A quinoline-based
hexadentate ligand, (S,S)-N,N,N′,N′-tetrakis(6-methoxy-2-quinolylmethyl)-1,2-diphenylethylenediamine
((S,S)-6-MeOTQPh2EN),
exhibits fluorescence enhancement at 498 nm upon addition of 1 equiv
of Zn2+ (I
Zn/I
0 = 12, φZn = 0.047) in aqueous DMF solution
(DMF/H2O = 2:1). Addition of 1 equiv of Cd2+ 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 Al3+, Cr3+, and Fe3+ 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-MeOTQPh2EN exhibits a Cd2+-selective fluorescence increase
at 405 nm in the presence of 1 equiv of metal ion (I
Cd/I
0 = 11.5, φCd = 0.022), while Zn2+ 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-MeOTQPh2EN in the presence of a large amount of Zn2+ and
Cd2+ become similar. This diastereomer-dependent, fluorescent
metal ion specificity is derived from the Zn2+-specific
intramolecular excimer formation in (S,S)-6-MeOTQPh2EN-Zn2+ complex and higher binding
affinity of meso-6-MeOTQPh2EN with Cd2+ in comparison to Zn2+. 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 Zn2+-specific fluorescence enhancement because
of the high metal binding affinity and intramolecular excimer forming
property derived from the rigid DACH backbone.