Synergistic π‐Hole Acceptor Effect and d–π Conjugation in Flexible Cu‐TCPP/g‐C ₃ N ₄ Heterojunctions for High‐Performance Oxygen‐Independent NO ₂ Sensing at Room Temperature

ABSTRACT The development of room‐temperature nitrogen dioxide (NO 2 ) sensors that simultaneously offer high sensitivity, excellent selectivity, mechanical flexibility, and, critically, oxygen‐independent operation remains a challenge for wearable applications. Here, we address this challenge by integrating a flexible copper (II) tetrakis (4‐carboxyphenyl) porphyrin/graphitic carbon nitride (Cu‐TCPP/g‐C 3 N 4 ) type‐II heterojunction as the active layer of a high‐performance NO 2 sensor. The resulting device exhibits an ultrawide detection range from ppm down to ppb levels, a high response of 58.3 toward 100 ppm NO 2 , rapid response/recovery times (4.3 s/9.2 s), excellent selectivity, and remarkable long‐term stability. Comprehensive structural characterization and mechanistic studies reveal an oxygen‐independent sensing pathway. The cooperative action of the π‐hole acceptor effect and d–π extended conjugation at the molecular interface shifts the gas‐sensing process from a conventional surface‐localized electron‐exchange regime to conjugation‐mediated electron extraction within the bulk of the heterojunction. This intrinsic mechanism enables the sensor to maintain high sensitivity even in oxygen‐free environments. In addition, the device preserves stable sensing performance under mechanical bending, underscoring its strong potential for wearable, flexible, and portable room‐temperature NO 2 detection.