Abstract

The modelling of the local structure of sol–gel derived Eu³⁺-based organic/inorganic hybrids is reported, based on Small-Angle X-ray Scattering (SAXS), photoluminescence and mid-infrared spectroscopy. The hybrid matrix of these organically modified silicates, classed as di-ureasils and termed U(2000) and U(600), is formed by poly(oxyethylene) (POE) chains of variable length grafted to siloxane domains by means of urea cross-linkages. Europium triflate, Eu(CF₃SO₃)₃, was incorporated in the two di-ureasil matrices with compositions 400 ≥ n ≥ 10, n is the molar ratio of ether oxygens per Eu³⁺. The SAXS data for undoped hybrids (n = ∞) show the presence of a well-defined peak attributed to the existence of a liquid-like spatial correlation of siloxane rich domains embedded in the polymer matrix and located at the ends of the organic segments. The obtained siloxane particle gyration radius Rg₁ is around 5 Å (error within 10%), whereas the interparticle distance d is 25 ± 2 Å and 40 ± 2 Å, for U(600) and U(2000), respectively. For the Eu³⁺-based nanocomposites the formation of a two-level hierarchical local structure is discerned. The primary level is constituted by strongly spatially correlated siloxane particles of gyration radius Rg₁ (4–6 and 3–8 Å, errors within 5%, for U(600)_nEu(CF₃SO₃)₃, 200 ≥ n ≥ 40, and U(2000)_nEu(CF₃SO₃)₃, 400 ≥ n ≥ 40, respectively) forming large clusters of gyration radius Rg₂ (≈75 ± 10 Å). The local coordination of Eu³⁺ in both di-ureasil series is described combining the SAXS, photoluminescence and mid-infrared results. In the di-ureasils containing long polymer chains, U(2000)_nEu(CF₃SO₃)₃, the cations interact exclusively with the carbonyl oxygens atoms of the urea bridges at the siloxane–POE interface. In the hybrids containing shorter chains, U(600)_nEu(CF₃SO₃)₃ with n ranging from 200 to 60, the Eu³⁺ ions interact solely with the ether-type oxygens of the polymer chains. Nevertheless, in this latter family of hybrids a distinct Eu³⁺ local site environment involving the urea cross-linkages is detected when the europium content is increased up to n = 40.