Photochromic reaction pathways of 1,1′-azobis-1,2,3-triazole: A CASSCF and spin-flip DFT study

This study employed multiconfigurational CASSCF and MS-CASPT2 methods to investigate the photoinduced isomerization mechanism of 1,1′-azobis-1,2,3-triazole. The MS-CASPT2//CASSCF computational results indicate that the photoisomerization reaction of 1,1′-azobis-1,2,3-triazole involves a non-adiabatic transition pathway through the S₂ state. After photoexcitation to the S₂ state, the molecule undergoes internal conversion to reach S₁-min, followed by a non-radiative transition back to the ground state. The S₁/S₀-CI and S₁-min have similar structures and close energies, which facilitates unidirectional rotation. The weak coupling between the ground state and excited states may be due to the strong electron-withdrawing nature of the N₈ chain. Additionally, using the MS-CASPT2//CASSCF computational results as a reference, we compared the differences in describing the photoisomerization process of this compound using the SF-DFT method, both qualitatively and quantitatively. The results demonstrate that the SF-DFT method significantly underestimates the energy of the S₁ state. These findings are expected to deepen the understanding of non-adiabatic transitions in the photoinduced rotation of high-nitrogen compounds and provide theoretical insights for other high-nitrogen compounds that may exhibit photochromism.