Layered assembly of alkoxy-substituted bis(trichlorosilanes) containing various organic bridges via hydrolysis of Si-Cl groups

Article abstract of DOI:10.1039/b512482d

Monoalkoxy derivatives of bis(trichlorosilanes) containing methylene, ethylene, and phenylene bridges (Cl₃Si-R′-SiCl ₂OC₁₆H₃₃, R′ = -CH₂-, -C ₂H₄-, -C₆H₄-) were synthesized and self-assembly of the amphiphilic hydrolyzed species ((HO)₃Si- R′-Si(OH)₂OC₁₆H₃₃) was investigated. Hydrolysis of all Si-Cl groups was confirmed by liquid-state ²⁹Si and ¹³C NMR while the alkoxy groups were retained. The self-assembly was induced either by casting the hydrolyzed solutions on glass substrates or by cooling. The structures of the products were characterized by X-ray diffraction (XRD), electron microscopies (TEM and SEM), and solid-state ²⁹Si and ¹³C NMR. The products obtained from methylene- and ethylene-bridged monomers have lamellar structures consisting of bridged polysilsesquioxane layers and all-trans hexadecanol layers, which means that alkoxy groups were cleaved during polycondensation. The large difference in the d values of these hybrids (5.84 nm and 3.40 nm) suggests the variation in the arrangement of hexadecanol molecules within the layers. In contrast, the phenylene-bridged monomer afforded a lamellar solid (d = 5.14 nm) consisting of monomeric species, where both silanol groups and alkoxy groups mostly remain intact. This is attributed to the relatively stronger interaction and hydrogen-bonding networks between hydrolyzed species. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b512482d

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www.rsc.org/materials | Journal of Materials Chemistry
Layered assembly of alkoxy-substituted bis(trichlorosilanes) containing
various organic bridges via hydrolysis of Si–Cl groups{
Yasuhiro Fujimoto,a Masaru Heishi,a Atsushi Shimojima*bc and Kazuyuki Kuroda*abc
Received 5th September 2005, Accepted 6th October 2005
First published as an Advance Article on the web 25th October 2005
DOI: 10.1039/b512482d
Monoalkoxy derivatives of bis(trichlorosilanes) containing methylene, ethylene, and phenylene
bridges (Cl3Si–R9–SiCl2OC16H33, R9 5 –CH2–, –C2H4–, –C6H4–) were synthesized and self-
assembly of the amphiphilic hydrolyzed species ((HO)3Si–R9–Si(OH)2OC16H33) was investigated.
Hydrolysis of all Si–Cl groups was confirmed by liquid-state 29Si and 13C NMR while the alkoxy
groups were retained. The self-assembly was induced either by casting the hydrolyzed solutions on
glass substrates or by cooling. The structures of the products were characterized by X-ray
diffraction (XRD), electron microscopies (TEM and SEM), and solid-state 29Si and 13C NMR.
The products obtained from methylene- and ethylene-bridged monomers have lamellar structures
consisting of bridged polysilsesquioxane layers and all-trans hexadecanol layers, which means that
alkoxy groups were cleaved during polycondensation. The large difference in the d values of these
hybrids (5.84 nm and 3.40 nm) suggests the variation in the arrangement of hexadecanol
molecules within the layers. In contrast, the phenylene-bridged monomer afforded a lamellar
solid (d 5 5.14 nm) consisting of monomeric species, where both silanol groups and alkoxy
groups mostly remain intact. This is attributed to the relatively stronger interaction and
hydrogen-bonding networks between hydrolyzed species.
Moreau et al. reported the synthesis of lamellar bridged
Introduction
silsesquioxanes by incorporating amido groups to make
use of the hydrogen bonding between molecules.11–14 By
Silica-based inorganic–organic hybrid materials have been
extensively studied because of their potential for a wide range
of applications.1 Organosilanes, containing Si–C and Si–OR
with molecular-scale periodicities in the frameworks have been
combining the above two approaches, hybrid mesostructures
synthesized by Inagaki et al.15–18
(or Si–Cl) bonds, are often used as precursors, and their
chemical design is very important for the creation of hybrids
with controlled structures and desirable properties.1,2
On the other hand, we have reported the formation of
lamellar hybrids from alkyltrialkoxysilanes via self-assembly
and polycondensation of amphiphilic hydrolyzed species.19
Precursors containing two trichloro- or trialkoxy-silyl groups,
X3Si–R9–SiX3 (X 5 Cl or alkoxy), form an interesting class
More recently, this approach has been extended to the use of
alkoxytrichlorosilanes (CnH2n+1OSiCl3, n 5 12–20).20 Self-
assembly of partially hydrolyzed monomers (alkoxysilane-
of hybrids, called bridged polysilsesquioxanes, in which
organic (R9) groups are homogeneously integrated as a part
of the frameworks.3,4
triols) followed by polycondensation and cleavage of alkoxy
Recently, much effort has been directed to the structural
organization of bridged silsesquioxanes at the nanometer
length scale. The use of surfactants as structure directing
agents allows the formation of various hybrid mesostructures
where organic groups are homogeneously distributed within
the frameworks.5–8 As an alternative approach, the self-
directed assembly based on weak interactions between organic
bridges has been demonstrated. Corriu et al. reported struc-
tural anisotropy in xerogels derived from precursors with
rigid organic bridges such as allyl groups.9,10 Furthermore,
groups led to the formation of lamellar silica (or silicic acid)–
alcohol nanocomposites. The resulting thin siloxane sheets are
useful as nanofillers for nanocomposites with organic poly-
mers. Moreover, esterification of the surface silanol groups
with alcohols affords organically modified hybrids which are
expected to have unique adsorption properties21 and a greater
affinity to organic solvents.22 Although Ying et al. developed a
ligand-assisted approach based on covalent linking of surfac-
tants to metal alkoxides, previous reports are limited to the
synthesis of mesostructured transition metal (Nb, Ta, and Zr)
oxides.23,24 Our method using alkoxylated chlorosilanes is
important as a new single precursor pathway to a wide range
of nanostructured silica-based materials by the molecular
design of chlorosilane moieties.
aDepartment of Applied Chemistry, Waseda University, Ohkubo-3,
Shinjuku-ku, Tokyo, 169-8555, Japan. E-mail: kuroda@waseda.jp;
Fax: +81-3-5286-3199; Tel: +81-3-5286-3199
bKagami Memorial Laboratory for Materials Science and Technology,
Waseda University, Nishiwaseda-2, Shinjuku-ku, Tokyo, 169-0051,
Japan
In this paper, we examined the monoalkoxy derivatives
of bis(trichlorosilanes) containing methylene, ethylene, and
phenylene bridges (Cl3Si–R9–SiCl2(OC16H33), R9 5 –CH2– (1),
–C2H4– (2), –C6H4– (3)) for possible starting monomers
for synthesizing self-organized bridged silsesquioxanes. The
cCREST, Japan Science and Technology Agency (JST), Honcho 4-1-8,
Kawaguchi-shi, Saitama, 332–0012, Japan
{ Electronic supplementary information (ESI) available: Fig. S1–S5.
See DOI: 10.1039/b512482d
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organically-bridged silsesquioxane layers should be utilized as
a completely new nanofiller as well as an organized building
block for well ordered nanomaterials. The structures of the
resulting products as well as the hydrolyzed intermediate
species were characterized in detail, and the effect of the kind
of bridging organic groups on the self-assembly behavior
was discussed.
Synthesis of Cl3Si–C6H4–SiCl2(OC16H33) (3)
1,4-Bis(trichlorosilyl)benzene was synthesized by the reaction
of the Grignard reagent of 1,4-dibromobenzene (Tokyo Kasei.
Co.) with tetrachlorosilane (Tokyo Kasei Co.). The reaction
with 1-hexadecanol was performed in a similar manner for 1.
The amounts of the reactants were as follows: 1-hexadecanol
(4.2 g) in THF (35 mL); 1,4-bis(trichlorosilyl)benzene (8.99 g)
in THF (35 mL). Although 1,4-bis(trichlorosilyl)benzene is a
crystalline solid at room temperature, 3 was obtained as a clear
liquid. 1H NMR (500 MHz, CDCl3): d (ppm) 0.86–0.89 (t, 3H;
CH3), 1.26 (m, 24H), 1.38 (m, 2H), 1.67–1.70 (m, 2H), 4.01–
4.04 (t, 2H; SiOCH2), 7.87 (br, Ph); 13C NMR (125.7 MHz,
CDCl3): d (ppm) 14.1, 22.7, 25.6, 29.2, 29.4, 29.6, 29.7, 31.6,
32.0, 65.3, 132.6, 133.5, 135.4, 136.3; 29Si NMR (99.3 MHz,
CDCl3): d (ppm) 21.4, 225.3.
Experimental
Materials
Bis(trichlorosilyl)methane (Sigma-Aldrich, .97%) and 1,2-
bis(trichlorosilyl)ethane (Sigma-Aldrich, .97%) were used
for the synthesis of 1 and 2, respectively. Tetrachlorosilane
(Tokyo Kasei Co., .98%) and 1,4-dibromobenzene (Wako
Pure Chemical Industries, .98%), and magnesium turnings
(Sigma-Aldrich, .98%) were used for synthesizing 1,4-bis(tri-
chlorosilyl)benzene. 1-Hexadecanol (.95%) was purchased
from Wako Pure Chemical Industries. Other chemicals
including aniline (Wako Pure Chemical Industries, .99%),
tetrahydrofuran (THF, Wako Pure Chemical Industries,
dehydrated, .99.5%), and hexane (Kanto Chemical Co.,
.96%) were used as received.
Hydrolysis and self-assembly processes
Alkoxy derivatives of organo(bis-trichlorosilanes) (1, 2, or 3,
ca. 1.0 g) in THF (10 mL) were added dropwise to the mixture
of THF (20 mL), aniline, and H2O under vigorous stirring in
an ice bath. The molar ratio of 1 (or 2 or 3) : H2O : aniline was
1.0 : 5.5 : 5.5. Aniline was added to neutralize evolved HCl,
which accelerates the polycondensation reaction. After stirring
the mixture for 1 h, the precipitates of aniline hydrochloride
were removed by filtration to give a clear solution. To this
solution, THF was then added so that the concentration of Si
species became ca. 0.025 mol L21. The self-assembly procedure
was performed either by casting or by cooling these hydrolyzed
solutions. The casting of the solutions on glass substrates
followed by air-drying at 10 uC for 2 days yielded slightly
yellowish thick films, and they were peeled off from the
substrates and gently pulverized before characterization. The
products derived from 1, 2, and 3 are denoted as 1H, 2H, and
3H, respectively. On the other hand, cooling the hydrolyzed
solutions to about 220 uC formed precipitates when 1 and 3
were used, whereas no precipitation occurred in the case of 2.
The precipitates were filtered, washed with THF, and air-dried
for 2 days to afford white powders.
Synthesis of Cl3Si–CH2–SiCl2(OC16H33) (1)
The monoalkoxy-substitution of 1,2-bis(trichlorosilyl)methane
was performed by the dropwise addition of 1-hexadecanol
(9.3 g) dissolved in 30 mL of THF to a solution of 1,2-
bis(trichlorosilyl)methane (10.9 g) in THF (20 mL). The
gaseous HCl formed by the reaction was allowed to go out
of the vessel. After stirring at room temperature for 3 h, the
solvent was removed under reduced pressure. The residue
was vacuum distilled to yield 1 as a clear liquid. 1H NMR
(500 MHz, CDCl3): d (ppm) 0.87–0.89 (t, 3H; CH3), 1.26
(m, 24H), 1.37 (m, 2H), 1.52–1.54 (t, 2H; SiCH2Si), 1.61–1.64
(m, 2H), 3.91–3.94 (t, 2H); 13C NMR (125.7 MHz, CDCl3):
d (ppm) 14.1, 18.2, 22.7, 22.5, 29.2, 29.4, 29.6, 29.7, 29.8,
31.4, 32.0, 65.2; 29Si NMR (99.3 MHz, CDCl3): d (ppm)
4.6, 220.0.
Characterization
Liquid-state 29Si and 13C NMR spectra were obtained on a
JEOL Lambda-500 spectrometer with resonance frequencies
of 99.25 and 125.65 MHz, respectively. The sample solutions
were put in 5 mm glass tubes, and tetramethylsilane (TMS)
was added as an internal reference. CDCl3 or THF-d8 was used
to obtain lock signals. A small amount of Cr(acac)3 was also
added as a relaxation agent for 29Si nuclei. Solid-state 29Si
magic-angle spinning (MAS) NMR analysis was performed on
a JEOL JNM-CMX-400 spectrometer at a resonance fre-
quency of 79.42 MHz, with a pulse width of 45u, and a recycle
delay of 100 s. Deconvolution of spectra was performed by
using a Gaussian function on Spinsight software ver. 4.3.2.
Solid-state 13C cross-polarization (CP)/MAS NMR spectra
were obtained on the same spectrometer at a resonance
frequency of 100.54 MHz with a contact time of 1.5 ms and a
recycle delay of 5 s. Chemical shifts for both 29Si and 13C
NMR were referenced to tetramethylsilane at 0 ppm. Powder
Synthesis of Cl3Si–C2H4–SiCl2(OC16H33) (2)
The ethylene-bridged monomer
2 was synthesized from
1,2-bis(trichlorosilyl)ethane and 1-hexadecanol in a similar
manner as for 1. The amounts of the reactants were as
follows: 1-hexadecanol (19.4 g) in THF (150 mL); 1,2-
bis(trichlorosilyl)ethane (23.8 g) in THF (70 mL). The
precursor 2 was obtained as a clear liquid. 1H NMR
(500 MHz, CDCl3): d (ppm) 0.87–0.89 (t, 3H; CH3), 1.26 (m,
24H), 1.31 (m, 2H), 1.51 (m, 2H; Cl3SiCH2CH2Si), 1.61–1.64
(m, 2H), 3.89–3.92 (t, 2H; SiOCH2); 13C NMR (125.7 MHz,
CDCl3): d (ppm) 11.5, 14.1, 16.3, 22.7, 25.5, 29.2, 29.4, 29.6,
29.7, 31.5, 32.0, 65.1; 29Si NMR (99.3 MHz, CDCl3): d (ppm)
12.5, 212.2.
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X-ray diffraction (XRD) patterns were recorded on a Mac
Science M03XHF22 diffractometer with Mn-filtered Fe Ka
radiation or on a Mac Science MXP3 diffractometer with
monochromated Cu Ka radiation. Transmission electron
microscopic (TEM) images were obtained by
a JEOL
JEM-2010 microscope at an accelerating voltage of 200 kV.
Field-emission scanning electron microscopic (FE-SEM)
observations were performed for the samples coated with
Pt–Pd using a Hitachi S-4500S field-emission microscope at
an accelerating voltage of 15 kV. Thermogravimetric (TG)
analysis was performed on a RIGAKU TG8120 instrument
under a dry air flow at a heating rate of 10 uC min21, and the
amounts of SiO2 fractions in the products were determined by
the residual weight after heating to 900 uC. The amounts of
organic constituents were determined by CHN analysis (Perkin
Elmer PE-2400).
Results
Hydrolysis of Si–Cl groups
Fig. 1 shows the liquid-state 29Si NMR spectra of the solutions
of 1, 2, and 3 after partial hydrolysis. The signals of the
unreacted monomers are not observed, and each solution
clearly shows two new signals. The chemical shifts of the
signals are 240.2 ppm and 241.1 ppm for the solution
of 1, 238.6 ppm and 239.4 ppm for 2, and 253.0 ppm and
254.4 ppm for 3. It is known that the 29Si signals of
trihydroxysilyl (2Si(OH)3) groups attached to alkyl or
alkylene groups usually appear at around 239 ppm, and shift
upfield (y1 ppm) when one hydroxy group is replaced by an
alkoxy group.19,25–27 From these facts, the two signals
observed for both 1 and 2 can be assigned to (HO)3SiC and
CSi(OH)2(OC16H33), generated by hydrolysis of Si–Cl groups
without the cleavage of alkoxy groups. The signals observed
for the system of 3 (Fig. 1c) can also be assigned to a similar
hydrolyzed monomer, although the signals are shifted con-
siderably upfield from those of 1 and 2. Actually, the signal at
253.0 ppm is close to the 29Si signals of phenylsilanetriol in
acetone (253.1 ppm)28 and bis(trihydroxysilyl)benzene in
DMSO (254.3 ppm).29 The other signal at 254.4 ppm is
assigned to the Si atom having an alkoxy group.
Fig. 2 Liquid-state 13C NMR spectra of the solutions obtained by
hydrolysis of (a) 1, (b) 2, and (c) 3.
The corresponding 13C NMR spectra (Fig. 2) display the
signals due to the bridging organic groups and hexadecyl
chains. It is important that the signals assigned to the a
carbon of –OC16H33 are observed at 62.6 ppm, 62.9 ppm,
and 62.8 ppm for the hydrolyzed solutions of 1, 2, and 3,
respectively. These chemical shifts are closer to those of
1-hexadecanol (62.5 ppm) than those of the unreacted
monomers (in CDCl3; 65.2 ppm, 65.1 ppm, and 65.3 ppm
for 1, 2, and 3). However, we confirmed that alkoxy groups are
not hydrolyzed, because a new set of signals were observed
when a small amount of C16OH was added to these hydrolyzed
solutions. The difference in the chemical shifts for the a
carbons before and after hydrolysis of Si–Cl groups should
be due to the variation in the electron density of adjacent Si
atoms and also to the difference in the solvent (CDCl3 or
THF). These results are consistent with the 29Si NMR data,
indicating that controlled hydrolysis of alkoxy derivatives of
bis(trichlorosilanes) can be performed in a similar manner for
alkoxytrichlorosilanes.20
Structure of the self-assembled products
The powder XRD patterns of the hybrid solids (1H, 2H,
and 3H) obtained by casting the hydrolyzed solutions of 1, 2,
and 3 are shown in Fig. 3. All of the samples exhibit the
patterns characteristic of lamellar structures. The hybrid 1H,
containing a methylene group, shows a peak at d 5 5.84 nm
and its second and third order reflections. The ethylene-
bridged hybrid 2H exhibits a much smaller d-spacing of
3.40 nm. Remarkably, 3H shows a relatively strong peak at
d 5 5.14 nm accompanying the higher order reflections up to
the tenth order.
Fig. 1 Liquid-state 29Si NMR spectra of the solutions obtained by
hydrolysis of (a) methylene-bridged monomer 1, (b) ethylene-bridged
monomer 2, and (c) phenylene-bridged monomer 3.
At higher 2h angles, all of the samples exhibit a peak at
d 5 0.41 nm, which corresponds to the distance between
closely packed alkyl chains, as reported for crystalline
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silica showed a distance of y0.44 nm between adjacent Si
atoms.15 However, the detailed structure of 3H is not yet
well understood because of the low quality of the XRD
data and the difficulty in obtaining large crystals required for
single-crystal X-ray analysis.
FE-SEM observation revealed that these products have
flake-like morphologies (Fig. 4). In particular, the angular
shape of 3H appears to reflect its crystalline nature. The TEM
images of these products (Fig. 5) show layered structures,
and their periodicities roughly correspond to the d-spacings
determined by XRD. Such striped patterns were typically
observed at the curved edges of the flake-like samples. The
presence of molecular periodicity in 3H was evidenced by the
several spots observed in the electron diffraction (ED) patterns
(Fig. S1, ESI{).
Fig. 6 shows the 13C CP/MAS NMR spectra of 1H, 2H,
and 3H. The signals of methylene and ethylene groups are
observed at 21 ppm and 5 ppm, and that of a phenylene group
is observed as split signals at around 132 ppm. In addition,
all of the hybrids show the signals of hexadecyl chains. The
internal methylene carbons are detected at 33.3 ppm, which
suggests the all-trans conformation of the interlayer alkyl
chains.30 It has been reported that the a carbons of alkoxy
groups (SiOCH2C) appear at y1.0–1.5 ppm downfield
from those of alcohol molecules (HOCH2C).20–22,31 In the
case of layered silicic acid–hexadecanol nanocomposites, the a
carbons of the interlayer hexadecanol molecules appear at
63.1 ppm and shift to 64.7 ppm after esterification to form
alkoxy groups.21 The hybrids 1H and 2H exhibit the signals
of the a carbons at 63.0 ppm and 63.1 ppm, respectively,
suggesting that the cleavage of alkoxy groups occurred to form
alcohol molecules. In contrast, 3H exhibits the signal due to
the a carbon at 64.2 ppm, which strongly suggests that the
alkoxy groups remain intact.
Fig. 3 Powder XRD patterns of the hybrids obtained by casting: (a)
1H, (b) 2H, and (c) 3H. The insets show the XRD patterns of higher
2h regions.
n-alkanes and long-chain amphiphiles. Importantly, the pattern
for 3H shows several additional peaks (e.g., d 5 0.50 nm,
0.45 nm, and 0.34 nm) that are not assigned to the higher
order reflections. We consider that these peaks arise from the
ordered arrangement of O3Si–C6H4–SiO3 units. It is note-
worthy that the d-spacing of 0.34 nm is almost identical to
the spacing between stacked phenylene rings reported for the
lamellar crystals of bis(trihydroxysilyl)benzene.29 Also, the
structural modeling of phenylene-bridged hybrid mesoporous
The 29Si MAS NMR spectra of 1H, 2H, and 3H are shown
in Fig. 7. The signals due to the T units (Tx; CSi(OH)3–x(OSi)x;
x 5 0–3) are observed. The absence of the signals due to Q
units (Qy; Si(OH)4–y(OSi)y; y 5 0–4) means that the cleavage
of Si–C bonds did not occur during the reaction. The hybrid
Fig. 4 FE-SEM images of (a) 1H, (b) 2H, and (c) 3H.
Fig. 5 Typical TEM images of (a) 1H, (b) 2H, and (c) 3H.
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Table 1 Ratios of total organic moieties (C16H33 + R9) per 2SiO2 unit
Mass% C16H33 + R9/2SiO2
Mass% C Mass% H Mass% N SiO2
ratio
1H 59.0
2H 53.0
3H 52.5
9.3
9.8
9.9
0.5
0.6
0.3
27.8
31.2
28.0
0.97
0.92
1.1
1H exhibits T1, T2, and T3 signals at 250 ppm, 259 ppm, and
267 ppm, respectively. Similarly, 2H shows the corresponding
three signals at 248 ppm (T1), 256 ppm (T2), and 267 ppm
(T3). Thus the formation of siloxane networks was confirmed.
On the other hand, the spectrum of 3H mainly shows the signal
due to the T0 unit (252 ppm) along with a small signal due to
the T1 unit (centered at 261 ppm),3,32 suggesting that the
polycondensation between silanol groups hardly occurred.
From this result, in combination with the 13C CP/MAS NMR
data showing the retention of the alkoxy groups, it is clear
that 3H consists of hydrolyzed monomers ((HO)3Si–C6H4–
Si(OH)2OC16H33). We confirmed that polycondensation pro-
ceeded when 3H was heated to 80 uC for 1 day, however, the
XRD peak due to the lamellar structure almost disappeared.
The composition analysis (Table 1) shows that the
(C16H33 + R9)/2SiO2 ratios of the products are about 1,
confirming that the products were formed by the assembly
of hydrolyzed monomers. The presence of nitrogen is
explained by the contamination of a small amount of aniline
hydrochloride formed as a nonvolatile by-product in the
hydrolysis step.
Fig. 6 Solid-state 13C CP/MAS NMR spectra of (a) 1H, (b) 2H, and
(c) 3H.
As we reported previously,20 the self-assembly (crystal-
lization) of alkoxysilanetriols was induced by cooling (220 uC)
the hydrolyzed solutions. In the present system, we found that
cooling of the hydrolyzed solutions instead of casting on glass
substrates formed precipitates when 1 and 3 were used. The
absence of nitrogen in the precipitates was confirmed by CHN
analysis. The XRD patterns of the precipitates revealed the
formation of lamellar phases with d-spacings of 5.84 nm and
5.33 nm (Fig. S2, ESI{). In addition, solid-state NMR analyses
(Fig. S3 and S4, ESI{) suggest that the products are essentially
the same as those obtained by casting. However, no precipita-
tion occurred in the system of 2.
Furthermore, the hybrids 1H, 2H, and 3H are obtained as
thin films by spin-coating the hydrolyzed solutions of 1, 2, and
3 on glass substrates. The XRD patterns of the films derived
from 1, 2, and 3 (Fig. S5, ESI{) show the peaks at d 5 5.66 nm,
3.34 nm, and 4.82 nm, respectively. However, all the films
exhibit no peak at d 5 0.41 nm (data not shown). This indicates
that the layers are to some extent oriented parallel to the
substrate surface (i.e., alkyl chains are not oriented parallel to
the surface). It should also be noted that the film derived
from 3 exhibits no higher order reflections of the peak at d 5
4.82 nm. This is probably due to the smaller size of the crystals
of 3H in the thin films compared to those formed by casting.
Discussion
The liquid-state 13C and 29Si NMR analyses of the hydrolyzed
Fig. 7 Solid-state 29Si MAS NMR spectra of (a) 1H, (b) 2H, and
(c) 3H.
solutions of 1, 2, and 3 revealed the formation of amphiphilic
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species containing a hexadecoxy group and five hydroxyl
groups attached to Si atoms. Upon casting these hydrolyzed
solutions on glass substrates, evaporation of the solvent (THF)
induces the self-assembly of these hydrolyzed species. On the
other hand, cooling the hydrolyzed solutions also induced the
self-assembly of hydrolyzed monomers even in the presence of
solvents because the solubility of the hydrolyzed monomers
decreased. However, the hydrolyzed solutions of 2 did not
afford precipitates on cooling, which implies the lower self-
assembling ability of the hydrolyzed monomers. We consider
that the higher flexibility of the ethylene bridge compared to
methylene and phenylene bridges disturbs the self-assembly of
hydrolyzed species into lamellar solids. Precipitation may
occur if the nature of solvent is changed or the hydrolyzed
solution is cooled far below 220 uC.
When alkylene-bridged monomers 1 and 2 were used,
cleavage of alkoxy groups occurred accompanying poly-
condensation between silanol groups to form siloxane
networks. Such a phenomenon was also observed when
alkoxytrichlorosilanes were used as precursors.19 The cleavage
of alkoxy groups proceeds by condensation between Si–OR
groups and neighboring Si–OH groups, and/or by the hydro-
lysis of Si–OR groups with water formed by condensation of
Si–OH groups.
arrangements and tilting angles of hexadecanol molecules (or
hexadecoxy groups) and O3Si–R9–SiO3 units, as well as to the
difference in the length of the bridging organic groups.
The d-spacing of 1H is larger by 0.75 nm than that of the
layered structure (d 5 5.09 nm) consisting of silicic acid
layers and bimolecular layers of perpendicularly oriented
hexadecanols (approximate thicknesses of 0.7 nm and 4.4 nm,
respectively).20 This indicates that 1H has a similar bilayer
structure where the silsesquioxane layer consists of bimole-
cular layers of O3/2Si–CH2–SiO3/2 units (2 6 ca. 0.7 nm)
(Fig. 8a). In contrast, 2H has a much smaller d-spacing than
that of 1H, which implies that hexadecanols are arranged in an
interdigitated monolayer (Fig. 8b). A bilayer arrangement of
hexadecanols is unlikely because it requires the long axis of the
alkyl chains to be highly tilted away from the plane normal.
The difference in the interlayer structures of 1H and 2H is
probably due to the difference in the surface areas occupied by
Si(OH)2–R9–Si(OH)3 (R9 5 CH2 or C2H4) units. The slightly
larger head group area of ethylene-bridged monomers
increases the average distance between alkoxy groups. As
a result, a monolayer arrangement rather than a bilayer
arrangement was favored for the close packing of alkyl chains.
It is apparent that 3H consists of bimolecular layers of
hexadecoxy groups because the d-spacing (5.14 nm) is much
larger than the extended molecular length of hydrolyzed
monomers (ca. 3 nm). We propose that the Si(OH)2–C6H4–
Si(OH)3 units also form bilayers (ca. 2 nm in thickness), and
hexadecoxy groups are tilted away from the plane normal
by ca. 42u to satisfy the layer thickness of ca. 3.14 nm (Fig. 8c).
In contrast, when assuming a monolayer arrangement of
Si(OH)2–C6H4–Si(OH)3 units (ca. 1 nm in thickness), the
thickness of the alkoxy layer is calculated to be 4.14 nm,
meaning that hexadecoxy groups are oriented almost perpen-
dicular to the layers. However, this model is inconsistent
with the generally accepted relationship between packing
and arrangement of interlayer organic species because the
monolayer arrangement of Si(OH)2–C6H4–Si(OH)3 units
leads to the increase of the average lateral distance between
alkoxy groups, which should cause a substantial tilting of
alkoxy groups.
The degree of polycondensation in the silsesquioxane layers
was calculated by the following equation: 1/3 6 I(T1) + 2/3 6
I(T2) + I(T3), where I(Tx) denotes the relative intensity ratio
(%) of the Tx signals in the 29Si MAS NMR spectra (Fig. 7).
The hybrids with alkylene bridges (1H and 2H) have the
similar values of about 65%. This is lower than that of layered
silica–alcohol nanocomposites (79%), which should be due to
the presence of bridging organics between two –Si(OH)3
groups. Interestingly, the phenylene-bridged hybrid 3H was a
crystalline solid composed of hydrolyzed monomers, and
neither polycondensation of silanol groups nor cleavage of
alkoxy groups occurred. This stability of hydrolyzed mono-
mers is attributed to the formation of a strongly hydrogen-
bonded network, as reported for the crystalline solids of
bis(trihydroxysilyl)benzene.29
The proposed structural models of the hybrids 1H, 2H, and
3H are illustrated in Fig. 8. All of the products have lamellar
structures where the alkyl chains adopt all-trans conforma-
tions, as evidenced by XRD and 13C CP/MAS NMR. The
variation of the d-spacings (5.84 nm, 3.40 nm, and 5.14 nm
for 1H, 2H, and 3H, respectively) is due to the difference in the
Recently, bis(trialkoxysilanes) with various organic bridges
have been found to form hybrid solids with short- or long-
range ordered structures. However, their formation requires
either rigid aromatic bridges or those containing hydrogen-
bonding units in the precursors. The present results
Fig. 8 Proposed structural models of the layered hybrids 1H, 2H, and 3H.
5156 | J. Mater. Chem., 2005, 15, 5151–5157
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We have demonstrated the formation of novel layered
materials from alkoxyderivatives of bis(trichlorosilanes)
having methylene, ethylene, and phenylene bridges. Self-
assembly and subsequent polycondensation behaviors of
amphiphilic hydrolyzed species depended on the kind of
bridging organic groups. The precursors containing methylene
and ethylene groups (1 and 2) afforded lamellar hybrids
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layers stacked alternately on a nanometer length scale.
However, the arrangement of interlayer alcohol molecules
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This work was supported in part by a Grant-in-Aid for
Center of Excellence (COE) Research ‘‘Molecular Nano-
Engineering’’, the 21st Century COE Program ‘‘Practical
Nano-Chemistry’’, and Encouraging Development Strategic
Research Centers Program ‘‘Establishment of Consolidated
Research Institute for Advanced Science and Medical Care’’
from the Ministry of Education, Culture, Sports, Science and
Technology of the Japanese Government.
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This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 5151–5157 | 5157