Response theory studies of triplet-state spectra and radiative lifetimes of naphthalene, quinoxaline, and phthalazine

Response theory calculations with complete account of spin-orbit coupling (SOC) are performed to predict spectra and radiative lifetimes for triplet states of naphthalene and two of the doubly N-substituted azanaphthalenes, quinoxaline and phthalazine. A quite long triplet-state lifetime for naphthalene is found, about 1 min, which is approximately twice the previously accepted value. For quinoxaline the pure electronic radiative lifetime is considerably shorter, on the order of 1 s, but still longer than the one given experimentally. The large increase of radiative lifetime on going from quinoxaline to phthalazine is well reproduced by the response calculations. Comparison with experimental data for total and resolved spin sublevel decay indicates a substantial role of vibronic coupling for the radiative lifetimes of the triplet states in the azanaphthalenes. The intensity of the T₂ ← S₀ transition in naphthalene is also predicted and is in good agreement with experimental measurements of pure crystals. A brief comparison between naphthalene and benzene with respect to the ground-state transition probabilities of the T₁ and T₂ states is given.