Zhu Hong-Yang, Li Quan-Song
Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing, 100081, China.
Chemphyschem. 2020 Mar 17;21(6):510-517. doi: 10.1002/cphc.202000049. Epub 2020 Feb 20.
As the first discovered organoboron compound with photochromic property, B(ppy)Mes (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)Mes to borirane isomerization mechanisms in the three lowest electronic states (S , S , and T ) 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 S state is dominant, which is initiated by the cleavage of the B-C bond. After overcoming a barrier of 0.5 eV, the reaction pathway leads to a conical intersection between the S and S states (S /S ) , from which the decay path may go back to the reactant B(ppy)Mes via a closed-shell intermediate (Int1-S ) or to the product borirane via a biradical intermediate (Int2-S ). Although triplet states are probably involved in the photoinduced process, the possibility of the photoisomerization in T state is very small owing to the weakly allowed S →T 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.
作为首个被发现的具有光致变色性质的有机硼化合物，B(ppy)Mes（ppy = 2-苯基吡啶，Mes = 均三甲苯基）展现出丰富的光化学性质，这为其在光电子领域的广泛应用奠定了坚实基础。在这项工作中，我们基于完全活性空间自洽场（CASSCF）及其二阶微扰（CASPT2）方法，并结合含时密度泛函理论（TD-DFT）计算，研究了B(ppy)Mes在三个最低电子态（S₀、S₁和T₁）下向硼杂环丙烷的异构化机理。我们的结果表明，S₁态的光异构化占主导，其由B - C键的断裂引发。在克服0.5 eV的势垒后，反应路径导致S₁和S₀态之间的锥形交叉点（S₁/S₀），从该交叉点出发，衰变路径可能通过闭壳中间体（Int1 - S₀）回到反应物B(ppy)Mes，或者通过双自由基中间体（Int2 - S₀）生成产物硼杂环丙烷。尽管三线态可能参与光诱导过程，但由于较弱允许的S₀→T₁系间窜越和高能量势垒（0.77 eV），T₁态光异构化的可能性非常小。此外，我们发现光异构化是热可逆的，这与实验观察结果一致。
