2-Anthryltriazolyl-Containing Multidentate Ligands: Zinc-Coordination Mediated Photophysical Processes and Potential in Live-Cell Imaging Applications

1,2,3-Triazol-4-yl (triazolyl)-containing tetradentate ligand 1 undergoes fluorescence enhancement upon binding to zinc ion (Zn²⁺) in both organic (acetonitrile) and aqueous solutions. A 1:1 complex of 1 with a trigonal bipyramidal Zn²⁺ was characterized by X-ray crystallography. The cyclic voltammogram (CV) of 1 suggests that an intramolecular photoinduced electron transfer (PET) process is thermodynamically feasible which would quench the fluorescence of the 2-anthryltriazolyl fluorophore. On the basis of the X-ray and CV data, it was initially postulated that the 1:1 binding between Zn²⁺ and ligand 1 shuts down the PET quenching pathway of the free ligand, which leads to the fluorescence enhancement of 1. However, the nuance of the interaction between 1 and Zn²⁺ was revealed by isothermal titration calorimetry (ITC) and ¹H NMR titration experiments. A two-step binding process was observed which proceeds through an intermediate species of 2:1 (ligand/Zn²⁺) stoichiometry. Upon close examination of the fluorescence spectra of 1 during the Zn²⁺ titration experiment, the fluorescence profile is in fact consistent with a two-step binding process in which a moderate fluorescence enhancement was observed during the early stage of the titration, followed by a bathochromic shift in conjunction with a more pronounced enhancement as Zn²⁺ concentration increases. The studies on compounds 2−5 support the amended hypothesis that upon increasing Zn²⁺ concentration, compound 1 first undergoes fluorescence enhancement because of the formation of a 2:1 (ligand to Zn²⁺) complex which slows down the PET quenching process. As Zn²⁺ concentration increases, the 2:1 complex is converted into a 1:1 complex which facilitates an intramolecular exciplex formation between the excited 2-anthryltriazolyl fluorophore and the Zn²⁺-bound pyridyl moiety. Finally, the potential of compound 1 as an intracellular fluorescent indicator for Zn²⁺ was evaluated. HeLa cells loaded with compound 1 grown in Zn²⁺-rich media show stronger fluorescence than those grown under Zn²⁺-deprived conditions, confirming the promise that the triazolyl-containing polyaza fluoroionophores can be developed into intracellular fluorescent indicators targeting biological Zn²⁺.