Self-assembly of amphiphilic POSS anchoring a short organic tail with uniform structure

Article abstract of DOI:10.1246/cl.150680

An amphiphilic polyhedral oligomeric silsesquioxane (POSS) with a uniform molecular weight was synthesized by anchoring short hydrophilic organic tail groups at a corner of the POSS. The amphiphiles self-assembled to form vesicles in water and nanosheets in organic solvents.

Full text of DOI:10.1246/cl.150680

CL-150680
Received: July 17, 2015 | Accepted: August 5, 2015 | Web Released: August 13, 2015
Self-assembly of Amphiphilic POSS Anchoring a Short Organic Tail with Uniform Structure
1
2
3
1,4
Misaki Takeda, Keita Kuroiwa, Masaya Mitsuishi, and Jun Matsui*
Graduate School of Science and Engineering, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560
1
2
Department of Nanoscience, Faculty of Engineering, Sojo University, 4-22-1 Ikeda, Nishi-ku, Kumamoto 860-0082
3
Institute for Multidisciplinary Research for Advanced Materials, Tohoku University,
2
-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577
4
Department of Material and Biological Chemistry, Yamagata University, 1-4-12 Kojirakawa-machi, Yamagata 990-8560
(
E-mail: jun_m@sci.kj.yamagata-u.ac.jp)
An amphiphilic polyhedral oligomeric silsesquioxane (POSS)
with a uniform molecular weight was synthesized by anchoring
short hydrophilic organic tail groups at a corner of the POSS.
The amphiphiles self-assembled to form vesicles in water and
nanosheets in organic solvents.
iBu
iBu
iBu
iBu
iBu
O
O
O
Si
Si
Si
Si
O
O
Si
O
O
Si
O
iBu
OH
iBu
O
O
iBu
Si
Si
O
i
O
iBu
OSi OH
O
+ HSiCl3
iBu
OSi
Si
O
O
O
SiH
O
Si
Si OH
Si O
iBu
O
iBu
iBu
iBu
POSS-H
iBu
iBu
iBu
O
O
Si
Si
Polyhedral oligomeric silsesquioxanes (POSSs), composed of
an inorganic core with organic functional groups, have attracted
a wealth of research in material science because of their unique
chemical composition and structure.1 High thermal stability
attributed to the inorganic cores together with ease of function-
alization, such as polymers and organic functional groups, renders
iBu
O
Si
O
O
O
Si
ii
O
iii
iBu
OSi
O
O
Si
O
O
O
OH
,2
Si
Si O
iBu
iBu
POSS-DEG
3
POSSs useful in nanofiller applications. Moreover, interest arises
iBu
iBu
iBu
O
O
Si
Si
from their cubic structure, which possesses multiple reaction sites
to act as building blocks for the synthesis of hybrid nanomaterials
iBu
O
Si
O
O
O
Si
O
O
N
iBu
OSi
Si
O
O
4
with unique structures, such as dendrimers and Janus mole-
Si
O
O
NH
O
O
N
Si O
iBu
5
­8
H
cules. Several groups have applied POSS as a “head” group of
amphiphilic materials.9 The size of the POSS “head” group
iBu
­15
POSS-DEG-Im
(1 nm in diameter) is several times larger than conventional head
groups of amphiphilic molecules, and thus the majority of POSS-
Scheme 1. Synthesis of POSS-DEG-Im. (i) triethylamine,
THF, 0 °C to rt, 72%; (ii) diethylene glycol vinyl ether,
Karstedt’s catalysis, toluene, 0 to 40 °C, 51%; (iii) histamine,
based amphiphiles use long polymer chains as organic “tail”
groups to form hybrid surfactants.1
6­18
Usually, a high degree of
homogeneity with respect to the polymer chain length is required
to synthesize hybrid surfactants.19 Therefore there is a prerequisite
to use controlled polymerization, which requires specific synthetic
techniques with restrictions on the monomer. Furthermore,
molecular weight distribution remains even when adopting
sophisticated polymerization chemistry. Syntheses of POSS-
based amphiphilic materials with simple organic tail groups are
1
0
,1¤-carbonyldiimidazole, 4-dimethylaminopyridine, CH2Cl2,
°C to rt, 98%.
Amphiphilic POSS with one simple organic tail (POSS-
DEG-Im) was synthesized from heptaisobutyl trisilanol POSS
(Scheme 1). Initially, trichlorosilane was reacted with the
trisilanol POSS to incorporate a Si­H group at a corner of POSS.
Thereafter, hydrosililation of the Si­H group with diethylene
glycol vinyl ether was performed to anchor the hydrophilic
organic tail group to the POSS. Finally, the OH group was
converted to histamine carbamate to produce POSS-DEG-Im
2
0­23
limited.
POSS-based amphiphiles with simple organic tails
have advantages relating to their simple synthesis process,
molecular uniformity, and versatility. For example, we have
reported that “core-coronae” type amphiphiles composed of
double-decker shape silsesquioxane (DDSQ) attached with
diethylene glycol groups. The amphiphile showed uniform
1
(ESI). The H NMR spectrum of the final product indicates the
molecular weight and self-assembled to form nanosheet or
presence of diethylene glycol (3.99­4.00 ppm) and an imidazole
group (6.82 and 7.56 ppm) (Figure S4). The FT-IR spectrum of
POSS-DEG-Im showed an absorption band related to caged Si­
nanorods at the air­water interface.2
4­27
The results indicate the
ease of preparing POSS-based amphiphiles through anchoring of
simple organic molecular tail groups. In this paper, we synthesize
an amphiphilic POSS, which was functionalized with one
diethylene glycol (DEG) group as an organic tail. DEG group
was selected as a hydrophilic tail group based on our previous
results of the “core-coronae” type amphiphile.24 Moreover,
¹
1
¹1
O­Si (1093 cm ) and carbamate groups (N­H at 1464 cm and
¹
1
C=O at 1717 cm ) (Figure S6). High-resolution mass spectrum
of the final product clearly matched the calculated value
(Figure S7). The spectroscopic data indicate the synthesis of
POSS-based amphiphiles having a uniform molecular weight.
Thermal properties of POSS-DEG-Im were characterized by
thermal gravimetric analysis and differential scanning calorimetry
measurements. A 5% weight loss of POSS-DEG-Im is observed at
259 °C (Figure S8a). The value is comparable to that reported in
histamine was attached to the end of DEG unit through carbamate
group as a hydrogen-bonding unit.2
5,27
It was observed that the
amphiphilic POSS formed a vesicle in the water. Moreover,
nanosheets were formed in organic solvents.
1560 | Chem. Lett. 2015, 44, 1560–1562 | doi:10.1246/cl.150680
© 2015 The Chemical Society of Japan

(
a)
(b)
(
a)
(b)
5
0 nm
0
.1 μm
0.2 μm
0.2 μm
Figure 1. TEM micrographs of POSS-DEG-Im vesicles
formed in water, (a) assembly of small vesicles and (b)
micrograph of a large vesicle.
Figure 2. TEM micrographs of self-assembled POSS-DEG-Im
in (a) methanol and (b) CH Cl .
2
2
double-decker-shaped polyhedral silsesquioxanes anchored with
2
8
four DEG units. Therefore, it was concluded that the DEG units
initially decompose. The differential scanning calorimetry curve
displays a melting temperature at 156 °C (Figure S8b), which
indicates that symmetry breaking of the molecular structure®
upon anchoring of the organic tail to POSS®has a marginal effect
on the crystallinity properties.29
(
b)
(
a)
The self-assembly of POSS-DEG-Im was initially performed
in the presence of water. POSS-DEG-Im is insoluble in water and
therefore mixed solvents were used. Then, 4.3 mg was dissolved
in mixed solvent of water/ethanol/THF (16:1:1 v/v). The
solution was observed to be a white dispersion. The dispersion
was drop cast onto a carbon-coated Cu mesh and dried under
vacuum. Transmission electron microscope (TEM) micrographs
of POSS-DEG-Im in water showed aggregation of vesicles having
diameters in the region of ca. 25 nm (Figure 1a). Furthermore,
we observed a small fraction (ca. 10%) of isolated vesicles
possessing diameters of ca. 100 nm (Figure 1b). The former
aggregated-type vesicle formation is more frequently observed,
which indicates that this is the thermodynamically stable phase.
The interaction between individual POSS groups together with
the hydrophilicity of the organic tail moieties results in the
formation of vesicles in water.
The self-assembled structure of POSS-DEG-Im was further
studied by varying the solvent. We assumed a strong POSS­POSS
interaction with hydrophilic DEG, with hydrogen bond groups
inducing self-assembly in organic solvents. To study the
assembled structure, POSS-DEG-Im was dissolved in chloroform,
toluene, cyclohexane, CH2Cl2, and methanol (1.1 mg/1 mL).
POSS-DEG-Im was readily dissolved in these solvents. There-
after, the solvent was dropped onto a TEM grid and vacuum dried.
The TEM micrograph of POSS-DEG-Im drop coated from
toluene showed an amorphous film (Figure S9). Similar images
were observed in the films prepared from chloroform and THF.
Conversely, TEM micrographs of POSS-DEG-Im in CH2Cl2 and
methanol showed unique self-assembled nanosheet structures
1
800 1760 1720 1680 1640
-1
Wavenumber/cm
Figure 3. FT-IR spectra of POSS-DEG-Im films prepared
from (a) methanol and (b) toluene solution.
1700 cm 1 (Figure 3). The former and the latter peaks are
attributed to free (non-hydrogen-bonded) and hydrogen-bonded
carbonyl groups, respectively.3
¹
0­32
FT-IR spectra of the assembly
prepared from methanol shows a stronger absorption at ca. 1700
¹
1
than ca. 1722 cm (Figure 3a), whereas the peak intensities in
the film prepared from toluene are similar (Figure 3b). The FT-
IR spectra indicate an increased presence of hydrogen bond
formation in the nanosheets. The data indicate that self-assembled
structures of POSS-DEG-Im can be easily tuned from hollow
vesicles to two-dimensional sheets as a function of solvent. This
property is unique to POSS-DEG-Im when compared with other
POSS-based amphiphiles, as generally self-assembly of such
structures is governed primarily by the chemical composition.19
POSS-DEG-Im has high mobility compared with other POSS-
based amphiphiles because of the short and flexible DEG tails:
the reason to allow tailoring of the self-assembled structure.
The sheet structures are composed of stacks of POSS-DEG-
Im bilayers. X-ray diffraction patterns of the nanosheets showed
a uniform peak with a corresponding d spacing of 10.7 ¡
(Figure 4), which arose from the end-to-end distance of POSS
moieties.33 Moreover, a cross section of AFM image of POSS-
DEG-Im cast film shows stacks of layered films with thickness of
2.9, 4.4, and 6.7 nm corresponding to three, four, and six stacks
of the bilayers (Figure S10). The length of the hydrophilic unit
was estimated to be ca. 14 ¡ from a molecular model optimized
with the PM6 method in MOPAC (Figure S11). Therefore, the
hydrophilic groups take on a tilted and interdigitated arrangement
in the bilayer (Figure 4).
(Figure 2). Considering that the self-assembly occurred in the
presence of a relatively polar solvent, the hydrophobic­hydro-
phobic interaction between the POSS groups initiates the self-
assembly with the DEG tail groups, together with hydrogen
bonding of carbamate groups, interacting to induce sheet-like
structures. Hydrogen bonding formation within the nanosheets
was confirmed by FT-IR spectra measurements. FT-IR spectra of
POSS-DEG-Im showed a C=O absorption peak close to 1721 and
In conclusion, an amphiphilic hybrid molecule composed of
POSS as an inorganic “head” group and DEG-Im as an organic
Chem. Lett. 2015, 44, 1560–1562 | doi:10.1246/cl.150680
© 2015 The Chemical Society of Japan | 1561

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1562 | Chem. Lett. 2015, 44, 1560–1562 | doi:10.1246/cl.150680
© 2015 The Chemical Society of Japan