Fullerene oxidation and clustering in solution induced by light.

We investigate the environmental stability of fullerene solutions by static and dynamic light scattering, FTIR, NMR and mass spectroscopies, and quantum chemical calculations. We find that visible light exposure of fullerene solutions in toluene, a good solvent, under ambient laboratory conditions results in C60 oxidation to form fullerene epoxides, and subsequently causes fullerene clustering in solution. The clusters grow with time, even in absence of further illumination, and can reach dimensions from ≈100 nm to the μm scale over ≈1 day. Static light scattering suggests that resulting aggregates are fractal, with a characteristic power law (d(f)) that increases from approximately 1.3 to 2.0 during light exposure. The clusters are bound by weak Coulombic interactions and are found to be reversible, disintegrating by mechanical agitation and thermal stress, and reforming over time. Our findings are relevant to the solution processing of composites and organic photovoltaics, whose reproducibility and performance requires control of fullerene solution stability under storage conditions.