Toward On-Demand Polymorphic Transitions of Organic Crystals via Side Chain and Lattice Dynamics Engineering

Controlling polymorphism,
namely, the occurrence of multiple crystal
forms for a given compound, is still an open technological challenge
that needs to be addressed for the reliable manufacturing of crystalline
functional materials. Here, we devised a series of 13 organic crystals
engineered to embody molecular fragments undergoing specific nanoscale
motion anticipated to drive cooperative order–disorder phase
transitions. By combining polarized optical microscopy coupled with
a heating/cooling stage, differential scanning calorimetry, X-ray
diffraction, low-frequency Raman spectroscopy, and calculations (density
functional theory and molecular dynamics), we proved the occurrence
of cooperative transitions in all the crystalline systems, and we
demonstrated how both the molecular structure and lattice dynamics
play crucial roles in these peculiar solid-to-solid transformations.
These results introduce an efficient strategy to design polymorphic
molecular crystalline materials endowed with specific molecular-scale
lattice and macroscopic dynamics.