Antje Kriltz

Silver halide-containing photochromic powders of SiO2 sol-gel glass

In order to develop a photochromic powder, an SiO2-gel based on tetraethoxysilane (TEOS) was doped with AgNO3 or AgNO2. The dried gel pieces were pulverized and treated with aqueous or ethanolic halide solutions for growing silver halide microcrystals. Both the introduction of cuprous compounds such as CuBr, CuCl, Cu2O, Cu(CH3COO)2·H2O or sulphur-containing compounds and the formation of AgBrxCl1-x crystals lead to variations of photochromic properties. X-ray diffraction measurements indicate that the average diameters of silver halide microcrystals are limited to 70–110nm. Ultraviolet–visible spectroscopic investigations show the reversibility of the darkening/fading process. Cu+-containing samples exhibit a remarkable fading even at room temperature.

Cuprous Halide Microcrystals Containing Photochromic Sol-Gel Glass

CuBr containing photochromic sol-gel glasses were synthesized by the introduction of CuBr dissolved in CH3CN into a sol-gel of Al2O3-SiO2 type. Microcrystals of CuBr were developed by a two step heat treatment, the first step in air initially at 280°C/2 h and then at 410°C/2 h for the removal of the organics and the second step at 700°C/2 h in nitrogen atmosphere for the reduction of the copper compounds to Cu+. Photometric measurements of the small gel pieces show a distinct darkening of 24% and a remarkable fading at room temperature of 13% within 10 minutes.

Reaction of “Non-Symmetrical” 1,2,4-Trithiolanes with [(phosphane)Pt0(η2-nbe)] Complexes†

Two unsymmetrical 1,2,4-trithiolanes (1d and 1e) were reacted with [Pt(0)(PPh(3))(2)(η(2)-nbe)] (6; nbe=norborn-2-ene) and [Pt(0)(dppn)(η(2)-nbe)] (11; dppn=1,8-bis(diphenylphosphanyl)naphthalene)), respectively. Their treatment with compound 6 resulted in the formation of six-membered platinacycles of type 7, which selectively underwent fragmentation into dithiolato complexes and thiobenzophenone (4b). The isolation of the first stable C-substituted member of this class of compounds (i.e., compound 7e) permitted kinetic studies of this process by using UV/Vis spectroscopy. These results, together with DFT calculations, allowed us to propose a mechanism for the reactions of compound 6 with 1,2,4-trithiolanes. In contrast, similar treatment of compound 11 with compounds 1d and 1e at room temperature did not result in any reaction. Heating the appropriate samples to 110 °C led to the formation of dithiolato complexes and η(2)-thioketone compounds, thus pointing to a thermally induced [3+2]-cycloreversion of the heterocycles as an initial step of the reaction.

IR-ATR investigation of surface anisotropy in silicate glasses

Several samples of flat soda-lime silicate glass were investigated by the Infrared Attenuated Total Reflection (IR-ATR) spectroscopy technique. The specular reflectance spectra together with the results of the performed dispersion analysis and the generated reflectance spectra, using Fresnel equations, suggest that the samples are isotropic. In contrast, spectra recorded by the ATR technique suggest an anisotropic structure on the surface of the specimen different from that in the bulk. This is established through differences in the s- and p-polarized IR-ATR spectra, which cannot be simply transformed into one another employing Fresnel formula for an isotropic case. It appears that this thin film having a structure different from the bulk is larger than the ATR effective penetration depth of the evanescent ray for each incidence angle above the critical one. The investigation suggests C2 symmetry of the SiO4 unit.

Glassy Photochromic Layers Produced by Sol-Gel Process

Abstract Inorganic - organic composite materials on the basis of Tetraethoxysilane/3-Glycidoxypropyltrimethoxysilane containing microcrystals of AgBr, AgCl or AgBr, Cl1−x were synthesized as photochromic coatings via the sol-gel route. The bonding of halide ions to components of the matrix system, such as Bromophenyltrimethoxysilane or Chloropropyltrimethoxysilane and of Ag+-ions with complexing agents such as 18-crown-6 and Aminopropyltriethoxysilane prevents the crystallization of silver halide crystals at room temperature and leads to a controlled growth of them. In the present work results of XRD- and AFM-measurements concerning crystal growth as function of the conditions of thermal treatment and UV-VIS spectroscopic investigations of the darkening and fading behaviour of the glassy coatings are shown.

Comparison of Photochromic Glasses by Darkening and Fading Spectra

Abstract Darkening and fading spectra of different types of photochromic glasses are compared and reaction schemes are discussed.

Determination of the size-distribution of CuBr nanocrystals in a glassy matrix by means of high-temperature UV–visible absorption spectroscopy

Abstract The size-distribution of CuBr nanocrystals embedded in a glassy matrix was investigated using high-temperature UV–visible absorption spectroscopy. The investigations were carried out with a photodiode array spectrometer in the temperature range from 25°C to 350°C. The absorbance of the excitonic z1/2 and z3 peaks of the CuBr nanocrystals decreases with increasing temperature. This effect is due to a size-dependent melting of the nanocrystals. We developed an analytical method, to calculate the size-distribution of the nanocrystals from the shape of the excitonic absorbance–temperature curve. The growth of a second crystal generation is observable at longer heat treatment times. Small-angle X-ray scattering (SAXS) investigations support these results. We find three crystal generations (one larger than the detection limit of our spectroscopical method). Gaussian functions are used to fit the size-distribution curves of each of these crystal generations. The sum of the distributions exhibits an increasing asymmetry during the ripening process.

Tailoring of Photochromic Glasses

Abstract Some ways to control and to improve the properties of silverfree photochromic glasses, including darkening level, fading rate and color, are discussed.

High-Temperature UV-Vis Spectroscopic Investigations on the Second Annealing Process of Photochromic Glasses

Abstract A second annealing process of silver- or cuprous halide microcrystals containing glass changes their photochromic behaviour. The second annealing is carried out both below Tg of the glass and below the melting point of the crystals. In situ UV-VIS spectroscopic measurements in this temperature range indicate changes in excitonic peaks of CuBr or CuCl microcrystals as function of temperature, annealing time and cooling rate. Room temperature spectra of irradiated samples show a direct dependence on the conditions of the second annealing process. The decrease in the transition temperature of these small crystals below the bulk value causes partiy transitions of the low temperature modification of the crystals into the high temperature modification, that does not contribute to excitonic absorptions and darkening ability.