Theoretical investigation of the Zn2+ detection mechanism based on the quinoline derivative of the Schiff-base receptor

The sensing mechanism of the quinoline-derived Schiff base HL (concentrated from 8-hydroxyquinoline with 2,4-dihydroxybenzaldehyde) as a highly selective fluorescent probe for Zn2+ was investigated by theoretical calculations with DFT and TDDFT. The conformations of the HL molecule, its ketone form and its Zinc complex structure, were optimized in the ground and excited states. The systems have been studied in depth in terms of structural parameters, frontier molecular orbitals, absorption and fluorescence spectra as well as potential energy curves analysis and approximately density gradient analysis. The present theoretical calculations propose a different detection mechanism from that proposed experimentally. The theoretical results predict that the fluorescence quenching in HL is attributed to the excited state intramolecular proton transfer (ESIPT) rather than the photoinduced electron transfer (PET) of benzene to electrons. When Zn2+ is introduced, Zn2+ takes the place of the H atom, creating a complex that blocks the ESIPT reaction and restores fluorescence.