Multifunctional Two-Dimensional Tetrazine-Based Polymer for an Inverse Electron Demand Diels–Alder Reaction and Room-Temperature NO₂ Sensing

A multifunctional tetrazine-integrated π-conjugated two-dimensional (2D) porous organic polymer (CMP) ( PTz ) has been developed by the self-condensation of benzene-1,3,5-tricarbonitrile in the presence of sulfur and hydrazine hydrate via the Pinner synthetic strategy. PTz has been further postmodified into a pyridazine polymer ( PPz ) through an inverse electron demand Diels–Alder (IEDDA) reaction using ethyl vinyl ether, which allows the tuning of structural, optical, and electronic properties. DFT revealed a 0.6 eV difference in band gap between the two polymers, with PTz having a narrow band gap of 1.6 eV and PPz having a band gap of 2.2 eV. Furthermore, PTz is a red solid with an absorption red edge at 620 nm, whereas PPz produces a yellow solid with a blue-shifted absorption red edge of 480 nm. 2D PTz was utilized to form a resistive sensor for detecting NO 2, a prominent environmental pollutant. PTz exhibited an excellent sensitivity toward NO 2, with its response varying from −8.4 to −34.2% for 2–10 ppm of NO 2 at room temperature. The detection limit of PTz was found to be 337 ppb, and the response time and recovery time ranged from 176 to 273 s and 307 to 443 s, respectively, for a concentration range of 2 to 10 ppm of NO 2 . The polymer exhibited high selectivity toward NO 2 among different tested analytes (CO, SF 6, NH 3, H 2 O, and H 2 S).