Enhanced near-infrared (NIR) luminescence from two structurally related heterobinuclear Na~(I)Yb~(III) eight-cooridnate and heterobinuclear Yb~(III)Na~(I) eight-coordinate (CN = 8) complexes is reported and compared to a nine-coordinate (CN = 9) homoleptic complex. For the heteroleptic complex, Yb(MPQ_(2))(acac), the Yb~(III) cation is coordinated to two tridentate 2-(5-methylpyridin-2-yl)-8-quinolinate (MPQ) anions, with a bidentate acetylacetonate (acac) anion completing the coordination sphere. Instead, the heterobinuclear NaYb(MPQ)_(4) complex comprises a total of four anionic MPQ ligands, two of which exhibit κ~(3)-coordination to the Yb~(III) cation. The remaining two MPQ anions are unidentate toward the lanthanide and form μ_(2)-bridges via the deprotonated quinolinate oxygens to a bound Na~(I) cation which is also coordinated to the remaining nitrogen donor atoms. The structural properties of these complexes were evaluated by single-crystal X-ray diffraction (SXRD), continuous shape measure (CShM) analysis, and ~(1)H NMR spectroscopy using a diamagnetic Lu~(III) analogue. The corresponding photophysical properties were examined in CH_(2)Cl_(2) solution by using absorption and emission spectroscopy. For both the complexes, characteristic Yb~(III) emission is observed at ca. 980 nm, with recorded photoluminescence quantum yields (Φ_(obs)) and NIR luminescence lifetimes (τ_(obs)) of 2.0 and 14.0 μs vs 1.5 and 11.6 μs for the NaYb(MPQ)_(4) and Yb(MPQ)_(2)(acac) complexes, respectively. Interestingly, the eight-coordinate Yb~(III) complexes both have higher photoluminescence quantum yields when compared to the homoleptic Yb(MPQ)_(3) complex, which has a reported quantum yield of 1.0 and a NIR lifetime determined herein of 13.3 μs under identical conditions. These results have been rationalized by considering the overall efficiency of the ligand-centered sensitization process (η_(sens) = Φ_(isc) × Φ_(eet)), together with subsequent radiative (k _(r)) and nonradiative (k _(nr)) deactivation of the Yb~(III) cation. Moreover, the efficiency of the intersystem crossing (Φ_(isc)) and electronic energy transfer (Φ_(eet)) processes involved in the antennae effect have been quantified for the new complexes using a combination of nanosecond and femtosecond transient absorption techniques and have been compared to our previous results using Ln(MPQ)_(3) complexes with Ln = Yb and Lu.
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