Photochemistry of 2-Naphthoyl Azide. An Ultrafast Time-Resolved UV–Vis and IR Spectroscopic and Computational Study

Abstract

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 (S(1)) was observed by both time-resolved techniques. Three processes are involved in the decay of the S(1) excited state of 2-naphthoyl azide: intersystem crossing, singlet nitrene formation, and isocyanate formation. The lifetime of the S(1) 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 S(1) state only upon 350 nm excitation (S(0) → 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 S(1) 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 S(1) 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.

Photolysis of 3,5‐diphenylisoxazole in argon matrices

“…Nitrenes are best described as electron‐deficient reactive intermediates and their reactivity is controlled by the nature of their substituents and their electron configuration . Generally, the triplet configuration of nitrenes is more stable than the corresponding singlet configuration, although nitrenes such as acylnitrenes have singlet ground states . Electron‐donating substituents stabilize triplet nitrene intermediates, whereas electron‐withdrawing substituents render them more reactive.…”

An ultrafast time‐resolved infrared and UV–vis spectroscopic and computational study of the photochemistry of acyl azides

“…Very recently, we reported the results of an fs time‐resolved UV–vis and IR study of the photochemistry of 2‐NpCON 3 in a series of solvents. Polar solvents were found to accelerate the S 1 state's decay significantly.…”

A Singlet Thiophosphoryl Nitrene and Its Interconversion with Thiazyl and Thionitroso Isomers

“…1 Upon either heat or irradiation, this class of energetic compounds can split off molecular nitrogen and form highly reactive nitrene intermediates. 2 The decomposition of covalent azides, in particular carbonyl, 3 phosphoryl, 4 and sulfonyl azides 5 have attracted enormous theoretical and experimental interest since the discovery of the Curtiusrearrangement from carbonyl azides to isocyanates as early as 1890. 6 Generally, a stepwise decomposition via α-oxonitrenes, RC(O)N, R 2 P(O)N, and RS(O) 2 N is energetically more favorable than the concerted pathway, and these α-oxonitrenes in their singlet state coexist with an oxazirine-like resonance structure due to intramolecular N-O stabilizing interaction.…”

“…6 Generally, a stepwise decomposition via α-oxonitrenes, RC(O)N, R 2 P(O)N, and RS(O) 2 N is energetically more favorable than the concerted pathway, and these α-oxonitrenes in their singlet state coexist with an oxazirine-like resonance structure due to intramolecular N-O stabilizing interaction. [3][4][5] However, due to complex reactions with solvents and Curtius rearrangement, only very few transient α-oxonitrenes have been detected using ultra-fast spectroscopy [3][4][5] and matrix isolation techniques. 7 Importantly, decomposition of α-oxoazides has provided unique access to N,O-containing species bearing novel structural and bonding properties such as N 2 CO, 8 OPN, 9 and O 2 SN, 10 in which the corresponding α-oxonitrenes were suggested to be involved.…”