Sensors and switches from supramolecular chemistry

Efficient sensors for metal ions can be made by assembling an appropriate receptor with a fluorescent fragment. The occurrence of the receptor/cation interaction is signalled through a sharp change of the fluorescence emission. The situation is esp. favorable in the case of transition metal ions, which are typically redox active and provide an electron release to the excited fluorophore (or uptake therefrom), causing fluorescence quenching. Thus, the supramol. design of fluorescent sensors should consider a ligating subunit able to promote the redox activity of the envisaged metal ion. An identical approach can be used for the synthesis of mol. redox switches, i.e. systems in which the distinctive property of a given subunit (e.g. fluorescence) is controlled by an adjacent control subunit, sensitive to the variation of the redox potential. Couples involving metal centers can be conveniently used to switch on/off the fluorescence of the proximate fluorophore through the variation of the redox potential.