Presence of Short Intermolecular Contacts Screens for Kinetic Stability in Packing Polymorphs

Polymorphism is pervasive
in molecular solids. While computational
predictions of the molecular polymorphic landscape have improved significantly,
identifying which polymorphs are preferentially accessed and experimentally
stable remains a challenge. We report a framework that correlates
short intermolecular contacts with polymorphic stability. The presence
of short contacts between neighboring molecules prevents structural
rearrangement and stabilizes the packing arrangement, even when the
stabilized polymorph is not enthalpically favored. In the absence
of such intermolecular short contacts, the molecules have added degrees
of freedom for structural rearrangement, and solid–solid polymorphic
transformations occur readily. Starting with a series of core-halogenated
naphthalene tetracarboxylic diimides, we establish this framework
with the packing polymorphs of more than 20 compounds, ranging from
molecular semiconductors to pharmaceutics and biological building
blocks. This framework, widely applicable across molecular solids,
can help refine computational predictions by identifying the polymorphs
that are kinetically stable.