Dual emitting [Yb(5,7ClQ) ₂(H5,7ClQ) ₂Cl]: Chemical and photophysical properties

The anhydrous hepta-coordinated [Yb(5,7ClQ) ₂(H5,7ClQ) ₂Cl] complex bearing two quinolinolato ligands in the lanthanide coordination sphere and two quinolinol ligands in the zwitterionic form as well as one chloride atom, has been synthesized and fully characterized. This compound exhibits dual luminescence in the near infrared (NIR; metal-centered) and in the visible (ligand-centered) spectral regions. Ligand emission is strongly influenced by π-π interactions in the crystal packing. Photophysical studies were applied to evaluate the quenching processes affecting the Yb ³⁺ NIR luminescence as well as to highlight the ligand-to-metal sensitization mechanism. The ligand-to-metal sensitization efficiency parameter was determined to be close to unity, thus showing that the quinolinolato ligand acts as a very efficient antenna funneling energy absorbed from UV and visible light toward Yb ³⁺. Time-resolved photoluminescence studies and transient absorption experiments clearly demonstrate that the ligand-to-metal energytransfer process occurs on an ultrafast timescale and involves the triplet states of the ligand. Very similar Yb ³⁺ emission lifetimes near 1 μm are observed for a solution in DMSO (7.85 μs) and for solid-state crystals (7.10 μs). The unusually short radiative lifetime of 690 μs, which was determined from spectroscopic data from a solution in DMSO, shows that the Yb ion in this complex has a remarkably high radiative strength. A low quantum yield of the NIR luminescence, Φ _tot=1.4×10 ^-2, was found. The deactivation rate of lanthanide ions through vibrational excitation of CH groups was assessed in the framework of the Fçrster theory of resonance energy transfer. According to this model, energy transfer to CH groups only partially explains the observed NIR luminescence quenching.