Structural Identification of 19 Purified Isomers of the OPV Acceptor Material bisPCBM by ¹³C NMR and UV-Vis Absorption Spectroscopy and High-Performance Liquid Chromatography

The molecular structures of 19 purified isomers of bis-phenyl-C₆₂-butyric acid methyl ester were identified by a combination of ¹³C NMR and UV-vis absorption spectroscopies and high-performance liquid chromatography (HPLC) retention time analysis. All 19 isomers are dicyclopropafullerenes (none are homofullerenes). There were seven isomers with C₁ molecular point-group symmetry, four with C_s, six with C₂, one with C_2v, and one with C_2h symmetry. The C_2h, C_2v, and all five nonequatorial C₁ isomers were unambiguously assigned to their respective HPLC fractions. For the other 12 isomers, the ¹³C NMR and UV-vis spectra placed them in six groups of two same-symmetry isomers. On the basis of the widely spaced HPLC retention times of the two isomers within each of these six groups, and the empirical inverse correlation between retention time and addend spacing, each isomer was assigned to its corresponding HPLC fraction. In addition, the missing trans-1 isomer was found, purified, and characterized.