Zn-Schiff's Base Complex as an "On-Off-On" Molecular Switch and a Fluorescence Probe for Cu and Ag Ions.

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 (HL) and its metal complexes with Zn, Cu and Ag ions. This study aims to investigate the behavior of HL and the binuclear Zn complex (1) as fluorescent probes for the detection of metal ions (Zn, Cu and Ag) using density functional theory (DFT) and time-dependent density functional theory (TDDFT). The six conformers of the HL ligand were optimized using the B3LYP/6-311 +  + G** level of theory, while the L-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 (Zn, Cu 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 Cu ions through the mechanism of intermolecular charge transfer (ICT) and as a molecular switch "On-Off-On" via the replacement of Cu by Ag ions, as proved experimentally.