Via UV-Vis spectroscopy the 19 isomers of bisPCBM were classified into 7 different groups, with each group containing 2 or 3 members. Each of the 7 groups corresponds to 1 of the 7 bond types identified by Hirsch (trans-1, trans-2, trans-3, trans-4, e, cis-3, cis-2). By examining the molecular structure of each of the 19 isomers, we conclude that 11 isomers have C1 symmetry, 3 isomers have Cs symmetry, 3 have C2 symmetry, 1 has C2h symmetry and 1 has C2v symmetry; and we have confirmed this prediction experimentally by 13C NMR spectroscopy. The two high-symmetry isomers, C2h and C2v, which correspond to the two polar isomers (where the two addends bridge the 1,9 and 52,60 bond of C60with the two addends having opposing relative orientations) correspond to the first and third HPLC fractions by retention time, respectively.
Unfortunately, the other isomers cannot be identified from the NMR spectra alone. However, as the UV-Vis spectra and 7 groups identified above reduce the uncertainty to no more than 3 isomers, when coupled with the symmetry identified from NMR spectra, some of the isomers may be confidently identified. For example, the 13th HPLC fraction belongs to the cis-2 group which contains one Cs and two C1 isomers. The NMR spectra shows that this 13th fraction is the Cs isomer, which involves the 32,33 bond of C60. By this method, the other Cs and C2isomers were also unambiguously confirmed.
The remaining unidentified isomers all have C1 symmetry and cannot be further identified by this method. For these isomers, the HOMOs and HOMO-LUMO gaps were calculated by DFT and TD-DFT methods. By comparing the experimental and calculated results, some, but not all, the C1 isomers were tentatively identified with reasonable confidence. For the remaining 8 C1 isomers the DFT calculations were not able to adequately distinguish, so the identification of these isomers will have to wait for single crystal analysis. A full description of the identification of the isomers will be presented in the meeting.