Photoreactivity of 1-azidostyrene and 3-phenyl-2H-azirine in acetonitrile and cryogenic matrices

• Irradiation of 1-azidostyrene yields 3-phenyl-2 H -azirine from its singlet excited state, bypassing triplet vinylnitrene formation. • Irradiation of 1-azidostyrene in argon matrices gives 3-phenyl-2 H -azirine and N-phenyl ketenimine, from its singlet excited state. • Short-wavelength irradiation of 3-phenyl-2 H -azirine in solution gives phenyl nitrene ylide; long-wavelength yields triplet vinylnitrene. Despite their versatile synthetic utility, vinyl azides have complex and poorly understood photochemistry. To address this, we investigated the photoreactivity of 1-azidostyrene 1 and 3-phenyl- 2H -azirine 2 in solution and cryogenic matrices. In argon matrices, irradiation of 1 at 254 nm yielded 2 , phenyl nitrile ylide 3 , and N-phenyl ketenimine 4 , whereas irradiation at wavelengths above 300 nm produced only 2 and 4 . Similarly, irradiation of 1 in 2-methyltetrahydrofuran (mTHF) glass at 77 K mainly yielded absorption corresponding to the formation of 2 (λ max ∼ 252 nm). In contrast, irradiation of 2 at wavelengths above 300 nm in Argon matrices yielded no photoproducts, whereas irradiation at 254 nm resulted in the formation of 3 . Furthermore, femto- and nano-second transient absorption and laser flash photolysis were performed to ascertain the transient species and reactive intermediates formed during the photochemical transformations of 1 and 2 . The ultrafast transient absorption spectroscopy of 1 resulted in a transient absorption band centered at ca. 472 nm with a time constant τ ∼ 22 ps, which was assigned to the first singlet excited state (S 1 ) of 1 . The nano-second flash photolysis of 1 (308 nm laser) generated 2 within the laser pulse (∼17 ns), and subsequently 2 is excited to yield triplet vinylnitrene 3 1N with an absorption centered at ∼ 440 nm. In contrast, the nano-second laser flash photolysis of 2 with 266 nm laser produced a weak absorption corresponding to 3 , whereas 308 nm laser yielded absorption due to triplet vinylnitrene 3 1N (λ max ∼ 440 nm). These findings demonstrate that the direct irradiation of 1 populates S 1 of 1 , which does not intersystem cross to form 3 1N , but instead decays to yield 2 . Density functional theory calculations supported the characteristics of the excited states and reactive intermediates formed upon irradiation of 1 and 2 .