Effects of Chirality and Counter Anions on the Structures and Solid‐State Properties of Radical Cation Salts of Benzo[ <i>b</i> ]Phenoxazine‐Based Double Heterohelicene

In this study, we prepared stable radical cation salts of racemic and enantiopure double hetero[5]helicene (1), consisting of two benzo[b]phenoxazine (BPO) molecules, using two different counter anions (SbCl₆ ^- and NTf₂ ^-). The effects of chirality and counter anions on the packing structure and solid-state electronic properties were investigated. X-ray single-crystal analysis revealed that the SbCl₆ ^- anion was positioned above the midpoint of 1^·+ in both the racemic and enantiopure crystals of 1^·+·SbCl₆ ^-. This alignment resulted from the matching between the curvature of 1^·+ and the spherical shape of the SbCl₆ ^- anion, preventing the formation of continuous π-π interactions among 1^·+. In contrast, 1D π-stacked columns of 1^·+ were present in both the racemic and enantiopure 1^·+·NTf₂ ^- crystals, as the NTf₂ ^- anions encased the π-stacked columns of 1^·+. All radical cation salts exhibited distinct melting points, and the enantiopure salts of 1^·+·NTf₂ ^- had notably low melting points of 393-395 K, which was 142 K lower than that of rac-1^·+·NTf₂ ^-. ESR and SQUID measurements of the enantiopure 1^·+·NTf₂ ^- crystals revealed significant changes in their magnetic properties upon crystal melting. In contrast, rac-1^·+·NTf₂ ^- displayed an unexpected crystal-crystal phase transition near room temperature, accompanied by a thermal hysteresis loop in its magnetic susceptibility.