Structural parameters of Zn(II) complexes of 8-hydroxyquinoline-based tripodal ligands affect fluorescence quantum yield

Tripodal ligands containing 8-hydroxyquinoline moieties have been reported with strong binding but perplexing fluorescence properties. Herein we describe the synthesis, photophysical properties and structural characterization of three 8-hydroxyquinoline-based fluorescent Zn(II) probes. Incorporation of the chromophore into a tripodal scaffold resulted in femtomolar sensitivity to the analyte. All three ligands were determined to bind Zn(II) in a 1:1 metal-ligand stoichiometry. Strong zinc affinity was complemented by more than 10-fold fluorescence turn-on. An X-ray crystallographic study revealed very different ligand geometries in the zinc complexes that may influence their spectroscopic behavior.