Surprisingly Bright Near-Infrared Luminescence and Short Radiative Lifetimes of Ytterbium in Hetero-Binuclear Yb-Na Chelates

New heterobinuclear lanthanide complexes with benzoxazole-substituted 8-hydroxyquinolines, [Ln(ligand)(2)(mu-ligand)(2)Na] (Ln: Yb, Lu), have been prepared and their structure established by X-ray crystallography, H-1 NMR spectroscopy, and photophysical studies. The complexes display efficient ligand-sensitized near-infrared luminescence of ytterbium at 9251075 nm with lifetimes and quantum yields as high as 22 mu s and 3.7%, in the solid state, and 20,us and 2.6% in CH2Cl2 solution, respectively. These quantum yields are the highest reported to date for ytterbium complexes with organic ligands containing C-H bonds. A long-wavelength and intense intraligand charge-transfer transition (lambda(max)=446-456 nm; epsilon approximate to 1.2 x 10(4) M-1 cm(-1)) allows for the excitation of infrared luminescence with visible light up to 600 nm. Remarkable features of these complexes include (i) quantitative ligand-to-Yb-III energy transfer resulting in high overall efficiency of the ytterbium luminescence, (ii) unusually short radiative lifetime of the Yb-III ion, 706-745 mu s for solutions in CH3Cl2, calculated from the f-f absorption spectra, and 513-635 mu s estimated for solid state samples from quantum yield and lifetime data, and (iii) the unexpected large influence of second-sphere composition on the radiative lifetime of ytterbium.