A versatile quinoxaline derivative serves as a colorimetric sensor for strongly acidic pH

A facile and reliable method to monitor strongly acidic pH was developed. The sensing mechanism was found to involve the protonation-deprotonation equilibrium of the synthesized probe HQphy (1) (N-phenyl-N-quinoxalin-2-ylmethylene-hydrazine) within a working range of pH 0.7-2.7. The eventual sensing of Fe3+ was the outcome of acidity imparted by [Fe(H2O)(6)](3+) ions in solution during the formation of [Fe(H2O)(5)(OH)](2+). The protonation-deprotonation phenomenon of HQphy was investigated using H-1 NMR and single crystal X-ray diffraction experiments. The protonated probe, H(2)Qphy(+), was crystallized with FeCl4-/ClO4- counter anions as [H(2)Qphy][FeCl4]H2O (2)/[H(2)Qphy][ClO4]H2O (3). A further complex containing the [H(2)Qphy] cation (4) was also formed. The complexes were characterized by SC-XRD experiments. Moreover, single crystal to single crystal transformation is observed between 1 and 3. In order to understand the sensing mechanism, various analytical studies, such as UV-Vis titration, ESI-MS spectrometry analysis and H-1 NMR, were carried out in detail. A theoretical study correlates well with the experimental data, where the (L) *(L) transition of the ligand is red shifted by 100 nm due to protonation of the quinoxaline moiety. The probe enables discrimination of trihalo acetic acid from its mono- and di-analogues.