Photochemistry of 2-naphthoyl azide. An ultrafast time-resolved UV-Vis and IR spectroscopic and computational study

The photochemistry of 2-naphthoyl azide was studied in various solvents by femtosecond time-resolved transient absorption spectroscopy with IR and UV-vis detection. The experimental findings were interpreted with the aid of computational studies. Using polar and nonpolar solvents, the formation and decay of the first singlet excited state (S₁) was observed by both time-resolved techniques. Three processes are involved in the decay of the S₁ excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S₁ state decreases significantly as the solvent polarity increases. In all solvents studied, isocyanate formation correlates with the decay of the azide S ₁ state. Nitrene formation correlates with the decay of the relaxed S₁ state only upon 350 nm excitation (S₀ → S ₁ excitation). When S_n (n ≥ 2) states are populated upon excitation (λ_ex = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S₁ and higher singlet excited states (S_n) of 2-naphthoyl azide. The data correlate with the results of electron density difference calculations that predict nitrene formation from the higher-energy singlet excited states, in addition to the S₁ state. For all of these experiments, no recovery of the ground state was observed up to 3 ns after photolysis, which indicates that both internal conversion and fluorescence have very low efficiencies.