Bis-Chalcones as Efficient Optical Chemosensors for Rapid and Visual Detection of Toxic Metal Ions

In continuation of our ongoing exploration of electronically delocalized Bis-chalcone scaffolds as versatile molecular platforms, the previously reported chemosensors 3-(4-(dimethylamino)benzylidene)pentane-2,4-dione (DBPD) and 3-(3-(4-dimethylamino)phenyl)allylidene)pentane-2,4-dione (DPAPD) were revisited to evaluate their metal-responsive properties beyond the initially characterized photophysical behavior. Although their synthesis and fundamental optical characteristics have been well established, systematic investigations into their metal-ion sensing capabilities have remained limited. Given the strong electron-donating nature of the dimethylamino groups conjugated with β-diketone moieties, which facilitate intramolecular charge transfer (ICT) and metal coordination, the present study examines the optical properties of DBPD and DPAPD in response to environmentally relevant metal ions, Cu²⁺, Co²⁺, Ni²⁺, Pb²⁺, Zn²⁺, Cd²⁺, and Fe³⁺. Distinct visual color changes were observed for Co²⁺, Cu²⁺, Ni²⁺, and Pb²⁺, whereas Zn²⁺, Cd²⁺, and Fe³⁺ elicited negligible optical response, highlighting the selective interactions of the Bis-chalcones with specific metal ions. The absorption measurements revealed characteristic bands at 455 nm (DBPD) and 474 nm (DPAPD). Upon metal ion addition, Co²⁺ enhanced the absorbance and induced bathochromic shifts, while Cu²⁺ and Ni²⁺ caused decreased absorbance and the emergence of new spectral features, indicative of complex formation and perturbation of the ICT process. Fluorescence studies further corroborated these interactions, showing metal-dependent quenching or enhancement, with DPAPD exhibiting a pronounced hypsochromic shift upon Ni²⁺ binding. Quantitative analyses using the Benesi-Hildebrand method and Job's plot confirmed 1:1 stoichiometry for the metal-chemosensor complexes and demonstrated the preferential binding of DBPD to Co²⁺ and DPAPD to Cu²⁺. To further probe the structural characteristics, solid-state complexes of DBPD-Co²⁺ and DPAPD-Cu²⁺ were isolated and characterized via molar conductance, Fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), and X-ray diffraction (XRD), confirming coordination interactions and complex stability. Reversibility studies revealed that the original optical properties of the chemosensors were fully restored upon addition of the chelating agent ethylenediaminetetraacetic acid disodium salt (EDTA), establishing the dynamic and non-permanent nature of the metal-ligand interactions suitable for reversible sensing applications. These findings establish DBPD and DPAPD as robust, selective, and reversible fluorescent chemosensors, capable of sensitive and cost-effective detection of specific metal ions in environmental and real-sample contexts, while their ICT-active frameworks provide a tunable platform for multifunctional optical applications.