The molecular structures of 19 purified isomers of bis-phenyl-C₆₂-butyric acid methyl ester (“bis[60]PCBM”) were identified by a combination of ¹³C NMR and UV-Vis absorption spectroscopy, and HPLC retention time analysis. The ¹³C NMR spectra indicated the symmetry of each isomer. It also showed that all 19 isomers are dicyclopropa-fullerenes (none are homo-fullerenes). This is because the fullerene bridgehead for both addends are sp³ hybridized; resonating between 75 and 85 ppm. According to UV-Vis spectroscopy, the 19 isomers were classified into 7 groups - each corresponding to one of the 7 bond types identified by Hirsch and co-workers (trans-1, trans-2, trans-3, trans-4, e, cis-3, cis-2).

All 58 nominal molecular structures of Bis[60]PCBM were accounted for by isolating the first addend as bridging fullerenes carbon 1 and 9 by default, after which the 2^nd addend may to one of the 29 remaining double bonds on C60 with two orientations relative to that of the 1^st addend. However, symmetry reduced this number to 36 physically distinguishable isomers. Many of these isomers have enantiomers, and hence cannot be spectroscopically distinguished; which reduces the number of effective isomers to 22. If these, 3 are not expected to form owing to steric hindrance because each of these three isomers involve the cis-1 bond whereby the bulky addends would be attached to two adjacent double bonds on the same 6-membered ring on C₆₀.

There were 7 isomers with C₁ molecular point group symmetry, 4 with C_S, 6 with C₂, 1 with C_2v and 1 with C_2h symmetry. The C_2h, C_2v, and all 5 non-equatorial C₁ molecular structures were unambiguously assigned to their respective HPLC fractions. For the other 12 isomers, the ¹³C NMR and UV-Vis spectra placed them in 6 groups of two same-symmetry isomers. Based on the widely spaced HPLC retention times of the two isomers within each of these 6 groups, and the empirical inverse correlation between retention time and addend spacing, each molecular structure was assigned to its corresponding HPLC fraction.

The information presented will enable those who follow and produce photovoltaic devices based purified isomers to know which specific isomer is being employed in their devices, develop a deeper understanding of morphological order/disorder within the acceptor phase, and thereby draw meaningful conclusions on why some isomers perform better than others.