3D Imaging Reveals Widespread Stacking Disorder in Single Crystal 2D Covalent Organic Frameworks

Although tailored porosity is a defining feature of layered, two-dimensional (2D) polymers known as 2D covalent organic frameworks (COFs), understanding the interplanar stacking of 2D COFs and their resulting three-dimensional (3D) pore structure remains challenging. Here, we use scanning transmission electron microscopy and ptychography, an emerging 3D angstrom-scale imaging method, to study single-crystalline particles of the imine-linked 2D COF TAPB-DMPDA. Previously assumed to adopt an average-eclipsed structure with only angstrom-level stacking disorder, we find the crystals contain widespread stacking disorder of larger magnitudes, including interplanar shifts up to a half unit cell and nanoscale inhomogeneities in stacking and tilt. 3D visualizations show pore channels are distorted by this stacking disorder. The extensive stacking disorder found in even high-quality 2D COFs has profound implications for envisioned applications and should motivate the development of design strategies to control their 3D structures.