A novel photo-responsive organogel based on azobenzene

Article abstract of DOI:10.1002/poc.1341

A novel triarmed cis-1,3,5-cyclohexanetricarboxamides gelator, functionalized by three azobenzene moieties grafted with three long alkyl chains, was designed and synthesized. The gelator can gel most of the organic solvents in low concentration. The morph

Full text of DOI:10.1002/poc.1341

Research Article
Received: 13 June 2007,
Revised: 7 January 2008,
Accepted: 9 January 2008,
Published online in Wiley InterScience: 12 March 2008
(www.interscience.wiley.com) DOI 10.1002/poc.1341
A novel photo-responsive organogel
based on azobenzene
a
a
a
a
a
*
Yifeng Zhou , Miao Xu , Junchen Wu , Tao Yi , Jiantao Han ,
Shuzhang Xiaoa, Fuyou Lia and Chunhui Huanga
*
A novel triarmed cis-1, 3, 5-cyclohexanetricarboxamides gelator, functionalized by three azobenzene moieties grafted
with three long alkyl chains, was designed and synthesized. The gelator can gel most of the organic solvents in low
concentration. The morphologies of the xerogels show one-dimensional aggregated bundles or helical fibres. The
azobenzene groups in the gel state form H-type aggregation and perform expected trans–cis photoisomerization with
a gel to sol phase transition upon irradiation of UV light. The main driving force for gelation is intermolecular
hydrogen bonding between amides and p–p interaction of azobenzene moieties. Copyright ß 2008 John Wiley &
Sons, Ltd.
Supplementary electronic material for this paper is available in Wiley InterScience at http://www.mrw.interscience.wiley.com/
suppmat/0894-3230/suppmat/
Keywords: azobenzene; helical structure; organogel; photochromism
system of two component photo-responsive gel.[24] However,
stimulus response operated in the self-assembled gel state is still
INTRODUCTION
The design of self-assembling supramolecules becomes the
charming target toward the development of new materials and
nanoscale devices.[1–6] As a novel class of self-assembled mate-
rials, low weight molecular (LWM) gels have received consider-
able attention in recent years because they can organize into
regular nanoarchitectures through specific non-covalent inter-
actions including hydrogen bonds, hydrophobic interactions,
p–p interactions and van der Waals forces.[7–13] The intermole-
cular hydrogen bonds or p–p interactions usually facilitate the
growth of linear, elongated aggregates in the gels. Chiral
diaminocyclohexane or cis-1, 3, 5-cyclohexanetricarboxamides
are important organogelator cores for the formation of chiral
organogel systems, which was reported by Hanabusa et al. for the
first time.[14,15] Shinkai et al. successfully transcribed the chiral,
helical structures of the organogels into silica gel and got chiral
silica with both right- and left-handed helical motif.[16,17]
Azobenzene moieties are known to all for their photo
responsive properties. The soft matter contained azobenzene
groups can potentially be applied in message storage and
transport, etc.[4] In the organogel systems, azobenzene groups
also acted as p system to enforce the interactions among the
molecules. Feringa reported the chiral recognition through
cooperative interactions between aggregates and gels of
bis(ureido)-cyclohexane skeleton containing azobenzene chro-
mophores.[18] Kato introduced azobenzene based chiral gelator
into liquid crystal systems to get photo responsive anisotropic
liquid-crystalline physical gels.[19,20] Zhao et al. achieved
electrically switchable diffraction gratings based on light-induced
reorganization process in two azobenzene contained gela-
tors.[21,22] In our previous work, we observed solvent tunable
morphologies in an azobenzene gelator with several competitive
or cooperative interactions[23] and we also developed a new
a challenge. In this work, we synthesized a novel triarmed cis-1, 3,
5-cyclohexanetricarboxamides gelator AZO-C16, functionalized
by three azobenzene moieties grafted with three long alkyl
chains (Scheme 1). The cooperative interactions of hydrogen
bonding, p–p interactions and van der Waals forces confirmed
the compound an effective gelator for various organic solvents.
The main attraction is that the self-assembly organization
resulted in a chiral helical structure in a supramolecular level
even using a non-chiral compound as a building block.
MATERIALS AND METHODS
General
All starting materials were obtained from commercial suppliers
and used as received. Moisture sensitive reactions were
performed under an atmosphere of dry argon. 1H NMR and
13C NMR spectra were recorded on a Mercuryplus, at 400 and
100 MHz, respectively. Proton chemical shifts are reported in
parts per million downfield from tetramethylsilane. Matrix-
assisted laser desorption ionization time-of-flight (MALDI-
TOF-MS) was recorded on an AXIMA-CFRPLVS mass spectrometer
(Shimadzu). Element analysis was carried out on a VARIOEL3
apparatus (ELEMENTAR).
*
Department of Chemistry & Laboratory of Advanced Materials, Fudan Uni-
versity, Shanghai 200433, China.
E-mail: yitao@fudan.edu.cn
a
Y. Zhou, M. Xu, J. Wu, T. Yi, J. Han, S. Xiao, F. Li, C. Huang
Department of Chemistry & Laboratory of Advanced Materials, Fudan Uni-
versity, Shanghai 200433, China
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Copyright ß 2008 John Wiley & Sons, Ltd.

A NOVEL PHOTO-RESPONSIVE ORGANOGEL
yield 4 as a white solid. The product was used in the next reaction
without further purification.
(1s,3s,5s)-N1,N3,N5-tris(4-((E)-(4-hexadecyloxy phenyl) diazenyl)
phenyl) cyclohexane-1,3,5- tricarboxamide (AZO-C16)
4 (0.22 g, 1 mmol) in 5 ml THF was dropped into 30 ml THF
solution of 3 (1.44 g, 3.3 mmol) under an atmosphere of dry
argon, then 0.5 ml triethylamine (TEA) was added. The reaction
mixture was stirred and refluxed overnight. The resulted
precipitate was then filtered and washed with THF to obtained
1
0.96 g of AZO-C16 (65% yields). M.p. 73–75 8C, H-NMR (CDCl3,
608C) d 8.18 (br, NH, 3H), 7.83–7.85 (m, ArH, 12H), 7.69–7.71 (m,
ArH, 6H), 6.95–6.98 (m, ArH, 6H), 4.02 (t, —OCH2—, 6H), 3.58 (m,
3H), 2.05 (m, 3H), 1.79–1.82 (m, 9H), 1.44–1.49 (m, 6H), 1.30
(s, —CH2—, 72H), 0.89 (t, —CH3, 9H). Anal. Calcd (%) for
C93H135N9O6: C 75.72, H 9.22, N 8.55; found: C 75.46, H 9.11, N 8.67.
MALDI-TOF-MS: 1475.1 [MþH]þ.
Instruments
Fourier transform infrared (FT-IR) spectra were performed using a
Nexus 470 spectrometer (Nicolet Company). UV–visible spectra
were recorded on an UV–vis 2550 spectroscope (Shimadzu). The
UV and visible irradiation was carried out by a CHF-XM550W
power system (China). Visible light was generated by a 500 W Xe
lamp with a standard band-pass filter 440 AF10 (Omega).
Irradiations in the UV region were effected at 365 nm. Scanning
electron microscopy (SEM) images of the xerogels were obtained
using an SSX-550 (Shimadzu) with an accelerating voltage of
15 kV. Samples were prepared by spinning the gels on glass slices
and coating with Au.
Scheme 1. Structure and synthesis process of compound AZO-C16
Synthesis of the gelator
(E)-4-(4-nitrophenyl diazenyl) phenol (1)
Yellow solid 1 was synthesized according to a previous
report[23,24] (95% yields). 1H NMR (CDCl3) d 8.36 (d, J ¼ 9.2 Hz,
2H), 7.93–7.99 (q, J ¼ 8.8 Hz, 4H), 6.98 (d, J ¼ 8.8 Hz, 2H).
(E)-1-(4-(hexadecyloxy) phenyl)-2-(4-nitrophenyl)diazene (2)
1-Bromohexadecane (6.1 ml, 20 mmol) was dropped into 50 ml of
dry acetone solution containing 1 (2.43 g, 10 mmol) and
anhydrous potassium (2.78 g, 20 mmol). The reaction mixture
was refluxed, stirred for 8 h and then poured into excess
water. 2[25] was obtained by filtration and purified by column
chromatography on silica gel with chloroform/petrol ether
RESULTS AND DISCUSSIONS
Gelation properties
A typical procedure for gelation test is described below: a
weighed compound was mixed with a certain organic solvent in a
sealed test tube and the mixture was heated until the solid was
dissolved. The resulting solution was then cooled to room
temperature. If the test tube with gelled sample was turned
upside down without flow, the sample was labelled as ‘gel’.[14] The
gelation properties of AZO-C16 were tested in 20 kinds of
solvents (Table 1). It shows excellent gelling capability with 13
kinds of the test solvents and all the gels were semitransparent
and stable for months (Fig. 1). These gelling solvents ranged from
hydrocarbon solvents and aromatic solvents to dipolar aprotic
solvents. This indicates that AZO-C16 acts as a versatile gelator.
1
(1:2, v/v) as the eluent, yield 4.20 g (90%). M.p. 87–88; H NMR
(CDCl3) d 8.36 (d, J ¼ 8.8 Hz, 2H), 7.97 (q, J ¼ 7.2 Hz, 4H), 7.02 (d,
J ¼ 7.2 Hz, 4H), 4.06 (t, J ¼ 6.8 Hz, 2H), 1.80–1.86 (m, 2H), 1.26–1.50
(m, 28H), 0.88 (t, J ¼ 6.4 Hz, 3H).
(E)-4-((4-hexadecyloxy phenyl)diazenyl)benzenamine (3)
Na2S ꢀ 9H2O (4.80 g, 20 mmol) was added to a suspension of 2
(2.72 g, 6 mmol) in 50 ml of H2O/dioxane (1:1, v/v). The reaction
mixture was stirred at 80 8C for 24 h. Crude 3 (2.62 g) was
obtained and purified by column chromatography on silica gel
1
with chloroform as the eluent, yield 60%. M.p. 88–89; H NMR
UV–vis spectra and the photochromic
properties of the gels
(CDCl3) d 8.83 (d, J ¼ 8.8 Hz, 2H), 7.76 (d, J ¼ 8.8 Hz, 2H), 6.98 (d,
J ¼ 9.2 Hz, 2H), 6.73 (d, J ¼ 8.4 Hz, 2H), 4.02 (t, J ¼ 6.4 Hz, 2H),
3.98 (s, 2H), 1.77–1.84 (m, 2H), 1.26–1.47 (m, 28H), 0.88 (t,
J ¼ 6.4 Hz, 3H).
Electronic UV–vis absorption spectra of the gels were studied to
obtain information about the aggregated state of azobenzene on
the molecular scale. The concentration of both dioxane and DMF
gels is 1.0 ꢁ 10ꢂ2 M and the concentration of both dioxane and
DMF solution is 1.0 ꢁ 10ꢂ5 M. Dioxane and DMF solutions of
AZO-C16 have maximum absorbance at 367 and 372 nm,
respectively, corresponding to the trans-azobenzene chromo-
phore (Fig. 2a and 2b). In the gel state of the corresponding
(1s, 3s, 5s)-Cyclohexane-1,3,5-tricarbonyl trichloride (4)
SOCl2 (1 ml; excess) was dropped into 10 ml of dry THF solution of
(1s, 3s, 5s)-cyclohexane-1, 3, 5-tricarboxylic acid (0.43 g, 2 mmol).
The reaction mixture was refluxed and stirred for 3 h. The solvent
and excess SOCl2 were evaporated under reduced pressure to
*
solvents, the absorption band of p–p transitions of the
J. Phys. Org. Chem. 2008, 21 338–343
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Y. ZHOU ET AL.
Table 1. Gelation properties of AZO-C16
Solvents
State
CGC
Solvents
State
CGC
Dichloromethane
Chloroform
Acetone
Acetonitrile
Ethyl acetate
Hexane
Cyclohexane
THF
Dioxane
G
G
I
I
G
I
8 mg/ml
10 mg/ml
—
Ethanol
1-Butanol
Benzene
Toluene
Xylene
Bromobenzene
1-Bromobutane
1-Bromohexane
1,2-Dichloroethane
DMF
I
I
—
—
G
G
G
G
G
G
G
G
10 mg/ml
8 mg/ml
10 mg/ml
10 mg/ml
10 mg/ml
10 mg/ml
10 mg/ml
6 mg/ml
—
6 mg/ml
—
—
10 mg/ml
8 mg/ml
—
I
G
G
I
Methanol
CGC, critical gelation concentration, the minimum concentration necessary for gelation of solvents; I, insoluble; G, gel.
azobenzene moiety became broad and the maximum absor-
bance blue shifted to 344 and 363 nm (Fig. 2c and 2d),
respectively. The situation for the solution and gel state in THF
solvent was similar to that of dioxane. This indicates that the
azobenzene groups form H-type aggregation in the gel state
(especially dioxane as the solvents). Under the irradiation of UV
light (365 nm), these bands of azobenzene obviously decreased
and two new bonds at 318 and 450 nm in these solvents were
observed. This indicated the transformation of the trans isomer to
cis isomer of the azobenzene groups. Sequentially, the stable gels
collapsed gradually and became homogeneous sol over 2 h.
Moreover, the absorption bands of the cis isomer in collapse gel
are almost the same with that of cis isomer in diluted solution,
which means that the photochromic process destroyed the
aggregates in the gel state, thus resulting in the collapse of the
gel. The sol was deposited instead of recovering to homogeneous
gel upon irradiation with the visible light of 440 nm for 30 min at
room temperature in DMF or dioxane. The gel state can be
restored again by heating the sample to solution (within 10 min,
95 and 105 8C for DMF and dioxane, respectively) and cooling to
room temperature. However, for CHCl3 gel (10 mg/ml) with Tg of
58 8C, the irradiation-induced sol could be directly back to the gel
by visible light irradiation at room temperature.
tration of 10 mg/ml, while THF xerogel was obtained from the gel
of 12 mg/ml. All of the micrographs of the xerogels from the
above three solvents show one-dimensional aggregation (Fig. 3),
but the solvent influences the morphology of the fibres to a
certain extent. In the DMF xerogel, one can consistently observe a
well-developed network structure composed of fibrous aggre-
gates, with the length of several micrometres and the diameters
of 100–200 nm (Fig. 3a). Some of the fibres self-assembled to form
right- and left-handed helical structure (Fig. 3a and 3b). This result
indicates the cyclohexyl core of the gelator induced the helically
self-aggregated morphology. To our surprise, the helical self-
assembly is not so clearly expressed at the supramolecular level in
THF and dioxane gels. In THF, the xerogel has a smooth belt
structure with the width of the belt in the range of 50–200 nm
(Fig. 3c). The xerogel from dioxane also shows one-dimensional
fibres, several of the fibres aggregated to form bundles with the
width about 500 nm (Fig. 3d). After a gel to sol transition by 2 h UV
irradiation (365 nm) in DMF, and followed by 30 min 440 nm
visible light irradiation, the obtained precipitates gave a
morphology as shown in Fig. 3e and 3f. It is obvious that the
precipitates are in fact also composed of fibre-like entities
(Fig. 3f). This implies that the packing of the molecules in the
precipitates is similar to that in the gel. However, as the overall
three-dimensional network of fibres cannot be fully developed at
a temperature far from Tg, the reversibility of a gel upon visible
light cannot be realized at room temperature in DMF. Whereas for
the gel with a lower Tg such as CHCl3 gel, the irradiation-induced
cis-sol can be reverted to gel even at room temperature (20 8C).
Morphologies of the xerogels
To obtain visual images of the assembled AZO-C16 from various
organic solvents, the morphologies of the xerogels obtained from
AZO-C16 were investigated by SEM. DMF and dioxane xerogels
were obtained from corresponding initial gels with a concen-
The structure investigation of the gels
To study the driving force of gel–sol process, temperature-
1
1
dependent H NMR spectra were examined. H NMR spectra of
AZO-C16 (ca. 10 mg/ml in CDCl3, Tg ¼ 58 8C) at 20, 40 and 60 8C
are shown in Fig. 4. 1H NMR spectrum of aromatic proton in
AZO-C16 displayed broad signals at room temperature and
became discriminable at higher temperature. The proton signal
of the amide groups (Ha, Scheme 1) at 20 8C was very weak. At
40 8C, a broad signal at about 8.38 ppm assigned to the protons of
amide groups was detected and high field shifted to 8.18 ppm at
a temperature of 60 8C. Hb also slightly went to the higher field
positions with the increase in the temperature (7.74–7.71 ppm
from 20 to 60 8C). Other proton signals have almost no change
Figure 1. Pictures of the sol–gel transition of AZO-C16 in DMF (10 mg/ml).
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Copyright ß 2008 John Wiley & Sons, Ltd.
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A NOVEL PHOTO-RESPONSIVE ORGANOGEL
Figure 2. Absorption spectral changes of AZO-C16 in the solutions of dioxane (a), DMF (b)
(1.0 ꢁ 10ꢂ5 M, 10 mm cell) and gels of dioxane (c), DMF (d) (1.0 ꢁ 10ꢂ2 M, 1 mm cell), under the
irradiation of 365 nm light at room temperature
with the temperature. These results implied that the inter-
powder, the NH and CO stretching modes of amide groups are
located at 3286 and 1659 cmꢂ1, respectively. These two peaks
only slightly move to 3284, 1657 cmꢂ1 in the THF gel, respectively.
Whereas in the THF solution (5 ꢁ 10ꢂ3 M), the IR signals include a
strong band at about 3386 cmꢂ1, attributed to the free NH
stretching vibrational band of amide.[15,26,27] The comparison of
the IR spectra among solution, gel and solid state indicates that
the formation of the hydrogen bond promotes the aggregation
of AZO-C16 molecules to form gel state; and the intermolecular
hydrogen bonds in the gel state are essentially similar to that of
the solid state.[11,19,26,27] The IR spectra of the gel from different
solvents of THF, dioxane and DMF are shown in Fig. 5B, from
which we can see that the two amide bands in THF and dioxane
gel are almost posited at same position. Whereas in the DMF gel,
molecular H-bonds were formed between neighbouring amide
groups at room temperature. The intermolecular H-bonds
became weaker by increase in the temperature, and as a result,
the proton signal of amide groups upfield shifted. After the
sample irradiated with 365 nm light for 3 h at room temperature,
the gel was translated to sol. From NMR spectrum of the cis-sol
sample, we can see that the amide proton Ha was obviously
upfield shifted to 7.97 ppm, while Hc and Hd were downfield
shifted to 7.87–7.85 ppm. These data indicate that the hydrogen
bonds were destroyed by trans–cis transition, thus resulted in a
gel to sol transition upon irradiation.
IR spectra of the gels were also studied to obtain the
information of intermolecular interaction of these molecules.
Figure 5A gives the comparison of the IR spectra of AZO-C16
powder and its corresponding gel and solution in THF. The IR
spectrum of pure THF was also measured as a reference. In the
—
both the NH and C O vibration move to a lower wavenumber
—
(3279, 1655 cmꢂ1, respectively). It is obvious that in a solvent
—
containing amide C O moiety such as DMF, the intermolecular
—
Figure 3. SEM images of xerogels from DMF (a, b, the arrow pointed to the right- and
left-hand helix), THF (c), dioxane (d), precipitates after a gel–sol transition by UV (365 nm) and
visible light (440 nm) in DMF (e, 50 mm and f, 10 mm)
J. Phys. Org. Chem. 2008, 21 338–343
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Y. ZHOU ET AL.
be deduced that the three amide-CO and NH of AZO-C16 can
direct themselves parallel to each other and perpendicular to the
cyclohexane ring by intermolecular hydrogen bonds as described
in Scheme 2a. Similar type of stacked amide hydrogen-bonding
arrangement has been observed in a crystal structure of cis-1, 3,
5-cyclohexanetricarboxamide derivative reported by Hamilton, in
which the molecules packed as a non-central arrangement in the
structure.[28] The presence of the azo group in the trans form is
important for the formation of the complete intermolecular
hydrogen bonding between amide as well as the p–p interaction
between azo. The irradiation of UV light could induce trans to cis
transition of the AZO group. As shown in Scheme 2b, the spatial
block in the cis isomer resulted in the incomplete hydrogen
bonding of AZO-C16, which destroyed the huge aggregate of
the molecules necessary for the gelation formation. As a result, a
gel to sol transition happened upon irradiation of the UV light in
the gel state of AZO-C16.
CONCLUSIONS
Figure 4. Partial 1H-NMR spectra of AZO-C16 (10 mg/ml) in CDCl3 at
temperatures of 20, 40, 60 8C and that after 365 nm irradiation for 3 h at
20 8C
In conclusion, by introducing cis-1, 3, 5-cyclohexanetricarbox-
amides core, azobenzene moieties and long alkyl chains into one
system, we obtained a versatile gelator which can widely gel
varieties of organic solvents. Electron microscopy and spectral
studies show that the gel formation is due to the aggregation of
AZO-C16 molecules into helical fibres or aggregated bundles,
through cooperative interactions of hydrogen bonding of amide
moieties, p–p interactions of azobenzene groups and hydro-
phobic interactions of alkyl chains. From this research, we have
obtained new insight toward the molecular design in the more
effective organogelator for organic solvents. Moreover, the gel is
hydrogen bonds were enhanced, which suggests that DMF may
participate in the formation of the aggregates.
As it pointed out in the above section, the cis-1, 3, 5-
cyclohexanetricarboxamides core reacted on the formation of
the hydrogen bonding; however, the p–p stacking of azobenzene
moieties and hydrophobic interactions of long alkyl chains may
also influence the large aggregate of the molecules in the gel
state (Scheme 2). By the molecular modelling studies,[2,15,28] it can
Figure 5. A: IR spectra of AZO-C16 in powder (a), the gel (b), (20 mg/ml), the solution (c,
5 ꢁ 10ꢂ3 M) from THF and the spectrum of pure THF (d). B: The IR spectra of the powder (a) and the
gels (20 mg/ml) of dioxane (b), DMF (c) and THF (d), respectively
Scheme 2. Schematic presentation of the intermolecular hydrogen bonds between trans AZO-C16 molecules in the gel state (a), the structure for the
cis AZO-C16 molecule (b)
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Copyright ß 2008 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2008, 21 338–343

A NOVEL PHOTO-RESPONSIVE ORGANOGEL
sensitive to light irradiation and performs photochromism and
gel–sol transition by UV irradiation at room temperature. The
stimuli responsive smart supramolecular system is important in
biological application, molecular (drug) release system, sensors,
molecular devices, etc. Therefore, this research affords a good
example for design and investigation of the environment
responsive soft materials.
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Acknowledgements
We thank National Science Foundation of China (20771027,
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(06PJ14016) and Shanghai Leading Academic Discipline Project
(B108) for financial support.
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J. Phys. Org. Chem. 2008, 21 338–343
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