Ordo ab Chao: Crystallographic Disorder as a Window into Ionic Liquid Structure

Despite being intended to resist crystallization, ionic liquids (ILs) quite frequently form solids, albeit with low melting points. The design motifs that lower melting points can leave measurable signatures in the solid state, manifesting as crystallographic disorder, and often give rise to metastable liquid states. While sometimes treated as experimental complications, such features provide a valuable window into the structural origins of IL phase behavior. Herein, we report a crystallographic study of a series of benzylated ILs that crystallize either directly or from long-lived supercooled melts. Single-crystal X-ray diffraction, supported by computational modeling and statistical analyses, provides structural evidence that conformational heterogeneity and interaction degeneracy can impede the propagation of long-range order. Competition among multiple energetically similar CH₃···π and related π-mediated interactions generates complex crystallographic landscapes in which multiple local arrangements coexist, leading to the formation of metastable liquid states prior to crystallization. Notably, across the series of structures examined, cations that successfully crystallize converge on a recurring effective molecular volume, achieved through static packing, conformational flexibility, or crystallographic disorder. Viewed in this context, disorder is not incidental but mechanistically informative, encoding how ILs accommodate volume, distribute interactions, and navigate the boundary between liquid persistence and crystalline order.