Abstract
A series of colorful binuclear Schiff bases derived from the different diamine bridges including 1,2- ethylenediamine (bis-Et-SA, bis-Et-4-NEt(2), bis-Et-5-NO(2), bis-Et-Naph), 1,2-phenylenediamine (bis-Ph-SA, bis-Ph-4-NEt(2), bis-Ph-5-NO(2), bis-Ph-Naph), dicyano-1,2-ethenediamine (bis-CN-SA, bis-CN-4-NEt(2), bis-CN-5-NO(2), bis-CN-Naph) have been designed and prepared. The optical properties of these binuclear Schiff base ligands were fully determined by UV-Vis absorption spectroscopy, fluorescence emission spectroscopy, and time-dependent-density functional theory (TD-DFT) calculations. The inclusion of D-A systems and/or π-extended systems in these binuclear Schiff base ligands not only enables adjustable RGB light absorption and emission spectra (300~700 nm) but also yields high fluorescence quantum efficiencies of up to 0.84 in MeCN solution. Then, with the ESIPT (excited-state intramolecular proton transfer) property, fluorescence analysis showed that the probe bis-Et-SA and bis-Ph-SA could recognize Zn(2+) via the "turn on" mode in the MeCN solution. During the detection process, bis-Et-SA and bis-Ph-SA demonstrate rapid response and high selectivity upon the addition of Zn(2+). The coordination of Zn(2+) with the oxygen atom and Schiff base nitrogen atom in a tetrahedral geometry is confirmed by Job's plot, FT-IR, and (1)H NMR spectroscopy. In addition, the paper test and Hela cells were successfully carried out to detect Zn(2+). Moreover, the sensitivity of bis-Et-SA and bis-Ph-SA is much better than that of those Schiff base ligands containing only one chelating unit [O^N^N^O].