Spectroscopic studies of europium(III) and gadolinium(III) tris-β-diketonate complexes with diazabutadiene ligands

Article abstract of DOI:10.1002/ejic.200400191

Adducts of the type Ln(NTA)₃·L [Ln = Eu, Gd; NTA = 1-(2-naphthoyl)-3,3,3-trifluoroacetonate; L = 1,4-diaza-1,3-butadiene RN=CHCH=NR (R = p-tolyl, o-tolyl)] were prepared by treatment of Ln(NTA) ₃·2H₂O with one equivalent of the chelating N-N ligand. The mixed ligand complexes were characterised by elemental analysis, thermogravimetric analysis, IR and Raman spectroscopy, and photoluminescence (PL) spectroscopy. Ab initio calculations were carried out in order to predict coordination geometries and also to interpret the vibrational spectra. For the europium(III) compounds, new Raman-active bands at approximately 440 and 479 cm^-1 are tentatively assigned to (vEu-N)_sym and (vEu-N)_as modes, respectively. A second band at 1185 cm^-1 is associated with the symmetric stretching of the N-C(ring) and C-C (DAB) bonds, and is shifted about 20 cm^-1 to a higher wavenumber relative to the corresponding band for the free ligand. The room-temperature PL spectra of the two Eu complexes are composed of the typical Eu³⁺ red emission, assigned to transitions between the first excited state ( ⁵D₀) and the ground multiplet (⁷F _0-4). Based on the room-temperature emission spectra and lifetime measurements, the quantum efficiencies of the ⁵D₀ Eu ₃₊ excited state were estimated and found to be quite low (e.g. 2-3%). The low efficiencies are attributed to the presence of thermally activated nonradiative channels involving ligand-to-metal charge-transfer (LMCT) states. Indeed, a low-lying LMCT band was detected for the Eu complex containing the ligand p-tolyl-DAB by comparison of the room-temperature diffuse reflectance spectrum for this complex with that of the corresponding Gd analogue. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.