Engineering Nickel(II) Porphyrin-Conjugated Polymers with Different Aryl meso-Substituents for n-Type and p-Type Ammonia Sensors

Conjugated polymers have revolutionized the field of conductometric gas sensors for sensing toxic gases arising from the fast urbanization and industrialization. In this work, we report the synthesis of a series of 5,15-diaryl Ni(II) porphyrin-conjugated polymers (pNiD(Aryl)P) and their integration as the top layer on an octafluorinated copper phthalocyanine (CuF₈Pc) sublayer to construct bilayer heterojunction (BLH) devices for ammonia sensing. For the first time, we report the pioneering demonstration of polarity engineering within a BLH device by manipulating the meso-substituent of the 5,15-diaryl Ni(II) porphyrin-conjugated polymer constituting the top layer of the CuF₈Pc/pNiD(Aryl)P BLH device. The BLH devices prepared from the 5,15-diaryl Ni(II) porphyrin-conjugated polymer bearing electron-donating meso-substituents as the top layer exhibit a p-type behavior, whereas an n-type behavior is observed for the BLH devices prepared from the 5,15-diaryl Ni(II) porphyrin-conjugated polymer bearing electron-withdrawing meso-substituents. Laser desorption ionization high-resolution mass spectrometry, UV/vis/NIR, and X-ray photoelectron spectroscopy studies provide evidence of a decrease in intramolecular dehydrogenative coupling in pNiD(Aryl)P bearing electron-withdrawing meso-substituents, resulting in low electrical conductivity of the thin films. Density functional theory calculations reveal noninvolvement of electron-withdrawing meso-substituents toward π-delocalization in the fused Ni(II) porphyrin tapes. Interestingly, all the CuF₈Pc/pNiD(Aryl)P BLH devices exhibit remarkable sensing response toward NH₃. Among all the devices, CuF₈Pc/pNiDPP displays the highest sensitivity of -1.17% ppm^-1 for NH₃, whereas CuF₈Pc/pNiDNapP and CuF₈Pc/pNiDCNPP exhibit the best limit of detection for NH₃, below 200 ppb. In addition, CuF₈Pc/pNiDCNPP shows short response and recovery times of 13 and 255 s, respectively, making this device highly suitable for deployment in emergency services.