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

Abstract

Multiple resonance (MR) boron–nitrogen doped polycyclic aromatic hydrocarbons (BN-PAHs) have shown compelling thermally activated delayed fluorescence (TADF), surpassing those of their hydrocarbon analogues. However, the structural variety of π-extended BN-PAHs remains narrow. In this study, we synthesized three double helical BN-doped nanographenes (BN-NGs), 2 a–2 c, and three heptagon-embedded BN-NGs, 1 a–1 c, by π-extension of the MR core. During the formation of 2 a, a nanographene with one heptagon (1 a) was obtained, whereas further dehydrocyclization of the [6]helicene units within 2 b and 2 c led to heptagon structures, yielding other two BN-NGs containing double heptagons (1 b and 1 c). These BN-NGs (2 a–2 c and 1 a–1 c) 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 a heptagon ring into 1 a–1 c expanded the conjugation, reduced the oxidation potentials, and yielded a more flexible framework compared to those of 2 a–2 c. The enantiomers of 2 a–2 c, 1 a, and 1 c were resolved and their chiroptical properties were studied. Notably, 1 a and 1 c exhibited increased chiroptical dissymmetry factors.