Role of organic linkers in controlling the deformation and fracture behavior of cyano-functionalized zeolitic imidazolate frameworks

Zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks whose structural flexibility facilitate many of their applications, but also renders their mechanical response a key challenge. Cyano substitution has been shown to modify the thermal behavior and glass-forming ability of a zeolitic imidazolate framework, ZIF-4, through electron-withdrawing effects, yet its influence on mechanical responses remains unknown. Using molecular dynamics simulations based on a machine learning force field, we here reveal how cyano groups alter the deformation and fracture behavior of ZIF-4 crystals and glasses. Pristine ZIF-4 crystals are found to accommodate tensile strain primarily through torsional rotation around the Zn–imidazole–Zn units, a hinge-like deformation mode that enables framework flexibility prior to Zn N coordination bond rupture. Cyano substitution suppresses this torsional freedom, increasing hinge rigidity and redirecting deformation toward Zn N bond stretching, altering how tensile strain is accommodated within the framework. The torsion-to-stretching transition is also observed in the corresponding glasses, demonstrating that the effect of –CN is insensitive to structural order. An electrostatic potential analysis further reveals that the electron-withdrawing nature of the cyano group enhances the electron density of the linkers, which restricts linker rotation through enhanced interactions, pointing to the electronic origin of the suppressed hinge flexibility. • We study tensile deformation and fracture in ZIF-4 crystals and glasses. • Results are obtained using molecular dynamics with machine learning force field. • Pristine ZIF-4 strains via Zn–Im–Zn hinge torsion before Zn–N rupture. • Cyano groups stiffen hinges, shifting strain from torsion to Zn–N stretching. • The torsion-to-stretching shift occurs in both crystals and glasses.