Tuning BN-Benzvalene Formation from BN-Dewar Benzene: Insights into Photochemical Synthesis

This study investigates the photochemical transformation of BN-Dewar benzene into BN-Benzvalene derivatives, providing a strategic route to heteroatom-containing valence isomers with unique electronic characteristics. We elucidate the excited-state reaction mechanism and associated structural reorganizations by employing time-dependent density functional theory (TD-DFT) and electron localization function (ELF) analysis.Vertical excitation to the first singlet excited state (𝑆1) induces notable weakening of the C=C and B–N bonds while concurrently strengthening the N–Si bond in silylsubstituted derivatives—a critical feature that facilitates the efficient formation of BN-Benzvalene products. Two minimum energy conical intersections (MECIs) mediate nonradiative decay: MECI1 drives the reaction forward via irreversible C2–B bond cleavage and C1–B bond formation, whereas MECI2 enables relaxation back to the BN-Dewar benzene ground state. Substitution at nitrogen with trialkylsilyl groups markedly enhances product yield by stabilizing charge redistribution and reducing the Franck–Condon excitation energy. The MECI1 pathway proceeds without an energetic barrier, strongly favoring BN-Benzvalene formation. ELF analysis further reveals that bond reorganization occurs through electron density migration rather than via radical intermediates. These findings offer detailed mechanistic insights and highlight the potential of photochemical strategies for constructing novel BN-heterocyclic frameworks.