Direct observation of a sulfonyl azide excited state and its decay processes by ultrafast time-resolved IR spectroscopy

The photochemistry of 2-naphthylsulfonyl azide (2-NpSO ₂N ₃) was studied by femtosecond time-resolved infrared (TR-IR) spectroscopy and with quantum chemical calculations. Photolysis of 2-NpSO ₂N ₃ with 330 nm light promotes 2-NpSO ₂N ₃ to its S ₁ state. The S ₁ excited state has a prominent azide vibrational band. This is the first direct observation of the S ₁ state of a sulfonyl azide, and this vibrational feature allows a mechanistic study of its decay processes. The S ₁ state decays to produce the singlet nitrene. Evidence for the formation of the pseudo-Curtius rearrangement product (2-NpNSO ₂) was inconclusive. The singlet sulfonylnitrene ¹(2-NpSO ₂N) is a short-lived species (τ ≈ 700 ± 300 ps in CCl ₄) that decays to the lower-energy and longer-lived triplet nitrene ³(2-NpSO ₂N). Internal conversion of the S ₁ excited state to the ground state S ₀ is an efficient deactivation process. Intersystem crossing of the S ₁ excited state to the azide triplet state contributes only modestly to deactivation of the S ₁ state of 2-NpSO ₂N ₃.