Picking a Pucker: Excited-State Charge Redistribution Navigates the BN-Reshaped Conical-Intersection Landscape in Azaborine Photochemistry
Year: 2026
Type: preprint
Abstract: Photoexcited benzene yields multiple isomers, whereas its BN isostere 1,2-azaborine selectively forms BN-Dewar benzene (BN-DB), despite both decaying through ringpuckered S1/S0 conical intersections (CIs). This selectivity is puzzling given that BNinduced asymmetry in 1,2-azaborine creates six distinct puckering sites and potentially multiple CIs, yet only a single photoproduct is observed. Here, we map the S1/S0 CI landscape of 1,2-azaborine using CASSCF and XMS-CASPT2 and identify eight N-, B-, and C-puckered CIs that open channels to various isomers including BN-DB and BN-benzvalene (BN-BV). BN incorporation therefore does not remove the excitedstate pathways of benzene but multiplies them. Of these CIs, only the N-puckered CIs leading to BN-DB lie below the S1 minimum, a preference we trace to S1 N→B charge redistribution. We then examine whether structural modifications that alter this redistribution can change CI accessibility, considering CH3 and F substitution and changes in B and N positions. Notably, 1,4-azaborine exhibits a qualitative shift from N→B to C→B charge redistribution. Consistently, C-and B-puckered CIs lie below the S1 minimum and may provide access to BN-BV, a versatile synthetic intermediate currently obtained through two photochemical steps. Together, these results suggest that S1 charge redistribution may be a key lever for steering azaborine photochemistry and for designing more direct routes to BN-BV.
Source: ChemRxiv
Authors: Annu Yadav, Mahesh Gudem
Institution: Indian Institute of Technology Dharwad
Language: English
Subfield: Physical and Theoretical Chemistry
Field: Chemistry
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7223143226