NO₂ Adsorption at Humid, Low-Concentration Environments Using Undulated and Metalated Covalent Organic Frameworks

Nitrogen dioxide (NO₂) pollution poses significant environmental and health risks, necessitating advanced materials for efficient capture under practical low-concentration and humid conditions. This study introduces a new class of undulated and metalated covalent organic frameworks (COFs; CuPc-AQ-COFs(M) (M = Co/Ni)) tailored for NO₂ adsorption. By integrating phthalocyanine units, dioxin linkages, and transition metal sites into a hydrophobic framework enriched with fluorine groups, the material achieves exceptional NO₂ capacities of 2.50 mmol g^-1 (dry) and 8.11 mmol g^-1 (wet; 75% RH) at 100 ppm-the highest reported for low-concentration capture. The undulated COFs architecture, featuring ≈1 nm pores and inclined AB stacking with an interlayer distance of ≈3.52 Å, ensures size-selective adsorption and structural stability. Metalation with Co/Ni enhances chemisorption via π-backbonding and charge transfer, while hydrophobicity prevents competitive water adsorption. The COF retains 46% capacity after five cycles, demonstrating robust regenerability. In situ DRIFTS and DFT calculations reveal the efficient NO₂ adsorption dominated by metal coordination, with Co/Ni sites exhibiting superior electron donation. This work establishes a blueprint for designing COFs that leverage structural tunability and metal integration to address air quality challenges, advancing their application in environmental remediation. © 2025 Wiley-VCH GmbH.