Fluorine-for-Hydrogen Substitution: A Chemical “Butterfly Effect” on Phase Transition, Dielectric, and Magnetic Behaviors

Abstract Smart molecular materials exhibiting solid–solid phase transitions are increasingly attractive for advanced technological applications due to their switchable physical properties. In this work, we designed two salts, [EMPyr][Ni(mnt)2] (1-H, [EMPyr]+ = N-ethyl-N-methylpyrrolidinium and mnt2– = maleonitriledithiolate) and [FEMPyr][Ni(mnt)2] (1-F, [FEMPyr]+ = N-(2-fluoroethyl)-N-methylpyrrolidinium), by combining a low-rotational-barrier pyrrolidinium-derived cation with the planar radical anion [Ni(mnt)2]−. These two compounds are structurally identical except for the substitution of a single hydrogen atom by fluorine on the cation. Both compounds undergo two-step solid–solid phase transitions, yielding three mutually isomorphic phases. Remarkably, despite their near-identical crystal packing, fluorination not only enhances thermal stability and raises the phase-transition temperatures but also induces markedly different dielectric and magnetic responses. This pronounced divergence in macroscopic properties arising from a single atomic substitution exemplifies a chemical “butterfly effect” in molecular materials. Our results demonstrate that even minimal chemical modifications can serve as a powerful and precise strategy for tuning structure–property relationships in molecular phase-transition systems.