Intrinsic mechanical properties of two-dimensional covalent organic frameworks.

Understanding the mechanical properties of two-dimensional covalent organic frameworks (2D COFs) is critical to their design for flexible devices, energy storage and catalysis applications. To date, only a limited number of 2D COFs have been examined, leaving the material design principles largely undefined. Furthermore, the measured results are often complicated by various extrinsic factors, causing difficulties in deciphering the underlying relationships. Here, we establish rules governing the intrinsic mechanical properties of 2D COFs based on molecular simulations of 86 structures under uniaxial tensile stress. Interestingly, we found that the mechanical properties of these nanoscale structures can be comprehended through principles traditionally applied to macroscopic objects. This enables quantitatively predicting the mechanical properties of 2D COFs based on their chemical linkage, topology, and pore dimensions, thereby facilitating material design. Counterintuitively, integrating rigid molecular groups into a 2D framework can potentially compromise overall mechanical strength by inducing imbalanced local strain. These findings pave the way for designing robust 2D COFs for diverse applications and serve as a solid foundation for fully unraveling the roles of various extrinsic factors in the future.