Switching of Fluorescent Zn/Cd Selectivity in ,,,'-Tetrakis(6-methoxy-2-quinolylmethyl)-1,2-diphenylethylenediamine by One Asymmetric Carbon Atom Inversion.

A quinoline-based hexadentate ligand, (,)-,,,-tetrakis(6-methoxy-2-quinolylmethyl)-1,2-diphenylethylenediamine ((,)-6-MeOTQPhEN), exhibits fluorescence enhancement at 498 nm upon addition of 1 equiv of Zn (/ = 12, φ = 0.047) in aqueous DMF solution (DMF/HO = 2:1). Addition of 1 equiv of Cd affords a much smaller fluorescence increase at the same wavelength (/ = 2.5, / = 21%). The trivalent metal ions such as Al, Cr, and Fe also exhibit fluorescence enhancement at 395 nm (/ = 22, / = 6 and / = 13). In contrast, -6-MeOTQPhEN exhibits a Cd-selective fluorescence increase at 405 nm in the presence of 1 equiv of metal ion (/ = 11.5, φ = 0.022), while Zn induces a smaller fluorescent response under the same experimental conditions (/ = 3.3, / = 29%). In this case, the fluorescence intensities of -6-MeOTQPhEN in the presence of a large amount of Zn and Cd become similar. This diastereomer-dependent, fluorescent metal ion specificity is derived from the Zn-specific intramolecular excimer formation in (,)-6-MeOTQPhEN-Zn complex and higher binding affinity of -6-MeOTQPhEN with Cd in comparison to Zn. The more conformationally restricted diastereomeric pair, namely, and -TQDACHs ( and -,,,-tetrakis(2-quinolylmethyl)-1,2-diaminocyclohexanes), both exhibit Zn-specific fluorescence enhancement because of the high metal binding affinity and intramolecular excimer forming property derived from the rigid DACH backbone.