Active Hydrogen Free, Z-Isomer Selective Isatin Derived “Turn on” Fluorescent Dual Anions Sensor

An efficient and anions fluorescence “on–off” sensor of 1-(prop-2-yn-1-yl)-3-(quinolin-3-ylimino)indolin-2-one (PQI) has been developed for the selective sensing of dual anions of F⁻ and NO₃⁻ ions in aqueous medium. Active hydrogen and Lewis acidic binding sites free, Z- isomer of isatin based π-conjugated quinoline exhibited excellent sensing activity against F⁻ and NO₃⁻ ions in UV light. The fluorescence turns on the process accomplished via the PET “on–off” mechanism. The interaction between probe molecule and anions is thought to be a non-covalent interaction of the low electron density covalently bonded N-methylene moiety of propargyl isatin (-N-CH₂-) of probe molecule with F⁻ ion and the terminal acidic proton of propargyl group of isatin (-C≡C-H) with NO₃⁻ ions. The modes of anions binding with PQI and plausible mechanisms are proposed by ¹H and ¹³C NMR titrations. The selectivity of anions sensing may be offered by the bucked structure of the Z-isomer. The calculated association constant values for PQI and F⁻ and NO₃⁻ are ions 2.5 × 10⁴ M⁻¹ and 2.2 × 10³ M⁻¹, respectively, indicating strong binding interaction between the PQI and anions. The association nature of anions and probes was analyzed by a Jobs plot and the finding indicates both F⁻ and NO₃⁻ ions are in 1:1 complexation with PQI. The limit of detection (LOD) of the probe with F⁻ and NO₃⁻ ions is calculated and is to be 6.91 × 10^–7 M and 9.93 × 10^–7 M, respectively. The proposed PQI fluorophore possesses a low limit of detection (LOD) for both F⁻ and NO₃⁻ ions which is within the WHO prescribed detection limit.