spectra of the aggregate solutions contain spectral features similar to features seen in the spectra of solid fullerene and are assigned to excimer emission. TEM images and diffraction patterns show that the aggregates consist of a number of crystallites. The C{dollar}sb{lcub}60{rcub}{dollar} aggregates have the same crystal structure as the powder while the C{dollar}sb{lcub}70{rcub}{dollar} aggregates' diffraction pattern does not correspond to structures reported for C{dollar}sb{lcub}70{rcub}{dollar} crystals. This work shows that aggregation must be taken into account when studying fullerene properties, even in dilute solutions and that many of the discrepancies in the literature can be attributed to aggregation.