Spectroscopic and DFT/TD-DFT studies on selective and sensitive fluorescent detection of Al(III) ion

Abstract

Spectroscopic investigation and Density Functional Theory (DFT) and Time-Dependent Density Functional Theory (TD-DFT) calculations have been carried out to delineate the mechanism of fluorescent detection of Al(III) by a new Schiff base (CN3) in an aqueous solution. CN3 was synthesized and characterized using 1H and 13C NMR and mass spectral techniques. Upon adding Al(III), the weak fluorescence of CN3 (512 nm) became enhanced with a slight blue shift to 502 nm. CN3 served as an efficient probe for highly selective and sensitive Al(III) detection without the interference of environmentally and biologically significant cations and anions. The limit of detection (LOD) was determined to be 1.3 µM. Job's continuous variation method of analysis suggested the formation of a 1:1 complex between CN3 and Al(III), which was confirmed as an octahedral complex by 27Al NMR and mass spectral data. The binding constant of the complex was found to be 7×104 M−1. The mode of coordination of CN3 with Al(III) was ascertained by 1H NMR titration experiments. CN3 served as an OONN-type tetradentate ligand to form a chelate, which is responsible for the enhancement of fluorescence of CN3 upon binding with Al(III) due to chelation enhanced fluorescence (CHEF) mechanism. The optical properties of CN3 and its Al(III) complex were corroborated well by DFT and TD-DFT calculations.