BO‐Fusion Modulates Excited‐State Energetics to Enable Heavy‐Atom‐Free Photosensitization

ABSTRACT Substitution of C═C bonds with B–E units (E = N, O, P, etc.) provides a powerful strategy to modulate the electronic structure of π‐conjugated frameworks with minimal skeletal perturbation. However, precise control over excited‐state processes through heteroatom incorporation remains challenging. Herein, we present a modular one‐pot cascade combining 1,1‐bromoboration with double electrophilic borylation to construct fully conjugated BO‐fused PAHs with diverse fusion topologies. Photophysical and computational studies reveal that the distinct BO‐fusion patterns induce pronounced changes in frontier molecular orbital energies and excited‐state landscapes, which correlate with their divergent photosensitization behavior. Specifically, linear BO‐fusion compresses the S 1 –T 2 energy gap in anthracene derivatives, promoting triplet formation via an S 1 →T 2 →T 1 pathway. This strategy enables the development of a heavy‐atom‐free anthracene‐based sensitizer ( 9a‐H) that combines singlet‐oxygen generation with improved photostability. These findings establish BO‐fused PAHs as tunable platforms for triplet‐state engineering and photosensitization.