Investigation Of Photophysical And Photochemical Reactions Of Selected Nitro-polycyclic Aromatic Hydrocarbon Derivatives Excited States

Nitro-polycyclic aromatic hydrocarbons?NPAHs?are persistent organic pollutants,formed from incompletely combustion processes and atmospheric reaction of polycyclic aromatic hydrocarbons?PAHs?.NPAHs have been identified to be more mutagenic and carcinogenic to organisms than their corresponding unsubstituted PAHs.Photolysis is the major degradation channel for NPAHs at ambient conditions.Irradiation of NPAHs populates excited singlet states,which usually undergo two deactivation channels.The first pathway is the ultrafast intersystem crossing to excited triplet states,and the other is generation of aryloxy radicals.The lowest excited triplet states of NPAHs have lifetimes of nanosecond to microsecond time scale,which makes them have potential to react with other molecules in ways such as,energy transfer,protonation,electron transfer and hydrogen abstraction.In this thesis,nanosecond transient absorption and resonance Raman spectrocopies with the help of TD-DFT and CASSCF calculations were used to investigate the properties and reactivities of T₁ states of selected NPAH derivatives in organic and aqueous solutions,as well as their detailed reaction mechanism.The selected NPAH derivatives include 2-methyl-1-nitronaphthalene?2Me1NN?,1-methyl-4-nitronaphthalene?1Me4NN?,1-methoxyl-4-nitronaphthalene?MeONN?and 2-Nitrofluorene?2NF?.The study results are summarized as following.?1?2-Methyl-1-naphthoxyl radical?2Me1NO×?and a novel reaction intermediate were observed,and the assignment of the latter species reveals the intramolecular hydrogen transfer reaction originated from the T₁ of 2Me1 NN.2Me1NN T₁ is not the precursor of 2Me1NO×.The reaction of 2Me1 NN T₁ with H2 O was fitted well to the third order reaction,so that the mechanism was tentatively assigned to a coupled proton and electron transfers through hydrogen bonding complex,followed by the water assistant deprotonation,resulting in radical anion.?2?Electron-donating groups of 1Me4 NN and MeONN make the NPAH molecules have “push-pull” configuration.Electron density on nitro group in MeONN is bigger than that of 1Me4 NN,due to the electron-donating ability of methoxyl and methyl group respectively,which influences T₁ reactivities of 1Me4 NN and MeONN.The kinetic results of triplet states with water and alcohol suggest that it is easier for 1Me4 NN T₁ to undergo electron transfer reaction,while protonation for MeONN counterpart.?3?2NF was observed to bear two triplet absorptions in polar solvent,both of which correlated with the solvent polarity.T₁ has the majority population in nonpolar solvent,and its potential energy surface could have conical intersection with dissociative state so that the lifetime of triplet state was pretty short.The triplet lifetime in protic polar solvent is longer and the absorbance intensity of T2 band increases than those in aprotic polar solvent,due to the hydrogen bonding between the nitro oxygen with the hydroxyl group of solvent molecule.