OFF-ON-OFF fluorescent response of N,N,N',N'-tetrakis(1-isoquinolylmethyl)-2-hydroxy-1,3-propanediamine (1-isoHTQHPN) toward Zn(2.).

An isoquinoline-based ligand, N,N,N',N'-tetrakis(1-isoquinolylmethyl)-2-hydroxy-1,3-propanediamine (1-isoHTQHPN), exhibits a fluorescence increase at 475 nm upon addition of 1 equiv. of Zn(2+) (IZn/I0 = 12, ϕZn = 0.023). This fluorescence enhancement turns and then decreases sharply after the addition of more than 1 equiv. of Zn(2+) reaching a constant minimum intensity with more than 2 equiv. of Zn(2+). In contrast, the fluorescence intensity at 353 nm continues to increase until the signal saturates at ca. 5 equiv. of Zn(2+). This observation can be explained by the formation of a fluorescent mononuclear complex ([Zn(1-isoHTQHPN)](2+)) followed by a non-fluorescent dinuclear complex ([Zn2(1-isoTQHPN)](3+)) at 475 nm during the titration of 1-isoHTQHPN with Zn(2+). Both the mono- and dinuclear complexes were characterized by UV-vis, fluorescence, (1)H NMR, ESI-MS, X-ray crystallography and TDDFT calculations. The fluorescence enhancement at 475 nm is Zn(2+)-specific; Cd(2+) induces a much smaller emission increase (ICd/I0 = 3.7, ICd/IZn = 31%). The Zn(2+)/Cd(2+) selectivity of the fluorescent response correlates with the difference in excimer-forming ability derived from the Cd-Nisoquinoline and Zn-Nisoquinoline bond distances.

Bis(2-quinolylmethyl)ethylenediaminediacetic acids (BQENDAs), TQEN-EDTA hybrid molecules as fluorescent zinc sensors.

Molecular hybrids of TQEN (N,N,N',N'-tetrakis(2-quinolylmethyl)ethylenediamine) and EDTA (ethylenediamine-N,N,N',N'-tetraacetic acid) were examined as fluorescent Zn(2+) sensors. Upon the addition of Zn(2+), N,N-BQENDA (N,N-bis(2-quinolylmethyl)ethylenediamine-N',N'-diacetic acid, 1a) exhibits a 30-fold emission enhancement at 456 nm (λex = 315 nm, ϕZn = 0.018) in buffer (HEPES, pH = 7.5, 100 mM KCl). The fluorescence enhancement is Zn(2+)-specific as Cd(2+) induces much smaller increases (ICd/I0 = 5 and ICd/IZn = 16%). These spectroscopic properties, as well as the excellent water-solubility, represent a significant improvement compared to the parent TQEN sensor (ϕZn = 0.007, ICd/IZn = 64%). The isoquinoline analog N,N-1-isoBQENDA (N,N-bis(1-isoquinolylmethyl)ethylenediamine-N',N'-diacetic acid, 1b) possesses a similar Zn(2+) fluorescence response to the parent 1-isoTQEN (N,N,N',N'-tetrakis(1-isoquinolylmethyl)ethylenediamine) sensor, but exhibits diminished fluorescence intensity. Apo 1a and 1b extract more than 50% of the Zn(2+) from an equimolar amount of [Zn(TPEN)](2+) (TPEN = N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine) or [Zn(EDTA)](2-), whereas TPEN and EDTA cannot effectively remove Zn(2+) from [Zn(1a)] and [Zn(1b)]. The reduction of steric crowding in [Zn(TQEN)](2+) resulting from the substitution of two quinolines with carboxylates enhances the interaction between the metal ion and the remaining quinoline nitrogen atoms. The stronger bonding interaction results in enhanced emission intensity, Zn(2+) selectivity and metal ion affinity. This is in contrast to [Zn(1-isoTQEN)](2+) where the isoquinoline-carboxylate replacement does not relieve any coordination distortion, therefore no significant changes in fluorescence or metal binding properties are observed.