Photooxidation of PC₆₀BM: new insights from spectroscopy

This joint experimental-theoretical spectroscopy study of the fullerene derivative PC₆₀BM ([6,6]-phenyl-C₆₀-butyric acid methyl ester) aims to improve the understanding of the effect of photooxidation on its electronic structure. We have studied spin-coated thin films of PC₆₀BM by X-ray Photoelectron Spectroscopy (XPS), Near-edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy, and Fourier Transform Infrared Spectroscopy (FTIR), before and after intentional exposure to simulated sunlight in air for different lengths of time. The π* resonance in the C1s NEXAFS spectrum was found to be a very sensitive probe for the early changes to the fullerene cage, while FTIR spectra, in combination with O1s NEXAFS spectra, enabled the identification of the oxidation products. The changes observed in the spectra obtained by these complementary methods were compared with the corresponding Density Functional Theory (DFT) calculated single-molecule spectra of a large set of in silico generated oxidation products of PC₆₀BM where oxygen atoms were attached to the C₆₀ cage. This comparison confirms that photooxidation of PC₆₀BM disrupts the conjugation of the fullerene cage by a transition from sp² to sp³-hybridized carbon and causes the formation of several oxidation products, earlier proposed for C₆₀. The agreement between experimental and calculated IR spectra suggests moreover the presence of dicarbonyl and anhydride structures on the fullerene cage, in combination with cage opening at the adsorption site. By including PC₆₀BM with physisorbed O₂ molecules on the cage in our theoretical description in order to model oxygen diffused through the film, the experimental O1s XPS and O1s NEXAFS spectra could be reproduced.