Singlet oxygen generation from secondary photoproducts of benzo[a]pyrene

We have investigated the photosensitized generation of singlet oxygen (¹Δ_g) from a variety of diones derived from benzo[a]pyrene (1), namely benzo[a]pyrene (6,12) dione (2), benzo[a]pyrene (1,6) dione (3), benzo[a]pyrene (3,6) dione (4), and benzo[a]pyrene (4,5) dione (5). The diones 2-4 are formed in the atmosphere as secondary photoproducts from benzo[a]pyrene. Benzo[a]pyrene (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 (Φ_Δ) 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 benzo[a]pyrene decreases with increasing polarity of the solvent. By contrast, the values for Φ_Δ 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.