Cd2+-sensiting bichromophore: Excimer emission from an EDTA-methylnaphthalene derivative

Chelating EDTA-based bichromophores, 1,4-bis(methylenecarboxy)-1,4-bis(N-1-naphthylmethylacetamide)-1,4- diazabutane (1) and its 2-napthyl isomer (2), were synthesized, and their fluorescence emission and complexation with Cd2+ and Zn2+ were studied. The fluorescence spectrum of 2 exhibited an emission band due to intramolecular excimer at about 400 nm in addition to a band from a monomeric (or isolated) chromophore at 335 nm. Complexation with Cd2+ sensitively intensified the excimer band and weakened the monomer band, as a result of formation of a [CdL2]2− type complex. In contrast, Zn2+ formed [ZnL]0, leading to depression of the excimer emission and enhancement of the monomer emission. The extent of the changes in emission intensities in the Cd2+ complex was larger than that of the Zn2+ complex contrary to the common metal-ion effect on fluorescence. Such a characteristic emission property was not observed for isomer 1. Ligand 2 has the high Cd-sensing capability that originates from monomer–excimer interconversion.