Irradiation of nanocrystals of azide 1 results in a solid-to-solid reaction that forms imine 2 in high chemical yield. In contrast, solution photolysis of azide 1 yields a mixture of products, with 7 as the major one. Laser flash photolysis (LFP) of a nanocrystalline suspension of azide 1 in water shows selective formation of benzoyl radical 4 (λmax ∼ 400 nm), which is short-lived (τ = 833 ns) as it intersystem crosses to form imine 2. In comparison, LFP of azide 1 in methanol reveals the formation of triplet alkylnitrene 10 (λmax ∼ 340 nm). The selectivity observed in the solid-state is related to stabilization of the triplet ketone with (n,π*) configuration by the crystal lattice, which results in α-cleavage being favored over triplet energy transfer to the azido chromophore. Both the solid-state and solution reaction mechanisms are further supported by density functional theory calculations. Thus, laser flash photolysis has been used to effectively elucidate the medium dependent reaction mechanisms of azide 1.

Photolysis of 3-azido-1-indenone (1) with a light-emitting diode (LED, λ = 405 nm) or mercury arc lamp (Pyrex) resulted in the formation of heterodimer 3 in excellent yield, through dimerization of triplet vinylnitrene 32. At ambient temperature, vinylnitrene 32 (λmax at 340 and 480 nm) was detected directly with laser flash photolysis of vinyl azide 1. The vinylnitrene intermediate was also characterized directly with IR and ESR spectroscopy in cryogenic matrices. The ESR spectrum of vinylnitrene 32 yielded a zero-field splitting parameter |D/hc| of 0.460 cm–1 and |E/hc| of 0.015 cm–1, which reveals that vinylnitrene 32 has significant 1,3-biradical character. The proposed mechanism for the formation and reactivity of triplet vinylnitrene 32 was supported with density functional theory (DFT) calculations, which highlight that the steric demand of the five-membered ring in vinylnitrene 32 prevents intersystem crossing to the corresponding azirine (10). CASSCF and CASPT2 calculations showed that the energy gap between the singlet and triplet configurations of vinylnitrene 2 is only 10 kcal/mol. In spite of this small energy gap, vinylnitrene 32 does not decay by intersystem crossing, but rather by dimerization. Thus, triplet vinylnitrenes can be selectively formed with visible light and used to form new C–N bonds in synthetic applications.

The photolysis of 2-azido-1,4-naphthoquinone (1) in argon matrices at 8 K results in the corresponding triplet vinylnitrene 32, which was detected directly by IR spectroscopy. Vinylnitrene 32 is stable in argon matrices but forms 2-cyanoindane-1,3-dione (3) upon further irradiation. Similarly, the irradiation of azide 1 in 2-methyltetrahydrofuran (MTHF) matrices at 5 K resulted in the ESR spectrum of vinylnitrene 32, which is stable up to at least 100 K. The zero-field splitting parameters for nitrene 32, D/hc = 0.7292 cm–1 and E/hc = 0.0048 cm–1, verify that it has significant 1,3-biradical character. Vinylnitrene 32 (λmax ∼ 460 nm, τ = 22 μs) is also observed directly in solution at ambient temperature with laser flash photolysis of 1. Density functional theory (DFT) calculations support the characterization of vinylnitrene 32 and the proposed mechanism for its formation. Because vinylnitrene 32 is relatively stable, it has potential use as a building-block for high-spin assemblies.

Nanosecond laser flash photolysis of o-hydroxyacetophenone (1a) and 2,4-dihydroxyacetophenone (1b) in ethanol and acetonitrile results in absorption due to triplet biradicals 2a (λmax 430 nm, τ ≈ 3 μs) and 2b (λmax 400 nm, τ ≈ 1 μs), respectively. Triplet biradical 2a intersystem crosses to form Z-3a (λmax 400 nm, τ ≈ 10 μs), whereas 2b forms both Z-3b and E-3b (λmax 350 nm, τ ≈ 5 and 72 μs). Quenching studies demonstrate that 3a,b are formed on both the singlet and triplet excited surface of 1a and 1b. In ethanol at 77 K, o-hydroxyacetophenone derivatives 1a and 1b show phosphorescence, as is typical for triplet ketones with (n,π*) configuration. The mechanism for the photoreactivity of 1a,b is supported by density functional calculations.

The irradiation of trans-vinylketones 1a–c yields the corresponding cis isomers 2a–c. Laser flash photolysis of 1a and 1b with 308 and 355 nm lasers results in their triplet ketones (T1K of 1), which rearrange to form triplet 1,2-biradicals 3a and 3b, respectively, whereas irradiation with a 266 nm laser produces their cis-isomers through singlet reactivity. Time-resolved IR spectroscopy of 1a with 266 nm irradiation confirmed that 2a is formed within the laser pulse. In comparison, laser flash photolysis of 1c with a 308 nm laser showed only the formation of 2c through singlet reactivity. At cryogenic temperatures, the irradiation of 1 also resulted in 2. DFT calculations were used to aid in the characterization of the excited states and biradicals involved in the cis–trans isomerization and to support the mechanism for the cis–trans isomerization on the triplet surface.

Photolysis of 3-methyl-2-phenyl-2H-azirine (1a) in argon-saturated acetonitrile does not yield any new products, whereas photolysis in oxygen-saturated acetonitrile yields benzaldehyde (2) by interception of vinylnitrene 5 with oxygen. Similarly, photolysis of 1a in the presence of bromoform allows the trapping of vinylnitrene 5, leading to the formation of 1-bromo-1-phenylpropan-2-one (4). Laser flash photolysis of 1a in argon-saturated acetonitrile (λ = 308 nm) results in a transient absorption with λmax at ∼440 nm due to the formation of triplet vinylnitrene 5. Likewise, irradiation of 1a in cryogenic argon matrixes through a Pyrex filter results in the formation of ketene imine 11, presumably through vinylnitrene 5. In contrast, photolysis of 2-methyl-3-phenyl-2H-azirine (1b) in acetonitrile yields heterocycles 6 and 7. Laser flash photolysis of 1b in acetonitrile shows a transient absorption with a maximum at 320 nm due to the formation of ylide 8, which has a lifetime on the order of several milliseconds. Similarly, photolysis of 1b in cryogenic argon matrixes results in ylide 8. Density functional theory calculations were performed to support the proposed mechanism for the photoreactivity of 1a and 1b and to aid in the characterization of the intermediates formed upon irradiation.

Photolysis of 1 in argon-saturated acetonitrile yields 2, whereas in oxygen-saturated acetonitrile small amounts of benzoic acid and benzamide are formed in addition to 2. Similarly, photolysis of 2 in argon-saturated acetonitrile results in 1 and a trace amount of 3, whereas in oxygen-saturated acetonitrile the major product is 1 in addition to the formation of small amounts of benzoic acid and benzamide. Laser flash photolysis of 1 results in an absorption due to triplet vinylnitrene 4 (broad absorption with λmax at 360 nm, τ = 1.8 μs, acetonitrile) that is formed with a rate constant of 1.2 × 107 s–1 and decays with a rate constant of 5.6 × 105 s–1. Laser flash photolysis of 2 in argon-saturated acetonitrile likewise results in the formation of triplet vinylnitrene 4 but also ylide 5 (λmax at 440 nm, τ = 13 μs). The rate constant for forming 4 in argon-saturated acetonitrile is 1.6 × 107 s–1. In oxygen-saturated acetonitrile, vinylnitrene 4 reacts to form the peroxide radical 6 (λmax 360 nm, ∼0.7 μs, acetonitrile) at a rate of 2 × 109 M–1 s–1. Density functional theory calculations were performed to aid in the characterization of vinylnitrene 4 and peroxide 6 and to support the proposed mechanism for the formation of these intermediates.

The irradiation of ester 1 in methanol and chloroform does not yield any photoproducts, whereas the photolysis of 1 in dry argon-saturated benzene produces cyclobutanol 4, which is converted to lactone 5 by the addition of HCl. Laser-flash photolysis of ester 1 demonstrates that 1 undergoes intramolecular H-atom abstraction to form the biradical 2 (λmax ∼ 310 nm, τ = 200 ns, benzene), which intersystem crosses to photoenols, Z-3 (λmax ∼ 380 nm, τ = 30–60 μs, benzene) and E-3 (λmax ∼ 380 nm, τ = 11 ms, benzene). Density functional theory calculations were performed to support the proposed mechanism for forming cyclobutanol 4 and to explain how steric demand facilitates photoenol E-3 to form cyclobutanol 4 rather than lactone 5.

The photoreactivity of (3-methyl-2H-azirin-2-yl)-phenylmethanone, 1, is wavelength-dependent (Singh et al. J. Am. Chem. Soc. 1972, 94, 1199−1206). Irradiation at short wavelengths yields 2P, whereas longer wavelengths produce 3P. Laser flash photolysis of 1 in acetonitrile using a 355 nm laser forms its triplet ketone (T1K, broad absorption with λmax ∼ 390–410 nm, τ ∼ 90 ns), which cleaves and yields triplet vinylnitrene 3 (broad absorption with λmax ∼ 380–400 nm, τ = 2 μs). Calculations (B3LYP/6-31+G(d)) reveal that T1K of 1 is located 67 kcal/mol above its ground state (S0) and has a long C–N bond (1.58 Å), and the calculated transition state to form 3 is only 1 kcal/mol higher in energy than T1K of 1. The calculations show that 3 has significant 1,3-carbon iminyl biradical character, which explains why 3 reacts efficiently with oxygen and decays by intersystem crossing to the singlet surface. Photolysis of 1 in argon matrixes at 14 K produced ketene imine 7, which presumably is formed from 3 intersystem crossing to 7. In comparison, photolysis of 1 in methanol with a 266 nm laser produces mainly ylide 2 (λmax ∼ 380 nm, τ ∼ 6 μs, acetonitrile), which decays to form 2P. Ylide 2 is formed via singlet reactivity of 1, and calculations show that the first singlet excited state of the azirine chromophore (S1A) is located 113 kcal/mol above its S0 and that the singlet excited state of the ketone (S1K) is 85 kcal/mol. Furthermore, the transition state for cleaving the C–C bond in 1 to form 2 is located 49 kcal/mol above the S0 of 1. Thus, we theorize that internal conversion of S1A to a vibrationally hot S0 of 1 forms 2, whereas intersystem crossing from S1K to T1K results in 3.

Photolysis of 1 in chloroform yielded 2 as the major product and a small quantity of 3. Laser flash photolysis demonstrated that upon irradiation, the first excited triplet state of the ketone (T1K) of 1 is formed and decayed to form radical 4, which has a λmax at 380 nm (τ = 2 μs). Radical 4 expelled a nitrogen molecule to yield imine radical 5 (λmax at 300 nm). Density functional theory (DFT) calculations showed that the transition state barrier for the formation of 5 is approximately 4 kcal/mol. In comparison, photolysis of 1 in argon matrices resulted in triplet nitrene 6, which was further characterized with 15N and D isotope labeling and DFT calculations. Prolonged irradiation of 6 yields triplet imine nitrene 7.

Photolysis of γ-azidobutyrophenone derivatives yields 1,4 ketyl biradicals via intramolecular H-atom abstraction. The 1,4 ketyl biradicals expel a nitrogen molecule to form 1,5 ketyl iminyl biradicals, which decay by ring closure to form a new carbon−nitrogen bond. The 1,5 ketyl iminyl biradicals were characterized with transient spectroscopy. In argon/nitrogen-saturated solutions, the biradicals have λmax ≈ 300 nm and τ = 15 μs. DFT-TD calculations were used to support the proposed mechanism for formation of the 1,5 ketyl iminyl radicals.

We report the first observation that the intermolecular triplet sensitization of alkyl azides leads to bimolecular reactivity, by forming triplet alkyl nitrenes. The intermolecular triplet-sensitized photolysis of 1-azidoadamantane with acetone, acetophenone and benzophenone leads to the formation of di-(adamantan-1-yl)-1,2-diazene as the major product via dimerization of triplet adamantan-1-yl nitrene. The triplet alkyl nitrene also abstracts a H-atom from the solvent to form adamantan-1-yl amine, adamantan-1-yl-benzyl amine and adamantan-1-yl benzylidene amine. The rates of the energy transfer from acetophenone and benzophenone to 1-azidodamantane are 1×108 and 5×106 M−1 s−1, respectively. Triplet-sensitized photolysis of benzyl azide with acetophenone gave methylene phenyl amine, dibenzyl amine and tribenzyl amine as the major products. Thus triplet benzyl nitrene abstracts a H-atom from the solvent to form dibenzyl amine and tribenzyl amine and rearranges to form methylene phenyl amine. The energy transfer rate between acetophenone and benzyl azide is diffusion controlled, or 2×109 M−1 s−1. Sensitized photolysis of benzyl azide with benzophenone yielded only benzylideneamine. Presumably, benzophenone abstracts a H-atom from benzyl azide and the resulting radical rearranges into benzylideneamine. The rate of the chemical quenching of benzophenone with benzyl azide is 5×107 M−1 s−1.