Chiral BN-Heteroacenes Embedded with B←N Coordination: Acid-Triggered Reversible Bond Switching and Optical Limiting Behavior.

Boron-nitrogen coordination (B←N) bonds have emerged as a versatile motif for engineering π-conjugated systems with tunable optoelectronic properties. Herein, we report the synthesis, structures, and properties of chiral heteroacenes (5a-5c) featuring B←N coordination within highly twisted π-frameworks. 5a-5c were synthesized via a four-step approach, where the final cyclization simultaneously forged six- and five-membered rings bearing both B-N covalent and B←N dative bonds. Crystal structure analysis revealed the distorted backbone and intrinsically chiral character of 5a-5c, which were successfully resolved into enantiomers by chiral-phase HPLC. The enantiomers of 5a-5c were characterized by circular dichroism spectroscopy, and the enantiomers (+)-5a and (-)-5a exhibit circularly polarized luminescence with the luminescence dissymmetry factors (glum) reaching 10-3 in both solid state and solution phase. Furthermore, the B←N coordination unit induces strong intramolecular charge-transfer (ICT) transitions, resulting in low-energy absorption band at 420-600 nm. The ICT band can be reversibly modulated by acid-base treatment through dynamic cleavage and restoration of B←N bond. Moreover, 5a-5c show strong third-order nonlinear optical responses, making them promising candidates for optical limiting materials. This study highlights the potential of B←N-embedded chiral heteroacenes as multifunctional materials.