Synthesis of ordered photoresponsive azobenzene-siloxane hybrids by self-assembly

Article abstract of DOI:10.1039/c3tc30587b

In this study, photoresponsive azobenzene-siloxane hybrids with lamellar structures were prepared by self-assembly using two types of alkoxysilane precursors, 4-[3-(triethoxysilyl)propoxy]azobenzene (P1) and 4-[3-(diethoxymethylsilyl)propoxy]azobenzene (P

Full text of DOI:10.1039/c3tc30587b

Journal of
Materials Chemistry C
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PAPER
Synthesis of ordered photoresponsive azobenzene–
siloxane hybrids by self-assembly†
Cite this: J. Mater. Chem. C, 2013, 1,
989
6
Sufang Guo, Ayae Sugawara-Narutaki, Tatsuya Okubo and Atsushi Shimojima‡*
In this study, photoresponsive azobenzene–siloxane hybrids with lamellar structures were prepared by self-
assembly using two types of alkoxysilane precursors, 4-[3-(triethoxysilyl)propoxy]azobenzene (P1) and 4-[3-
(
diethoxymethylsilyl)propoxy]azobenzene (P2). The films H1 and H2 were prepared by spin-coating
hydrolyzed solutions of P1 and P2, respectively, on a glass substrate followed by heating to induce
polycondensation. X-ray diffraction patterns revealed that H1 and H2 have lamellar structures with
different d-spacings (3.20 nm and 2.37 nm, respectively), suggesting that the arrangements of the
azobenzene moieties are different. These samples show slight but reversible changes in the d-spacings
under photo-irradiation. Under UV irradiation, H1 shows a slight decrease in d-spacing, while H2 shows
a slight increase. Such changes were caused by trans–cis isomerization of a part of the azobenzene
moieties in the films, as confirmed by UV-vis absorption spectroscopy. These processes were reversible,
with the d-spacings recovering their original values under visible light irradiation. Furthermore, P1 and
0
0
P2 were co-hydrolyzed and polycondensed with tetraethoxysilane to give lamellar films (H1 and H2 )
Received 29th March 2013
Accepted 26th August 2013
0
0
showing a higher degree of trans–cis photoisomerization of the azobenzene moieties. Both H1 and H2
show increase in the d-spacings after soaking in various organic solvents. Possible structural models
have been proposed to explain these photoresponsive properties of the azobenzene–siloxane
nanohybrids, which will find potential applications as smart sensors and adsorbents in future.
DOI: 10.1039/c3tc30587b
www.rsc.org/MaterialsC
should be guaranteed to realize efficient photoisomerization.
The aforementioned polymer-based materials have satised
1
Introduction
Photoresponsive materials have attracted increasing attention these requirements; for example, azobenzene moieties are
1
–7
owing to their potential applications in smart devices. Azo- unidirectionally aligned in the lm by a rubbing procedure, and
benzene is a typical photoresponsive molecule that undergoes the exibility of the polymer chains gives them adequate free
photochemical conversion between trans and cis isomers, volume and mobility.
8
accompanied by large changes in the molecular shape and size.
Silica-based inorganic–organic hybrid materials have been
extensively studied because of their advantages of high thermal
and chemical stabilities, good mechanical properties and high
Azobenzene-containing photoresponsive organic polymers such
9
10
as liquid crystal elastomers (LCEs), liquid crystal gels (LCGs)
11
16
and polymer particles showing reversible size, shape or colour
changes or bending–unbending motions under irradiation have of photoresponsive smart materials. Ogawa et al. and Inoue
been reported. Moreover, photoresponsive materials capable of et al. have reported the intercalation of azobenzene derivatives
various dynamic motions, such as directed motion, oscilla- between layered inorganic crystals and demonstrated their
transparency. Azobenzene–silica hybrids could be a new class
17
18
12
13
14
15
tion,
rolling
and inchworm movement,
have been interesting photoresponsive behaviours, such as photo-induced
prepared. The requirements to achieve such unique properties changes in the basal spacings and adsorption properties. On
are as follows: (1) azobenzene moieties should be arranged in
order or selectively excited with polarized light to convert the
molecular-level shape change or motions into the macroscopic
the other hand, self-assembly techniques in combination with
sol–gel chemistry provide more diverse control over the struc-
ture, composition and morphology of silica-based hybrid
scale and (2) free volume and mobility of azobenzene moieties materials.19–22 Azobenzene moieties are graed to the inner wall
of mesoporous silica either by co-condensation of azobenzene-
modied alkoxysilane and tetraethoxysilane (TEOS) in the
Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo,
23,24
Bunkyo-ku, Tokyo 113-8656, Japan. E-mail: shimoji@chemsys.t.u-tokyo.ac.jp; Fax: presence of a surfactant
or by post-modication of meso-
25
+
†
1
‡
81-3-5800-3806; Tel: +81-3-5841-7348
porous silica to achieve dynamic pore size changes and
controlled release of small molecules. Periodic mesoporous
organosilicas (PMOs) in which azobenzene moieties are
Electronic supplementary information (ESI) available: Fig. S1–S6. See DOI:
0.1039/c3tc30587b
Present address: Department of Applied Chemistry, Waseda University, 3-4-1
26,27
embedded in the wall have also been prepared.
Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan. E-mail: shimojima@waseda.jp
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Furthermore, ordered azobenzene–silica hybrid materials can yield of 69%) were obtained aer recrystallization from EtOH.
1
be prepared by surfactant-free self-assembly processes. Liu
H NMR (d, 270 MHz, CDCl ): 4.62 (d, 2H, OCH CH]CH ),
3 2 2
28
et al. reported the self-assembly of bis-trialkoxysilylated azo- 5.30–5.48 (d, 2H, OCH CH]CH ), 6.01–6.15 (m, 1H, OCH CH]
2
2
2
benzene into a lamellar structure by intermolecular hydrogen CH ), 7.01–7.05 (d, 2H, ArH), 7.40–7.53 (m, 3H, ArH), 7.86–7.94
2
1
3
bonding. Unfortunately, this lamellar hybrid shows no photo- (m, 4H, ArH). C NMR (d, 67.8 MHz, CDCl
3
): 69.02, 114.94,
responsive properties, probably owing to the low mobility and 118.05, 122.54, 124.70, 129.01, 130.35, 132.73, 147.07, 152.76,
lack of free volume between azobenzene moieties.
161.03.
In this study, novel photoresponsive azobenzene–siloxane
hybrid materials with lamellar structures have been prepared
using triethoxysilyl- and diethoxysilyl-functionalized azobenzene
precursors (denoted as P1 and P2, respectively) by a surfactant-
free self-assembly process (Scheme 1). Lamellar hybrid lms (H1
and H2) with different arrangements of azobenzene moieties
were formed via self-assembly of hydrolyzed precursors, showing
different photoresponsive properties, i.e. reversible but opposite
changes in the d-spacings under UV/vis irradiation. Further-
more, co-condensation of these precursors with TEOS leads to
lamellar hybrids (H1 and H2 ) exhibiting more efficient trans–cis
isomerization of azobenzene moieties. The results of detailed
structural characterization as well as the photoresponsive
properties of these new types of ordered azobenzene–siloxane
hybrid materials have been presented in this paper.
Synthesis of 4-[3-(triethoxysilyl)propoxy]azobenzene (P1)
Hydrosilylation of 4-allyloxy-azobenzene (0.704 g, 0.003 mol)
with an excess amount of triethoxysilane (2.324 g, 0.015 mol)
was performed in toluene (15 ml) in the presence of Pt as a
ꢃ5
ꢁ
catalyst (0.03 g, 3 ꢂ 10 mol). The mixture was stirred at 70 C
for 24 h under a nitrogen atmosphere, and the solvent and
unreacted triethoxysilane were removed in vacuo. P1 was
obtained as a red liquid (1.09 g, 90% yield) aer purication
0
0
using gel permeation chromatography (GPC) with chloroform
1
as the eluent. H NMR (d, 270 MHz, CDCl ): 0.76–0.83 (t, 2H,
3
OCH
2
CH
2
CH
CH
CH
2
Si), 1.21–1.27 (t, 9H, SiOCH
2
CH
3
), 1.89–2.00 (m,
CH ), 4.00–4.06
Si), 6.98–7.01 (t, 2H, ArH), 7.25–7.53 (m,
2H, OCH
2
2
CH
2
Si), 3.81–3.90 (m, 6H, SiOCH
2
3
(t, 2H, OCH
2
2
CH
2
1
3
3
6
1
H, ArH), 7.86–7.92 (t, 4H, ArH). C NMR (d, 67.8 MHz, CDCl
.51, 18.32, 22.76, 58.47, 70.18, 114.72, 122.54, 124.76, 129.02,
30.29, 146.88, 152.82, 161.67. Si NMR (d, 53.45 MHz, CDCl ):
3
):
2
Experimental
2
9
3
+
Materials
ꢃ45.6. ESI-MS: m/z: 403.2047 [M + H] .
The chemicals 4-phenylazophenol (98.0%), allyl bromide
(>98.0%) and N,N-dimethylformamide (DMF, dehydrated,
Synthesis of 4-[3-(diethoxymethylsilyl)propoxy]azobenzene
>99.5%) were purchased from Wako Pure Chemical Industries.
(P2)
Sodium hydride (NaH) (60% dispersion in paraffin liquid),
triethoxysilane (>97.0%) and diethoxymethylsilane (>95.0%)
were purchased from Tokyo Chemical Industry. Platinum (0)-
The chemical 4-allyloxy-azobenzene (0.483 g, 0.002 mol) dis-
solved in toluene (15 ml) was mixed with diethoxymethylsilane
ꢃ5
(
1.34 g, 0.010 mol) and 0.02 g (2 ꢂ 10 mol) of Pt catalyst. The
1,3-divinyl-1,1,3,3-tetramethyldisiloxane complex in xylene (Pt
ꢁ
mixture was stirred at 70 C for one day. A red liquid (0.69 g;
ꢀ
2%) was purchased from Sigma-Aldrich. All chemicals were
used without further purication.
yield of 93%) was obtained aer solvent evaporation followed by
1
purication using GPC. H NMR (d, 270 MHz, CDCl
3
): 0.14–0.18
Si), 1.20–1.25 (t,
CH Si), 3.74–3.83
(
s, 3H, SiCH
3
), 0.74–0.80 (t, 2H, OCH
), 1.85–1.64 (m, 2H, OCH
2
CH
2 2
CH
Synthesis of 4-allyloxy-azobenzene
6
H, SiOCH CH
2
3
2
CH
2
2
In a 100 ml Schlenk ask, 4-phenylazophenol (1.98 g, 0.010 mol) (m, 4H, SiOCH CH ), 3.99–4.04 (t, 2H, OCH CH CH Si), 6.98–
2
3
2
2
2
was dissolved in DMF (25 ml). Activated NaH (1.80 g, 0.045 mol) 7.01 (t, 2H, ArH), 7.25–7.52 (m, 3H, ArH), 7.86–7.94 (t, 4H, ArH).
1
3
dispersed in DMF (30 ml) was added to the ask. Aer stirring at
C NMR (d, 67.8 MHz, CDCl ): ꢃ4.87, 9.97, 18.42, 22.81, 58.19,
3
room temperature for 2 h, allyl bromide (3.63 g, 0.030 mol) was 70.40, 114.71, 122.54, 124.75, 129.02, 130.28, 146.88, 152.80,
ꢁ
29
added, and the mixture was stirred at 60 C for one day under a 161.64. Si NMR (d, 53.45 MHz, CDCl ): ꢃ5.2. ESI-MS: m/z:
3
+
nitrogen atmosphere. The product was extracted with ethyl 373.1942 [M + H] .
acetate and washed with cold water. Evaporation of ethyl acetate
gave a dark-red, crude solid product. Yellow crystals (1.65 g;
Preparation of azobenzene–siloxane hybrid lms
An aqueous solution of HCl was added to THF solutions of
P1 and P2, and the mixtures (molar compositions of P1 : THF :
H
1
2
O : HCl ¼ 1 : 50 : 15 : 0.03 and P2 : THF : H
2
O : HCl ¼
: 50 : 10 : 0.004) were stirred at room temperature for 40 and
60 min, respectively. A portion of the mixtures was spin-coated
on glass substrates to obtain thin lms. Turbid, yellowish lms
(
H1 and H2) were obtained from P1 and P2 aer heating at 120
ꢁ
and 60 C, respectively, to induce polycondensation. Cast lms
were also prepared from P1 and P2 on glass substrates under
similar conditions. They were pulverized and heated at 120 and
Scheme 1 Structures of diethoxysilyl- and triethoxysilyl-azobenzene precursors
P1 and P2) and preparation of four types of lamellar, azobenzene–siloxane
hybrid films by self-assembly with and without TEOS.
(
ꢁ
80 C for solid-state NMR and MS characterizations.
6
990 | J. Mater. Chem. C, 2013, 1, 6989–6995
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0
0
H1 and H2 were prepared by co-hydrolysis and poly-
condensation of P1 and P2, respectively, with TEOS. TEOS was
added to the previously stirred (ꢀ5 min) mixtures of P1 or P2,
THF, HCl and H O. Further stirring was conducted for 1.5 h (for
2
P1) and 3 h (for P2) before spin-coating on glass substrates.
ꢁ
Aer heating at 120 C, transparent, yellow lms were obtained.
0
0
Molar ratios of the mixtures for preparing H1 and H2
were P1 : TEOS : THF : H
O : HCl ¼ 1 : 4 : 50 : 19 : 0.05 and
P2 : TEOS : THF : H
O : HCl ¼ 1 : 4 : 50 : 19 : 0.02, respectively.
2
2
Preparation of powder samples from P1 and P2
Fig. 1 XRD patterns of azobenzene–siloxane hybrid films (a) H1 and (b) H2
prepared by hydrolysis and polycondensation of P1 and P2, respectively.
An aqueous solution of HCl was added to the EtOH solutions of
P1 and P2. The molar ratios of the mixtures were
P1 : EtOH : H O : HCl ¼ 1 : 75 : 38 : 0.02 and P2 : EtOH : H O :
2
2
HCl ¼ 1 : 100 : 10 : 0.02. Under stirring at room temperature for
several hours, yellow precipitates were formed. The precipitates
ꢁ
were collected by ltration and heated at 120 C for 4 h to
induce polycondensation.
Characterization
1
13
29
Liquid-state H-, C- and Si-NMR spectra were recorded on a
JEOL JNM-270 spectrometer at 270, 67.8 and 53.45 MHz,
3
respectively, using CDCl as the solvent and tetramethylsilane
1
3
29
as the internal reference. Solid-state C and Si CP/MAS NMR
spectra were recorded using a JEOL CMX-300 spectrometer at
resonance frequencies of 75.57 and 59.7 MHz, respectively. X-
ray diffraction (XRD) patterns were obtained using a RIGAKU
UltimaIV diffractometer with Cu Ka radiation. Fourier trans-
form infrared (FTIR) spectra were recorded using a JASCO FT/
IR-6100 spectrometer by the KBr pellet technique. UV/vis
absorption spectra were recorded using a JASCO V-670 instru-
ment. Morphologies of the samples were observed on a eld-
emission scanning electron microscope (FE-SEM, Hitachi S-900)
with an accelerating voltage of 6 kV. Before the observation, the
samples were sputter-coated with Pt. Transmission electron
microscopy (TEM) observations were conducted on a JEOL JEM-
Fig. 2 FE-SEM images of the top surfaces of (a) H1 and (b) H2.
surface consisting of stacked thin plates. The TEM image of H1
(Fig. 3) clearly shows the lamellar structure with a periodicity of
ca. 3.2 nm, which is consistent with the XRD result. Unfortu-
nately, TEM observation of H2 was unsuccessful because it was
susceptible to electron beam damage.
H1 and H2 prepared as cast lms show similar XRD patterns
13
29
(
Fig. S1 in the ESI†). Solid-state C and Si CP/MAS NMR
spectra of these samples aer pulverization are shown in Fig. 4.
13
The C CP/MAS NMR spectra show peaks assigned to azo-
benzene and propylene linkers. No peaks of ethoxy groups are
2000EXII at an accelerating voltage of 200 kV. Sample lms were
observed, conrming complete hydrolysis of both precursors.
pulverized from the substrates, dispersed in ethanol (for H1) or
water (for H2) and dropped on microgrids (Cu mesh). A
SUPERCURE-204S UV light source (San-ei electric) with the
29
The Si CP/MAS NMR spectrum of H1 shows the peaks at ꢃ48,
1
2
ꢃ56 and ꢃ66 ppm, corresponding to T (CSi(OSi)(OH)
2
), T
3
(CSi(OSi)
2
(OH)) and T (CSi(OSi)
3
), respectively, suggesting that
ꢃ2
intensity in the range of 67–76 mW cm was used for UV and
29
condensation has proceeded. On the other hand, the Si CP/
visible light irradiation of the samples. UV cut (HOYA L-420 nm)
and UV pass (HOYA U-340 nm) lters were used.
1
MAS NMR spectrum of H2 shows a single D peak at ꢃ13 ppm.
3
Results and discussion
Characterizations of H1 and H2
XRD patterns of the thin lms of H1 and H2 (Fig. 1) show
lamellar-structured features with different d-spacings. H1
exhibits a sharp peak corresponding to the d-spacing of 3.20 nm
with higher order reections, indicating highly uniform meso-
scale periodicity. On the other hand, H2 shows a different
prole with a much smaller d-spacing of 2.37 nm. The macro-
scopic morphologies of these lms are also different. FE-SEM
images of the top surfaces of H1 and H2 are shown in Fig. 2. H1
shows a wrinkled surface morphology, while H2 has a rough Fig. 3 TEM image of H1.
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Fig. 5 UV-vis absorption spectra of H1 (left) and H2 (right): (a, red) before
irradiation, (b, blue) after UV irradiation for 5 min and (c, green) after subsequent
visible light irradiation for 5 min.
barely visible for the lms, possibly because their lamellar
structures are oriented parallel to the substrate.
Photoresponsive properties of H1 and H2
Azobenzene molecules can undergo reversible trans–cis isom-
erization under UV/vis light irradiation and show a change in
the molecular length from 0.90 to 0.55 nm. This process is facile
in solution because of the high mobility of azobenzene.
However, in the solid state, the mobility of azobenzene
decreases considerably, especially when both phenyl groups are
covalently bonded to polymer networks, which might induce
severe interference in photoisomerization.
In the systems presented here, even aer the formation of
siloxane networks, one end of the azobenzene moieties is still
free to move. The UV-vis absorption spectra of H1 and H2 aer
UV/vis irradiation are shown in Fig. 5. Before irradiation
1
3
29
Fig. 4 Solid-state C CP/MAS NMR (left) and Si CP/MAS NMR (right) spectra
of (a) H1 and (b) H2.
29
This sample is soluble in organic solvents such as THF, and
electrospray ionization mass spectrometry (ESI-MS) revealed the
formation of dimers (main peaks: 615.2459: [M + H] , 637.2278:
M + Na] ). Thus, H2 is a type of molecular assembly, probably
stabilized by intermolecular hydrogen bonding between silanol
groups.
+
+
[
(Fig. 5a), the thermally more stable trans isomer of azobenzene
It is inferred that lamellar structures are formed by evapo-
ration-induced self-assembly of hydrolyzed P1 and P2 mono-
mers during spin-coating. Aer hydrolysis, ethoxyl groups of the
precursors are converted to silanol groups (Si–OH). The hydro-
phobic interactions between the amphiphilic hydrolyzed
monomers could be a driving force for self-assembly. Also, p–p
interactions between benzene rings may be effective for the
formation of the lamellar structures. Subsequent heat treat-
ment promoted polycondensation of the silanol groups while
maintaining the lamellar structure. The d-spacings slightly
decreased from 3.39 to 3.20 nm (H1) and from 2.59 to 2.37 nm
is plentiful in the lms, showing a strong absorption at ca.
8
340 nm owing to p–p* transitions. Aer 5 min of UV irradia-
tion (Fig. 5b), the intensity of the 340 nm peak decreases, while
that of a small peak at ca. 440 nm attributed to the forbidden n–
p* transition of the metastable cis isomer increases, indicating
partial trans to cis photoisomerization. No further change was
observed when the irradiation time was prolonged. Aer a
subsequent 5 min of visible light irradiation (Fig. 5c), the
reverse cis to trans process occurred, which is conrmed by the
recovery of the 340 nm peak and the decrease in the intensity of
the 440 nm peak. Repeated UV/vis irradiation cycles induce the
same changes in the UV-vis spectra.
(H2) upon heating. Heat-induced polycondensation was
conrmed by FTIR spectroscopy (Fig. S2 in the ESI†). Before
The photoresponsive properties of H1 and H2 were further
investigated by studying their structural changes under UV/vis
irradiation. Fig. 6 (le) shows the XRD patterns of H1 (a) before
ꢃ1
heating, peaks assigned to Si–OH vibrations at ca. 900 cm and
the stretching vibration of hydrogen bonded SiO–H at
ꢃ1
3
280 cm are observed. Aer heating, the intensity of these
bands decreased considerably, and the intensity of the band
ꢃ1
corresponding to Si–O–Si vibration at ca. 1020 cm increased.
Similar lamellar hybrids were also obtained as powders by
hydrolysis in an EtOH solution. Layered, plate-like morphol-
ogies are clearly observed by FE-SEM (Fig. S3 in the ESI†). The
XRD patterns of these powder samples show peaks character-
istic of lamellar structures (Fig. S4 in the ESI†), similar to those
of the lm samples H1 and H2. Their d-spacings are slightly
different from those of the lm samples, which may be due to
the different reaction conditions. The additional broad peaks at
Fig. 6 Variations in the XRD patterns (left) and d-spacings (right) of H1 upon
photo-irradiation: (a) before irradiation, (b) after the first cycle and (c) after the
ꢁ
19–24 (d ¼ 0.37–0.47 nm) might be arising from the stacking of
the azobenzene moieties by p–p interactions. These peaks are second cycle of UV (solid) and visible (dash) light irradiation.
6
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Fig. 7 Variations of XRD patterns (left) and d-spacing (right) of H2 upon photo-
irradiation: (a) before irradiation, (b) after the first cycle and (c) after the second
cycle of UV (solid line) and visible (dashed line) light irradiation.
Fig. 8 Possible structural models of (a) H1 and (b) H2 formed by self-assembly
and polycondensation of P1 and P2, respectively.
irradiation, (b) aer the rst cycle and (c) the second cycle of UV
(
solid line) and visible light (dashed line) irradiation. Reversible
single azobenzene molecules (ꢀ0.45 nm). It was speculated that
photoisomerization of a part of azobenzene moieties triggers a
slight change in the overall tilt angles in the lamellar hybrids.
For H1, trans to cis isomerization of azobenzene induced a
decrease in the tilt angle relative to the siloxane layers. On the
other hand, because of the smaller tilt angle of azobenzene in
H2 before irradiation, the formation of the cis isomers may
increase the tilt angle under UV irradiation.
The occurrence of d-spacing changes for these hybrid lms
suggests the exible properties of the networks, which not only
enables the photoisomerization of azobenzene moieties but
also changes the lamellar periodicities. This is inherently
different from that of azobenzene-graed mesoporous silica
and similar to the organic polymer-based azobenzene-contain-
ing lms or elastomers.
d-spacing changes are observed for each cycle. Aer UV irradi-
ation, the intensity of the peak decreased, accompanying a
slight decrease in the d-spacing. When visible light was subse-
quently applied, the position of the peak was almost recovered.
This process could be repeated for several cycles. The variation
in the d-spacing of H1 in these processes is shown in Fig. 6
(
right), showing a ca. 0.05 nm decrease under UV irradiation.
Interestingly, the phenomena observed for H2 were contrary.
Reversible changes in the d-spacing of H2 under two cycles of
UV/vis irradiation are shown in Fig. 7. Aer UV irradiation, the
(001) and (003) peaks shied to lower angles and partially (rst
cycle) and fully (aer second cycle) recovered aer visible light
irradiation. There is an increase of ca. 0.03 nm in the d-spacing
under UV irradiation.
Although the uctuation ranges are very small, to the best of
our knowledge, this is the rst report on the reversible change
in the d-spacings of self-assembled organosiloxane materials.
0
0
Structure and properties of H1 and H2
0
By utilizing precursors with different numbers of alkoxy groups, Co-condensation of TEOS with P1 and P2 gave lms (H1 and
different tendencies of d-spacing changes (decreasing or H2 ) with high transparency. The FE-SEM images of these lms
0
increasing) are observed. Such photoresponsive behaviour is show uniform surface morphologies (Fig. S5 in the ESI†); these
thought to be caused by reversible partial trans–cis photo- are quite different from the rough morphologies of the lms
isomerization of azobenzene moieties, as evidenced by the UV- prepared without TEOS (cf. Fig. 2). Their XRD patterns (Fig. 9a)
vis spectra (Fig. 5). This result is in contrast to a previous report show sharp peaks with different d-spacings (2.79 and 3.80 nm
0
0
0
in which trans to cis isomerization is severely inhibited owing to for H1 and H2 , respectively). The larger d-spacing of H2
extensive hydrogen bonding interactions.
compared to H2 (2.37 nm) can be reasonably explained by the
increased thickness of the siloxane layers by TEOS incorpora-
0
tion. In contrast, the smaller d-spacing of H1 compared to H1
(
Structural models of H1 and H2
3.20 nm) strongly suggests that the arrangement of azobenzene
To explain the differences in the structures and photo-
responsive properties of H1 and H2, structural models are
proposed (Fig. 8). Lamellar lms consist of inorganic siloxane
layers and organic azobenzene layers. The azobenzene groups
should be in bilayer arrangements because the d-spacings are
larger than the molecular lengths of hydrolyzed P1 and P2. The
different d-spacings for H1 (3.20 nm) and H2 (2.37 nm) can be
explained by the different tilt angles of the azobenzene moieties
relative to the siloxane layers. The angles are calculated to be
was changed from a bilayer to an interdigitated monolayer,
possibly owing to the increase of the lateral distance between
hydrolyzed H1 molecules by co-condensation with TEOS. UV-vis
ꢁ
about 90 and 40 for H1 and H2, respectively. Such a difference
is presumably due to the steric inuence of the methyl group
attached to the Si atoms and/or due to the difference in the
number of Si–OH groups in the hydrolyzed P1 and P2.
The photo-induced changes in the d-spacings are much
0
0
Fig. 9 XRD patterns of H1 (left) and H2 (right): (a) before and (b) after swelling
smaller than those expected from trans–cis isomerization of with dioxane.
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present study offers a facile approach to fabricate a new class of
azobenzene-based photoresponsive materials with highly orga-
nized structures. Further design of such self-assembled hybrid
materials is important in order to realize smart materials
showing drastic changes in the structure and shape in response
to irradiation.
Acknowledgements
0
0
Fig. 10 UV-vis absorption spectra of H1 and H2 (a, red) before irradiation, (b,
blue) after 2 min of UV light irradiation and (c, green) after a subsequent 2 min of
visible light irradiation.
We are grateful to Prof. Makoto Fujita, Dr Sota Sato and Mr
Hiroyuki Yokoyama (The University of Tokyo) for the ESI-MS
measurements. TEM observation was conducted at the Center
for Nano Lithography & Analysis, The University of Tokyo,
supported by the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), Japan. This study was partly supported
by a Grant-in-Aid for Scientic Research on Innovative Areas
absorption spectra of these lms (Fig. 10) show higher degrees
of trans–cis isomerization of azobenzene moieties under UV/vis
irradiation.
Another inuence of adding TEOS is that the materials swell
when exposed to organic solvents. XRD patterns of H1 and H2
were measured aer soaking in dioxane for 1 h (Fig. 9b). Both
samples have maintained the lamellar structures but show large
increases (0.54 and 0.45 nm) in d-spacings. Efficient trans–cis
isomerizations were still observed for these swollen lms
“New Polymeric Materials Based on Element-Blocks (No. 2401)”
0
0
(no. 25102508) from the MEXT. S.G. acknowledges the nancial
support offered by the Monbukagakusho Scholarship and by
the Global Center for Excellence for Mechanical Systems Inno-
vation (GMSI, The University of Tokyo).
(
Fig. S6 in the ESI†). The lms shrank aer drying, as conrmed
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