The Tuning of Structure Types and Switchable Dielectric Behavior in Pyridine Bromocuprate (II) Complexes

The origins of dielectric phase transitions stem from both flexible organic building blocks and moderate intermolecular interactions enabling molecular reorientations. Herein the zero-dimensional [C₅NH₆]₂[CuBr₄] exhibits a structural phase transition coupled with dielectric switching triggered by the order-disorder transformations of pyridinium cations. But for the coordination compound [CuBr₂(C₅NH₅)₂] and supramolecular host-guest compound [(C₅NH₆)₂(18-crown-6)₂][CuBr₄], structural transformations are suppressed by stronger intermolecular interactions.

Stimuli-responsive phase transition materials exhibiting reversible switching between distinct dielectric states have emerged as promising candidates for applications in memory storage, logic circuits, and smart sensors. However, a fundamental challenge remaining is the lack of in-depth understanding of the structure-property relationships that govern phase transitions. Herein, a comparative study of three pyridine-based copper (II) bromide compounds is given, [C₅NH₆]₂[CuBr₄] (1), [CuBr₂(C₅NH₅)₂] (2), and [(C₅NH₆)₂(18-crown-6)₂][CuBr₄] (3), falling into three different structural categories: a zero-dimensional organic–inorganic hybrid, a coordination complex, and a supramolecular host-guest compound, respectively. Among them, only compound 1 has a distinct order–disorder phase transition accompanied by switchable dielectric property. Structural analyses and Hirshfeld surface studies reveal that the loosely packed hybrid framework of compound 1 enables moderate intermolecular interactions and sufficient motional freedom of the pyridinium cations, thereby dominating the phase transition. In contrast, for compounds 2 and 3, such molecular motions are significantly suppressed by the stronger coordination bonding or hydrogen bonds. These findings emphasize the decisive role of competition between molecular motions and suitable intermolecular interactions in modulating structural phase transitions, offering valuable insights into the crystal engineering of functional materials with dielectric switching behaviors.

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