Photochemistry of 2-Naphthoyl azide. An ultrafast time-resolved UV-Vis and IR spectroscopic and computational study
Résumé
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 S1 state. Nitrene formation correlates with the decay of the relaxed S1 state only upon 350 nm excitation (S0 → S1 excitation). When Sn (n ≥ 2) states are populated upon excitation (λex = 270 nm), most nitrene formation takes place within a few picoseconds through the hot S1 and higher singlet excited states (Sn) 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.