Isoquinoline-based TQEN family as TPEN-derived fluorescent zinc sensors

The hexadentate nitrogen ligands 1-isoTQEN ( N,N,N',N'-tetrakis(1-isoquinolylmethyl)ethylenediamine) and 3-isoTQEN ( N,N,N',N'-tetrakis(3-isoquinolylmethyl)ethylenediamine) have been prepared. The structures of these ligands are based on that of TPEN ( N,N,N',N'-tetrakis(2-pyridylmethyl)ethylenediamine). The introduction of a benzene ring into TPEN affords fluorescence ability upon zinc-ion binding. Compared to the quinoline isomer TQEN, isoquinoline derivatives 1-isoTQEN and 3-isoTQEN exhibit a lower-energy shift in the excitation and emission wavelengths and an enhanced fluorescence intensity, probably because of the energy-transfer mechanism between adjacent isoquinoline rings. Importantly, an increase in the Zn (2+)/Cd (2+) discriminating ability and a reduction in the background fluorescence induced by pH were also achieved for isoquinoline derivatives. The zinc-ion-induced fluorescence of these isoTQENs was not quenched by an addition of TPEN, which demonstrates the significantly high zinc-ion binding ability of these isoTQEN ligands.