Photogeneration and Direct Observation of a 2H‐phosphirene and a Closed‐shell Singlet Arylphosphenium Ion

Phosphenium ions are highly reactive species with the formula of RR'P⁺. Although phosphenium ions were proposed as the invaluable intermediates of various important chemical processes, the methods to generate and characterize them were very limited. In this study, a novel photoprecursor to give rise to the arylphosphenium ions, 1-(naphthalen-2-yl)phosphirane, was synthesized and investigated by femtosecond to nanosecond transient absorption spectroscopies, photoproduct analysis, and density functional theory calculations. Subsequent to photoexcitation, the S₁ state of the photoprecursor of interest is populated from the higher excited states, accompanied by the generation of an unexpected 2H-phosphirene, 7bH-naphtho[1,2-b]phosphirene. After that, the resulted 2H-phosphirene is protonated by H₂O in the solvent to generate the naphthalen-2-yl(hydryl)phosphenium ion, which can be attacked by H₂O and oxidized by O₂ to give naphthalen-2-ylphosphinic acid as the major final product. The presence of these transient species is supported by density functional theory calculation and control experiments with different solvents. This study provides an understanding of the reactivity of arylphosphenium ions as well as 2H-phosphirenes, and provides a novel method to generate and characterize these reactive species, which may help to further reveal their reactivity and explore their chemistry and use in a range of applications.