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Photoenols, formed through photoinduced intra-molecular H atom abstraction in o-alkyl-substituted arylketones,typically have limited utility as reactive intermediates owing to fastreversion to the starting material. Herein, we introduced an azidogroup on the o-alkyl substituent to render the photoreactionirreversible. Irradiation of 2-azidomethylbenzophenone (1) inmethanol yielded 2-(hydroxy(phenyl)methyl)benzonitrile (2). Laser flash photolysis of 1 revealed the formation of biradical 3Br1followed by intersystem crossing to photoenols Z-3 (τ ∼ 3.3 μs) and E-3 (τ > 45 μs), both of which reverted to 1. Alternatively, 3Br1could lose N2 to form 3Br2 (not detected), which decays to 2. In cryogenic argon matrices, irradiation of 1 yielded nitrene 31N and 2but no photoenols, likely because Z-3 regenerated 1. Both ESR spectroscopy and absorption analysis in methyltetrahydrofuran (80K) confirmed 31N formation. Upon prolonged irradiation, the absorbance of 31N decreased, whereas that of 3 remained unchangedand that of 2 increased. Thus, TK of 1 is proposed to form 3Br1 via H atom abstraction, with subsequent intersystem crossing to 3competing with the loss of N2 to generate 3Br2. DFT calculations revealed a small energy gap (∼2 kcal/mol) between the triplet andsinglet configurations of Br2, supporting a mechanism in which 3Br2 intersystem crosses to yield 2
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Photoreactivity of 1-azidostyrene and 3-phenyl-2H-azirine in acetonitrile and cryogenic matrices
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.
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- Award ID(s):
- 2400277
- PAR ID:
- 10582339
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Photochemistry and Photobiology A: Chemistry
- Volume:
- 467
- ISSN:
- 1010-6030
- Page Range / eLocation ID:
- 116427
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1016/j.jphotochem.2025.116427
