Strategies for a fluorescent sensor with receptor and fluorophore designed for the recognition of heavy metal ions

Abstract The ligand DC-18-ane-N2O4 (7,16-bis(6,7-dimethoxy-1-benzopyran-2-one-4-methyl)-1,4,10,13-tetraoxa-7,16-diazacyclooctadecane) is synthesized and its fluorescence responses to the binding of a set of metal ions (Zn2+, Cd2+, Hg2+, Pb2+, Cu2+ and Ag+) are investigated. Among the metal-ligand complexes studied here, the crystal structures of [Ag(DC-18-ane-N2O4)]ClO4.H2O, [Pb(DC-18-ane-N2O4)(ClO4)2H2O] 3.5H2O and [DC-18-ane-N2O4)H2](ClO4)2.ClCH2CH2Cl are reported. Design of DC-18-ane-N2O4 as a selective turn-on sensor for heavy metal ions is based on two important factors: 1) target metal ions can be selectively complexed according to their sizes, and 2) quenching of fluorescence is related to the ability to form π contacts between the complexed metal ion and the fluorophore of the sensor. The fluorescence study shows that the size of macrocyclic 18-ane-N2O4 is too large for the Zn2+ to bind tightly, but right for Cd2+. This leads to a strong CHEF (Chelation enhanced fluorescence) effect for Cd2+, but not for Zn2+, which renders DC-18-ane-N2O4 a selective Cd2+ sensor over the ubiquitous Zn2+. In fact, it is found that Zn2+ induces fluorescence quenching, which is quite unusual as Zn2+ almost always causes a CHEF effect. For the heavier and larger metal ions tested in this work, the fluorescence intensities are found to be in line with the ability to form π contacts with the fluorophore: Cd2+