Silsesquioxane
Encyclopedia
A silsesquioxane is a compound with the empirical
chemical formula
RSiO3/2 where Si is the element silicon
, O is oxygen
and R is either hydrogen
or an alkyl, alkene
, aryl
, arylene
group.These materials can be used as a support for catalysts, and recently on pH-responsive material. Silsesquioxanes can have a cage like structure, see Figure 1, which is most commonly cubes, hexagonal prisms, octagonal prisms, decagonal and dodecagonal prisms.
, materials, catalysis
and bioengineering. The extent of this variation is largely due to the molecules themselves, which have been found to form a number of different structure types. Though the basic formula for all silsesquioxanes has been found to be RSiO3/2 with the identity of R typically being alkyl or organo-functional groups, the combined structure of these RSiO3/2 units vary depending on synthesis methods, starting materials and the catalyst used. The four most common silsesquioxane structures are cage structures in which the units form a cage of n units in a designated Tn cage, partial cage structures, seen in Figure 2, in which the aforementioned cages are formed but lack complete connection of all units in the cage, ladder structures in which two long chains composed of RSiO3/2 units are connected at regular intervals by Si-O-Si bonds, and finally random structures which include RSiO3/2 unit connections without any organized structure formation.
The high three dimensional symmetry and nanometer size makes silsesquioxanes very useful building blocks for nanocomposites. The diversity of possible functional groups along with their controlled orientation in 3-D space allows for highly tailored nanometer-by-nanometer construction in all three dimensions with these unique nanobuilding blocks. The silica core gives rigidity and thermal stability that provides mechanical and thermal properties surpassing typical organics. Combining the robust core with the functionalities of the attached organic groups can also change the physical properties of the SQs allowing for easier processing than typical ceramics. The mixture of organic and inorganic functionalities can lead to the creation of novel nanocomposite materials that exhibit properties intermediate and superior to those of traditional polymer
and ceramic
properties. Tailored SQs provide a solution where processing conditions prevent ceramic-like materials from being plausible or mechanical requirements prevent polymer-like materials from being useful. The significance of SQs can be seen from their vast application possibilities in diverse fields including aerospace, antimicrobials, photonics, microelectronics, semiconductors, cosmetics and catalysis science.
The basic silsesquioxane synthesis methods introduced by those who pioneered its formation in the silicone industry typically involved producing trichlorosilane
precursors. These precursors were often formed by the reaction of methylene chloride or hydrogen chloride
with silicon metal in the presence of a metal catalyst. The subsequent reaction used to form the silsesquioxanes were typically metal-catalyzed hydrosilylation reactions with chloro- or alkylsilanes or organometallic coupling reactions with chlorosilanes. The choice of metal catalyst is dependent on the selected R substituents to be attached (for instance, with the addition of alkyl groups larger than methyl or organo-functional groups, platinum
is used as a catalyst). Some work has been done for limited alterations of substituent addition, but in general there is to date no known method to direct or control the formation of particular substitutional isomers when introducing two or more different substituents in a silsesquioxane cage synthesis.
When characterizing silsesquioxanes the typical methods involved are x-ray diffraction, nuclear magnetic resonance
spectroscopy (proton
, carbon
and silicon
), and infrared radiation spectroscopy, though SEM
and TEM
have been used for visualization in crystal growth studies. Features of general importance when describing silsesquioxane compounds are the number of RSiO3/2 units in the cage compounds and the degree to which the silsesquioxane is condensed. In a fully condensed silsesquioxane compound, the general formula is RaSiaO(1.5a–0.5b)(OH)b with b=0, indicating that all oxygen atoms in the compound are bridging silicon atoms. A less condensed silsesquioxane compound (b>0) is indicative of the compound having less coordinated Si-O-Si connections, thus in general how condensed the compound is gives the nature of the macroscopic molecule (i.e. polymeric forms are typically highly condensed- almost fully connected networks). It is worth noting that when the cage formations of silsesquioxanes are produced, though many different sizes are possible (i.e. T8, T10, T12), the most preferred formation are the cubic T8 compounds due to the high stability of the Si4O4 rings in the cage.
Many polymeric forms of silsesquioxanes have been developed with varying molecular weights and synthesis methods. The first high molecular weight tractable polymeric silsesquioxane was a ladder type repeating unit, seen in Figure 3, polyphenylsilsequioxane, reported by Brown et al. in 1960. Brown’s findings were used as a basis for further research and synthesis variations by plethora of additional research groups investigating polyphenylsilsequioxanes. Though many alterations were made, the origin synthesis proposed by Brown involved a three step process outlined as follows: (1) the hydrolysis of phenyltrichlorosilane in a solvent with excess water to give a hydrolyzate, (2) equilibration of the hydrolyzate with potassium hydroxide at a low concentration and temperature to give the prepolymer, and (3) equilibration of the prepolymer at a high concentration and temperature to give the final polymeric form. It was found that the critical factors to increase the polymer weight were high concentration and temperature during the equilibration of the prepolymer. Another noteworthy milestone in the silsesquioxane polymeric materials is the development of soluble and stable polymethylsilsesquioxane with high molecular weights by Japan Synthetic Rubber. This polymer which, unlike its phenyl derivative, gels easily during the course of its synthesis, has found a wide range of alternative applications including cosmetics, resins, and chemical amplification resist for electron beam lithography.
Another scientific development in the field of silsequioxanes was the first synthesis of hydridosilsesquioxanes by Frye and Collins. Hydridosilsesquioxanes are a silsesquioxane type with only hydrogen substituents on the silicon, and are thus purely inorganic compounds. Initial synthesis methods involved the adding benzene solutions of trichlorosilane to a mixture of benzene, concentrated sulfuric acid, and fuming sulfuric acid to yield the T10-T16 oligomers. The T8 oligomer was also synthesized, but by the reaction of trimethylsilane with a mixture of acetic acid, cyclohexane, and hydrochloric acid. It has been found that these compounds can be converted to silica coatings for application in environmental protection, and for application as an interlayer dielectric for integrated circuits.
The bridged polysilsesquioxanes were developed originally to produce controlled porosity in structures. Bridging refers to structures where two or more –SiO(3/2) units are attached by the same organic fragment to form molecular composites. These are most readily prepared from molecular building blocks that contain two or more trifunctional silyl groups attached to non-hydrolysable silicon-carbon bonds, with typical sol-gel processing. Monomers are usually dissolved in miscible solvent of water, with hydrolysis and condensation reactions catalyzed by acid, base or fluoride. The catalyst changes the physical properties of the silsesquioxane structures. Acid catalysts give clear, brittle solids, and base catalysts give opaque solids. It was found that mesopore size is proportional with the length of the bridge.
Synthesis of organosilsesquioxane films for semiconducting devices can be summarized as follows. A trichlorosilane is added drop-wise to distilled water and some non-polar solvent such as hexanes at 0°C. Reaction left to stir for some time to allow precipitate to form, which is then filtered. Hexane is then added to the aqueous reaction medium to extract the product. This general reaction gives a basic synthesis for hydrogen silsesquioxanes. These reactions often use platinum catalysts such as chloroplatinic acid to get desired properties. Commercially available silsesquioxanes can then be modified to alkylated silsesquioxanes by the cross-metathesis of alkenes with readily available vinyl-substituted silsesquioxanes.
In order to form a low k dielectric film, copolymers of alkylsilanes are copolymerized with trichlorosilane, with properties being controlled by the ratios of each. These polymers are then separated by molecular weight, since only low molecular weight polymers can be applied by Chemical Vapor Deposition
(CVD) to a device. This is usually obtained by heating above the vapor pressure in a vacuum. There are also many other methods of applying these thin films for semiconductor devices such as spin coating, dip-coating, and spraying. The resulting material would have a molecular formula of [R-SiO1.5]x[H-SiO1.5]y with x+y being an integer between 5 and 30. The methods described for forming thin films are useful in filling in empty space in electric materials as well as giving them an even surface.
There has also been interest in applying caged silsesquioxanes to these materials. Poly(methylsilsesquioxane), as mentioned above is an example of such a species. These materials give cage structures of varying sizes that are controlled by the synthetic processing. In general hydrolyzing hydrido- or organo- trichlorosilanes forms cages. Temperatures are below room temperature, and the system is kept dilute to favor intramolecular condensations. Condensation rates have also been found to slow by hydrogen bonding solvents. In general, caged structures are formed by kinetic not thermodynamic control.
In the application of light emitting diodes, there have been many more recent advances in synthetic techniques and functionalization of cubic silsesquioxanes. One of the first precursors used in light emitting application was octadimethylsiloxysilsesquioxane, which can be prepared in yields of >90% by treating tetraethoxysilane or rice hull ash with tetramethylammonium hydroxide followed by dimethylchlorosilane. The general method of hydrolyzing organotrichlorosilanes is still effective here. Recent research is looking at the effects of phenyl(silsesquioxane) structures, which can be functionalized to have a light emitting component from the organic end, through aromatic substitution reactions. When brominated or aminated, these structures can be coupled with epoxies, aldehydes, and bromoaromatics. The main goal is to attach these silsesquioxanes to π-conjugated polymer systems. Which can be done through the same functionalization methods mentioned above. These methods can use copolymerization techniques, Grignard reagents, and different coupling strategies. There has also been research on the ability of conjugated dendrimer silsesquioxanes to behave as light emitting materials. Though, highly branched substituents tend to have π-π interactions, which hinder high luminescent quantum yield
.
It has been demonstrated by many research groups that chemically incorporating silsesquioxanes, can improve materials properties such as solubility, amorphousness, thermal and oxidative stability. This in turn leads to improved OLED device efficiencies and lifetimes. Whether the strategy involves linking the silsesquioxane cage to a polymer backbone to minimize aggregation, or linking active moieties to the rigid silsesquioxane core to form amorphous materials, it is clear that improved properties can be achieved.
, IBM
, Honeywell
, Japan Rubber Co, Hitachi
, Mayaterials, Hybrid Plastics, University of Michigan
and University of California-Irvine. These materials can be used in semiconductor
devices as both semiconducting materials, with tuned functionality resulting from the attached organic groups, or as insulators in their native forms to form spacing in layers in semiconducting devices. These materials tend to have low dielectric constants (k), which makes them good thin film insulators. As organic OLEDs, polyhedral oligomeric silsesquioxanes make up an inorganic core with peripheral organic emitters making up the peripheral of a complex. This incorporation allows for an improved stability and an enhancement in electroluminescence
properties.
The first example of a type of silsesquioxane that has found application for interlayer dielectric applications is poly(hydridosilsesquioxane), which represents a linked-cage structure, which is sold under the name Fox Flowable Oxide. These hydrogen silsesquioxanes are readily used for ceramic coatings in devices such as semiconductors and can be found not only in the linked-cage (see Figure 5), but also in the ladder form as well (see Figure 4). These compounds are often applied to an electronic device with organic solvent through evaporative techniques for thin-film coating. These devices can be difficult to prepare due to the fact that silsesquioxanes can be unstable in solvents and it is difficult to control film thickness.
Methylsilsesquioxane materials are useful as spin-on-glass (SOG) dielectrics. Bridged silsesquioxanes have been used for quantum confined nano-size semiconductors. Silsesquioxane resins have also been used for these applications because they have high dielectric strengths, low dielectric constants, high volume resistivities, and low dissipation factors, making them very suitable for electronics applications. These resins have heat and fire resistant properties, which can be used to make fiber-reinforced composites for electrical laminates.
With electronics getting smaller and smaller, the need for materials that keep these devices from short-circuiting is growing in demand. Single microchips contain thousands of interconnecting transistors that when overlapped, can cause interference problems, power dissipation and voltage issues. Silsesquioxane properties have the ability to prevent short-circuiting by acting as a rigid, insulating spacer; preventing corrosion or oxidation of metal conductors. They can level uneven topography, and fill gaps between closely spaced conductors. These films are easily applied through solvent evaporation with hydrogen silsesquioxane resins, and turned ceramic by heating the substrate in air. These films are known as interlevel dielectric (ILD) and protective overcoat films (PO).
Industrial applications of OLEDs have a limited application. Traditional OLEDs do not typically contain inorganic materials, however due to the instability of OLEDs on their own, research is being conducted to look at hybrid materials that increase the stability of these compounds. Polyhedral oligomeric silsesquioxanes have been looked at in order to form an inorganic core. These compounds give better mechanical properties and stability, with an organic matrix for good optical and electrical properties. The mechanisms of degradation in these devices is not well understood, but it is believed that material defect understanding is important for understanding the optical and electronic
properties.
The most frequently employed starting silsesquioxane for metal complex synthesis is the trisilanol derivative Cy7Si7O9(OH)3, originally reported by Brown and Vogt, which is synthesized from trichlorocyclohexylsilane, but can take several years to run to completion. This resulted in only a few incompletely condensed silsesquioxanes available in useful quantities for research. Consequently most research has been focused on the trisilanol derivative Cy7Si7O9(OH)3and its cyclopentyl-substituted analog. However recently Feher et al. have developed an acid-mediated cleavage of fully condensed silsesquioxane frameworks like Cy8Si8O12. The process results in silanediols that can further be used to create new metallasilsesquioxanes.
The general preparation for metal-silsesquioxane derivatives involves treating the parent silanol and the desired metal halide in the presence of a base like triethylamine. The product metallasilsesquioxane can frequently be readily isolated by fractional crystallization. Another route of synthesis involves first deprotonating the trisilanol group, however this has proven to be somewhat difficult. Initial attempts by Feher et al. to deprotonate trisilanols with sodium t-butoxide did so, but the products were unstable for an extended period of time. More recently it was found that deprotonated trisilanols could successfully be prepared if the right base was used. Feher et al. determined three equivalents of LiN(SiMe3)2 were effective, with the product potentially even precipitating out depending on the solvent. Aspinall et al. later succeeded in doing the same using three equivalents of n-BuLi in hexanes and further results indicate that alkali metal derivatives of deprotonated silsesquioxanes could also be prepared using alkali metalbis(trimethylsilyl) amides.
Much of modern research is focused on the synthesis of metallasilsesquioxanes that containing metals that have not been done before and the potential application of the metallasilsesquioxanes as catalysts, as it has become accepted that metallasilsesquioxanes are good silica-supported transition metal catalysts. For example Edelmann et al. successfully synthesized and analyzed the first beryllium silsesquioxane, [Cy7Si7SO12BeLi]2.2THF. The use of silsesquioxanes to make homogeneous models for heterogeneous catalysts is another area research, allowing a better understanding of the system to be reached.
, epoxidation and Diels-Alder reactions of enones. A number of metallasilsesquioxanes have been reported that can polymerize ethene, which generally contain chromium
as the metal, due to its well-known use in industry in the Phillips catalyst. The catalyst can be easily activated with trimethylaluminum and typically proceeds for several hundred to more than 3200 turnovers. Vanadium
complexes as well as Ziegler-Natta type catalysts have also been employed as ethene polymerization catalysts. The coordination of metals to the silsesquioxane framework gives electrophilic centers that are approximately as electron-withdrawing as a CF3 group, leading to increased catalytic activity. Complexes of silsesquioxane and molybdenum and tungsten have been reported as alkene metathesis catalysts. The metallasilsesquioxanes complex with tungsten
has even been recently used for ring-opening metathesis polymerization of norborene. The epoxidation of alkenes by the metallasilsesquioxanes is currently an area of high interest, with many groups focused on its development. Crocker et al. have reported catalysts for alkene epoxidation that use peroxide. Abbenhius et al. have recently reported using silsesquioxanes for the construction of polyoxometalaltes as well, which have potential for use in oxidation processes that currently use environmentally friendly oxidizing agents like O2 and H2O2. The first reported catalytic alkene epoxidation was done by a titanium
complex, epoxidizing compounds like cyclooctene and norbornene nearly quantitatively. Further research has revealed the silsesquioxane-based complexes can promote Diels-Alder reactions, as well as other Lewis acid catalyzed reactions like Oppenauer oxidation and Meerwein-Pondorf-Verley reductions. The application of silsesquioxanes as catalysts as barely begun and with easier methods for syntheses of the complexes being published further applications can be anticipated.
coatings. Such coatings have numerous applications, as there is a continually increasing demand for materials that will keep people safe and healthy. Organic ammonium salts are well-known for their antimicrobial properties, meaning their ability to kill or inhibit the growth of harmful microbes. Specifically, quaternary ammonium salts (QAS) are used as disinfectants, antiseptics, and antifoulants that kill bacteria, fungi, and algae, but are not harmful to humans and animals. Several research groups have functionalized polyhedral oligomeric silsesquioxanes with QASs. The relatively small size of the silsesquioxane molecule, 2-5 nm, allows a QAS functionalized molecule to have a charge density similar to dendrimers and thus the antimicrobial efficacy is prominent.
Chojnowski et al. explored the quaternization of dimethyl-n-octylamine by octa(3-chloropropylsilsesquioxane), (T-ClPr)8, and transformed almost all eight chloropropyl groups into ionic quaternary ammonium chloride functions. The synthesis is based on a three-step hydrolytic polycondensation process of 3-chlo- ropropyltrimethoxysilane. The resulting material exhibited antimicrobial efficacy for the prevention of growth of both Gram-positive
and Gram-negative bacteria.
More recently, Majumdar et al. synthesized an array of QAS functionalized polyhedral oligomeric silsesquioxanes (Q-POSS). These researchers varied the alkyl chain length from –C12H25 to –C18H37 and varied the counter ion between chloride, bromide, and iodine. The first reaction was the hydrosilylation between allydimethlamine and octasilane polyhedral oliomeric silsesquioxane via Karstedt’s catalyst to make a tertiaryamino-functinoalized silsesquioxane. The second step was the quaternization of the tertiaryamino groups with an alkyl halide. The alkyl halides used were 1-iodooctadecane, 1-bromohexadecane, and 1-chloroctadecane.
Empirical formula
In chemistry, the empirical formula of a chemical compound is the simplest positive integer ratio of atoms of each element present in a compound. An empirical formula makes no reference to isomerism, structure, or absolute number of atoms. The empirical formula is used as standard for most ionic...
chemical formula
Chemical formula
A chemical formula or molecular formula is a way of expressing information about the atoms that constitute a particular chemical compound....
RSiO3/2 where Si is the element silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
, O is oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
and R is either hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
or an alkyl, alkene
Alkene
In organic chemistry, an alkene, olefin, or olefine is an unsaturated chemical compound containing at least one carbon-to-carbon double bond...
, aryl
Aryl
In the context of organic molecules, aryl refers to any functional group or substituent derived from an aromatic ring, be it phenyl, naphthyl, thienyl, indolyl, etc....
, arylene
Arylene
An arylene or arenediyl is a substituent of an organic compound that is derived from an aromatic hydrocarbon that has had a hydrogen atom removed from two ring carbon atoms, such as phenylene....
group.These materials can be used as a support for catalysts, and recently on pH-responsive material. Silsesquioxanes can have a cage like structure, see Figure 1, which is most commonly cubes, hexagonal prisms, octagonal prisms, decagonal and dodecagonal prisms.
Background
Since their initial discovery, silsesquioxanes have been a frequent and productive topic of research that has become interwoven into many fields of science, including energyEnergy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
, materials, catalysis
Catalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
and bioengineering. The extent of this variation is largely due to the molecules themselves, which have been found to form a number of different structure types. Though the basic formula for all silsesquioxanes has been found to be RSiO3/2 with the identity of R typically being alkyl or organo-functional groups, the combined structure of these RSiO3/2 units vary depending on synthesis methods, starting materials and the catalyst used. The four most common silsesquioxane structures are cage structures in which the units form a cage of n units in a designated Tn cage, partial cage structures, seen in Figure 2, in which the aforementioned cages are formed but lack complete connection of all units in the cage, ladder structures in which two long chains composed of RSiO3/2 units are connected at regular intervals by Si-O-Si bonds, and finally random structures which include RSiO3/2 unit connections without any organized structure formation.
The high three dimensional symmetry and nanometer size makes silsesquioxanes very useful building blocks for nanocomposites. The diversity of possible functional groups along with their controlled orientation in 3-D space allows for highly tailored nanometer-by-nanometer construction in all three dimensions with these unique nanobuilding blocks. The silica core gives rigidity and thermal stability that provides mechanical and thermal properties surpassing typical organics. Combining the robust core with the functionalities of the attached organic groups can also change the physical properties of the SQs allowing for easier processing than typical ceramics. The mixture of organic and inorganic functionalities can lead to the creation of novel nanocomposite materials that exhibit properties intermediate and superior to those of traditional polymer
Polymer
A polymer is a large molecule composed of repeating structural units. These subunits are typically connected by covalent chemical bonds...
and ceramic
Ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...
properties. Tailored SQs provide a solution where processing conditions prevent ceramic-like materials from being plausible or mechanical requirements prevent polymer-like materials from being useful. The significance of SQs can be seen from their vast application possibilities in diverse fields including aerospace, antimicrobials, photonics, microelectronics, semiconductors, cosmetics and catalysis science.
Chemical Structure and Synthesis
The structure of silsesquioxanes depends on the preparation method. To simplify, a silicon with a hydrolytically stable organic substituent and three easily hydrolyzed groups such as chlorine or alkoxy groups, which are reacted with water and an acid or base catalyst. The final structure depends on the function of the concentration of initial monomer, concentration of water, temperature, type of catalyst, and the nature of the non-hydrolyzing substituent. This can be seen in the following equations. Solvent hydrogen bonding can have a large effect on rates and types of molecular condensations.The basic silsesquioxane synthesis methods introduced by those who pioneered its formation in the silicone industry typically involved producing trichlorosilane
Trichlorosilane
Trichlorosilane is a chemical compound containing silicon, hydrogen, and chlorine. At high temperatures, it decomposes to produce silicon, and therefore purified trichlorosilane is the principal source of ultrapure silicon in the semiconductor industry. In water, it rapidly decomposes to produce...
precursors. These precursors were often formed by the reaction of methylene chloride or hydrogen chloride
Hydrogen chloride
The compound hydrogen chloride has the formula HCl. At room temperature, it is a colorless gas, which forms white fumes of hydrochloric acid upon contact with atmospheric humidity. Hydrogen chloride gas and hydrochloric acid are important in technology and industry...
with silicon metal in the presence of a metal catalyst. The subsequent reaction used to form the silsesquioxanes were typically metal-catalyzed hydrosilylation reactions with chloro- or alkylsilanes or organometallic coupling reactions with chlorosilanes. The choice of metal catalyst is dependent on the selected R substituents to be attached (for instance, with the addition of alkyl groups larger than methyl or organo-functional groups, platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
is used as a catalyst). Some work has been done for limited alterations of substituent addition, but in general there is to date no known method to direct or control the formation of particular substitutional isomers when introducing two or more different substituents in a silsesquioxane cage synthesis.
When characterizing silsesquioxanes the typical methods involved are x-ray diffraction, nuclear magnetic resonance
Nuclear magnetic resonance
Nuclear magnetic resonance is a physical phenomenon in which magnetic nuclei in a magnetic field absorb and re-emit electromagnetic radiation...
spectroscopy (proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
, carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
and silicon
Silicon
Silicon is a chemical element with the symbol Si and atomic number 14. A tetravalent metalloid, it is less reactive than its chemical analog carbon, the nonmetal directly above it in the periodic table, but more reactive than germanium, the metalloid directly below it in the table...
), and infrared radiation spectroscopy, though SEM
Scanning electron microscope
A scanning electron microscope is a type of electron microscope that images a sample by scanning it with a high-energy beam of electrons in a raster scan pattern...
and TEM
Transmission electron microscopy
Transmission electron microscopy is a microscopy technique whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through...
have been used for visualization in crystal growth studies. Features of general importance when describing silsesquioxane compounds are the number of RSiO3/2 units in the cage compounds and the degree to which the silsesquioxane is condensed. In a fully condensed silsesquioxane compound, the general formula is RaSiaO(1.5a–0.5b)(OH)b with b=0, indicating that all oxygen atoms in the compound are bridging silicon atoms. A less condensed silsesquioxane compound (b>0) is indicative of the compound having less coordinated Si-O-Si connections, thus in general how condensed the compound is gives the nature of the macroscopic molecule (i.e. polymeric forms are typically highly condensed- almost fully connected networks). It is worth noting that when the cage formations of silsesquioxanes are produced, though many different sizes are possible (i.e. T8, T10, T12), the most preferred formation are the cubic T8 compounds due to the high stability of the Si4O4 rings in the cage.
Many polymeric forms of silsesquioxanes have been developed with varying molecular weights and synthesis methods. The first high molecular weight tractable polymeric silsesquioxane was a ladder type repeating unit, seen in Figure 3, polyphenylsilsequioxane, reported by Brown et al. in 1960. Brown’s findings were used as a basis for further research and synthesis variations by plethora of additional research groups investigating polyphenylsilsequioxanes. Though many alterations were made, the origin synthesis proposed by Brown involved a three step process outlined as follows: (1) the hydrolysis of phenyltrichlorosilane in a solvent with excess water to give a hydrolyzate, (2) equilibration of the hydrolyzate with potassium hydroxide at a low concentration and temperature to give the prepolymer, and (3) equilibration of the prepolymer at a high concentration and temperature to give the final polymeric form. It was found that the critical factors to increase the polymer weight were high concentration and temperature during the equilibration of the prepolymer. Another noteworthy milestone in the silsesquioxane polymeric materials is the development of soluble and stable polymethylsilsesquioxane with high molecular weights by Japan Synthetic Rubber. This polymer which, unlike its phenyl derivative, gels easily during the course of its synthesis, has found a wide range of alternative applications including cosmetics, resins, and chemical amplification resist for electron beam lithography.
Another scientific development in the field of silsequioxanes was the first synthesis of hydridosilsesquioxanes by Frye and Collins. Hydridosilsesquioxanes are a silsesquioxane type with only hydrogen substituents on the silicon, and are thus purely inorganic compounds. Initial synthesis methods involved the adding benzene solutions of trichlorosilane to a mixture of benzene, concentrated sulfuric acid, and fuming sulfuric acid to yield the T10-T16 oligomers. The T8 oligomer was also synthesized, but by the reaction of trimethylsilane with a mixture of acetic acid, cyclohexane, and hydrochloric acid. It has been found that these compounds can be converted to silica coatings for application in environmental protection, and for application as an interlayer dielectric for integrated circuits.
Silsesquioxanes for Electronics
The synthesis of silsesquioxane materials for electronics applications can be quite detailed, with many variations occurring through the desired shapes of these structures as well as the different organic groups attached to these structures.The bridged polysilsesquioxanes were developed originally to produce controlled porosity in structures. Bridging refers to structures where two or more –SiO(3/2) units are attached by the same organic fragment to form molecular composites. These are most readily prepared from molecular building blocks that contain two or more trifunctional silyl groups attached to non-hydrolysable silicon-carbon bonds, with typical sol-gel processing. Monomers are usually dissolved in miscible solvent of water, with hydrolysis and condensation reactions catalyzed by acid, base or fluoride. The catalyst changes the physical properties of the silsesquioxane structures. Acid catalysts give clear, brittle solids, and base catalysts give opaque solids. It was found that mesopore size is proportional with the length of the bridge.
Synthesis of organosilsesquioxane films for semiconducting devices can be summarized as follows. A trichlorosilane is added drop-wise to distilled water and some non-polar solvent such as hexanes at 0°C. Reaction left to stir for some time to allow precipitate to form, which is then filtered. Hexane is then added to the aqueous reaction medium to extract the product. This general reaction gives a basic synthesis for hydrogen silsesquioxanes. These reactions often use platinum catalysts such as chloroplatinic acid to get desired properties. Commercially available silsesquioxanes can then be modified to alkylated silsesquioxanes by the cross-metathesis of alkenes with readily available vinyl-substituted silsesquioxanes.
In order to form a low k dielectric film, copolymers of alkylsilanes are copolymerized with trichlorosilane, with properties being controlled by the ratios of each. These polymers are then separated by molecular weight, since only low molecular weight polymers can be applied by Chemical Vapor Deposition
Chemical vapor deposition
Chemical vapor deposition is a chemical process used to produce high-purity, high-performance solid materials. The process is often used in the semiconductor industry to produce thin films. In a typical CVD process, the wafer is exposed to one or more volatile precursors, which react and/or...
(CVD) to a device. This is usually obtained by heating above the vapor pressure in a vacuum. There are also many other methods of applying these thin films for semiconductor devices such as spin coating, dip-coating, and spraying. The resulting material would have a molecular formula of [R-SiO1.5]x[H-SiO1.5]y with x+y being an integer between 5 and 30. The methods described for forming thin films are useful in filling in empty space in electric materials as well as giving them an even surface.
There has also been interest in applying caged silsesquioxanes to these materials. Poly(methylsilsesquioxane), as mentioned above is an example of such a species. These materials give cage structures of varying sizes that are controlled by the synthetic processing. In general hydrolyzing hydrido- or organo- trichlorosilanes forms cages. Temperatures are below room temperature, and the system is kept dilute to favor intramolecular condensations. Condensation rates have also been found to slow by hydrogen bonding solvents. In general, caged structures are formed by kinetic not thermodynamic control.
In the application of light emitting diodes, there have been many more recent advances in synthetic techniques and functionalization of cubic silsesquioxanes. One of the first precursors used in light emitting application was octadimethylsiloxysilsesquioxane, which can be prepared in yields of >90% by treating tetraethoxysilane or rice hull ash with tetramethylammonium hydroxide followed by dimethylchlorosilane. The general method of hydrolyzing organotrichlorosilanes is still effective here. Recent research is looking at the effects of phenyl(silsesquioxane) structures, which can be functionalized to have a light emitting component from the organic end, through aromatic substitution reactions. When brominated or aminated, these structures can be coupled with epoxies, aldehydes, and bromoaromatics. The main goal is to attach these silsesquioxanes to π-conjugated polymer systems. Which can be done through the same functionalization methods mentioned above. These methods can use copolymerization techniques, Grignard reagents, and different coupling strategies. There has also been research on the ability of conjugated dendrimer silsesquioxanes to behave as light emitting materials. Though, highly branched substituents tend to have π-π interactions, which hinder high luminescent quantum yield
Quantum yield
The quantum yield of a radiation-induced process is the number of times that a defined event occurs per photon absorbed by the system. The "event" may represent a chemical reaction, for example the decomposition of a reactant molecule:...
.
It has been demonstrated by many research groups that chemically incorporating silsesquioxanes, can improve materials properties such as solubility, amorphousness, thermal and oxidative stability. This in turn leads to improved OLED device efficiencies and lifetimes. Whether the strategy involves linking the silsesquioxane cage to a polymer backbone to minimize aggregation, or linking active moieties to the rigid silsesquioxane core to form amorphous materials, it is clear that improved properties can be achieved.
Applications for Electronics
Extensive research on silsesquioxanes as semiconductors, insulators and organic light emitting diodes (OLED) has been done by many companies and Universities, including Dow CorningDow Corning
Dow Corning is a multinational corporation headquartered in Midland, Michigan, USA. Dow Corning specializes in silicon and silicone-based technology, offering more than 7,000 products and services...
, IBM
IBM
International Business Machines Corporation or IBM is an American multinational technology and consulting corporation headquartered in Armonk, New York, United States. IBM manufactures and sells computer hardware and software, and it offers infrastructure, hosting and consulting services in areas...
, Honeywell
Honeywell
Honeywell International, Inc. is a major conglomerate company that produces a variety of consumer products, engineering services, and aerospace systems for a wide variety of customers, from private consumers to major corporations and governments....
, Japan Rubber Co, Hitachi
Hitachi
Hitachi is a multinational corporation specializing in high-technology.Hitachi may also refer to:*Hitachi, Ibaraki, Japan*Hitachi province, former province of Japan*Prince Hitachi and Princess Hitachi, members of the Japanese imperial family...
, Mayaterials, Hybrid Plastics, University of Michigan
University of Michigan
The University of Michigan is a public research university located in Ann Arbor, Michigan in the United States. It is the state's oldest university and the flagship campus of the University of Michigan...
and University of California-Irvine. These materials can be used in semiconductor
Semiconductor
A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...
devices as both semiconducting materials, with tuned functionality resulting from the attached organic groups, or as insulators in their native forms to form spacing in layers in semiconducting devices. These materials tend to have low dielectric constants (k), which makes them good thin film insulators. As organic OLEDs, polyhedral oligomeric silsesquioxanes make up an inorganic core with peripheral organic emitters making up the peripheral of a complex. This incorporation allows for an improved stability and an enhancement in electroluminescence
Electroluminescence
Electroluminescence is an optical phenomenon and electrical phenomenon in which a material emits light in response to the passage of an electric current or to a strong electric field...
properties.
The first example of a type of silsesquioxane that has found application for interlayer dielectric applications is poly(hydridosilsesquioxane), which represents a linked-cage structure, which is sold under the name Fox Flowable Oxide. These hydrogen silsesquioxanes are readily used for ceramic coatings in devices such as semiconductors and can be found not only in the linked-cage (see Figure 5), but also in the ladder form as well (see Figure 4). These compounds are often applied to an electronic device with organic solvent through evaporative techniques for thin-film coating. These devices can be difficult to prepare due to the fact that silsesquioxanes can be unstable in solvents and it is difficult to control film thickness.
Methylsilsesquioxane materials are useful as spin-on-glass (SOG) dielectrics. Bridged silsesquioxanes have been used for quantum confined nano-size semiconductors. Silsesquioxane resins have also been used for these applications because they have high dielectric strengths, low dielectric constants, high volume resistivities, and low dissipation factors, making them very suitable for electronics applications. These resins have heat and fire resistant properties, which can be used to make fiber-reinforced composites for electrical laminates.
With electronics getting smaller and smaller, the need for materials that keep these devices from short-circuiting is growing in demand. Single microchips contain thousands of interconnecting transistors that when overlapped, can cause interference problems, power dissipation and voltage issues. Silsesquioxane properties have the ability to prevent short-circuiting by acting as a rigid, insulating spacer; preventing corrosion or oxidation of metal conductors. They can level uneven topography, and fill gaps between closely spaced conductors. These films are easily applied through solvent evaporation with hydrogen silsesquioxane resins, and turned ceramic by heating the substrate in air. These films are known as interlevel dielectric (ILD) and protective overcoat films (PO).
Industrial applications of OLEDs have a limited application. Traditional OLEDs do not typically contain inorganic materials, however due to the instability of OLEDs on their own, research is being conducted to look at hybrid materials that increase the stability of these compounds. Polyhedral oligomeric silsesquioxanes have been looked at in order to form an inorganic core. These compounds give better mechanical properties and stability, with an organic matrix for good optical and electrical properties. The mechanisms of degradation in these devices is not well understood, but it is believed that material defect understanding is important for understanding the optical and electronic
Electron configuration
In atomic physics and quantum chemistry, electron configuration is the arrangement of electrons of an atom, a molecule, or other physical structure...
properties.
Silsesquioxane Catalysis
Another area involving silsesquioxanes that has experienced increased scientific development recently is the study of metal coordination of silsesquioxanes, resulting in metallasilsesquioxanes, which have found application as catalysts. Incompletely condensed silsesquioxanes like Cy7Si7O9(OH)3 are similar in structure to β-tridymite and β-cristobalite, making them good models for the silanol sites on silica surfaces. The structures of silsesquioxanes make them ideal for metal coordination as well, due to the fixed orientation of the silanol groups and also the siloxane bridges which can interact with the metal. Additionally the silsesquioxane can be modified for even better metal coordination via simply silylation reactions. Research has shown that silsesquioxanes can bind with numerous main group and transition group metals, including Na, Li, and Be.The most frequently employed starting silsesquioxane for metal complex synthesis is the trisilanol derivative Cy7Si7O9(OH)3, originally reported by Brown and Vogt, which is synthesized from trichlorocyclohexylsilane, but can take several years to run to completion. This resulted in only a few incompletely condensed silsesquioxanes available in useful quantities for research. Consequently most research has been focused on the trisilanol derivative Cy7Si7O9(OH)3and its cyclopentyl-substituted analog. However recently Feher et al. have developed an acid-mediated cleavage of fully condensed silsesquioxane frameworks like Cy8Si8O12. The process results in silanediols that can further be used to create new metallasilsesquioxanes.
The general preparation for metal-silsesquioxane derivatives involves treating the parent silanol and the desired metal halide in the presence of a base like triethylamine. The product metallasilsesquioxane can frequently be readily isolated by fractional crystallization. Another route of synthesis involves first deprotonating the trisilanol group, however this has proven to be somewhat difficult. Initial attempts by Feher et al. to deprotonate trisilanols with sodium t-butoxide did so, but the products were unstable for an extended period of time. More recently it was found that deprotonated trisilanols could successfully be prepared if the right base was used. Feher et al. determined three equivalents of LiN(SiMe3)2 were effective, with the product potentially even precipitating out depending on the solvent. Aspinall et al. later succeeded in doing the same using three equivalents of n-BuLi in hexanes and further results indicate that alkali metal derivatives of deprotonated silsesquioxanes could also be prepared using alkali metalbis(trimethylsilyl) amides.
Much of modern research is focused on the synthesis of metallasilsesquioxanes that containing metals that have not been done before and the potential application of the metallasilsesquioxanes as catalysts, as it has become accepted that metallasilsesquioxanes are good silica-supported transition metal catalysts. For example Edelmann et al. successfully synthesized and analyzed the first beryllium silsesquioxane, [Cy7Si7SO12BeLi]2.2THF. The use of silsesquioxanes to make homogeneous models for heterogeneous catalysts is another area research, allowing a better understanding of the system to be reached.
Applications in Catalysis
Metallasilsesquioxanes have found wide use as catalysts for both homogeneous and heterogeneous systems. The complexes have found application as catalysts for alkene metathesis, polymerizationPolymerization
In polymer chemistry, polymerization is a process of reacting monomer molecules together in a chemical reaction to form three-dimensional networks or polymer chains...
, epoxidation and Diels-Alder reactions of enones. A number of metallasilsesquioxanes have been reported that can polymerize ethene, which generally contain chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
as the metal, due to its well-known use in industry in the Phillips catalyst. The catalyst can be easily activated with trimethylaluminum and typically proceeds for several hundred to more than 3200 turnovers. Vanadium
Vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery gray, ductile and malleable transition metal. The formation of an oxide layer stabilizes the metal against oxidation. The element is found only in chemically combined form in nature...
complexes as well as Ziegler-Natta type catalysts have also been employed as ethene polymerization catalysts. The coordination of metals to the silsesquioxane framework gives electrophilic centers that are approximately as electron-withdrawing as a CF3 group, leading to increased catalytic activity. Complexes of silsesquioxane and molybdenum and tungsten have been reported as alkene metathesis catalysts. The metallasilsesquioxanes complex with tungsten
Tungsten
Tungsten , also known as wolfram , is a chemical element with the chemical symbol W and atomic number 74.A hard, rare metal under standard conditions when uncombined, tungsten is found naturally on Earth only in chemical compounds. It was identified as a new element in 1781, and first isolated as...
has even been recently used for ring-opening metathesis polymerization of norborene. The epoxidation of alkenes by the metallasilsesquioxanes is currently an area of high interest, with many groups focused on its development. Crocker et al. have reported catalysts for alkene epoxidation that use peroxide. Abbenhius et al. have recently reported using silsesquioxanes for the construction of polyoxometalaltes as well, which have potential for use in oxidation processes that currently use environmentally friendly oxidizing agents like O2 and H2O2. The first reported catalytic alkene epoxidation was done by a titanium
Titanium
Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant transition metal with a silver color....
complex, epoxidizing compounds like cyclooctene and norbornene nearly quantitatively. Further research has revealed the silsesquioxane-based complexes can promote Diels-Alder reactions, as well as other Lewis acid catalyzed reactions like Oppenauer oxidation and Meerwein-Pondorf-Verley reductions. The application of silsesquioxanes as catalysts as barely begun and with easier methods for syntheses of the complexes being published further applications can be anticipated.
Antimicrobial Silsesquioxanes
In recent research efforts silsesquioxanes have been functionalized with biocidal groups to produce antimicrobialAntimicrobial
An anti-microbial is a substance that kills or inhibits the growth of microorganisms such as bacteria, fungi, or protozoans. Antimicrobial drugs either kill microbes or prevent the growth of microbes...
coatings. Such coatings have numerous applications, as there is a continually increasing demand for materials that will keep people safe and healthy. Organic ammonium salts are well-known for their antimicrobial properties, meaning their ability to kill or inhibit the growth of harmful microbes. Specifically, quaternary ammonium salts (QAS) are used as disinfectants, antiseptics, and antifoulants that kill bacteria, fungi, and algae, but are not harmful to humans and animals. Several research groups have functionalized polyhedral oligomeric silsesquioxanes with QASs. The relatively small size of the silsesquioxane molecule, 2-5 nm, allows a QAS functionalized molecule to have a charge density similar to dendrimers and thus the antimicrobial efficacy is prominent.
Chojnowski et al. explored the quaternization of dimethyl-n-octylamine by octa(3-chloropropylsilsesquioxane), (T-ClPr)8, and transformed almost all eight chloropropyl groups into ionic quaternary ammonium chloride functions. The synthesis is based on a three-step hydrolytic polycondensation process of 3-chlo- ropropyltrimethoxysilane. The resulting material exhibited antimicrobial efficacy for the prevention of growth of both Gram-positive
Gram-positive
Gram-positive bacteria are those that are stained dark blue or violet by Gram staining. This is in contrast to Gram-negative bacteria, which cannot retain the crystal violet stain, instead taking up the counterstain and appearing red or pink...
and Gram-negative bacteria.
More recently, Majumdar et al. synthesized an array of QAS functionalized polyhedral oligomeric silsesquioxanes (Q-POSS). These researchers varied the alkyl chain length from –C12H25 to –C18H37 and varied the counter ion between chloride, bromide, and iodine. The first reaction was the hydrosilylation between allydimethlamine and octasilane polyhedral oliomeric silsesquioxane via Karstedt’s catalyst to make a tertiaryamino-functinoalized silsesquioxane. The second step was the quaternization of the tertiaryamino groups with an alkyl halide. The alkyl halides used were 1-iodooctadecane, 1-bromohexadecane, and 1-chloroctadecane.