Laser flash photolysis of nanocrystalline α-azido-p-methoxy-acetophenone

Irradiation of nanocrystals of azide 1 results in a solid-to-solid reaction that forms imine 2 in high chemical yield. In contrast, solution photolysis of azide 1 yields a mixture of products, with 7 as the major one. Laser flash photolysis (LFP) of a nanocrystalline suspension of azide 1 in water shows selective formation of benzoyl radical 4 (lambda(max) similar to 400 nm), which is short-lived (tau = 833 ns) as it intersystem crosses to form imine 2. In comparison, LFP of azide 1 in methanol reveals the formation of triplet alkylnitrene 10 (lambda(max) similar to 340 nm). The selectivity observed in the solid-state is related to stabilization of the triplet ketone with (n, pi*) configuration by the crystal lattice, which results in alpha-cleavage being favored over triplet energy transfer to the azido chromophore. Both the solid-state and solution reaction mechanisms are further supported by density functional theory calculations. Thus, laser flash photolysis has been used to effectively elucidate the medium dependent reaction mechanisms of azide 1.