Thermal [6,6] --> [6,6] Isomerization and Decomposition of PCBM (Phenyl-C61-butyric Acid Methyl Ester)

For the first time, the thermal stability limits of one of the most highly cited and well-studied fullerene derivative electron acceptors, phenyl-C ₆₁-butyric acid methyl ester (PCBM), have been investigated under thermal annealing and vapor deposition conditions. Significant decomposition is observed when PCBM is heated, even briefly, to and beyond its melting temperature in an inert atmosphere, as evidenced and quantified here by proton nuclear magnetic resonance, atmospheric-pressure chemical ionization mass spectrometry, and UV-vis spectroscopy, as well as high-performance liquid chromatography. The major thermally induced decomposition product of PCBM has been isolated, characterized, and identified as a new pentacyclic [6,6]-addition motif isomer of PCBM (iso-PCBM). Cyclic voltammetry studies show no difference in electrochemical properties between PCBM and iso-PCBM, and our quantum chemical calculations predict the new isomer to be ∼43 kJ/mol more thermodynamically stable than PCBM.