Christine Reinhard and Hans U. Güdel
High-Resolution Optical Spectroscopy of Na₃[Ln(dpa)₃]·13H₂O with Ln = Er³⁺, Tm³⁺, Yb³⁺
Inorg. Chem. 41, 1048-1055 (2002)                       

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 ¹P-¹P* 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 ²F_5/2 and ²F_7/2 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.

Cover Picture: High-resolution optical spectroscopy was performed on the compounds Na₃[Ln(dpa)₃]·13H₂O (Ln = Er³⁺, Tm³⁺, Yb³⁺). The cover shows the molecule, 15 K crystal excitation and luminescence spectra of the Yb³⁺ complex, and a single-configurational coordinate picture, which accounts for the excitation and relaxation processes in [Yb(dpa)₃]^3-.