Quinodimethane embedded expanded helicenes and their open-shell diradical dications/dianions

Abstract

Helicenes and expanded helical π-conjugated molecules consisting of aromatic benzene rings or heterocycles have been widely studied, but the incorporation of a quinoidal conjugated unit in such π-systems, which is expected to provide intriguing electronic properties, remains limited. Herein, we report the synthesis and redox properties of two quinodimethane-embedded expanded [11]- and [13]helicenes with respectively eleven and thirteen fused rings. Their helical-shaped geometry structures were unambiguously elucidated by X-ray crystallographic analysis. Both molecules exhibit a small energy gap and amphoteric redox behavior with multiple redox waves. Moreover, a fast racemization process for cyclopenta-fused expanded [11]helicene was observed above 173 K by variable temperature nuclear magnetic resonance (NMR), with a small inversion barrier, while cyclopenta-fused expanded [13]helicene possesses a moderate racemization barrier (18.09 ± 1.72 kcal mol⁻¹ at coalescence temperature T_c = 360 K), as demonstrated by dynamic NMR spectroscopy. On the other hand, both compounds can be oxidized or reduced by NOSbF₆ or sodium anthracenide to the respective radical cations, radical anions, dications and dianions. Compared with closed-shell neutral compounds, their dications and dianions show significant open-shell singlet diradical character with a small singlet–triplet energy gap. This work provides some insights into the design and synthesis of novel helical π-systems with tunable properties.