Chelating 5′-(p-hydroxyphenyl)pyridylthiazoles as ratiometric fluorescence probes for d10 metal ions

Three fluorescent 5′-(p-hydroxyphenyl)pyridylthiazoles (HPPT) with different chelating groups at the 4′ position were synthesized and evaluated for their ability to detect transition metal ions in acetonitrile and aqueous buffers, based on changes in fluorescence intensity and intramolecular charge transfer (ICT). Both 4’-O-picolyloxy-HPPT (Pic-HPPT) and 4’-O-(o-carboxypicolyl)-HPPT (CPic-HPPT) respond strongly to Zn(II), Cd(II), and Pb(II) in CH₃CN with a bathochromic shift in emission up to 68 nm, whereas 4’-O-carboxymethyl-HPPT (CM-HPPT) is unresponsive. Only CPic-HPPT responds to d¹⁰ metal ions in aqueous phosphate buffered solution (PBS, pH 7.4), attributable to the added chelating power of the ortho-carboxylate. CPic-HPPT forms a 2:1 complex with Zn(II) and a 1:1 complex with Cd(II) and Pb(II) in CH₃CN, whereas a 1:1 complex forms with Zn(II), Cd(II), and Hg(II) ions in PBS. X-ray structural analysis of 1:1 Pic-HPPT–metal ion complexes reveals a planar tridentate binding motif with Zn(II) but a nonplanar tridentate geometry with the larger Cd(II) ion. Fluorescence titration of CPic-HPPT in PBS with Zn(NO₃)₂ established sub-micromolar sensitivity with a limit of quantitation at 50 nM. These results show that CPic-HPPT has promise as a fluorescent probe for d¹⁰ metal ions in physiologically relevant media.