Enantiopure Crystallization From Achiral Building Blocks Under Supercooled Conditions

The emergence of homochirality in the absence of external chiral influences remains a fundamental challenge in studies of abiogenesis and asymmetric materials synthesis. Here it is shown that both the conglomerate polymorph of the supramolecular salt (BTBA)FeCl₄ (BTBA = benzyl(tributyl)ammonium) (1, a racemic mixture of 1-P3₁ and 1-P3₂), and its achiral polymorph (2, P2₁/c) exclusively convert into 100% 1-P3₁ or 100% 1-P3₂ upon melt crystallization at a cooling rate of 3600 K h^-1. Differential scanning calorimetry (DSC) reveals that, prior to this transformation, racemic mixtures of 1 convert into 3 (P2₁/c), an achiral polymorph distinct from both 1 and 2, whereas 2 crystallizes directly without intermediates. Nucleation experiments conducted at varying degrees of supercooling show that deeper supercooling effectively impedes molecular reorganization into stable crystal nuclei and, in particular, suppresses secondary nucleation by restricting molecular diffusion, thereby enabling a single chiral nucleus to dominate crystal growth. These results establish melt supercooling as a solvent- and template-free strategy for accessing homochiral materials and reveal how the balance between thermodynamic and kinetic factors governs spontaneous mirror-symmetry breaking (SMSB).