A heterocyclic hydrazone ligand, diacetyl monoxime-2-pyridyl hydrazone, HL, 1, was investigated as a new chromogenic agent for selective detection of Fe³⁺. The ligand 1, undergoes 1 : 2 complexation with Fe³⁺ and Ni²⁺ to form complexes [Fe^III(HL)₂]Cl₃, 1a and [Ni^II(HL)₂]Cl₂, 1b respectively. The iron(III) complex 1a gives a characteristic absorption peak at 487 nm with distinct reddish-pink colouration. The change in colour can easily be distinguished from other metal complexes by the naked eye. No obvious interference was observed in presence of other metal ions (Na⁺, K⁺, Ca²⁺, Mg²⁺, Al³⁺, Mn²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Co²⁺, Pb²⁺, Hg²⁺, Cd²⁺). The bands appearing in the UV region (200-340 nm) are characteristics of the ligand, HL, 1. In the complexes [Fe^III(HL)₂]Cl₃, 1a, and [Ni^II(HL)₂]Cl₂, 1b, these ligand centered bands are accompanied by multiple bands extending into the visible region (350-500 nm). The association constants ($K_{\text{ass}}$, UV-Vis) were found to be $(6.4865 \pm 0.004)\times 10^5$ for the complex 1a and $(1.1960 \pm 0.002)\times 10^5$ for the complex 1b at 298 K determined by the UV-Vis spectroscopy. On excitation at 285 nm, the ligand HL, 1 strongly emits at 364 nm due to an intraligand¹($\pi - \pi^\ast$) transition. The complexes are luminescent ($\lambda_{\text{ex}}$ 285 nm, $\lambda_{\text{em}}$ 365 nm) with $F/F_0$ 0.75 for 1a and 0.81 for 1b. In both the cases, the 1 : 2 binding is confirmed by Job’s method. Molecular structure of the complex 1b has been determined by single crystal X-ray diffraction studies. Here, two crystallographically distinct but metrically very similar molecules making an enantiomeric pair constitute the asymmetric unit in which both metal atoms are tris chelated in meridional geometry.

A new quinoline based hydrazone was synthesized via a condensation reaction and characterized by NMR, mass and single crystal X-ray diffraction studies. It was investigated for suitability as a reversible ratiometric fluorescent pH sensor in acidic pH region. The sensor exhibits intramolecular charge transfer (ICT) type photophysical changes upon protonation of the quinoline ring. No significant interference on emission behavior was observed in the presence of various metal ions.