A Fluorescent Chemosensor for Detection pH and Cu2+ Ion Base on 7-((2-Aminoethyl)amino)-5-Bromo-6-Hydroxy-1-Methylquinolin-1-ium-3-Sulfonate: Experimental and DFT Calculation.

A quinoline derivative 7-((2-aminoethyl)amino)-5-bromo-6-hydroxy-1-methylquinolin-1-ium-3-sulfonate (QEt) containing quinoline ring, - ${\text{SO}}_{\text{3}}^{\text{-}}$ sulfonate, -OH phenol, and amine groups was synthesized and studied luminescence properties. The aqueous solutions QEt 10µM change luminescence color from green (λ_em = 490 nm) to yellow (λ_em = 563 nm) as increasing pH and the intensity at a peak of 563 nm is linearly proportional with pH value in the range of pH = 3,0-4,0. The QEt solution can be used as a chemosensor for Cu²⁺ with an LOD value at 0.66 $\mu M$ . Along with the experiment, the structure, absorption and emission spectra of QEt have been investigated by TD-DFT calculation. The result shows that the absorption band centered at 420 nm is due to the electron transition from HOMO to LUMO (π → π*). The results also help to assign emission band centered at 490 nm is due to the S₁ → S₀ transition (LUMO → HOMO singlet transition), at 563 nm is due to the T₁ → S₀ transition (LUMO → HOMO triplet transition). The dependence of the relative intensity of each emission peak on pH, which is experimentally recorded, is explained based on the results of theoretical TD-DFT calculation.