Effect of electron-donating substitution on the triplet state reactivities of 1-nitronaphthalene

Nitro-polycyclic aromatic hydrocarbons (nitro-PAHs), often found in polluted air, are carcinogenic and mutagenic. The nitro group increases the spin-orbit coupling and results in the lowest excited triplet (T₁) on the picosecond time scale with a high yield. The electron-donating substituents have a significant influence on the photophysics and photochemistry of nitro-PAHs. We used transient absorption spectroscopy and kinetic analysis to investigate the reactivities of the T₁ state 1-methoxy-4-nitronaphthalene (³MeO-NN) and 1-methyl-4-nitronaphthalene (³Me-NN). The results show that the methoxy and methyl substitutions have a minor effect on their hydrogen abstraction and electron accepting abilities. The main distinction is their reaction rates towards protons. The second order rate constant of ³MeO-NN towards protons is three orders of magnitude greater than that of ³Me-NN, indicating that ³MeO-NN has a stronger hydrogen bond accepting ability. The kinetic analysis reveals that the dimer of 2,2,2-trifluoroethanol participates in the reaction with ³MeO-NN. These results suggest that the formation of the hydrogen-bonded complex is responsible for the unusually short lifetime of ³MeO-NN in methanol solution and the lack of hydrogen abstraction radicals during the decay of ³MeO-NN in methanol.