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

Abstract

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 CC 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.