A Selective and ''Off-On'' Fluorescent Chemosensor Based on Fluorescein for Al³⁺: Synthesis, Characterization, Spectroscopy Analyses, and DFT Calculation

An efficient fluorescent cation chemosensor based on fluorescein L4 was well prepared and identified with spectroscopy analyses. UV-vis and fluorescence measurements examined the analyte complexation of the L4 with various cations, demonstrating a clear tendency to Al³⁺ ion. In the Job plot study, a stoichiometry ratio of a complex between L4 and Al³⁺ ion was determined to be 1: 2 (L4: Al³⁺). A stoichiometry ratio of complex between L4 and Al³⁺ ion was determined to be 1: 2 (L4: Al³⁺) using the Job plot. The association constant (K_a) of the L4-Al³⁺ complex was found 2.8 × 10⁷ M^-2. The obtained limit of detection (LOD) value (1.37 × 10^-6 M for Al³⁺) exhibited the considerable sensitivity of the chemosensor L4 to Al³⁺ ion. DFT/TD-DFT calculations have also been employed to support the binding mode and photophysical properties of the complexation of chemosensor L4 to Al³⁺ ion and also to investigate the enhancement of L4 fluorescence by Al³⁺ ion.