Gayathri M, D Sriram, Nilanjan Dey
A systematically engineered series of donor-modulated hydrazone-based fluorescent amphiphiles bearing indole, benzothiophene, and quinoline terminal heterocycles was developed to elucidate the cooperative roles of molecular electronics and supramolecular self-assembly in Cu2+ sensing. The amphiphilic hydrazone scaffold integrated multidentate N/O donor sites, extended π-conjugation, and hydrogen-bonding motifs to promote aggregation-assisted analyte recognition in aqueous media. Among the series, the indole-functionalized derivative (compound-1) exhibited the most favourable donor-acceptor coupling, strongest intramolecular charge-transfer (ICT) character, and highest fluorescence output, as supported by DFT calculations and global reactivity analysis. Compound 1 formed supramolecular aggregates in water (DLS: ∼480 nm) and displayed pronounced multi-stimuli responsiveness, including solvatochromic, halochromic, and thermoresponsive fluorescence behaviour. Upon Cu2+ binding, compound 1 underwent ratiometric UV-visible changes, 6.8-fold fluorescence quenching, and emission blue-shift (464 → 414 nm), with selective response over competing metal ions. Mechanistic studies involving lifetime analysis, FT-IR, 1H NMR, DLS, and reversibility assays established Cu2+-induced N,O-chelation accompanied by aggregate reorganization and ICT suppression as the operative sensing mechanism. The probe further enabled ultrasensitive Cu2+ detection in real water samples with low detection limits (0.08-0.10 μM), excellent recoveries (96.2-104.8%), and high precision (<5% RSD), highlighting its practical utility for environmental monitoring.