Hitting Two Molecules with One Photon; Photooxidation 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 the most efficient singlet oxygen (1O2) photosensitizer ever known. Yet, there has been no reported evidence of 1O2 reacting with its original C60 photosensitizer, which we refer to as ‘oxidation with self-sensitized 1O2’. The literature suggests both oxygen and C60 must be at excited states to undergo reaction, implying a 2-photon process: first, oxygen is photosensitized in the C60 and O2 complex (1C60·1O2); second, 1C60* is photogenerated (1C60*·1O2). However, this scheme is unlikely to occur in a solvent, where 1O2 would be rapidly quenched by solvent molecules before the second photon is absorbed by C60. Here, we substantiate a single-photon oxidation mechanism via self-sensitized 1O2 in solvents above a photon energy of 3.7 eV. Using phosphorescence (1270 nm) excitation spectroscopy and kinetics measurements, we identify the transition, 1Ag → 21Hu, as the driver of photooxidation. Subsequently, intersystem crossing generates the higher energy triplet transient, 3C60**·3O2, which then converts to 1C60*·1O2. Both excited by the same photon, at the same place and same time, C60 and O2 readily react. Such triplet-triplet annihilation, yielding two simultaneously-excited singlets, is unique. This scheme is stimulating for the conception of novel efficient photochemical processes, implemented with C60 or other photosensitizers. 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.