Spontaneous chiral symmetry breaking in a single crystal

Abstract

Chiral symmetry breaking (CSB) under nonequilibrium open conditions is a ubiquitous phenomenon in the universe, whereas molecular-level CSB is limited. Only the preferential enrichment and Viedma ripening that occur during the crystallization process from a solution are known. Herein, we discovered the third category of CSB, which is complete CSB within a single crystal. A racemic crystal of 3-(4-(benzo[d]thiazol-2-yl)phenyl)-10-propyl-10H-phenothiazine, a phenothiazine derivative with dynamic chirality, undergoes a single-crystal-to-single-crystal structural transition to a chiral crystal. Furthermore, the chirality after the transition is able to be controlled by solid-seeding of a chiral crystal. The resulting chiral single crystals exhibited circularly polarized luminescence (CPL) properties (g _lum = 8.9 × 10^-4). This discovery provides a simple model of CSB and stimuli-responsive materials involving the CSB phenomenon.

Fig. 1. CSB phenomena at the molecular level. (a) Preferential enrichment, (b) Viedma ripening, and (c) structural transition from an achiral crystal to a chiral crystal.

Fig. 2. (a) Selective preparation conditions for forms I and II from solution. (b) PXRD patterns for form I grown from EtOAc and form II grown from CH₂Cl₂/heptane (black lines), and simulated patterns for forms I and II (red lines). Crystal structures of (c–e) form I and (f–h) form II. Anisotropic displacement ellipsoid plots of (c) form I and (f) form II are drawn with 50% probability. The benzothiazole moiety was disordered into syn and anti forms, referring to the relative positions of the sulphur atoms on the phenothiazine and benzothiazole. Downward views of the packing structures along the 2₁ screw axes for (d) form I and (g) form II. The space filling models in red and blue indicate the R and S isomers, respectively. The side views from the 2₁ screw axes for (e) form I and (h) form II.

Fig. 3. (a) DSC curves for forms I and II. The thermograms for form I and the second heating of form II are plotted with a vertical offset of 4 mW for clarity. The heating rate was 5 K min⁻¹. (b) VT-PXRD patterns for form II. The observed patterns for forms I and II are indicated. The orange and blue shaded regions indicate the characteristic peaks for forms I and II, respectively. (c) POM images during the SC–SC structural transformation from form II to I at 180 °C. The scale bar represents 100 μm. (d) POM images during chirality transfer from form I to II by solid-seeding at 150 °C. (e–g) POM images of typical progression behaviour when the transition started from the contact point. Areas with yellow frames indicate a seed crystal. The arrows indicate the starting point of the transition. Areas with green frames indicate where the transition occurred. (h) POM image during the transition initiated from the edge of the crystal. (i) POM image during the transition initiated from multiple points.

Fig. 4. (a) Emission and excitation spectra, displayed as dotted lines and shaded areas, respectively. Yellow: CHCl₃ solution (excited at 330 nm for the emission spectrum, observed at 550 nm for the excitation spectrum); blue: form I (excited at 385 nm, observed at 500 nm); green: form II (excited at 385 nm, observed at 500 nm). (b) CD and (c) CPL (excited at 350 nm) spectra for form I (red: R-isomer and blue: S-isomer). (d) CD and (e) CPL spectra (excited at 350 nm) for the crystals after the structural transition (red: R-isomer and blue: S-isomer).

Fig. 5. Schematic illustration of the structural transition. During the transition, racemization by the flipping of phenothiazine, rotation of benzothiazole, and drastic changes in the molecular arrangement occurred.