1. Field of the Invention
The present invention relates to a singlet oxygen generating process and also to a process of oxidizing fullerenes with the singlet oxygen.
2. Description of Related Art
Oxidation of fullerenes is a key for synthesis of organic compounds. Oxidation of fullerenes with molecular oxygen is conventionally performed by a photo-chemical process using organic dyes as photo-sensitizers. Use of the molecular oxygen as an oxidant has been regarded as promising means for clean reactions with less environmental burdens, compared with use of toxic organic or inorganic oxidants.
Fullerenes, which are recently applied to various technical fields, are also effective photo-sensitizers, as reported in Ref. No. 1. According to Ref. No. 1, singlet photo-excited fullerenes are further excited to a triplet state by intersystem crossing. The excitation is accompanied with energy transfer to oxygen, and oxygen is sequentially excited to a singlet state.
By the way, fullerenes represented by C60, C70 or C84 are carbon clusters with closed shell structures (hereinafter, referred to as “carbon cages”) and have peculiar physical properties originated in the specified carbon cages. For instance in a medical field, big powers for generation of singlet oxygen enable employment of fullerenes as anti-cancer drugs or anti-HIV proteases in combination with selective drug-delivery systems. In a cosmetic field, fullerenes are used as anti-active oxygen medicines similar to vitamin C due to radical-repairing powers. Moreover, development of fullerenes is also researched and examined by bonding other elements or groups to some of cage-forming carbon atoms or by enclosing other elements or groups in the carbon cages so as to change properties of fullerenes themselves.
Among oxidation processes using fullerenes as photo-sensitizers, oxidation of fullerenes is the simplest system. Since fullerene oxide has excellent activity, its applicability as an additive to battery electrodes using LUMO (a lowest unoccupied molecular orbital) lower than fullerenes is also expected, as well as precursors for functional fullerene derivatives. For instance, fullerene oxides are produced by liquid-state photo-oxidation as noted in Ref. No. 2 or by chemical oxidation with oxidants, e.g. m-chloroperbenzoic acid, as noted in Ref. No. 3. Ref. No. 4 reports another fullerene oxidizing process, wherein fullerenes are irradiated with microwaves in a solid state mixed with m-chloroperbenzoic acid.                Ref. No. 1 Michael Orfanopoulous, Spiros Kambourakis, Tetrahedron Lett. (1994) 35, 1945        Ref. No. 2 Kathleen M Creegan, John L. Robbins, Win K. Robbins, John M. Millar, Rexford D. Sherwood, Paul J. Tindall, Donald M. Cox, J. Am. Chem. Soc. (1992) 114, 1103        Ref. No. 3 Alan L. Balch, David A. Costa, Bruce C. Noll, Marilyn M. Olmstead, J. Am. Chem. Soc. (1995) 177, 8926        Ref. No. 4 Weon Bae Ko, Sung Ho Hwang, Ju Hyun Ahn, Elastomer, (2005) 40, 45.        
Any method of Ref. Nos. 1-3 premises liquid-state reactions, which are unavoidably accompanied with heavy environmental burdens and unsuitable for massive synthesis of fullerene oxides since huge amounts of solvents are consumed for dissolution of sparingly-soluble fullerenes. Besides, photo-irradiation is essential for the reactions.
The other method of Ref. No. 4 is based on solid-state reactions and limits a reaction field between solids. As a result, objective compounds are produced with poor yields, and organic or inorganic oxidants were still required.