Solvent Effect on the Fluorescence Response of Hydroxycoumarin Bearing a Dipicolylamine Binding Site to Metal Ions

Abstract

The fluorescence behavior of a probe dpa-HC, which has a coumarin derivative that acts as a fluorophore and a dipicolylamine (DPA) unit that functions as a metal ion-recognition site, was investigated with various metal ions in aqueous and several non-aqueous solvents. In aqueous solution, the fluorescence of dpa-HC was enhanced by Zn²⁺ and Cd²⁺, but was quenched by other metal ions. On the other hand, in an acetonitrile solution, only Mg²⁺ enhanced the fluorescence, and the addition of a small amount of water quenched this fluorescence. This dramatic selectivity change is explained by stabilization of a metal-dpa-HC complex due to acetonitrile coordination and ON-OFF switching of the intramolecular photoinduced electron transfer (PET) from the nitrogen lone pair of DPA to the coumarin derivative.