Structural stability and elastic properties of prototypical covalent organic frameworks

► We found that 2D COFs with layered structures prefer shifted AA stacking, which is different from the straight AA stacking that was generally proposed previously. ► The shear moduli of 2D COFs are exceedingly small (<0.5 GPa), comparable to highly defective graphite, suggesting that the layer-layer coupling in 2D COFs is rather weak, and stacking faults may widely exist in the actual materials. ► Representative 3D COFs were found to exhibit relatively low elastic stiffness overall, presumably due to their high porosity and low density. ► COF-108, the least dense crystal known, exhibits rather low bulk and shear moduli, raising the question as to whether or not COF-108 can keep its structural integrity after fully removing the guest molecules from the as-synthesized sample. ► Our findings provide important structural and physical details to be considered in the further development of COF materials.