Structural and photophysical studies of highly stable lanthanide complexes of tripodal 8-hydroxyquinolinate ligands based on 1,4,7-triazacyclononane

The tripodal H(3)thqtcn ligand allows the synthesis of well-defined neutral monomeric syn-tris(hydroxyquinolinate) complexes of lanthanides. Pure [Ln(thqtcn)] complexes (Ln = Nd, 1; Er, 2; Yb, 3) of the triply deprotonated ligand thqtcn(3-) were prepared. Crystallographic characterization was carried out for complexes 1 and 3, showing that the ligand is flexible enough to wrap around Ln(III) of different size with a tricapped trigonal-prism coordination geometry. The partially protonated H(1.5)thqtcn(1.5-) ligand also binds strongly to Ln(III) ions in methanol and water (at pH approximately 5). The X-ray diffraction study shows that protonated complexes crystallize as chiral dimers of formula [Ln(H(1.5)thqtcn)](2)(OTf)(3) x 3 MeOH (Ln = Nd, 4; Yb, 5) in which two equivalent monomeric complexes of the partially protonated H(1.5)thqtcn(1.5-) are bridged by very strong hydrogen bonds between the phenol oxygen atoms. The ligand thqtcn(3-) sensitizes efficiently the near-infrared emission of Er, Nd (0.10% Qy), and Yb (0.60% Qy). For the first time, the effect of ligand protonation on the efficiency of the solid-state luminescence emission of lanthanides complexes is demonstrated by the decrease of the luminescence quantum yield observed for [Yb(H(1.5)thqtcn)](2)(OTf)(3) (0.26%) with respect to [Yb(thqtcn)] (0.60%). The water-soluble H(3)thqtcn-SO(3) analogue of H(3)thqtcn and its lanthanide complexes has been prepared. The solution quantum yields of the thqtcn-SO(3)(3-) complexes were measured in water at pH 7.4 (0.016% for Nd(III) and 0.14% for Yb(III)) and in deuterated water (Nd, 0.047%; Yb, 0.55%), and they are among the highest reported in the literature for Yb(III) in aqueous solutions. The high thermodynamic and kinetic stability in water at physiological pH of the gadolinium complex of thqtcn-SO(3)(3-) indicate that the lanthanide complexes of thqtcn(3-) and thqtcn-SO(3)(3-) are highly resistant to hydrolysis and therefore are well suited for the development of luminescent devices and for application as probes in biomedical imaging.