Multi-targeted fluorescent probes for detection of Zn(II) and Cu(II) ions based on ESIPT mechanism.

With the rapid development of industry, it is pretty critical to detect the heavy metals. Recently, the 2-(2-hydroxyphenyl) benzothiazole derivatives with different numbers of rotatable phenyl m (m = 1,2,3) fluorescent probes HLm were synthesized. However, the theoretical analysis of the mechanism was still missing. In this work, we have systematically researched the mechanisms of excited state intramolecular proton transfer (ESIPT) and the detection of Cu(II), Zn(II) ions for the hydrogen-bond system HLm though quantum chemistry methods. By bond parameters and the minimum energy pathways analyses, the proton of this system was probed directly transfer without barrier. Bond parameters, real space function at bond critical point, the frontier molecular orbital, electron spectra and orbital interaction diagram were carried to elucidate response to Cu(II), Zn(II) ions. In addition, comparing the Gibbs free energy variation of the complexation reaction between fluorescent probes and ions, it can be proved that the number of rotatable benzene rings affects the response ability of the probe to target ions.