Silsesquioxanes (SQs: RSiO1.5), one class of the materials containing Si–O framework, have attracted much attention in the research fields of materials chemistry for academic and application reasons (Baney et al., 1995; Loy et al., 2000) because they can contain the various functional groups as side chains (R) and are inorganic materials indicating the remarkable compatibility with organic materials such as polymers (Choi et al., 2001; Kim & Chujo, 2003; Yu et al., 2010; Wang et al., 2011), in addition to having superior thermal, mechanical, and chemical properties derived from Si–O framework with high bond energy compared with C–C bonds.
However, even though various kinds of polyhedral oligomeric SQs (POSS) are known as SQs with controlled molecular structures (Scheme 1b) (Feher & Wyndham, 1998; Laine et al., 1998; Zhang et al., 2007; Cordes et al., 2010; Tanaka et al., 2010; Tanaka & Chujo, 2012), the regularly structured polySQs (PSQs) have only been obtained in the limited cases. This is because PSQs are prepared by polycondensation of the trifunctional silane monomers such as organotrialkoxysilanes and organotrichlorosilanes. These multifunctional monomers generally result in the formation of insoluble polymers with irregular three-dimensional network structures of Si–O–Si bonds (Scheme 1a) (Loy et al., 2000). If the molecular, conformational, and higher-ordered structures of PSQs can be controlled, they are expected to be applicable for a wide range of materials research fields.
Ladder-structured PSQs are one of a few PSQs with controlled molecular structures (Scheme 1c) (Brown et al., 1960; Brown et al., 1964; Unno et al., 2005; Zhang et al., 2006; Seki et al., 2010). These structures are classified into two types: “perfect ladder structure” (Scheme 1c-i) and “ladder-like structure” (Scheme 1c-ii) (Abe & Gunji, 2004). The latter has slight defects in Si–O–Si bond main-chain. Even though oligomeric SQ with “perfect ladder structure” could be synthesized by a step-by-step method and its characterization methods were established (Unno et al., 2002), it is difficult to prepare PSQs with such structures and to characterize them. In most cases, the ladder-structured PSQs probably do not have “perfect ladder structure” but have “ladder-like structure” as shown in Scheme 1c-ii.
The ladder-like PSQs exhibit rigidity and anisotropy in addition to the aforementioned superior physical properties of SQs because they have one-dimensional network structures of Si–O–Si bonds. Furthermore, ladder-like PSQs are colorless materials due to no absorptions of Si–O–Si bonds in the visible light region and can be used as transparent solutions owing to good solubility in any solvents. These properties of ladder-like PSQs would be useful for versatile applications, in particular, as inorganic fillers due to hybridization with organic functional materials.
Furthermore, control of the higher-ordered structures (nanostructures) of PSQs would also be significant to apply to various supramolecular organic–inorganic hybrid materials. For example, it has been reported that trifunctional silane monomers containing long alkyl chains were hydrolyzed to form amphiphilic molecules having silanol groups, resulting in the formation of multilayered PSQs by polycondensation (Scheme 1d) (Parikh et al., 1997; Shimojima et al., 1997). Another method for controlling higher-ordered structures of PSQs is sol–gel reaction (hydrolytic polycondensation) of 1,4-bis(trialkoxysilyl)benzene as a monomer in the presence of surfactants (Inagaki et al., 2002). The resulting material has a hexagonal array of mesopores and crystal-like frameworks. Self-organization of long alkyl chains by hydrophobic interactions is a driving force to form such regular higher-ordered structures.
From the aforementioned background, it is evident that the development of PSQs with regularly controlled molecular and higher-ordered structures is one of the important issues for research fields of SQs. In this chapter, therefore, the author first describes the preparation of cationic PSQs with controlled molecular and higher-ordered structures by simple sol–gel method and their detailed characterizations in section 2. The resulting PSQs indicated the anion-exchange properties due to existence of cationic functional groups as side-chains. Then, the author also describes the anion-exchange behaviors with various organic and inorganic compounds such as anionic surfactants (section 3), a polymer (section 4), layered clay minerals (section 5), and a dye molecule (section 6) to obtain the functional hybrid materials.
2. Preparation of ammonium group-containing ladder-like PSQs with hexagonally stacked structures
The author and co-workers have developed a preparation method for ammonium group-containing PSQs with controlled molecular and higher-ordered structures by hydrolytic polycondensation (sol–gel reaction) of organotrialkoxysilane monomers containing amino groups in aqueous inorganic acids (Kaneko et al., 2004a; Kaneko et al., 2005a; Kaneko & Iyi, 2007; Kaneko & Iyi, 2010).
2.1. Sol–gel reaction of 3-aminopropyltrimethoxysilane
Preparation of ladder-like PSQs with hexagonally stacked structures was achieved by sol–gel reaction of organotrimethoxysilanes containing amino groups. The first example of monomer to form these materials was 3-aminopropyltrimethoxysilane (APTMOS) (Kaneko et al., 2004a).
The sol–gel reaction of APTMOS was performed by stirring in an acid,
For the XRD measurements, the films of the products on the glasses were obtained by drying the aqueous product solutions spread on flat glass substrates. The XRD pattern of PSQ-NH3+Cl– film showed diffraction peaks with the
The TEM image of the product showed a stripe pattern, indicating that the rod-like PSQs were stacked parallel. In addition, the SEM image showed the aggregate, which lined up in a regular direction. This indicates that nano-ordered regular structure of PSQ affected the structures in the micro-ordered aggregate. Furthermore, the real image of PSQ-NH3+Cl– obtained by lyophilization seemed to be reflected by the regular nano- and micro-structures. The Brunauer–Emmett–Teller (BET) surface area of the product evaluated by the nitrogen adsorption–desorption isotherm measurement at 77K was much too small (
So far, decisive analysis methods to prove the regular molecular structure such as ladder-like structure of PSQ have not been established. Therefore, necessary evidences for confirmation of the structures were collected by performing multiple analyses. The ladder-like PSQ simultaneously satisfies the following conditions: i) to be soluble in any solvents, ii) relatively high molecular weight, iii) relatively narrow molecular width, and iv) observation of large T3 peak and small T2 peak in 29Si NMR spectrum.
The PSQ-NH3+Cl– had a rod-like structure with relatively small diameter (
It was supposed that ladder-like PSQ-NH3+Cl– was twisted to form rigid rod-like structure in the solid state, resulting in the formation of the hexagonally stacked structure. The driving force for the formation of the twisted conformation is probably intramolecular charge repulsion between the ammonium cations in side-chain groups of PSQ-NH3+Cl–. Therefore, to investigate correlation between intramolecular charge repulsion and regular structure of PSQ-NH3+Cl–, the XRD measurements were performed with changing RH. Stability of the ammonium cations is affected with RH because of hydration with water molecules. The XRD patterns of PSQ-NH3+Cl– with RH higher than 30% indicated three diffraction peaks derived from hexagonal phase, meanwhile those with RH lower than 20% did not show such the diffraction peaks. Because these results indicate the presence of correlation between the formation of the hexagonally stacked structure and higher RH, it is assumed that the twisted structure caused by intramolecular charge repulsion between the ammonium cations is plausible conformation of PSQ-NH3+Cl–.
On the basis of the aforementioned results, it was considered that self-organization of the ion pair composed of ammonium cations in the side-chains of the PSQ and chloride anions was the driving force for the formation of regular molecular, conformational, and higher-ordered structures.
2.2. Sol–gel reaction of (3-(2-aminoethylamino)propyl)trimethoxysilane
As another monomer, organotrialkoxysilane containing two amino groups,
The XRD pattern of the resulting product film showed three diffraction peaks with the
3. Ion-exchange behaviors of ladder-like PSQ with fatty acids
Because PSQ-NH3+Cl– has ammonium groups as side-chains and chloride anions (Cl–) as counterions, the anion-exchange property can be expected. Therefore, the ion-exchange reaction of the PSQs was performed with anionic organic compounds and the higher-ordered structures of the resulting products were characterized. First, a fatty acid salt such as
On the other hand, when the ion-exchange reactions were performed using the fatty acid salts containing longer alkyl chains (
4. Preparation of organic–inorganic polymer hybrids by ion-exchange reaction of ladder-like PSQ with organic polymer
Hybrids composed of organic and inorganic materials usually exhibit improved performance properties compared with conventional composites, mixtures on a micrometer scale (µm), due to their unique phase morphology and improved interfacial properties. For these reasons, nanostructured organic–inorganic hybrids have attracted considerable attention from both fundamental research and applications points of view (Usuki et al., 1993; Chujo, 1996). In particular, the organic–inorganic hybrids obtained from the synthetic polymers as organic species are important industrial materials.
In this section, the author describes the preparation of an organic–inorganic polymer hybrid (PSQ–PAA hybrid) with regular higher-ordered structure composed of the aforementioned PSQ-NH3+Cl–as the inorganic species and poly(acrylic acid sodium salt) (PAA-Na) as the organic species by the ion-exchange reaction (Kaneko et al., 2005b). To obtain PSQ–PAA hybrid, the ion-exchange reaction was performed by pouring PSQ-NH3+Cl– aqueous solution into PAA-Na aqueous solution (Scheme 3b).
The IR spectrum of PSQ–PAA hybrid displayed absorptions at 1044 and 1144 cm–1 assigned to the Si–O bond of the PSQ, and absorptions at 1407 and 1558 cm–1 due to the COO–of the PAA, indicating that the product consisted of both organic and inorganic polymers. The CHN elemental analysis data showed that the C/N molar ratio for the product was 5.92. From this value, the ratio of the functional groups of the two polymers,
The XRD pattern of the resulting insoluble product showed formation of a hexagonal phase and the
5. Preparation of nanoporous materials composed of ladder-like PSQs and layered clay minerals by ion-exchange reaction
There have been intense research activities on layered silicates pillared with inorganic or organic clusters, which are called pillared interlayer clays (PILCs) (Otsuka, 1997; Kloprogge, 1998; Ding et al., 2001). PILCs have much higher surface areas and pore volumes than the original clays. Such properties make them useful catalysts, ion-exchangers, and adsorbents. These materials are normally prepared by the intercalative ion-exchange of layered clay minerals with a variety of nano-sized pillars, such as organic ions (Ogawa et al., 1994), inorganic ions (Pichowicz & Mokaya, 2004), and sol particles (Yamanaka et al., 1992).
Even though the preparation of various polymers/clay hybrids (Pinnavaia & Beall, 2000) has already been reported, there have been a few studies on the preparation of PILCs using polymers. Because the polymers usually have a flexible structure, expansion of the interlayer space of clays by polymer incorporation is not enough for providing more free space.
The aforementioned PSQ-NH3+Cl– has motivated the author and co-workers to develop new PILCs, because it has rigidity and bulkiness to expand the interlayer of clays and ability to intercalate molecules into the interlayer of anionic clays by ion-exchange reaction due to the presence of ammonium cations in side-chains of PSQ-NH3+Cl–. In this section, therefore, the author describes the preparation of a clay pillared with rod-like cationic PSQ-NH3+Cl–(Scheme 4) (Kaneko et al., 2004b).
The synthesis was performed by pouring the PSQ-NH3+Cl–aqueous solution into an aqueous suspension of Na-saturated saponite (Na-SAP). After the mixture was vigorously stirred for 2 h at room temperature, the product was collected by filtration, washed with water, and then dried under reduced pressure at room temperature to obtain the clay pillared with PSQ-NH3+Cl–(PSQ–SAP). The resulting product was characterized by IR, CHN elemental analysis, XRD, and nitrogen adsorption–desorption isotherm measurements.
The IR spectrum of PSQ–SAP showed an absorption band at
From the nitrogen adsorption–desorption isotherms at 77K, the surface area and pore volume of PSQ–SAP derived from the
When a clay mineral with high CEC such as Li-saturated taeniolite was employed, such a porous material was not obtained by combination with PSQ-NH3+Cl– (BET surface area of the resulting product was
6. Preparation of chiral ladder-like PSQs and hybridization with dye compounds by ion-exchange reaction
Self-assembled hybrids formed by noncovalent interactions between photofunctional compounds and chiral molecules have attracted much attention because of their potential applications in circularly polarized luminescent (CPL) materials. To achieve the preparation of these hybrids by chiral induction from chiral molecules to photofunctional compounds, several combinations have been investigated with respect to supramolecular organization,
6.1. Copolycondensation of 3-aminopropyltrialkoxysilane with chiral group-containing trialkoxysilane
To prepare chiral ladder-like PSQs, the introduction of a chiral moiety into the PSQs was first investigated by acid-catalyzed copolycondensation of two organotrialkoxysilanes containing chiral and amino groups, respectively (Kaneko & Iyi, 2009).
The chiral trialkoxysilanes (R- and S-Monomers) were synthesized by reaction of 3-(triethoxysilyl)propyl isocyanate with (R)-(+)- or (S)-(–)-1-phenylethylamine in dichloromethane at room temperature for 15 min, respectively, followed by evaporation of the dichloromethane. The sol–gel copolycondensation of the resulting chiral trialkoxysilanes with 3-aminopropyltriethoxysilane (APTEOS) (feed molar ratio is 1 : 9) was performed in a mixed solvent of aqueous hydrochloric acid and methanol by heating in an open system until the solvent was completely evaporated (Scheme 5a). The products were isolated as the fractions insoluble in acetone, washed with acetone and chloroform, and then dried under reduced pressure at room temperature to yield the white powdered PSQs.
The 1H NMR spectra in D2O at 60 ºC of the products showed the signals indicating the presence of both the components of the chiral trialkoxysilanes and APTEOS; this indicates that the products were copolymers composed of both trialkoxysilanes. The unit ratio of the chiral groups to the ammonium chloride groups in the products was calculated to be 6 : 94. Therefore, the resulting PSQs are denoted as R6- and S6-PSQs, according to the stereostructure and functionality of the chiral groups. The IR spectra of R6- and S6-PSQs showed absorptions at 1620 cm–1 attributed to the C=O bond of the urea groups, indicating the existence of the units of chiral groups in the products.
In the IR spectra of R6- and S6-PSQs, large absorption bands at 1135 and 1040 cm–1 assigned to the Si–O–Si bonds were observed, indicating the formation of PSQs. In addition, the 29Si NMR spectra in DMSO-
The XRD patterns of the films of R6- and S6-PSQs showed three diffraction peaks with the
As aforementioned, R6- and S6-PSQs had rod-like structures with relatively small diameter (
The vibrational circular dichroism (VCD) spectroscopy, which is the extension of the electronic circular dichroism (ECD) into the IR region, is powerful technique to obtain conformational information of chiral molecules (Tang et al., 2007). The VCD spectra of R6- and S6-PSQs showed the reversed absorptions at
6.2. Polymer reaction of chiral group-containing compound with PSQ-NH3+Cl–
To prepare ladder-like PSQs containing higher compositional ratios of chiral side-chain groups, the aforementioned acid-catalyzed copolycondensation was investigated with a higher feed molar ratio of chiral trialkoxysilanes (>20%). However, the resulting PSQs were insoluble in all solvents owing to the formation of irregular three-dimensional cross-linked network structures. This is because the number of ion pairs formed by the amino groups of APTEOS and the acid-catalysts decreased.
Therefore, to prepare soluble ladder-like PSQs containing higher ratio of chiral groups,
Synthesis was performed by reaction of (R)-(+)- or (S)-(–)-1-phenylethyl isocyanate (R- or S-PEI) with PSQ-NH3+Cl– in the presence of triethylamine in DMSO/water (9 : 1 (v : v)) mixed solvent at room temperature for 10 min (Scheme 5b). The products were isolated by precipitation in acetone. The compositional ratios of chiral groups to ammonium chloride groups in the resulting products were estimated from their 1H NMR spectra and were found to depend on the feed molar ratio of PEI to ammonium chloride group of PSQ-NH3+Cl–. Here, soluble PSQs with the compositional ratio of chiral groups to ammonium chloride groups =
The diffraction peaks in the XRD patterns of the PSQs were broadened compared with those of PSQ-NH3+Cl–. This is due to the decrease in the number of ion pairs,
The chiral conformations of many kinds of helical polymers are stabilized by intramolecular interaction,
6.3. Chiral induction behavior of chiral PSQ to dye compound
These chiral ladder-like PSQs were applied to chiral inductors for hybridization with a dye compound such as an anionic achiral porphyrin by the ion-exchange reaction (Kaneko & Iyi, 2009; Kaneko et al., 2011).In this section, the author describes the chiral induction behavior from the aforementioned chiral ladder-like PSQs into an anionic achiral porphyrin such as tetraphenylporphine tetrasulfonic acid (TPPS).
In the UV–Vis spectra of TPPS/R6- and S6-PSQs aqueous mixtures (4 μmol/L and 100 μmol unit/L, respectively), absorptions due to the Soret band of TPPS in these mixtures were blue-shifted (to 400 nm) compared with that of TPPS alone indicated monomeric state with protonated (at 434 nm) and deprotonated (at 414 nm) species (Fig. 4a).These results indicate that the negatively charged TPPS formed H-aggregates along the positively charged ammonium groups as side-chains of the PSQs.
The ECD spectra of these TPPS/PSQs aqueous mixtures showed the reversed absorptions due to the Soret bands of TPPS-aggregates, corresponding to R6- and S6-PSQs as templates, respectively (Fig. 4b), indicating that TPPS-aggregates have chiralities induced from the chiral PSQs.
However, these mixtures did not show fluorescence emissions due to self-quenching of the excited state of the TPPS-aggregate. Therefore, to inhibit the formation of H-aggregates of TPPS by extension of the distance between the ammonium chloride side chains of PSQs, same analyses were performed using R80- and S80-PSQs as chiral inductors.
ECD spectra of TPPS/R80- and S80-PSQs mixtures in methanol (4 μmol/L and 100 μmol unit/L, respectively) exhibited the reverse absorptions at 418 nm (Fig. 4d), corresponding to the absorptions assigned to the Soret bands of TPPS in the UV–Vis spectrum (Fig. 4c). The absorption wavelength of this mixture was almost same as that of dilute methanol solution of sole TPPS (concentration = 4 μmol/L), indicating that TPPS maintained the monomeric state in the mixture. These results indicate that the chiral induction from PSQs to TPPS was achieved without formation of H-aggregate of TPPS. In the present mixture, because of no formation of H-aggregate, the fluorescence spectrum excited at 420 nm showed an emission peak at 654 nm.
In this chapter, the author described the preparation of cationic ladder-like PSQs with hexagonally stacked structures by the sol–gel reaction of amino group-containing trialkoxysilane monomers using acid-catalysts. Then, their detailed characterizations were performed. The resulting PSQs indicated the anion-exchange properties due to existence of cationic functional groups as side-chains. Therefore, it is also described that the anion-exchange behaviors with various organic and inorganic compounds such as anionic surfactants, a polymer, layered clay minerals, and a dye molecule to obtain the functional hybrid materials.
The author gratefully acknowledges Dr. Nobuo Iyi of National Institute for Materials Science (NIMS) for causing my research interests in the sol–gel chemistry, silsesquioxanes, and organic–inorganic hybrid materials. The author is indebted to the co-workers, whose names are found in references from this chapter, for their enthusiasistic collaborations.
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