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-NpSO2N3) was studied by femtosecond time-resolved infrared (TR-IR) spectroscopy and with quantum chemical calculations. Photolysis of 2-NpSO2N3 with 330 nm light promotes 2-NpSO2N3 to its S-1 This is the first direct observation of the S-1 state of a sulfonyl azide, state. The S-1 excited state has a prominent azide vibrational band. 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-NpNSO2) was inconclusive. The singlet sulfonylnitrene (1)(2-NpSO2N) is a short-lived species (tau approximate to 700 +/- 300 ps in CCl4) that decays to the lower-energy and longer-lived triplet nitrene (3)(2-NpSO2N). Internal conversion of the S, 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-NpSO2N3.