Single-photon oxidation of C(60) by self-sensitized singlet oxygen

C(60) is regarded as the most efficient singlet oxygen ((1)O(2)) photosensitizer. Yet, its oxidation by self-sensitized (1)O(2) remains unclear. The literature hints both oxygen and C(60) must be at excited states to react, implying a two-photon process: first, oxygen is photosensitized ((1)C(60)•(1)O(2)); second, C(60) is photoexcited ((1)[Formula: see text] •(1)O(2)). However, this scheme is not plausible in a solvent, which would quench (1)O(2) rapidly before the second photon is absorbed. Here, we uncover a single-photon oxidation mechanism via self-sensitized (1)O(2) in solvents above an excitation energy of 3.7 eV. Using excitation spectroscopies and kinetics analysis, we deduce photoexcitation of a higher energy transient, (3)[Formula: see text] •(3)O(2), converting to (1)[Formula: see text] •(1)O(2). Such triplet-triplet annihilation, yielding two simultaneously-excited singlets, is unique. Additionally, rate constants derived from this study allow us to predict a C(60) half-life of about a minute in the atmosphere, possibly explaining the scarceness of C(60) in the environment.