Solvent-dependent photophysical and anion responsive properties of one glutamide gelator

Article abstract of DOI:10.1039/c1sm05431g

One gelator consisting of four amide units and one chromophore was designed and synthesized. These four amide groups could induce the self-assembly of the gelator molecules in one-dimensional (1-D) direction. Two cyano and one nitro terminal groups were a

Full text of DOI:10.1039/c1sm05431g

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PAPER
Solvent-dependent photophysical and anion responsive properties of one
glutamide gelator†
Pengchong Xue, Yuan Zhang,b Junhui Jia,a Defang Xu,a Xiaofei Zhang,a Xingliang Liu,a Huipeng Zhou,a
a
*
a
a
Peng Zhang, Ran Lu, Makoto Takafujic and Hirotaka Iharac
*
Received 11th March 2011, Accepted 28th June 2011
DOI: 10.1039/c1sm05431g
One gelator consisting of four amide units and one chromophore was designed and synthesized. These
four amide groups could induce the self-assembly of the gelator molecules in one-dimensional (1-D)
direction. Two cyano and one nitro terminal groups were also appended into the chromophore as
electron withdrawing groups to make the gelator’s own typical D–p–A electron structure. As a result, it
was found that the color of the gel phases, the sol–gel phase transition temperature, microscopic
morphologies, emission behaviour and response to anions were dependent on the solvent because of the
different packing models and solvation effect in different solvents. For example, the color of the DMSO
gel was orange, but ODCB gel possessed a yellow color. In addition, multicolor emission could be
realized in DMSO gel systems, but ODCB gels only emitted yellow fluorescence. Moreover, the gelator
was responsive to Fꢀ, H2PO4ꢀ and OAcꢀ in DMSO, but in ODCB it selectively responded to Fꢀ.
solvents can influence the thermotropic, photophysical and
Introduction
anion-responsive properties of the gels formed by a glutamide
Smart materials and systems have been attracting increasing
attention because they can significantly change their mechanical
properties (such as shape, stiffness, and viscosity), or their
thermal, optical, or electromagnetic properties, in a predictable
or controllable manner in response to their environment, such as
stress, temperature, moisture, pH, electric or magnetic fields.1 In
particular, smart functional organogels, formed by a large
amount of organic solvents and a low-molecular mass compound
with low concentration (typically <2 wt%), have been widely
developed in recent years, because their bulk shape and proper-
ties can be either switched or tuned by an external chemical or
physical stimulus, such as metal ions, anions, small organic
compounds, proton, light irradiation, oxidation or reduction
reaction, sound and temperature by introducing unique func-
tional moieties.2 It is well known that a given gelator can only
gelatinize some special solvents. Sometimes, a little difference
between solvent structures can lead to obvious differences in gel
properties, such as the sol–gel phase transition temperature,
morphologies of the molecular aggregates and rheological
behaviour.3 In this paper, we would like to show how the selected
derivative with electron-withdrawing groups.
Based on the results obtained in our previous works,4 in which
two gelators with electron withdrawing and donating groups
were reported and their photophysical properties in gel phases
could be tuned by solvents. It can be confirmed that D–p–A
structure of chromophores in gelator molecules is an important
factor to achieve adjustable photophysical properties of gels
through changing solvents with different polarities. In addition,
so far most of the anion-responsive gels reported comprise anion
receptors, such as amide, amine or urea units, which also act as
hydrogen bond sites that support molecular assembly. More-
over, the works reported by other groups suggested that it was
a sensible choice that the anion acceptor was directly linked to
chromophores so that the response of the gels to anion stimulus
was visible to the naked eye due to obvious color changes.5
Therefore, we designed and synthesized one new gelator 1, as
shown in Scheme 1. It contains three aliphatic amide groups and
one aromatic amide moiety, which induce the self-assembly of
gelator molecules in one-dimensional (1-D) direction.6
aState Key Laboratory of Supramolecular Structure and Materials College
of Chemistry, Jilin University, 2699# Changchun, China. E-mail:
xuepengchong@jlu.edu.cn; luran@jlu.edu.cn
bCollege of Chemistry, Beijing Normal University, Beijing, 100875, China
cDepartment of Applied Chemistry and Biochemistry, Kumamoto
University, 2-39-1 Kurokami, Kumamoto, Japan
† Electronic supplementary information (ESI) available: Frontier
orbitals, IR, FL sepctra of gels and absorption spectra with anions. See
DOI: 10.1039/c1sm05431g
Scheme 1 Molecular structure of 1 and markings of anion-binding sites
and electron withdrawing units.
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Moreover, four NH units can also act as anion acceptors. Two
cyano and one nitro terminal groups were also appended as
electron withdrawing groups to enhance the acidity and
responsive activity of aromatic NH toward anions, and give 1 its
own typical D–p–A electron structure (Fig. S1†). The results
indicate that the color of the gel phases, microscopic morphol-
ogies, emission behavior and response to anions could be
adjusted by solvents, which makes us further understand the role
of solvent on controlling extraordinary properties of the gelator.
Experimental section
Instruments
Scheme 2 Synthesis route of 1.
Infrared spectra were measured using a Nicolet-360 FT-IR
spectrometer by incorporating the samples in KBr disks. The
UV-vis spectra were determined on a Mapada UV-1800pc
spectrophotometer. The absorption spectra of the DMSO and
DMF system at high concentration were measured with 0.1 mm
glass cell. C, H, and N elemental analyses were performed on
a Perkin-Elmer 240C elemental analyzer. Photoluminescence
measurements were taken on a Shimadzu RF-5301 Lumines-
cence Spectrometer. X-Ray diffraction (XRD): diffraction
patterns were carried out on a Rigaku D/max-rA X-ray
diffractometer with graphite monochromatized Cu-Ka radiation
69.12, H 3.56, N 10.20. 1H NMR (500 MHz, TMS, CDCl3): 10.10
(s, 1 H), 8.34 (d, J ¼ 9.0 Hz, 2 H), 8.09 (d, J ¼ 8.0 Hz 2 H), 8.02
(d, J ¼ 8 Hz 2 H), 7.89 (d, J ¼ 19.0 Hz, 2H), 7.89 (s, 1H).
N1-((1,3-Bis(dodecylcarbamoyl)propylcarbamoyl))-N5-(4-(1-
cyano-2-(4-(2-cyano-2-(4-nitrophenyl)vinyl)phenyl) vinyl)phenyl)
glutaramide (1). Compound 3 (0.48 g, 0.68 mmol) and 2 (0.19 g,
0.68 mmol) were dissolved in ethanol (50 mL). After the solution
was heated and refluxed for 30 min, a few drops of tetrabutyl-
ammonium hydroxide (TBAOH) (2 M solution in water) were
added. The mixture was refluxed overnight. Orange solid was
obtained after filtration and washing using water and ethanol.
After dried under vacuum, orange product (0.5 g, 76%) was
ꢀ
(l ¼ 1.5418 A). The accelerating voltage was set at 50 kV, with
100 mA fluxes at a scanning rate of 0.02 deg. sꢀ1 in the 2 q range
of 1–10ꢁ. The XRD patterns of the xerogel were obtained by
casting wet gel samples in a glass flask followed by evaporation
of the solvent naturally. 1H NMR spectra were recorded on
a Mercury plus 500 MHz. SEM images were carried out on
a Japan Hitachi model X-650 scanning electron microscope.
TEM images were observed with a Hitachi H-8100 apparatus by
wiping the samples onto a 200-mesh carbon coated copper grid
followed by naturally evaporating the solvent.
ꢁ
obtained. Mp >200 C, IR: 3285, 3067, 2922, 2852, 2218, 1645,
1637, 1521 cmꢀ1
. Element analysis (%): calculated for
C58H79N7O6: C, 71.80; H, 8.21; N, 10.10; found: C, 71.70; H,
1
ꢁ
8.31; N, 10.02. H NMR (500 MHz, TMS, DMSO-d6, 60 C):
10.01 (s, 1 H, NH), 8.36 (d, J ¼ 8.5 Hz, 2 H), 9.30 (s, 1 H), 8.12 (d,
J ¼ 8 Hz, 2 H), 8.07 (d, J ¼ 8.5 Hz, 4 H), 7.96 (s, 1 H), 7.76–7.72
(m, 5 H), 7.62 (m, 1 H), 7.55 (m,1 H), 4.19 (m, 1 H), 3.01 (m, 4 H),
2.36 (t, J ¼ 7 Hz, 2 H), 2.22 (t, J ¼ 7 Hz, 2 H), 2.06 (t, J ¼ 7 Hz,
2 H), 1.88–1.68 (m, 4 H), 1.37 (m, 4 H), 1.22 (m, 36 H), 0.84
(t, J ¼ 7 Hz, 6 H).
Gelation test of organic fluids
The solution containing certain weighed gelator 1 and organic
solvent in a sealed test tube was heated with 1 cm diameter in an
oil bath until the solid was dissolved completely. After the
solution was allowed to stand at room temperature for 6 h, the
state of the mixture was evaluated by the ‘‘stable to inversion of
a test tube’’ method.
Results and discussion
Gelation properties investigation
The gelation ability of 1 in organic solvents was checked by the
standard heating-and-cooling method. We found that 1 was
insoluble in many solvents, such as cyclohexane, benzene, THF,
butyl acetate etc, even upon heating (Table 1). However, it could
be dissolved in hot ethanol and n-octanol. Moreover, the trans-
parent yellow gel phase could be observed when the hot solutions
of 1 in aromatic solvents with high boiling point, such as
o-dichlorobenzene (ODCB), mesitylene and anisole, were cooled
to room temperature. Furthermore, hot bright yellow DMSO
and DMF solutions of 1 could be also transformed into opaque
orange gels at high concentration.
Synthetic procedures and characterizations
Molecule 3 was synthesized according to the literature proce-
dures.6 The synthesis route of 1 was shown in Scheme 2.
(Z)-3-(4-Formylphenyl)-2-(4-nitrophenyl)acrylonitrile (2).
A
mixture of 4-nitro phenylacetonitrile (1.4 g, 8.6 mmol), tereph-
thalaldehyde (2.35 g, 14.5 mmol) and ethanol (20 mL) was heated
until the solid was completely dissolved. A few drops of tetra-
butylammonium hydroxide (TBAOH) (2 M solution in water)
were added and the solution was further refluxed for 2 h. Yellow
solid (1.4 g, 58% in yield) was obtained by heat filtration and
washing several times by ethanol. Mp 196–198 ꢁC. IR: 3110,
3057, 2233, 1689, 1604 and 1514 cmꢀ1. Elemental analysis (%):
calculated for C16H10N2O3: C, 69.06; H, 3.62; N, 10.07; found: C
It is well known, the type of gelation solvent strongly depends
on the solvation degree of solvent to gelators or its solubility in
solvent. However, their solubility in a given solvent indeed
depends on the molecular structures of the gelators. For
example, T. Yi reported two gelators with amide and cholesterol
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Table 1 Gelation ability of 1 in various solvents.a
Solvent
Status
Solvent
Status
Cyclohexane
n-Hexane
Petroleum ether
Benzene
Toluene
I
I
I
I
I
I
I
I
I
I
Ethyl acetate
Ethanol
Methanol
n-Octanol
Xylene
DMSO
DMF
Mesitylene
Anisole
o-Dichlorobenzene
I
S
I
S
I
THF
G (1.5b)
G (3)
G (0.5)
G (0.5)
G (0.5)
Chloroform
Dichloromethane
Acetone
Butyl acetate
a
b
G: gel; S: soluble; I: insoluble. Minimum gel concentration (mM)
necessary for gelation.
Fig. 1 Plots of Tgel versus the concentration of 1 in DMSO and ODCB.
units, which could form a gel in apolar and polar solvents.7 It
indicates that the existence of polar and apolar moieties in
gelator molecules is important to induce the gelator to gelatinize
a wide range of solvents. Another typical example is amino acid
gelators with long alkyl chains.2 For example, M. Suzuki
reported L-valine and L-isoleucine gelators, which also gelati-
nized many types of solvents, such as hexane, menthol, ethyl
acetate, toluene, DMSO, and so on.8 In our case, the gelator 1
has four polar amide units, an apolar long alkyl chain and
aromatic moieties. Considering the existence of four amide units,
which makes the gelator more polar, 1 has poor solubility in
apolar solvents, such as benzene, toluene, even on heating.
Eventually, 1 can only be dissolved in aromatic solvents with
high boiling point and gelatinizes them at low concentration. On
the other hand, 1 has large solubility in DMSO and DMF, so
minimum gel concentrations necessary for gelation in DMSO
and DMF are higher than those in aromatic solvents.
influenced by the solvent. Because 1 has low solubility in ODCB,
the Tgel is relatively higher. On the other hand, 1 has large
solubility in DMSO. As a result, DMSO gel possesses low Tgel at
the same concentration.9
Self-assembled characteristic in gel phases
Because a visible color change was observed in DMSO during the
gelation, the gelation processes in the two solvents were investi-
gated by their absorption spectra. As shown in Fig. 2, the
maximal absorption peak of 1 in hot DMSO (3 mM) was located
at 399 nm, which decreased and blue shifted to 364 nm after the
gel phase was formed, indicating a shift of 35 nm. The peak at
399 nm in hot ODCB solution (0.5 mM) disappeared and a new
peak at 347 nm (blue-shift of 52 nm) appeared when the sample
was cooled down and transformed into the gel phase. It suggests
that the H-aggregate (face-to-face packing) between aromatic
moieties is formed during gelation in DMSO and ODCB, and the
p–p interactions between chromophores of 1 in the ODCB gel
phase is stronger than that in the DMSO gel.10 In other words,
the distance between adjacent aromatic moieties in the ODCB gel
phase is smaller than that in the DMSO gel. It is worth noting
that during the gelation process in DMSO the absorption band
was gradually broadened accompanied with a blue shift of the
absorption band, and the visible absorption band from 460 to
535 nm (maxmium at 458 nm) became stronger and stronger,
which can explain why the color of the DMSO solution is yellow,
but the DMSO gel is orange. On the other hand, ODCB gel only
possessed very weak absorption in this region, so its color
remained yellow.
The color difference of the gels in two kinds of solvents with
different polarities suggests that photophysical properties of gels
should be different and influenced by the solvent. DMSO and
ODCB were selected as testing solvents to observe the influence
of the solvent on the photophysical and anion-responsive prop-
erties of gels.
Thermotropic behaviors
First, we examined the influence of solvent on the sol–gel phase
transition temperature (Tgel). Tgel was determined by a conve-
nient ball-drop method. The result shows that the Tgel values rise
nonlinearly with increasing gelator concentration in two solvent
systems (Fig. 1). However, the increasing Tgel of the ODCB gel
with concentration is more rapid than that of the DMSO gel. For
example, ODCB gel at a concentration of 1.0 mM transformed
into solution when temperature reached 66 ꢁC. Tgel of the ODCB
gel at a concentration of 2.0 mM increased by 57 ꢁC and reached
Owing to the formation of H-aggregate and molecular
chirality, CD spectroscopy might be considered an appropriate
method for further researching the assembly process. Strong
bisignated induced CD band (negative Cotton effect) in the
ꢁ
123 C. On the other hand, when the gelator concentration was
increased from 1.4 mM to 2.8 mM, the corresponding Tgel of the
DMSO gel slowly went up from 56 ꢁC to 88 ꢁC. Moreover, it is
also evident that Tgel of ODCB gel is higher than that of the
DMSO gel at the same concentration of 1. For example, when
the concentration of 1 was maintained at 2.0 mM, Tgel of the
ODCB gel reached 123 ꢁC, while Tgel of the DMSO gel was
78 ꢁC. Therefore, the thermotropic behavior of gel could be
region of 300–500 nm (lmax
lq
¼
335, lmin
¼
375, and
¼ 350 nm) was found in ODCB gel. DMSO gel also
¼
0
possessed negative Cotton effect, in which the zero-crossing was
located at 374 nm. The results of the UV-vis and CD spectral
investigation indicate that the chromophores in molecule 1 are
packed in a counter-clockwise direction in DMSO and ODCB
gel phases.11 Interestingly, DMSO gel showed a strong additional
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solution the absorption peaks of amide I located at 1687 and
1665 cmꢀ1, which shifted to 1660 and 1637 cmꢀ1, respectively, in
DMSO wet gel, and shifted to 1657 and 1634 cmꢀ1, respectively,
in ODCB wet gel. Moreover, the absorption bands of NH
appeared at 3299 and 3288 cmꢀ1 in DMSO and ODCB wet gels,
respectively. The result indicates that all the amide groups are
involved in intermolecular hydrogen bonds,12 and the interaction
between amide units in ODCB gel is stronger than that in DMSO
gel, and the closer distance between adjacent amide groups in
ODCB gel than that in DMSO gel is present.13
NMR spectra were also used to study the hydrogen bonds
between amide units. Fig. 3 shows the temperature-dependent 1H
NMR spectra of 1 in DMSO-d6. The peaks of the chromophore
and amide moieties were invisible at 20 ꢁC, suggesting that these
groups existed in aggregated form at this temperature. They
appeared and gradually became sharper and stronger with
increasing temperature, indicating that more and more gelator
molecules were detached from aggregation and present in
monomeric form. Notably, NMR signals for the four kinds of
amide N–H proton not only became stronger, but also shifted
toward higher field with increasing temperature because of
weakening of hydrogen bond strength. Moreover, the NMR
Fig. 2 Absorption spectral change of 1 during cooling process in DMSO
(a, 90-20 ꢁC, 3.0 mM) and in ODCB (b, 160-20 ꢁC, 0.5 mM), (c) and CD
spectra of DMSO (3.0 mM) and ODCB (0.5 mM) gels at 20 ꢁC.
peak at 500 nm, which is consistent with its absorption spectrum
in this region. It further suggests that the absorption band from
460–540 nm originates from the molecules aggregating and
packing models in DMSO and ODCB are different.
Furthermore, FT-IR spectra were measured to investigate the
interaction between amide units in the gel phase. To exactly
clarify the hydrogen bonds in different solvents, IR spectra of
wet gels and hot solution were obtained. It was found that in hot
Fig. 3 (a) Variable temperature 1H NMR spectra of DMSO-d6 gel from
20 to 60 ꢁC, [1] ¼ 1.67 mM. (b) Concentration-dependent 1H NMR
spectra of 1 in DMSO-d6 at 60 ꢁC.
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spectra of the samples at different concentrations were also
compared and were shown in Fig. 3b. It is very clear that the
peaks of the four types of amide groups from the low-concen-
tration sample are located at a higher field region relative to those
observed at a high concentration. These results support the fact
that all the amide groups form unidirectional intermolecular
hydrogen bonds,14 being in agreement with the results obtained
by FT-IR spectra. These observations exhibit that the hydrogen
bonds between amide moieties in both solvents play an impor-
tant role on the gel formation.
The morphologies of gels in DMSO and ODCB were investi-
gated by using scanning and transmission electron microscopes
(SEM and TEM). SEM images showed that the extended fibrillar
network structure was formed by numerous intertwined fibers
with high aspect ratio in both gels. It suggests that gelator
molecules tend to self-assemble along the 1-D fibrillar direction.
Moreover, the width of fibers from the DMSO gel are larger than
those from the ODCB gel. TEM images of DMSO and ODCB
xerogels also displayed fibrous structures. The fibers in DMSO
held solid morphology. On the other hand, the thick fibers were
composed of many thin fibers with a width of about 4.5 nm in
ODCB, which were loosely stacked together (Fig. 4d).
Fig. 5 Small-angle XRD patterns of xerogels of 1 from DMSO and
ODCB.
ODCB xerogel are also indicative of poorer long-range ordering
of molecular stacking than that in the DMSO gel.
Once the 1-D self-assembly in the gel phase had been estab-
lished, the question arose as to why the colors of DMSO and
ODCB gels are different and how the molecules are packed
within the superstructures. The results of the UV-vis and IR
absorption spectra, NMR and electron microscopy experiments
clearly illuminate that many aromatic and amide groups interact
with each other to form one-dimensional stacking along the
direction of fibrous growth. To further reveal detailed informa-
tion of molecular stacking, the geometric optimization of the
gelator molecule was performed by using the AM1 force field. It
was found that the molecular length of 1 in extended confor-
mation was about 4.4 nm (Fig. S3†), which is shorter than the
layer period (5.08 nm). It is possible that gelators adopt a bimo-
lecular layer structure. Considering the fact the formation of
H-aggregation in DMSO and ODCB gels and the present of an
additional absorption peak at 458 nm in DMSO gel, we give the
molecular stacking models in DMSO and ODCB gels. As shown
in Fig. 6, when the hot DMSO and ODCB solutions are cooled
down, gelator molecules aggregate together to form 1-D face-to-
face packing (H-aggregate) driven by the p–p interaction and
XRD spectra can be used to detect the regularity of molecular
packing. The samples were prepared by casting wet gels on glass
plates and further natural evaporation. As shown in Fig. 5, the
XRD diffraction pattern of DMSO xerogel was characterized by
four reflection peaks at 5.08, 2.54, 1.67 and 1.31 nm in the low
angle region, illustrating that a lamellar organization with
5.08 nm of interlayer distance is formed in the DMSO gel
phase.15 On the other hand, two weak diffraction peaks for
ODCB xerogel at about 5.08 and 2.54 nm could be observed,
implying that it is possible that gelator molecules adopt the
similar packing types in long range in DMSO and ODCB.
Moreover, it was very clear that the full width at half maximum
(FWHM) of the ODCB xerogel was larger than that of the
DMSO gel. The weaker diffraction and wider FWHM of the
Fig. 4 SEM and TEM images of xerogels from DMSO (a, c) and ODCB
Fig. 6 The schematic representation of stacking models in DMSO and
(b, d), respectively.
ODCB gels.
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hydrogen bonds. Because of the solvophobic effect of the
aromatic chromophore group of gelator to polar DMSO, such
1-D H-aggregates further construct tilting and interpenetrating
bimolecular superstructure, leading to J-aggregates (head-to-tail)
in another direction (Fig. 6). Therefore, a red-shifted absorption
band at 458 nm can be found in the DMSO gel.16 On the other
hand, in ODCB, better solvation of ODCB to the aromatic
moiety of gelator induces a large distance between the 1-D
H-aggregates and results in poor J-aggregation in another
direction. Finally, a weak absorbance above 450 nm is observed.
This assumption can be confirmed by the observations for DMF
and anisole gels (Fig. S4 and Fig. S5†). In polar DMF,
morphologies and absorption spectra are similar to those in
DMSO. Anisole gel possesses similar behaviors to the ODCB gel.
Fluorescence properties of gels
When the orange powder of 1 in DMSO was heated to form
a clear bright yellow solution (6.6 mM), strong green fluores-
cence with a emission maxima of 530 nm, ascribed to the emis-
sion of molecules in monomeric form, was observed. When the
hot solution was naturally cooled, the fluorescence color of the
sample showed a substantial color change from green to yellow,
and to orange, accompanied by a gradual decrease in emission
intensity. Finally weak red emission with a maxima at 605 nm
was observed in the gel phase (Fig. 7 and S6†). On the other
hand, the hot solution of 1 in ODCB (4.0 mM) emitted intense
bluish-green fluorescence with a maxima at 512 nm, and the gel
showed enhanced yellow emission (lem ¼ 567 nm, Fig. 7,
Fig. S7†). The large red shift in the emission spectra during the
self-assembly of 1 in two solvent systems indicates the delocal-
ization of the exciton across the self-assembled aggregates.
Because exciton can be migrated efficiently within the aggregates
of different HOMO–LUMO levels, fluorescence originates
mainly from aggregates of lower energy, resulting in a red shift in
emission wavelength.17 Interestingly, it also was found that the
DMSO gels with various emission color (green, yellow, orange
and red) could be realized by simply varying the concentration of
1 (Fig. 8a). On the contrary, in the range of the measured
concentration of 1 the ODCB gels possessed similar emission
bands (Fig. 8b, Fig. S7†). To further understand the fluorescence
difference in the two solvents, their concentration-dependent
UV-vis spectra were measured. As shown in Fig. 8c, the maximal
absorption peaks of the DMSO gels related to gelator concen-
tration are gradually shifted to the longer wavelength region with
decreasing concentration. It indicates the weakening p–p inter-
action between chromophores or the larger distance between
adjacent chromophores at lower concentration because the
Fig. 8 Concentration-dependent fluorescence and absorption spectra of
1 in DMSO (a, c) and ODCB (b, d), respectively, at room temperature.
lex ¼ 400 nm.
Fig. 7 Photos of the gelation process of 1 in DMSO (6.0 mM) and
ODCB (2.0 mM) under irradiation by 365 nm light.
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gelator has higher solubility, or lower aggregation tendency in
DMSO, which induces inefficient exciton migration to lower
aggregates of lower energy and promotes blue shift of emission
band relative to those in high concentration.18 On the other hand,
the maximal absorption peaks of ODCB gels with different
concentrations of 1 kept unchanged other than a slight increase
in the range of 400 to 500 nm (Fig. 8d). In other words, the
aggregated trend of 1 in ODCB is very strong so that almost
gelator molecules self-assemble together to form aggregates and
the distances between adjacent chromophores are similar at
different concentrations, inducing similar exciton migration
within one-dimensional aggregates and further resulting in
similar emission bands for ODCB gels with different concen-
trations. By carefully observing emission spectra of ODCB gels,
a small blue shift of emission peaks at low concentration could be
found and the fluorescence intensity of gels with lower concen-
tration generally are stronger than those of gels with higher
concentration. It further proves the delocalization of the exciton
across the self-assembled aggregates. These results declare that
the solvent system is important in driving the gelator associations
and then controlling the photophysical properties of gels. To
further give more compelling evidences, concentration-depen-
dent UV-vis and emission spectra of DMF and anisole were also
measured. As shown in Fig. S4, the absorption and emission
spectra of 1 in DMF strongly depends on the gelator concen-
tration, which is similar to those in DMSO.† In another aromatic
solvent, anisole, the spectroscopic behaviors of 1 look like those
in ODCB. More interestingly, when a small amount of polar
DMSO was added into the ODCB gel, yellow ODCB gel was
changed into orange gel (Fig. S8†). For example, while 50 mL of
DMSO was injected into ODCB gel (1 mL) and the mixture was
heated and cooled down to form a gel, the maximal absorption
peak exhibited at 369 nm and a new band from 450 to 500 nm
appeared, which resembles that in neat DMSO. Moreover, the
fluorescence was quenched and shift to 600 nm. When additional
50 mL of DMSO was added, the mixture had analogous
absorption and emission spectra to those with low concentration
in DMSO. This result further confirms the important role of the
interaction between solvent and gelator on controlling photo-
physical characteristics.
Anion responsive properties
Fig. 9 The absorption spectra of 1 in DMSO (a, 0.5 mM) and ODCB (b,
0. 5 mM) upon the addition of different amounts of TBAF, and photos of
1 in DMSO (3.3 mM) and ODCB (1.0 mM) with and without anions.
Insets show the plots of absorbance at 590 nm (a), and 510 nm (b) vs.
[TBAF]/[1].
The anion responsive properties of the gels of 1 toward a number
ꢀ
of selected target anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4 and
ꢀ
HSO4 as tetrabutylammonium (TBA) slats) were examined in
DMSO and ODCB.19 As we expected, the organogel of 1
exhibited excellent gel–sol transition according to the stimuli of
anions. In DMSO, the hot solution of 1 (3 mM) could not be
changed into a gel upon the addition of 10 eq. TBAF even when
the sample was left over one day. Moreover, the solution turned
into a dense red (Fig. 9). However, the gel state of 1 in DMSO
could be preserved after adding the same amount of other halide
anions, H2PO4ꢀ, AcOꢀ and HSO4ꢀ. It suggests that the anions,
not the TBA cation, are responsible for the transform of the gel
to solution and color change.20 Interestingly, the DMSO gel
changed its color from orange to red upon the addition of
H2PO4ꢀ, and the color of the gel only became deeper after
adding AcOꢀ (Fig. S9†). The addition of other anions did not
influence the gel color. More interestingly, the hot yellow DMSO
solution with less than 6 eq. TBAF could still transform into red
gel, but the color of the red gel became deeper and deeper with
increasing TBAF concentration (Fig. S9†). To further take
insight into the anion response in DMSO, their UV-vis spectra
were obtained. Consequently, it was found that a broad
absorption band with two maxima at 560 and 595 nm appeared
upon the addition of TBAF and gradually raised with the
simultaneous decrease in absorbance of the band at 399 nm as
the concentration of TBAF was increased. Finally, the
8302 | Soft Matter, 2011, 7, 8296–8304
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absorption band at 399 nm completely disappeared upon the
addition of 10 eq. TBAF. This result suggests that more and
more gelator molecules were bound with Fꢀ while TBAF was
continuously added. Therefore, the gel phase could be preserved
in the condition of low TBAF concentration because of enough
free gelator molecules to self-assemble and gelatinize the solvent.
When more than 6 eq. TBAF was added, the amount of free
gelators was too little to form gel phase. At the same time, it was
found that the absorption band at 399 nm was still very strong
even after adding 10 eq. H2PO4ꢀ and AcOꢀ, and the bands at 560
and 595 nm were weak (Fig. S10†), which can explain the exis-
Ministry; The National Natural Science Foundation of China
(20874034 and 51073068); 973 Program (2009CB939701); Open
Project of State Key Laboratory of Supramolecular Structure
and Materials (SKLSSM200901).
Notes and references
1 (a) M. V. Gandhi and B. S. Thompson, Smart materials and
structures, Chapman
& Hall, London; New York, 1992; (b)
J. Wang, A. Kulago, W. R. Browne and B. L. Feringa, J. Am.
Chem. Soc., 2010, 132, 4191.
2 (a) P. Terech and R. G. Weiss, Chem. Rev., 1997, 97, 3133; (b)
D. J. Abdallah and R. G. Weiss, Adv. Mater., 2000, 12, 1237; (c)
J. H. van Esch and B. L. Feringa, Angew. Chem., Int. Ed., 2000, 39,
2263; (d) L. A. Estroff and A. D. Hamilton, Chem. Rev., 2004, 104,
1201; (e) N. M. Sangeetha and U. Maitra, Chem. Soc. Rev., 2005,
34, 821; (f) A. R. Hirst and D. K. Smith, Chem.–Eur. J., 2005, 11,
5496; (g) A. Brizard, R. Oda and I. Huc, Top. Curr. Chem., 2005,
256, 167; (h) F. Fages, Angew. Chem., Int. Ed., 2006, 45, 1680; (i)
M. George and R. G. Weiss, Acc. Chem. Res., 2006, 39, 489; (j)
D. K. Smith, Adv. Mater., 2006, 18, 2773; (k) K. Sada,
M. Takeuchi, N. Fujita, M. Numata and S. Shinkai, Chem. Soc.
Rev., 2007, 36, 415; (l) A. Ajayaghosh and V. K. Praveen, Acc.
Chem. Res., 2007, 40, 644; (m) Z. Yang and B. Xu, J. Mater.
Chem., 2007, 17, 2385; (n) A. R. Hirst, B. Escuder, J. F. Miravet
and D. K. Smith, Angew. Chem., Int. Ed., 2008, 47, 8002; (o)
M. M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed,
Chem. Rev., 2010, 110, 1960.
3 (a) M. Bielejewski and J. Tritt-Goc, Langmuir, 2010, 26, 17459; (b)
N. Yan, G. He, H. Zhang, L. Ding and Y. Fang, Langmuir, 2010,
26, 5909; (c) W. Edwards, C. A. Lagadec and D. K. Smith, Soft
Matter, 2011, 7, 110; (d) S. Bhattacharya and A. Pal, J. Phys.
Chem. B, 2008, 112, 4918; (e) J. Puigmarti-Luis, A. P. del Pino,
V. Laukhin, L. N. Feldborg, C. Rovira, E. Laukhina and
D. B. Amabilino, J. Mater. Chem., 2010, 20, 466; (f) J. Makarevic,
M. Jokic, M. Peric, V. Tomisic, B. Kojic-Prodic and M. Zinic,
Chem.–Eur. J., 2001, 7, 3328; (g) Y. Huang, F. Liao, W. Zheng,
Xi. Liu, X. Wu, X. Hong and S. C. Tsang, Langmuir, 2010, 26,
3106; (h) P. Zhu, X. Yan, Y. Su, Y. Yang and J. Li, Chem.–Eur. J.,
2010, 16, 3176.
ꢀ
tence of gel after the addition of 10 eq. H2PO4 and AcOꢀ.
According to the absorbance at 560 nm, the anion affinities
ꢀ
sequence of 1 is Fꢀ > H2PO4 > AcOꢀ [ Clꢀ, Brꢀ, Iꢀ and
HSO4 .
ꢀ 21 It is well known that the size of anions is an important
factor when organic ligands, which can form a cavity by the
receptor binding sites, are used to selectively recognize these
anions because of the complementary size of cavity and anion.22
However, in our case, due to the absence of such a cavity for the
gelator, basicity of the anion determines the association constant.
Therefore, fluoride anion has the largest binding ability.
In ODCB, only Fꢀ was able to prevent gel formation. Similarly
to those in DMSO, a small amount of TBAF could not prohibit
the formation of gel phase (Fig. S9†). 10 eq. TBAF led to
a brown solution. Moreover, the absorption spectrum was
different from that in DMSO. Two new peaks at 490 and 650 nm
in the visible region in the ODCB solution with Fꢀ were found
(Fig. 9b). In contrast, the UV-vis spectra showed negligible
changes after the addition of other anions (Fig. S12). This
spectral observation indicates that 1 has weaker affinity to anions
in ODCB than that in DMSO, but possesses excellent selectivity
to anions.23 It is well known that the anions generally have
stronger basicity in DMSO than that in ODCB.24 Therefore, 1 in
DMSO can respond to three anions (Fꢀ, H2PO4ꢀ, AcOꢀ), but in
ODCB it selectively responds to Fꢀ. Furthermore, the different
responses of gels to different anions in the same solvent originate
from the difference of inherent basicity of anions.25
4 (a) P. C. Xue, R. Lu, X. C. Yang, L. Zhao, D. F. Xu, Y. Liu, H,
Z. Zhang, H. Nomoto, M. Takafuji and H. Ihara, Chem.–Eur. J.,
2009, 15, 9824; (b) Y. Xu, P. C. Xue, D. F. Xu, X. F. Zhang,
X. L. Liu, H. P. Zhou, J. H. Jia, X. C. Yang, F. Y. Wang and
R. Lu, Org. Biomol. Chem., 2010, 8, 4289.
5 (a) H. Maeda, Chem.–Eur. J., 2008, 14, 11274; (b) D. F. Xu, X. L. Liu,
R. Lu, P. C. Xue, X. F. Zhang, H. P. Zhou and J. H. Jia, Org. Biomol.
Chem., 2011, 9, 1523; (c) T. H. Kim, N. Y. Kwon and T. S. Lee,
Tetrahedron Lett., 2010, 51, 5596; (d) M. Teng, G. Kuang, X. Jia,
M. Gao, Y. Li and Y. Wei, J. Mater. Chem., 2009, 19, 5648.
6 (a) H. Jintoku, T. Sagawa, M. Takafuji and H. Ihara, Org. Biomol.
Chem., 2009, 7, 2430; (b) P. C. Xue, R. Lu, L. Zhao, D. F. Xu,
X. F. Zhang, K. C. Li, Z. G. Song, X. C. Yang, M. Takafuji and
H. Ihara, Langmuir, 2010, 26, 6669.
7 X. Yu, Q. Liu, J. Wu, M. Zhang, X. Cao, S. Zhang, Q. Wang, L. Chen
and T. Yi, Chem.–Eur. J., 2010, 16, 9099.
8 M. Suzuki, T. Sato, A. Kurose, H. Shirai and K. Hanabusa,
Tetrahedron Lett., 2005, 46, 2741.
9 (a) J. van Esch, F. Schoonbeek, M. de Loos, H. Kooijman,
A. L. Spek, R. M. Kellogg and B. L. Feringa, Chem.–Eur. J., 1999,
5, 937; (b) G. Ghini, L. Lascialfari, C. Vinattieri, S. Cicchi,
A. Brandi, D. Berti, F. Betti, P. Baglioni and M. Mannini, Soft
Matter, 2009, 5, 1863.
10 (a) P. C. Xue, R. Lu, Y. Huang, M. Jin, C. H. Tan, C. H. Bao,
Z. M. Wang and Y. Y. Zhao, Langmuir, 2004, 20, 6470; (b)
X. Wang, L. Xing, W. Cao, X. Li, B. Chen, C. Tung and L. Wu,
Langmuir, 2010, 27, 774.
Conclusions
In conclusion, 1 was designed and synthesized. It was found that
ODCB gel possessed higher Tgel relative to DMSO gel at the
same concentration. Hot DMSO solution of 1 could transform
into an orange gel, but the color of the ODCB gel was yellow.
Moreover, tunable multicolor emission from green to red could
be easily achieved by adjusting concentration of gelator in
DMSO. In contrast, ODCB gels only emitted yellow fluores-
cence, being independent of concentration of the gelator. In
addition, the anion-responsive behaviors of gels were also rela-
tive to solvent. 1 could respond to three anions in DMSO, but
only Fꢀ could bind with 1 and transform the yellow gel into
a brown solution in ODCB. This result is fascinating because it
supports a strategy to design a novel gelator whose smart
properties can be adjusted by solvents.
11 (a) P. Babu, N. M. Sangeetha, P. Vijaykumar, U. Maitra, K. Rissanen
and A. R. Raju, Chem.–Eur. J., 2003, 9, 1922; (b) J. Cui, A. Liu,
Y. Guan, J. Zheng, Z. Shen and X. Wan, Langmuir, 2010, 26, 3615.
12 (a) X. Zhu, Y. Li, P. Duan and M. Liu, Chem.–Eur. J., 2010, 16, 8034;
(b) L. H. Su, C. Y. Bao, R. Lu, Y. L. Chen, T. H. Xu, D. P. Song,
C. H. Tan, T. S. Shi and Y. Y. Zhao, Org. Biomol. Chem., 2006, 4,
Acknowledgements
This work was supported by the Scientific Research Foundation
for the Returned Overseas Chinese Scholars, State Education
This journal is ª The Royal Society of Chemistry 2011
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View Article Online
2591; (c) M. Jacksona and H. H. Mantsch, Can. J. Chem., 1991, 69,
1639; (d) M. Suzuki, M. Yumoto, M. Kimura, H. Shirai and
K. Hanabusa, New J. Chem., 2002, 26, 817.
125, 15941; (d) K. Y. Peng, S. A. Chen and W. S. Fann, J. Am.
Chem. Soc., 2001, 123, 11388.
18 (a) S. J. George and A. Ajayaghosh, Chem.–Eur. J., 2005, 11, 3217; (b)
ꢂ
ꢁ
L. Frkanec and M. Zinic, Chem. Commun., 2010, 46, 522; (c)
ꢁ
13 (a) V. de Zea Bermudez, L. D. Carlos and L. Alcacer, Chem. Mater.,
€
F. Wurthner, C. Thalacker, S. Diele and C. Tschierske, Chem.–Eur.
€
1999, 11, 569; (b) M. Shirakawa, s. Kawano, N. Fujita, K. Sada and
S. Shinkai, J. Org. Chem., 2003, 68, 5037; (c) M. Suzuki, M. Yumoto,
M. Kimura, H. Shirai and K. Hanabusa, Chem.–Eur. J., 2003, 9, 348.
14 (a) X. Yang, G. Zhang, D. Zhang and D. Zhu, Langmuir, 2010, 26,
11720; (b) S. Samai, J. Dey and K. Biradha, Soft Matter, 2011, 7,
2121; (c) K. Yabuuchi, E. Marfo-Owusu and T. Kato, Org. Biomol.
Chem., 2003, 1, 3464; (d) M. Suzuki, T. Nigawara, M. Yumoto,
M. Kimura, H. Shirai and K. Hanabusa, Org. Biomol. Chem., 2003, 1,
4124; (e) M. M. Rahman, M. Czaun, M. Takafuji and H. Ihara,
Chem.–Eur. J., 2008, 14, 1312; (f) V. Berl, M. Schmutz, M. J. Krische,
R. G. Khoury and J. M. Lehn, Chem.–Eur. J., 2002, 8, 1227.
15 (a) Y. L. Chen, Y. X. Lv, Y. Han, B. Zhu, F. Zhang, Z. S. Bo and
C. Y. Liu, Langmuir, 2009, 25, 8548; (b) Y. Wu, S. Wu, X. Tian,
X. Wang, W. Wu, G. Zou and Q. Zhang, Soft Matter, 2011, 7, 716;
(c) J. Liu, J. Yan, X. Yuan, K. Liu, J. Peng and Y. Fang, J. Colloid
Interface Sci., 2008, 318, 397; (d) C. Y. Bao, R. Lu, M. Jin,
P. C. Xue, C. H. Tan, T. H. Xu, G. F. Liu and Y. Y. Zhao, Chem.–
Eur. J., 2006, 12, 3287.
J., 2001, 7, 2245; (d) F. Wurthner, Z. Chen, F. J. M. Hoeben,
P. Osswald, C. C. You, P. Jonkheijm, J. v. Herrikhuyzen,
A. P. H. J. Schenning, P. P. A. M. van der Schoot, E. W. Meijer,
E. H. A. Beckers, S. C. J. Meskers and R. A. J. Janssen, J. Am.
Chem. Soc., 2004, 126, 10611; (e) S. Kawano, N. Fujita and
S. Shinkai, Chem.–Eur. J., 2005, 11, 4735.
ꢂ
ꢂ
ꢁ
ꢁ
19 (a) Z. Dzolic, M. Cametti, A. D. Cort, L. Mandolini and M. Zinic,
Chem. Commun., 2007, 3535; (b) M. O. M. Piepenbrock,
G. O. Lloyd, N. Clarke and J. W. Steed, Chem. Commun., 2008,
2644; (c) G. O. Lloyd and J. W. Steed, Nat. Chem., 2009, 1, 437.
20 J. Lium, Y. Yang, C. Chen and J. Ma, Langmuir, 2010, 26, 9040.
21 Y. Zhang, Q. Lin, T. Wei, X. Qin and Y. Li, Chem. Commun., 2009,
6074.
22 (a) N. Singh and D. O. Jang, Org. Lett., 2007, 9, 1991; (b) J. Kang and
J. Kim, Tetrahedron Lett., 2005, 46, 1759; (c) E. B. Veale and
T. Gunnlaugsson, J. Org. Chem., 2008, 73, 8073; (d) P, A. Gale,
Chem. Commun., 2011, 47, 82; (e) P. A. Gale, Chem. Soc. Rev.,
2010, 39, 3746; (f) L. M. Hancock and P. D. Beer, Chem.–Eur. J.,
2009, 15, 42.
16 H. Jintoku, T. Sagawa, T. Sawada, M. Takafuji, H. Hachisako and
H. Ihara, Tetrahedron Lett., 2008, 49, 3987.
€
17 (a) H. Lin, R. Camacho, Y. Tian, T. E. Kaiser, F. Wurthner and
23 T. H. Kim, M. S. Choi, B. Sohn, S. Park, W. S. Lyoo and T. S. Lee,
Chem. Commun., 2008, 2364.
24 V. Amendola, D. Esteban-Gomez, L. Fabbrizzi and M. Licchelli, Acc.
Chem. Res., 2006, 39, 343.
25 M. Cametti and K. Rissanen, Chem. Commun., 2009, 2809.
I. G. Scheblykin, Nano Lett., 2010, 10, 620; (b) A. Ajayaghosh and
V. K. Praveen, Acc. Chem. Res., 2007, 40, 644; (c) P. Jonkheijm,
F. J. M. Hoeben, R. Kleppinger, J. van Herrikhuyzen,
A. P. H. J. Schenning and E. W. Meijer, J. Am. Chem. Soc., 2003,
8304 | Soft Matter, 2011, 7, 8296–8304
This journal is ª The Royal Society of Chemistry 2011