A novel coumarin-based ligand: a turn-off and highly selective fluorescent chemosensor for Cu2+ in water

Abstract

A novel and structurally simple coumarin-based ligand (L) containing a picolinamide moiety was developed for fluorogenic detection of transition metal ions in water. Investigations of the absorption and emission spectra of L in the presence of different metal ions showed that the ligand has higher sensitivity and specificity towards Cu²⁺ in water over a number of metal ions. The presence of cupric ions induced significant fluorescence quenching in the emission spectrum of L which was assigned to the transfer of excitation energy from the fluorophore to the metal d-orbital and/or charge transfer from the fluorophore to the metal ion. NMR studies of the ligand upon continuous addition of Cu²⁺ indicated that the interaction with Cu²⁺ involved deprotonation of the hydroxyl group attached to coumarin. The latter affected the electronic properties of the coumarin system and the changes were detected in the emission spectrum of the ligand. The binding ratio of the L–Cu²⁺ complex was investigated through Job's plot analysis which indicated a 1 : 1 stoichiometric ratio between L and Cu²⁺ with an association constant of 2.33 × 10⁶ M. TD-DFT calculations were performed to explain the electronic configuration of the ligand L, its possible Cu²⁺ complex and a possible quenching mechanism. Analytical application of the ligand L showed that the proposed method could be used successfully for the detection and quantification of Cu²⁺ in real samples at micromolar levels.