Zn2+-Schiff’s Base Complex as an “On–Off-On” Molecular Switch and a Fluorescence Probe for Cu2+ and Ag+ Ions

Shaaban A Elroby,Saadullah G Aziz,Abdesslem Jedidi,Bashair Abdullah Banaser,Osman I Osman,Walid I Hassan

doi:10.1007/s10895-021-02864-4

Abstract

The present study presents a thorough theoretical analysis of the electronic structure and conformational preference of Schiff's base ligand N,N-bis(2-hydroxybenzilidene)-2,4,6-trimethyl benzene-1,3-diamine (H2L) and its metal complexes with Zn2+, Cu2+ and Ag+ ions. This study aims to investigate the behavior of H2L and the binuclear Zn2+ complex (1) as fluorescent probes for the detection of metal ions (Zn2+, Cu2+ and Ag+) using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The six conformers of the H2L ligand were optimized using the B3LYP/6-311 + + G** level of theory, while the L-2-metal complexes were optimized by applying the B3LYP functional with the LANL2DZ/6-311 + + G** mixed basis set. The gas-phase and solvated Enol-cis isomer (E-cis) was found to be the most stable species. The absorption spectra of the E-cis isomer and its metal complexes were simulated using B3LYP, CAM-B3LYP, M06-2X and ωB97X functionals with a 6-311 + + G** basis set for C, O, N and H atoms and a LANL2DZ basis set for the metal ions (Zn2+, Cu2+ and Ag+). The computational results of the B3LYP functional were in excellent agreement with the experimental results. Hence, it was adopted for performing the emission calculations. The results indicated that metal complex (1) can act as a fluorescent chemosensor for the detection of Ag+ and Cu2+ ions through the mechanism of intermolecular charge transfer (ICT) and as a molecular switch "On-Off-On" via the replacement of Cu2+ by Ag+ ions, as proved experimentally.

Highlights

Due to their operational simplicity, high sensitivity, and high response speed, fluorescent probes have been widely applied in diverse fields such as chemical biology, medicinal chemistry, analytical chemistry, and molecular biology [1]
Its isomers result from the rotation around single bonds, e.g. Enol-cis isomer (E-cis) and E-trans; while the tautomers originate from the difference in functional groups e.g. E-cis and K-cis
The geometry optimization has been performed using B3LYP functional which was adopted as the best functional for the absorption and emission calculations

Summary

Introduction

Due to their operational simplicity, high sensitivity, and high response speed, fluorescent probes have been widely applied in diverse fields such as chemical biology, medicinal chemistry, analytical chemistry, and molecular biology [1]. The fluorescent probes use either specific host-guest interactions or selective chemical reactions to affect changes in the fluorescence properties. If the interaction between the host and guest leads to an irreversible chemical reaction, the fluorescent probe is known as a chemodosimeter [2]. The chemical compounds with azomethine (-CH=N-) groups are called Schiff’s bases They are excellent classes of ligands and enjoyed a popular use in coordination chemistry because they form coordinate bonds with metal ions to produce coordination metal complexes. The metal complexes incorporating Schiff’s bases have played important roles in the advancement of inorganic chemistry. They were discovered by Hugo Schiff’s (Germanist) [3]. Schiff’s bases have considerable chemical and biological importance because of their easy method of preparation, excellent chelating ability and synthetic flexibility [4,5]

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