A supramolecular strategy for the control of thermal expansion in molecular crystals

Abstract

Precise control of thermal expansion in molecular crystals remains a a significant challenge in the design of hybrid materials with tailored functionalities. Here we report a supramolecular design that achieves uniaxial zero thermal expansion (ZTE) in a molecular crystal of ((⁺H₃N–C₂H₄)₂O)(benzo[18]crown-6)₂([Ni(dmit)₂]⁻)₂. The wheel–axle-type supramolecular cations self-assemble into one-dimensional chains and further connect into two-dimensional (2D) layers via C-H•••π interactions. These interactions play a significant role in suppressing the dynamic behavior of the crown ether units. At low temperatures (110–190 K), restricted molecular motion results in a remarkably small coefficient of linear thermal expansion (CLTE) of −0.99 × 10⁻⁶ K⁻¹. Upon heating, the progressive weakening of supramolecular interactions triggers enhanced rotational dynamics, culminating in pronounced negative thermal expansion with a CLTE of −75.6 × 10⁻⁶ K⁻¹ in the high-temperature phase (270–320 K). Concurrently, [Ni(dmit)₂]⁻ anions form a 2D antiferromagnetic network, consistent with a 2D Heisenberg antiferromagnetic model. This study highlights that engineering noncovalent interactions offers a strategy for controlling thermal expansion in molecular materials, without relying on rigid covalent frameworks. Our approach provides new insights into designing multifunctional crystalline solids that couple thermal responsiveness with emergent magnetic behavior.