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

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 x 10(6) 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.