Photochemical site-selective synthesis of [70]methanofullerenes.

Methanofullerenes such as the well-known [70]PCBM are commonly synthesized under harsh conditions to obtain the product as a mixture of site-isomers (namely alpha, beta and minor gamma) due to the D-5h symmetry of the C-70 cage. We report the first site-selective synthesis of [70]methanofullerenes under light irradiation and low temperatures, thus avoiding time-consuming and highly expensive HPLC separations. Pure major site-isomers alpha-[70]PCBM and alpha-[70]DPM have been thus efficiently prepared including the crystal structure of 5b. Photovoltaic preliminary results revealed a slightly beneficial performance for alpha-pure [70]PCBM site-isomer devices.