Ligand-Assisted Rational Design and Supramolecular Tectonics toward Highly Luminescent Eu³⁺-Containing Organic−Inorganic Hybrids

The ligands−assisted rational design of a Eu 3+ -containing organic−inorganic hybrid (di-ureasil) displaying the highest emission quantum yield (0.60 ± 0.06) reported so far is introduced. Two new lanthanide complexes were synthesized in which the metal (Eu 3+ or Gd 3+ ) first coordination sphere is formed by three β-diketonate ligands (btfa is the 4,4,4-trifluoro-l-phenyl-1,3-butanedionate ion) and two methanol (MeOH) molecules. The complexes also comprise the bpeta (1,2−bis(4−pyridyl)ethane) ligand. The Eu 3+ complex crystal structure, determined by single crystal X−ray diffraction, confirms the hydrogen bonding ability, the high conformational flexibility and the versatile binding mode of the N,N′-bidentate bpeta in the architecture of the crystals. The bpeta spacers act as bridges, promoting the formation of supramolecular dimeric [Eu(btfa) 3 (MeOH) 2 ] 2 bpeta 2 species via the establishment of highly directional and strong hydrogen−bonds between the bpeta N atoms and the OH groups of the MeOH molecules. The synthesis of the Eu 3+ -doped di-ureasils is an efficient three-step concerted process that results in the destruction of the bpeta-driven self-assembled dimeric units and the formation of a new complex in which the di-ureasil structure plays the role of ligand: (i) one or two labile MeOH molecules are released from the ion local environment and replaced by the oxygen atoms of the carbonyl groups of the urea cross-links; (ii) an increase in the degree of order of the poly(oxyethylene) (POE) chains occurs concomitantly; (iii) the bpeta ligand remains in the neighborhood of the newly formed Eu 3+ complex. The synergy between the absorption ability of the btfa and bpeta cromophores and the hybrid’s emitting centers creates additional and efficient bpeta-to-hybrid and bpeta-to-btfa transfer channels that optimize the metal sensitization process contributing for the large measured emission quantum yield.