Halogen bond-modulated solid-state reordering and symmetry breaking of azahelicenes

Spontaneous symmetry breaking plays an essential role in revealing the origin of homochirality in nature. However, previous studies have prevalently focused on the emergence of symmetry breaking upon the aggregation of discrete molecules in solution. Herein, we report a unique solid-state symmetry breaking process of dynamically chiral azahelicenes that emerged in vacuum-driven transformation of halogen bond-woven crystals. Aza[6]helicene with fixed chirality is first employed to co-crystallized with various halides to explore the strength of intermolecular halogen bonds. Due to the weak feature of the halogen bonding, the halides in the cocrystals can be completely removed under vacuum at an elevated temperature, allowing the residual aza[6]helicene molecules to reorder into single crystals. Particularly, the micron sheet of the cocrystal formed with 1,3,5-trifluoro-2,4,6-triiodobenzene directly transforms into tightly packed slim blocks. Further, aza[4]helicene which possesses chiral conformations but rapidly enantiomerizes in solution is adopted to prepare halogen bond-woven cocrystals. While all the cocrystals are comprised by equal amount of P and M isomers, the solid formed after the removal of pentafluoroiodobenzene is found to be chiroptically active. The aza[4]helicene molecules released from the halogen bond network solely adopt one chiral conformation upon reordering and symmetry breaking instantly occurs in a solid state. The Cotton effects gradually increase with the extension of vacuum–heating treatment, indicating a unidirectional transformation of the chiral conformations and an amplification of symmetry breaking during the solid-state reorganization. Precise manipulation for the absolute configuration of the solid-state symmetry breaking is further accomplished by using aza[6]helicene as a chiral inducer. This work provides new insights into the origin of homochiral solid biostructures and expands the pathway to versatile chiral organic materials from achiral/racemic precursors.