Characterization of the Pore Environments in Covalent Organic Frameworks by Dynamic Spin Probe Exchange

Understanding the internal pore properties of crystalline covalent organic frameworks (COFs) is crucial for chemically fine-tuning these porous materials and optimizing them for applications. Typical characterization based on adsorption isotherms, such as N₂-adsorption can in some cases be limited due to the cryogenic temperatures that are used, limiting molecular motion and thereby giving a static picture. We synthesized alkyl and triethylene glycol (TEG) functionalized COFs, which, despite high crystallinity, showed low BET values owing to the large side chains leading to pore blocking. We applied continuous-wave electron paramagnetic resonance (CW EPR) spectroscopy to shed light on their pore properties. Using different EPR-active radicals as spin probes, the adsorption affinity, the type and strength of interactions, the local concentrations of the guest radicals, and the polarity difference between alkyl- and TEG-containing pores could be sensed and quantified. We could identify the spectral signatures of multiple non-covalent interactions from the adsorbed radical species, including hydrogen bonds, dipolar, dispersion, and π-π-interactions. This study demonstrates the potential of CW EPR spectroscopy to characterize COF pore environments and pore-guest interactions with radicals in suspensions. It opens the door to a new complementary methodology for pore characterization.