A Comparative Photophysical and Photochemical Study of Nitropyrene Isomers Occurring in the Environment

Abstract

Ground state absorption, first excited-singlet state, and properties of reactive intermediates of mononitropyrene isomers encountered in the atmospheric aerosol have been studied under different conditions that could mimic the environment. The nitro group can present different orientation relative to the pyrene ring depending on its geometric location and could induce differences in the photochemistry of the isomers. The 2-NO 2 Py isomer has the largest red shift and lowest oscillator strength in the UV-visible band associated with the nitrogroup. The isomers show very low fluorescence yields (10 −3 -10 −4 ). Only 1-NO 2 Py and 4-NO 2 Py have phosphorescence emission (Φ p ≈ 10 −4 ), indicating that the lowest triplet state decays mainly through effective radiationless channels. Laser photolysis produces a low lying triplet state (τ T = 10 −5 -10 −6 s), a long-lived pyrenoxy radical, and a PyNO 2 H radical in solvents in which the triplet can abstract a hydrogen atom. Similar triplet yields were calculated (0.1-0.6) for the isomers while significant differences in the relative yield of the long-lived species were determined. Differences in the quenching rate constants of the triplet by water and phenols suggest a strong hydrogen bond interaction with the nitro group in the C-2 position, which provides for radiationless deactivation routes.

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 could be explained if in 2-propanol the ground state 1-NO 2 Py is weakly H-bonded to 2-propanol, not allowing for the approach of the ArOH. 48 …”

On the Primary Reaction Pathways in the Photochemistry of Nitro-Polycyclic Aromatic Hydrocarbons

“…Importantly, NPAHs are frequently more toxic than their parent PAHs [8,20], and photochemical degradation of a number of NPAHs leads, in turn, to photoproducts, some of which are more toxic than their parent compounds [8,20,21]. However, the light-induced transformation mechanisms of NPAHs are still under debate and knowledge of their fates in the environment is of current interest [22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39]. Fundamental gaps remain in our knowledge of the elementary steps that lead to product formation with regard to the competition among available reaction pathways, the intermediate species involved in the photochemical transformations, and the degradation rates as a function of compound structure, added co-solutes, and micro-environment [26,34,35,[40][41][42][43].…”

Synthesis, Crystal Packing, and Ambipolar Carrier Transport Property of Twisted Dibenzo[g,p]chrysenes

“…The emission maximum wavelengths of NO 2 ‐DBC 7 and Me‐NO 2 ‐DBC 9 are 541 nm and 608 nm, respectively, while those of the other DBC derivatives appear at shorter wavelengths (between 395 nm and 424 nm). The nitro groups significantly perturb the excited states 14. The quantum yields of DBC emission vary.…”