π‐Extension of Multiple Resonance Core: Double Helical and Heptagon Embedded Nanographenes

Abstract

Multiple resonance (MR) boron–nitrogen doped polycyclic aromatic hydrocarbons (BN‐PAHs) showed compelling thermally activated delayed fluorescence (TADF), surpassing those of their hydrocarbon analogs. However, the structural variety of π‐extended BN‐PAHs remains narrow. In this study, we synthesized three double helical BN‐doped nanographenes (BN‐NGs), 2a–2c, via the π‐extension of the MR core. During the formation of 2a, a nanographene with one heptagon (1a) was obtained, whereas subsequent dehydrocyclization of the [6]helicene units within 2b–2c led to heptagon structures, yielding other two BN‐NGs containing double heptagons (1b–1c). These BN‐NGs (2a–2c and 1a–1c) showed pronounced redshifts of 100–190 nm compared to the parent MR core while preserving the TADF characteristics and prolonging the delayed fluorescence lifetime to the millisecond level. Furthermore, the integration of heptagon ring into 1a–1c expanded the conjugation, reduced the oxidation potentials, and yielded a more flexible framework compared to those of 2a–2c. The enantiomers of 2a–2c, 1a, and 1c were resolved and their chiroptical properties were studied. Notably, 1a and 1c exhibited the increased chiroptical dissymmetry factors.