EuIII-Doping of Lamellar Bilayer and Amorphous Mono-Amide Cross-Linked Alkyl/Siloxane Hybrids

Two structurally different but chemically similar families of alkyl/siloxane mono-amidosil hosts, represented by m-A(x) [where x = 14 or 8 represents the number of CH2 groups of the pendant alkyl chains directly bonded to the carbonyl group of the amide cross-link] have been doped with a wide range of concentrations of Eu(CF3SO3)3. Mono-amidosils m-A(x)nEu(CF3SO3)3 with n ≥ 10 (where n is the molar ratio of carbonyl groups per Eu3+ ion) have been analyzed. The m-A(8)nEu(CF3SO3)3 mono-amidosils are transparent and amorphous films, in which the alkyl chains adopt gauche conformations. In contrast, the m-A(14)nEu(CF3SO3)3 mono-amidosils are solid powders; here the lamellar bilayer hierarchical structure of m-A(14) coexists with a new lamellar phase in which the Eu3+ ions are bonded to carbonyl oxygen atoms of the amide groups. At n = 10 the hydrogen-bonded associations formed are highly ordered and considerably stronger than those found in the less concentrated hybrids and in the nondoped matrices. “Free” and weakly coordinated triflate ions occur in all the mono-amidosil samples. The hybrids are white light emitters (maximum quantum yield: 0.08 ± 0.01), presenting a broad emission band in the blue/purplish-blue spectral region (ascribed to the hybrid host) superimposed on the 5D0→7F0–4 Eu3+ intra-4f6 transitions. Two Eu3+ local coordination sites (named A and B) have been discerned in both systems. Site A is attributed to weakly coordinated Eu3+/CF3SO3– ion pairs, whereas site B involves Eu3+ coordination to the oxygen atoms of the C=O groups, of the CF3SO3– ions and of the water molecules. For site B, the long-range order of the hybrid host induces distinct features in the energy of the 5D0→7F0–4 transitions, the 5D0 lifetime and the degree of covalency of the Eu3+–first-ligand bonds