Chiral symmetry breaking in NaClO3 crystallization from an aqueous solution with perturbations has been of great interest. To understand the mechanism, several models focusing on the early stage of the crystallization have been proposed. However, they are ambiguous because the early stage has been barely explored directly. Here, we investigate the early stages of the crystallization process driven by droplet evaporation using a combination of direct in situ microscopic observations and cryogenic single-crystal XRD experiments. We demonstrate that an achiral crystal having P21/a symmetry, which is newly discovered for a solution growth, first appears in the droplet and then transforms into the chiral crystals. Additionally, determination of the lattice constants by XRD experiments (a = 8.42 Å, b = 5.26 Å, c = 6.70 Å, β = 109.71°) revealed that the achiral phase should be identical to Phase III (a = 8.78 Å, b = 5.17 Å, c = 6.83 Å, β = 110°), which is a high-temperature phase from a melt growth of NaClO3. We advocate further assessment of the achiral crystal and a new pathway for the formation of chiral crystals via crystalline phase transition from achiral Phase III.

We demonstrate that a statistically-significant chiral bias in NaClO 3 chiral crystallization can be provoked by inducing nucleation via the optical trapping of Ag nano-aggregates using a continuous wave visible circularly polarized laser (λ = 532 nm). The laser was focused at the interface between air and an unsaturated NaClO 3 aqueous solution containing Ag nanoparticles. The "dominant" enantiomorph was switchable by changing the handedness of the incident circularly polarized laser, indicating that the chiral bias is enantioselective. Moreover, it has been found that the resulting crystal enantiomeric excess (CEE) reached approximately 25%. The CEE is much higher than the typical enantiomeric excess (EE) in the asymmetric photosynthesis of organic compounds ranging from 0.5 to 2%. The efficient induction of the nucleation and the large chiral bias imply the contribution of localized surface plasmon resonance of the Ag nanoaggregates to chiral nucleation. Our method has potential to offer the benefit for studies on the spatiotemporal nucleation control, optical resolution of chiral compounds and biohomochirality.

Chiral symmetry breaking during the chiral crystallization from a sodium chlorate (NaClO3) aqueous solution is an intriguing phenomenon because it provides insights into the prebiotic process of biohomochirality. However, a mechanism of the emergence and amplification of chirality remains controversial, especially for crystallization from highly supersaturated solution, and one of the hypotheses proposed before is a transition toward the homochiral state during the early stages of crystallization. In this contribution, we directly examined the early stage of crystallization by in situ polarized-light microscopy. The observation revealed that achiral crystals, which appear prior to the formation of chiral crystals, transform to the chiral crystal through two kinds of polymorphic transformations: (1) martensitic transformation (MT) and (2) solution-mediated phase transition (SMPT). The SMPT is remarkably facilitated by contact with a chiral crystal. Notably, the resulting enantiomorph through contact-facilitated SMPT is strongly directed by the contacting enantiomorph. In contrast, the MT yields two enantiomorphs in equal probability. The emergence and amplification of chirality has generally been considered to be a result of direct nucleation of a chiral crystal and its fragmentation. In contrast, our observations provide a possibility that the MT and contact-facilitated SMPT play a role for the emergence and amplification of chirality, respectively.