Incorporation of PbF₂ into Heavy Metal Oxide Borate Glasses. Structural Studies by Solid State NMR

A series of heavy metal oxide (HMO) glasses with composition 26.66B₂O₃-16GeO₂-4 Bi₂O₃-(53.33-<i>x</i>)­PbO-<i>x</i>PbF₂ (0 ≤ <i>x</i> ≤ 40) were prepared and characterized with respect to their bulk (glass transition and crystallization temperatures, densities, molar volumes) and spectroscopic properties. Homogeneous glasses are formed up to <i>x</i> = 30, while crystallization of β-PbF₂ takes place at higher contents. Substitution of PbO by PbF₂ shifts the optical band gap toward higher energies, thereby extending the UV transmission window significantly toward higher frequencies. Raman and infrared absorption spectra can be interpreted in conjunction with published reference data. Using ¹¹B and ¹⁹F high-resolution solid state NMR as well as ¹¹B/¹⁹F double resonance methodologies, we develop a quantitative structural description of this material. The fraction of four-coordinate boron is found to be moderately higher compared to that in glasses with the same PbO/B₂O₃ ratios, suggesting some participation of PbF₂ in the network transformation process. This suggestion is confirmed by the ¹⁹F NMR spectra. While the majority of the fluoride ions is present as ionic fluoride, ∼20% of the fluorine inventory acts as a network modifier, resulting in the formation of four-coordinate BO_3/2F^– units. These units can be identified by ¹⁹F­{¹¹B} rotational echo double resonance and ¹¹B­{¹⁹F} cross-polarization magic angle spinning (CPMAS) data. These results provide the first unambiguous evidence of B–F bonding in a PbF₂-modified glass system. The majority of the fluoride ions are found in a lead-dominated environment. ¹⁹F–¹⁹F homonuclear dipolar second moments measured by spin echo decay spectroscopy are quantitatively consistent with a model in which these ions are randomly distributed within the network modifier subdomain consisting of PbO, Bi₂O₃, and PbF₂. This model, which implies both the features of atomic scale mixing with the network former borate species and some degree of fluoride ion clustering, is consistent with all of the experimental data obtained on these glasses

Network Structure and Rare-Earth Ion Local Environments in Fluoride Phosphate Photonic Glasses Studied by Solid-State NMR and Electron Paramagnetic Resonance Spectroscopies

A detailed structural investigation of a series of fluoride phosphate laser glasses with nominal composition 25BaF₂–25SrF₂–(30 – <i>x</i>)­Al­(PO₃)₃–<i>x</i>AlF₃–(20 – <i>z</i>)­YF₃:<i>z</i>REF₃ with <i>x</i> = 25, 20, 15 and 10, RE = Yb and Eu, and 0 ≤ <i>z</i> ≤ 1.0 has been conducted using Raman, solid-state nuclear magnetic resonance (NMR), and electron paramagnetic resonance (EPR) spectroscopies. The network structure is dominated by the preferred formation of aluminum-to-phosphorus linkages, which have been quantified by means of ²⁷Al/³¹P NMR double-resonance techniques. The fluoride ions are found in mixed Al/Y/Ba/Sr environments accommodating the luminescent dopant species as well. The local environments of the rare-earth species have been studied by pulsed EPR spectroscopy of the Yb³⁺ spin probe (<i>S</i> = ¹/₂), revealing composition-dependent echo-detected lineshapes and strong hyperfine coupling with ¹⁹F nuclei in hyperfine sublevel correlation (HYSCORE) spectra consistent with the formation of Yb³⁺–F bonds. In addition, photoluminescence spectra of Eu³⁺-doped samples reveal that the ⁵D₀ → ⁷F₂/⁵D₀ → ⁷F₁ transitions intensity ratio, the normalized phonon sideband intensities in the excitation spectra, and excited state ⁵D₀ lifetime values are systematically dependent on fluoride content. Altogether, these results indicate that the rare-earth ions are found in a mixed fluoride/phosphate environment, to which the fluoride ions make the dominant contribution. Nevertheless, even at the highest fluoride levels (<i>x</i> = 25), the data suggest residual rare-earth–phosphate coordination

Reaching Biocompatibility with Nanoclays: Eliminating the Cytotoxicity of Ir(III) Complexes

Cyclometalated Ir^III complexes are promising candidates for biomedical applications but high cytotoxicity limits their use as imaging and sensing agents. We herein introduce the use of Laponite as carrier for triplet-emitting cyclometalated Ir^III complexes. Laponite is a versatile nanoplatform because of its biocompatibility, dispersion stability and large surface area that readily adsorbs functional nonpolar and cationic molecules. These inorganic–organic hybrid nanomaterials mask cytotoxicity, show efficient cell uptake and increase luminescent properties and photostability. By camouflaging intrinsic cytotoxicity, this simple method potentially extends the palette of available imaging and sensing dyes to any metal–organic complexes, especially those that are usually cytotoxic

Structural Studies of Fluoroborate Laser Glasses by Solid State NMR and EPR Spectroscopies

The structure of glasses in the systems (100 – <i>x</i>)­B₂O₃–<i>x</i>PbF₂ (<i>x</i> = 30, 40, and 50) and 50B₂O₃–(50 – <i>x</i>)­PbO–<i>x</i>PbF₂ (<i>x</i> = 5, 10, 15, 20, 25, 30, 35, 40, and 45) has been studied by solid state NMR and EPR spectroscopies. On the basis of ¹¹B and ¹⁹F high resolution solid state NMR as well as on ¹¹B/¹⁹F double resonance results, we develop a quantitative structural description on the atomic scale. ¹⁹F NMR results indicate a systematic dependence of the fluoride speciation on PbF₂ content: At low <i>x</i>-values, F^– ions are predominantly found on BO_3/2F^– units, whereas, at higher <i>x</i>-values, fluoride tends to be sequestrated into amorphous domains rich in PbF₂. In addition, both pulsed EPR studies of Yb³⁺ doped glasses and photophysical studies of Eu³⁺ doped samples indicate a mixed fluoride/borate coordination of the rare-earth ions and the absence of nanophase segregation effects