Effects of Light Energy and Reducing Agents on C 60 ‐Mediated Photosensitizing Reactions

Many biomolecules contain photoactive reducing agents, such as reduced nicotinamide adenine dinucleotide ( NADH ) and 6‐thioguanine (6‐ TG ) incorporated into DNA through drug metabolism. These reducing agents may produce reactive oxygen species under UVA irradiation or act as electron donors in various media. The interactions of C 60 fullerenes with biological reductants and light energy, especially via the Type‐I electron‐transfer mechanism, are not fully understood although these factors are often involved in toxicity assessments. The two reductants employed in this work were NADH for aqueous solutions and 6‐ TG for organic solvents. Using steady‐state photolysis and electrochemical techniques, we showed that under visible light irradiation, the presence of reducing agents enhanced C 60 ‐mediated Type‐I reactions that generate superoxide anion (O 2 .− ) at the expense of singlet oxygen ( 1 O 2 ) production. The quantum yield of O 2 .− production upon visible light irradiation of C 60 is estimated below 0.2 in dipolar aprotic media, indicating that the majority of triplet C 60 deactivate via Type‐ II pathway. Upon UVA irradiation, however, both C 60 and NADH undergo photochemical reactions to produce O 2 .− , which could lead to a possible synergistic toxicity effects. C 60 photosensitization via Type‐I pathway is not observed in the absence of reducing agents.