High-Resolution Optical Spectroscopy of Na3[Ln(dpa)3]·13H2O with Ln = Er3+, Tm3+, Yb3+

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Abstract
The title compounds were synthesized and studied by solution and single-crystal absorption, luminescence, and excitation spectroscopy. The f−f luminescence is induced in the Tm3+ and Yb3+ complexes in solution by exciting into the 1Π−1Π* absorptions of the ligand in the UV. A single-configurational coordinate model is proposed to rationalize the nonradiative relaxation step from ligand-centered to metal-centered excited states in [Yb(dpa)3]3- (dpa = 2,6-pyridinedicarboxylate). Direct f−f excitation is used in crystals of Na3[Tm(dpa)3]·13H2O and Na3[Yb(dpa)3]·13H2O to induce f−f luminescence. From low-temperature, high-resolution absorption, luminescence, and excitation spectra, the ligand-field splittings in the relevant states can be determined. It was impossible to induce NIR to VIS upconversion in any of the complexes. This is mainly due to the fact that nonradiative relaxation among the f−f excited states is highly competitive, even in [Yb(dpa)3]3- with an energy gap between 2F5/2 and 2F7/2 of about 10000 cm-1. It can be rationalized on the basis of an adapted energy gap law. No luminescence at all could be detected in Na3[Er(dpa)3]·13H2O.