Photodegradation Mechanisms of 1-Nitropyrene, an Environmental Pollutant: The Effect of Organic Solvents, Water, Oxygen, Phenols, and Polycyclic Aromatics on the Destruction and Product Yields

This work describes studies of the photodegradation mechanism of 1-nitropyrene (1-NO2Py) in a chemical model system consisting of an organic solvent and known constituents of an aerosol particle. Photoproducts such as 1-hydroxypyrene (1-OHPy), 1-hydroxy-x-nitropyrenes (1-OH-x-NO2Py), 1-nitrosopyrene, and 1,6- and 1,8-pyrenediones were identified by high-performance liquid chromatography (HPLC) and HPLC/mass spectrometry (HPLC/MS) techniques, and their quantum yields show a significant dependence on the type of solvent. The photodegradation quantum yield of 1-NO2Py, ϕ(-1-NO2Py), was larger in toluene, benzene, and polar protic solvents (10–3) in comparison with nonpolar and polar aprotic solvents, where the yield is on the order of 10–4. In solvents with an abstractable hydrogen atom, the products formed in higher yields were 1-OHPy and 1-OH-x-NO2Py. These represent 60–80% of the photodestruction yield and result from abstraction and recombination reactions of the pyrenoxy radical, an intermediate postulated to be formed as a result of a nitro–nitrite rearrangement in nitroaromatics. The small O2 effect in the photodegradation yield and the quenching experiments with azulene demonstrate the small contribution of the 3(π,π*) state in the 1-NO2Py photoreaction. The nitrosopyrene product was not observed under these conditions, demonstrating the participation of the 3(π,π*) state in its formation. In the presence various phenol aerosol constituents, the photodegradation yield increased by 10-fold in all solvents. This effect is partly ascribed to the reaction of the 3(π,π*) state with the phenol. The effect of water resulted in the reduction of the 1-NO2Py photodegradation yield and of its photoproducts. The phototodegradation of 1-NO2Py was also studied in a viscous solvent, hexadecane, and it was determined that this medium does not inhibit its photodecay.