Lariat-crown ether based fluorescence sensors for heavy metal ions

Exploratory investigations were conducted to probe several aspects of a new strategy for the design of metal ion fluorescence sensors. The results of the investigation show that lariat-crown ethers that contain amine and thioether side chains, and a naphthalene chromophore can be efficiently prepared by using sequences that rely on key single electron transfer promoted photocyclization reactions. Members of this novel family of lariat-crown ethers serve as selective fluorescence sensors for the divalent metal cation of Mg, Hg, and Pb. The response of the sensors to the divalent metal ion is modulated by the nature of the heteroatom(s) incorporated into the side chains. Specifically, lariat-crown ethers that contain tertiary amine groups in their side chains display an off–on type response to Mg(II), Hg(II), and Pb(II). In contrast, thioether side chain containing lariat-crown ethers behave differently in that their fluorescence intensities decrease in the presence of increasing concentrations of these divalent metal cations. These responses can be understood on the basis of selective divalent metal ion induced disruption of intramolecular single electron transfer (SET)-quenching (for side chain amine containing lariat-crown ethers) and the enhancement of intersystem crossing (for side chain thioether containing lariat-crown ethers) of the singlet excited state of the naphthalene fluorophore.