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 Pb2+ 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 Pb2+ the stoichiometry depends on the counterion present. Crystal structure data for the complex of Pb(CF3-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. H-1 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.