4-HYDROXYPHENYLPYRIDYLTHIAZOLES AS RATIOMETRIC FLUORESCENT PROBES AND PIEZOCHROMIC STUDIES ON TETRANUCLEAR COPPER AND PYRAZOLATE PHOSPHORS

Part I: Three fluorescent 5’-(p-hydroxyphenyl)pyridylthiazole (HPPT) derivatives 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) responded strongly to Zn(II), Cd(II), and Pb(II) in CH₃CN with bathochromic shifts in emission up to 68 nm, whereas 4’-O-carboxymethyl-HPPT (CM-HPPT) was unresponsive. Only CPic-HPPT responded 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 formed 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 was formed with Zn(II), Cd(II), and Hg(II) ions in PBS. X-ray structural analysis of 1:1 4’-O-picolyl-HPPT–metal ion complexes revealed a planar tridentate binding motif with Zn(II) but a nonplanar tridentate geometry with the larger Cd(II) ion. A fluorescence titration study of CPic-HPPT in PBS with Zn(NO₃)₂ established sub-micromolar sensitivity with a limit of quantitation at 50 nM. Density functional theory (DFT) calculations confirmed increased polarization in HPPT upon metal ion binding. These results show that CPic-HPPT is a useful fluorescent probe for d¹⁰ metal ions in physiologically relevant media.

Part II: The solid-state photoluminescence of tetranuclear Cu(I)–pyrazolate (TCP) complexes was studied under hydrostatic pressure up to 3 GPa using a sapphire anvil cell, with the hypothesis of inducing hypsochromic shifts in emission through pressure-induced rigidochromism. All TCP complexes tested produced pressure-induced blueshifts of varying degree, with most producing a reversible redshift upon release of pressure. Two deep-blue-emitting TCPs (Cu₄(Cl-pz)₄, all-exo polymorph of Cu₄(Et-pz)₄) exhibited hypsochromic shifts as high as 37 nm into the blue–violet range, and two green-emitting TCPs (Cu₄(H-pz)₄, Cu₄(CF₃-pz)₄) exhibited blueshifts up to 17 nm. The 1,3-di-exo polymorph of Cu₄(Et-pz)₄) produced a bimodal piezoluminescent response with a hypsochromic shift up to 92 nm (from 532 to 440 nm) yet was able to return to its original wavelength upon release of pressure. The sizable difference in hypsochromic shifts for blue- and green-emissive TCPs correlates with the ground-state geometry and aspect ratio of the Cu₄ core: TCPs with planar, close-packed Cu₄ cores experienced larger blueshifts than those with puckered cores with gaps between Cu atoms.