On the origin of ultrafast nonradiative transitions in nitro-polycyclic aromatic hydrocarbons: Excited-state dynamics in 1-nitronaphthalene

The electronic energy relaxation of 1-nitronaphthalene was studied in nonpolar, aprotic, and protic solvents in the time window from femtoseconds to microseconds. Excitation at 340 or 360 nm populates the Franck–Condon $S1(ππ∗)$ state, which is proposed to bifurcate into two essentially barrierless nonradiative decay channels with sub-200 fs lifetimes. The first main decay channel connects the $S1$ state with a receiver $Tn$ state that has considerable $nπ∗$ character. The receiver $Tn$ state undergoes internal conversion to populate the vibrationally excited $T1(ππ∗)$ state in 2–4 ps. It is shown that vibrational cooling dynamics in the $T1$ state depends on the solvent used, with average lifetimes in the range from 6 to 12 ps. Furthermore, solvation dynamics competes effectively with vibrational cooling in the triplet manifold in primary alcohols. The relaxed $T1$ state undergoes intersystem crossing back to the ground state within a few microseconds in $N2$-saturated solutions in all the solvents studied. The second minor channel involves conformational relaxation of the bright $S1$ state (primarily rotation of the $NO2$-group) to populate a dissociative singlet state with significant charge-transfer character and negligible oscillator strength. This dissociative channel is proposed to be responsible for the observed photochemistry in 1-nitronaphthalene. Ground- and excited-state calculations at the density functional level of theory that include bulk and explicit solvent effects lend support to the proposed mechanism where the fluorescent $S1$ state decays rapidly and irreversibly to dark excited states. A four-state kinetic model is proposed that satisfactorily explains the origin of the nonradiative electronic relaxation pathways in 1-nitronaphthalene.