Role of the ancillary ligand N,N-dimethylaminoethanol in the sensitization of EuIII and TbIII luminescence in dimeric β-diketonates

Two types of dimeric complexes [Ln(2)(hfa)(6)(mu(2)-O(CH2)(2)NHMe2)(2)] and [Ln(thd)(2)(mu(2),eta(2)-O(CH2)(2)NMe2)](2) (Ln = Y-III, Eu-III, Gd-III Th-III, Tm-III, Lu-III; hfa(-) = hexafluoroacetylacetonato, thd(-) = dipivaloylmethanato) are obtained by reacting [Ln(hfa)(3)(H2O)(2)] and [Ln(thd)(3)], respectively, with NN-dimethylaminoethanol in toluene and are fully characterized. X-ray single crystal analysis performed for the Th-III compounds confirms their dimeric structure. The coordination mode of NN-dimethylaminoethanol depends on the nature of the beta-diketonate. In [Tb-2(hfa)(6)(mu(2)-O(CH2)(2)NHMe2)(2)], eight-coordinate Th-III ions adopt distorted square antiprismatic coordination environments and are O-bridged by two zwitterionic N,N-dimethylaminoethanol ligands with a Tb1 center dot center dot center dot Tb2 separation of 3.684(1) angstrom. In [Tb(thd)(2)(mu 2,eta(2)-O(CH2)(2)NMe2)](2), the N,N-dimethylaminoethanol acts as chelating-bridging O,N-donor anion and the Th-III ions are seven-coordinate; the Th1 center dot center dot center dot Th1A separation amounts to 3.735(2) angstrom within centrosymmetric dimers. The dimeric complexes are thermally stable up to 180 degrees C, as shown by thermogravimetric analysis, and their volatility is sufficient for quantitative sublimation under reduced pressure. The Eu-III and Tb-III dimers display metal-centered luminescence, particularly [Eu-2(hfa)(6)(O(CH2)(2)NHMe2)(2)] (quantum yield Q(Ln)(L) = 58%) and [Tb(thd)(2)(O(CH2)(2)NMe2)](2) (32%). Consideration of energy migration paths within the dimers, based on the study of both pure and Eu-III- or Th-III-doped (0.01-0.1 mol%) Lu-III analogues, leads to the conclusion that both the beta-diketone and NN-dimethylaminoethanol ligands contribute significantly to the sensitization process of the Eu-III luminescence. The ancillary ligand increases considerably the luminescence of [Eu-2(hfa)(6)(O(CH2)(2)NHMe2)(2)], compared to [Ln(hfa)(3)(H2O)(2)], through the formation of intra-ligand states while it is detrimental to Th-III luminescence in both beta-diketonates. Thin films of the most luminescent compound [Eu-2(hfa)(6)(O(CH2)(2)NHMe2)(2)] obtained by vacuum sublimation display photophysical properties analogous to those of the solid-state sample, thus opening perspectives for applications in electroluminescent devices.