Understanding of Photo-Induced Reversible Rearrangement from Borepin to Borirane

The first reversible photoisomerization between a borepin and a borirane was reported in the photo-induced reactions of B(npy)Ar₂ (npy=2-(naphthalen-1-yl) pyridine, Ar=phenyl or electron rich aryl; S. Wang, et al. Angew. Chem. Int. Ed. 2019, 58, 6683–6687). In this work, the detailed mechanisms of the unprecedented reversible photoisomerization between the borepin (compound a) and the borirane (compound b) of B(npy)Ph₂ in the first excited singlet (S₁) state and the ground (S₀) state were studied by carrying out calculations with the complete active space self-consistent field (CASSCF) and its second-order perturbation (CASPT2) methods combined with time-dependent density functional theory (TD-DFT). The calculation results show that photoexcitation of a-S₀ at 365 nm and b-S₀ at 450 nm populate their S₁ state with evident charge transfer characteristics. The photoisomerization is triggered in the S₁ state and ends in the S₀ state, at which the intersection points in a (S₁/S₀)_x intersection seam participate in and promote phenyl migration and ring-closure processes. Furthermore, we reveal that the not large energy difference (less than 0.6 eV) and similar conjugation properties of π electrons between a-S₀ and b-S₀ are responsible for their unique photo-reversible reactivity, compared with those of the isomers of the thermally reversible compound B(ppy)Mes₂. Our results contribute to an understanding of the excited-state reactivity of organoboron compounds and will be useful to support the design of new boron-based photo-responsive materials.