Ab Initio Study of Decay Dynamics of 1-Nitronaphthalene Initiated from the S2(ππ* + nNOπ*) 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 SFC(ππ*) to a receiver, higher-energy triplet state Tn(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: S2-FC-1NN → S2-MIN-1NN or S2T3-MIN-1NN → T3-MIN-1NN or T3T2-MIN-1NN→ T2-MIN-1NN or T2T1-MIN-1NN → T1-MIN-1NN. This explains the high yield of T1-1NN 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 T1-1NN state via T2T1-MIN-1NN, one leads to T1-MIN-1NN, while the other to T1-MIN-ISO to produce Ar-O• and NO• radicals.