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.