Study Of The Reactivities Within The Lowest Excited Triplet State Of Selected Nitroaromatic Compounds Based On Transient Absorption Spectroscopy

Nitroaromatic compounds are widespread air pollutants that have received a lot of attention due to their carcinogenicity and mutagenicity.Nitroaromatics have a high intersystem crossing efficiency,and can produce the lowest excited triplet state（T₁）with high yield upon excitation.The T₁states have a long lifetime,making it highly reactive to environmental acids,bases,hydrogen donors,electron donors,etc.,which are the main reaction channels of nitroaromatics in the environment after exposure to light.In this dissertation,the model molecules of 1-methoxy-4-nitronaphthalene（Me O-NN）,1-methyl-4-nitronaphthalene（Me-NN）,4-amino-4’-nitrobiphenyl（NH₂-BP-NO₂）,4-nitrobiphenyl（NO₂-BP）,and 2-bromo-5-nitrothiophene（2-Br-5-NO₂-TP）were selected to reveal the microscopic reaction mechanisms of their T₁state in the electron transfer,proton transfer and hydrogen abstraction reactions,as well as the effect of substituents,aromatic ring structure and solvent properties on these reactivities.The nanosecond transient absorption spectroscopic technique and density functional theoretical calculation methods were used to acquire the spectral and kinetic information of short-lived intermediates.The second-order rate constants for the electron transfer reactions of T₁state Me O-NN（³Me O-NN）and T₁state Me-NN（³Me-NN）with a series of electron donors were obtained.Using the Rehm-Weller equation to fit the relationship between the rate constants of electron transfer reactions and the oxidation potential of the electron donors,the reduction potentials of³Me O-NN and ³Me-NN were measured to be 1.22～1.24 V,and the recombination energies were 6.76～7.72 k J/mol.Meanwhile,the rate constants of hydrogen transfer reactions of ³Me O-NN and ³Me-NN with 1,4-cyclohexadiene were obtained as（9.77±0.37）×10⁵and（1.12±0.05）×10⁶M^-1s^-1,respectively,and the rate constants of proton transfer reactions with perchloric acid were（1.75±0.08）×10⁹and（3.53±0.42）×10⁶M^-1s^-1.The reaction rate results indicate that the methoxy and methyl substitutions have a little influence on the oxidation ability and hydrogen abstraction ability of the T₁states,whereas the methoxy group make the T₁state more basic,implying that it has a stronger ability to obtain hydrogen bonds.Besides,the kinetic results of the reaction of ³Me O-NN with trifluoroethanol illustrated the involvement of trifluoroethanol dimer in the reaction.These results well explain that the lifetime of ³Me O-NN in methanol solution of only 300 ns is caused by the formation of hydrogen bonding complexes with methanol molecule.In contrast,³Me-NN undergoes hydrogen transfer reaction in methanol solution to produce radicals（Me-NNH·）.The results of fluorescence and low-temperature phosphorescence confirm the hydrogen-bonding complexes of ³Me O-NN and methanol.The hydrogen-bonding complexes formed between NH₂-BP-NO₂,NO₂-BP and alcohol molecules are different due to the presence of the amino substitution,resulting in a significant difference in the absorption changes as the former is blue-shift and the latter has a relatively enhanced at longer wavelength of absorption band.The second-order reaction rate constants of the lowest excited triplet state ³（NH₂-BP-NO₂）with 1,4-cyclohexadiene in tetrahydrofuran were（1.55±0.08）×10⁵M^-1s^-1,which was faster than in methanol.Therefore,³（NH₂-BP-NO₂）has the electron configuration ofππ*in above solutions.In addition,in acetonitrile and methanol solutions,the change of electron transfer rates of ³（NH₂-BP-NO₂）with a series of electron donors was faster than that in tetrahydrofuran.In acetonitrile and tetrahydrofuran solutions ³（NH₂-BP-NO₂）and TBA undergoes proton transfer to generate ³（NH^--BP-NO₂）with the absorption maximum at 450 nm.The fluorescence quantum yield of NH₂-BP-NO₂were 0.0065 and<0.001 in tetrahydrofuran and acetonitrile,respectively,which suggests that the solvent can regulate the excited state relaxation of NH₂-BP-NO₂.Due to the electron-rich property of the thiophene ring,the basicity of ³（2-Br-5-NO₂-TP）is much greater than that in the ground state,and the protonation product is ³（2-Br-5-NO₂H⁺-TP）.The Ware’s formula was used to analyze the rate constant of the reaction of ³（2-Br-5-NO₂-TP）with acid to obtain the p K_a^*=2.36 for ³（2-Br-5-NO₂H⁺-TP）.The reduction potential of the³（2-Br-5-NO₂-TP）was calculated to be（1.58±0.04）V using the Rehm-Weller equation to fit the relationship between the rate constants of electron transfer reactions and the oxidation potential of the electron donor.³（2-Br-5-NO₂-TP）with phenol undergoes a proton-coupled electron transfer reaction to form the radical（2-Br-5-NO₂H·-TP）with the absorption maximum at 290-300 nm.The above spectroscopic and kinetic results for the reactions of T₁states of nitroaromatic compounds with acids,bases,electron donors,and hydrogen donors reveal the effect of substituents,aromatic ring structures and solvent properties on the reaction channels and reactivity of T₁state molecules.These results provide an experimental basis for the study of photochemical reactions of various nitroaromatic compounds in the environment.