A 2025 Chemical Science study (Oketani et al.) reported the first complete chiral symmetry breaking (CSB) confined to a single crystal: a metastable racemic crystal of the dynamically chiral phenothiazine derivative 3-(4-(benzo[d]thiazol-2-yl)phenyl)-10-propyl-10H-phenothiazine undergoes a thermally induced single-crystal-to-single-crystal (SC-SC) transition into a homochiral conglomerate crystal, with the product chirality controllable by solid seeding. The discovery left a precisely defined open problem: there is no theoretical framework or predictive criterion identifying which single-component molecular crystals can undergo such transitions; the microscopic mechanism is unknown — why nucleation initiates at seed contact points in some crystals but at remote sites or multiple sites in others, why some crystals remain superheated, and how a local stereochemical inversion propagates across a whole crystal in seconds without loss of single crystallinity; static lattice-energy calculations give nearly identical energies for the two forms, so the transition pathway and thermodynamic driving forces remain to be established beyond such static calculations; and the phenomenon is demonstrated for only one scaffold. The problem is to convert this single observation into validated predictive and mechanistic knowledge: a criterion for CSB-capable crystals, a quantitative account of nucleation and propagation, and resolution of the transition pathway and thermodynamic driving forces.

Background

Chiral symmetry breaking (CSB) is the process by which a metastable, mirror-symmetric (racemic) system spontaneously converges to a single handedness. At the molecular level only two CSB phenomena were known before 2025, and both operate in nonequilibrium open solid–liquid systems. The first is preferential enrichment (Ushio, Tamura et al., Angew. Chem. 1996; Tamura et al., JACS 2002), in which polymorphic transitions during crystallization of racemic compounds from supersaturated solutions leave the deposited crystals nearly racemic while enriching the mother liquor in one enantiomer. The second is Viedma ripening (Kondepudi et al., Science 1990; Viedma, Phys. Rev. Lett. 2005), in which grinding a suspension of conglomerate crystals of a racemizing compound amplifies one enantiomer in the solid phase (reviewed by Sögütoglu et al., Chem. Soc. Rev. 2015). A conglomerate is a physical mixture of separate single-enantiomer crystals, as opposed to a racemic compound whose crystal contains both enantiomers related by a center or mirror of symmetry; chiral crystals crystallize in the Sohncke space groups, the 65 space groups containing only proper symmetry operations. Quantitative theory exists for these solution-based processes — e.g. thermodynamic-kinetic feedback models (Viedma, Astrobiology 2007) and exact population-balance conditions for deracemization (Deck, Hosseinalipour, Mazzotti, JACS 2024) — but all of it relies on growth–dissolution cycles mediated by solvent and solution-phase racemization. Oketani, Okada, Takaji, Shigemitsu, Kida, Nakashima, and Hisaki (Chem. Sci. 2025, 16, 17621; ChemRxiv preprint 2025) discovered a third category: complete CSB inside one crystal, with no solvent involved. The compound, 3-(4-(benzo[d]thiazol-2-yl)phenyl)-10-propyl-10H-phenothiazine, is dynamically chiral — it has point chirality at the phenothiazine nitrogen and racemizes rapidly in solution at room temperature by ring inversion of the phenothiazine core, so the molecule is achiral on the solution timescale. Its metastable racemic form II (space group Pna2₁) transforms on heating near 150–160 °C, in a single-crystal-to-single-crystal (SC-SC) manner that preserves single crystallinity, into the stable chiral conglomerate form I (space group P2₁); single-crystal X-ray diffraction shows that one of the two enantiomers in the racemic crystal inverts so that the entire crystal converges to a single chirality. Heating a form II crystal in contact with a form I seed of predetermined chirality controls the product handedness with over 95% probability (10 trials per enantiomer), while transitions nucleating away from the contact point give random chirality; some crystals superheat well above the transition temperature, and transitions occasionally start at remote or multiple points. The proposed qualitative mechanism — transient spaces generated by thermal vibration of the molecules and the lattice enabling phenothiazine inversion (calculated barrier 27.7 kJ·mol⁻¹ on a model compound) and an approximately 90° collective molecular rotation — is unsupported by dynamics: the computed lattice energies of the two forms are nearly identical, which the authors themselves note makes the thermodynamic stability undecidable by such static calculations, and the chiral product crystals show circularly polarized luminescence (CPL, an emission dissymmetry measured by the luminescence dissymmetry factor g_lum = 8.9×10⁻⁴), motivating materials applications. A significant precedent exists: Nath, Nilapwar, and Nangia (Cryst. Growth Des. 2012) found that racemic, centrosymmetric polymorphs (P2₁/n, Pbca) of 2,6-di-t-butylditolylfuchsone convert on heating into a homochiral P2₁ polymorph by crystal-to-crystal transformations whose microscopic mechanism has never been established — although in that system all examined chiral batches showed the same absolute handedness rather than stochastic selection, and the conformationally chiral fuchsone (rotation barrier ≈ 5 kcal·mol⁻¹) is configurationally frozen at room temperature. Other adjacent systems — crystal-to-crystal transitions between chiral and non-chiral space groups with mechano-responsive luminescence (Jin, Seki, Ito, JACS 2017), and racemic-to-homochiral conversion of Ag40 silver nanoclusters by seeded solution recrystallization with rapid configurational flipping in solution (Ma et al., JPCL 2023) — likewise involve either achiral molecules or solution-mediated steps, so neither supplies a mechanism or a predictive criterion for the single-component solid-state case.

Problem Statement

The objective is to convert the single 2025 observation of spontaneous single-crystal-to-single-crystal chiral symmetry breaking into predictive, mechanistic knowledge. A complete answer delivers, as jointly supporting parts of one objective: (1) a theoretical framework or predictive criterion — which may draw on candidate ingredient classes including intramolecular inversion and rotation barriers, lattice dynamics, and collective packing degrees of freedom — that identifies which single-component molecular crystals (a metastable racemic compound with an accessible chiral conglomerate polymorph) can undergo complete spontaneous SC-SC CSB, with its domain of applicability stated and tested against the phenothiazine derivative of Oketani et al. as the confirmed positive instance, against related racemic-to-chiral crystal-to-crystal systems such as the fuchsone of Nath et al., in which the examined chiral batches all showed the same absolute handedness rather than stochastic symmetry breaking, so as to discriminate whether the criterion correctly separates these behaviours, and against at least two additional prospective candidates, stating the predicted outcome for each; (2) a quantitative microscopic account of the transition, namely what determines the nucleation site statistics (why transitions initiate at seed contact points in some crystals, at remote or multiple sites in others, and why some crystals remain superheated above the transition temperature), whether, and if so how, transient lattice vacancies and spaces play a role, and the kinetics and thermomechanical coupling by which a local stereochemical inversion propagates across an entire crystal within seconds to tens of seconds without loss of single crystallinity; and (3) resolution of the transition pathway and the thermodynamic ordering that static lattice-energy calculations — nearly degenerate for the two forms — cannot decide. All three parts are mandatory and jointly constitute a complete answer; resolving fewer than all three does not resolve the task. For part (2), a quantitative account requires explicit, checkable estimates or predictions for each named observable rather than a qualitative restatement of the proposed mechanism; for part (3), the thermodynamic ordering and the transition pathway must be derived rather than asserted, since the nearly degenerate static lattice energies leave both undecided. Delivering the criterion of part (1) alone, or merely restating the ordering already stated in Background, does not resolve the task.

References

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