Hybrid Inorganic–Organic Phosphorus(V) Dihydrazone as Colorimetric and Fluorometric Probe for the Detection of Fe2+ and Zn2+ Cations: Synthesis, Physicochemical, Photophysical, NMR, Cyclic Voltammetry, and DFT Studies

Abstract

A new hybrid inorganic–organic probe, phenylphosphonothioic salicylic dihydrazone (PPSD), was designed, synthesized, and characterized as Schiff base via simple condensation reaction using two units of salicylaldehyde with phenylphosphonothioic dihydrazide (PPDH). In the colorimetric studies, PPSD in methanol medium responded as probe to detect cations iron (Fe²⁺) and zinc (Zn²⁺). During attempts to venture PPSD as probe with series of cations (Fe²⁺, Co²⁺, Ni²⁺, Ca²⁺, Li⁺, Zn²⁺, Cu²⁺, Na⁺, Al³⁺, and Ce²⁺), detection of cations was observed specifically with Zn²⁺ and Fe²⁺ cations. Herein, the colorless probe medium was identified to turn into dark blue with Fe²⁺ cations, whereas Zn²⁺ cations showed fluorescent green under UV light. To validate these colorimetric detections, Fe²⁺ cations were studied by UV-vis and Zn²⁺ cations by photoluminescence. To validate the colorimetric detection of Fe²⁺ and Zn²⁺ cations, UV-vis absorbance analysis was performed for Fe²⁺ cations while Zn²⁺ cations found to be supported by photoluminescence data. Based on absorbance and fluorescence titration studies, the detection limits (limit of detection [LOD]) for Fe²⁺ (9.7789 × 10⁻⁵ M) and Zn²⁺ (9.6340 × 10⁻⁵ M) cations were established, respectively. The binding constants for Fe²⁺ and Zn²⁺ cations were determined to be 7.92 × 10⁴ M⁻¹ and 2.76 × 10⁸ M⁻¹, from the Benesi–Hildebrand (B-H) equation. Consequently, PPSD showed a photoluminescence quantum yield (ɸ = 0.038), which increased approximately 11-fold (ɸ = 0.417) due to its interaction with Zn²⁺ cations. FTIR, multinuclear NMR (¹H, ¹³C, and ³¹P), photoluminescence, UV-vis, NMR titration, HRSEM, EDS, cyclic voltammetry, and DFT studies were used to characterize structures, sensing abilities and binding studies of PPSD with Fe²⁺/Zn²⁺ cations.