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

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 S₁-T₂ energy gap in anthracene derivatives, promoting triplet formation via an S₁→T₂→T₁ 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.