Phase Transition Triggered by Ordering of Unique Pendulum-Like Motions in a Supramolecular Complex: Potassium Hydrogen Bis(dichloroacetate)-18-Crown‑6

A novel supramolecular phase change material (PCM) potassium hydrogen bis­(dichloroacetate)-18-crown-6 (1), which undergoes a reversible second-order phase transition at 181.8 K (Tc), has been successfully synthesized and grown as bulk crystals. DSC measurements confirm its reversible phase transition with a thermal hysteresis of 1.6 K. Dielectric permittivities also display obvious anomalies approaching Tc, being characteristic for the reversible phase transition. Variable-temperature X-ray single-crystal diffractions demonstrate that 1 behaves as a molecular rotor above room temperature in which the chlorine atoms rotate around the C–C axis of the CHCl2COO–/CHCl2COOH exhibiting distinct pendulum-like motions, and the (18-crown-6)·K+ part acts as a stator. Further studies reveal that the origin of its phase transition was ascribed to the moving close together of the whole set molecules from the original position, which is induced by the order–disorder transformation of the pendulum-like motions of the CHCl2COO–/CHCl2COOH units between the LT phase and RT phase. We believe that all the results would urge the exploration of new phase change functional materials.