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 signalling subunit) has been linked to either a cyclic or a noncyclic quadridentate ligand (the receptor). Occurrence of the metal-receptor interaction is signalled through the quenching of anthracene fluorescence. When the receptor (i.e., the dioxote-tramine subunit of sensors 2 and 3) is able to promote the one-electron oxidation of the metal, quenching takes place through a photoinduced metal-to-fluorophore electron-transfer mechanism. In the case of sensors containing a tetraamine binding subunit (4 and 5), quenching proceeds by an energy-transfer process. Selective metal binding and recognition can be achieved by varying the pH, and metal ions can be distinguished (e.g., Cu-II from Ni-II) by spectrofluorimetric titration experiments in buffered solutions. Whereas systems 2, 3 and 5 show reversible metal binding behaviour, the cyclam-containing system 4 irreversibly incorporates transition metals (due to the kinetic macrocyclic effect) and cannot work properly as a sensor.