Hydrazone-Derived Schiff Bases as Dual-Mode Sensors: Fluorescence Turn-On Detection of Zn²⁺ and Colorimetric Detection of Zn²⁺ and Cu²⁺ Ions.

Three novel Schiff base chemosensors (HZ-1, HZ-2, and HZ-3) were synthesized by condensing 4-hydroxy-[1,1'-biphenyl]-3-carbaldehyde with distinct hydrazine derivatives: (Z)-(anthracen-9-ylmethylene)hydrazine, (Z)-(pyren-1-ylmethylene)hydrazine, and (Z)-4-(hydrazonomethyl)-N,N-diphenylaniline. Structural variation was confined to the hydrazine moiety, enabling a systematic investigation of its influence on photophysical and sensing properties. UV-Vis absorption and fluorescence studies revealed marked differences in optical behavior, with HZ-1 and HZ-3 chemosensors showing significant fluorescence enhancement upon selective metal ion binding as compared with HZ-2. The limit of detection (LOD) for each sensor was in the micromolar range, confirming their high sensitivity. Job's plot analysis indicated a predominant 2:1 ligand-to-metal stoichiometry. Complementary density functional theory (DFT) calculations provided insights into frontier molecular orbitals and electron density distributions, rationalizing the observed spectral variations. Overall, this study demonstrates how precise modification of the hydrazine fragment can tailor the electronic environment and sensing performance of Schiff bases, offering valuable design strategies for efficient fluorescent metal ion chemosensors.