Fullerene cation-mediated demethylation/cyclization to give 5- and 7-membered cyclo[60]fullerene derivatives

Abstract

Single-bonded fullerene dimers, (C₆₀R)₂ (R = (2-methoxyphenyl)dimethylsilylmethyl, (isopropoxy)dimethylsilylmethyl, and 2-methoxy-5-methylphenyl), afforded fullerene derivative cations (RC₆₀⁺) in the presence of an oxidant (I₂, CuCl₂, or CuBr₂), leading to 5- and 7-membered cyclo[60]fullerene derivatives such as cyclo[60]fullerenes containing a Si–O bond, cyclo[60]fullerenes containing a C₆₀–O bond, and dihydrobenzofurano[60]fullerene. The reaction mechanism involved thermal homolytic cleavage to give fullerene radicals, oxidation of the radicals to generate fullerene cations, and then demethylative cyclization in which the fullerene cationic centers interact with an aromatic group or the oxygen atoms of methoxy groups to produce the cyclized products. The obtained cyclo[60]fullerene derivatives were used in bulk-heterojunction organic solar cells and showed moderate power conversion efficiency and respectable open-circuit voltage.