Sequential Phase Transitions with Switchable Dielectric Constant in a Metal‐Free Ionic Crystal

An intriguing area for developing smart devices is metal‐free materials. These materials have simple preparation, low processing temperature and environmentally‐friendly processing of the crystal. Stimuli‐responsive materials, which have been developed switchable property of dielectric constant in most inorganic‐organic hybrid compounds, are rarely found in metal‐free materials. Herein, a metal‐free ionic crystal compound, [3R‐(+)‐3‐AP][ClO4]2 (1, 3R‐(+)‐3‐AP = 3R‐(+)‐3‐aminopyrrolidine cation), was synthesized and characterized. The structure is featured by zero‐dimensional supramolecular block structure built from the [3R‐(+)‐3‐AP][ClO4]2 units. Compound 1 undergoes sequential reversible phase transitions at 350/373 K and 430/441 K, respectively. Variable‐temperature single crystal structural analysis and dielectric constant measurements reveal changes of the crystal lattice dynamics, i.e., the twisting motion of the perchlorate anions and the dynamic changes of the [3R‐(+)‐3‐AP] cations, are the origin of the phase transitions. This finding reveals that the amphidynamic metal‐free crystal can evolve rich electrical properties once the dynamic components (especially the polar ones) undergo order‐disorder transitions, such as temperature‐responsive dielectric switching. Static‐to‐dynamic transition in metal‐free ionic crystal [3R‐(+)‐3‐AP][ClO4]2 (3R‐(+)‐3‐AP = 3R‐(+)‐3‐aminopyrrolidine cation) exhibits sequential switch‐type characteristics during the dielectric transitions.