Multiconfigurational non-adiabatic molecular dynamics towards photochemical-N₂-extrusion reactions in borodiazenes

We used state-of-the-art quantum-chemical calculations to study the photodenitrogenation of 1-methylborodiazene to 1-methylborirane. Vertical excitation energy calculations indicate that the S₀ → S₁ (n_N → π_NN*) and S₀ → S₂ (n_N → 2p_B) transitions are accessible (318 nm and 287 nm, respectively), and the S₀ → S₁ transition is the bright state. The minimum-energy path indicates near-degeneracy between the S₁ and S₂ surfaces in the Franck-Condon region, leading to the steepest descent path on the S₁ state to be towards fluorescent decay rather than denitrogenation. Our simulations indicate that denitrogenation primarily occurs shortly after an S₁/S₀ hopping event (79% of trajectories) and proceeds via three distinct pathways. The dominant pathway involves boron pyramidalization and partial π_NN isomerization in the S₁ state, with N₂ elimination shortly after an S₁/S₀ hop. Minor pathways include either π_NN isomerization or boron pyramidalization, with N₂ elimination occurring on the S₀ and S₁ states, respectively. We also observed that some trajectories rearrange to form a diazoborete intermediate. Our results show that the labile N₂ group can be used to access base-free boriranes.