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+ < Pb2+ < Hg2+ < Ag+ where Cd2+ exhibits a strong CHEF effect and Ag+ induces complete quenching of fluorescence. Since the coumarin fluorophore is a weak π contactor, only Ag+ shows fluorescence quenching. Therefore, our results show that two design principles tested in this work may be used to optimize the ion selectivity without interfering each other. Detailed fluorescence quenching and enhancing mechanisms for Pb2+ and Zn2+ are further discussed via density functional theory (DFT) and time-dependent DFT.