Entropy driven disorder–order transition of a metal–organic framework with frustrated flexibility

Abstract

Flexible metal–organic frameworks (MOFs), showing a reversible phase change behavior in response to guest adsorption or temperature, provide unique opportunities for molecular separation or energy storage applications. Herein, we investigate the complex guest- and temperature-responsive behavior of a functionalized MOF-5 derivative. The material is characterized by a geometrically rigid network structure that is decorated with dispersion energy donating hexyloxy substituents. Distinguished by the phenomenon of frustrated flexibility, the functionalized MOF-5 derivative switches between a highly crystalline, cubic structure and a semi-crystalline, aperiodically distorted structure depending on guest adsorption and temperature. Via a combination of several variable temperature global and local structure techniques (x-ray diffraction, x-ray total scattering, and Fourier-transform IR spectroscopy), detailed insights into the complementary disorder–order transitions of the framework backbone and the dangling hexyloxy substituents are provided. Our results set the stage for the discovery of new responsive MOFs exhibiting a more complex phase change behavior interfacing periodic and aperiodic structural changes.