Chelation-enhanced fluorescence chemosensing of Pb(II), an inherently quenching metal ion.

Pb(II) ion serves as a quencher of anthracene fluorescence both intermolecularly and in intracomplex systems reported to date. The advantages of intensimetric analyses showing increasing signal require the design of mechanistically novel fluorescent chemosensors capable of yielding enhanced fluorescence upon chelation of inherently quenching metals. We synthesized both 2- and 9-derivatives of anthracene bearing the N-methylthiohydroxamate ligand, which is capable of quenching fluorescence in the uncomplexed form and which shows some selectivity for Pb(II). Complexation of the 2-derivative to Pb(II) results in rapid metal ion-catalyzed hydrolysis, rendering this compound useless as a sensor with real-time response. However, complexation of the 9-derivative with Pb(II) results in a 13-fold fluorescence increase, reversible upon dissociation of the metal ion. While blood lead analysis is a major potential application for fluorescent chemosensors, the present compound is insufficiently selective for this use.