A medium effect in the photolysis of phenyl azide in low-temperature matrices

Abstract Measurements have been made of the relative rates of formation of azacyclohepta-1,2,4,6-tetraene and disappearance of the starting material in the photolysis of phenyl azide in Ar, CH 4 and 3-methylpentane matrices. With narrow band light ( λ = 280±5 nm), formation of the ring expanded product lagged behind the decomposition of the azide by a distinct and reproducible amount in the early stages of the photolyses in CH 4 and 3-methylpentane. Full arc photolysis ( λ > 200 nm), on the other hand, resulted in a linear relationship between growth of product and disappearance of starting azide. No similar wavelength dependence was observed for the photolysis in argon. This is clear evidence that the photolysis in the hydrocarbon matrices proceeds in part to triplet phenylnitrene, which can subsequently absorb a second photon to give the azacycloheptatetraene. Vibrational coupling between the initially formed singlet phenylnitrene and the host material, which should be much more efficient in CH 4 and 3-methylpentane than in the monatomic argon, is proposed to account for the observed difference in behaviour in the matrix hosts.