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.
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