Ab Initio Study of Decay Dynamics of 1-Nitronaphthalene Initiated from the S₂(ππ* + n_NOπ*) State

Irradiation of nitro-PAHs in solution at ambient conditions leads to formation of its lowest excited triplet, dissociation intermediates nitrogen oxide (NO^•) and aryloxy radical (Ar-O^•). Experimental and theoretical studies demonstrated that Franck-Condon excited singlet state S_FC(ππ*) to a receiver, higher-energy triplet state T_n(nπ*) controlled the ultrafast population of the triplet state and, hence, the slight fluorescence yield of nitronaphthalenes. However, the detailed information about the curve-crossings of potential energy surfaces and the major channels for forming T₁ species and Ar-O^• radical were unclear. Here, by using the CASSCF//CASPT2 method, an efficient decay channel is revealed: S_2-FC-1NN → S_2-MIN-1NN or S₂T_3-MIN-1NN → T_3-MIN-1NN or T₃T_2-MIN-1NN→ T_2-MIN-1NN or T₂T_1-MIN-1NN → T_1-MIN-1NN. This explains the high yield of T_1-1NN species and minor yield of Ar-O^• and NO^• radicals. The calculation results suggest the bifurcation processes take place predominantly after the internal conversion to the T_1-1NN state via T₂T_1-MIN-1NN, one leads to T_1-MIN-1NN, while the other to T_1-MIN-ISO to produce Ar-O^• and NO^• radicals.