Photooxidation in Solution and Liquid-like Surfaces of Organic Aerosols

In 1963, E. J. Bowen published his classic review The Photochemistry of Aromatic Hydrocarbon Solutions, in which he described two major reaction pathways for PAHs irradiated in organic solvents photodimerization and photooxidation mediated by the addition of singlet molecular oxygen, 02 ) (or simply 102), to a PAH (e.g., anthracene). For details on the spectroscopy and photochemistry of this lowest electronically excited singlet state of molecular oxygen, see Chapter 4.A, the monograph by Wayne (1988), and his review article (1994). For compilations of quantum yields of formation and of rate constants for the decay and reactions of 02( A), see Wilkinson et al., 1993 and 1995, respectively. 

The 02( Ag) can then react with another BaP molecule (or a different PAH receptor molecule in a POM mixture), e.g., 

Note The symbols S0, S, and T, represent the ground electronic state and first excited singlet and triplet states, respectively, of BaP. 02( Ag) is 22.5 kcal mol-1 above the ground state of 02, while for BaP the S, and T, states are 71 and 42 kcal mol-1 above the S0 ground state, respectively. 

This product suggests a singlet oxygen mediated mechanism for quinone formation (Lee-Ruff et al., 1986). 

Subsequently, Lee-Ruff and Wang (1991) conducted a similar study of 6-methylbenzo[a]pyrene, a methyl isomer whose carcinogenicity is approximately equal to that of BaP. It photooxidized 20 times faster than BaP and, in addition to quinones, formed as a major product a seco ketone analogous to XXXIII from BaP. Its formation (as with BaP) is ascribed to a ( -mediated mechanism. 

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