Solid-State NMR reveals mixed side-chain organization across pores in amphiphilic covalent organic frameworks

Covalent organic frameworks (COFs) are a promising class of materials for advanced applications due to their well-organized backbone and pores. Amphiphilic side chains that are prone to self-organization by phase separation are envisioned to lead to chemically heterogeneous pores with different microenvironments with different philicities. Previously, these microenvironments were only indirectly investigated and thus the microphase separation in COFs has remained inconclusive. We probe the local structure of a dual-chain functionalized, amphiphilic COF where the pore size and side chain length is expected to lead to through-pore interactions. 13C and 1H MAS NMR experiments, including 2D 1H-1H spin-diffusion exchange, proved that close spatial proximity and dynamic interactions between the chemically distinct side chains exist for a majority of the side chains, excluding the possibility of significant phase separation. These results indicate that a mixed arrangement in which polar and non-polar chains coexist within the same pores predominates. Our study demonstrates the power of ssNMR in elucidating the structure of amphiphilic COFs at the molecular level and provides new insights into the design of frameworks with chemically heterogeneous pores.