Design, Synthesis and Photophysical Studies of Novel Dye-Based Donor–Acceptor Systems for Selective Detection of Cu²⁺ Ions

Abstract

The selective and sensitive detection of metal ions and ligands is crucial across environmental, biological and industrial domains. Traditional analytical methods often suffer from low sensitivity, poor selectivity and complex instrumentation. To overcome these limitations, dye-based probes have emerged as versatile tools, capable of producing measurable optical changes in the presence of target analytes. This study focuses on the design and synthesis of novel donor–acceptor (naphthalimide–dansyl) dyads and their evaluation as ratiometric fluorescent probes for Cu²⁺ ion detection. The dyads were characterized using spectroscopic techniques (UV-Vis, fluorescence, FTIR, ¹H NMR) and X-ray crystallography. Their photophysical behavior demonstrated efficient Förster resonance energy transfer (FRET) in the ground state, which was selectively disrupted upon Cu²⁺ binding. The probes exhibited high selectivity and sensitivity toward Cu²⁺ ions compared to other mono- and divalent metals, forming 2:1 stoichiometric complexes with strong binding constants. These interactions were confirmed through Job’s plot, Benesi–Hildebrand analysis and MALDI-TOF mass spectrometry. The study also explored the impact of sulfonamide groups and spacer length on binding affinity. Such dye-based systems have significant potential as ratiometric sensors for environmental monitoring, biomedical applications and photodynamic research.