Benzothiadiazole-Based Diarylamines as a Fluoride Sensor: Prevention of Fluoride Induced Decomposition of Receptor Molecule by Complex Formation with Cu2+

Benzothiadiazole based diarylamine TBT-HH and its derivatives TBT-HN and TBT-HM, having an electron withdrawing nitro group and an electron donating methoxy group at para phenylamine position, respectively, were synthesized and used as a fluoride sensor. All the compounds show well-defined visible color changes in the presence of fluoride ion; the orange colour of TBT-HN becomes blue and the pink colored TBT-HH and TBT-HM becomes colourless. While the presence of F- red shifts the absorption maximum by 174 nm to the near IR region for comparatively electron deficient nitro substituent, for the electron rich receptors TBT-HH and TBT-HM blue shifted absorption was observed. H-1-NMR titration experiments of all the receptors confirm deprotonation of the N-H proton by fluoride ion. The deprotonated state of the receptor was stabilized by electron withdrawing nitro substituent whereas that of electron donating substituents destabilizes the deprotonated state and subsequently leads to its decomposition, which is prevented by Cu2+ ion through the complex formation. This hypothesis has been used to develop sensor strips based on these compounds, which shows a color change in the presence of fluoride ion in aqueous solution. Our investigations suggest that the presence of suitable electron deficient substitution would lead to the deprotonation of the diarlyamines, which can subsequently be used for several applications. In rare cases, fluoride induced decomposition of molecules can be prevented by this kind complex formation reaction.