Dual role of a di-urethanesil hybrid doped with europium β-diketonate complexes containing either waterligands or a bulky chelating ligand

Abstract

In the present work we report the unusual role played by a sol-gel derived di-urethane cross-linked poly(oxyethylene) (POE)/siloxane (di-urethanesil, d-Ut(600)) hybrid matrix in the immobilization of the β-diketonate aquocomplex Eu(btfac)₃(H₂O)₂ (btfac⁻ is 4,4,4-trifluoro-1-phenyl-2,4-butanedionate) and in the quasi preservation in the hybrid of the ⁵D₀ quantum efficiency q(⁵D₀) value displayed by this complex in the isolated state (0.12 versus 0.18, respectively). We demonstrate that the d-Ut(600) framework acts as an inert (although optically active) support towards Eu(btfac)₃(H₂O)₂, enabling Eu³⁺ sensitization by the btfac⁻ ligands and energy transfer between the hybrid host excited states and the lanthanide intra-4f⁶ levels, while the number of Eu³⁺-coordinated water molecules remains constant. We also provide evidence that the incorporation of the Eu(btfac)₃phen (phen is 1,10-phenantroline) complex into the same hybrid matrix is disadvantageous from the ⁵D₀ quantum efficiency standpoint, because of the severe steric hindrance that emerges between the POE chains of the host structure and the bulky phen molecules, leading to the expulsion of these ligands from the Eu³⁺ first coordination sphere and to their replacement by two water molecules. As a consequence, embedding of Eu(btfac)₃phen in d-Ut(600) results in a significant reduction of the q(⁵D₀) value of the isolated complex (from 0.47 to 0.16). The behaviour observed in the presence of this bidentate chelating ligand correlates well with the practically identical q(⁵D₀) values and number of Eu³⁺-coordinated water molecules (ca. 2) estimated for the two hybrids.