Relaxation and Photochemistry of Nitroaromatic Compounds: Intersystem Crossing through ¹ππ* to Higher ³ππ* States, and NO^• Dissociation in 9-Nitroanthracene─A Theoretical Study

Determination of the photodegradation pathways of nitroaromatic compounds, known for their mutagenic properties and toxicity, is a relevant topic in atmospheric chemistry. In the present theoretical study, mechanisms for the photophysical relaxation and NO^• dissociation of 9-nitroanthracene (9-NA) are proposed that challenge the commonly assumed pathways based on the El-Sayed rules. The analysis of the stationary points on the potential energy surfaces obtained with multiconfigurational methods indicates that after light absorption and subsequent relaxation of the S₁ state, the system undergoes ultrafast intersystem crossing to T₂, which serves as a gate-state to the triplet manifold due to favorable energetic couplings. This occurs despite the nature of the singlet and triplet states being ¹ππ* and ³ππ*, where the receiver triplet involves NO₂ orbitals that are tilted from the polyaromatic plane, with no involvement of the ³nπ state in the process. After the singlet to triplet manifold crossing, the system evolves along two possible trajectories. One leads to the global minimum of T₁ (phosphorescent end state) and the other involves the dissociation into antryloxy and NO^• radicals. Overall, the information obtained is in agreement with steady-state and time-resolved spectroscopic data reported for 9-NA. Furthermore, it suggests that the deactivation mechanism of nitroaromatic compounds can take place between ¹ππ* and ³ππ* states, which opens a new landscape for the rationalization of the photophysics of these and other systems.