Nonalternant B,N‐Embedded Helical Nanographenes Containing Azepines: Programmable Synthesis, Responsive Chiroptical Properties and Spontaneous Resolution into a Single‐Handed Helix

Abstract

AbstractHeteroatom‐embedded helical nanographenes (NGs) constitute an important and appealing class of intrinsically chiral materials. In this work, a series of B,N‐embedded helical NGs (BN‐HNGs) bearing azepines was synthesized via stepwise regioselective cyclodehydrogenation. First, the phenyl‐ or nitrogen‐bridged dimers were efficiently clipped into highly congested model compounds 1 and 2. Later, the controllable Scholl reactions of the tetraphenyl‐tethered precursor generated 1, 7 or 8 new C−C bonds, thereby establishing a robust method for the preparation of nonalternant BN‐HNGs with up to 31 fused rings. The helical bilayer nature was unambiguously verified by X‐ray diffraction analysis. The helical chirality was transferred to the stereogenic boron centers upon fluoride coordination, with a concave‐concave structure to comply with the bilayer skeleton. Notably, the largest nonalternant BN‐HNG (6) spontaneously resolved into a homochiral 41 helix structure as a molecular spiral staircase during crystallization via conglomerate formation at the single‐crystal scale. The large twisted C2‐symmetric π‐surface and the dynamic chiral skeleton induced by curved azepines might have synergistic effects on self‐recognition of enantiomers of 6 to achieve the intriguing spontaneous resolution behavior. The chiroptical properties of the enantiomer of 6 were further investigated, revealing that 6 had a strong chiroptical response in the visible range (400–700 nm).