Reticulating Local Dynamics in Three-Dimensional Covalent Organic Frameworks
In covalent organic frameworks (COFs), the geometry of the building units predetermines the structure of the framework, yet comparable design principles relating the COF linkers to the local motions that govern guest adsorption and diffusion as well as mechanical response have remained beyond reach. Here, we combine synchrotron X-ray pair distribution function (PDF) analysis with molecular dynamics (MD) simulations accelerated by machine learning interatomic potentials to resolve and compare the local dynamics of two three-dimensional imine-linked COFs, chosen to differ in the geometry and rigidity of one of their two building units. Both are constructed from the same 4-connected square planar linker, $4,4',4'',4'''$-(pyrene-1,3,6,8-tetrayl)tetraaniline (PyTTA), reticulated with the 12-connected nanographene hexakis{[3,5-bis($p$-formylphenyl)-4,6-dimethoxyphenyl]}hexabenzocoronene to give COF-412, {[(HBC-LA$_{12}$)(PyTTA)$_3$]}$_{\mathit{imine}}$, and with the 6-connected triptycene 2,3,6,7,14,15-hexakis(4-formylphenyl)triptycene to give COF-564, {[(Trip-CHO)$_2$(PyTTA)$_3$]}$_{\mathit{imine}}$. The PDF-MD approach resolves the amplitude, the time scale, and the spatial extent of the motion of all COF linkers, separating the limited libration of the fused aromatic cores from the larger reorientational tilts of the pendant aryl rings they carry and from the translation of whole linkers within the net. The simulations further reveal that exchanging the nanographene- for the triptycene-based linker tunes the equilibrium tilt and the positional probability density of PyTTA, raising the tilt amplitude of its linker partner, and, notably, strengthens the coupling between the translations of neighbouring linkers. This approach provides a blueprint for understanding local dynamical properties in COFs, and shows them to be selectable through the choice of building unit and amenable to design.