The photoluminescence and the local structure of sol-gel derived organic-inorganic hybrids, so-called ureasils, are discussed. Their host matrix is a silica-based network to which different numbers of oxyethylene repeat unitss8.5, 15.5, and 40.5 for U(600), U(900), and U(2000), respectivelysare covalently grafted by means of urea linkages. The small-angle X-ray scattering (SAXS) results suggest a diphasic structure for the morphology of the hybrids induced by local phase separation between siliceous domains and polymeric regions. The estimated interdomain distances, ranging from 27 Å for U(600) to 59-64 Å for U (2000), indicate that the three ureasils are greatly homogeneous on the SAXS scale. The luminescence spectra show a broad light emission (2.0-4.1 eV) with a blue band at ∼2.6 eV and a purplish-blue one at ∼2.8-3.0 eV, clearly distinguished by time-resolved spectroscopy. The energies of these two components are related to the dimension of the backbone inorganic skeleton. The local structure of these amorphous siliceous regions is depicted as a planar structure that combines different proportions of six to eight silica-based chains (blue emission) with three to four organically modified Si-O environments (purplish-blue emission). The calculated coherent diffraction lengths of the siliceous domains for U(600), U(900), and U(2000)s16.6, 16.1, and 20.5 Å, respectivelyspoints to an increase of the overall disorder of the inorganic backbone as the quantity of oxyethylene chains increase from 8.5 to 40.5.

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The luminescence spectra and extended x-ray-absorption fine-structure ͑EXAFS͒ measurements of a series of Eu 3ϩ -based organic/inorganic xerogels were reported and related to the local coordination of the lanthanide cations. The hybrid matrix of these organically modified silicates, classed as U (2000) ureasils, is a siliceous network to which short organic chains containing oxyethylene units are covalently grafted by means of urea bridges. The luminescent centers were incorporated as europium triflate, Eu͑CF 3 SO 3 ͒ 3 , and europium bromide, EuBr 3 , with concentrations 200уnу20 and nϭ80, 40, and 30, respectively-where n is the number of ether oxygens in the polymer chains per Eu 3ϩ cation. EXAFS measurements were carried out in some of the U (2000) n Eu͑CF 3 SO 3 ͒ 3 xerogels ͑nϭ200, 80, 60, and 40͒. The obtained coordination numbers N ranging from 12.8, nϭ200, to 9.7, nϭ40, whereas the average Eu 3ϩ first neighbors distance R is 2.48-2.49 Å. The emission spectra of these multiwavelength phosphors superpose a broad green-blue band to a series of yellow-red narrow 5 D 0 → 7 F 0 -4 Eu 3ϩ lines and to the eye the hybrids appeared to be white, even at room temperature. The ability to tune the emission of the xerogels to colors across the chromaticity diagram is achieved by changing the excitation wavelength and the amount of salt incorporated in the hybrid host. The local environment of Eu 3ϩ is described as a continuous distribution of closely similar low-symmetry network sites. The cations are coordinated by the carbonyl groups of the urea moieties, water molecules, and, for U (2000) n Eu͑CF 3 SO 3 ͒ 3 , by the SO 3 end groups of the triflate anions. No spectral evidences have been found for the coordination by the ether oxygens of the polyether chains. A mean radius for the first coordination shell of Eu 3ϩ is calculated on the basis of the emission energy assignments. The results obtained for U (2000) n Eu͑CF 3 SO 3 ͒ 3 , 2.4 Å for 90 уnу40 and 2.6 and 2.5 Å for nϭ30 and 20, respectively, are in good agreement with the values calculated from EXAFS measurements. The energy of the intraconfigurational charge-transfer transitions, the redshift of the 5 D 0 → 7 F 0 line, with respect to the value calculated for gaseous Eu 3ϩ , and the hypersensitive ratio between the 5 D 0 → 7 F 2 and 5 D 0 → 7 F 1 transitions, point out a rather low covalency nature of the Eu 3ϩ first coordination shell in these xerogels, comparing to the case of analogous polymer electrolytes modified by europium bromide. ͓S0163-1829͑99͒03138-0͔