Highly luminescent tris(β-diketonate)europium(III) complexes immobilized in a functionalized mesoporous silica

The mesoporous silica MCM-41 functionalized with a chelating pyrazolylpyridine ligand (MCM-41-L2) was used as a support for the immobilization of tris(beta-diketonate) complexes Ln(NTA)(3) [Ln = Eu, Gd; NTA = 1-(2-naphthoyl)-3,3,3-trifluoroacetonate]. The derivatized materials were characterized by powder X-ray diffraction, N-2 adsorption, FTIR and IT Raman spectroscopy, Eu L-3-edge X-ray absorption fine structure (XAFS), diffuse reflectance, and photoluminescence spectroscopy. The spectroscopic studies, supported by ab initio calculations, provide strong evidence that the immobilized europium(III) complex is 8-coordinate, with a local coordination environment that is similar to that for a model complex containing the ligand ethyl[3-(2-pyridyl)-1-pyrazolyl] acetate (L1). No emission from the pyrazolylpyridine ligands is observed in the room-temperature emission spectrum of MCM-41-L2/Eu, despite the fact that only about one-third are engaged in coordination with Eu3+ ions. In contrast, the pyrazolylpyridine groups in the precursor ligand-silica exhibit efficient emission. Furthermore, the radiance value measured for MCM-41-L2/Eu (0.33 mu W cm(-2)) is only about one-half of that measured for the complex Eu(NTA)(3)center dot L1 (0.73 mu W cm(-2)), even though the concentration of emitting centers in the MCM material is much lower. The results point to the existence of an unusual two-step intermolecular energy transfer between "free" and complexed ligands in MCM-41-L2/Eu, culminating in the observation of enhanced Eu3+ luminescence.