Triplet Sensitized Photodenitrogenation of Δ-1,2,3-Triazolines To Form Aziridines in Solution and in the Crystalline State: Observation of the Triplet 1,3-Alkyl-aminyl Biradical.

Taking advantage of an operationally simple technique to perform transmission pump-probe spectroscopy in crystalline solids, based on the use nanocrystalline suspensions in water, we analyzed the intermediates in the photodenitrogenation of a Δ-1,2,3-triazoline bearing a benzophenone group that served as an internal triplet sensitizer. Measurements carried out in acetonitrile solution revealed the formation of a transient with a λ= 570 nm with a lifetime of 70 ns. Measurements in the solid state displayed an analogous blue-shifted transient with a λ= 510 nm that first grows and then decays with time constants of 63 and 270 ns, respectively. Based on the comparison of the observed transient spectra with the one obtained from an independently generated aminyl radical, we assign it to the corresponding 1,3,-alkyl-aminyl biradical. We conclude that triplet state denitrogenation and the subsequent intersystem crossing-limited product formation are slower in the solid state than in solution.