TAMEisoquin, a novel tripodal fluorescent zinc sensor with high Zn(II) affinity and Zn(II)/Cd(II) selective fluorescence response: Synthesis, coordination geometry, spectroscopy, and comparative response to biometal ions

Abstract

Herein we report a novel fluorescent zinc sensor 1,1,1-tris (aminomethylethane)N,N′,N″-(2-methylisoquinoline) (TAMEisoquin), [Zn(TAMEisoquin)]2+ complex and its structural and photophysical studies. TAMEisoquin was obtained from reduction of templated [Ni(TAMEisoquin-trisimine)]2+ complex to give [Ni(TAMEisoquin)]2+, which was subsequently decomposed with excess sodium cyanide to afford TAMEisoquin. A competition reaction between TAMEisoquin and the well-known Zn(II) chelator (N,N,N′, N′-tetrakis(2-pyridylmethyl)ethylenediamine (TPEN) at pH 7.2 indicated a conditional dissociation constant for [Zn(TAMEisoquin)]2+ of Kd=1.4×10−15M. The affinity of TAMEisoquin for Zn(II) is indicative of a ligand design exploiting ring size, preorganization and chelate effects to potentially detect picomolar and femtomolar zinc concentrations. X-ray crystallographic analysis of [Zn(TAMEisoquin)]2+ showed four unique cations that preferred a distorted octahedral geometry with trigonal twist angles that range between 34(2)° and 43.1(8)°. Moreover, [Zn(TAMEisoquin)]2+ complex showed Zn–N bond distances slightly similar to our previously reported [Zn(TAMEpyr)]2+ complex, being Zn–Nheterocyclic, 2.160(1)Å and Zn–Naliphatic, 2.178(4)Å versus 2.128(2)Å and 2.169(2)Å respectively for [Zn(TAMEpyr)]2+. The addition of Zn(II) to TAMEisoquin displayed 11-fold fluorescence enhancement, selective by comparison to divalent biometal ions and Cd(II) which induces only 16% of the Zn(II) response.