Planar Positive-Zero-Negative Thermal Expansion Transition in Crystalline Supramolecular Rotors

Zerothermal expansion materials are crucial for manufacturinghigh-precision devices, but most of these materials discovered sofar are inorganic. Organic zero thermal expansion materials are raredue to the need for precise control over the complex supramolecularinteractions between organic groups in order to modulate their thermalexpansion properties. Here, we report a novel organic crystallinesupramolecular rotor, [(3-chloro-1-adamantylammonium)-(18-crown-6)]-ClO4, which exhibits planar positive-zero-negativethermal expansion along the a- and c-axes, with zero thermal expansion behavior in the range of 250-317K. X-ray single-crystal structures, differential scanning calorimetry(DSC) measurements, and molecular dynamics simulations disclosed thatsuch unique transition in thermal expansion results from the static-swing-rotationtransition of 18-crown-6 in the crystal structure.