Single-photon oxidation of C60 by self-sensitized singlet oxygen

Abstract

Thanks to its unique properties, C60 has remarkably impacted nanoscience in the past two decades. Specifically, it is regarded as the most efficient singlet oxygen (1O2) photosensitizer. Yet, its oxidation by self-sensitized 1O2 has not been known. The literature hints both oxygen and C60 must be at excited states to react, implying a 2-photon process: first, oxygen is photosensitized (1C60 1O2); second, 1C60* is photogenerated (1C60* 1O2). However, this scheme is not plausible in a solvent, which would quench 1O2 rapidly before the second photon is absorbed. Here, we uncover a single-photon oxidation mechanism via self-sensitized 1O2 in solvents above an excitation energy of 3.7 eV. Using excitation spectroscopies and kinetics analysis, we deduce photoexcitation of a higher energy transient, 3C60** 3O2, converting to 1C60* 1O2. Such triplet-triplet annihilation, yielding two simultaneously excited singlets, is unique. It may empower other novel photochemistries with higher efficiency. Additionally, rate constants derived from this study allow us to predict a half-life of about a minute in the atmosphere, possibly explaining the scarceness of C60 in the environment.