Substituent effect on discriminative detection of Mg2+ and Zn2+ ions by two related coumarin-based chemosensors

Abstract

Two closely related coumarin-based chemosensors, L1 with a –NO2 group and L2 with an −I group, have been designed as the fluorescent sensors. Chemosensors L1 and L2 have been utilized for the selective detection of Mg2+ and Zn2+ ions, respectively. The non-fluorescent L1 and L2 exhibit ‘turn-on’ fluorescence response with Mg2+ and Zn2+ ions, respectively. The absorption spectrum of L1 in presence of Mg2+ ion exhibits a 12 nm blue-shift while L2 shows a 5 nm red-shift with Zn2+ ion. Chemosensors L1 and L2 show noteworthy detection limits of 0.10 and 0.35 μM and binding constants (Kb; ×104 M−1) of 0.45 and 1.14 for Mg2+ and Zn2+, respectively. The chemosensor-metal binding was studied by mass and proton NMR spectra while a 2:1 stoichiometry was confirmed by Job’s plot. The crystal structure of [(L2)2Zn] complex further confirms a 2:1 stoichiometry as well as binding mode of the chemosensors. Both chemosensors further worked as the colorimetric sensors for the detection of Mg2+ and Zn2+ ions. For both chemosensors, reversibility was achieved by using ATP while logic gates were also generated. Chemosensor L2 was found to be AIEE active in MeOH-H2O and EtOH-H2O solvent systems. Water promoted H-type aggregation by hindering the CN bond rotation which enhanced the emission. The formation of aggregates was confirmed by the dynamic light scattering and florescence lifetime measurements.