Photoresponsive organogel based on supramolecular assembly of tris(phenylisoxazolyl)benzene

Article abstract of DOI:10.1071/CH09653

Azobenzene-substituted tris(phenylisoxazolyl)benzene 1 was developed as a photoresponsive gelator. A detailed study of the self-assembly of the trans- and cis-isomers in solution revealed that the planar trans-isomer assembled to form molecular stacks along its C ₃ axis, whereas the cis-isomer did not owing to steric requirements. Based on diffusion ordered spectroscopy experiments, the size of the aggregates formed from the trans-isomer were roughly four times as large as those of the cis-isomer. The photoinduced gel-to-sol transition was achieved by irradiation with UV light at 360 nm. Solid-state morphologies of the trans- and cis-isomers were quite contrastive; the trans-isomer created fibrous supramolecular networks with a lot of voids in which solvent molecules could be immobilized, whereas the cis-isomer never created such fibrous morphologies. The transcis structural change of the azobenzene moieties obviously regulates the gelation ability of 1. CSIRO 2010.

Full text of DOI:10.1071/CH09653

RESEARCH FRONT
Full Paper
CSIRO PUBLISHING
Aust. J. Chem. 2010, 63, 640–645
www.publish.csiro.au/journals/ajc
Photoresponsive Organogel Based on Supramolecular
Assembly of Tris(phenylisoxazolyl)benzene
Takeharu HainoA,B and Hiroshi SaitoA
ADepartment of Chemistry, Graduate School of Science, Hiroshima University,
1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8526, Japan.
BCorresponding author. Email: haino@sci.hiroshima-u.ac.jp
Azobenzene-substituted tris(phenylisoxazolyl)benzene 1 was developed as a photoresponsive gelator. A detailed study
of the self-assembly of the trans- and cis-isomers in solution revealed that the planar trans-isomer assembled to form
molecular stacks along its C3 axis, whereas the cis-isomer did not owing to steric requirements. Based on diffusion ordered
spectroscopy experiments, the size of the aggregates formed from the trans-isomer were roughly four times as large as
those of the cis-isomer. The photoinduced gel-to-sol transition was achieved by irradiation with UV light at 360 nm. Solid-
state morphologies of the trans- and cis-isomers were quite contrastive; the trans-isomer created fibrous supramolecular
networks with a lot of voids in which solvent molecules could be immobilized, whereas the cis-isomer never created
such fibrous morphologies. The trans–cis structural change of the azobenzene moieties obviously regulates the gelation
ability of 1.
Manuscript received: 15 December 2009.
Manuscript accepted: 9 February 2010.
Introduction
organogel.[22,23] The gel formed from the trans-isomer shows
the phase transition to the sol on the trans–cis local structural
change of the azobenzene moiety induced by UV irradiation.
Recently, we have reported that the self-assembly of
tris(phenylisoxazolyl)benzene driven by intermolecular dipole–
dipoleinteractionsformssupramolecularhelicalfibresthatactas
an organogelator.[24,25] Introduction of a photoresponsive func-
tionality onto the periphery of the tris(phenylisoxazolyl)benzene
should be a way to create such smart materials (Fig. 1). Here,
we report a new photoresponsive tris(phenylisoxazolyl)benzene
derivative 1 possessing three azobenzene units on its periphery
as well as a study of its photoresponsive self-assembly behaviour
in solution.
Self-assembly of functional molecules into supramolecular
architectures provides a powerful approach to create new
nanoscale materials and devices. Their properties rely on the
nature of their supramolecular architectures in which the molec-
ular constituents are spatially well organized. Control of these
organization processes by chemical or physical stimuli repre-
sents a prime scientific challenge and opens up a new way to
develop smart materials and devices.[1] Integration of photo-
chromic units as an addressable functionality onto the molec-
ular constituents holds considerable promise in achieving the
reversible control of the organization process on a molecu-
lar level.[2] Low-molecular-weight organic gelators (LMOGs)[3]
possessing photochromic units are a novel class of such smart
materials and have become a cutting-edge area in the field
of material chemistry.[4–12] They assemble into supramolecular
nanoarchitectures via specific non-covalent interactions includ-
ing hydrogen-bonding, hydrophobic, π-π, and van der Waals
interactions.These interactions are quite sensitive to their molec-
ular structures; structural change induced by external stimuli
can vary the intermolecular non-covalent interactions in the
assembled state. The properties of smart gels based on photo-
responsive LMOGs are reversibly controlled on irradiation with
different wavelengths of light.Azobenzene shows a unique light-
induced trans–cis isomerization. Irradiating azobenzene units
with UV light causes them to isomerize from the trans to
the cis configuration; thus, it is known as a potential photo-
responsive functionality that can be conveniently introduced
onto an LMOG core. There is precedence for photoresponsive
LMOGs possessing an azobenzene moiety.[13–21] Shinkai et al.
have reported pioneering work; cholesterol derivatives possess-
ing an azobenzene moiety are found to be a photoswitchable
Results and Discussion
Photoisomerization of trans-1 was examined at a concentra-
tion of 0.10 mmol L−1 in ethyl acetate (Fig. 2). The solution
displayed a sharp absorption band at 363 nm and a shoulder
at ∼450 nm, interpreted as the π–π* and n–π* transitions of
the azobenzene moieties. On irradiation at 360 nm, the band at
360 nmdecreasedandanewbroadbandsimultaneouslyemerged
at 445 nm. Isosbestic points were observed at 316 and 427 nm,
indicating that the photoisomerization was a unimolecular pro-
cess in which only two chemical species were present. Thus, the
trans-isomer was converted to form the cis-isomer as a primary
product. Moreover, the cis-isomer turned back into the trans-
isomer by irradiation with 460-nm light; the spectral changes
completelytracedbackthetrans–cis photoisomerization.The1H
NMR spectra of trans-1 before and after the photoisomerization
provide further evidence for the structural changesof the azoben-
zene moieties (Fig. 3). After photoirradiation at 360 nm, most of
© CSIRO 2010
10.1071/CH09653
0004-9425/10/040640

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Photoresponsive Organogel Based on Supramolecular Assembly of Tris(phenylisoxazolyl)benzene
641
OC10H21
N
N
C10H21O
N
N
N
N
O
O
O
N
O
N
N
O
C10H21O
OC10H21
N
N
N
O
N
N
N N
N N
trans-1
cis-1
OC10H21
OC10H21
Fig. 1. Schematic representation of trans–cis isomerization of 1.
(a)
(b)
0 min
1 min
0 min
1 min
2 min
3 min
4 min
5 min
10 min
30 min
1
1
2 min
3 min
4 min
5 min
10 min
30 min
0.5
0.5
0
0
300
400
500
600
300
400
Wavelength [nm]
500
600
Wavelength [nm]
Fig. 2. Absorption spectral changes of trans-1 (0.10 mmol L−1) in ethyl acetate: (a) the process of trans–cis isomerization under irradiation with 360-nm
light; (b) the subsequent reverse isomerization process by irradiating with 450-nm light. A 1-mm optical path-length cell was used.
Hf
He
the proton signals of trans-1 shifted upfield, and the amount
of cis-azobenzene reached ∼90%. Under visible light, the cis-
isomer isomerized back to the trans-isomer with the amount in
∼80% yield.
Hd
Hc
(c)
N
O
CHC9H19
N
O
Ha
N
Hg
Hb
The π-π stacking interactions of trans-1 in solution were
evidenced by a self-association study using 1H NMR spec-
troscopy. The 1H NMR spectra of trans-1 are concentration-
dependent in [D1]chloroform (Fig. 4a). The aromatic signals
moved upfield on increasing the concentration across a range
of 0.198–10.0 mmol L−1. Plotting the chemical shift changes
of the protons versus the concentration of trans-1 produced
hyperbolic curves. By applying the isodesmic model,[26–28] non-
linear curve-fitting analysis of the curves produced the estimated
complexation-induced shifts (Ha −2.31; Hb −2.22; Hc −1.47;
Hd −1.04; He −0.90; Hf −0.82; Hg −0.92 ppm) and the asso-
ciation constant (KE 13.4 0.8 L mol−1). The complexation-
induced shifts decreased with increasing distance of the protons
from the C3 axis of trans-1, indicating that flat aromatic com-
pound trans-1 stacks in a columnar fashion along the C3 axis. In
contrast, the 1H NMR spectra of the cis-isomer did not show con-
centration dependence in the concentration range (Fig. 4b). This
result implies that the intermolecular π-π stacking interactions
to produce supramolecular aggregates are weakened owing to
the photoisomerization of the azobenzene moieties.
H
b H
Ha
eHf
Hg
(b)
(a)
H
Hac,d
b Hf
H
He
Hg
8
6
4
d [ppm]
Fig. 3. 1H NMR spectra of: (a) trans-l (4.90 mmol L−1) in [D1]chloroform;
(b) the solution on exposure to 360-nm light for several hours; and (c) the
solution resulting from solution (b) irradiated with 450-nm light.
To obtain the molecular structures of the trans- and cis-
isomers, density functional theory (DFT) calculations were
carried out at the B3LYP/6–31G* level using the Gaussian 03
program[29] (Fig. 5).The results supported that trans-1 can adopt

RESEARCH FRONT
642
T. Haino and H. Saito
H
b He
Hc,d He
Hb Hf
Hf
Ha
Ha
(a)
(d)
(e)
(b)
(c)
9
(f)
7
9
7
d [ppm]
d [ppm]
Fig. 4. 1H NMR spectra of 1: (a) 1.17; (b) 2.40; (c) 4.90; (d) 1.17; (e) 2.40; and (f) 4.90 mmol L−1 at 298 K in [D1]chloroform: the solutions (a)–(c) are
before irradiation with UV light, and (d)–(f) are after irradiation with 365-nm light for several hours.
Table 1. Gelation properties of trans-1
G, gel; P, precipitation; and I, insoluble. Critical gelation concentrations
[mg mL−1] are shown in parentheses
a planar conformation, creating an extended flat aromatic sur-
face that is responsible for its self-assembly, driven by π-π
stacking interactions. In contrast, a planar conformation of cis-1
can no longer be maintained owing to the steric requirement of
the aromatic rings adjacent to the azo groups; its π-π stacking-
driven self-assembly could be impeded by intermolecular steric
interactions. These structural characteristics should lead to the
photoresponsive properties of the supramolecular assemblies
of 1.
Solvent
State
Solvents
State
Hexane
Cyclohexane
Benzene
Dichloromethane
Chloroform
Ether
Ethyl acetate
Acetonitrile
Ethanol
I
P
G(5)
P
P
1
G(20)
I
I
Iso-propanol
Tetrahydrofuran
Dimethylformamide
Dimethylsulfoxide
Acetone
Toluene
Butanol
Decanol
I
G(3)
P
P
P
P
I
To gain insight into the self-assembling behaviour of trans-
and cis-isomers in solution, we determined their diffusion
coefficients at a concentration of 10.0 mmol L−1 by pulsed
field gradient NMR using a bipolar pulse pair stimulated
echo pulse sequence. The influence of increasing the mag-
netic field gradient strength was monitored by the intensity of
the aromatic proton signals. The signal intensities of the pro-
tons for both isomers decayed as a function of the gradient
strengths. The given data were fitted to the Stejskal–Tarnner
equation to give diffusion coefficients of 2.79 0.08 × 10−10
and 4.96 0.02 × 10−10 m2 s−1 for trans- and cis-isomers. The
diffusion coefficient of the trans-isomer is remarkably dimin-
ished compared with that of the cis-isomer; the trans-isomer
most likely forms sizable aggregates, whereas the cis-isomer
cannot at the same concentrations. Whereas the relative size
of the aggregates remains to be determined, direct measure-
ment of their size is challenging. Recently, Lehn et al. reported
the systematic investigation of the translational diffusion coeffi-
cients of various double-stranded metallohelicates according to
their nuclearities, varying from one to five, by using diffusion
ordered spectroscopy (DOSY) experiments.[30] The diffusion
coefficients (Ds) were inversely proportional to the cylindri-
cal elongation of the metallohelicates along the principal axis.
Assuming that the aggregation of 1 occurs in a columnar fash-
ion, the aggregates could be more than tetrameric on the basis
of the relative ratio of Dtrans/Dcis = 0.563.
P
of 336 K, whereas those in THF and in ethyl acetate displayed
Tgs of 318 and 301 K. Photoresponsive behaviour of the gel was
examined. The turbid gel in benzene was stable when the tube
was turned upside down. Under irradiation with UV light at
360 nm for several hours, the gel became transparent, indicating
that the trans-isomer isomerized to the cis-isomer, causing the
collapse of the stable gel. The effective self-assembling ability
of the trans-isomer is most likely responsible for the gel for-
mation, whereas the reasonably weaker molecular interaction
of the cis-isomer destroys the higher order of supramolecular
assemblies.
Morphologies of the films prepared by casting solutions of
trans- and cis-isomers in THF, benzene, and ethyl acetate were
investigated by scanning electron microscopy (SEM) and atomic
force microscopy (AFM). In xerogels from THF and benzene,
the fibrous network formed by self-assembly of trans-1 is obvi-
ously evident (Fig. 6a, b, d, e, g, h). The thin fibres formed from
trans-1 are polydisperse and consist of bundles of thinner fibres
that are irregularly entangled and twisted. The entanglement and
bundles of the fibres causes junctures with fusion and splitting,
creating a large number of voids in which solvent molecules are
accommodated and immobilized. By contrast, the fibrous net-
work of the gel formed from a solution of ethyl acetate is well
developed, and each fibre retains a hexagonal shape with a high
aspect ratio, indicating that they are formed as a result of a highly
anisotropic growth process.
The gelation ability of trans-1 was evaluated in a variety
of solvents (Table 1). A suspension of 1 in a solvent was
heated to give a clear solution, which was then slowly cooled
to room temperature. After standing for 1 h, turbid gels formed
and exhibited no gravitational flow. The gelation was entirely
thermoreversible.Trans-1 exhibited a high gelation performance
in a limited range of organic solvents: benzene, ethyl acetate, and
THF, whereas many other solvents did not yield gels. The crit-
ical gelation concentration varied in the range of 3–20% w/v,
depending on the solvent. The stabilities of the gels in these
solvents were evaluated by critical gelation temperatures (Tg).
The gel in benzene was most stable, and showed the highest Tg
Photoinduced isomerization of the azobenzene moieties gave
rise to a dramatic transition of nanostructured morphology in
the film. After irradiation with UV light at 360 nm of solutions
of trans-1 both in THF and benzene, the solutions was cast on

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Photoresponsive Organogel Based on Supramolecular Assembly of Tris(phenylisoxazolyl)benzene
643
(a)
(b)
glass plates, and films were prepared. The SEM images showed
that the fibrous morphologies completely disappeared and
film-like morphologies formed (Fig. 6c, f). From the ethyl
acetate solution of cis-1, a prepared cast film on a glass plate was
not suitable for an SEM measurement owing to its low solubility
in the solvent. AFM instead of SEM allowed the observation of
a morphological transition between the trans- and cis-isomers
in the solid state. The solutions of the trans- and cis-isomers
Fig. 5. Calculated structures of (a) trans-1, and (b) cis-1.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
Fig. 6. Field emission scanning electron microscope and atomic force microscopy images of the cast films on glass or mica plates: (a)–(b) and (d)–(e), and
(g)–(i) prepared from a solution of trans-1 in THF, benzene, and ethyl acetate, respectively; (c), (f), and (j) prepared from the solutions after irradiation with
360-nm light.

RESEARCH FRONT
644
T. Haino and H. Saito
were cast on freshly cleaved mica plates and dried. The resultant
thin films were measured by AMF to obtain the morphologies
(Fig. 6i, j). Observed morphologies were quite contrastive; the
fibrils formed on a mica surface were highly entangled and cre-
atedvoids, whereasthefibrousmorphologycompletelyvanished
and any characteristic nanostructure was not observed in the film
prepared from the solution of the cis-isomer.
for 48 h, the precipitate was filtered off and washed with
CH2Cl2 to give the desired product as a white solid (1.78 g,
43%). mp 213–214◦C. δH (300 MHz, [D7]DMF) 9.70 (s, 3H),
8.64 (s, 3H), 7.98 (d, 6H, J 8.7), 7.81 (d, 6H, J 8.7), 1.53
(s, 27H). δC (75 MHz, [D7]DMF) 168.6, 163.3, 153.5,
142.5, 129.7, 127.8, 124.1, 122.8, 118.7, 100.4, 79.9, 28.1.
νmax(KBr)/cm−1 3322, 2779, 1703, 1615, 1535, 1509, 1463,
1432, 1390, 1368, 1316, 1231, 1159, 1051, 1019. m/z (high
resolution-mass spectrometry fast atom bombardment (HR-MS
FAB)) Anal. Calc. for C48H49N6O9 853.3561 [M + H+]. Found
853.3566.
Conclusion
Self-assembling properties of tris(isoxazolyl)benzene 1 possess-
ing azobenzene groups at its periphery have been investigated
and were found to alter in response to the trans–cis photo-
isomerization of the azobenzene moiety. In solution, trans-1
assembled in columnar fashion. Diffusion-ordered NMR spec-
troscopy supported the fact that oligomeric assemblies exist in
solution. On irradiation with UV light at 360 nm, the assemblies
were disrupted because the trans-azobenzene moieties were con-
verted to their cis configuration and the resulting cis-azobenzene
moieties no longer adopted a planar structure. These structural
features observed in solution were confirmed in the solid state.
SEM and AMF measurements of the films of trans-1 prepared
on a glass and a mica surface provide evidence that the fibrous
assemblies formed an entanglement that contained voids respon-
sible for immobilizing solvent molecules, whereas the images
from the cis-isomer were contrastive; the fibres and the voids
disappeared and no fibrous morphology was obtained. This fact
suggested that the photoresponse of the supramolecular gel orig-
inated from the photoinduced trans-to-cis structural transition of
the azobenzene moieties.
1,3,5-Tris[3-(4-aminophenyl)isoxazol-5-yl]benzene 3
A suspension of 2 (1.69 g, 1.98 mmol) in trifluoroacetic acid
(100 mL) was stirred for 1 h. To the reaction mixture was then
added a cooled solution of sodium hydroxide (53.85 g, 1.35 mol)
in water (300 mL). After stirring at 5◦C for 1 h, the precip-
itate was filtered off, and washed with water, methanol, and
CHCl3 to give the desired product as a white solid (1.10 g,
99%). mp >300◦C. δH (300 MHz, [D7]DMF) 8.57 (s, 3H), 7.81
(s, 3H), 7.74 (d, 6H, J 8.1), 6.85 (d, 6H, J 8.1), 5.75 (s, 6H). δC
(75 MHz, [D7]DMF) 168.1, 163.9, 151.9, 130.0, 128.3, 123.8,
116.4, 114.4, 100.0. νmax(KBr)/cm−1 3341, 3353, 3225, 3115,
3031, 1609, 1561, 1532, 1468, 1438, 1389, 1291, 1252, 1181.
m/z (HR-MS FAB)Anal. Calc. for C33H24N6O3 552.1910 [M+].
Found 552.1901.
1,3,5-Tris[3-(4-{(4-hydroxyphenyl)diazenyl}phenyl)
isoxazol-5-yl]benzene 4
A suspension of 3 (660 mg, 1.19 mmol) in water (60 mL) was
cooled to 5◦C and conc. HCl (1.04 mL, 35.83 mmol) was added.
To the reaction mixture was added a cooled solution of NaNO2
(271 mg, 3.94 mmol) in water (4 mL). After stirring at 5◦C for
1 h, a solution of phenol (0.35 mL, 3.98 mmol) and sodium
hydroxide (215 mg, 5.37 mmol) in water was added dropwise to
the reaction mixture over 10 min and the mixture was stirred at
room temperature for 3 h.After the reaction was complete, dilute
acetic acid was added to the reaction mixture and the pH adjusted
to 4. The precipitate was filtered off and washed with water,
then methanol, to give the desired product as an orange solid
(991 mg, 95%). mp >300◦C. δH (300 MHz, [D7]DMF) 8.30
(s, 3H), 8.01 (d, 6H, J 8.0), 7.78–7.99 (m, 12H), 7.74
(s, 3H), 7.06 (d, 6H, J 8.0). δC (75 MHz, [D7]DMF) 168.7,
162.7, 162.2, 153.7, 146.3, 130.6, 129.2, 127.9, 125.6, 123.7,
123.2, 116.4, 100.4. νmax(KBr)/cm−1 3125, 1587, 1559, 1504,
1459, 1433, 1377, 1227, 1138. m/z (HR-MS FAB) Anal. Calc.
for C51H32N9O6 866.2476 [M − H−]. Found 866.2475.
Experimental
General
All chemicals were used without further purification unless
otherwise specified. Proton and carbon NMR spectra were
recorded on aVarian Mercury-300 spectrometer. IR spectra were
obtained on a JASCO Fourier-transform (FT)-IR-420S. UV-
visible absorption spectra were measured on a JASCO V-560
spectrometer. Photoisomerization was carried out by a JASCO
FP-6500 fluorescence spectrometer. Mass spectra were obtained
with a JEOL JMX-SX 102A. Melting points were measured
with Yanagimoto micro melting point apparatus and are uncor-
rected. Elemental analyses were performed on a Perkin–Elmer
2400CHN elemental analyzer. AFM measurements were car-
ried out using an Agilent 5100 AFM system. SEM images were
obtained on Hitachi S-5200 field emission scanning electron
microscope.
SEM and AFM Sample Preparation
Solutions of trans-1 (∼10−4 mol L−1) were prepared in benzene,
ethyl acetate, and tetrahydrofuran. Trans-to-cis isomerization of
trans-1 was carried out by irradiation with monochromatic light
of 360 nm at 293 K using a fluorescence spectrometer. Solu-
tions of the trans- and cis-isomers were cast on glass or mica
plates.Then, theresultantfilmswerequicklydriedunderreduced
pressure. These were subjected to microscope measurements.
1,3,5-Tris[3-(4-{(4-decyloxyphenyl)diazenyl}phenyl)
isoxazol-5-yl]benzene 1
To a solution of 4 (978 mg, 1.13 mmol) in DMF (80 mL) was
added 1-iododecane (1 mL, 4.69 mmol) and K2CO3 (1.56 g,
11.29 mmol). After stirring at 120◦C for 48 h, the reaction mix-
ture was poured into water (300 mL).The precipitate was filtered
off and washed with water, methanol, and hexane to give the
desired product as an orange solid (1.353 g, 93%). mp 188–
190◦C. δH (300 MHz, CDCl3) 8.35 (s, 3H), 8.04 (d, 6H, J 9.0),
7.99 (d, 6H, J 9.0), 7.94 (d, 6H, J 9.0), 7.10 (s, 3H), 7.01
(d, 6H, J 9.0), 4.04 (t, 6H, J 6.6), 1.76–1.90 (m, 6H), 1.20–
1.60 (m, 42H), 0.89 (t, 9H, J 6.9). δC (75 MHz, CDCl3) 168.1,
162.4, 162.0, 153.5, 146.7, 129.8, 128.8, 127.4, 125.6, 125.0,
123.1, 114.6, 98.9, 68.4, 31.9, 29.6, 29.5, 29.3, 29.2, 26.0, 22.7,
1,3,5-Tris[3-(4-{tert-butoxycarbonylamino}phenyl)isoxazol-
5-yl]benzene 2
To a solution of triethynylbenzene (732 mg, 4.88 mmol) and
4-(tert-butoxycarbonylamino)benzohydroximinoly chloride[31]
(3.60 g, 13.30 mmol) in CH2Cl2 (200 mL) was added triethyl-
amine(5.57 mL, 39.91 mmol).Afterstirringatroomtemperature

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645
14.1. νmax(KBr)/cm−1 2924, 2582, 1601, 1581, 1559, 1500,
1468, 1433, 1387, 1297, 1252, 1140, 1106, 1015. m/z (HR-MS
FAB)Anal. Calc. for C81H94N9O6 1288.7327 [M + H+]. Found
1288.7317.
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Acknowledgements
This work is supported by Grant-in-Aid for Scientific Research
(no. 18350065, no. 21350066) of Japan Society for the Promotion of Science.
We acknowledge financial support from Yamada Science Foundation.
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