Hidden blue emission in [60]fullerene stemming from weak π–π aggregation

The unusual UV/blue-green fluorescence of [60]fullerene at micromolar concentrations in ethyl acetate is investigated using steady-state and time-resolved fluorescence techniques. This broad emission exhibits longer lifetimes (≈10 ns) than the well-known weak red fluorescence typically associated with C₆₀ (≈1 ns). The relative intensity of the blue and red emissions is influenced by several factors, including C₆₀ concentration, sonication, solvent polarity, the presence of additives (e.g., inert gases or oxygen), and excitation wavelength. Evidence from time-resolved fluorescence anisotropy and computational studies suggests that the unconventional emission originates from the formation of ∼2 nm-sized fluorophores due to weak intermolecular π–π van der Waals interactions, which become ineffective at concentrations below 2 μM, this concentration being the estimated value for the solubility of C₆₀ in ethyl acetate. Interestingly, since the intense and long-lived unconventional emission of aggregated C₆₀ is sensitive to molecular oxygen or solvent polarity, a novel functionality for the singlet excited state of this emblematic molecule in its aggregated state as a luminophore, a fluorescent probe or a photosensitizer has been unveiled.