Ultrafast Photoelimination of Nitrogen from Upper Excited States of Diazoalkanes and the Fate of Carbenes Formed in the Reaction.

The photochemical reactivity of diphenyldiazomethane and phenyl 1- and 2-adamantyl diazomethanes and , respectively, was investigated by transient absorption spectroscopy (TA). Photoelimination of N upon UV excitation takes place in the anti-Kasha ultrafast photochemical reaction from the upper excited singlet states to deliver singlet carbenes, which were, in the case of and , detected by fs-TA. The reactivity of the carbenes differs with respect to the substituent at the carbene center. The singlet in a nonpolar solvent delivers the triplet carbene by intersystem crossing (ISC). Singlet does not undergo ISC but reacts in the intermolecular insertion reactions into C-H bonds. has an α-C-H bond next to the carbene center and reacts rapidly in the intramolecular C-H insertion reaction to deliver alkene, precluding its detection by fs-TA. However, the isolation of ketone photoproducts from is highly indicative of triplet 's intermediate formation. The TA spectra from the S-S states of were computed using time-dependent density functional theory, while the multiconfigurational perturbation theory to the second order was used for the absorption spectra of the corresponding singlet and triplet carbenes. The modeled and measured spectra are in good agreement, and the computations corroborate the assignments of the key short-lived intermediates.