Benzo‐Fused Double [7]Carbohelicene: Synthesis, Structures, and Physicochemical Properties

A benzo‐fused double [7]carbohelicene (D7H) was synthesized through a regioselective cyclodehydrogenation of a tetranaphthyl‐p‐terphenyl‐based precursor. The twisted (D7H‐1) and anti‐folded (D7H‐2) conformers of D7H were separated by recrystallization, and their double helicene structures with overlapping terminal benzene rings were unambiguously elucidated by X‐ray crystallography. A record‐high isomerization barrier (46.0 kcal mol−1) in double helicenes was estimated based on density functional theory (DFT) calculation, which resulted in the excellent conformational stability of D7H. The physicochemical properties of D7H‐1 and D7H‐2 were investigated by UV/Vis absorption spectroscopy and cyclic voltammetry, displaying the variation of electronic structure upon conformational changes. The optical resolution of the racemic D7H‐1 was carried out by chiral HPLC, offering enantiopure D7H‐1‐(P,P) and D7H‐1‐(M,M), which were further characterized by circular dichroism spectroscopy. Carbon‐rich compounds: An all‐carbon double [7]helicene has been synthesized by regioselective cyclodehydrogenation. The compound represents the highest homolog of the double carbohelicenes. The twisted conformers D7H‐1 and D7H‐2 were separated by recrystallization, and their double helicene structures were elucidated by X‐ray crystallography.