Vibrational Quenching in Near-Infrared Emitting Lanthanide Complexes: A Quantitative Experimental Study and Novel Insights

Two series of novel NIR-emissive complexes of Nd3+, Sm3+, Er3+ and Yb3+ with two different β-diketonate ligands (L1=4,4,4-trifluoro-1-phenyl-1,3-butadione and L2=4,4,4-trifluoro-1-(4-chlorophenyl)-1,3-butadione) are reported. The neutral triphenylphosphine oxide (tppo) ligand was used to replace coordinated water molecules in the first coordination sphere of the as-obtained [Ln(L1(2))3(H2O)2] complexes to afford water-free [Ln(L1(2))3(tppo)2] molecular species. Upon replacement of water molecules by tppo units, the NIR emission lifetimes of the Nd3+, Er3+and Sm3+complexes increase by about one order of magnitude up to values of ≈9, 8 and 113 ms while Yb3+ complexes reach intrinsic quantum yields as high as to ΦYb=6.5 %., which are remarkably high for fully hydrogenated complexes. Vibrational quenching by CH and OH oscillators has been quantitatively assessed by implementing the Förster's model of resonance energy transfer on the basis of experimental data. This study demonstrates that highly efficient NIR-emitting lanthanide complexes can be obtained with facile, cheap and accessible syntheses through a rational design.