Engineering Thermoresponsive Dielectric Properties and Phase Transitions via Confinement of Polar Cations in Photoisomerizable Nitroprusside Frameworks

Crystalline materials capable of responding to both heat and light stimuli offer unique opportunities for tunable dielectric behavior but have remained scarcely explored. Using a hydrogen-bond-guided supramolecular strategy, we synthesized a family of nitroprusside-based hybrids, (A)(NH₄)[Fe(CN)₅(NO)] (A = azetidinium (<b>1</b>), pyrrolidinium (<b>2</b>), piperidinium (<b>3</b>)), that exhibit reversible thermo-induced dielectric switching and intrinsic photoisomerization capability. Thermal, structural, and dielectric analyses reveal that all compounds exhibit a reversible thermally induced phase transition with dielectric switching driven by order-disorder transitions of polar cyclic ammonium cations. Compound <b>1</b> shows a one-step phase transition near 230 K, whereas <b>2</b> and <b>3</b> display multistep transitions with distinct pathways, with transition temperatures increasing systematically with cation size to 391 K. Variable-temperature single-crystal X-ray diffraction combined with molecular dynamics simulations elucidates that phase transitions and dielectric anomalies arise from sequential unlocking of rotational degrees of freedom within a hydrogen-bonded framework. Notably, the [Fe(CN)₅(NO)]^2- units preserve reversible photoinduced nitrosyl Fe-NO ↔ Fe-ON linkage isomerization, confirmed by IR spectra, suggesting the potential for light-driven dielectric modulation. This work establishes a clear structure-dynamics-property relationship linking organic cation dynamics in confined space to macroscopic dielectric behavior and highlights the promise of nitroprusside-based frameworks as dual thermo- and photoresponsive dielectric materials.