Sol-gel synthesis of ladder polysilsesquioxanes forming chiral conformations and hexagonal stacking structures

Article abstract of DOI:10.1039/b910345g

Ladder polysilsesquioxanes containing ammonium chloride and chiral groups as side-chains were prepared by sol-gel copolycondensation of 3-aminopropyltriethoxysilane and chiral organotriethoxysilane under acidic conditions, forming chiral conformations and

Full text of DOI:10.1039/b910345g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Sol–gel synthesis of ladder polysilsesquioxanes forming chiral conformations
and hexagonal stacking structures
a
Yoshiro Kaneko and Nobuo Iyib
*
Received 27th May 2009, Accepted 21st July 2009
First published as an Advance Article on the web 11th August 2009
DOI: 10.1039/b910345g
Ladder polysilsesquioxanes containing ammonium chloride and chiral groups as side-chains were
prepared by sol–gel copolycondensation of 3-aminopropyltriethoxysilane and chiral
organotriethoxysilane under acidic conditions, forming chiral conformations and hexagonal stacking
structures.
proposed as a driving force for the formation of regular higher-
Introduction
ordered structures.
The important functions of biological macromolecules such as
Recently, we have developed a new preparation method of
proteins, nucleic acids, and polysaccharides are known to be
SiO-based materials with regularly controlled higher-ordered
reflected by not only their primary structures but also secondary
structure, i.e., the sol–gel polycondensation of amino-
and higher-ordered structures.1 Therefore, precise control of
alkyltrialkoxysilanes in aqueous solutions of inorganic strong
such hierarchical structures of the synthetic materials is one of
acids, such as hydrochloric and nitric acids, without using the
the important subjects in the research field of materials chem-
surfactants and long alkyl chain moieties.8 The resulting poly-
istry, for academic and application reasons.
siloxanes (polysilsesquioxanes) had rodlike structures forming
The preparation of siloxane (SiO)-based materials, e.g., sil-
hexagonal phases in the solid state, and were soluble in water to
sesquioxanes (RSiO1.5) and silicas (SiO2), is an extensively
obtain transparent solutions due to the presence of hydrophilic
investigated area in materials chemistry due to their superior
ammonium groups on the surface of each rod. Self-organization
thermal, mechanical, and chemical properties. Hydrolysis and
of an ion complex prepared from the amino group of the orga-
(poly)condensation, i.e., sol–gel reaction, of alkoxysilanes is well-
notrialkoxysilane and an inorganic acid used as a catalyst is a key
known as one of the important methods for the preparation of
SiO-based materials.2 However, it is generally difficult to control
addition, the primary structure of this polysilsesquioxane was
factor for the formation of a regular higher-ordered structure. In
the structures of SiO-based materials by using the sol–gel
assigned to a ladder structure,8c and the ladder was assumed to be
method. The (organo)alkoxysilane starting materials for
twisted to form the rodlike molecule.
synthesizing silsesquioxanes and silicas are tri- and tetrafunc-
Thus, we have investigated the control of primary and higher-
tional monomers, which result in the formation of irregular
ordered structures of SiO-based materials, but still there remains
network structures composed of siloxane bonds.
the problem of controlling the secondary structure in the nano-
So far, some attempts for control of these materials have been
metre scale, e.g., formation of a chiral conformation that is a motif
reported. Polyhedral oligomeric silsesquioxanes (POSS) and
for biological macromolecules suchas DNA, protein, and amylose.
ladder polysilsesquioxanes, synthesized from organo-
Wesuppose thatcontrolofthesecondarystructurewouldbeoneof
trialkoxysilanes, are known to be SiO-based materials with
the important subjects in the next stage of sol–gel chemistry. Such
controlled primary structures.3 Another is the sol–gel poly-
a secondary structure of the biological macromolecules is generally
condensation of alkoxysilanes in the presence of surfactants.4
caused by chiral moieties of the monomer units.1
This is one of the methods for controlling higher-ordered struc-
In this study, to control not only the primary and higher-
tures of SiO-based materials. The surfactants forming micelles of
ordered structures of SiO-based materials, but also their nano-
various hypermorphs, such as hexagonal and lamellar phases, act
scale secondary structures, we investigated the introduction of
as templates and allow the sol–gel polycondensation of alkoxy-
chiral organotrialkoxysilane as a monomer into the sol–gel
silanes to form silica–surfactant hybrid materials with regular
polycondensation of aminoalkyltrialkoxysilane under acidic
higher-ordered structures. In addition, the sol–gel poly-
conditions. Consequently, we found that ladder poly-
condensation of trichlorosilane,5 trialkoxysilane,6 and oligosi-
silsesquioxanes containing ammonium chloride and chiral groups
loxane7 covalently bonding to long alkyl chains has been
as side-chains could be prepared, which formed chiral confor-
performed to obtain polysilsesquioxanes and silicas with regular
mations as well as hexagonal stacking structures (Scheme 1).
higher-ordered structures. Self-organization of the surfactants
and long alkyl chains by hydrophobic interactions has been
Experimental
aGraduate School of Science and Engineering, Kagoshima University,
Materials
1-21-40 Korimoto, Kagoshima 890-0065, Japan. E-mail: ykaneko@eng.
kagoshima-u.ac.jp; Fax: +81 99 285 7856; Tel: +81 99 285 7843
bNational Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba,
Ibaraki 305-0044, Japan
All reagents and solvents were commercially purchased and used
without further purification.
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temperature using a Jasco V-650Q1 spectrophotometer and
a Jasco J-820 spectropolarimeter, respectively.
Results and discussion
Preparation and characterization of the polysilsesquioxanes (R- and
S-polymers)
The chiral monomers (R- and S-monomers) were synthesized
by reaction of TEOSPI with R- and S-PEAs in dichloro-
methane at room temperature for 15 min, respectively, followed
by evaporation of the dichloromethane (Scheme 2). The sol–gel
copolycondensations of APTEOS with the resulting chiral
monomers (feed molar ratio is 9 : 1) were performed in a mixed
solvent of aqueous hydrochloric acid (1.0 mol/L) and methanol
by heating (ca. 60–70 ꢁC) in an open system until the solvent
was completely evaporated (Scheme 2). The products were
isolated as fractions insoluble in acetone, washed with acetone
and chloroform, and then dried under reduced pressure at
room temperature to yield the white powdered poly-
silsesquioxanes (R- and S-polymers). The products were soluble
in water and dimethyl sulfoxide (DMSO), but insoluble in
typical organic solvents such as methanol, acetone, chloroform,
and n-hexane.
The 1H NMR spectra, in D2O at 60 ꢁC, of the products show
the signals indicating the presence of both the components of
APTEOS and the chiral monomers (Fig. 1); this indicates that
the products were copolymers composed of APTEOS and chiral
monomers. The unit ratios of the ammonium chloride groups to
the chiral groups in the products were calculated to be 94 : 6 by
the integrated ratios of the signal a due to –CH2Si to the signal g
due to the phenyl group. The IR spectra of the products show
absorptions at 1620 cmꢀ1 attributed to the C]O bond of the
urea groups (Fig. 2), indicating the existence of the units of chiral
groups in the products.
Scheme 1 Synthetic strategy for a ladder polysilsesquioxane forming
a chiral conformation and hexagonal stacking structure by the sol–gel
method.
Preparation of the polysilsesquioxanes
A typical experimental procedure for the polysilsesquioxane
(R- or S-polymer) was as follows. A solution of (R)-(+)- or
(S)-(ꢀ)-1-phenylethylamine (R- or S-PEA: 2.5 mmol ¼ 0.303 g)
in dichloromethane (10 mL) was added to 3-(triethoxy-
silyl)propyl isocyanate (TEOSPI: 2.5 mmol ¼ 0.618 g) with
stirring at room temperature. After the solution was stirred
further for 15 min, dichloromethane was evaporated by heating.
To the resulting product, methanol (60 mL) and a solution of
3-aminopropyltriethoxysilane (APTEOS: 22.5 mmol ¼ 4.981 g)
in 1.0 mol/L aqueous hydrochloric acid (37.5 mL) were succes-
sively added with stirring at room temperature. The reaction
solution was stirred for 1 h at 50 ꢁC, followed by heating to 60–70
ꢁC in the open system until the solvent was completely evapo-
ꢁ
rated. After the product was left for 2 h at 100 C, it was dis-
solved in water (30 mL) and the product solution was poured
into acetone (300 mL) to precipitate the powdered product. The
precipitated product was isolated by filtration, washed with
acetone and chloroform, and then dried under reduced pressure
at room temperature to yield 3.968 g of white-powdered R- or
S-polymer. 1H NMR (400 MHz, D2O): d 7.48–7.20 (5H, br, Ph),
d 4.81–4.61 (1H, br, PhCH(CH3)NHC(¼O)NH–), d 3.31–2.76
(2H, br, –NHC(¼O)NHCH2– and NH3CH2–), d 2.04–1.49 (2H,
br, –NHC(¼O)NHCH2CH2CH2Si– and NH3CH2CH2CH2Si–),
d 1.47–1.25 (3H, br, –CH3), d 1.05–0.45 (2H, br, –CH2Si–). IR:
1620 cmꢀ1 (C]O of the urea group), 1135 and 1040 cmꢀ1 (Si–O).
Measurements
The 1H and 29Si NMR spectra were recorded using a JEOL
ECX-400 spectrometer. The IR spectra were recorded using
a Shimadzu FTIR-8400 spectrometer. The molecular weights
(Mw) of the products were estimated by the Zimm plot method
using static light scattering (SLS) equipment; an Otsuka Elec-
tronics DLS-8000. The X-ray diffraction (XRD) measurements
were performed at a scanning speed of 2q ¼ 0.2 ꢁ/min using
a Rigaku Geigerflex RAD-IIB diffractometer with Ni-filtered Cu
Ka radiation (¼ 0.15418 nm). The vibrational circular dichroism
(VCD) spectra were measured in a CaF2 cell (cell length ¼ 0.1
mm) at room temperature using a Jasco FVS-4000 spectrometer.
The UV-Vis and electronic circular dichroism (ECD) spectra
were measured in a quartz cell (cell length ¼ 10 mm) at room
Scheme 2 Preparation of the polysilsesquioxanes by sol–gel copoly-
condensation.
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Fig. 3 29Si NMR spectra in DMSO-d6 at 40 ꢁC of (a) R-polymer and (b)
S-polymer. Chemical shifts were referenced to tetramethylsilane (TMS)
(d 0.00).
Fig. 1 1H NMR spectra in D2O at 60 ꢁC of (a) R-polymer and (b)
S-polymer. Chemical shifts were referenced to sodium 2,2-dimethyl-2-
silapentane-5-sulfonate (DSS) (d 0.00).
Higher-ordered structure of R- and S-polymers
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 profiles of the
product films show three diffraction peaks with the d-value ratio
of 1 : 1/O3 : 1/2, indicating that the products have regular higher-
ordered structures such as hexagonal phases (Fig. 4). Because the
products were soluble in water and DMSO, we supposed that
these hexagonal phases originated not from porous-type struc-
tures but from the stacking of rodlike polymers. The diameters of
the rodlike products calculated from d-values of (100) peaks
(1.47–1.48 nm) were assessed to be ca. 1.7 nm.
The intensities of the diffraction peaks of the present product
(R-polymer: Fig. 5b) were weaker than those of the poly-
silsesquioxane without a chiral group, as shown in our previous
study (Fig. 5c),8a although these peaks indicated the formation of
hexagonal phases. On the other hand, the intensities of the
diffraction peaks of the polysilsesquioxane containing a higher
ratio of chiral group (ca. 10%) become very weak (Fig. 5a),
Fig. 2 IR spectra of (a) R-polymer and (b) S-polymer.
The IR and 29Si NMR spectrometries are useful analytical
methods to confirm the formation of Si–O bonds. In the IR
spectra of the products, large absorption bands at 1135 and
1040 cmꢀ1 attributable to the Si–O–Si bonds were observed
(Fig. 2), indicating the formation of polysilsesquioxanes. In
addition, the 29Si NMR spectra in DMSO-d6 at 40 ꢁC of the
products exhibit the signals in the regions of T3, which means Si
atoms with three siloxane bonds (Fig. 3). These results indicate
that the progress of the sol–gel copolycondensation of APTEOS
and the chiral monomers, and the formation of Si–O–Si bonds, is
complete. The regularity of the primary structure of the products
is discussed in detail hereinafter. The average molecular weights
(Mw) of the products (R- and S-polymers) estimated by the
Zimm plot method using a SLS apparatus were assessed to be
10 700 and 9800, respectively, indicating that the products were
not oligomeric compounds but macromolecules.
Fig. 4 XRD patterns of (a) R-polymer and (b) S-polymer. Relative
humidity on XRD measurements was ca. 60%.
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Fig. 5 XRD patterns of R-polymers containing different ratios of chiral
groups: (a) 10%, (b) 6%, and (c) 0%. The amount of each product on glass
was ca. 10 mg/cm2. Relative humidity on XRD measurements was ca.
50%.
Fig. 6 IR spectra in DMSO of (a) R-polymer and (b) S-polymer, and
VCD spectra in DMSO of (c) R-polymer and (d) S-polymer. Concen-
trations of polymer solutions in DMSO were 0.1 mol unit/L.
respectively (Fig. 6c, d), corresponding to the absorptions
assigned to the Si–O bond of the polymer’s main-chains in the IR
spectra (Fig. 6a, b). These results indicate that the present poly-
silsesquioxanes had chiral conformations of main-chains.
indicating a relatively irregular higher-ordered structure. This is
probably because the contents of ion complexes, i.e., ammonium
chloride groups, decreased. The ion complex has an important
role for the formation of a regular higher-ordered structure in
this reaction system. In addition, the product obtained from
a much higher feed molar ratio of the chiral monomer (20%) was
insoluble in water. Therefore, the following characterizations
were performed using the polysilsesquioxanes containing ca. 6%
chiral groups, as described above.
Formation of chiral aggregates of achiral porphyrin induced from
R- and S-polymers
The self-assembly of achiral chromophores such as porphyrins to
form chiral aggregates has attracted much attention because of
its potential application to optoelectronic materials.10 To obtain
chiral aggregates of porphyrins, chiral templates to support self-
assembly of the porphyrins, such as biopolymers11 and chiral
synthetic polymers,12 have often been employed. In this study, we
investigated the formation of chiral aggregates of an anionic
achiral porphyrin such as tetraphenylporphine tetrasulfonic acid
(TPPS) using the present polysilsesquioxanes.
Primary structure of R- and S-polymers
The primary structures of the polysilsesquioxanes have often
been characterized by IR spectrometry, and it has been suggested
that ladder polysilsesquioxanes have exhibited two absorption
bands due to the Si–O–Si bond at ca. 1140 and 1050 cmꢀ1 3a,b,e
In
.
fact, the IR spectra of the present products show two absorption
bands at 1135 and 1040 cmꢀ1 (Fig. 2). In addition, to support the
ladder structure of the products, we considered the results of the
XRD and 29Si NMR measurements. The diameters of the rodlike
polysilsesquioxanes were extremely small (ca. 1.7 nm), confirmed
by the XRD measurements (Fig. 4), in spite of their highly dense
Si–O–Si bond network structures (only T3 signal), characterized
The UV-Vis spectra of aqueous mixtures of TPPS/poly-
silsesquioxanes (4 mmol/L and 100 mmol unit/L, respectively) are
shown in Fig. 7a, b. Absorptions due to the Soret band of TPPS
in these mixtures are blue-shifted (to 400 nm) compared with that
of TPPS alone indicated a monomeric state with protonated (at
434 nm) and deprotonated (at 414 nm) species. These results
indicate that the negatively charged TPPS formed H-aggregates
along the positively charged ammonium groups as side-chains of
the polysilsesquioxanes.
by the 29Si NMR measurements in DMSO-d6 at 40 C (Fig. 3).
ꢁ
Both the results of the XRD and 29Si NMR provide convincing
evidence of the formation of regular ladder structures as shown
in Scheme 1. On the basis of all results obtained by IR, XRD, and
29Si NMR, we proposed that the primary structures of the
products were ladder structures.
The ECD spectra of these TPPS/polysilsesquioxanes aqueous
mixtures show the reversed absorptions due to the Soret bands of
TPPS-aggregates, corresponding to R- and S-polymers as
templates, respectively (Fig. 7c, d), indicating that TPPS-aggre-
gates have chiralities induced from R- and S-polymers.
Secondary structure of R- and S-polymers
To confirm that these induced circular dichroisms (ICD) of the
TPPS-aggregates were not directly derived from chiral groups in
the polysilsesquioxanes, the ECD measurements of the aqueous
mixtures of TPPS and the chiral molecules existing in the poly-
silsesquioxanes, i.e., R- and S-PEAs, (4 mmol/L and 6 mmol/L,
The VCD spectroscopy, which is an extension of the ECD into
the IR region, is a powerful technique to obtain conformational
information of chiral molecules.9 The VCD spectra of R- and S-
polymers show the reversed absorptions at ca. 1140–1165 cmꢀ1
,
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was added into the aforementioned aqueous mixture (TPPS ¼ 4
mmol/L, PEA ¼ 6 mmol/L, and achiral polysilsesquioxane ¼ 100
mmol unit/L, respectively), the ECD spectra of this mixed solu-
tion do not also show the absorption band indicating the
chirality of TPPS although H-aggregates of TPPS were formed
(Fig. 9). These results indicate that the reversed absorptions due
to chiral aggregates of TPPS as shown in Fig. 7 were induced not
directly from chiral groups in the polysilsesquioxanes, but from
their chiral conformations.
Proposed formation mechanism
Based on all data described above, we regarded the present
products as polysilsesquioxanes with hierarchically controlled
primary (ladder), secondary (chiral conformation), and higher-
ordered (hexagonal stacking) structures. Next, we propose the
formation mechanism of the present polysilsesquioxanes.
Hydrochloric acid acted not only as a catalyst for sol–gel poly-
condensation, but also as a reagent for the formation of the ion
complex with the amino groups of APTEOS. First, the regular
primary structure such as the ladder structure of poly-
silsesquioxanes would be formed by self-organization of the ion
complex during the sol–gel polycondensation. Then, the rodlike
molecules formed by twisting ladder polysilsesquioxanes stacked
to form regular higher-ordered structures such as the hexagonal
phase. The driving force for the formation of a rodlike structure
by twisting ladder polysilsesquioxanes may be a mutual repul-
sion between the positively charged ammonium groups as the
side-chains. During the formation of the rodlike structures, the
twisting direction of the ladder polysilsesquioxanes would be
Fig. 7 UV-Vis spectra of aqueous mixtures: (a) TPPS + R-polymer, (b)
TPPS + S-polymer, and ECD spectra of aqueous mixtures: (c) TPPS +
R-polymer, (d) TPPS + S-polymer. Concentrations of aqueous solutions
of TPPS and polysilsesquioxanes were 4 mmol/L and 100 mmol unit/L,
respectively.
respectively) were performed. As expected, these spectra do not
show absorptions due to the chirality of TPPS (Fig. 8). In
addition, when the polysilsesquioxane without the chiral group
Fig. 9 UV-Vis spectra of aqueous mixtures: (a) TPPS + R-PEA + achiral
polysilsesquioxane, (b) TPPS + S-PEA + achiral polysilsesquioxane, and
ECD spectra of aqueous mixtures: (c) TPPS + R-PEA + achiral poly-
Fig. 8 UV-Vis spectra of aqueous mixtures: (a) TPPS + R-PEA, (b)
TPPS + S-PEA, and ECD spectra of aqueous mixtures: (c) TPPS +
R-PEA, (d) TPPS + S-PEA. Concentrations of aqueous solutions of
TPPS and PEA were 4 mmol/L and 6 mmol/L, respectively.
silsesquioxane, (d) TPPS
+ S-PEA + achiral polysilsesquioxane.
Concentrations of aqueous solutions of TPPS, PEA, and achiral poly-
silsesquioxanewere4 mmol/L, 6 mmol/L, and100 mmolunit/L, respectively.
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Conclusions
We have reported the preparation of ladder polysilsesquioxanes
containing ammonium chloride and chiral groups, forming chiral
conformations and hexagonal stacking structures. This was
achieved by sol–gel copolycondensation of 3-amino-
propyltriethoxysilane and chiral organotriethoxysilane in
a mixed solvent of aqueous hydrochloric acid and methanol. The
1
structures of the resulting products were characterized by H
NMR, IR, 29Si NMR, SLS, XRD, VCD measurements. In
addition, we investigated the formation of chiral aggregates of
TPPS using the resulting polysilsesquioxanes. The ECD and UV-
Vis spectra of aqueous mixtures of TPPS/polysilsesquioxanes
indicated the formation of chiral H-aggregates of TPPS along the
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Acknowledgements
This work was supported in part by a fund from Iketani Science
and Technology Foundation and a Grant-in-Aid for Young
Scientists (B) (No. 21750127) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan. We wish to
thank Mr S. Nakamura of Otsuka Electronics Co., Ltd. for
support in SLS measurements.
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