Quinoline derivatives containing 5-membered heterocycles for metal ion detection

Abstract

The design and synthesis of fluorescent sensors for metal ions are interesting because of their high sensitivity, high selectivity and imaging capability derivable for analysis of biological and environmental systems. Herein, ester and amide derivatives containing 5-membered heterocyclic aromatic and aliphatic rings of 8-hydroxyquinoline and 8-aminoquinoline, respectively, are evaluated as fluorescent sensors for metal cations to explore their effects on the metal ion binding selectivity. In THF solution, the amide derivative of 8-aminoquinoline bearing L-proline (AQPro) exhibited fluorescence enhancement, a remarkable 18-fold increase of the quantum yield, with Zn²⁺. The fluorescence enhancement of the complexation between AQPro and Zn²⁺ involves the chelation enhanced fluorescence (CHEF) and suppressed photo induced electron transfer (PET) which is promoted by the deprotonation of the amide proton as confirmed by ¹H NMR, MS absorption and emission spectroscopy. The Job’s plot of fluorescence intensity and ¹H NMR spectroscopy revealed that both 1:1 and 2:1 stoichiometric binding ratio between AQPro and Zn²⁺ are possible and gave the association constant (Ka) of 5.0 x 10² M⁻¹ and 1.5 x 10⁴ M⁻² for the 1:1 and 2:1 complexation, respectively. Furthermore, the limit of fluorescence detection of Zn²⁺ with AQPro in THF solution was as low as 25.7 nM.