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 (λ_max = 446−456 nm; ε ≈ 1.2 × 10⁴ 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 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.