Auxetic Behavior in JUC‐530: The Effect of Interstitial Molecules on Framework Deformation

In this work a comprehensive computational investigation of thedeformation mechanism governing auxetic behavior in the three‐dimensional covalent organic framework (COF) JUC‐530 is presented. 3D COFs are crystalline porous polymers constructed from organic building blocks linked by covalent bonds, resulting in 3D frameworks with permanent porosity and tunable topology. Through molecular modeling studies using consistent‐valence force field and polymer consistent force field, the empty framework is shown to exhibit a negative on‐axis Poisson's ratio of approximately −0.30 in the (001) plane, arising from a cooperative tetrahedral‐flattening mechanism involving nested small and large tetrahedral motifs linked through rigid linkers. Loading the framework with nitrogen, methane, or benzene suppresses this auxetic response, shifting the Poisson's ratio toward less negative values. This suppression originates from guest‐induced asymmetry in the deformation of the tetrahedral motifs, together with possible steric and/or π–π or CH–π interactions. The interplay between topology, linker rigidity, and guest–framework interactions shows that the auxetic behavior of JUC‐530 is a tunable property sensitive to the chemical environment of the pores.