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., CuII from NiII) 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. When a copper or nickel ion complexes with a sensor of type 1 (below), which contains a diamine‐diamide receptor, a metal‐to‐fluorophore electron‐transfer process quenches the fluorescence of the anthracene subunit and signals recognition of CuII and NiII ions. When the sensor contains a tetramine receptor, 3 d metal recognition is still indicated by anthracene fluorescence quenching, but this now occurs by an energy‐transfer mechanism.