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 (1), pyrrolidinium (2), piperidinium (3)), 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 1 shows a one-step phase transition near 230 K, whereas 2 and 3 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.
Fu et al. (Wed,) studied this question.