Materials Science Forum Vols. 514-516

Abstract: The synthesis of thienyl- substituted pyrrole azo dyes and their UV-visible, solvatochromic and electrochemical properties are described. In agreement with the solvatochromic data and also with the electrochemical study the new donor-acceptor systems synthesized could have applications in NLO.

Abstract: Organically-modified silica xerogels from 3-aminopropyltrimethoxysilane (APTES) and 3-isocyanatepropyltriethoxysilane (ICPTES) have been synthesized through carboxylic acid (formic acid, acetic acid and valeric acid) solvolysis. The resulting hybrid materials have been characterized by powder X-ray diffraction, mid-infrared spectroscopy, 29Si and 13C nuclear magnetic resonance, and photoluminescence spectroscopy. The results show that urea cross-links have been formed in these hybrids. The luminescence features depend on the selected carboxylic acids. For example, comparatively to the hybrids derived from formic and acetic acid solvolysis, valeric acid shows a red-shift of the emission features.

Abstract: Photoluminescence of di-ureasil hybrids containing homogeneously dispersed Ag nanoparticles, covered with silica shells was investigated at room temperature and compared with emission of undoped hybrids. Metal-doped di-ureasils show a broad emission band in the blue spectral region similar to what has been observed in pure di-ureasils but with lower relative intensity. A broad minimum arises around 438 nm for Ag@SiO2 - containing di-ureasils which coincides with the peak of plasmon resonance band in the absorption spectrum. This behaviour can be attributed to inner filtering effect, which manifests itself as absorption by metal nanoparticles of light emitted from the matrix. Low-temperature time-resolved spectroscopy and lifetime measurements demonstrate that changes are induced in the time-scale behind the di-ureasil host emission when the nanoparticles are incorporated. In particular, the lifetime values for Ag@SiO2- doped di-ureasils are smaller than those measured for the undoped host, suggesting that energy transfer might exist between the hybrid's emitting centres (NH groups and siliceous nanodomains) and the nanoparticles.

Abstract: Two organo-bridged silsequioxanes derived from the same molecular precursor were synthesized with completely different local structure, namely long-range ordering. The photoluminescence features (emission, excitation, and time-resolved modes) were studied in the temperature range 13-300 K and compared with those of the molecular precursor. The effects of the self-assembling of the nanobuild blocks on these emission properties were discussed in terms of the magnitude of the hydrogen bonds between adjacent organic groups.

Abstract: The thermal transformation of Eu3+-doped and undoped microporous titanosilicate AM-3 has been reported. AM-3 is stable up to 600 °C and transforms into the analogue of the mineral narsarsukite at 800 °C. The narsarsukite obtained from AM-3 is purer than that prepared from the titanosilicates ETS-10, ETS-4 and AM-1, and is suitable for hosting optically-active Ln3+ ions. The materials have been studied by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDS), solid-state magic-angle spinning nuclear magnetic resonance (MAS NMR), Raman and photoluminescence pectroscopies.

Abstract: The present study is devoted to synthesis and investigation of photoluminescence in the nanosized Bi4Ti3O12 (BTO) thin films. The randomly oriented BTO thin films have been obtained by a modified hybrid sol-gel process. Bismuth nitrate and titanium (IV) butoxide were used as the starting materials. The thin films were deposited on silicon, glass and stainless steel by spin coating and crystallized by the conventional thermal annealing at temperature T=650°C. Formation of the layer perovskite-type structure of the orthorhombic symmetry was confirmed by X-ray diffraction method. Conservation of the chemical composition was investigated by energy dispersive spectroscopy. The UV luminescence has been observed in BTO thin films at room temperature. Results on luminescence excitation and emission in bismuth titanate thin films are given in the present paper.

Abstract: EPR and TEM investigations on sublimated solid Alq3, [tris(8- hydroxyquinoline)aluminum(III)] are reported. Different paramagnetic centers in microcrystalline Alq3 fractions as synthesized and annealed are observed. Their nature, thermal treatment dependence, as well as correlation with the crystal structure are analyzed. The existence of randomly oriented triplets and quartets in annealed Alq3 fractions is evidenced from the temperature dependence of X- and Q-band EPR spectra. The spin Hamiltonian parameters are given and proposals for the model structures are made.

Abstract: The UV illumination effects in KTaO3 single crystals with Cu2+ impurity ions have been studied by electron paramagnetic resonance method. Single crystals both “as-grown” and thermally treated in oxygen atmosphere were investigated. It was found that the EPR signal of the Cu2+ centers decreases in the “as-grown” crystals during UV-irradiation at 365 nm, in the wholeinvestigated temperature range (3.7- 300 K). On the contrary, for the oxidized specimens a sizeable increase of the EPR signal of Cu2+ centers (except the signal from exchange coupled pair centers) is observed under UV illumination in some temperature regions.

Abstract: Nanometric ferrihydrite, maghemite and magnetite particles formed within an organicinorganic hybrid matrix were obtained by the sol-gel process. In contrast to precipitation techniques, sol-gel process appears as suitable way to achieve size-controlled nanoscopic magnetic particles anchored in a hybrid structure. The hybrid matrix here reported, named di-ureasil, is composed of poly(oxyethylene) chains grafted to siloxane groups by means of urea cross-linkages. The formation of ferrihydrite particles was achieved incorporating iron nitrate during the sol-gel process, at low pH. The formation of maghemite takes place after the incorporation of a mixture of Fe3+ and Fe2+ ions and treatment with an ammonia solution, after the sol-gel process. Magnetite nanoparticles are formed after the incorporation of Fe2+ ions and treatment with ammonia at 80°C. The AC magnetic susceptibility shows thermal irreversibility with a blocking temperature TB≈13K and ≈25K depending on frequency for the ferrihydrite and maghemite particles, respectively. The magnetite nanoparticles are blocked at room temperature. Above the irreversibility the magnetization of ferrihydrite and maghemite follows a Langevin function modified with a linear term, as found in antiferromagnetic and ferrimagnetic particles.

Abstract: Thienyl- and bithienyl-1,3-benzothiazoles 1 and 2 were synthesised by reacting various formyl thienyl and bithienyl derivatives with o-aminobenzenethiol in moderate to excellent yields. Evaluation of the solvatochromic and fluorescence properties of these compounds was carried out. Due to their strong fluorescence and also the strong push-pull character, benzothiazole derivatives 1 and 2 can be used as potential NLO materials or as fluorescent markers.