BN‐Isosteres of Nonacene with Antiaromatic B 2 C 4 and N 2 C 4 Heterocycles: Synthesis and Strong Luminescence**

Abstract Embedding both boron and nitrogen into the backbone of acenes to generate their isoelectronic structures has significantly enriched the acene chemistry to offer appealing properties. However, only small BN‐heteroacenes have been extensively investigated, with BN‐heptacenes as the hitherto longest homologue. Herein, we report the synthesis of three new nonacene BN‐isosteres via incorporating a pair of antiaromatic B 2 C 4 and N 2 C 4 heterocycles, representing a new length record for BN‐heteroacenes. The distance between the B 2 C 4 and N 2 C 4 rings affects the contribution of the charge‐separated resonance forms, leading to tunable antiaromaticity of the two heterocycles. The adjusted local antiaromaticity manifests substantial influence on the molecular orbital arrangement, and consequently, the radiative transition rate of BN‐3 is greatly enhanced compared with BN‐1 and BN‐2 , realizing a high fluorescence quantum yield of 92 %. This work provides a novel design concept of large acene BN‐isosteres and reveals the importance of BN/CC isosterism on their luminescent properties.