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

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.