Chemosensors for Lead(II) and Alkali Metal Ions Based on Self-Assembling Fluorescence Enhancement (SAFE)

A class of bis-crown ether fluorescent chemosensors for detection of trace amounts of Na +, K +, and Cs + and Pb 2+ was investigated. A distyrylbenzene derivative with crown ether rings of varying size attached to each benzene (12-crown-4 ( 1 ), 15-crown-5 ( 2 ), and 18-crown-6 ( 3 )) can form intermolecular sandwich structures with Na +, K +, and Cs + ions, respectively. In addition, bis-crown 2 forms unique sandwich complexes with Pb 2+; the stoichiometry depends on the counterion present. Crystal structure data for the complex of Pb(CF 3 COO) 2 with 2 indicate a unique sandwich structure having dilead tetrakis(trifluoroacetate) bridging two crowns on separate ligands. The bis-crown/alkali metal ion complexes have 1:1 stoichiometry in each case. 1 H NMR spectra and MALDI-TOF mass spectra clearly demonstrate that formation of the self-assembled sandwich structure is associated with the fluorescence enhancement for sensor molecules 2 and 3 with K + and Cs +, respectively. Sensor molecule 1, however, has an additional fluorescence enhancement mechanism, which involves contributions from the formation of excimers.