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

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A quinoline-based hexadentate ligand, ( S, S )- N, N, N′, N′ -tetrakis(6-methoxy-2-quinolylmethyl)-1,2-diphenylethylenediamine (( S, S )-6-MeOTQPh 2 EN), exhibits fluorescence enhancement at 498 nm upon addition of 1 equiv of Zn 2+ ( I Zn / I 0 = 12, φ Zn = 0.047) in aqueous DMF solution (DMF/H 2 O = 2:1). Addition of 1 equiv of Cd 2+ affords a much smaller fluorescence increase at the same wavelength ( I Cd / I 0 = 2.5, I Cd / I Zn = 21%). The trivalent metal ions such as Al 3+, Cr 3+, and Fe 3+ also exhibit fluorescence enhancement at 395 nm ( I Al / I 0 = 22, I Cr / I 0 = 6 and I Fe3+ / I 0 = 13). In contrast, meso -6-MeOTQPh 2 EN exhibits a Cd 2+ -selective fluorescence increase at 405 nm in the presence of 1 equiv of metal ion ( I Cd / I 0 = 11.5, φ Cd = 0.022), while Zn 2+ induces a smaller fluorescent response under the same experimental conditions ( I Zn / I 0 = 3.3, I Zn / I Cd = 29%). In this case, the fluorescence intensities of meso -6-MeOTQPh 2 EN in the presence of a large amount of Zn 2+ and Cd 2+ become similar. This diastereomer-dependent, fluorescent metal ion specificity is derived from the Zn 2+ -specific intramolecular excimer formation in ( S, S )-6-MeOTQPh 2 EN-Zn 2+ complex and higher binding affinity of meso -6-MeOTQPh 2 EN with Cd 2+ in comparison to Zn 2+ . The more conformationally restricted diastereomeric pair, namely, cis- and trans -TQDACHs ( cis- and trans - N, N, N′, N′ -tetrakis(2-quinolylmethyl)-1,2-diaminocyclohexanes), both exhibit Zn 2+ -specific fluorescence enhancement because of the high metal binding affinity and intramolecular excimer forming property derived from the rigid DACH backbone.