Colossal Anisotropic Thermal Expansion in a Diazo‐Functionalized Compound with Switchable Solid‐State Behavior

Abstract

Achieving substantial anisotropic thermal expansion (TE) in solid‐state materials is challenging as most materials undergo volumetric expansion upon heating. Here, we describe colossal, anisotropic TE in crystals of an organic compound functionalized with two azo groups. Interestingly, the material exhibits distinct and switchable TE behaviors within different temperature regions. At high temperature, two‐dimensional, area zero TE and colossal, positive linear TE (α=211 MK−1) are attained due to dynamic motion, while at low temperature, moderate positive TE occurs in all directions. Investigation of the solid‐state motion showed the change in enthalpy and entropy are quite different in the two temperature regions and solid‐state NMR experiments support motion in the solid. Cycling experiments demonstrate that the solid‐state motions and TE behaviors are completely reversible. These results reveal strategies for designing significant anisotropic and switchable behaviors in solid‐state materials.

Dynamic Order–Disorder Reversible Phase Transition and Anisotropic Thermal Expansions in a Single-Component Organic Molecular Crystal

“…Although a recent study based on a database search estimates that ∼34% of organic crystals may exhibit NTE, experimental reports are rare in the literature . A very few cases of single-component organic materials with colossal PTE and NTE effects, i.e., coefficient of thermal expansion, CTE > ± 100 × 10 –6 K –1 , are reported in the literature. ,, Various inorganic systems such as oxides ,,,, and metal cyanide, , coordination polymers, , and metal–organic frameworks ,,, are known to exhibit NTE and ZTE characteristics. ZTE behavior in organic molecular materials is seldom due to the difficulty in controlling the ubiquitous complex supramolecular interactions .…”

“…22 A very few cases of single-component organic materials with colossal PTE and NTE effects, i.e., coefficient of thermal expansion, CTE > ± 100 × 10 −6 K −1 , are reported in the literature. 2,12,23 Various inorganic systems such as oxides 6,8,9,24,25 and metal cyanide, 3,26 coordination polymers, 19,27 and metal−organic frameworks 4,9,28,29 istics. ZTE behavior in organic molecular materials is seldom due to the difficulty in controlling the ubiquitous complex supramolecular interactions.…”