This presentation reports on new cyclization reaction by using fullerene cation to obtain 5- and 7-membered cyclo[60]fullerene derivatives. In this talk, mainly we focused on development of methodology to obtain new fullerene derivatives utilizing cationic fullerene intermediate, and discussed reaction mechanism involving fullerene cation which can be coordinated by benzene rings or the oxygen atom, affording migration of the phenyl groups and activation of the methyl group of the methoxy group for demethylative cyclization.^[1]

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 radical, oxidation of the radical to generate fullerene cation, 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. Obtained cyclo[60]fullerene derivatives were used in bulk-heterojunctoin organic solar cells and showed moderate power conversion efficiency and respectable open-circuit voltage.

[1] Y. Matsuo, K. Ogumi, Y. Zhang, H. Okada, T. Nakagawa, H. Ueno, A. Gocho, E. Nakamura, J. Mater. Chem. A, in press.