The Origin of Delayed Polymorphism in Molecular Crystals Under Mechanochemical Conditions

We show that mechanochemically driven polymorphic transformations can require extremely long induction periods, which can be tuned from hours to days by changing ball milling energy. The robust design and interpretation of ball milling experiments must account for this unexpected kinetics that arises from energetic phenomena unique to the solid state. Detailed thermal analysis, combined with DFT simulations, indicates that these marked induction periods are associated with processes of mechanical activation. Correspondingly, we show that the pre-activation of reagents can also lead to marked changes in the length of induction periods. Our findings demonstrate a new dimension for exerting control over polymorphic transformations in organic crystals. We expect mechanical activation to have a much broader implication across organic solid-state mechanochemistry. The extended induction period associated with mechanochemical polymorphism arises from the phenomenon of mechanical activation, which preceeds the transformation.image

Automated summary

We demonstrate that mechanochemically driven polymorphic transformations can have significantly long induction periods, which can be adjusted by changing the energy of ball milling. The design and interpretation of ball milling experiments need to consider the unexpected kinetics arising from unique energetic phenomena in the solid state. Through detailed thermal analysis and DFT simulations, we find that these long induction periods are associated with mechanical activation processes. Additionally, we show that pre-activation of reagents can also affect the length of induction periods. This discovery opens up a new dimension for controlling polymorphic transformations in organic crystals, and we expect mechanical activation to have broader implications in organic solid-state mechanochemistry.