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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 (λmax ~ 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 nanosecond 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 (S1) 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 31N 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 31N (λmax ~ 440 nm). These findings demonstrate that the direct irradiation of 1 populates S1 of 1, which does not intersystem cross to form 31N, 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. 

Solid-state photoreactions are generally controlled by the rigid and ordered nature of crystals. Herein, the solution and solid-state photoreactivities of carbonylbis(4,1-phenylene)dicarbonazidate (1) were investigated to elucidate the solid-state reaction mechanism. Irradiation of 1 in methanol yielded primarily the corresponding amine, whereas irradiation in the solid state gave a mixture of photoproducts. Laser flash photolysis in methanol showed the formation of the triplet ketone (TK) of 1 (τ ∼ 99 ns), which decayed to triplet nitrene 31N (τ ∼ 464 ns), as assigned by comparison to its calculated spectrum. Laser flash photolysis of a nanocrystalline suspension and diffuse reflectance laser flash photolysis also revealed the formation of TK of 1 (τ ∼ 106 ns) and 31N (τ ∼ 806 ns). Electron spin resonance spectroscopy and phosphorescence measurements further verified the formation of 31N and the TK of 1, respectively. In methanol, 31N decays by H atom abstraction. However, in the solid state, 31N is sufficiently long lived to thermally populate its singlet configuration (11N). Insertion of 11N into the phenyl ring to produce oxazolone competes with 31N cleavage to form a radical pair. Notably, 1 did not exhibit photodynamic behavior, likely because the photoreaction occurs only on the crystal surfaces. 

ABSTRACT Triplet arylnitrenes may provide direct access to aryl azo‐dimers, which have broad commercial applicability. Herein, the photolysis ofp‐azidostilbene (1) in argon‐saturated methanol yielded stilbene azo‐dimer (2) through the dimerization of tripletp‐nitrenostilbene (³1N). The formation of³1Nwas verified by electron paramagnetic resonance spectroscopy and absorption spectroscopy (λ_max ~ 375 nm) in cryogenic 2‐methyltetrahydrofuran matrices. At ambient temperature, laser flash photolysis of1in methanol formed³1N(λ_max ~ 370 nm, 2.85 × 10⁷ s⁻¹). On shorter timescales, a transient absorption (λ_max ~ 390 nm) that decayed with a similar rate constant (3.11 × 10⁷ s⁻¹) was assigned to a triplet excited state (T) of1. Density functional theory calculations yielded three configurations for T of1, with the unpaired electrons on the azido (T_A) or stilbene moiety (T_Tw, twisted and T_Fl, flat). The transient was assigned to T_Twbased on its calculated spectrum. CASPT2 calculations gave a singlet–triplet energy gap of 16.6 kcal mol⁻¹for1 N; thus, intersystem crossing of¹1Nto³1Nis unlikely at ambient temperature, supporting the formation of³1Nfrom T of1. Thus, sustainable synthetic methods for aryl azo‐dimers can be developed using the visible‐light irradiation of aryl azides to form triplet arylnitrenes.