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(μ-ligand)2Na] (Ln: Yb, Lu), have been prepared and their structure established by X-ray crystallography, 1H NMR spectroscopy, and photophysical studies. The complexes display efficient ligand-sensitized near-infrared luminescence of ytterbium at 925-1075 nm with lifetimes and quantum yields as high as 22 μs and 3.7%, in the solid state, and 20 μs 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 ($\lambda_{max}=446-456 nm; \varepsilon \approx 1.2\times 10^4 M^{-1} cm^{-1}$) allows for the excitation of infrared luminescence with visible light up to 600 nm. Remarkable features of these complexes include (i) quantitative ligand-to-YbIII energy transfer resulting in high overall efficiency of the ytterbium luminescence, (ii) unusually short radiative lifetime of the YbIII ion, 706-745 μs for solutions in CH2Cl2, calculated from the f-f absorption spectra, and 513-635 μs 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.