White light emission of Eu3+-based hybrid xerogels

The luminescence spectra and extended x-ray-absorption fine-structure (EXAF S) measurements of a series of Eu3+-based organic/inorganic xerogels were r eported and related to the local coordination of the lanthanide cations. Th e 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 lumi nescent centers were incorporated as europium triflate, Eu(CF3SO3)(3), and europium bromide, EuBr3, with concentrations 200 greater than or equal to n greater than or equal to 20 and n = 80, 40, and 30, respectively-where n i s the number of ether oxygens in the polymer chains per Eu3+ cation. EXAFS measurements were carried out in some of the U(2000)(n)Eu(CF3SO3)(3) xeroge ls (n = 200, 80, 60, and 40). The obtained coordination numbers N ranging f rom 12.8, n = 200, to 9.7, n = 40, whereas the average Eu3+ first neighbors distance R is 2.38-2.49 Angstrom. The emission spectra of these multiwavel ength phosphors superpose a broad green-blue band to a series of yellow-red narrow (5)Do-->F-7(0-4) Eu3+ 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 hybri d host. The local environment of Eu3+ is described as a continuous distribu tion of closely similar low-symmetry network sites. The cations are coordin ated by the carbonyl groups of the urea moieties, water molecules, and, for U(2000)(n)Eu(CF3SO3)(3), by the SO3 end groups of the triflate anions. No spectral evidences have been found for the coordination by the ether oxygen s of the polyether chains. A mean radius for the first coordination shell o f Eu3+ is calculated on the basis of the emission energy assignments. The r esults obtained for U(2000)(n)Eu(CF3SO3)(3), 2.4 Angstrom for 90 greater th an or equal to n greater than or equal to 40 and 2.6 and 2.5 Angstrom for n = 30 and 20. respectively, are in good agreement with the values calculate d from EXAFS measurements. The energy of the intraconfigurational charge-tr ansfer transitions, the redshift of the D-5(0)-->F-7(0) line, with respect to the value calculated for gaseous Eu3+, and the hypersensitive ratio betw een the D-5(0)-->F-7(2) and D-5(0)-->F-7(1) transitions, point out a rather low covalency nature of the Eu3+ first coordination shell in these xerogel s, comparing to the case of analogous polymer electrolytes modified by euro pium bromide. [S0163-1829(99)03138-0].