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

Abstract

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 ¹H NMR and single crystal X-ray diffraction experiments. The protonated probe, H₂Qphy⁺, was crystallized with FeCl₄⁻/ClO₄⁻ counter anions as [H₂Qphy][FeCl₄]·H₂O (2)/[H₂Qphy][ClO₄]·H₂O (3). A further complex containing the [H₂Qphy] 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 ¹H NMR, were carried out in detail. A theoretical study correlates well with the experimental data, where the π(L) → π*(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.