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

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.