Formation of two- and three-dimensional hybrid mesostructures from branched siloxane molecules

Article abstract of DOI:10.1021/ja903176k

(Chemical Equation Presented) We report the design of a new precursor having three branching disiloxane units capable of forming 3D mesostructures with a cubic Pm-3n and its orthorhombic and tetragonal variants Cmmm and P4 ₂/mnm, in addition to a conventional 2D hexagonal (p6mm) mesostructure, thus creating a novel research area of mesostructural design in silica-organic nanohybrid materials.

Full text of DOI:10.1021/ja903176k

Published on Web 06/24/2009
Formation of Two- and Three-Dimensional Hybrid Mesostructures from
Branched Siloxane Molecules
†
‡
§,|
§
§,|
Shigeru Sakamoto, Atsushi Shimojima, Keiichi Miyasaka, Juanfang Ruan, Osamu Terasaki,
,
†
and Kazuyuki Kuroda*
Department of Applied Chemistry, Waseda UniVersity, Ohkubo-3, Shinjuku-ku, Tokyo 169-8555, Japan,
Department of Chemical System Engineering, The UniVersity of Tokyo, Hongo-3, Bunkyo-ku, Tokyo 113-8656, Japan,
Structural Chemistry, Arrhenius Laboratory, Stockholm UniVersity, 10691 Stockholm, Sweden, and Graduate School of
EEWS, Korea AdVanced Institute of Science and Technology, Daejeon 305-701, Republic of Korea
Received April 21, 2009; E-mail: kuroda@waseda.jp
The self-assembly route to hybrid inorganic-organic solids is
Scheme 1. Self-Assembly of the Precursor 1 into Two Types of
Hybrid Mesostructures
of significant interest not only for fundamental research but also
1
-3
for the production of novel functional materials.
Recently,
ordered silica-based hybrid materials have been synthesized from
organoalkoxysilanes by taking advantage of their intermolecular
interactions and ability to form siloxane network by hydrolysis and
4
,5
polycondensation. The molecular design based on rich organo-
silicon chemistry would permit fine-tuning of compositions, struc-
tures, and properties of the products. The design of amphiphilic
siloxane-based molecules is promising for producing hybrid me-
sostructures with a long-range order. The structure can be controlled
in part by varying the size of the siloxane head as well as the alkyl
4
,6
chain length. For example, trialkoxy- or trichloro-(alkyl)silanes
6
form lamellar solids via hydrolysis and polycondensation, whereas
to give a xerogel film (1H ). Similarly, a hybrid solid, 1H , was
A
B
tris(trialkoxysilyloxy)-(alkyl)silanes (C
-13 (hereafter called “tetrasiloxane precursors”), can form mes-
ophases consisting of rod-like assemblies such as 2D hexagonal
n
H
2n+1Si(OSi(OMe)
3
)
3
, n )
prepared by diluting with only THF under otherwise identical
conditions.
1
0
6
The powder XRD pattern of 1H (Figure 1a) shows several peaks
A
7
6a,8
phase. In view of the geometrical packing parameter,
further
due to mesoscale periodicity. The TEM images (Figure 2) show
enlargement of the siloxane head should favor the formation of
spherical assemblies, leading to structural diversity and complexity
arising from packing variations; however, no such higher curvature
mesophases have been observed for amphiphilic organosiloxanes
without the aid of surfactants.
that the material comprises different packing domains of cages with
1
0
space groups of P4
among which the P4
2
/mnm, Pm-3n, and their modulated mixture,
/mnm is dominant. The packing of the cage
2
domains is very coherent, which is evidenced by the FFT diffrac-
tograms (insets of Figure 2) that display well-resolved spots despite
the presence of the mesostructural modulation and the relevant
Here we report the design of a new precursor having three
branching disiloxane units (1 in Scheme 1) capable of forming 3D
mesostructures with a cubic Pm-3n and its orthorhombic and
10
stacking faults. On the basis of the prevailing P4 /mnm symmetry,
2
the lattice constants of the unit cell calculated from the FFT are a
tetragonal variants Cmmm and P4
2
/mnm. A 2D hexagonal (p6mm)
) 12.8 nm and c ) 5.3 nm. These lattice constants are presumed
mesostructure was also formed by control of water content during
self-assembly. The large, branched siloxane head of 1 is essential
for the formation of such 3D mesophases, which have only been
found in organic molecules and block copolymers with specific
to give the indexing of the present XRD peaks in Figure 1a. The
packings with P4 /mnm and Cmmm are explained from that with
2
Pm-3n by the occurrence of two-dimensionally periodic planar
11
faults. This same packing behavior and the relationship have since
9
9a
geometries, thus creating a novel research area of mesostructural
been described for liquid crystals and a mesoporous silica (AMS-
design in silica-organic nanohybrid materials.
9
) prepared with anionic surfactants and costructure directing
1
2
We have developed a facile route to the heptasiloxane precursor
agents.
7
1
by one-step silylation of decylsilanetriol with disiloxane species
In contrast, the 2D hexagonal (p6mm) structure of 1H
B
was
where -OMe groups of hexamethoxydisiloxane ((MeO)
3
SiOSi-
revealed by XRD and TEM showing either honeycomb or striped
patterns depending on the electron incidents. The d10 spacing (2.69
nm) is smaller than that for the 2D hexagonal structure derived
from the tetrasiloxane precursor with the same chain length (d10
10
(
OMe) ) are partly replaced by more reactive -Cl groups. Suc-
3
10
cessful synthesis of 1 was confirmed by NMR and MS. Hydrolysis
of 1 proceeded almost completely under acidic conditions in THF
as confirmed by disappearance of the SiOCH
NMR spectrum. After diluting this hydrolyzed solution with more
THF and water, the solution was cast and dried on a glass substrate
)
1
3
7
3
signals in the
C
2
B
.92 nm), suggesting that 1H has a smaller diameter of the rod-
like assemblies. Although the larger siloxane head of 1 should lead
to a thicker siloxane wall, the internal diameter should decrease
because of the increased lateral distance and therefore lowered
density of alkyl chains in the rods. These structural features were
actually confirmed by nitrogen sorption data of the porous silica
†
Waseda University.
‡
The University of Tokyo.
§
Stockholm University.
Korea Advanced Institute of Science and Technology.
|
10
samples obtained by calcination.
9
634 9 J. AM. CHEM. SOC. 2009, 131, 9634–9635
10.1021/ja903176k CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
The present results suggest that alkylsiloxane mesophase is
governed by both the size and configuration of the siloxane head.
In contrast to the formation of well-ordered 3D mesophases from
n 8 7
1, cubic-octasiloxane precursors (C H2n+1Si O12(OEt) ) yielded 2D
hexagonal phases when n ) 16-20 and the structure became
14
disordered upon decreasing the chain length to n ) 10. Although
the number of Si atoms is similar among these two precursors, major
differences exist in the rigidity and the number of hydrolyzable
alkoxy groups. The maximum number of -OH groups generated
by hydrolysis is much larger for 1 (15 -OH per molecule) than
for the octasiloxane precursors (7 -OH per molecule). Thus, the
hydrolyzed 1 should be more amphiphilic and, hence, capable of
forming well-ordered mesostructures.
In conclusion, we have demonstrated the formation of 3D
tetragonal (and cubic) as well as 2D hexagonal mesostructures from
a branched heptasiloxane precursor, providing deeper insight into
the relationship between molecular structure and self-assembled
mesostructures of alkylsiloxane amphiphiles. The modification of
the organic counterpart is under study to achieve more sophisticated
structures with specific functions.
Figure 1. (a) Powder XRD pattern (collected in Spring-8, Japan) of 1H
which is indexed based on the cell parameter determined from TEM
observation. (b) 29Si MAS NMR spectrum of 1H
A
,
A
.
Acknowledgment. This work was supported in part by the
Global COE program, a Grant-in-Aid for Scientific Research on
Priority Areas from MEXT, Japan, and the WCU program of the
Korean Government (MEST). O.T. acknowledges the Knut and
Alice Wallenberg Foundation for the support to EM centre of
Stockholm University. We are grateful to Mr. S. Nasu, Ms. M.
Sakurai, Mr. K. Kawahara (Waseda Univ.), and Ms. J. Du (Queen’s
Univ., Canada) for experimental help and suggestions.
Figure 2. TEM images and FFT diffractograms of 1H
A
taken by a JEM-
2
/mnm space group.
3010 microscope; (a) [001] and (b) [110] directions of P4
The italic symbols indicate the lattice vectors of the tetragonal lattice. Scale
bar: 20 nm.
Supporting Information Available: Experimental section, ad-
ditional XRD, NMR, IR, and TEM data. This material is available free
of charge via the Internet at http://pubs.acs.org.
A B
In both 1H and 1H , alkyl chains are in a similarly disordered
state with mixed trans-gauche conformations, as suggested by the
-1
10
position of the VasCH
2
bands (2930 cm ) in the IR spectra. Also,
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JA903176K
J. AM. CHEM. SOC. 9 VOL. 131, NO. 28, 2009 9635