Redox-Coupled Charge Transfer in an Ionic Cu(I) Coordination Polymer for Room-Temperature NO₂ and SO₂ Sensing

Conducting coordination polymers (CPs) have emerged as promising materials for chemiresistive gas sensing at ambient conditions. In this work, we report three ionic semiconducting Cu(I) CPs constructed from (pyridin-4-yl)-N-(4H-1,2,4-triazol-4-yl)methenamine (PTMA), oxydi(2,1-phenylene)bis(diphenylphosphine), incorporating BF₄ ^- (CP1, C₄₄H₃₅BCuF₄N₅OP₂), PF₆ ^- (CP2, C₄₄H₃₅CuF₆N₅OP₃), and ClO₄ ^- (CP3, C₄₄H₃₅ClCuN₅O₅P₂) counteranions. Among these, CP1 shows outstanding dual-gas sensing capability toward SO₂ and NO₂ at ambient conditions, achieving strong chemiresistive responses with excellent selectivity, fast response-recovery behavior and stable cycling performance. Comparative studies using CP2 and CP3 reveal minimal counteranion influence, indicating that the sensing response is predominantly driven by interactions at the Cu-ligand framework. Complementary experimental measurements and theoretical calculations reveal that adsorption of electron-accepting analytes perturbs the electron density on Cu(I), inducing partial oxidation of Cu(I) to Cu(II) and modulating charge transfer along the polymer backbone without disrupting structural stability, thereby accounting for the observed resistance modulation. Overall, these findings highlight the potential of Cu(I) systems as a high-efficiency platform for room-temperature dual-gas sensing.