From BN-Dewar benzene to BN-benzvalene: a computational exploration of photoisomerization mechanisms

This study explores the photochemical conversion of BN-Dewar benzene into BN-benzvalene derivatives, offering a strategic route to heteroatom-containing valence isomers with distinctive electronic properties. Using time-dependent density functional theory (TD-DFT) and electron localization function (ELF) analyses, the excited-state mechanism and associated structural rearrangements were elucidated. Vertical excitation to the S₁ state was found to weaken the CC and B-N bonds while strengthening the N-Si bond in silyl-substituted derivatives, a key factor enabling efficient BN-benzvalene formation. Two minimum energy conical intersections MECI1 and MECI2 govern the deactivation pathways: MECI1 promotes irreversible C2-B bond cleavage and C1-B bond formation, driving the system toward BN-Benzvalene, whereas MECI2 enables relaxation back to the BN-Dewar benzene reactant. Nitrogen substitution, particularly with trialkylsilyl groups, significantly enhances the reaction yield by stabilizing charge redistribution and lowering Franck-Condon excitation energies. Nonradiative decay via MECI1 proceeds barrierlessly, favoring the production of BN-benzvalene. Finally, ELF analysis reveals that bond formation occurs through electron density migration rather than via radical intermediates.