Synthesis, thermal stability and photoresponsive behaviors of azobenzene-tethered polyhedral oligomeric silsesquioxanes

Article abstract of DOI:10.1039/c1nj20577c

A series of azobenzene-tethered polyhedral oligomeric silsesquioxane (POSS) derivatives, i.e. monoazobenzene-substituted POSS (MonoAzo-POSS), bisazobenzene-substituted POSS (BisAzo-POSS) and triazobenzene-substituted POSS (TriAzo-POSS), were synthesized through the amidation acidylation of aminopropylisobutyl POSS and benzoic acid derivatives (AzoMs) with one, two and three azobenzene groups (AzoM1, AzoM2 and AzoM3). Their structures were characterized by FT-IR, ¹H NMR, ¹³C NMR and mass spectra, and their thermal stability and photoresponsive behaviors in DMF solutions were evaluated with TGA, XRD and UV-vis spectra, respectively. The results indicated that the thermal stability and photoisomerization of azobenzenes could be effectively controlled by their molecular structure. In MonoAzo-POSS, the large steric hindrance of POSS destroys the molecular ordering and limits the molecular packing, contributing to its poor thermal stability. And the low molecular ordering of MonoAzo-POSS offers an azo group with large free space, and its trans-cis photoisomerization rate increases accordingly. But, in BisAzo-POSS and TriAzo-POSS, the incorporation of POSS units does not impact on the regularity of azobenzenes obviously, and the hindrance effect of nanosize POSS on the molecular motion plays a primary role in increasing their high thermal stability. Their photoisomerization rates decrease due to the steric hindrance of POSS and the unfolding structure of the azo moieties in BisAzo-POSS and TriAzo-POSS.

Full text of DOI:10.1039/c1nj20577c

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PAPER
Synthesis, thermal stability and photoresponsive behaviors of
azobenzene-tethered polyhedral oligomeric silsesquioxanesw
Jinlan Zhou,a Yongchen Zhao,a Kaichao Yu,a Xingping Zhou*a and Xiaolin Xieab
Received (in Montpellier, France) 4th July 2011, Accepted 8th September 2011
DOI: 10.1039/c1nj20577c
A series of azobenzene-tethered polyhedral oligomeric silsesquioxane (POSS) derivatives, i.e.
monoazobenzene-substituted POSS (MonoAzo-POSS), bisazobenzene-substituted POSS
(BisAzo-POSS) and triazobenzene-substituted POSS (TriAzo-POSS), were synthesized through
the amidation acidylation of aminopropylisobutyl POSS and benzoic acid derivatives (AzoMs)
with one, two and three azobenzene groups (AzoM1, AzoM2 and AzoM3). Their structures were
1
characterized by FT-IR, H NMR, 13C NMR and mass spectra, and their thermal stability and
photoresponsive behaviors in DMF solutions were evaluated with TGA, XRD and UV-vis
spectra, respectively. The results indicated that the thermal stability and photoisomerization of
azobenzenes could be effectively controlled by their molecular structure. In MonoAzo-POSS, the
large steric hindrance of POSS destroys the molecular ordering and limits the molecular packing,
contributing to its poor thermal stability. And the low molecular ordering of MonoAzo-POSS
offers an azo group with large free space, and its trans–cis photoisomerization rate increases
accordingly. But, in BisAzo-POSS and TriAzo-POSS, the incorporation of POSS units does not
impact on the regularity of azobenzenes obviously, and the hindrance effect of nanosize POSS
on the molecular motion plays a primary role in increasing their high thermal stability.
Their photoisomerization rates decrease due to the steric hindrance of POSS and the
unfolding structure of the azo moieties in BisAzo-POSS and TriAzo-POSS.
effect of free volume distribution on the reactivity of azobenzene
1. Introduction
in polycarbonate. Sasaki et al.13 studied the photoisomeriza-
tion and thermal isomerization behaviors of azobenzenes in
nematic and smectic polymer liquid crystals (PLCs), and
found that the azobenzenes show faster photoisomerization
in the former than in the latter since the nematic matrix
possesses more inner free volume than the smectic one. In
order to enhance the dispersion of azobenzene in the polymer
matrix, the azobenzene units are usually bonded in the
polymer chains or attached on the side chains covalently.4
However, the thermo- and photo-stability of those materials
are not enough at higher temperatures due to the rotational
freedom of the chromophores and the motion of polymer
chains.7 Recently, azobenzene chromophores have been
incorporated into the inorganic silica network by the sol–gel
process,14,15 the formed organic–inorganic hybrid materials
show good thermal stability, excellent nonlinear optical
properties, and reversible photoisomerization. But the sol–gel
glass film is more rigid, and there exists a strong interaction
between its precursor tetraethyl orthosilicate (TEOS) and
azobenzene, leading to the partial suppression of the isomeri-
zation of azobenzene.16
Photoresponsive molecular materials with azobenzene (Azo)
units are promising for applications such as photochemical
switches,1,2 optical information data-storages,3,4 sensors,5
liquid crystal display,6 optical limiting,7,8 site-related drug
release,9 and light-driven machines or actuators.10–12 For high
performance devices, the used materials require high thermal
stability, low fatigue, rapid response, high sensitivity, and
excellent processability. Mita et al.2 prepared polycarbonate/
azobenzene composite films and investigated their photo-
isomerization. They found that the azobenzene in poly-
carbonate at room temperature has the same trans–cis
photoisomerization rate as in ethyl acetate and ethanol up to
a conversion less than 86%, and then the trans–cis photo-
isomerization deviates from the first-order kinetics due to the
a Hubei Key Laboratory of Materials Chemistry and Service Failure,
School of Chemistry and Chemical Engineering, Huazhong
University of Science and Technology, Wuhan, 430074, China.
E-mail: xpzhou@mail.hust.edu.cn; Fax: +86-27-8754-3632;
Tel: +86-27-8755-8194
b National Anti-counterfeit Engineering Research Center, Huazhong
University of Science and Technology, Wuhan, 430074, China
w Electronic supplementary information (ESI) available: Experimental
details, FTIR, 1H NMR and Raman spectra. See DOI: 10.1039/
c1nj20577c
Polyhedral oligomeric silsesquioxane (POSS) is a unique
modifier to high-performance materials due to its unique
three-dimensional structure.17,18 In POSS molecules, the rigid
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inorganic core provides high stiffness, thermo- and photo-
stability, the organic corner groups offer excellent solubility,
processability and compatibility with other materials. Many
POSS-based functional materials, such as liquid crystal (LC)
materials,19–22 light-emitting materials,23–25 dental restorative
materials26 and ionic liquid,27 have been prepared. And they
are usually based on mono- or octa-substituted POSSs. The
incorporation of POSS moieties into organic molecules or
polymers may have dramatic effects on their properties, these
hybrid materials display superior properties to the organic
material alone. Thermal stability and photoresponsive
behaviors of azobenzene compounds depend strongly on their
structure and size, but the effect of the molecular regularity on
properties of POSS-based azobenzene hybrids has not been
found. In order to explore the relationship of the molecular
structure and its thermostability, photoisomerization and
potential application in optical data storage and optical
switches, we first designed and synthesized a series of
azobenzene-tethered POSS derivatives (Azo-POSSs), i.e.
monoazobenzene substituted POSS (MonoAzo-POSS),
bisazobenzene substituted POSS (BisAzo-POSS) and triazo-
benzene substituted POSS (TriAzo-POSS), as shown in Fig. 1,
and then studied their thermal stability and photoresponsive
behaviors.
2. Results and discussion
2.1 Synthesis and structure of AzoMs and their hybrids with
POSS
AzoMs, NH2-POSS and Azo-POSSs were successfully synthe-
sized according to the procedures in Scheme 1. Azobenzene
monomers (AzoMs), including 4-(6-(4-(4-methoxy)phenylazo)-
phenoxy) hexyloxybenzoic acid (designated as AzoM1), 3,4-bis
(6-(4-(4-methoxy)phenylazo)phenoxy)hexyloxybenzoic acid
(designated as AzoM2), and 3,4,5-tri(6-(4-(4-methoxy)-
phenylazo)phenoxy)hexyloxybenzoic acid (designated as
AzoM3), were synthesized by a substitution reaction of 1-bromo-
6-(4-methoxyazobenzene-40-oxy) hexane (BrMAB) with
methyl-4-hydroxybenzoate,
methyl-3,4-dihydroxybenzoate
and methyl-3,4,5-trihydroxy-benzoate, respectively. Synthetic
procedures of BrMAB were reported by Mitsuoka et al. and
Yang et al.28,29 However, the shortcomings include the
requirement for recrystallization and relatively low yield
(from 46% to 52%) making them inefficient in preparing BrMAB.
In this work, BrMAB was prepared efficiently by prolonging
the time for alkylation reaction of 4-hydroxy-40-methoxyazo-
benzene (HMAB) with 1,6-dibromohexane from 24 h to
36 h and changing purification procedures. Compared
with the previous methods, our method not only gives a higher
Fig. 1 Chemical structures of MonoAzo-POSS, BisAzo-POSS and
TriAzo-POSS.
Scheme 1 Synthetic procedures of AzoMs, NH2-POSS and Azo-POSSs.
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Fig. 2 1H NMR spectra of AzoM2, AzoM3 and ester of AzoM1.
yield (70%), but also offers attractive features such as opera-
tional convenience, dispensing with the recrystallization
process.
cyclo[9,5,1,13,9,1,7,1315,15]octasiloxane-1-yl)-propyl-4-(6-(4-(4-
methoxy)phenylazo)phenoxy)hexyloxybenzamide) (MonoAzo-
POSS),
(N-(3-(3,5,7,9,11,13,15-hepta-isobutyl)pentacyclo-
1
From H NMR spectra of AzoM2, AzoM3 and methyl-4-
[9,5,1,13,9,1,7,1315,15]octasiloxane-1-yl)-propyl-3,4-bis(6-(4-(4-
methoxy)phenylazo)phenoxy)hexyloxybenzamide) (BisAzo-POSS)
(6-(4-(4-methoxy)phenylazo)phenoxy)hexyloxybenzoate (ester
of AzoM1) (Fig. 2), there exists a singlet peak at 3.75–3.87 ppm
(–OCH3 on the aromatic rings), multiplet peaks at
6.95–8.2 ppm (Ar–H), triplet peaks at 3.95–4.08 ppm
(–OCH2– on the aromatic rings) and multiplet peaks at
1.48–1.90 ppm (–CH2CH2–) with the ratio of the integrated
areas of the absorption peaks in accordance with the proton
number of the expected compounds, respectively. These details
and FT-IR spectra are given in ESI.w It is worth noting that no
NMR spectrum of AzoM1 was recorded due to its poor
solubility in ordinary solvents, such as CDCl3, CD3OD,
(CD3)2CO and DMSO. These results indicate that AzoMs
are consistent with the proposed chemical structures.
and
(N-(3-(3,5,7,9,11,13,15-hepta-isobutyl)pentacyclo-
[9,5,1,13,9,1,7,1315,15]octasiloxane-1-yl)-propyl-3,4,5-tri(6-(4-(4-
methoxy)-phenylazo)phenoxy)hexyloxybenzamide) (TriAzo-
POSS) were synthesized by reaction of 3-(3,5,7,9,11,13,15-
heptaisobutylpentacyclo[9,5,1,1,3,91,7,131,5,15]octasiloxane-1-yl)-
propylamine (NH2-POSS) with the acyl chlorides of AzoMs.
The starting compound for NH2-POSS was prepared via
catalytic hydrolysis and condensation of isobutyltrimethoxy-
silane to yield incompletely condensed 1,3,5,7,9,11,14-
heptaisobutyltricyclo[7,3,3,15,14]heptasiloxane-3,7,11-trisilanol
(trisilanolisobutyl-POSS) in the presence of lithium hydroxide,
and then corner-capping reaction between trisilanolisobutyl-
POSS and 3-aminopropyltrimethoxysilane was employed to
obtain NH2-POSS.30,31
As shown in Scheme 1, three kinds of azobenzene-tethered
i.e.
POSSs,
(N-(3-(3,5,7,9,11,13,15-heptaisobutyl)penta-
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2956 cmꢀ1 are due to the vibration frequency of the C–H
bond. The characteristic absorption bands in AzoM2 are
located at B3448 cmꢀ1 for u (O–H), 1467 and 1601 cmꢀ1
for u (aromatic ring) and 1684 cmꢀ1 for u (CQO), respectively.
Especially, the absorption band at 1633 cmꢀ1 is due to
carbonyl stretching vibration (amide I) in BisAzo-POSS. This
proves that AzoM2 was anchored onto the POSS, and FT-IR
spectra of other Azo-POSSs are provided in ESI.w
Fig. 4 shows 1H NMR spectra of MonoAzo-POSS, BisAzo-
POSS and TriAzo-POSS. For example, the 1H NMR
spectrum of BisAzo-POSS is composed of the resonance peaks
of the protons, such as aromatic rings d (ppm): 6.97
(dd, Ar–H, 8H), 7.84 (m, Ar–H, 8H), 6.97 (dd, Ar–H, 8H),
7.84 (m, Ar–H, 8H), 6.83 (dd, Ar–H, 1H) and 7.42 (dd, Ar–H,
1H), aliphatic hydrocarbon groups d (ppm): 3.84 (s, –OCH3, 6H),
4.03 (t, –OCH2–, 8H), 1.83 (m, –OCH2CH2–, –SiCH2CH,
15H), 1.58 (m, –CH2CH2–, 8H), 3.42 (t, –NHCH2, 2H), 1.71
(m, –NHCH2CH2, 2H), 0.63 (m, –SiCH2CH2, 2H), 0.59
(m, –SiCH2CH, 14H), 0.98 (D, –CH(CH3)2, 42H), and amide
group d (ppm): 5.99 (s, NH, 1H). Additionally, in terms of the
integration intensities of protons in the aliphatic methyl,
Fig. 3 FT-IR spectra of NH2-POSS, BisAzo-POSS and AzoM2.
Fig. 3 shows FT-IR spectra of NH2-POSS, BisAzo-POSS
and AzoM2, the typical u (Si–O) strong bands at about
1112 cmꢀ1 and characteristic bands between 2873 and
Fig. 4 1H NMR spectra of MonoAzo-POSS, BisAzo-POSS and TriAzo-POSS.
c
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Fig. 5 13C NMR spectra of MonoAzo-POSS, BisAzo-POSS and TriAzo-POSS.
methylene and methenyl moiety to protons of aromatic rings,
it can be judged that the POSS compounds possess one propyl
group, seven isobutyl groups and one AzoM2 group. 1H NMR
spectra of MonoAzo-POSS and TriAzo-POSS are similar to
that of BisAzo-POSS.
Fig. 5 shows 13C NMR spectra of MonoAzo-POSS,
BisAzo-POSS and TriAzo-POSS. For example, the character-
istic CQO signal is observed at 167.07 ppm in BisAzo-POSS,
the peaks at 114.65, 114.15, 124.34, 146.98, 146.95, 161.54,
161.15, 119.15, 112.3,151.74, 149.00, 113.01, 127.60 ppm
are assigned to aromatic carbons, and all kinds of aliphatic
carbon peaks are observed at 42.32, 25.82, 23.85, 23.15,
22.48, 9.53, 55.50, 68.98, 68.14, 29.69, 29.10 ppm. Also,
13C NMR spectra of MonoAzo-POSS and TriAzo-POSS
are similar to that of BisAzo-POSS. Detail data are shown
in ESI.w
Furthermore, their ESI-HRMS and FAB-MS are as follows:
FAB-MS for MonoAzo-POSS, m/z, calcd C57H98N3Si8O16
([M + H]+): 1304.51 found: 1304.7; HRMS-MS for BisAzo-
POSS, calcd C76H119N5Si8O19Na ([M + Na]+): 1653.6566,
found: 1653.6621; HRMS-MS for TriAzo-POSS calcd
C95H141N7Si8O22Na ([M + Na]+): 1979.8198, found: 1979.8175.
All the experimental results are in good agreement with the
expected molecular weights and structures (see ESIw). In
summary, the synthesized MonoAzo-POSS, BisAzo-POSS
and TriAzo-POSS are in good accordance with their structures.
2.2 Thermal properties of AzoMs, MonoAzo-POSS,
BisAzo-POSS and TriAzo-POSS
Fig. 6 shows TGA and DTG curves of AzoMs and MonoAzo-
POSS, BisAzo-POSS and TriAzo-POSS. Obviously, there are
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used to compare their thermal stability. From Table 1, Td and
Tmax1 of MonoAzo-POSS are, respectively, 234 and 261 1C,
lower than 340 and 383 1C of AzoM1, indicating that the
stability of MonoAzo-POSS is poorer than that of AzoM1.
However, BisAzo-POSS and TriAzo-POSS have better stabi-
lity than AzoM2 and AzoM3. These results confirm that the
thermal stability of the hybrids with POSS is dominated by the
following two factors. On the one hand, the incorporation of
the inorganic POSS unit restricts the molecular motion,23,34
increases the heat capacity and provides a barrier to further
heat attack,35,36 leading to high thermal stability. On the other
hand, the incorporation of the inorganic POSS unit also
destroys the molecular ordering structure of azobenzenes
and decreases the molecular package density, leading to poor
thermal stability.37 As shown in Scheme 1, AzoM1 is most
symmetrical, and there exist binary intermolecular hydrogen
bonds,38 leading to high molecular package density, which is
confirmed by its high decomposition temperature and melting
point. After two or three azo groups are introduced in
azobenzenes, the regularity of AzoM2 and AzoM3 is
destroyed, leading to lower decomposition temperature and
melting point than those of AzoM1. Especially, the steric
hindrance of the POSS unit is higher than that of azo groups,
and the POSS unit acts as an inert diluent to decrease the self-
association interaction of molecules,39,40 which leads to the
lowest decomposition temperature and melting point of
MonoAzo-POSS. For BisAzo-POSS and TriAzo-POSS, their
Td and Tmax1 values increase to (368, 300 1C) and (394, 385 1C)
from (310, 265 1C) and (368, 363 1C) of AzoM2 and AzoM3,
indicating that the incorporation of POSS units effectively
improves the thermal stability of BisAzo-POSS and
TriAzo-POSS due to the enhanced molecular regularity and
restriction effects of POSS units on molecular motion.
Fig. 6 TGA (a) and DTG (b) curves of AzoMs, MonoAzo-POSS,
BisAzo-POSS and TriAzo-POSS.
three designated, respectively, as Tmax1, Tmax2 and Tmax3. They
are related to Tmax’s of the cleavage of NQN bonds, the
degradation of carboxyphenyl groups, and the breakdown of
the backbone.28 And MonoAzo-POSS, BisAzo-POSS and
TriAzo-POSS show the different thermal decomposition
behaviors with AzoM1, AzoM2 and AzoM3. Generally, the
initial thermal decomposition temperature (Td) is defined as
the temperature at 5% mass loss. The above calculated values,
residue yield and melting point are summarized in Table 1.
To understand the effect of molecular structure on the
packing arrangement, the XRD patterns of AzoM1, AzoM2,
MonoAzo-POSS and BisAzo-POSS (Fig. 7) were measured at
room temperature. For AzoM1, there exist multiple sharp
peaks with the four-order diffraction at 2y = 1.721, 3.411,
5.091 (overlapping with a peak at 5.331) and 7.111, corres-
ponding to a layer spacing of 5.13 nm. While the three-order
diffraction at 2y = 5.331, 10.641 and 15.201 characterizes
another layer spacing of 1.66 nm of AzoM1. The two peaks
at 2y = 20.581 (d = 0.43 nm) and 24.371 (d = 0.37 nm) are
ascribed to the packing of azo units and the typical p–p
stacking of aryl units, respectively.41–43 And the diffraction
peak at 2y = 19.201 (d = 0.46 nm) is attributed to the close-
packing of the alkyl chains.43,44 These results illustrate that
AzoM1 has a high-ordered layered structure. For AzoM2, the
From Table 1, AzoM1 has the highest Td, Tmax1 and Tmax3
,
and AzoM2 shows the highest Tmax2, which indicates that the
number of azo groups affect the molecular regularity of
azobenzenes, leading to their different thermal stabilities. Since
the cleavage of peripheral arms (420–500 1C) and the break-
down of the Si–O–Si cage (500–700 1C) in the POSS unit are
overlapped with the decompositions of carboxyphenyl groups
and the azobenzene backbone,32,33 Td and Tmax1 values are
Table 1 Thermal stability of AzoMs and their hybrids with POSS
Compounds
Td/1C
Tmax1/1C
Tmax2/1C
Tmax3/1C
Residue yield (%)
Melting point/1C
AzoM1
MonoAzo-POSS
AzoM2
BisAzo-POSS
AzoM3
TriAzo-POSS
340
234
310
368
265
300
383
261
368
394
363
385
401
375
442
478
436
456
575
481
518
540
538
539
9.60
22.40
9.74
31.00
0.01
26.10
Ba
165–167
183–185
184–185
158–159
175–177
a
Melting point of AzoM1 is close to its decomposition temperature.
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(2y = 20.581, d = 0.43 nm) and the p–p stacking of aryl units
(2y = 24.391, d = 0.37 nm) in MonoAzo-POSS are weaker
and broader than those in AzoM1, and the periodic peaks with
multi-order diffraction disappear. These results confirm that
the incorporation of POSS destroys the symmetry of AzoM1
and decreases the molecular package density of MonoAzo-
POSS. Interestingly, it can be seen that there exist the
enhanced diffraction peaks of azo units at 2y = 19.511 (d =
0.46 nm) and the p–p stacking of aryl units at 2y = 22.411
(d = 0.40 nm), and the three-order diffraction at 2y = 5.481,
11.051 and 16.511 (overlapping with the peak at 19.511)
corresponding to a layer spacing of 1.61 nm in BisAzo-POSS.
The results indicate that BisAzo-POSS has higher ordering
than MonoAzo-POSS.
To further substantiate the above XRD results, the opti-
mized geometries of AzoM1, AzoM2, MonoAzo-POSS and
BisAzo-POSS were calculated by molecular modeling soft-
ware (Hyper-Chem 7.5). Fig. 8 shows their energy-minimized
structures. The calculated van der Waals chain length (l) of
AzoM1 is 2.45 nm. Due to the binary intermolecular hydrogen
bonds, AzoM1 exists as a dimer with a layer spacing length (d)
as shown in Fig. 9. Obviously, d is double the length of l
(4.90 nm) and binary hydrogen bonds, and close to the layer
spacing of 5.13 nm obtained by XRD. Additionally, the
calculated interplanar spacing lengths of AzoM1 and AzoM2
are 1.56 nm (see Fig. 8A and B), close to the above 1.66 nm
and 1.60 nm determined by XRD, which suggests that AzoM1
and AzoM2 are lamellar (see Fig. 9A and B). As shown in
Fig. 8C, the azo units in MonoAzo-POSS are bent towards
POSS via the flexible –O(CH2)6O– spacer, the formed large
steric hindrance limits the packing of MonoAzo-POSS as
shown in Fig. 9C, leading to the weak diffraction peaks of
azo and aryl units. However, different from those in MonoAzo-
POSS, azo and aryl units in BisAzo-POSS unfold as shown in
Fig. 8D, and they easily form an effective packing as shown in
Fig. 9D.
Fig. 7 XRD patterns of AzoM1, AzoM2, MonoAzo-POSS and
BisAzo-POSS.
diffraction peaks of azo units (2y = 20.111, d = 0.44 nm), the
p–p stacking of aryl units (2y = 23.891, d = 0.37 nm), the
close-packing of the alkyl chains (2y = 21.281, d = 0.42 nm)
are similar to those in AzoM1. However, AzoM2 only has
three-order diffraction at 2y = 5.531, 11.021 and 15.961
corresponding to a layer spacing of 1.60 nm. The results
confirm that AzoM2 exhibits lower ordering than that of
AzoM1. Additionally, for MonoAzo-POSS, the peak at 2y =
8.121 (d = 1.09 nm) reflects the size of POSS units,45 and the
peaks at 2y = 7.671 (d = 1.15 nm), 8.831 (d = 1.01 nm), 12.91
(d = 0.69 nm), 19.61 (d = 0.45 nm) and 24.41 (d =
0.36 nm) are related to the rhombohedral crystal structure
of POSS.43,45,46 However, the diffraction peaks of azo units
Fig. 8 Energy-minimized structures of AzoM1 (A), AzoM2 (B), MonoAzo-POSS (C) and BisAzo-POSS (D).
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Fig. 9 Possible aggregation models of AzoM1 (A), AzoM2 (B), MonoAzo-POSS (C) and BisAzo-POSS (D).
2.3 Photochemical behaviors
trans-azobenzene units, the p–p* and n–p* transition of the
cis-isomer. Under UV irradiation, the absorbance at 360 nm
decreases significantly while the absorbances at 310 and
460 nm increase slightly due to trans–cis photoisomerization.
AzoM1, AzoM2, MonoAzo-POSS and BisAzo-POSS show
similar features.
To examine the photochemical trans–cis isomerization of
AzoMs and their hybrids, their solutions in DMF were
irradiated with 365 nm unpolarized UV light until they
reached a photostationary state. Typically, Fig. 10 shows the
changes in UV-vis spectra of AzoM3 and TriAzo-POSS with
different irradiation times. The absorption peaks at 360, 310
and 460 nm are, respectively, related to the p–p* transition of
Fig. 10 UV-vis spectra of AzoM3 (A) and TriAzo-POSS (B) in DMF
upon irradiation with 365 nm UV light at room temperature. Arrows
indicate changes upon irradiation.
Fig. 11 ln[(A0 ꢀ AN)/(At ꢀ AN)]-t plots of AzoMs and their hybrids
with POSS.
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Table 2 Photoisomerization rates of AzoMs and their hybrids with POSS
Compounds
k/sꢀ1
AzoM1
MonoAzo-POSS
AzoM2
BisAzo-POSS
AzoM3
TriAzo-POSS
0.20
0.24
0.22
0.17
0.25
0.20
The first-order rate constant (k) is determined by fitting the
experimental data to the following equation:13,47
potassium carbonate, sodium nitrite, 1,6-dibromohexane,
sodium hydroxide, lithium hydroxide, absolute methanol,
petroleum ether, acetone, anhydrous magnesium sulfate,
chloroform, dichloromethane, ethyl acetate, tetrahydrofuran,
dimethyl sulfoxide, and other chemicals were used as received
from Sinopharm Group Chemical Reagent Co., Ltd, Shanghai,
China.
A0 ꢀ A
1
1
ln
¼ kt
At ꢀ A
where At, A0, and AN are absorbances at 360 nm at time t,
time zero, and infinite time, respectively.
Fig. 11 shows the relationship between ln[(A0 ꢀ AN)/
(At ꢀ AN)] and t. Obviously, their trans–cis isomerizations
obey the first-order reaction, the corresponding k values are
listed in Table 2.
Measurements. Fourier transform infrared (FT-IR) spectra
were recorded on a Bruker Equinox 55 spectrometer with a
disc of KBr. 1H NMR and 13C NMR spectra were recorded on
a Bruker AV400 spectrometer with TMS as an internal
standard. Electrospray ionization high-resolution mass
spectroscopy (ESI-HRMS) was carried out on a Perkin-Elmer
QSTAR mass spectrometer, fast atom bombardment mass
spectrometry (FAB-MS) was carried out on a Finnigan
SSQ-710 mass spectrometer. Thermal gravimetric analysis
(TGA) was conducted on a TGA-7 Perkin-Elmer calorimeter
under argon flow (20 mL minꢀ1) at 10 1C minꢀ1. X-Ray
diffraction (XRD) was performed using an X’Pert PRO
powder diffractometer with CuKa radiation (0.15406 A) with
the 2y range of 1–401 at room temperature. UV-vis absorption
spectra were recorded at room temperature in DMF solution
on a diode-array spectrophotometer (Shimadzu UV-2550).
The spectral region 550–280 nm was examined by using a cell
path length of 10 mm. Photoisomerization experiments were
carried out by using 365 nm light from an ultraviolet lamp
(32 W, ELC-500, light exposure system electrolite corporation,
US). Melting point data were collected on a WRS-1A Melting-
point Apparatus with a heating rate of 11 minꢀ1. Calculations
were made utilizing the Spartan program. This calculation
package provides optimized geometries constructed by using
the semi-empirical quantum chemistry AM1 method.
From Table 2, the k value of AzoM1 (0.20 sꢀ1) is smaller
than those of AzoM2 (0.22 sꢀ1) and AzoM3 (0.25 sꢀ1),
indicating that the low molecular ordering of azobenzene
endows azo moieties with large free volume and high mobility,
and their trans–cis photoisomerization rates increase accord-
ingly. Similarly, the incorporation of POSS units also
decreases the molecular ordering of azobenzene, leading to
the fast photoisomerization rate (0.24 sꢀ1) of MonoAzo-
POSS. However, as discussed above, the azo moieties in
BisAzo-POSS and TriAzo-POSS unfold, leading to small free
volume for their mobility. Additionally, POSS units also limit
their mobility, the k values of the BisAzo-POSS and TriAzo-
POSS decrease to 0.17 and 0.20 sꢀ1 accordingly. These results
indicate that the photoisomerization of azobenzenes can be
effectively controlled by their molecular structure.
3. Conclusions
The thermal stability and photoisomerization of azobenzenes
could be effectively controlled by their molecular structure.
The large steric hindrance of POSS destroys the molecular
ordering of AzoM1, limits the molecular packing and offers
azo moieties with a large free space, leading to poor thermal
stability and high trans–cis photoisomerization of MonoAzo-
POSS. But, the incorporation of POSS units does not impact
on the regularity of azobenzenes obviously, and the hindrance
effect of nanosize POSS on the molecular motion plays a main
role in increasing the high thermal stability of BisAzo-POSS
and TriAzo-POSS. Their photoisomerization rates decrease
due to the steric hindrance of POSS and the unfolding
structure of the azo moieties in BisAzo-POSS and TriAzo-
POSS.
4.2 Synthesis
4-Hydroxy-40-methoxyazobenzene (HMAB) was synthesized
according to a method similar to that of ref. 28 and 29. Yield:
76.98%. IR (KBr, cmꢀ1), 3324 (u, O–H), 3100–3050 (u, C–H,
aromatic), 2980–2850 (u, C–H, CH2, and CH3), 1602–1470
(u, CQC, aromatic), 1247–1040 (u, C–O). 1395 (u, NQN), and
1383 (d, C–H, CH2, and CH3), 842 (doop, p-Ar).
Synthesis of BrMAB. HMAB (11.4 g, 0.05 mol), 1,6-dibromo-
hexane (73.2 g, 0.30 mol), and anhydrous potassium carbonate
(22.175 g, 161 mmol) were added to acetone (500 mL) under
magnetic stirring, and the reaction mixture was heated to
75 1C and refluxed for 36 h. After the reaction was complete,
the hot solution was immediately filtered and the inorganic
residues were washed thoroughly with hot acetone. The
filtrates were collected and distilled under vacuum to remove
acetone, and the remaining concentrated extracts were poured
into cooled petroleum ether (60–90 1C). The precipi-
tated product was filtered off and washed twice with petroleum
ether to yield 13.7 g of yellow squamose crystals. Yield: 70%.
4. Experimental section
4.1 Materials and methods
Materials. Isobutyltrimethoxysilane ((CH3)2CHCH2Si(OCH3)3)
(Alfa Aesar 98%) was used without further purification.
Absolute ethanol was dried by sodium, and dimethylform-
amide (DMF) was purified by vacuum distillation before
use. Sulfuric acid (98%), nitric acid (67%), hydrochloric
acid (36.5%), thionyl chloride, phenol, 4-methoxyaniline,
c
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New J. Chem., 2011, 35, 2781–2792 2789

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IR (KBr, cmꢀ1) 3100–3050 (u, C–H, aromatic), 2936–2863
(u, C–H, CH2, and CH3), 1602–1470 (u, CQC, Ar), 1395
(u, NQN), 1247–1040 (u, C–O). 842 (doop, p-Ar), 1H NMR
(400 MHz, CDCl3) d: 3.91 (s, –OCH3, 3H), 7.01 (dd, Ar–H,
4H, ortho to O), 7.88 (dt, Ar–H, 4H, ortho to N), 4.02
(t, –OCH2–, 2H), 1.93 (m, –OCH2CH2–, 2H), 1.6
(m, –CH2CH2–, 4H), 1.84 (m, –CH2CH2Br, 2H), 3.45
AzoM2 as a yellow powder (3.2 g, yield: 82.7%). IR
(KBr, cmꢀ1), 3448 (u, O–H), 2935–2857 (u, C–H, CH2, and
CH3), 1684 (u, CQO), 1601–1467 (u, CQC, Ar), 1246–1029
(u, C–O). 1392 (u, NQN), 842 (doop, p-Ar), 1H NMR
(400 MHz, Pr–d5) d: 3.75 (s, –OCH3, 6H), 7.13 (d, Ar–H,
8H, ortho to O), 8.15 (d, Ar–H, 8H, ortho to N), 4.05
(t, –OCH2–, 4H), 3.95 (t, –CH2O–, 4H) 1.90 (m, –OCH2CH2–,
8H), 1.48 (m, –CH2CH2–, 8H), 8.15 (d, Ar–H, 1H,), 8.2
1
(t, –CH2Br, 2H). Its FT-IR and H NMR spectra are shown
in Fig. S1 and S2 (ESIw).
1
(d, Ar–H, 1H), 7.2 (d, Ar–H, 1H). Its FT-IR and H NMR
spectra are shown in Fig. S5 and S6 (ESIw).
Synthesis of ester of AzoM1. The alkyloxylation reaction
was carried out by refluxing methyl-4-hydroxybenzoate
(0.76 g, 5 mmol), BrMAB (2.346 g, 6 mmol), K2CO3
(0.828 g, 6 mmol), and KI (0.166 g, 1 mmol) in 2-butanone
(150 mL), and the reaction mixture was stirred at 75 1C for
24 h. After the reaction, the mixed solution was concentrated
in vacuo, and the residue was poured into dilute HCl. The
yellow precipitate was filtered, and washed with water until the
liquid phase became neutral and dried in vacuo to afford a
yellow solid. The crude product was recrystallized from CHCl3
and dried under vacuum to get 1.80 g of a yellow powder.
Yield: 97.3%. IR (KBr, cmꢀ1), 2936–2864 (u, C–H, CH2, and
CH3), 1652 (u, CQO), 1601–1468 (u, CQC, Ar), 1246–1028
(u, C–O). 1395 (u, NQN), 842 (doop, p-Ar), 1H NMR
(400 MHz, CDCl3) d: 3.87 (s, –OCH3, 6H), 6.95 (dd, Ar–H,
4H, ortho to O), 7.86 (m, Ar–H, 4H, ortho to N), 4.08
(t, –OCH2–, 4H), 1.84 (m, –OCH2CH2–, 4H), 1.59
(m, –CH2CH2–, 4H), 6.88 (dd, Ar–H, 2H, ortho to O), 7.99
Synthesis of AzoM3. AzoM3 was obtained as an orange
powder, in a similar procedure as described for the synthesis of
AzoM1 reflux for 20 h. The crude product was purified by
silica gel column chromatography eluting with CH2Cl2/MeOH
(20/1) to give 3.685 g of AzoM3. Yield: 67% IR (KBr, cmꢀ1),
3449 (u, O–H), 2937–2861 (u, C–H, CH2, and CH3), 1684
(u, CQO), 1596–1465 (u, CQC, Ar), 1248–1028 (u, C–O). 1391
1
(u, NQN), 840 (doop, p-Ar), H NMR (400 MHz, Pr–d5) d:
3.85 (s, –OCH3, 9H), 6.95 (d, Ar–H, 12H, ortho to O), 7.83
(d, Ar–H, 12H, ortho to N), 4.07 (t, –OCH2–, 6H), 3.99
(t, –CH2O–, 6H) 1.83 (m, –OCH2CH2–, 12H), 1.77 (m,
–CH2CH2–, 12H), 7.34 (d, Ar–H, 2H). Its FT-IR and
1H NMR spectra are shown in Fig. S7 and S8 (ESIw).
Synthesis of trisilanolisobutyl-POSS. Isobutyltrimethoxy-
silane (46.65 g, 261.6 mmol), acetone (250 mL), distilled water
(4 g, 222 mmol), and LiOHꢁH2O (5.0 g, 119.1 mmol) were
charged in a three-necked flask equipped with a reflux con-
denser and a magnetic stirrer. The mixture was refluxed in an
oil bath for 20 h with vigorous stirring and was acidified by
quenching it into 1 mol Lꢀ1 HCl (aq) (250 mL) and stirring for
2 h. The resulting solid was filtered and washed two times with
CH3CN (2 ꢂ 80 mL) and air dried. 32.36 g of the product was
isolated in 73.8% yield. IR (KBr, cmꢀ1), 3250 (u, O–H),
2955–2872 (u, C–H, CH2, and CH3), 1111 (u, Si–O). Its
FT-IR spectrum is shown in Fig. S9 (ESIw).
1
(dd, Ar–H, 2H, ortho to COOCH3). Its H NMR spectrum is
shown in Fig. S3 (ESIw).
Synthesis of esters of AzoM2 and AzoM3. Esters of AzoM2
and AzoM3 were synthesized according to the synthesis of
AzoM1. Yield for esters of AzoM2: 98.4%. 2935–2860
(u, C–H, CH2, and CH3), 1686 (u, CQO), 1601–1467
(u, CQC, Ar), 1246–1028 (u, C–O). 1392 (u, NQN), 842
(doop, p-Ar). Yield for ester of AzoM3: 99.35%.
Synthesis of AzoM1. To a solution of ester of AzoM1
(2.31 g, 5 mmol) in 250 mL of the EtOH/THF mixture was
added a solution of NaOH (4 g, 100 mmol) in deionized water
(100 mL), and the reactants was stirred and refluxed for 8 h.
The mixed solution was concentrated in vacuo. The residues
were poured into dilute HCl at 0 1C, and the reaction mixture
was acidified to pH o 7 with aqueous HCl solution. The
yellow precipitate was filtered and washed with water until the
liquid phase became neutral. After drying, the crude product
was recrystallized from CHCl3 and dried in vacuo to get 1.97 g
of AzoM1 as a yellow powder. Yield 87.9%. No NMR
spectrum was recorded because the solubility of the final
compound was too low for ordinary solvents (CDCl3,
CD3OD, (CD3)2CO, DMSO and so on). IR (KBr, cmꢀ1),
3313 (u, O–H), 2936–2864 (u, C–H, CH2, and CH3), 1656
(u, CQO), 1601–1468 (u, CQC, Ar), 1246–1029 (u, C–O). 1395
(u, NQN), 842 (doop, p-Ar). Its FT-IR spectrum is shown in
Fig. S4 (ESIw).
Synthesis of NH2-POSS. Trisilanolisobutyl-POSS (20 g,
25.20 mmol) was dissolved in ethanol (125 mL) followed by
addition of 3-aminopropyltrimethoxysilane (4.35 g, 25.20 mmol)
and tetraethylammonium hydroxide (0.39 g, 0.52 mmol of a
20% methanol solution). The solution was stirred at 20 1C for
36 h. The solvent was evaporated and the product was washed
with acetonitrile, recovered, and dried to yield 17.1 g, 77.4% of
the product as a white solid. IR (KBr, cmꢀ1), 3567–3503
(u, N–H), 2956–2873 (u, C–H, CH2, and CH3), 1112
1
(u, Si–O), H NMR (400 MHz, CDCl3) d: 2.6 (s, –CH2NH2,
2H), 1.9 (m, –CH2CH, –CH2CH2, 9H), 0.6 (t, Si–CH2–, 16H),
1.0 (d, –CH(CH3)2, 42H), 7.1 (d, –NH2, 2H). Its FT-IR
spectrum is shown in Fig. S10 (ESIw).
Synthesis of AzoCOCl1, AzoCOCl2 and AzoCOCl3. It
should be noted that the acyl chlorides were induced on
AzoCOCl1, AzoCOCl2 and AzoCOCl3 by reaction of AzoM1,
AzoM2 and AzoM3 with excess thionyl chloride (SOCl2),
respectively. Typically for AzoCOCl1, AzoM1 (1.12 g,
2.5 mmol) was suspended in 50 mL of CH2Cl2 and a catalytic
amount of DMF was added, and then SOCl2 (0.893 g,
7.38 mmol) was added dropwise under stirring. The reaction
was carried out for 4 h at room temperature, and at 40 1C for
Synthesis of AzoM2. Following the procedure for AzoM1,
AzoM2 was obtained from an ester of AzoM2 under reflux for
12 h. The crude product was purified using silica gel column
chromatography eluting with CH2Cl2/MeOH (15/1) to give
c
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another 4 h under reflux conditions till the reactant became a
clear homogenous solution. Then CH2Cl2 and the excessive
SOCl2 were evaporated under vacuum carefully. The resulting
orange acyl chloride was dissolved in 80 mL of CH2Cl2.
AzoCOCl2 and AzoCOCl3 were synthesized by the same
procedure as described for the preparation of AzoCOCl1.
Synthesis of MonoAzo-POSS. A three-neck 50 mL flask was
charged with NH2-POSS (1.746 g, 2 mmol), CH2Cl2 (10 mL)
and triethylamine (0.40 mL, 2.86 mmol) in order and cooled to
0 1C with stirring. The acyl chloride solution (AzoCOCl1) was
added into chilled amine solution; the flask containing the acyl
chloride was treated with an additional 5 mL of CH2Cl2 and
again added into the amine solution. The resulting mixture
was allowed to stir for 2 h at 0 1C, and then refluxed for
another 4 h. The light brown phase was washed with sat. aq.
NaHCO3 (5 ꢂ 100 mL), distilled water (3 ꢂ 100 mL), dried
(MgSO4), filtered and concentrated under reduced pressure to
get a yellow solid. The crude product was purified using silica
gel column chromatography eluting with CH2Cl2/MeOH (30/1)
to give MonoAzo-POSS as a yellow powder (1.59 g, 61%). IR
(KBr, cmꢀ1), 3289 (u, N–H), 2955–2871 (u, C–H, CH2, and
CH3), 1109 (u, Si–O), 1636 (u, CQO), 1604–1469 (u, CQC,
Ar), 1401 (u, NQN), 1254–1170 (u, C–O), 840 (d00P, p-Ar),
1H NMR (400 MHz, CDCl3) d: 3.89 (s, –OCH3, 3H), 6.98 (dd,
Ar–H, 4H, ortho to O), 7.88 (m, Ar–H, 4H, ortho to N), 4.08
(t, –OCH2–, 4H), 1.84 (m, –OCH2CH2–, –SiCH2CH, 11H),
1.58 (m, –CH2CH2–, 4H), 6.89 (dd, Ar–H, 2H, ortho to O),
7.70 (dd, Ar–H, 2H, ortho to CONH–), 5.99 (s, NH, 1H), 3.42
(t, –NHCH2, 2H), 1.69 (m, –NHCH2CH2, 2H), 0.64
(m, –SiCH2, CH2, –2H), 0.59 (m, –SiCH2, CH, –14H), 0.98
(D, –CH(CH3)2, –42H), 13C NMR (400 MHz, CDCl3,), d
166.98 (CQO), 114.69 (o-ArC to –OCH3), 114.20 (o-ArC to
–O(CH2)6–), 124.36 (m-ArC to –OCH3 and –O(CH2)6–),
147.07 (p-ArC to –OCH3), 146.95 (p-ArC to –O(CH2)6–),
161.61 (ArC next to –OCH3 and next to –O(CH2)6–), 161.17
(p-ArC to –CONH–), 131.59 (o-ArC to –CONH–), 128.57
(m-ArC to –CONH–), 127.07 (ArC next to –CONH–), 42.23
(NCH2(CH2)2), 25.70 (SiCH2CH(CH3)2), 23.86 (SiCH2CH-
(CH3)2), 23.14 (NCH2CH2CH2), 22.52 (SiCH2CH(CH3)2),
9.52 (N(CH2)2CH2), 55.57 (–OCH3), 68.13, 68.00
(dd, Ar–H, 1H), 6.01 (s, NH, 1H), 3.42 (t, –NHCH2, 2H), 1.71
(m, –NHCH2CH2, 2H), 0.63 (m, –SiCH2, CH2, –2H), 0.59
(m, –SiCH2, CH, –14H), 0.98(D, –CH(CH3)2, –42H), 13C
NMR (400 MHz, CDCl3) d: 167.07 (CQO),114.65 (o-ArC to
–OCH3), 114.15 (o-ArC to –O(CH2)6–), 124.34 (m-ArC to
–OCH3 and –O(CH2)6–), 146.98 (p-ArC to –OCH3), 146.95
(p-ArC to –O(CH2)6–), 161.54 (ArC next to –OCH3), 161.15
(ArC next to –O(CH2)6–), 119.15, 112.3 (o-ArC to –CONH–),
151.74 (p-ArC to –CONH–), 149.00 (m-ArC (next to
–O(CH2)6–) to –CONH–), 113.01 (m-ArC to –CONH–),
127.60 (ArC next to –CONH–), 42.32 (NCH2(CH2)2), 25.82
(SiCH2CH(CH3)2), 23.85 (SiCH2CH(CH3)2), 23.15 (NCH2-
CH2CH2), 22.48 (SiCH2CH(CH3)2), 9.53 (N(CH2)2CH2),
55.50 (–OCH3), 68.98, 68.14 (–OCH2(CH2)4CH2O–), 29.69
(–OCH2CH2(CH2)2CH2CH2O–), 29.10 (–O(CH2)2CH2CH2-
(CH2)2O–), HRMS-MS calcd C76H119N5Si8O19Na ([M +
Na]+): 1653.6566, found: 1653.6621. Spectra of TriAzo-
POSS: IR (KBr, cmꢀ1), 3289 (u, N–H), 2951–2870 (u, C–H,
CH2, and CH3), 1107 (u, Si–O), 1634 (u, CQO), 1594–1465
(u, CQC, Ar), 1392 (u, NQN), 1250 (u, C–O), 840 (d00P, p-Ar).
1H NMR (400 MHz, CDCl3) d: 3.86 (s, –OCH3, 9H), 6.97
(m, Ar–H, 14H, ortho to –OCH3, ortho to –OCH2–, ortho to
–CONH), 7.83 (m, Ar–H, 12H, ortho to N), 4.00 (t, –OCH2–,
12H), 1.84 (m, –OCH2CH2–, –SiCH2CH, 19H), 1.57 (m,
–CH2CH2–, 12H), 5.99 (s, NH, 1H), 3.42 (t, –NHCH2, 2H),
1.66 (m, –NHCH2CH2, 2H), 0.65 (m, –SiCH2, CH2CH2NH,
2H), 0.59 (m, –SiCH2, CH, 14H), 0.98 (D, –CH(CH3)2, 42H),
13C NMR (400 MHz, CDCl3) d: 167.29(CQO), 114.65 (o-ArC
to –OCH3), 114.16 (o-ArC to –O(CH2)6–), 124.36 (m-ArC to
–OCH3 and –O(CH2)6–), 146.97 (p-ArC to –OCH3), 146.94
(p-ArC to –O(CH2)6–), 161.54 (ArC next to –OCH3), 161.13
(ArC next to –O(CH2)6–), 105.83 (o-ArC to –CONH–), 152.79
(m-ArC to –CONH–), 141.04 (p-ArC to –CONH–), 130.13
(ArC next to –CONH–), 42.52 (NCH2(CH2)2), 25.72 (SiCH2-
CH(CH3)2), 23.92 (SiCH2CH(CH3)2), 23.2 (NCH2CH2CH2),
22.53 (SiCH2CH(CH3)2), 9.59 (N(CH2)2CH2), 55.50 (–OCH3),
68.99, 68.19 (–OCH2(CH2)4CH2O–), 31.94 (–OCH2CH2-
(CH2)2CH2CH2O–), 29.36 (–O(CH2)2CH2CH2(CH2)2O–),
HRMS-MS calcd C95H141N7Si8O22Na ([M
+
Na]+):
1979.8198, found: 1979.8175. Their FT-IR, 1H NMR,
13C NMR and ESI-HRMS spectra are shown in Fig. S15
and S22 (ESIw), respectively.
(–OCH2(CH2)4CH2O–),
CH2O–), 29.12 (–O(CH2)2CH2CH2(CH2)2O–), FAB-MS,
29.17
(–OCH2CH2(CH2)2CH2-
1
m/z(%): 1304.7 ([M + H]+). Its FT-IR, H NMR, 13C NMR
and FAB-MS spectra are shown in Fig. S11 and S14 (ESIw).
Synthesis of BisAzo-POSS and TriAzo-POSS. BisAzo-
POSS and TriAzo-POSS were obtained as orange powder
via a similar procedure as described for the synthesis of
MonoAzo-POSS, using NH2-POSS reacted with AzoCOCl2
and AzoCOCl3, respectively, and the crude products were
purified using silica gel column chromatography eluting with
CH2Cl2/MeOH (35/1) to yield BisAzo-POSS (1.92 g, 59%)
and TriAzo-POSS (2.47 g, 63%). Spectra of BisAzo-POSS: IR
(KBr, cmꢀ1), 3289 (u, N–H), 2954–2870 (u, C–H, CH2, and
CH3), 1108 (u, Si–O), 1633 (u, CQO), 1599–1469 (u, CQC,
Ar), 1400 (u, NQN), 1256–1167 (u, C–O), 840 (d00P, p-Ar),
1H NMR (400 MHz, CDCl3) d: 3.84 (s, –OCH3, 6H), 6.97
(dd, Ar–H, 8H, ortho to O), 7.84 (m, Ar–H, 8H, ortho to N),
4.03 (t, –OCH2–, 8H), 1.83 (m, –OCH2CH2–, –SiCH2CH,
15H), 1.58 (m, –CH2CH2–, 8H), 6.83 (dd, Ar–H, 1H), 7.42
Acknowledgements
We wish to acknowledge the financial support by the Out-
standing Youth Fund of the National Natural Science
Foundation of China (50825301), and by the Fund from the
State Key Laboratory of Plastic Forming Simulation and
Moulding Technology at Huazhong University of Science
and Technology (HUST). We thank the HUST Analytical
and Testing Center for allowing us to use its facilities.
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