Single-Atom Halogen Substitution in Covalent Organic Frameworks Enables σ-Hole-Driven CO 2 Adsorption

Covalent organic frameworks (COFs) are promising materials for CO2 adsorption owing to their tunable porosity and modular functionality. The detailed atomic-scale understanding of CO2 adsorption has yet to be achieved. Here, we report a systematic study on halogenated N,N,N′,N′-tetraphenyl-1,4-phenylenediamine (Wurster, W)-anthracene (A) COFs (W-A-X, X = H, Cl, Br, I) designed to isolate the effect of halogen atoms on CO2 sorption behavior. All halogen-functionalized COFs exhibit significantly higher CO2 uptake and increased isosteric heats of adsorption compared to the W-A-H COF. To elucidate the origin of this enhanced affinity, density functional theory (DFT) calculations were performed on molecular fragments and, for the first time, on extended COF frameworks, enabling direct insight into host–guest interactions within the lattice. In W-A-H, CO2 binds primarily via N(δ–)···C(δ+) interactions at the imine linkage, whereas halogenated derivatives introduce additional adsorption sites through σ-holes, localized regions of positive electrostatic potential on halogen atoms, allowing X−σ(δ+)···O(δ–) interactions with CO2. The interaction strength follows the trend I > Br > Cl, consistent with halogen polarizability. These findings demonstrate σ-hole-mediated adsorption in COFs and establish halogenation as a powerful molecular design strategy to tune host–guest electrostatics for enhanced CO2 uptake.