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.硼氮配位（Boron–nitrogen coordination (B←N) bonds）键已成为构建可调谐光电子性质π共轭体系（π-conjugated systems）的通用结构基元。本文报道了在高度扭曲π骨架中内嵌B←N配位键的手性杂并苯（chiral heteroacenes）5a–5c的合成、结构与光电性质。5a–5c通过四步合成路线制备，最终的环化反应同时构筑了同时带有B-N共价键与B←N配位键的六元环与五元环。晶体结构分析揭示了5a–5c具有扭曲的分子骨架与固有手性特征，通过手性高效液相色谱（chiral-phase HPLC）成功实现了其对映异构体的拆分。采用圆二色谱法（circular dichroism spectroscopy）对5a–5c的对映异构体进行表征，其中(+)-5a与(-)-5a展现出圆偏振发光（circularly polarized luminescence）性能，其发光不对称因子（luminescence dissymmetry factors, glum）在固态与溶液相中均达到10^-3量级。此外，B←N配位单元可诱导强烈的分子内电荷转移（ICT）跃迁，在420–600 nm波长范围内形成低能吸收带。该ICT吸收带可通过酸碱处理，借助B←N键的动态断裂与重建实现可逆调控。进一步研究发现，5a–5c具有显著的三阶非线性光学响应，有望成为光限幅材料（optical limiting materials）的候选体系。本研究凸显了内嵌B←N配位键的手性杂并苯作为多功能材料的应用潜力。