Counting the Isomers and Estimation of Anisotropy of Polarizability of the Selected C₆₀ and C₇₀ Bisadducts Promising for Organic Solar Cells

Currently, bisadducts of C60 and C70 fullerenes are widely studied as electron-acceptor materials for organic solar cells. These compounds are usually used as mixtures of the positional isomers. However, as recently shown, the separate use of the purified isomers with lowest anisotropies of polarizability may enhance solar cell output parameters. To predict the structures of the compounds appropriate for this purpose, we calculated anisotropies of polarizability of four classes of fullerene bisadducts, namely, bis-[60]PCBM, [60]OQMF, bis-[70]PCBM, and [70]OQMF (18, 16, 41, and 42 positional isomers, respectively). As found, the anisotropies quadratically correlate with the interaddend distances in fullerene bisadducts, whereas there are no obvious correlations between the structures and lowest unoccupied molecular orbital levels, traditionally used for assessing the efficiency of candidates for organic solar cell electron acceptors. According to our calculations, bisadducts bis-[60]PCBM-ee-1, [60]OQMF-cis-3.2, [60]OQMF-trans-4.2, cc(1.1)cc(2'.1)-bis-[70]PCBM, and cc1cc(2'.1)-[70]OQMF have the lowest anisotropies of polarizability. These compounds have a primary interest for synthesis, purification, and further separate testing in solar cells. The structures of these adducts have a common feature, which we describe with the "not so close and not so far" rule: the distances between the addends in the most isotropic fullerene bisaddicts should be medium among the possible values. These are ee, ef, cis-3, and trans-4 positions in the case of the C60 bisadducts and cc bonds placed on the different poles and the same hemisphere of the C70 skeleton.