Effects of Chirality and Counter Anions on the Structures and Solid-State Properties of Radical Cation Salts of Benzo[b]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.