Comprehensive Study of Poly(2,3,5,6-tetrafluoroaniline): From Electrosynthesis to Heterojunctions and Ammonia Sensing

In this work, we report for the first time on a comprehensive study of poly(2,3,5,6-tetrafluoroaniline) (PTFANI). Contrary to the nonfluorinated polyaniline (PANI) or its analogues bearing one fluorine atom, PTFANI is a poorly conductive material. We present a comprehensive study of the electrosynthesized PTFANI from its monomer in an acidic aqueous medium. PTFANI was fully characterized by a potential-pH diagram, spectroelectrochemistry, and electrochemical quartz crystal microbalance (EQCM) measurements, as well as by a morphological study. Combined with the X-ray photoelectron spectroscopy (XPS) analysis, it allowed us to understand the redox properties of this polymer compared to those of the unsubstituted PANI. At pH < 1.85, no proton transfer occurred during the electrochemical process, but the insertion of anions at the site of the protonated imines was demonstrated through the EQCM and XPS experiments. PTFANI showed a lower ratio of 1 ClO₄^- per 3 2,3,5,6-tetrafluoroaniline units compared to that of PANI. The behavior at pH > 1.85 was different; no anion upload was observed during the electron transfer, but 1 H⁺ per electron was involved during the transition between the leucoemeraldine and emeraldine base forms. It should also be noted that the oxidation of the emeraldine into the pernigraniline form was not accessible in PTFANI because of the electron-withdrawing effects of the fluorine atoms. However, we took advantage of the unique behavior of PTFANI to build heterojunctions, by combining with a highly conductive molecular material, namely lutetium bisphthalocyanine, LuPc₂. The obtained double-lateral heterojunction exhibited a particularly interesting sensitivity to ammonia, even under humid atmospheres, with a limit of detection of 450 ppb. This work paves the way for the use of PTFANI in other electronic devices and as a sensor not only in the field of air quality monitoring but also in the field of health diagnosis in measuring the human breath.