Silicon-oxygen Bonds Research Articles

Mesoporous materials incorporating a zinc(II) complex: Synthesis and direct luminescence quantum yield determination

New luminescent inorganic–organic hybrid materials incorporating the luminescent zinc(II) complex ZnL 2 ( λ em = 457 nm and Φ em = 4.4% reference values for ZnL 2; HL = chelating ligand resulting from the reaction between salicylaldehyde and 3-aminopropyltriethoxysilane), covalently bonded to different types of mesoporous silica hosts (namely MCM-41, MCM-48 and SBA-15), were prepared via both the methods of grafting post-synthesis (GPS) and one-pot synthesis (OPS). The products obtained, which form the GPS [(GPS)(Zn/MCM-41), (GPS)(Zn/MCM-48), (GPS)(Zn/SBA-15)] and the OPS [(OPS)(Zn/MCM-41), (OPS)(Zn/MCM-48), (OPS)(Zn/SBA-15)] series, contain the ZnL 2 guest covalently bonded to the silica framework through silicon–oxygen bonds formed when the silane group is placed at the periphery of the Zn(II) coordination sphere. GPS and OPS materials were characterized by powder X-ray diffraction, N 2 adsorption/desorption, thermogravimetric analysis (TGA) and UV/vis spectroscopy. For the new mesoporous materials the emission quantum yield (EQY) was measured by means of an integrating sphere combined with a spectrofluorimeter. The ZnL 2 loading (measured by the ZnL 2/SiO 2 ratio calculated from TGA data) for MCM-41 appears to be independent of the synthesis procedure, whereas, for both MCM-48 and SBA-15, the ZnL 2/SiO 2 ratio of the materials obtained via OPS is about four times higher than products obtained from GPS. The ZnL 2 loaded GPS and OPS series show λ em maxima at about 485 and 455 nm, respectively. Moreover, with reference to EQY (GPS)(Zn/SBA-15) and (OPS)(Zn/SBA-15), although featuring ZnL 2/SiO 2 ratios of 0.13 and 0.45, respectively, they showed similar EQY values: 2% and 5%. On the contrary, (GPS)(Zn/MCM-41) and (OPS)(Zn/MCM-41) which give similar ZnL 2/SiO 2 ratios (0.09 and 0.14) exhibit very different EQY, i.e. 2% and 22%, respectively.

On the biocatalytic cleavage of silicon–oxygen bonds: A substrate structural approach to investigating the cleavage of protecting group silyl ethers by serine-triad hydrolases

The biotransformation of compounds containing silicon has recently been a subject of much interest. In this study, a variety of commercially available serine hydrolases were tested for their ability to catalyse the hydrolysis of the silicon–ether bond in a variety of silyl ethers. The hydrolysis of trimethylethoxysilane in buffer was not found to be accelerated by the presence of trypsin, chymotrypsin, or a variety of other lipase and protease enzymes. Cleavage of a range of alternative silyl ether substrates, including a trimethylsilyl (TMS) ether, by these hydrolases was also not observed, but, interestingly, only two of the enzymes tested were able to cleave a t-butyl α,α,α-carboxylate that was approximately isosteric with the TMS-protected substrate. This suggests that the cleavage of Si–O bonds by serine hydrolases, such as the cathepsin homolog silicatein-α, may be in part limited by steric effects, as the reactive centre in the substrate is always, by analogy to C-centred substrates, tertiary, and thus inherently sterically demanding regardless of the putative catalytic competence of the enzymes.

Determination of descriptors for organosilicon compounds by gas chromatography and non-aqueous liquid–liquid partitioning

Measurements of retention factors by gas chromatography on up to 10 complementary stationary phases at up to 5 temperatures for each stationary phase and liquid–liquid partition coefficients in three biphasic organic solvent systems ( n-hexane-acetonitrile, n-heptane- N, N-dimethylformamide and n-heptane-2,2,2-trifluoroethanol) were used to estimate solute descriptors for 54 organosilicon compounds for use in the solvation parameter model. Many of the E descriptor values (electron lone pair interactions) are negative for simple siloxanes and silanes indicating that these compound bind electron lone pairs more tightly than n-alkanes. Silanes and siloxanes with alkyl groups have near zero dipolarity/polarizability ( S descriptor). The S descriptor is only modest for simple phenylsilanes, silazanes, silanols, orthosilicates, and alkoxides. All organosilicon compounds with silicon-oxygen bonds are reasonably strong hydrogen-bond bases ( B descriptor) but only the silanol group is a reasonably strong hydrogen-bond acid ( A descriptor). Silanes (Si H) and silazanes (Si NH Si) are weak hydrogen-bond acids. Cavity formation and dispersion interactions ( V or L descriptor) are often the main component of solvation models for siloxanes and silanes that have simple alkyl and aromatic substituents. A number of physicochemical properties (vapor pressure, aqueous solubility, biphasic partition coefficients, sorption coefficients, etc.) for linear and cyclic dimethylsiloxanes can be reliably predicted from their descriptors in established models for organic compounds.

Improvement of luminescence degradation in pure water of nanocrystalline silicon particles covered by a hydrogenated amorphous carbon layer

The stability of luminescence in pure water of surface-modified nanocrystalline silicon (nc-Si) particles has been studied. The surfaces of nc-Si particles were modified with more stable silicon-oxygen (Si-O) and silicon-carbon (Si-C) bonds by the formation of a thin native oxide layer and a hydrogenated amorphous carbon (a-C:H) layer consisting of both disordered regions and graphite bands. The peak position of luminescence in pure water of the sample with the a-C:H layer was similar to that in the sample without the a-C:H layer. Moreover, the degradation rate of the luminescence intensity of the sample with the a-C:H layer was less than that of the sample without the a-C:H layer, for immersion in pure water above 100 h. The luminescence in pure water of the a-C:H coated sample could be seen with the naked eye even after aging for 200 h. The degradation of the luminescence in pure water can be improved remarkably by the formation of Si-O and Si-C bonds with strong bond strengths on the surfaces of the nc-Si particles.

Effect of Self-Biased Nitrous Oxide Plasma on Plasma-Enhanced Chemical-Vapor-Deposited Silicon Oxide

A new plasma treatment method was proposed to improve silicon oxide films. The improvement of the films was attributed to the introduction of oxygen atoms, which were extracted from self-biased nitrous oxide plasma. The introduced oxygen atoms decreased the number of defect sites in the silicon oxide film by forming new silicon-oxygen bonds. After the proposed nitrous oxide plasma treatment, the nonstoichiometric silicon oxide film showed good quality similarly to the silicon dioxide film. However, when the self-bias was very high, extra nitrogen atoms were generated from the nitrous oxide plasma and were incorporated into the silicon oxide film. This generated many defect sites degrading the quality of the silicon oxide film.

Stress in silicon interlayers at the SiOx∕Ge interface

Materials such as germanium display an advantage relative to silicon in terms of carrier mobilities but form poor quality interfaces to oxides. By sandwiching silicon layers between a germanium substrate and the oxide, advantages of the silicon oxide/silicon (SiOx∕Si) interface can be retained combined with the advantage of a high mobility germanium substrate. Using density functional theory calculations, stress within the silicon interlayer is quantified for different interlayer thicknesses revealing that for up to three silicon layers, the stress in the interlayer is compensated for by the energy gained by forming silicon-oxygen bonds at the interface.

Asymmetry of Si-O-Si bridge bond and atom groups in silicates

Analysis of silicon-oxygen distances and bond angles in simple silicates containing asymmetric bridge bonds Si-O...Si shows that the silicates with Si: O proportions of 1: 3, 1: 2.5, and 1: 2 are built of atom groups SiO 3 2− , Si2O 5 2− , and SiO2, respectively. The atoms in groups are linked through ordinary bonds Si-O or double bonds Si=O. The atoms of neighboring groups are linked by intergroup bonds Si...O which are longer than the ordinary bonds. The valences of the silicon and oxygen atoms are saturated on account of intragroup bonds; intergroup bonds are hypervalent. The structure of silicates is an analogue of the structure of compounds with intermolecular hydrogen bonds: in the latter, the atom valences are saturated by intramolecular bonds and intermolecular bonds are also hypervalent. In both cases, intergroup (intermolecular) bonds have the Coulombic nature

Conformational rigidity of silicon-stereogenic silanes in asymmetric catalysis: A comparative study.

In recent years, cyclic silicon-stereogenic silanes were successfully employed as stereoinducers in transition metal-catalyzed asymmetric transformations as exemplified by (1) the hydrosilylation of alkenes constituting a chirality transfer from silicon to carbon and (2) the kinetic resolution of racemic mixtures of alcohols by dehydrogenative silicon-oxygen coupling. In this investigation, a cyclic and a structurally related acyclic silane with silicon-centered chirality were compared using the above-mentioned model reactions. The stereochemical outcome of these pairs of reactions was correlated with and rationalized by the current mechanistic pictures. An acyclic silicon-stereogenic silane is also capable of inducing excellent chirality transfer (ct) in a palladium-catalyzed intermolecular carbon-silicon bond formation yet silicon incorporated into a cyclic framework is required in the copper-catalyzed silicon-oxygen bond forming reaction.

A new strategy for the synthesis of taurine derivatives using the ‘safety-catch’ principle for the protection of sulfonic acids

The safety-catch principle has been applied for the development of a new method for protecting sulfonic acids. 2,2-Dimethylsuccinic acid was reduced to 2,2-dimethylbutane-1,4-diol, which was selectively silylated to give 4-(tert-butyldiphenylsilanyloxy)-2,2-dimethylbutan-1-ol. Reaction of the latter compound with 2-chloroethanesulfonyl chloride in the presence of triethylamine afforded 4-(tert-butyldiphenylsilyloxy)-2,2-dimethylbutyl ethenesulfonate directly. The ethenesulfonate underwent Michael-type addition with secondary amines to give protected derivatives of taurine (2-aminoethanesulfonic acid). Deprotection was achieved on treatment with tetrabutylammonium fluoride, whereby cleavage of the silicon-oxygen bond led to an intermediate alkoxide that immediately cyclised to 2,2-dimethyltetrahydrofuran with liberation of a sulfonate. Pure sulfonic acids were obtained from the crude product by ion exchange chromatography on a strongly basic resin, which was eluted with aqueous acetic acid. The method developed should be generally applicable to the protection of sulfonic acids and is amenable to a multiparallel format.

Effect of neutron irradiation on the optical properties and lattice dynamics of α-SiO2

The effect of reactor irradiation on the IR reflectivity spectrum of α-SiO2 single crystals is studied in a broad range of fast-neutron fluences. Data are presented on the irradiation-induced changes in the spectral characteristics of antisymmetric and degenerate vibrational modes of bridge bonds and in dynamic parameters. Critical lattice dynamics is revealed near the antisymmetric stretching and degenerate modes. The ionic polarizability and dynamic constants of the crystals are shown to be nonlinear functions of neutron fluence. The results are interpreted in terms of radiation damage, a radiation-induced phase transition, and breaking and stretching of some of the silicon-oxygen bonds. A mechanism of radiation-induced structural changes is discussed.

Spectral characteristics and force constants of irradiated α-SiO2 single crystals

The IR-radiation reflectance spectra of α-SiO2 single crystals exposed, in a reactor, to fast neutrons with fluences changing in a wide range have been investigated. The regularities of the change in spectral characteristics of the antisymmetric and degenerate vibrations of the bridge bonds of the crystals studied and in the dynamic parameters of these crystals in the process of their irradiation have been determined. The critical dynamics of the crystal lattice in the region of antisymmetric stretching and degenerate modes and the extremum radiation kinetics of the ion-polarizability parameters and the force constants of the indicated crystals were detected. The changes in some characteristics of these crystals, arising on their irradiation as a result of the damage of the substance, the rupture and extension of some silicon—oxygen bonds, and the transformation of the structure of α-SiO2 single crystals by two mechanisms, have been considered.

Optical and electronic properties of HWCVD and PECVD silicon films irradiated using excimer and Nd:Yag lasers

Thin silicon film samples were deposited using HWCVD and PECVD techniques to study the influence of laser annealing on their optical and electronic properties. Samples were annealed in air using a XeCl excimer and Nd:Yag lasers. Excimer laser annealing (ELA) at 50 to 222 mJ/cm 2 increased conductivity in PECVD films by 2 to 3 orders of magnitude and in HWCVD films by 1 to 2 orders of magnitude. ELA was also seen to decrease the optical gap in PECVD films by 0.5 eV and HWCVD films by 0.15 eV. Silicon–oxygen bond content was higher in as-deposited HWCVD films than PECVD films. Hydrogen content (at.%) in PECVD films was higher than HWCVD for higher H dilution ratios. A Nd:Yag laser 3-beam interference pattern was used to produce a periodic array of crystals in both PECVD and HWCVD films.

A Facile β-Cyclodextrin-Catalyzed Oxidative Deprotection of tert-Butyl­dimethylsilyl (TBDMS) Ethers with NBS in Water

Treatment of TBDMS ethers with NBS in the presence of β-cyclodextrin in water results, for the first time, in the cleavage of the silicon-oxygen bond, carbonyl compounds are obtained upon oxidation.

Cobalt-Mediated Regioselective Synthesis of Substituted Tetrahydroquinolines

A regioselective synthesis of polycyclic substituted ­pyridines is reported. Key step is the cobalt-catalyzed intramole­cular cyclization of diynenitriles, tethered by a silicon oxygen bond. Subsequent opening of the Si-O ring led then to the related tetra­hydroquinolines.

Radiation-Induced Phase Transition of Quartz

The effect of high-energy, high-flux neutron irradiation on the phonon spectrum and structure of α-quartz is studied using IR reflectivity, Raman scattering, and x-ray diffraction measurements. The results demonstrate that neutron irradiation to a fluence of about 7 × 1019 n/cm2 transfers low-quartz to a state similar in structure to high-quartz. The dynamics and mechanism of the radiation-induced phase transition are discussed in relation to the associated changes in silicon–oxygen bond distances and angles.

Mie-Grüneisen Analysis of the Molecular Bonding States in Silica Which Impact Time-Dependent Dielectric Breakdown

An effective molecular dipole-moment of 7-13eA is routinely observed during time-dependent dielectric breakdown (TDDB) testing of silica-based dielectrics. This rather narrow range of values is independent of oxide thickness, oxide test area, electrode materials, polarity, test temperature and oxide-growth conditions and strongly suggests a fundamental molecular property. A Mie-Gruneisen analysis of the molecular bonding states indicates that the upper-end of the effective dipole-moment range (13eA) is associated with a stretched silicon-oxygen bond while the lower-end (7eA) is consistent with a hole-captured silicon-oxygen bond. This would serve as molecular evidence that both current and field play important roles in TDDB.

Determination of the nature of molecular bonding in silica from time-dependent dielectric breakdown data

An effective molecular dipole moment of 7–13 e Å is routinely observed during time-dependent dielectric breakdown testing of silica-based dielectrics. A Mie-Grüneisen analysis of the molecular bonding states indicates that the upper end of the effective dipole moment range (13 e Å) is associated with a stretched silicon-oxygen bond while the lower end (7 e Å) is consistent with a hole-captured silicon-oxygen bond.

Control of molecular weight, stereochemistry and higher order structure of siloxane-containing polymers and their functional design

We describe the precision synthesis schemes of siloxane-containing polymers, i.e., the control of their molecular weight, stereoregularity, and higher-order structures. First, we found a new catalytic dehydrocoupling reaction of water with bis(dimethylsilyl)benzene to give poly(phenylene-disiloxane). Together with this reaction, we applied hetero-condensations to the synthesis of thermally stable poly(arylene-siloxane)s. The dehydrocoupling reaction was applied to the synthesis of syndiotactic poly(methylphenylsiloxane) and poly(silsesquioxane)s, which we also prepared by hydrolysis and deaminative condensation reactions. We discuss the tendency for loop formation to occur in the synthesis of poly(silsesquioxane) by hydrolysis, and provide comments on the design of functionality of the polymers produced. By taking advantage of the low energy barrier to rotation in the silicon-oxygen bond, we designed selective oxygen-permeable membrane materials and liquid crystalline materials. The low surface free energy of siloxane-containing systems allows surface modification of a blend film and the design of holographic grating materials.

Review: Synthetic methodologies in siloxanes

AbstractThis review describes the various types and synthetic strategies for the production of siloxanes. A description is given of the various synthetic methods for the production of organic compounds with silicon–oxygen bonds. The different types of siloxane are described, with the emphasis being on their structural aspects. Copyright © 2004 John Wiley & Sons, Ltd.

Non-stoichiometric silicon oxide deposited at low gaseous N 2O/SiH 4 ratios

Semi-insulating polycrystalline silicon films were deposited using different N 2O/SiH 4 ratios, different pressures, at 650 and 700 °C, in a low-pressure chemical vapor deposition furnace. The deposition rate of the films increased with increasing pressure, increasing temperature and decreasing N 2O/SiH 4 ratio. At higher N 2O/SiH 4 ratios, more oxygen is incorporated in the film. Oxygen rich films had lower conductivities and lower dielectric constants than silicon rich films. The annealing of the films, at only 430 °C, decreased the conductivity indicating that more silicon-oxygen bonds are formed. The films also proved to be photoconductive.