Novel quinoline-fused quinolone derivatives: synthesis, photophysical studies, solvatochromism and chemosensing applications toward Cu2+, Fe3+, and Hg2+ ions

Abstract

A series of novel quinoline-fused quinolone-based fluorescent probes have been designed and synthesized via Zn-catalysed one-pot aerobic oxidative annulation reactions using o-vinylaniline and isatin derivatives as starting materials. The photophysical studies of the synthesized molecules revealed a positive fluorescent solvatochromism with moderate to excellent quantum yields (Φ_F = 0.024–0.695) and large Stokes shifts (6520–13 783 cm⁻¹). Probes 5e and 6 exhibited a fluorescence “Turn-Off” response upon binding with Fe³⁺, Cu²⁺, and Hg²⁺, displaying strong selectivity over other metal ions. The detection limits of probe 6 were 52.5 nM (for Cu²⁺, linearity 8–28 µM), 34.88 nM (for Fe³⁺, linearity 6–30 µM) and 16.5 nM (for Hg²⁺, linearity 2–10 µM), well below or comparable to the USEPA standards for drinking water. Mechanistic studies including Job's plot, ¹H and ¹³C NMR titrations, IR, HRMS analyses, DFT and TCSPC studies confirmed a 1 : 2 metal–ligand complex formation in which Cu²⁺/Fe³⁺ ions coordinate with the amide (–CONH–) group, suppressing intramolecular charge transfer (ICT) within the donor–π–acceptor (D–π–A) system, leading to fluorescence quenching. The sensor's practical applicability was demonstrated in real water samples (accuracy rate of 100 ± 4%) and in the development of encryption ink.