Structural Insights into the Mechanical Behavior of Large-Area 2D Covalent Organic Framework Nanofilms.

Two-dimensional covalent organic frameworks (2D COFs) are periodic, permanently porous, lightweight solids with remarkable structural modularity, enabling precise control over their properties. As thin films, they have shown promising applications in chemical separations and organic electronics, making it crucial to understand their stability under mechanical stress. Here, we investigate how two different chemical linkages commonly used for 2D COFs, specifically imine and enamine, influence the mechanical properties of nanoscale thick films. Centimeter-scale 2D COF films with a thickness below 100 nm were synthesized by a condensation reaction at a liquid-liquid interface and subsequently transferred onto patterned substrates for mechanical testing. By employing a custom-made nanotensile testing platform, we achieved a comprehensive mechanical characterization of freestanding 2D COF films over a large area (0.5 mm), a size relevant for device applications. The enamine-linked COF exhibits a higher Young's modulus and tensile strength but a lower fracture strain compared to the imine-linked COF, a difference attributed to the tightly stacked structure of the enamine-linked COF, as confirmed by molecular dynamics simulations. This distinct mechanical behavior reveals a fundamental relationship between the linkage chemistry of 2D COF and their mechanical properties, providing valuable insights that can drive the development of strong and durable thin-film devices based on 2D COFs.