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 (ΦF = 0.024-0.695) and large Stokes shifts (6520-13783 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), that 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.