Self-assembly and anion-response of azobenzene-based L-valinamide derivative

Article abstract of DOI:10.1016/j.dyepig.2018.04.010

An azobenzene-based L-valinamide derivative was synthesized, and its gelation ability and self-assembly in organic solvents were investigated. Results suggested that it is an excellent gelator and formed organogels in many solvents, such as 3-pentanone, aniline, o-dichlorobenzene (ODCB), CH₂Cl₂, THF, ethanol, DMSO, and DMF. Its self-assembly in ODCB gel was studied. Transmission electron microscopic observation suggested that the gelator can self-assemble into one-dimensional nanofibers in the gel, and this phenomenon is driven by hydrogen bonding between amide units and π-π interaction between azobenzene moieties. With the increase in gelator concentration, the gel-to-sol phase transition temperature increased and the gelation time of the solvent shortened. Moreover, the gel exhibit anion response. A gel-to-sol phase transition was found after fluoride anion was added, exhibiting selective response to F^?.

Full text of DOI:10.1016/j.dyepig.2018.04.010

Accepted Manuscript
Self-assembly and anion-response of azobenzene-based L-valinamide derivative
Kechang Li, Pengchong Xue
PII:
S0143-7208(17)32459-2
10.1016/j.dyepig.2018.04.010
DYPI 6671
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 1 December 2017
Revised Date: 30 March 2018
Accepted Date: 8 April 2018
Please cite this article as: Li K, Xue P, Self-assembly and anion-response of azobenzene-based L-
valinamide derivative, Dyes and Pigments (2018), doi: 10.1016/j.dyepig.2018.04.010.
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ACCEPTED MANUSCRIPT

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Self-assembly and anion-response of azobenzene-based
L-valinamide derivative
Kechang Li,a, Pengchong Xueb
a College of Chemistry, Jilin University, No. 2699, Qianjin Street, Changchun, 130012, P. R. China
b Tianjin Key Laboratory of Structure and Performance for Functional Molecules, Key Laboratory
of Inorganic-Organic Hybrid Functional Material Chemistry, Ministry of Education, College of
Chemistry, Tianjin Normal University, No. 393, Bin Shui West Road, Tianjin, 300387, P.R. China.
Abstract: An azobenzene-based L-valinamide derivative was synthesized, and its
gelation ability and self-assembly in organic solvents were investigated. Results
suggested that it is an excellent gelator and formed organogels in many solvents, such
as 3-pentanone, aniline, o-dichlorobenzene (ODCB), CH2Cl2, THF, ethanol, DMSO,
and DMF. Its self-assembly in ODCB gel was studied. Transmission electron
microscopic observation suggested that the gelator can self-assemble into
one-dimensional nanofibers in the gel, and this phenomenon is driven by hydrogen
bonding between amide units and π-π interaction between azobenzene moieties. With
the increase in gelator concentration, the gel-to-sol phase transition temperature
increased and the gelation time of the solvent shortened. Moreover, the gel exhibit
anion response. A gel-to-sol phase transition was found after fluoride anion was added,
exhibiting selective response to F‒.
Keywords: gelator; self-assembly; anion; stimulus-response
1. Introduction
Supramolecular organogels formed by a low-molecular weight gelator recently
attracted a growing attention because of their unique self-assemblies and wide
applications. Different functional groups are appended to the molecular
structures of gelators to impart specific applications, such as sensors (including
gas,5-8 anion,9-10 cation11-12 and neutral molecules13), drug releases systems,14-15
medical treatments,16-17 stimuli-responsive soft materials,18-20 solar cell,21
molecular recognition,22 field effect transistors,23 enantioselective sensing24 and
Corresponding author: likc@jlu.edu.cn

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dyes absorbing.25-26 Gelators form various assemblies, such as nanofibers,
nanotubes, nanoribbons, nanorods, and plate-like structures, and the formation
of these assemblies is driven by weak intermolecular interactions (van der
Waals force, π-π interaction, hydrogen bonding, coordination, halogen bonding,
and charge transfer interactions). These assemblies form a three-dimensional
(3D) network to prevent solvent from flowing, resulting in gel formation.
Generally, chiral carbon, hydrogen bonding moiety, and long alkyl chain are
important factors that induce a gelator to self-assemble into a 3D network in
different solvents.27 A chiral center prevents molecules from orderly stacking
and then forming large aggregation or deposition.28 The hydrogen bonding
groups may help a gelator to self-assemble into a 1D superstructure.29-30 The
long alkyl chain not only can adjust the solubility but also promotes
crosslinking of 1D aggregate through nodes in coordination with the chiral
center.31
In this work, based on the above designed principle, an L-valinamide derivative
with dodecyl group as long alkyl chain, amide moieties as hydrogen bonding
units and chiral center (1) was designed and synthesized. Its gelation ability and
anion response were investigated. It was found to be an excellent gelator in
many kinds of organic solvents, including aromatic hydrocarbon, ketone,
aromatic amine, aromatic ketone, alcohol, chloroform, THF, and polar DMF,
and DMSO. Moreover, its self-assembly in gel was investigated too. The results
indicated that the gelator formed 1D nanofibers in gel. FT-IR and UV

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absorption spectra revealed that the main driving forces of gel formation were
hydrogen bonding and π-π interaction. Gelator concentration determined the
gel-to-sol phase transition temperature (Tgel) and the gelation speed. Moreover,
a gel-to-sol phase transition was observed upon the addition of fluoride anion.
2. Results and discussion
2.1 Synthesis of gelator 1
Compound
The reaction of
yielded compound
compound with a moderate yield (78%). Compound
obtained through an amidation reaction of with succinic anhydride at a yield
of 93%. Compound was prepared via a condensation reaction of and 6,
2
was synthesized in an aqueous solution as previously described.32
and laurylamine in the presence of EDC·HCl in dry CH2Cl2
. A deprotection process of by diethylamine provided
could be easily
2
3
3
4
6
5
1
4
producing a moderate yield (66%).
2.2 Gelation ability
Through a standard heating and cooling method,33 the gelation ability of
investigated, and the results are listed in Table 1. Compound is insoluble or
1 was
1
has a low solubility in measured solvents at room temperature and poorly
soluble in nonpolar alkane (such as hexane and cyclohexane), petroleum ether,
acetone, benzene, and toluene even upon heating. Although
1 dissolves in ethyl
acetate upon heating, only yellowish deposition was observed when the
solution was cooled to room temperature. Fortunately, the gel phases in other

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solvents were formed while the hot solution cools. The gelation solvents
include aromatic solvents, for instance mesitylene and
(ODCB), aromatic amines, and aromatic ketone. Stable gels formed in CH2Cl2,
CHCl3, THF, and ethanol with low boiling points. Moreover, can gelate
DMSO and DMF. The above results clearly indicated that the long alkyl chain
o-dichlorobenzene
1
and chiral amino acid indeed rendered
solvents.
1 an excellent gelator of many organic
Table 1. Gel properties of 1 in organic solvents.a
Solvent
Statusb
Solvent
Status
TG
Hexane
I
Aniline
Cyclohexane
Petroleum ether
Ethyl acetate
Acetone
I
N-Methylaniline
TG
I
N-Dimethylaniline TG
I
CH2Cl2
TG
TG
TG
OG
TG
OG
TG
TG
P
CHCl3
3-Pentanone
OG
THF
2,4-Dimethylpentanone OG
Ethanol
Acetophenone
DMF
Benzene
I
Toluene
I
Mesitylene
o-Dichlorobenzene
TG
TG
DMSO
Benzyl alcohol
a TG : transplant gel; OG: opaque gel; S: soluble; I: insoluble; P:
precipitate. b Gelator = 5.0 mg/mL.
2.3 Gel-to-sol phase transition temperature
When the formed gels were heated to a given temperature and transformed into
a sol, this temperature was defined as Tgel.34 While the hot sol was cooled to
room temperature, a gel phase reappeared, demonstrating thermal reversibility.
Here, the influence of gelator concentration on Tgel was examined. Fig. 1a
shows the relationship between Tgel and gelator concentration. Tgel values
increase nonlinearly with increasing gelator concentration. Tgel reaches 78 ºC

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when concentration is 1.0 mg/mL. The system with a concentration of 2.5
mg/mL did not maintain a gel phase when the temperature increased to 94 °C.
If the gel-to-sol transition is comparable to the melting of crystals, the phase
transition enthalpy can be estimated by the following equation:
ꢅꢆꢇ ∆ꢉ
1
= ꢄ
+
ꢃ
Tꢀꢁꢂ
∆ꢈ ∆ꢈ
Fig. 1. Plots of (a) Tgel versus concentration of 1 and (b) 1/Tgel versus the natural
logarithm of the concentration of 1 in ODCB.
where C is the molar gel concentration, ∆H and ∆S are the standard enthalpy
and entropy for the sol–gel transition, respectively, and R is the gas
constant.35-37 The reciprocal melting temperature was linearly dependent on the
natural logarithm of the concentration (Fig. 1b). Based on the fitted linear
equation, ∆H and ∆S were ‒6.31
×
104 J mol-1 (‒109.4 J g-1) and -126.2 J mol-1

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K-1. The Gibbs free-energy change ꢀG and the aggregation constant K at room
temperature were ‒25.5 kJ mol-1 and 2.89
×
104 M-1, respectively. Such a
moderate aggregation constant is possibly responsible for the good gelation
in many solvents.38
abilities of
1
2.4 Morphological observation
Fig. 2. TEM image of ODCB xerogel.
To observe the morphology of self-assemblies of the gel phases of 1, we
obtained a transmission electron microscopic (TEM) image of the xerogel film.
Fig. 2 shows the TEM image of ODCB xerogel, in which long nanofibers with
widths of 25-100 nm are observed. Moreover, wide fibers consist of thin fibers.
This result suggests that 1 tends to self-assemble to from 1D aggregate in the
gel phase.39
2.5 FT-IR and 1H NMR spectra
As discussed above, amide moieties may promote the formation of 1D aggregate
through intermolecular hydrogen bonding; we thus obtained the FT-IR spectrum of
xerogel film. As shown in Fig. 3, the stretching vibrational peak of N-H is located at
3288 cm-1. The vibration peaks at 1664 and 1635 cm-1 are assigned to the aromatic

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Fig. 3. (a) FT-IR spectrum of ODCB xerogel film, and (b) concentration-dependent
NMR spectra in CDCl3.
and aliphatic C=O vibrational peaks, respectively.40 The locations of these vibrational
peaks indicate that all of the amide groups are involved in hydrogen bonding.41 In
addition, the antisymmetric and symmetric stretching vibrational bands of CH2 in
xerogel film are located at 2921 and 2851 cm-1, respectively, implying that the alkyl
chains adopt an all-trans extended conformation,42 and the van der Waals interaction
among the alkyl chains plays an important role in the self-assembly of 1. IR spectral

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results also reveal that intermolecular hydrogen bonding is a major driving force in
gel formation. In addition, the IR spectrum of the as-synthesized solid was measured
and found to be the same as that of gel, indicating that there is intermolecular
hydrogen bonding in as-synthesized solid. Fig. 3b shows the concentration-dependent
NMR spectra in CDCl3. It is clear that the peaks ascribed to three N-H groups in
higher concentration shifted to low field. This result suggests that hydrogen bonding
in higher concentration became stronger relative to those in low concentration.
2.6 UV-Vis spectral change during gelation.
To monitor the interaction between azobenzene moieties during gelation, we
investigated the UV-Vis absorption spectral change of
gelation. As shown in Fig. 4a, the maximal absorption peak in the hot solution
is located at 355 nm and slightly increased and red-shifted to 357 nm after
1 in ODCB during
1
min; this phenomenon is ascribed to planarization of azobenzene moiety.43 This
peak rapidly decreased with time and remained nearly unchanged after 10 min.
Moreover, a new peak at 314 nm, which is ascribed to aggregates, appeared and
gradually increased during gelation. An isoabsorptive point at 326 nm was
observed, suggesting an equilibrium reaction of the two components. The
disappearance of the peak at 357 nm and the appearance of a new peak at 314
nm clearly revealed a large blueshift of 43 nm, suggesting that the gelators
stack together in a face-to-face H-aggregate.44 Moreover, absorption spectral
shift of 41 nm illustrates that strong π-π interactions exist between azobenzene
moieties, which is a main driving force for molecular aggregation in organic

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solvents. Based on these results, a suggested packing process for
1 during
gelation is shown in Fig. 5. Molecules as monomers exist in hot solution.
During gelation molecules stack together in a parallel fashion and all three
amide groups are involved in intermolecular hydrogen bonding. A gelator
forms six hydrogen bonds with two adjacent gelators. Moreover, azobenzene
moieties form H-aggregates through hydrogen bonding. In addition, XRD
spectrum was measured, but no obvious diffraction peaks were observed. So,
stacking model in the long-distance period is unknown.
Fig. 4. (a) UV/Vis absorption spectral changes during gelation in ODCB (1.5 mg/mL)
with an interval of 1 min as the hot solution was cooled from 100 °C to 20 °C. Inset is
the photo of ODCB gel. (b) Normalized absorption spectra of 1 in gels with different
concentrations.

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Fig. 5. Schematic of possible packing of 1 during gelation.
Fig. 6. Dynamic changes of absorbance of 1 at 360 nm during gelation at different
concentrations. The sample firstly was heated to 120 ºC and then cooled naturally to
room temperature.

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Fig. 4b shows the normalized absorption spectra of gels in different ODCB
concentrations. The absorbance at around 350 nm from monomeric molecules
gradually weakened with increasing concentration. This result suggests that the
proportion of monomeric molecules relative to that of aggregates decreases
under increased concentration. Furthermore, the gelation speed of
1 in ODCB
was dependent on the concentration of the compound. As shown in Fig. 6, the
absorbance at 360 nm at a concentration of 1.5 mg/mL increases first within 18
s and then rapidly decreases. When the concentration decreases to 1.2 mg/mL,
the arising time in absorbance extends to 20 s, and the descending rate of the
absorbance slows down, indicating a slow gelation process. If the concentration
further decreases to 1.0 mg/mL, the absorbance slowly starts down after 52 s.
These results imply that a low gelator concentration requires a low temperature
to form gel, resulting in a slow gelation speed.

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Fig. 7. Absorption spectra of 1 in ODCB (1.0 mg/mL) upon addition of (a) 1 equiv.
different anions and (b) in presence of fluoride anions with different concentrations.
2.7 Anion response of gel
The anion responsive properties of
1
gel toward different anions (F‒, Cl‒, Br‒,
‒
I‒, AcO‒ and H2PO4 as tetrabutylammonium (TBA) salts) were examined in
DMSO. It was found that the gel was destroyed and changed into a sol when
the TBAF solution was added on surface of gel and stayed for 1 h. However,
other TBA salts did not induce such phase transition. Moreover, fluoride anion

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can prevent the gel formation. The hot solution of
could not be changed into a gel upon the addition of 1 equiv. TBAF even when
the solution was left over one day. The gel state of in ODCB could be
1
(1.0 mg/mL) in ODCB
1
maintained after adding the same amount of other halide anions. It suggests that
the anions, not the TBA cation, should be responsible for the phase transition
from a gel to a sol.10b Absorption spectra were further used to investigate this
phenomenon. As shown in Fig. 7a, the absorption spectra in the presence of Cl‒,
Br‒, I‒, AcO‒ are similar to that of the neat gel. The absorption peak at 314 nm,
ascribing to H-aggregate, disappeared in the presence of 1.0 equiv F‒ , and a
new peak at 378 nm emerged. This result suggests that F‒ may destroy the
molecular aggregate.46 The absorption peak at 314 nm decreased and the
‒
absorption at 360 nm emerged while 1.0 equiv. H2PO4 existed, indicating that
‒
H2PO4 could partly destroy aggregate. In addition, it was found that a sol
‒
‒
phase was observed when 5.0 equiv. H2PO4 existed, suggesting that H2PO4
may destroy a gel phase in higher concentration. Because the absorption peak
located at 357 nm in hot solution, meaning a small red-shift of 21 nm, the
hydrogen bonding between amide and F‒ should be responsible for the
dissociation of gelator’s aggregate. The reason of selective response to fluoride
anion is due to weaker ability of other anions to form hydrogen bond with
amide group. Furthermore, the F‒ concentration strongly impacted the phase
transition. When the ratio of F‒ to
1 was lower than 0.5, a gel phase could be
still kept. Fig. 7b shows the concentration-dependent absorption spectra. The

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absorption band at 314 nm gradually decreased and the absorbance at more than
365 nm gradually increased upon the addition of TBAF. While the ratio of F‒ to
1
reached 0.5, the peak intensity at 366 nm was stronger than that at 314 nm. At
the moment, there were enough fibrous aggregates to hold a gel phase. As a
result, only a sol was observed.
Conclusions
An azobenzene-based L-valinamide derivative bearing a long alkyl chain, chiral
carbon, and amide moieties was synthesized, and the self-assembly of this
compound into a gel was investigated. The results indicate that the synthesized
compound is an excellent gelator and can form gels in various organic solvents,
such as, aromatic solvents, ketone, alcohols, halohydrocarbons, THF, DMF,
and DMSO. The gelator may self-assemble into nanofiber in gels, and this
process is driven by multiple hydrogen bonds and π-π interactions. Tgel and gel
speed are strongly dependent on gelator concentration. Moreover, the gel
exhibited selective response to fluoride anion. The addition of fluoride anion
could result in a gel-to-sol phase transition. Our findings suggest that
introduction of long alkyl chain, chiral carbon, and amide groups render a
compound to be an excellent gelator and the existence of amide group may
promote a gel to response anions.
Experiment section

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Instruments and experimental methods: All the raw materials were used
without further purification. All the solvents as analytical reagents were
purchased from Beijing Chemical Works (Beijing, China), and were used
without further purification. Infrared spectra were measured with a Nicolet- 360
Fourier transform infrared (FT-IR) spectrometer by incorporating the samples
in KBr disks. The UV-vis spectra were determined on a Mapada UV-1800pc
spectrophotometer. C, H, and N elemental analyses were performed on a
1
Perkin-Elmer 240C elemental analyzer. H NMR spectra were recorded on
Mercury plus 500 MHz.
TEM investigation: A piece of the gels was placed on a carbon-coated copper
grid (400 mesh) followed by naturally evaporating the solvent. The TEM
specimens were examined with a Hitachi mode H600 A-2 apparatus with an
accelerating voltage of 100 kV.
Gelation test of organic fluids: The solution of gelator (pre-weighed) in
organic solvent was heated in a sealed test tube (diameter = 1 cm) in an oil bath
until the solid was dissolved. The solution was allowed to stand at room
temperature for 6 h, and the state of the mixture was evaluated by the “stable to
inversion of a test tube” method.
Light response of gel: The gel at a concentration of 1.0 mg/mL was prepared
in a cell with 1 mm optical path, aged for 6 h and inverted, and then exposed to
a high pressure mercury lamp (500 W) with a 365 nm filter. UV-vis absorption
spectra of the gels before and after illumination were measured.

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Gelator synthesis:
Scheme 1. Synthesis route of 1.
were synthesized by the reported procedure.45,46
(S)-(9H-fluoren-9-yl)methyl-1-(dodecylamino)-3-methyl-1-oxobutan-2-ylca
rbamate
N-Fmoc-L-valine (2) and 5
(3)
N-Fmoc-L-valine (2.79 g), DMAP (0.2 g) and laurylamine (1.58 g) were
dissolved in 100 mL dried CH2Cl2. EDC·HCl (2.0 g) was added into the above
solution at 0 ºC. The mixture was stirred at room temperature for 24 h. After the
solvent was removed, ethanol (100 mL) was added and the mixture was treated
for 5 min in an ultrasonic cleaner. The compound was obtained by filtration and
washing with ethanol. Yield = 71%. mp = 146-148 ºC. FT-IR (cm-1): 3219,
1
3065, 2956, 2920, 2851, 1692, 1648, 1549, 1250, 740. H NMR (500 MHz,
CDCl3): δ 7.76 (d, J = 7.5 Hz, 2H), 7.59 (d, J = 7.5 Hz, 2H), 7.40 (t, J = 7.5 Hz,
2H), 7.31 (td, J = 7.5, 1.1 Hz, 2H), 5.77 (s, 1H), 5.37 (d, J = 8.9 Hz, 1H), 4.40
(dt, J = 17.6, 10.4 Hz, 2H), 4.22 (t, J = 6.9 Hz, 1H), 3.88 (t, J = 7.8 Hz, 1H),
3.45 – 3.04 (m, 2H), 2.21 – 2.01 (m, 1H), 1.49 (t, J = 7.3 Hz, 2H), 1.34–1.18

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(m, 18H), 0.95 (t, J = 7.2 Hz, 6H), 0.88 (t, J = 7.0 Hz, 3H). Element analysis for
C32H46N2O3: C, 75.85; H, 9.15; N, 5.53; found: C, 75.82; H, 9.19; N, 5.51.
(S)-2-amino-N-dodecyl-3-methylbutanamide
(4)
3
(2.0 g) and diethylamine (5 mL) was dissolved in CH2Cl2 (20 mL) and the
mixture was heated for 1 h at 50 ºC. After the solvent was removed and the
residues was treated by column chromatography (silica, CH2Cl2/methanol =
10/1 (V/V)). Yield = 78 %. mp = 44-46 ºC. FT-IR (cm-1): 3372, 3297, 3087,
1
2954, 2850, 1642, 1556, 1463. H NMR (500 MHz, CDCl3): δ 3.24 (m, 3H),
2.31 (m, 1H), 1.50 (p, J = 7.2 Hz), 1.35-1.21 (m, 18H), 0.99 (d, J = 7.0 Hz, 3H),
0.88 (t, J = 6.8 Hz, 3H), 0.83 (d, J = 6.9 Hz, 3H). Element analysis for
C17H35N2O: C, 72.03; H, 12.44; N, 9.88; found: C, 72.07; H, 12.41; N, 9.89.
(E)-4-oxo-4-(4-(p-tolyldiazenyl)phenylamino)butanoic acid
(6)
5
(0.5 g, 2.4 mmol) and succinic anhydride (0.5 g, 5.0 mmol) were dissolved
in dried THF (10 mL) and the mixture were refluxed 24 h. The solvent was
removed, 10 ml CHCl3 was added and the mixture was treated for 5 min in an
ultrasonic cleaner. The solid was obtained by filtration. Yield: 93%. mp =
205-206 ºC. FT-IR (cm-1): 3430, 3309, 3042, 2925, 1718, 1668, 1594, 1533,
1
844. H NMR (500 MHz, DMSO-d6): δ 12.15 (s, 1H), 10.31 (s, 1H), 7.85 (d, J
= 7.6 Hz, 2H), 7.80 (d, J = 8.6 Hz, 2H), 7.76 (d, J = 6.8 Hz, 2H), 7.38 (d, J =
7.7 Hz, 2H), 3.32 (s, 2H), 2.62 (t, J = 6.5 Hz, 3H), 2.55 (t, J = 6.5 Hz, 3H), 2.40
(s, 4H). Element analysis for C17H17N3O3：C, 65.58; H, 5.50; N, 13.50; found:
C, 65.50; H, 5.55; N, 13.49.

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(E)-N1-(1-(dodecylamino)-3-methyl-1-oxobutan-2-yl)-N4-(4-(p-tolyldiazeny
l)phenyl)succinamide
(1)
4
(0.23 g，0.8 mmol), 6 (0.26 g，0.84 mmol) and a small amount of DMAP
were dissolved in 20 mL dried THF. EDC·HCl (0.2 g) was added into the
above solution at 0 ºC. The mixture was stirred at room temperature for 48 h.
The compound was obtained by filtration and then washing with ethanol. Yield
= 66%. mp = 237-239 ºC. FT-IR (cm-1): 3288, 3098, 2958, 2921, 2851, 1664,
1635, 1528, 845. 1H NMR (500 MHz, DMSO-d6): δ 10.31 (d, 1H) , 7.91 (d, J =
8.9 Hz, 1H), 7.85 (d, J = 8.6 Hz, 2H), 7.80 (d, J = 8.7 Hz, 3H), 7.77 (d, J = 8.1
Hz, 2H), 7.39 (d, J = 8.0 Hz, 2H), 4.09 (t, J = 7.9 Hz, 1H), 3.17 – 2.91 (m, 2H),
2.63 (m, 2H), 2.54 (m, 2H), 2.41 (s, 3H), 1.99 (m, 1H), 1.37 (m, 2H), 1.32-1.14
(m, 18H), 0.84 (m, 9H). Element analysis for C34H51N5O3：C, 70.68; H, 8.90;
N, 12.12; found: C, 70.73; H, 8.78; N, 12.17.
Acknowledgements
This work was financially supported by the National Natural Science
Foundation of China (21103067), the Youth Science Foundation of Jilin
Province (20130522134JH).
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1. An azobenzene-based L-valinamide derivative was found to form gels in various
solvents.
2. The hydrogen bonding between amide units and π-π interaction between
azobenzene moieties were confirmed to be driving force of gel formation.
3. The gel exhibits selective response to fluoride anion.