FLUORESCENT SENSORS FOR TRANSITION-METALS BASED ON ELECTRON-TRANSFER AND ENERGY-TRANSFER MECHANISMS

Fluorescent sensors for 3 d divalent metal ions have been designed by means of a supramolecular approach: an anthracene fragment (the signal ling subunit) has been linked to either a cyclic or a noncyclic quadri dentate ligand (the receptor). Occurrence of the metal-receptor intera ction is signalled through the quenching of anthracene fluorescence. W hen the receptor (i.e., the dioxote-tramine subunit of sensors 2 and 3 ) is able to promote the one-electron oxidation of the metal, quenchin g takes place through a photoinduced metal-to-fluorophore electron-tra nsfer mechanism. In the case of sensors containing a tetraamine bindin g subunit (4 and 5), quenching proceeds by an energy-transfer process. Selective metal binding and recognition can be achieved by varying th e pH, and metal ions can be distinguished (e.g., Cu-II from Ni-II) by spectrofluorimetric titration experiments in buffered solutions. Where as systems 2, 3 and 5 show reversible metal binding behaviour, the cyc lam-containing system 4 irreversibly incorporates transition metals (d ue to the kinetic macrocyclic effect) and cannot work properly as a se nsor.