Spontaneous chiral symmetry breaking in a single crystal

Chiral symmetry breaking (CSB) under the 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 chirality, undergoes a single-crystal-to-single-crystal structural transition to a chiral crystal. Furthermore, the chirality after the transition could 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 provide a simple model of CSB and stimuli-responsive materials involving the CSB phenomenon.