Nanostructuration of phenylenevinylenediimide-bridged silsesquioxane: from electroluminescent molecular J-aggregates to photoresponsive polymeric H-aggregates

A new approach to control molecular aggregation of pi-conjugated chromophores in the solid state has been investigated. Our strategy was to use a modifiable bulky fragment which should induce a J-aggregation and offer the possibility to reach an H-aggregation upon its chemical modification by lateral slip of pi-conjugated molecules. The chosen fragment for that purpose was the hydrolyzable triethoxysilane function (Si(OEt)3). Our objective was to design and synthesize electroluminescent or solar cell hybrid organic-inorganic materials by the sol-gel process applied to a bifunctionalized silane. With this intention, the synthesis of the sol-gel processable phenylenevinylenediimide silsesquioxane 6 was accomplished and the study of spin-coated thin films of the pure silane precursor subjected or not to the sol-gel process has been carried out. Optical properties of 6 are consistent with the formation of J-aggregates in the solid state due to the steric hindrance introduced by the triethoxysilane units. Conversely, the spectroscopic behavior observed for the hybrid film 6F is attributed to an H-aggregation corresponding to a "card pack" orientation of the distyrylbenzeneimide chromophores in the compressed silicate network. Morevover, 6 and 6F also exhibited different electronic behaviors: light-emitting diodes exhibited high brightness with the native precursor 6 and almost no light output with the sol-gel processed silsesquioxane 6F. Photovoltaic cells showed the opposite behavior with low photocurrent generation in the precursor case and higher photocurrents with the sol-gel processed layers. These results provide a deeper understanding of the present self-assembly process that is strongly governed by the molecular packing of the oligosiloxane precursor.     

Utilization of a combination of weak hydrogen-bonding interactions and phase segregation to yield highly thermosensitive supramolecular polymers

Supramolecular polymerization, i.e., the self-assembly of polymer-like materials through the utilization of the noncovalent bond, is a developing area of research. In this paper, we report the synthesis and investigation of nucleobase-terminated (N6-anisoyl-adenine and N4-(4-tert-butylbenzoyl)cytosine) low molecular weight poly(THF) macromonomers (<2000 g mol(-1)). Even though the degree of interaction between the nucleobase derivatives is very low (<5 M(-1)) these macromonomers self-assemble in the solid state to yield materials with film and fiber-forming capability. While the mechanical properties of films of both materials show extreme temperature sensitivity, resulting in the formation of very low viscosity melts, they do behave differently, which is attributed to the nature of the self-assembly controlled by the nucleobase. A combination of FT-IR, WAXD, and rheological experiments was carried out to further investigate the nature of the self-assembly in these systems. The studies demonstrate that a combination of phase segregation between the hard nucleobase chain ends and the soft poly(THF) core combined with aromatic amide hydrogen bonding is utilized to yield the highly thermosensitive supramolecular polymeric materials. In addition, analysis of the data suggests that the rheological properties of these supramolecular materials is controlled by the disengagement rate of the nucleobase chain ends from the "hard" phase, which, if shown to be general, provides a design criteria in the development of more thermally responsive materials.     

1H MAS-NMR at High Spinning Rate: An Interesting Characterization Tool for Hybrid Materials

With increasing magnetic fields and spinning rates, it becomes possible to obtain partly resolved proton spectra in solid state materials using a simple excitation sequence. We present high resolution spectra obtained at high field (14 T) and high spinning rates (30 kHz) for two types of hybrid organic/inorganic compounds: silica matrices doped with pH indicators and mesostructured hybrid silicates templated by surfactant molecules. These first results show that proton solid state NMR at high field and spinning rate should become a tool of choice to characterize interactions between guest molecules and host matrix in sol-gel hybrid materials.     

Advances in Characterisation Methods for Sol-Gel Derived Materials: High Resolution Solid State Nuclear Magnetic Resonance

The objective of this paper is to discuss the recent advances in high resolution solid state Nuclear Magnetic Resonance (NMR) to characterise sol-gel derived materials. The first part will be focused on methods to describe M _i --O--M _j connectivities in sol-gel networks. After a brief review on techniques based on scalar J-coupling, we will focus on the use of ¹⁷O solid state NMR experiments. Then in a second part, techniques to probe through-space interactions in hybrid networks will be described, with a particular emphasis on the use of high resolution ¹H solid state NMR sequences and related two-dimensional homonuclear or heteronuclear correlations.     

Solid state hydrolysis/polycondensation of alkoxysilane: access to crystal-like silicon-based hybrid materials

Transformation of a crystallized molecular solid to a periodically organized covalent extended solid was obtained by a solvent-free process of an organosilane. Hydrolysis and further polycondensation occur in the solid state; in addition this route leads to crystal-like silicon-based hybrid materials with a higher periodicity than material prepared by sol-gel process.     

Fullerenes in Sol-Gel Materials

The family of fullerene molecules is composed of a large variety of compounds that have been synthesized following the discovery of C₆₀ in 1985. The chemistry of fullerenes, developed in these last years, has allowed designing the properties of this family of molecules for specific applications in materials science. One of the main tasks to build up solid state devices based on fullerenes is the synthesis of materials doped with a highly dispersed and homogeneous distribution of fullerenes. Many of the peculiar photophysical properties, such as the reverse saturable absorption used to obtain a solid state optical limiter, are in fact lost in the aggregates of fullerenes. Sol-gel processing allows preparing inorganic oxides and hybrid organic-inorganic materials at low temperatures and presents an interesting alternative to organic polymers to entrap molecules of the fullerene family in a solid matrix. Porous inorganic solids and aerogels are also important classes of materials that can be synthesized via sol-gel and can act as hosts of fullerenes. In the present article we have reviewed the main achievements of sol-gel processing of fullerene based nanocomposite materials.     

Ion-driven ATP pump by self-organized hybrid membrane materials

We report new hybrid organic-inorganic materials, based on macrocyclic receptors 1-3 self-organized in tubular superstructures prepared by sol-gel process. Fourier transform infrared (FTIR) and NMR spectroscopic analyses demonstrate that the self-organization by hydrogen bonding of organogel superstructures of 2 and 3 were preserved in the hybrid materials throughout the sol-gel process. The molecular arrangement of heteroditopic receptors defines a particularly attractive functional transport device for both cation (tubular macrocycles) and anion (sandwich-urea) directional-diffusion transport mechanism in the hybrid membrane material. This system has been employed successfully to design a solid dense membrane, functioning as an ion-powered adenosine triphosphate (ATP(2)(-)) pump, and illustrates how a self-organized hybrid material performs interesting and potentially useful functions.     

Selective chemisorption of carbon monoxide by organic-inorganic hybrid materials incorporating cobalt(III) corroles as sensing components

Twenty-one hybrid materials incorporating cobalt(III) corrole complexes were synthesized by a sol-gel process or by grafting the metallocorrole onto a mesostructured silica of the SBA-15 type. All the materials show an almost infinite selectivity for carbon monoxide with respect to dinitrogen and dioxygen in the low-pressure domain where the chemisorption phenomenon is predominant. This peculiar property is of prime importance for an application as a CO sensor. The selectivity slightly decreases at high pressures where nonselective physisorption phenomena mainly occur. The percentage of active sites for CO chemisorption ranges from 22 to 64 %. This low percentage may be attributable to interactions between the cobalt(III) corroles with silanol or siloxane groups remaining at the surface of the materials which prevent further coordination of the CO molecule. Notably, the most efficient materials are those prepared in the presence of a protecting ligand (pyridine) during the gelation or the grafting process. The removal of this ligand after the gelation process releases a cavity around the cobalt ion that favors the coordination of a carbon monoxide molecule. The CO adsorption properties of the SBA-15 hybrid were not affected over a period of several months thus indicating a high stability of the material. Conversely, the xerogel capacities slowly decrease owing to the evolution of the material structure.     

Porous Siloxane-Silica Hybrid Materials by Sol-Gel Processing

Porous hybrid materials have been fabricated by sol-gel processing of tetraethoxysilane (TEOS) and 1,3,5,7-tetramethyl-tetrakis(ethyltriethoxysilane)-cyclotetrasiloxane (1) in the presence of the cationic surfactant, cetyltrimethylammonium bromide (CTAB). The chemical and physical properties of these materials have been analysed by FT-IR spectroscopy, solid state ²⁹Si NMR spectroscopy, powder X-ray diffraction and nitrogen adsorption-desorption studies. FT-IR spectroscopy established that the CTAB surfactant can be extracted from a crushed gel using ethanol as a solvent. Solid state ²⁹Si NMR spectroscopy showed the presence of D, T and Q species as expected from the structure of the precursors. Broad bands observed for the D units at –18 ppm and the T units at –63 ppm suggested that the cyclotetrasiloxane was held in a rigid environment and bound to the Q species of the silica matrix derived from the TEOS. NMR spectroscopy confirmed that solvent extraction resulted in further condensation of the silica matrix. Powder X-ray diffraction indicated that the materials possess short-range order and small domain sizes, as shown by broad diffraction peaks. The condensation induced by solvent extraction led to a decrease in the lattice and domain size of the samples, generally resulting in a less ordered material. Nitrogen adsorption-desorption isotherms were typical of microporous materials with pore diameters of 18 Å and a narrow size distribution.     

Protein micropatterning on bifunctional organic-inorganic sol-gel hybrid materials

Active protein micropatterns and microarrays made by selective localization are popular candidates for medical diagnostics, such as biosensors, bioMEMS, and basic protein studies. In this paper, we present a simple fabrication process of thick (approximately 20 microm) protein micropatterning using capillary force lithography with bifunctional sol-gel hybrid materials. Because bifunctional sol-gel hybrid material can have both an amine function for linking with protein and a methacryl function for photocuring, proteins such as streptavidin can be immobilized directly on thick bifunctional sol-gel hybrid micropatterns. Another advantage of the bifunctional sol-gel hybrid materials is the high selective stability of the amine group on bifunctional sol-gel hybrid patterns. Because amine function is regularly contained in each siloxane oligomers, immobilizing sites for streptavidin are widely distributed on the surface of thick hybrid micropatterns. The micropatterning processes of active proteins using efficient bifunctional sol-gel hybrid materials will be useful for the development of future bioengineered systems because they can save several processing steps and reduce costs.     

The current state of sol-gel technology

The current state of sol-gel technology has been reviewed mainly from the standpoint of microstructures of materials which can be achieved by the sol-gel method. It has been shown that the sol-gel method makes it possible to produce a great variety of high technology materials by providing the existing substances with a significant microstructure and producing the material with a novel microstructure. There are microstructures characterized by micropores, preferential crystal orientation, inorganic-organic and inorganic-inorganic composites of hybrid nature and gradient composition. These characteristic microstructures are related to the properties and applications of the materials.     

Photo- and electroluminescence in thin films of covalently bonded azomethin-zinc/SiO2 hybrid materials

We report the design and chemical synthesis of covalently bonded azomethin-zinc/SiO(2) hybrid transparent thin films with photoluminescent and electroluminescent emissions in condensed solid state by a sol-gel approach.     

Tryptophan photooxidation promoted by new hybrid materials prepared by condensation of naphthalene imides with silicate by the sol–gel process

Three novel hybrid organic/inorganic materials were synthesized from 4-substituted (NO₂, Br, H) 1,8-naphthalene imide-N-propyltriethoxysilane by the sol–gel process. These materials were obtained as a xerogel and partially characterized. The ability to photosensitize the oxidation and degradation of tryptophan indole ring by these materials was studied through photophysical and photochemical techniques. Although the derivatives containing Br and NO₂ as substituent do not cause efficient tryptophan photodamage, the hybrid material obtained from 1,8-naphthalic anhydride is very efficient to promote tryptophan photooxidation. By using laser flash photolysis it was possible to verify the presence of naphthalene imide transient radical species. The presence of oxygen causes an increase of the yield of radical formation. These results suggest that the mechanism of photodegradation of tryptophan occurs by type I, i.e. the transient radical (TrpH⁺) formed by the direct reaction of the triplet state of the naphthalene imide moiety with tryptophan. Thus a inorganic–organic hybrid material that can be used to promote the oxidation of biomolecules was obtained.     

Hybrid organic-inorganic catalytic mesoporous materials with proton sponges as building blocks

Non-ordered organic-inorganic mesoporous hybrid materials with basic sites have been synthesized following a fluoride-catalysed sol-gel process at neutral pH and low temperatures that avoids the use of structural directing agents (SDAs). Proton sponges have been used as the organic builder of the hybrids, while the inorganic part corresponds to silica tetrahedra. The proton sponges are diamines that exhibit very high basicity and, after functionalization, have been introduced as part of the walls of the mesoporous silica by one-pot synthesis. Several hybrids with different organic loadings have been synthesized and characterized by gas adsorption, thermogravimetric and elemental analysis, solid state MAS-NMR and FTIR spectroscopy. These hybrids show high activity as base catalysts and can be recycled.     

Unusual inorganic phase formation in ultraviolet‐curable organic–inorganic hybrid films

Inorganic–organic hybrid materials were prepared with a cycloaliphatic epoxide adduct of linseed oil with tetraethylorthosilicate (TEOS) oligomers via a cationic UV‐curing process. The TEOS oligomers were prepared in the presence of water and ethanol with hydrochloric acid as a catalyst. The TEOS oligomers were characterized with ¹H and ²⁹Si NMR and matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry. Hybrid films were cured, and the dynamic mechanical and thermal properties of the hybrid films were evaluated as a function of the TEOS oligomer content. The morphology of the hybrid films was examined with atomic force microscopy, transmission electron microscopy, and small‐angle light scattering. The microscopy and dynamic mechanical data indicated that the hybrid films were heterogeneous materials with various inorganic particle sizes dispersed within the organic matrix. In addition, ²⁹Si solid‐state NMR spectroscopy was used to investigate the coupling between the silicate region and organic regions. A schematic model is proposed to address structural features of hybrid materials. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1607–1623, 2005     

Tailoring the catalytic performance of sol-gel-encapsulated tetra-n-propylammonium perruthenate (TPAP) in aerobic oxidation of alcohols

Sol-gel nanohybrid silica particles organically modified and doped with the ruthenium species tetra-n-propylammonium perruthenate (TPAP) are highly efficient catalysts for the selective oxidation of alcohols to carbonyl groups with O(2) at low pressure in toluene. The materials are easily prepared by a one-step sol-gel process, and their catalytic performance can be optimised by tailoring the conditions of their synthesis by hydrolytic co-polycondensation of tetramethoxysilane (TMOS) and alkyltrimethoxysilanes R-Si(OMe)(3) in the presence of TPAP. Eventually, heterogeneous catalysts considerably more active than the unsupported perruthenate were obtained, while also being leach-proof and recyclable. The correlation between the materials' activity, surface polarity and textural properties suggests valuable information on the chemical behaviour of sol-gel catalytic materials in oxidation catalysis; this is of interest in view of the importance of efficient solid catalysts for the selective oxidation of alcohols with O(2).     

New Microporous and Mesoporous Materials: Elucidation of Their Structures by Electron Microscopy

We have recently published three papers (P. Wagner et al., Phys. Chem. 103 (1999) 8245; S. Inagaki et al., J. Am. Chem. Soc. 121 (1999) 9611; A. Carlsson et al., J. Electron Microscopy 48 (1999) 795) that herald a new approach to structural solutions in micro- and mesoporous solid state materials. Among these materials are the first hybrid inorganic–organic mesoporous materials, synthesized using the organosilane compound 1,2-bis(trimethoxysilyl) ethane (BTME). Both organic and inorganic fragments are distributed completely uniformly at the molecular scale in the mesoporous walls. Two distinct phases with two- and three-dimensional (2d- and 3d-) hexagonal periodic pore-arrangements have been detected. We have also recently reported another new cubic hybrid phase with a decaoctahedral crystal morphology. Two new approaches for solving the structures of porous materials from either electron diffraction (ED) or high-resolution electron microscope (HREM) observations have also been developed. The former was successfully applied by combining direct methods for structural analysis of the new microporous crystal SSZ-48, which crystallizes only in very small crystals. The latter technique was applied to 3d-structural analysis of the mesoporous material MCM-48. The structure solutions, in the latter case, are obtained uniquely without preassumed models or parameterization, unlike previous reports.     

Solid polymer electrolytes. VI. Microstructure of organic–inorganic hybrid networks composed of 3‐glycidoxypropyltrimethoxysilane and LiClO4 and affected by different synthetic routes

Hybrid organic–inorganic materials derived from 3‐glycidoxypropyltrimethoxylsilane were prepared via two different synthetic routes: (1) the HCl‐catalyzed sol–gel approach of silane followed by the lithium perchlorate (LiClO₄)/HCl‐catalyzed opening of epoxide and (2) the simultaneous gelation of tin/LiClO₄‐catalyzed silane/epoxide groups. LiClO₄ catalyzed the epoxide polymerization, and its effects on the structures of these hybrid materials were studied by NMR. The structure of the inorganic side was probed by solid‐state ²⁹Si NMR spectroscopy, and the characterizations of the organic side and the chemical processes involved in the different synthetic routes were performed with solid‐state cross‐polarity/magic‐angle‐spinning ¹³C NMR. The different synthetic routes significantly affected the polymerization behaviors of the organic and inorganic sides in the presence of LiClO₄. A larger amount of LiClO₄ promoted the opening of epoxide and led to the formation of longer poly(ethylene oxide) chains via the HCl‐catalyzed sol–gel approach, whereas in the case of the tin‐catalyzed approach, the faster polymerization of the inorganic side hindered the growth of the organic network. The addition of LiClO₄ was proven to be without crystalline salt present in the hybrid networks by wide‐angle X‐ray powder diffraction. Also, the interactions between the ions and hybrid host, examined with Fourier transform infrared and ⁷Li proton‐decoupled magic‐angle‐spinning NMR, further demonstrated that extensive ion aggregation existed in these hybrid materials. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 151–161, 2004     

Characterization of sol-gel immobilized lipases

The immobilization of lipases within a chemically inert hydrophobic sol-gel support, which is prepared by polycondensation of hydrolyzed tetramethoxysilane (TMOS) and methyltrimethoxysilane (MTMS) or iso-butyltrimethoxysilane (iso-BTMS), results in heterocatalysts. The heterocatalysts so prepared showed a dramatically enhanced catalytic activity and stability as measured by the hydrolysis and transesterification of soybean oil. The lipase/sol-gel materials were characterized by nitrogen adsorption to determine their specific surface area. Solid state NMR was used to reveal the degree of cross-linking of the sol-gel materials. Scanning electron microscopy and atomic force microscopy were used to observe the morphology of the biocatalysts. Transmission electron microscopy and confocal microscopy were used to investigate the enzyme distribution within the sol-gel materials. The characterization studies showed that the most active lipase-containing sol-gel was a non-porous amorphous material with enzyme randomly distributed throughout the sol-gel material. The activity of the immobilized enzyme did not correlate to the degree of cross-linking or the specific surface area of the sol-gel materials. The highly retained activity of the immobilized enzyme was more likely attributed to the conformational changes of the enzyme during the immobilization, which result in enzyme's fixation in a more favorable conformation and to the lipophilic environment of the hybrid matrix structure which facilitates the transport of the hydrophobic substrate to the active sites.     

Design of In Vitro Bioactive Hybrid Materials from the First Generation of Amine Dendrimers as Nanobuilding Blocks

In this work, the first generation of poly(propyleneimine) dendrimers were functionalized with alkoxysilane terminal groups and subjected to one of two different sol–gel process that followed two different catalytic pathways, that is base‐ or acid‐catalyzed pathways. Thus, two series of new organic–inorganic hybrid materials were obtained in the form of monolithic pieces with differences in terms of both morphology and silanol content, which originated from the different sol–gel pathway that was followed. Moreover, calcium ions were added into the hybrid composition to promote in vitro bioactivity and phosphorous sources were used during the sol–gel step to obtain an earlier bioactive response. Characterization of these organic–inorganic hybrid materials was performed by means of thermogravimetric and elemental analyses, Fourier transform infrared spectroscopy (FTIR), solid state ¹³C, ²⁹Si and ³¹P magic‐angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, N₂‐adsorption isotherms, mercury‐intrusion porosimetry, and ζ‐potential measurements. The in vitro bioactivity of the dendritic hybrid networks was evaluated by soaking the materials in simulated body fluid and the results were explained in terms of the composition of the hybrids and the sol–gel route that was followed to prepare them.