Light-Assisted Akamptisomerization: Excited-State Bond-Angle Reflection (ESBAR) as a Molecular Photoswitching Element in B ₂ OF ₂ –Porphyrins

Akamptisomerism arises from bond-angle reflection (BAR) at a B-O-B bridge in low-symmetry B₂OF₂ porphyrins and leads to isolable diastereisomers in the ground state. Herein, we present the first theoretical investigation of the excited-state bond-angle reflection (ESBAR). TD-DFT calculations show that the BAR activation barrier decreases from 21.3 kcal mol^-1 (0.924 eV) in the ground-state (S₀) to 15.3 kcal mol^-1 (0.663 eV) in the triplet excited-state (T₁), rendering the process fluxional upon excitation. The feasibility of singlet-triplet intersystem crossing is supported by near-isoenergetic singlet and triplet states and enhanced spin-orbit coupling. These results provide substantial evidence that akamptisomerism could be facilitated in the excited state, highlighting ESBAR as a potential photoswitching mechanism in porphyrinoid systems.