Singlet oxygen generation from secondary photoproducts of benzoapyrene.

We have investigated the photosensitized generation of singlet oxygen (1Δg) from a variety of diones derived from benzoapyrene (1), namely benzoapyrene (6,12) dione (2), benzoapyrene (1,6) dione (3), benzoapyrene (3,6) dione (4), and benzoapyrene (4,5) dione (5). The diones 2-4 are formed in the atmosphere as secondary photoproducts from benzoapyrene. Benzoapyrene (4,5) dione (5) was additionally selected for this study due to its much larger dipole moment relative to the other diones. All of these diones are excellent sensitizers with quantum yields of singlet oxygen generation (ΦΔ) ranging from 0.77 to near unity. Singlet oxygen appears to be generated primarily from the triplet excited state. Quantum yields were measured in four solvents with different polarity. The singlet oxygen quantum yield for benzoapyrene decreases with increasing polarity of the solvent. By contrast, the values for ΦΔ for the diones 2-5 increase slightly with increasing polarity of the solvent. Unlike substituted anthracene derivatives, all of the diones 2-5 show very little singlet oxygen quenching, as singlet oxygen lifetimes showed no decrease compared with the literature values when diones 2-5 were used for singlet oxygen generation. The large singlet oxygen quantum yields for diones 2-5, and the lack of singlet oxygen quenching implies that, in the presence of these compounds in the environment, photosensitized generation of singlet oxygen generation could be significant.