Rationally designed anion-responsive-organogels: Sensing F-via reversible color changes in gel-gel states with specific selectivity

Article abstract of DOI:10.1039/c4sm00841c

Through the rational introduction of the multi self-assembly driving forces and F^- sensing sites into a gelator molecule, low-molecular-weight organogelators L1 and L2 were designed and synthesized. L1 and L2 showed excellent gelation ability i

Full text of DOI:10.1039/c4sm00841c

Soft Matter
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Rationally designed anion-responsive-organogels:
sensing Fꢀ via reversible color changes in gel–gel
states with specific selectivity†
a
Cite this: Soft Matter, 2014, 10, 5715
Qi Lin, Xin Zhu,a Yong-Peng Fu,a You-Ming Zhang,a Ran Fang,b Li-Zi Yangb
*
a
*
and Tai-Bao Wei
Through the rational introduction of the multi self-assembly driving forces and Fꢀ sensing sites into a
gelator molecule, low-molecular-weight organogelators L1 and L2 were designed and synthesized. L1
and L2 showed excellent gelation ability in DMF and DMSO. They could form stable organogels (OGL1
and OGL2) in DMF and DMSO with very low critical gelation concentrations. OGL1 and OGL2 could act
as anion-responsive organogels (AROGs). Unlike most of the reported AROGs showing gel–sol phase
transition according to the anions' stimulation, OGL1 could colorimetrically sense Fꢀ under gel–gel
states. Upon addition of Fꢀ, OGL1 showed dramatic color changes, while the color could be recovered
by adding H+. Moreover, OGL1 showed specific selectivity for Fꢀ, other common anions and cations
could not lead to any similar response. What deserves to be mentioned is that the report on specific
sensing of anions under gel–gel states is very scarce. The gel–gel state recognition can endow the
organogel OGL1 with the merits of facile and efficient properties for rapid detection of Fꢀ. Therefore,
OGL1 could act as a Fꢀ responsive smart material.
Received 17th April 2014
Accepted 28th May 2014
DOI: 10.1039/c4sm00841c
www.rsc.org/softmatter
catalysis, and biomimetic systems.20–24 Despite the great prog-
ress made recently in this eld, there are two big challenges in
1. Introduction
the eld of designing novel organogels. One is how to improve
the gelator's gelation ability and make sure that the designed
gelator can self-assemble into the desired organogel in certain
solutions and the other is how to selectively detect a given
chemical stimulus.
Smart materials which can sense, process, and actuate a
response to an external change without assistance have been
attracting considerable attention due to their wide range of
potential applications in sensors, biomaterials, surface science,
displays, etc.1–3 Among them, stimuli-responsive supramolec-
ular organogels are one kind of the most attractive examples.4–10
Supramolecular organogels are formed by assembling low-
molecular-weight gelators (LMWGs) into physically crosslinked
three-dimensional networks with solvent molecules entrapped
inside through noncovalent intermolecular interactions, such
as hydrogen bonding, p–p stacking, van der Waals (vdW),
electrostatic interactions, and so on.11–19 Due to the weak nature
of these forces, the gel–sol transition of organogels is thermally
reversible and can be further tuned by other physical and
chemical stimuli. These intriguing properties of organogels
endow them with promising applications in optoelectronic
devices, template syntheses, drug delivery, tissue engineering,
In addition, anions play a fundamental role in chemical,
biological and environmental processes. More and more
interest has been attracted in selective recognition and sensing
of anions via articial receptors.25–35 Among the anions, uoride
anions are one of the most attractive targets because of their
considerable signicance for health and environmental
issues.36 Therefore, there is a great demand for the development
of methods that can rapidly, sensitively, and selectively detect
the uoride anion.37,38 As we all know, an important feature of
the chemosensor is its specic selectivity toward the analyte.
Although considerable efforts have been made for the devel-
opment of anion-responsive organogel (AROG),4–10,12,15,39–44 few
AROGs could detect uoride anions with specic selectivity.
Moreover, common AROGs realize the anion recognition via
a reversible or irreversible gel–sol phase transition because the
anion could be competitively bound to the gelator's self-
assembly sites (e.g., hydrogen bonds or electrostatic interac-
tions sites) and induce the dis-assembly of the organogel. To
date, the reports of supramolecular organogel which could
colorimetrically or uorescently detect anions under gel–gel
states without sol–gel phase change are very scarce. What is
aKey Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education
of China, Key Laboratory of Polymer Materials of Gansu Province, College of Chemistry
and Chemical Engineering, Northwest Normal University, Lanzhou, Gansu, 730070,
P. R. China. E-mail: linqi2004@126.com; weitaibao@126.com
bCollege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou,
Gansu, 730070, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4sm00841c
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more the gel–gel state recognition can endow the AROGs with
the merits of facile and efficient properties for rapid detection
of the target anion.
2. Experimental section
Synthesis of L1
In view of this, as a part of our research interest in molec-
ular recognition,43,45–49 herein, we report an AROG which could
selectively recognize Fꢀ via a dramatic color change under gel–
gel states, the color could be recovered by adding H+. It is
interesting to note that in the Fꢀ recognition process, the
organogel did not carry out gel–sol phase change. In addition,
other anions (such as AcOꢀ, H2PO4ꢀ, Clꢀ, Brꢀ, Iꢀ, HSO4ꢀ and
3,4-Bis(hexadecyloxy)benzaldehyde was synthesized according
to literature methods.50,51 3,4-Bis(hexadecyloxy)benzaldehyde
(5.0 mmol), 4-nitrobenzenehydrazine (5.0 mmol) and a catalytic
amount of p-toluenesulfonic acid (p-Ts) were mixed in hot
absolute ethanol (30 mL). The solution was stirred under reux
conditions for 8 hours. Aer being cooled to room temperature,
the yellow precipitate was ltered and then recrystallized with
CHCl3–EtOH to get compound L1 (yield, 72%) as an orange
ClO4ꢀ) and common cations (such as Mg2+, Ca2+, Cr3+, Fe3+
,
Co2+, Ni2+, Cu2+, Zn2+, Ag+, Cd2+, Hg2+ and Pb2+) could not lead
to any similar response. Our strategy for the design of gelator
L1 (Scheme 1) is as follows. Firstly, in order to achieve specic
selectivity for Fꢀ, we rationally selected and introduced an Fꢀ
sensing group into the gelator molecule. There are lots of
functional groups have been reported as Fꢀ sensing groups,
like urea, thiourea, amide, phenol, pyrrole, acylhydrazone and
arylhydrazone and so on.37,38 Among them, the hydrazone
group (–CH]N–NH–), as a sense site, possesses several
merits. For example, there are two different hydrogen bond
donor sites (H–C]N– and –N–H) in the same group, both of
which can act as anion binding sites as well as self-assembly
sites. Furthermore, owing to the fact that the acidity of “–N–H”
is stronger than that of “H–C]N–”, the hydrazone group can
sense Fꢀ not only by forming hydrogen bonds (Fꢀ/H–C]N–)
but also by a deprotonation process (deprotonation of –N–H).
As a result, it can afford two kinds of sense approaches and
enhance the sense selectivity for Fꢀ. In the light of these
considerations, we introduced hydrazone groups into the
gelator as Fꢀ sensing sites and hydrogen bond donor groups.
Secondly, in order to obtain easy gelation and a relatively
stable gelator, we introduced a 3,4-bis(hexadecyloxy)phenyl
group as a self-assembly group to provide strong vdW forces.
In addition, since the arylhydrazone group and the 3,4-bis(h-
exadecyloxy)phenyl group were introduced into the same
gelator molecule, the gelator possesses three kinds of self-
assembly driving forces, including vdW forces, hydrogen
bonding and p–p stacking interactions. These designs
provided the gelator L1 with stronger gelation ability. Finally,
with the aim to achieve colorimetric sense for Fꢀ, a nitro-
1
powdery product: H NMR (CDCl3, 400 MHz) d 8.18 (d, J ¼ 6.9
Hz, 2H, ArH), 7.91 (s, 1H, –NH), 7.70 (s, 1H, –N]CH–), 7.33 (s,
1H, ArH), 7.10–7.07 (m, 3H, ArH), 6.87 (d, J ¼ 6.3 Hz, 1H, ArH),
4.06 (m, 4H, –OCH2), 1.86 (m, 4H, –OCH2CH2), 1.51 (m, 4H,
–CH2CH3),1.35 (m, 48H, –C12H24), 0.87 (t, J ¼ 5.4 Hz, 6H,
–CH2CH3); 13C NMR (CDCl3, 150 MHz) d 150.85, 149.56, 149.37,
141.75, 139.93, 126.86, 126.25, 121.29, 112.75, 111.43, 110.26,
69.23, 31.88, 29.36, 25.99, 22.65, 14.10; IR (KBr, cmꢀ1) v: 3505
(N–H), 2919, 2850 (C–H), 1596 (C]N), 1516, 1468 (C]C), 1111
(Ar–O–C). Anal. calcd for C45H75N3O4: C 74.85, H 10.47, N 5.82;
found C, 74.88, H, 10.41, N, 5.85. MS: m/z: 722.9 [L1 + H]+; calcd
for C45H76N3O4: 722.6.
Synthesis of L2
The compound L2 (yield, 85%) was prepared by a similar
procedure: 1H NMR (CDCl3, 400 MHz) d 11.30 (s, 1H, –NH), 9.16
(s, 1H, –N]CH–), 8.35 (d, J ¼ 7.2 Hz, 1H, ArH), 8.06 (d, J ¼ 11.6
Hz, 2H, ArH), 7.38 (s, 1H, ArH), 7.19 (d, J ¼ 8.4 Hz, 1H, ArH),
6.91 (d, J ¼ 8.4 Hz, 1H, ArH), 4.10–4.04 (m, 4H, –OCH2), 1.89–
1.84 (m, 4H, –OCH2CH2), 1.53–1.49 (m, 4H, –CH2CH3), 1.37–
1.25 (m, 48H, –C12H24), 0.89 (t, J ¼ 6.4 Hz, 6H, –CH2CH3). 13C
NMR (CDCl3, 150 MHz) d 152.09, 149.46, 148.35, 144.72, 137.79,
137.23, 129.93, 128.97, 125.67, 122.76, 116.56, 112.60, 110.82,
69.32, 31.91, 29.66, 29.36, 25.99, 22.68, 14.11; IR (KBr, cmꢀ1) v:
3443 (N–H), 2920, 2849 (C–H), 1616 (C]N), 1508, 1466 (C]C),
1133 (Ar–O–C). Anal. calcd for C45H74N4O6: C 70.46, H 9.72, N
7.30; found C, 70.48, H, 9.70, N, 7.25. MS: m/z: 767.9 [L2 + H]+;
calcd for C45H75N4O6: 767.6.
phenyl group was introduced into the gelator as a colorimetric 3. Results and discussion
signal group.
The gelation abilities of gelators L1 and L2 were examined in
various solvents by means of the “stable to inversion of a test
Scheme 1 Structure and synthesis of the gelators L1 and L2.
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Table 1 Gelation properties of L1 and L2
sulfoxide (DMSO) and N,N-dimethylformamide (DMF). More
interestingly, L1 and L2 exhibited excellent gelation abilities
and very low CGCs in DMSO, and the CGCs of L1 and L2 were
0.6% and 0.2% respectively. Moreover, in DMSO, the sol–gel
transition was very rapid. The whole transition process only
took less than 3 min aer heating of the DMSO solution was
stopped. The gel–sol transition temperature (Tgel) was measured
by using the ‘tilted tube’ method. Regularly increasing the
concentration of L1 and L2 led the Tgel to reach a limit of 79 ꢁC
and 111 ꢁC respectively (Fig. 1). These gels were found stable in
closed tubes for at least 3 months at 25 ꢁC. According to the
CGCs and Tgel of L1 and L2, we could nd that L2 was more
stable than L1, because L2 contained more nitro groups than L1
and the nitro groups could act as the hydrogen bond acceptor
L1
L2
Solvent
Statea (CGCb, CGTc)
State (CGC, CGT)
n-Amyl alcohol
Ethanol
I
P
I
I
I
I
Acetone
S
Acetonitrile
DMSO
P
OG (0.6%, 58 ꢁC)
OG (0.2%, 75 ꢁC)
Toluene
S
S
P
S
P
P
I
Petroleum ether
THF
P
S
Chloroform
Dichloromethane
Cyclohexanol
DMF
S
WG
I
OG (3.1%, 38 ꢁC)
OG (1.7%, 65 ꢁC) for these gelators.
Since gelators L1 and L2 could form stable organogel in
DMSO, a series of experiments were carried out to investigate
the anion response capability of the organogels formed by
gelators L1 and L2 in DMSO (named OGL1 and OGL2 respec-
tively). As we expected, OGL1 and OGL2 exhibited excellent
color changes according to the stimulus of the uoride ion. As
shown in Fig. 2, upon addition of Fꢀ (5 equiv., solid Bu4NF) to
OGL1 at 25 ꢁC, a dramatic color change from yellow to dark
brown instantly took place on the surface of OGL1. The color of
whole gels changed in ca. 4 hours with Fꢀ diffusion into the
gels. Additionally, the color of OGL1 could be recovered by
adding proton solutions such as HClO4 solution, MeOH or
water. It is very interesting that unlike most of the reported
AROGs which showed gel to solution phase transition according
to the uoride anion's stimulation, the gel state of OGL1 did not
show any changes in the whole Fꢀ response process. The Tgel of
Fꢀ-contained gel is 52 ꢁC and the hot solution of Fꢀ-contained
gel could recover to the gel states aer cooling the sol to room
temperature. This special stability could be attributed to the
cooperation of the above-mentioned multi self-assembly forces
(vdW forces, hydrogen bonding and p–p stacking interactions)
we rationally introduced into the gelators. Simply stated,
because there are three kinds of self-assembly driving forces in
the same gelator, even if the hydrogen bonds were destroyed by
Fꢀ, the remaining two forces could maintain the organogels' gel
states. Meanwhile, owing to the breaking of hydrogen bonds,
the Tgel of OGL1 changed from 79 ꢁC to 52 ꢁC. Under the same
conditions, OGL2 showed similar response for Fꢀ.
a
OG, opaque gel; S, solution; I, insoluble; WG, weak gel; P, precipitate.
CGC is the critical gelation concentration (w/v%, 10 mg mLꢀ1 ¼ 1%).
CGT is the gelation temperature (ꢁC) at critical gelation concentration.
b
c
tube” method. The corresponding critical gelation concentra-
tions (CGCs) at room temperature were also measured (Table 1).
Gelators L1 and L2 could gel polar aprotic solvents dimethyl
Fig. 1 Plots of Tgel versus the concentration of OGL1 and OGL2
obtained from DMSO.
Fig. 2 (a) Organogel OGL1 (0.8%, in DMSO); (b) immediately after addition of solid TBAF (5 equiv.); (c) after 30 min; (d) after 90 min; (e) after 150
min; (f and g) after 220 min; (h) addition of 0.05 mL HClO4 (0.01 M) after 10 min; (i) after 20 min.
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The selectivity of organogels for anions was also investi-
Paper
gated. As shown in Fig. 3, when DMSO solutions of various
anions (Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, HSO4ꢀ and ClO4ꢀ) were
added to the small amounts of organogel OGL1 on a spot plate,
only Fꢀ could induce an instant color change of OGL1. Other
anions could not induce any color or organogels' state change,
which indicated that OGL1 could instantly sense Fꢀ with
specic selectivity. The selectivity tests were also carried out in
the DMSO solution of gelator L1. As shown in Fig. 4, in DMSO
solution, the gelator L1 could selectively respond to Fꢀ by color
change. In the corresponding UV-vis spectra, upon addition of
Fꢀ, the absorption of L1 at 430 nm shied to 570 nm. Similarly,
the color and the UV-vis absorption could be restored by adding
Fig. 5 UV-vis spectra of L1 (2.0 ꢂ 10ꢀ55 M) in DMSO upon addition of
10 equiv. of various cations (Mg2+, Ca2+, Cr3+, Fe3+, Co2+, Ni2+, Cu2+
Zn2+, Ag+, Cd2+, Hg2+ and Pb2+).
,
proton solutions (Fig. S1a in the ESI†). As shown in Fig. 4 andꢀ5,
other anions (Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, HSO4 and ClO4
)
ꢀ
and common cations (such as Mg2+, Ca2+, Cr3+, Fe3+, Co2+, Ni2+,
Cu2+, Zn2+, Ag+, Cd2+, Hg2+ and Pb2+) could not cause any
signicant color or spectral response. Therefore, the gelator L1
could respond to Fꢀ with specic selectivity. However, under
similar conditions, the addition of AcOꢀ or H2PO4ꢀ also led to
the color of OGL2 changing from yellow to red (Fig. 4) and the
color and UV-vis absorption could be restored by adding proton
solutions too (Fig. S1b iꢀn the ESI†). Thus, OGL2 could respond
to Fꢀ, AcOꢀ and H2PO4 without specic selectivity.
According to these results we can nd that L1 and L2 showed
different anion response selectivities. Because in the molecule
of L2, there is an intramolecular hydrogen bond between
hydrazone –N–H and ortho nitro groups (Fig. 6). This hydrogen
bond enhanced the acidity of the –N–H group. Therefore, the
–N–H group in L2 could carry out a deprotonation process not
only with Fꢀ but also with AcOꢀ and H2PO4ꢀ. This assumption
was conrmed by 1H NMR spectra. The –N–H proton chemical
shi of L1 appeared at 7.91 ppm, while in L2 the –N–H signal
appeared at 11.30 ppm, which indicated that the –N–H group of
L2 more easily deprotonates than that of L1.
The anion response mechanisms of L1 and L2 were carefully
investigated by various methods. According to the absorption
titration experiments (Fig. 7a), upon gradual addition of Fꢀ to
the solution of L1, an isosbestic point at 475 nm was observed.
Moreover, in Job's plots (Fig. 7b), the absorbance value
approached the maximum when the molar fraction of L1 was
0.5, which demonstrated the formation of a 1 : 1 complex
between L1 and Fꢀ. By the Benesi–Hildebrand equation tting52
Fig. 3 Color changes immediately after the addition of various anions
(one drop, ca. 0.02 mL) to organogel OGL1 (0.8%, in DMSO) on a spot
plate.
Fig. 4 UV-vis spectral response of (a) L1; (b) L2 (2.0 ꢂ 10ꢀ5 M) in DMSO upon addition of 50 equiv. of various anions. Inset: photograph of (ꢀa)
L1; (b) L2 (2.0 ꢂ 10ꢀ5 M) upon adding 50 equiv. of various anions in DMSO, from left to right: none, Fꢀ, Clꢀ, Brꢀ, Iꢀ, AcOꢀ, H2PO4ꢀ, HSO4
,
ꢀ
and ClO4
.
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Fig. 6 A possible mechanism of L1 and L2 response to Fꢀ or anions.
Fig. 7 (a) UV-vis spectral titration of L1 (2.0 ꢂ 10ꢀ5 M) with Fꢀ in DMSO solution. (b) Job's plot for the complex of L1 and Fꢀ (in DMSO solution),
which indicated that the stoichiometry of L1–Fꢀ is 1 : 1.
at lmax ¼ 571 nm for L1, the association constant Ka of L1 with shown in Fig. 8, in the Fꢀ-1H NMR titration spectra, before the
Fꢀ was obtained as 1.67 ꢂ 103 Mꢀ1
.
addition of Fꢀ, chemical shis of the N–H and –HC]N– protons
The further L1–Fꢀ reaction mechanism was investigated by on L1 appeared at d 7.91 and 7.70 ppm. Aer the addition of 0.2
Fꢀ-1H NMR titration, 19F NMR and ESI-MS experiments. As equiv. of Fꢀ, the signal of these two protons shied downeld.
Fig. 8 Partial 1H NMR spectra of L1 (0.01 M) in CDCl3 upon addition of Fꢀ. (a) Free; (b) 0.2 equiv. of Fꢀ; (c) 0.5 equiv. of Fꢀ; (d) 1.0 equiv. of Fꢀ.
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With the continuous addition of Fꢀ, the N–H signal completely
disappeared; meanwhile, the –HC]N– signal shied downeld
at d 8.30 ppm. These phenomena indicated that the N–H group
might undergo a deprotonation process and the –HC]N– group
formed a Fꢀ/H–C]N– hydrogen bond (Fig. 6). The deproto-
nation process induced the color changes of the L1 solution and
OGL1. This process could be conrmed by adding proton solu-
tions to OGL1 containing Fꢀ. Aer adding proton solutions, the
color of the solution or gel turned yellow again.
The presumed Fꢀ response mechanism was also supported
by 19F NMR and EI-Mass. As shown in Fig. 9, in the 19F NMR
spectra of TBAF, the signal of Fꢀ appeared at ꢀ129.36 ppm.
While aer the addition of 50 equiv. TBAF into the CDCl3
solution of L1, two 19F signals appeared in the corresponding
19F NMR spectra at ꢀ129.03 and ꢀ81.56 ppm respectively. The
signal at ꢀ126.03 was attributed to the excess TBAF and the
signal at ꢀ81.56 was attributed to the L1–Fꢀ complex. Mean-
while, in the ESI-MS spectra of sensor L1, the [L1 + H]+ peak
appeared at 722.9 (m/z calcd ¼ 722.6). However, when 50 equiv.
of Fꢀ was added to the solution of L1, a new peak appeared at
764.7, coinciding well with that for the species [L1 + Fꢀ + Na+ +
H]+ (m/z calcd ¼ 764.6) and indicating the formation of the
stabilized anionic species L1–Fꢀ.
Fig. 11 FT-IR spectra of powdered L1 and xerogel of OGL1 (0.8% in
DMSO).
experiments of L1 and L2 with AcOꢀ were carried out. As shown
in Fig. S2,† chemical shis of the N–H and –HC]N– protons on
L1 appeared at d 7.91 and 7.70 ppm. Aer the addition of
0.5 equiv. of AcOꢀ, the signal of these two protons shied
downeld. Aer the addition of 2 equiv. of AcOꢀ, the signal of
these two protons shied to 10.66 and 8.03 ppm, respectively.
Moreover, in order to provide more adequate evidence for
the possible anion response mechanism, the 1H NMR titration
Fig. 9 Partial 19F NMR spectra of (a) TBAF and (b) L1 + TBAF (50 equiv.).
Fig. 10 Partial 1H NMR spectra of L1 in CDCl3 with different concentrations (a) 0.6 mM; (b) 2.8 mM; (c) 13.9 mM; (d) 27.7 mM.
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–N–H group. Usually, the deprotonation of the N–H group
on the hydrazone moiety would induce the proton signal of
–HC]N– shied upeld while the formation of hydrogen
bonds would induce the proton signal of –HC]N– shied
downeld. When the two events took place simultaneously, the
signal of the –HC]N– group would not show an obvious shi.
Upon addition of AcOꢀ, the –HC]N– proton signal of L2 did
not show obvious upeld-shi, which indicated that AcOꢀ
formed hydrogen bonds with the –HC]N– group (Fig. 6).
1
The self-assembly mechanism was investigated by H NMR,
FT-IR, DFT calculations, X-ray diffraction and SEM. According
to the results of concentration dependent 1H NMR experiments
(Fig. 10), the NH resonance signals gradually shied downeld
as the concentration of L1 rose. Moreover, in the FT-IR spectra
(Fig. 11) the N–H vibration absorption of powdery L1 appeared
at 3505 cmꢀ1 while it shied to 3470 cmꢀ1 in xerogel. These
results revealed that the NH groups formed hydrogen bonds in
the gelation process.
Fig. 12 Optimized self-assembly model of OGL1 obtained by DFT
calculations.
These presumptions were conrmed by the optimized self-
assembly model obtained by DFT calculations. As shown in
Fig. 12, the nitro group on one gelator formed two intermolec-
ular hydrogen bonds with the NH and CH groups on the other
˚
gelator molecule (bond length: –N–H/O, 14 A; –C–H/O,
˚
2.508 A). There were p–p stacking interactions in neighbouring
nitrophenyl groups, and the distance between the two phenyl
˚
groups was 3.84 A. Therefore, the gelator L1 could self-assemble
into stable organogel via the hydrogen bonds, p–p stacking
interactions, and vdW interactions.
Moreover, the X-ray diffraction patterns (Fig. 13) of OGL1
xerogel showed periodical diffraction peaks, indicating that L1
indeed assembled into an ordered structure. The d-spacing of
Fig. 13 Powder X-ray diffraction patterns for powdered L1 and xerogel
of OGL1 (0.8% in DMSO).
˚
3.80 A at 2q ¼ 23.42 suggested that p–p stacking existed
between the phenyl groups of gelator L1, which also supported
the self-assembly assumption.
The SEM images of OGL1 showed a network structure con-
sisting of many entangled tape-like structures. These tapes were
approximately 10–30 nm in width and several mm long
(Fig. 14a). It was very interesting that although the macro-phase
of OGL1 could not change under the stimulation of Fꢀ, at the
micro-level, the self-assembly process was actually inuenced
by Fꢀ. As shown in (Fig. 14b), upon addition of Fꢀ, the entan-
gled tape-like structure (Fig. 14a) of OGL1 changed to a rugate
layer structure (Fig. 14b). This phenomenon could be attributed
These results indicated that the N–H and –HC]N– protons on
L1 formed stable hydrogen bonds with AcOꢀ. Thus, unlike that
Fꢀ could induce the deprotonation of N–H on L1, AcOꢀ could
not deprotonate N–H of L1. Therefore, AcOꢀ could not lead to
the color change of L1 and OGL1. The similar experiments were
1
carried out for L2 with AcOꢀ. As shown in Fig. S3,† in the H
NMR spectra of L2, the N–H and –HC]N– protons on L2 were at
d 11.3 and 9.16 ppm. Upon addition of AcOꢀ, the N–H signal
at 11.3 ppm disappeared, indicating the deprotonation of the
Fig. 14 SEM images of (a) xerogel of OGL1 (0.8%, in DMSO); (b) OGL1 xerogel treated with Fꢀ in situ; (c) OGL1 xerogel treated with Fꢀ in situ, then
added HClO4 (0.01 M).
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to the deprotonation of the N–H group by the attack of Fꢀ. 12 L. E. Buerklea and S. J. Rowan, Chem. Soc. Rev., 2012, 41,
When the N–H group was deprotonated, the hydrogen bonds 6089.
between the gelators were destroyed, and the gelators were 13 G. C. Yu, X. Z. Yan, C. Y. Han and F. H. Huang, Chem. Soc.
assembled by vdW forces among long alkyl chains and p–p Rev., 2013, 42, 6697.
stacking interactions among nitrophenyl groups. Therefore, the 14 Y. L. Liu, Z. Q. Wang and X. Zhang, Chem. Soc. Rev., 2012, 41,
micro-morphology of OGL1 changed as mentioned above. 5922.
When adding the proton solutions (HClO4, H2O, etc.) to OGL1 15 M.-O. M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed,
containing Fꢀ, the micro-morphology was restored (Fig. 14c).
Chem. Rev., 2010, 110, 1960.
16 J. W. Steed, Chem. Commun., 2011, 47, 1379.
17 J. Liu, Yu. Feng, Z. X. Liu, Z. C. Yan, Y. M. He, C. Y. Liu and
Q. H. Fan, Chem.–Asian J., 2013, 8, 572.
4. Conclusion
In summary, we have developed an excellent organogel OGL1 18 J. Zeng, W. Wang, P. Deng, W. Feng, J. Zhou, Y. Yang,
which could act as a Fꢀ responsive smart material for instant
L. Yuan, K. Yamato and B. Gong, Org. Lett., 2011, 13, 3798.
colorimetric detection of Fꢀ with specic selectivity under gel– 19 X. Li, Y. Fang, P. Deng, J. Hu, T. Li, W. Feng and L. Yuan, Org.
gel states as well as in DMSO solutions. The color change Lett., 2011, 13, 4628.
induced by Fꢀ could be recovered by adding proton solutions. 20 J. H. Esch and B. L. Feringa, Angew. Chem., Int. Ed., 2000, 39,
Other anions and common cations did not cause any similar 2263.
stimuli response. With the cooperation of multi self-assembly 21 A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew.
driving forces we rationally introduced into the gelator, L1 Chem., Int. Ed., 2008, 47, 8002.
showed excellent gelation capability in DMSO and DMF. Of 22 H. L. Liu, P. C. Zhang, M. J. Liu, S. T. Wang and L. Jiang, Adv.
particular signicance, in the Fꢀ sensing process, the organogel
Mater., 2013, 25, 4477.
did not carry out any gel–sol phase change at the macro-level, 23 P. D. Wadhavane, R. E. Galian, M. A. Izquierdo, J. Aguilera-
´
which is very rare in the research of anion responsive organo-
Sigalat, F. Galindo, L. Schmidt, M. I. Burguete, J. Perez-
gels. The gel–gel state recognition has the merits of facile and
Prieto and S. V. Luis, J. Am. Chem. Soc., 2012, 134, 20554.
efficient properties. The experimental study presented herein 24 C.-T. Chen, C.-H. Chen and T.-G. Ong, J. Am. Chem. Soc.,
has important potential for the fabrication of smart materials in
anion sensing applications.
2013, 135, 5294.
25 C. Caltagirone and P. A. Gale, Chem. Soc. Rev., 2009, 38, 520.
26 S. Kubik, Chem. Soc. Rev., 2009, 38, 585.
´
´
27 P. A. Gale, R. Perez-Tomas and R. Quesada, Acc. Chem. Res.,
Acknowledgements
2013, 46, 2801.
´
We are grateful for nancial support from the National Natural 28 S. V. Krivovichev, O. Mentre, O. I. Siidra, M. Colmont and
Science Foundation of China (NSFC) (nos 21064006; 21161018; S. K. Filatov, Chem. Rev., 2013, 113, 6459.
21262032), the Natural Science Foundation of Gansu Province 29 P. A. Gale, Acc. Chem. Res., 2011, 44, 216.
(1308RJZA221) and the Program for Changjiang Scholars and 30 Y. M. Yang, Q. Zhao, W. Feng and F. Y. Li, Chem. Rev., 2013,
Innovative Research Team in the University of Ministry of
Education of China (IRT1177).
113, 192.
31 B. Wu, F. Cui, Y. Lei, S. Li, N. de Sousa Amadeu, C. Janiak,
Y.-J. Lin, L.-H. Weng, Y.-Y. Wang and X.-J. Yang, Angew.
Chem., Int. Ed., 2013, 52, 5096.
32 C. Jia, B. Wu, S. Li, X. Huang, Q. Zhao, Q.-S. Li and X.-J. Yang,
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