Tuning Electronic Skeleton Properties of Porphyrin Covalent Organic Frameworks (COFs) for High-Performance Selective Gas Chemiresistive Sensing

Covalent organic frameworks (COFs) have emerged as promising platforms for chemiresistive gas sensing due to their intrinsic porosity and tunable electronic structures. However, achieving high sensitivity, low detection limits, and long-term stability simultaneously remains challenging. Herein, we report a skeleton engineering strategy applied to three isostructural porphyrin-based metal free COFs, synthesized with skeleton linkers bearing methyl, hydrogen, or fluorine substituents, enabling systematic tuning of their electronic properties. Incorporation of electron-withdrawing fluorine atoms reduces the intrinsic conductivity to an optimal level, thereby amplifying the resistance change upon NO₂ exposure. Consequently, the fluorinated COF exhibits an exceptional sensing response (ΔI/I₀ = 379.5 at 20 ppm) and a low detection limit of 7.8 ppb under ambient conditions, with operational stability maintained for over 75 days. These results provide a rational design strategy, demonstrating that skeleton engineering can effectively improve sensitivity, selectivity, and stability in COF-based gas sensors.