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.