White light emission of (formula presented)-based hybrid xerogels

Abstract

The luminescence spectra and extended x-ray-absorption fine-structure (EXAFS) measurements of a series of (Formula presented)-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 (Formula presented) 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, (Formula presented) and europium bromide, (Formula presented) with concentrations (Formula presented) and (Formula presented) 40, and 30, respectively—where (Formula presented) is the number of ether oxygens in the polymer chains per (Formula presented) cation. EXAFS measurements were carried out in some of the (Formula presented) xerogels ((Formula presented) 80, 60, and 40). The obtained coordination numbers (Formula presented) ranging from 12.8, (Formula presented) to 9.7, (Formula presented) whereas the average (Formula presented) first neighbors distance (Formula presented) 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 (Formula presented) (Formula presented) 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 (Formula presented) 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 (Formula presented) by the (Formula presented) 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 (Formula presented) is calculated on the basis of the emission energy assignments. The results obtained for (Formula presented) 2.4 Å for (Formula presented) and 2.6 and 2.5 Å for (Formula presented) 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 (Formula presented) line, with respect to the value calculated for gaseous (Formula presented) and the hypersensitive ratio between the (Formula presented) and (Formula presented) transitions, point out a rather low covalency nature of the (Formula presented) first coordination shell in these xerogels, comparing to the case of analogous polymer electrolytes modified by europium bromide. © 1999 The American Physical Society.