Completely solvent-free synthesis of double heterohelicenes and their further ring fusion using mechanochemical reaction†

In this study, we developed a simple and efficient method for synthesizing double heterohelicenes (DHHs) composed of two heteroacenes bearing an NH group, such as benzo[b]phenoxazine (BPO) and dibenzo[b,i]phenoxazines (DBPO), using mechanochemical oxidative C–N coupling reactions, allowing complete solvent-free synthesis from commercially available compounds. Our new synthetic method afforded more than 1 g of DHH, which has a high dissymmetry factor for circularly polarized luminescence (g_CPL) of >1 × 10⁻², in a one-pot mechanochemical reaction using BPO as a reactant. In addition, mechanochemical oxidative coupling also allows for further fusion reactions of DHHs, leading to semi- or fully planarized molecules, which have not been previously achieved through solution-phase reactions. We isolated semi-planarized heterohelicenes 5 and 6 and determined their structures using single-crystal X-ray analysis. Compounds 5 and 6 exhibited enhanced electron donor properties compared to DHHs 3 and 4. The enantiomers of 6 exhibited clear CPL emissions with a |g_CPL| value of 2 × 10⁻³. The magnitudes of the transition magnetic dipole moment (TMDM) of 5 and 6 increased compared to those of 3 and 4. Transition moment density analysis revealed that large TMDM densities appeared on the newly formed C–C bonds, providing a unique molecular design guideline for enhancing the magnitude of the TMDM without expanding the molecular structure.