Kazunori Matsui

Fluorescence of pyrene in sol-gel silica derived from triethoxysilane

Abstract The fluorescence of pyrene probe molecules was measured in a sol-gel reaction system of triethoxysilane (HTEOS)/tetraethoxysilane (TEOS). There was minimal change in the ratio of the intensity of the third peak to the first peak (I3/I1) for the fluorescence spectra in the starting sol-gel solutions for various mixtures of HTEOS and TEOS. On the contrary, I3/I1 in silica xerogels increased linearly as HTEOS increased. These results indicate that the solvent composition mainly determines the environmental polarity of pyrene probes in initial solutions, while the HTEOS composition determines the polarity in xerogels, and that those silica xerogels derived from HTEOS are hydropholic. The infrared spectra of these xerogels suggest that the hydrophobic character of the xerogels derived from HTEOS is largely ascribed to the reduction of SiOH groups and adsorbed water molecules on these groups.

Polyaniline/Silica Hybrid Composite Gels Prepared by the Sol-Gel Process

The preparation of polyaniline/silica hybrid gel was studied by chemical polymerization of aniline with sodium dodecyl sulfate (SDS) during the sol-gel processing. As a typical procedure, aqueous solutions of SDS, aniline, HCl, and tetramethoxysilane (TMOS) were mixed. Then ammonium peroxodisulfate (APS) in an acidic aqueous solution was added as initiator oxidant into the sol-gel systems at different sol-gel stages: (A) just after mixing (sol), (B) after gelation (wet gel), and (C) after 7 d (dried gel). As for the condition (A), several green areas and several tens of dark green spots with absorption peaks at 343, 422, and 802 nm were seen in the xerogel, indicating that emeraldine salt type polyaniline is aggregated in the silica xerogel. Polymerization of aniline little occurred in the dried gel (C), possibly because APS could not soak the silica pores and/or aniline could not polymerize in the gel. The xerogel from (B) showed a homogeneously green color with peaks at 339, 425, and 769 nm, showing that emeraldine salt type polyaniline can be uniformly dispersed in sol-gel silica.

Codoping effect of Nd3+ on luminescence properties of tris(2,2′-bipyridine)ruthenium (II) in sol–gel systems of SiO2

Abstract The luminescence spectra and lifetime of tris(2,2′-bipyridine)ruthenium (II), Ru(bpy) 3 2+ were studied in the sol–gel reaction systems of tetramethoxysilane (TMOS) codoped with Nd 3+ . The luminescence spectrum showed a dip at the wavelength corresponding to a Nd 3+ absorption band in xerogels. In addition, the decay rate of the Ru(bpy) 3 2+ luminescence increased with increasing Nd 3+ concentration. These results suggest the presence of radiative and nonradiative energy transfer processes from excited Ru(bpy) 3 2+ to Nd 3+ .

Luminescence of Silica-Pillared Clays and Gels Derived from Aminofunctional Silanes

Silica-pillared clays were prepared by reacting Na-montmorillonite aqueous suspensions with (3-aminopropyl)triethoxysilane (APTEOS). It was indicated that cubic octamers [NH3+(CH2)3SiO3/2]8 derived from APTEOS are intercalated with two different conformations of the alkyl chains. Namely, the cubic octamers are intercalated with the alkyl chains parallel to the silicate layers at low concentration of APTEOS, and then intercalated with the alkyl chains normal to the silicate layers with increase in APTEOS. Gels were prepared from the reaction of APTEOS with acetic acid and by the conventional sol-gel process using (3-aminopropyl)trimethoxysilane (APTMOS). The luminescence spectra of the gel prepared by the sol-gel process and the intercalated clays were similar to that of silica gels. The gel from APTEOS with acetic acid showed broad visible luminescence. The luminescence spectra and the structures of these materials were described.

ESR STUDIES OF UV-IRRADIATED SILICA GELS CONTAINING A FUNCTIONAL GROUP AT SILICON

Abstract Modified silica gels were prepared by the sol-gel process and studied by ESR following UV irradiation at 77 K. The gels were prepared by acidic hydrolysis of tetraethoxysilane (Si(OC2H5)4), triethoxysilane (HSi(OC2H5)3) and methyltri-ethoxysilane (CH3Si(OC2H5)3). The gels showed different ESR spectra, this being due to photolysis of residual solvent and SiH, SiCH3 and SiOC2H5 groups. The sharp signal observed in UV-irradiated gels prepared from triethoxysilane was a point of particular interest and the resonance of which was ascribed to the E′ center from its g value (2.001) and saturation properties.

Formation and Entrapment of Tris(8-hydroxyquinoline)aluminum from 8-Hydroxyquinoline in Anodic Porous Alumina

The formation and entrapment of tris(8-hydroxyquinoline)aluminum (Alq3) molecules on the surface of anodic porous alumina (APA) immersed in an ethanol solution of 8-hydroxyquinoline (HQ) were investigated by absorption, fluorescence, and Raman spectroscopies. The effects of the selected APA preparation conditions (galvanostatic or potentiostatic anodization method, anodizing current and voltage values, one- or two-step anodizing process, and sulfuric acid electrolyte concentration) on the adsorption and desorption of Alq3 species were examined. Among the listed parameters, sulfuric acid concentration was the most important factor in determining the Alq3 adsorption characteristics. The Alq3 content measured after desorption under galvanostatic conditions was 2.5 times larger than that obtained under potentiostatic ones, regardless of the adsorbed quantities. The obtained results suggest the existence of at least two types of adsorption sites on the APA surface characterized by different magnitudes of the Alq3 bonding strength. The related fluorescence spectra contained two peaks at wavelengths of 480 and 505 nm, which could be attributed to isolated Alq3 species inside nanovoids and aggregated Alq3 clusters in the pores of APA, respectively. The former species were attached to the adsorption sites with higher binding energies, whereas the latter ones were bound to the APA surface more weakly. Similar results were obtained for the Alq3 species formed from the HQ solution, which quantitatively exceeded the number of the Alq3 species adsorbed from the Alq3 solution. Alq3 molecules were formed in the HQ solution during the reaction of HQ molecules with the Al3+ ions in the oxide dissolution zone near the oxide/electrolyte interface through the cracks and the Al3+ ions adsorbed on surface of pore and cracks. In addition, it was suggested that HQ molecules could penetrate the nanovoids more easily than Alq3 species because of their smaller sizes, which resulted in higher magnitudes of the adsorption.

Reduction of Cu2+ to Cu+ in Xerogels Prepared from (3-Aminopropyl)Trimethoxysilane

Silica gels doped with Cu2+ ions were prepared from the (3-aminopropyl) trimethoxysilane (APTMOS)/tetraethoxysilane (TEOS) systems. Sols showed a broad absorption peak at 640 nm, suggesting 3–5 coordination of the aminopropyl groups to Cu2+. For gels prepared from APTMOS and dried at room temperature, the 640 nm peak decreased and a red-shifted absorption appeared below 400 nm within a few months. The luminescence spectra of the xerogels showed emission bands at 430–470 and 510 nm. The former and latter bands are ascribed to Cu+ monomer and dimer emissions, respectively. These results indicate that Cu2+ ions are reduced to Cu+. When xerogels were prepared from APTMOS/TEOS = 1 (vol/vol), the color of xerogels was blue with an absorption peak at around 670 nm, indicating no reduction of Cu2+ ions.

ESR Study of a Nitroxide Radical in Sol-Gel Glasses

A spin probe TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidinyl-1-oxy) was dissolved in a tetraethyl orthosilicate sol-gel reaction system and measured by electron spin resonance spectroscopy at 295 K. The nitrogen hyperfine coupling constant was from 1.64–1.66 mT in the sol-gel solutions. The values were sensitive to the ethanol-to-water ratio of the solutions. The hyperfine coupling constant in the xerogels was 1.70 mT, which was almost the same as that in water, indicating that the probe molecules were trapped in silica pores with water adsorbed on the silica surfaces. The motion of TEMPOL in the xerogels was considerably slower than in the sol-gel solutions. The local viscosity estimated was from 70–90 cP. The ESR spectra of TEMPOL were altered during the sol-gel process, indicating that adsorbed water on the silicas surfaces has an important role for trapping organic molecules in sol-gel glasses.

Photochromic film of 6‐nitro‐1’, 3’,3’‐trimethylspiro‐[2H‐1‐benzopyran ‐2, 2’‐indoline] prepared by plasma polymerization

Polymeric film of 6‐nitro‐1’, 3’, 3’‐trimethylspiro‐[2H‐1‐benzopyran‐2, 2’‐indoline](SP) was prepared by a plasma polymerization process and showed good surface morphology. It was found by ultraviolet‐visible and infrared absorption studies that the molecular structure of SP after polymerization revealed a structure almost identical to that of the original compound. The film showed reversible photoisomerization and, accordingly, photochromism. These photochromic properties were much better in the polymerized film than for film prepared by the evaporation process.

Blue Light Emission from Silica Films Prepared by Plasma-Enhanced Chemical Vapor Deposition Using Triethoxysilane

Film preparation was carried out by plasma-enhanced chemical vapor deposition using triethoxysilane (TES)/O2, TES/N2, and TES/Ar systems. Films deposited at 50° C showed IR bands due to silica networks and organic groups such as Si-H, Si-OEt and C=O. For the TES/O2 system, the intensities of the signals due to organic groups decreased with an increase in substrate temperature, resulting in conventional silica spectra above 200° C. The organic groups still remained at 200° C for films prepared from TES/N2, and Si-H groups were observed even at 350° C. The films exhibited blue photoluminescence at room temperature. Relative intensity of the photoluminescence was related to IR absorption intensity of the Si-H groups, suggesting that O-Si-H complexes or defects induced by Si-H may be the cause of photoluminescence.