Abstract
C60 is regarded as the most efficient singlet oxygen (1O2) photosensitizer. Yet, its oxidation by self-sensitized 1O2 remains unclear. The literature hints both oxygen and C60 must be at excited states to react, implying a two-photon process: first, oxygen is photosensitized (1C60•1O2); second, C60 is photoexcited (1{mathrm{C}}_{60}^{ast}•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, 3{mathrm{C}}_{60}^{{ast}{ast}}•3O2, converting to 1{mathrm{C}}_{60}^{ast}•1O2. Such triplet-triplet annihilation, yielding two simultaneously-excited singlets, is unique. Additionally, rate constants derived from this study allow us to predict a C60 half-life of about a minute in the atmosphere, possibly explaining the scarceness of C60 in the environment.
Highlights
C60 is regarded as the most efficient singlet oxygen (1O2) photosensitizer
C60 is subject to photodegradation in these applications at ambient temperature[9,10]
The initial work on photooxidation (PO) of C60 credited it to ozonation[11,12,13], which is limited to excitation wavelengths shorter than 240 nm for photogeneration of O314
Summary
C60 is regarded as the most efficient singlet oxygen (1O2) photosensitizer. Yet, its oxidation by self-sensitized 1O2 remains unclear. The spectrum of the excitation source (Fig. 1c) consists of a major narrow band peaking at 310 nm with no emissions below 250 nm.
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