A series of selective and sensitive fluorescent sensors based on a thiophen-2-yl-benzothiazole unit for Hg2+

Different types of fluorescent probes for Hg2+ based on the 5-thiophen-2-yl-benzothiazole derivatives (TBT, 2-thiophen-2-yl-benzothiazole; CTBT, 9-(5-benzothiazol-2-yl-thiophen-2-yl)-9H-carbazole; DTBT, 2-(5-acenaphthen-5-yl-thiophen-2-yl)-benzothiazole; and NTBT, 6-(5-benzothiazol-2-yl-thiophen-2-yl)-2-hexyl-benzo[de]isoquinoline-1,3-dione) were realized by changing the subsituents. Multisignaling changes were observed through UV-vis absorption and fluorescence spectra upon addition of Hg2+. The addition of Hg2+ on TBT induced a remarkable OFF/ON-type fluorescence signaling behavior. With the introduction of electron-donating carbazole, the detection behavior of CTBT was just the reverse of what TBT displayed, while a ratiometric fluorescent probe based on DTBT for Hg2+ was prepared by introducing the weak electron-drawing group acenaphthene. Moreover, the dichloromethane solution of CTBT and DTBT showed a distinct color change from colorless to yellow with the addition of Hg2+, which indicated that these two probes would facilitate the naked-eye detection of Hg2+. However, both UV-vis absorption and fluorescence spectra of NTBT with strong electron-withdrawing substituents showed no obvious change upon addition of Hg2+. It would be concluded that the electronic properties of the molecule was one of the vital factors in designing chemosensor molecules.