Off-Stoichiometric Synthesis of Porphyrin-Based Covalent Organic Frameworks for Structural Modulation and Enhanced Chemiresistive NO2 Sensing

Covalent organic frameworks (COFs) with high porosity and tunable structures have emerged as promising materials for energy and environmental applications. Their synthetic versatility enables precise structural control, allowing tailored properties and functions. Herein, we report a green, aqueous synthesis of porphyrin-based COFs through stoichiometric tuning of amine and aldehyde monomers under ambient conditions. The resulting COFs display well- defined nanoscale morphologies with tunable porosity and crystallinity. Notably, off- stoichiometric conditions led to decreased crystallinity and surface area, indicating the formation of defect-rich structures. COFs synthesized under amine- and porphyrin-rich 2 conditions demonstrated exceptional chemiresistive NO2 sensing performance, with responses up to ~8000 at 100 ppm and a detection limit as low as ~12 ppb. The sensors also showed rapid response and recovery, excellent recyclability, and long-term stability. In situ DRIFTS measurements and DFT calculations revealed strong interactions between NO2 molecules and the imine and porphyrin units, along with charge transfer from nitrogen sites to NO2, accounting for the enhanced sensing behavior. This study establishes a facile, sustainable strategy for the structural modulation of COFs toward high-performance gas sensing applications.