Ratiometric and Selective Fluorescent Sensor for Zn2+ as an “Off–On–Off” Switch and Logic Gate

A new chemosensor, 2,3,15,16-tetrakis­(pyridin-2-yl)-7,8,10,11,20,21,23,24-octahydro­[1,4,7,10,13,16]­hexaoxacyclooctadecino­[2,3-g:11,12-g′]­diquinoxaline (1), containing 2,3-bis­(pyridin-2-yl)­quinoxaline and crown ether moieties, has been designed and found to be a ratiometric and selective fluorescent detector of Zn2+ over a wide range of tested metal ions. The addition of Zn2+ to the solution of 1 in acetonitrile induced the formation of a 1:2 ligand–metal complex, 1-Zn2+, which exhibits a remarkable enhanced fluorescent emission centered at 460 nm, with the disappearance of the fluorescent emission of 1 centered at 396 nm due to the mechanism of internal charge transfer. In contrast, the presence of K+ results in the fluorescence quenching of 1 and 1-Zn2+ through the photoinduced electron-transfer mechanism. These results demonstrate that 1 can perform as not only an INHIBIT logic gate but also an “off–on–off” molecular switch triggered by Zn2+ and K+. The structure of complex 1-Zn2+ has been characterized by single-crystal X-ray crystallography, mass spectrometry, and 1H NMR titration experiments. Density functional theory calculation results on 1 and the 1-Zn2+ complex are well consistent with the experimental results.