Insights into the Photoinduced Isomerization Mechanisms of a N,C–Chelate Organoboron Compound: A Theoretical Study

As the first discovered organoboron compound with photochromic property, B(ppy)Mes₂ (ppy=2-phenylpyridine, Mes=mesityl) displays rich photochemistry that constitutes a solid foundation for wide applications in optoelectronic fields. In this work, we investigated the B(ppy)Mes₂ to borirane isomerization mechanisms in the three lowest electronic states (S₀, S₁, and T₁) based on the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT) calculations. Our results show that the photoisomerization in the S₁ state is dominant, which is initiated by the cleavage of the B-C_ppy bond. After overcoming a barrier of 0.5 eV, the reaction pathway leads to a conical intersection between the S₁ and S₀ states (S₁/S₀)_x, from which the decay path may go back to the reactant B(ppy)Mes₂ via a closed-shell intermediate (Int1-S₀) or to the product borirane via a biradical intermediate (Int2-S₀). Although triplet states are probably involved in the photoinduced process, the possibility of the photoisomerization in T₁ state is very small owing to the weakly allowed S₁→T₁ intersystem crossing and the high energy barrier (0.77 eV). In addition, we found the photoisomerization is thermally reversible, which is consistent with the experimental observations.