Exploring photochemistry of p-bromophenylsulfonyl, p-tolylsulfonyl and methylsulfonyl azides by ultrafast UV-pump–IR-probe spectroscopy and computations

Abstract

The photochemistry of p-bromophenylsulfonyl azide (BsN₃), p-tolylsulfonyl azide (TsN₃) and methylsulfonyl azide (MsN₃) was studied by femtosecond time-resolved infrared spectroscopy with CH₂Cl₂ and CCl₄ as solvents along with quantum chemical calculations. The photolysis of these azides after 267 nm light excitation leads to the population of each respective azide S₁ excited state. Decay of the S₁ excited state gives rise to singlet nitrene formation. In the case of BsN₃, the decay was found to correlate with the formation of a pseudo-Curtius photoproduct (PCP) BrC₆H₄NSO₂. Transient electronic ground states of the three azides on their way to singlet nitrenes and PCPs were shown by locating the corresponding transition states on the potential energy surfaces. The lifetime of singlet ¹(BsN) and ¹(TsN) nitrenes is τ_S = ∼20 ps in CH₂Cl₂ and ∼700 ps in CCl₄. Singlet ¹(MsN) was not detected. Due to fast intersystem crossing (ISC), singlet nitrenes are converted into the triplet spin isomers lying lower in energy, the formation time constants being equal to the corresponding singlet nitrene lifetime. The formation of ³(MsN) was shown and the formation time constant in CH₂Cl₂ was found to be τ_ISC = 34 ± 3 ps. Internal conversion of the S₁ excited state to the ground state of the azide was low (Φ ≈ 0.15) for BsN₃ and TsN₃ and was not found in the case of MsN₃.