Recognition of switching on or off fluorescence emission spectrum on the Schiff-bases as a Mg2+ chemosensor: A first principle DFT and TD-DFT study

Abstract

Today, due to convenience, rapid diagnosis and etc., chemosensors are widely used to detect metal ions. In the current research, through DFT and TD-DFT methods, 8-hydroxyquinoline-5-carbaldehyde-(benzotriazol-1′-acetyl) (QCH) ligand was used to identify Mg2+ cation and investigate its fluorescence emission spectrum mechanism. In order to investigate electronic properties, natural bond orbital (NBO) analysis, reduced density gradient (RDG) plots and electron localization function (ELF) as well as, changes in energy gap of HOMO and LUMO orbitals in the ground state, in water solution were used. Energy gap underwent change about ΔE = 0.17 eV in the presence of Mg2+ cation. Using TD-DFT calculations the optical absorption and emission of QCH ligand was investigated in the presence and absence of Mg2+cation. QCH ligand lacks emission spectrum but in the presence of Mg2+cation shows an emission spectrum in the wavelength of λEmi = 611 nm. Using electron-hole theory, photoinduced electron transfer (PET) process and fluorescence emission spectrum mechanism were investigated. Turning on emission spectrum in the presence of Mg2+ cation is due to stopping charge transfer from chelator to fluorophore.