Chalcone-based Turn-Off Chemosensor for Selective and Susceptible Detection of Fe2+ Ions: Spectroscopic and DFT Investigations.

Herein, in this report we are introducing newly synthesized chalcone derivative, "(E)-1-phenyl-3-(4-((5-(((Z)-thiophen-2-ylmethylene)amino)-1,3,4-thiadiazol-2-yl)thio)phenyl)prop-2-en-1-one" (5), as a chemosensor to detect Fe²⁺ metal ions in HEPES buffer solution of pH 7.5. Spectroscopic techniques were used to confirm the synthesized sensor. To determine the chemical reactivity and molecular stability of the probe, a frontier molecular orbitals investigation was carried out. A molecular electrostatic potential map was investigated to know the binding site of 5 for metal ion coordination. The theoretical absorption and fluorescence emission properties were estimated and correlated with the experimental observations. The sensor showed excellent selectivity for Fe²⁺ compared to all other studied metal ions. The fluorescence binding studies were carried out by adding different amounts of Fe²⁺ ions for a fixed concentration of probe 5. The inclusion of Fe²⁺ ions resulted in a decrease in fluorescence intensity with a bathochromic shift of emission wavelength of 5 due to the 5-Fe²⁺ complexation. The binding affinity value for the probe was found to be 576.2 M^-1 with the help of the Stern-Volmer plot. The Job's plot and mass spectra supported the 2:1 (5: Fe²⁺) stoichiometry of complex formation. The detection limit and limit of quantification of 5 for Fe²⁺ were calculated to be 4.79 × 10^-5 M and 14.54 × 10^-5 M. Further, in addition to this, the photophysical parameters such as fluorescence lifetime of 5 and 5-Fe²⁺ complex measured to be 0.1439 and 0.1574 ns. The quantum yield of 5 and 5-Fe²⁺ was found to be 0.0398 and 0.0376. All these experimental findings revealed that probe 5 has excellent selectivity and sensitivity for Fe²⁺ ions.