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.