Encoding Polymorphism by Incorporating Lattice Dynamics to Design Molecular Materials

Polymorphism in molecular crystals arises from the interplay between conformational strain, intramolecular forces, and collective lattice dynamics, yet its control remains largely empirical. Here, we show that deliberate molecular design that takes into account the dynamics of the constituent molecular building blocks programs polymorphic tendencies in molecular solids. A modular series of compounds that combine rigid aromatic cores with mobile imine linkers and rotatable tert-butyl moieties yields multiple thermally-accessible polymorphs with distinct low-energy vibrational signatures. Variable-temperature X-ray diffraction and low-frequency Raman spectroscopy reveal that each molecular design establishes characteristic low-frequency dynamics that correlate directly with macroscopic phase behavior. As more static motifs are introduced, the accessible low-energy coordinates contract and phase transitions become reversible, demonstrating that intramolecular design shapes the collective motions that accompany structural reorganization. Interestingly, the direct measurement of the low-frequency dynamics highlights that molecular motions are thermally-activated before the thermodynamic phase transitions, revealing internal motions that precede cooperative structural transformations. Together, these results provide a direct and designable connection between lattice dynamics and polymorphism, and that by tuning the motions of the molecular building blocks polymorphism can be effectively programmed. The principles established here are general, and can be extended to other chemical platforms to explore how vibrational energy landscapes are related to bulk dynamic phenomena.