Interaction Hierarchy and Polymorphic Structure-Property Dynamics in Luminescent Molecular Crystals.

Solid-state phase transitions provide a powerful platform for translating subtle molecular-level interactions into macroscopic functional responses; however, rational design strategies that enable predictable control over such transitions remain limited. Herein, we report polymorphic structure-property switching behaviors associated with competing intermolecular interactions in luminescent molecular crystals. Cyano-β-substituted distyrylbenzene derivatives bearing bromo and methoxy side chains were designed to incorporate competing homotypic and heterotypic noncovalent interactions with distinct interaction characteristics. Single-crystal analyses reveal that subtle differences in the hierarchical ordering of dispersion-, dipole-dipole-, and electrostatically dominated interactions give rise to polymorphic crystal structures with distinct molecular orientations and photoluminescence properties. Thermal and mechanical stimuli induce distinct phase transitions: an irreversible thermally induced single-crystal-to-single-crystal transition and a pseudo-reversible mechanochemical pathway via an amorphous intermediate, both directly visualized as pronounced emission color changes. Kinetic and thermodynamic controls over polymorph formation are elucidated through a combination of structural analysis, photophysical measurements, and crystal framework calculations. This study suggests that multifunctional molecular side chains enable access to diverse interaction landscapes, allowing multiple structure-property switching pathways to be encoded within a single molecular framework. The presented framework provides a qualitative perspective for interpreting dynamic structural behaviors in molecular crystals.