Quasiracemic Crystallization of Luminescent Chiral Molecules: Mechanochromic Luminescence and Sign Inversion of Circularly Polarized Luminescence

The arrangement of molecules plays a crucial role in governing the luminescent properties of organic crystals. While quasiracemic crystallization, based on the co-crystallization of two structurally similar but nonidentical enantiomeric molecules, offers a promising strategy for organizing distinct species within a single crystal, its potential for tuning luminescent behavior remains largely unexplored. Herein, we have prepared quasiracemic crystals from a pair of enantiomeric binaphthyl derivatives functionalized with either ethynylnaphthalene or ethynylbenzothiadiazole groups. The molecular arrangement of the quasiracemic crystal is analogous to that of the racemic ethynylnaphthalene derivative, whereas the quasiracemic crystals exhibit mechanochromic luminescence (MCL) behavior similar to that of the racemic ethynylbenzothiadiazole derivative. Notably, the quasiracemic crystals exhibit circularly polarized luminescence (CPL), and the CPL sign is inverted relative to that of the enantiopure crystal of the ethynylbenzothiadiazole derivative. These results highlight quasiracemic crystallization as a new strategy for modulating MCL and CPL properties.