Ultrafast Photochemical Reaction Dynamics of 3-Phenyl-1,4,2-dioxazol-5-one Revealed by Femtosecond Time-Resolved Infrared Spectroscopy

Dioxazolones are important precursors for generating nitrenes (highly reactive intermediates widely used for carbon–nitrogen bond formation in organic synthesis) upon exposure to light or heat. The photochemical reaction dynamics of 3-phenyl-1,4,2-dioxazol-5-one in CHCl3 were investigated using femtosecond time-resolved infrared spectroscopy and electronic structure calculations. Photoexcitation at 267 nm rapidly populates an excited singlet state that serves as the key branching point for subsequent photophysical and photochemical processes. Transient infrared spectra reveal the formation of carbon dioxide, phenyl isocyanate, and singlet benzoyl nitrene through their characteristic vibrational features. Kinetic analysis shows that decarboxylation from the excited singlet state occurs with a time constant of 4.7 ± 1 ns, producing phenyl isocyanate and benzoyl nitrene with time constants of 8.1 ± 2 ns and 11 ± 3 ns, respectively. Competing relaxation pathways include internal conversion to the ground state (7.5 ± 2 ns) and intersystem crossing to the T1 state (25 ± 5 ns). The T1 state relaxes to the ground state (350 ± 30 ns) without contributing to product formation. These results demonstrate that both isocyanate and nitrene products originate from the S1 state and provide detailed mechanistic insight into the competing pathways governing dioxazolone photochemistry in solution.