Photochemical Synthesis of Planar-to-Helical BN Acenes

BN-isosteric acenes offer new opportunities for structural and property modulation, but the synthesis and conformational control of highly fused BN acenes remain challenging. Here we develop a photochemical strategy to convert BN heterocyclic precursors into highly fused BN acenes. By varying substituent positions and ring-fusion orientations, we achieve a conformational evolution from planar to double-helical structures. Despite pronounced backbone distortion, the six-membered BN rings retain substantial local aromaticity. Conformational modulation gives rise to diverse photophysical properties, including long-lived phosphorescence and persistent afterglow. Enhanced air stability arises from the synergistic effects of steric shielding of the boron centres by the double-helical architecture and HOMO energy lowering upon π-extension. This work establishes a new route to highly fused BN acenes and provides opportunities for tuning their three-dimensional structures and properties.