Fluorescent Sensors for Transition Metals Based on Electron‐Transfer and Energy‐Transfer Mechanisms

Fluorescent sensors for 3d 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 dioxotetramine 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.