N-diethylaminosalicylidene based turn-on fluorescent Schiff base chemosensor for Al3+ ion: Synthesis, characterisation and DFT/TD-DFT studies

A Schiffbase chemosensor (H3L) based on the 4-(N,N- Diethylamino)salicylaldehyde moiety hasbeen synthesized and characterized using several spectroscopic techniques i.e. H-1-NMR, FT-IR, Mass and UVvisible. The chemosensor shows high selectivity for Al3+ ion through 'turn-on' cyan colour fluorescence over a variety of metal ions i.e. Ni2+, Zn2+, Cd2+, Mn2+ ions except Co2+, Cu2+ and Fe3+ ions. The chemosensor exhibits 20 fold intensity enhancement at 481 nm (lambda(ex) = 378 nm) against Al3+ ion until saturation point. Additionally, the detection limit and stability constant were found to be 1.2 x 10(-6) M and 5.2 x 10(5) M-1, respectively with the low equivalent range of 1-34 mu M. The chemosensor also displays similar fluorescence behaviour for Al3+ ion in a variety of solvents and solvent mixtures. The quantum yield (Phi(F)) values for H3L and Al3+-complex were observed to be 0.08 and 0.29, respectively. The Job's plot calculations show that, H3L binds to Al3+ ion in 2:1 ratio. H-1-NMR titration analysis indicates that the chemosensor undergoes mono-deprotonation while binding to Al3+ ion. The geometry of the chemosensor and Al3+-complex were optimised and some thermodynamic parameters were calculated using Density Functional Theory (DFT). The Gibbs free energy of complex was calculated to be -1272.00 kcal/mol. Further, the optical and electronic properties of the proposed complex were explained with the help of Time Dependent-Density Functional Theory (TD-DFT). (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

A Schiffbase chemosensor based on the 4-(N,N-Diethylamino)salicylaldehyde moiety has been synthesized and shows high selectivity for Al3+ ion. The chemosensor exhibits significant intensity enhancement for Al3+ ion and demonstrates fluorescence behavior in a variety of solvents. The optical and electronic properties of the complex were explained using Time Dependent-Density Functional Theory (TD-DFT).