Elucidating the effects of isoelectronic atomic substitution on the excited-state dynamics and reactivity of aromatic compounds.

Photochemistry of aromatic compounds has attracted significant interest for their distinctive excited-state behavior and potential applications as energy materials. Isoelectronic atomic substitution, which involves replacing ring atoms while maintaining the same number of electrons in aromatic compounds (e.g., from C-C to B-N), offers a promising strategy for modulating these properties by tuning electronic structures. However, a fundamental understanding of its effect on the excited-state dynamics and reactivity remains elusive. In this study, we investigate the photochemical properties of oxaborine, using benzene and azaborine as references, to elucidate the effects of isoelectronic atomic substitution. Potential energy surface (PES) calculations reveal that oxaborine undergoes barrierless ring distortion toward S1/S0 conical intersection (CI), followed by barrierless Dewar isomerization directly from the S1/S0 CI. Using nonadiabatic molecular dynamics simulations, we observe a rapid S1 decay time (125 fs) and a notably high Dewar isomerization yield (23.3%), consistent with the results of PES calculations, highlighting the unique photochemical properties of oxaborine. Orbital-level analysis reveals that atomic substitution breaks the π orbital degeneracy and destabilizes the S1 state, explaining the much faster S1 decay for azaborine and oxaborine. The p orbital energy modulations from O-B or N-B substitution weaken the meta-bridge bond while enhancing para-bridge C-C interactions, favoring Dewar isomerization. These findings offer a comprehensive understanding of the effect of isoelectronic atomic substitution in aromatic compounds and demonstrate its potential as a design strategy for controlling photochemical properties.