White light emission ofEu3+-based hybrid xerogels

The luminescence spectra and extended x-ray-absorption fine-structure (EXAFS) measurements of a series of ${\mathrm{Eu}}^{3+}$-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 $U\left(2000\right)\mathrm{}$ 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, ${\mathrm{E}\mathrm{u}(\mathrm{C}\mathrm{F}}_3{\mathrm{SO}}_3{)}_3,$ and europium bromide, ${\mathrm{EuBr}}_3,$ with concentrations $200>~n>~20$ and $n=80,$ 40, and 30, respectively—where $n$ is the number of ether oxygens in the polymer chains per ${\mathrm{Eu}}^{3+}$ cation. EXAFS measurements were carried out in some of the ${U\left(2000\right)\mathrm{}}_n{\mathrm{E}\mathrm{u}(\mathrm{C}\mathrm{F}}_3{\mathrm{SO}}_3{)}_3$ xerogels ($n=200,$ 80, 60, and 40). The obtained coordination numbers $N$ ranging from 12.8, $n=200,$ to 9.7, $n=40,$ whereas the average ${\mathrm{Eu}}^{3+}$ 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 ${}^5{\overrightarrow{D_0}}^7{F}_{0–4}$ ${\mathrm{Eu}}^{3+}$ 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 ${\mathrm{Eu}}^{3+}$ 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 ${U\left(2000\right)\mathrm{}}_n{\mathrm{E}\mathrm{u}(\mathrm{C}\mathrm{F}}_3{\mathrm{SO}}_3{)}_3,$ by the ${\mathrm{SO}}_3$ 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 ${\mathrm{Eu}}^{3+}$ is calculated on the basis of the emission energy assignments. The results obtained for ${U\left(2000\right)\mathrm{}}_n{\mathrm{E}\mathrm{u}(\mathrm{C}\mathrm{F}}_3{\mathrm{SO}}_3{)}_3,$ 2.4 Å for $90>~n>~40$ and 2.6 and 2.5 Å for $n=30\mathrm{}$ 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 ${}^5{\overrightarrow{D_0}}^7{F}_0$ line, with respect to the value calculated for gaseous ${\mathrm{Eu}}^{3+},$ and the hypersensitive ratio between the ${}^5{\overrightarrow{D_0}}^7{F}_2$ and ${}^5{\overrightarrow{D_0}}^7{F}_1$ transitions, point out a rather low covalency nature of the ${\mathrm{Eu}}^{3+}$ first coordination shell in these xerogels, comparing to the case of analogous polymer electrolytes modified by europium bromide.