A novel smart organogel which could allow a two channel anion response by proton controlled reversible sol-gel transition and color changes

Article abstract of DOI:10.1039/b911125e

A super gelator (F3) with an excellent gelation ability was synthesized; the organogel of F3 could allow two channel recognition of F^-, AcO^- and H₂PO₄^- through proton controlled reversible sol-gel tra

Full text of DOI:10.1039/b911125e

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A novel smart organogel which could allow a two channel anion response
by proton controlled reversible sol–gel transition and color changesw
You-Ming Zhang, Qi Lin, Tai-Bao Wei,* Xiao-Ping Qin and Yan Li
Received (in Cambridge, UK) 5th June 2009, Accepted 11th August 2009
First published as an Advance Article on the web 27th August 2009
DOI: 10.1039/b911125e
A super gelator (F3) with an excellent gelation ability was
synthesized; the organogel of F3 could allow two channel
However, amide or urea units often act as anion binding sites
in anion receptors. Therefore in organogel, if the hydrogen
bonding donor NH sites in the amide or urea units interact
with anions, the gelators may not form stable assemblies,
resulting in the transformation of the gels, for example,
into a solution. In this case, the organogel is so-called
anion-responsive gel.7e
recognition of FÀ, AcOÀ and H2PO4 through proton
À
controlled reversible sol–gel transition and color changes.
Recently, low-molecular mass organogelators (LMOG) have
attracted substantial attention1 in supramolecular chemistry
and materials science due to their unique characteristics and
wide range of potential applications. For example, organogels
could be used as templates of nano-scale inorganic materials,2
organic soft materials,3 optical sensors,4 slow drug-delivery
systems5 and so on. In an organogel the gelator molecules
self-assemble into nanoscale superstructures, such as fibers,
rods, and ribbons through weak noncovalent interactions
(i.e., hydrogen bonding, p–p stacking, van der Waals, coordination,
and charge-transfer interactions).1,6 Of particular interest for
organogel material science are ‘‘smart gels’’, i.e. gels whose
properties can be controlled reversibly or irreversibly in
response to changes in external chemical,7a–g,8 photochemical,9
thermal stimuli6,10 or sound.7h To date, there have been
reports on organogels that are responsive to pH changes,8
light,9 catalysis,11 and cation12 recognition. However, only a
few reports currently exist on the controllable gelation systems
based on reversible changes of a gelator molecule induced by
anions stimuli.7a–g
As an attempt to obtain a smart anion-responsive gel
with potential anion sense applications, we have designed
and synthesized a gelator (F3) bearing phenol O–H and
acylhydrazone N–H groups (Scheme 1). Interestingly, F3
could form very stable organogel in N,N-dimethyl formamide
(DMF). It is exciting that the organogel of F3 could allow two
channels response for FÀ, AcOÀ and H2PO4À through proton
controlled reversible gel–sol transition and color changes.
The most impressive gelation ability of F3 was observed in
polar solvents such as DMF and DMSO (Table 1). As a
general procedure, the powdery F3 was completely dissolved
in hot DMF (Z15.6 mM), after cooling to ambient temperature,
the vessel was turned upside down and the fluidity of the
system was absent, it was denoted as a gel (Fig. 1a). However,
much to our surprise, in DMF, the sol–gel transition is very
rapid. The whole transition process only takes less than 2 min
after stop heating the DMF solution (23.3 mM). The DMF gel
is very stable, for example, the gel–sol transition temperature,
T
gel, was measured by using the ‘tilted tube’ method and
Over the past thirty years, more and more interest has been
attracted to the selective recognition and sensing of anions via
artificial receptors, because anions play fundamental roles in
chemical, biological and environmental process.13 Although
previous work developed a wide variety of anion sensors based
on electrostatic interactions, hydrogen bond donor groups,
Lewis acid groups and hydrophobic interactions, it is still a
challenge to design and synthesize anion sensors with single
selectivity and high sensitivity for certain anions.13,14
Moreover, most of these anion sensors have been employed
in solution by means of spectroscopic instrumentation. This
will significantly restrict the practical applications of these
anion sensors. For simplicity, convenience and low cost, the
fabrication of small molecular anion sensors into colorimetric
test kits is highly demanding.13,15 It is interesting that most of
the organogels reported thus far using amide or urea units as
hydrogen bonding sites that support molecular assembly.
increasing regularly with concentration of F3 and reaching a
limit at 151 1C from a 23.3 mM gel. The gel was quite stable
for more than one year at room temperature without phase
separation.
Moreover, F3 could act as a super gelator for dimethyl-
sulfoxide (DMSO). In DMSO, the gelation took place at a low
F3 concentration of only 3.1 mM (0.2 wt%). Such an ability to
gelate solvents at concentrations lower than 1 wt% (15.6 mM
for F3) is classified as super-gelation.16 Gel formation was not
observed in other solvents or solvent mixtures such as aliphatic
College of Chemistry and Chemical Engineering, Key Laboratory of
Polymer Materials of Gansu Province, Northwest Normal University,
Lanzhou, Gansu, 730070, P. R. China. E-mail: weitaibao@126.com;
Tel: +86(931)7970394
w Electronic supplementary information (ESI) available: Synthesis
and characters, optimized self-assemble model, XRD patterns of
xerogel, FT-IR and UV-vis spectra. See DOI: 10.1039/b911125e
Scheme 1
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Fig. 1 (a) Organogel formed from a solution of F3 in DMF (23.3 mM);
(b) Immediately after addition of solid TBAF (5 eqv.); (c) after 2 min;
(d) after 8 min; (e) and (f) after 15 min; (g) Immediately after addition
of 0.30 mL HClO4 (0.1 M) to the above obtained solution; (h) The
‘‘stable-to-inversion test’’ succeeds after addition of HClO4 2 min.
Fig. 3 (a) and (b) AFM images of xerogel from DMSO gel of F3
(scale bar: (a) = 1 mm; (b) = 200 nm); (c) and (d) SEM images of
xerogel from DMF gel of F3 ((scale bar: (c) = 1 mm; (d) = 100 nm)).
solution, the bands of O–H and N–H were combined into a
broad band which appeared at 3482 cmÀ1. However, in
organogel, the broad band of O–H and N–H shifted to
3450 cmÀ1, which indicate the O–H and N–H groups participate
in the formation of intermolecular hydrogen bonds among
the gelator molecules. An optimized hydrogen bonding
self-assemble model obtained by DFT studies agrees with
the evidences (Fig. S2 in the ESIw). Moreover, the self-
assembled structure of the organogel is also supported by
XRD analysis (Fig. S3 in the ESIw). The xerogel of F3
exhibited well-resolved X-ray diffraction patterns that were
characteristic of the long-range ordering of the molecules.
As we expected, the organogel of F3 exhibited excellent
reversible gel–sol transition according to the stimulus of
anions. As shown in Fig. 1b–f, the addition of FÀ (5 equiv.)
as its TBA salt (solid Bu4NF) to the DMF gel of F3 at 20 1C
effected a gradual decomposition of the gelatinous state in
ca. 15 min, yielding an wine-colored solution. Similarly, the
Fig. 2 (a) The solution obtained from the DMF gel reacted with solid
TBAF salt (5 eqv.); (b) immediately after addition of methanol
(0.20 mL); (c) after 1 min; (d) after 2 min; (e) after 7 min; (f) after
8 min (the small amount of solution is excess methanol).
Table 1 Gelation properties of F3
Solvent
Concentration/mM
Phasea
Gelating time
Tgel/1C
DMF
DMF
DMF
DMSO
DMSO
DMSO
DMSO
o15.6
15.6
23.3
SP
—
—
98
151
—
45
56
92
OG
OG
S
TG
GP
GP
5 min
2 min
—
ca. 1 month
ca. 1 day
10 min
o3.1
3.1
15.6
462.2
a
SP, solution and precipitate; OG, opaque gel; S, solution; TG,
translucent gel; GP, gel and precipitate.
solvents, ethers, ethanol, and tetrahydrofuran due to the poor
solubility of F3. In order to investigate the influence of the
O–H group and the molecular structure on the gelating
abilities of F3, we also designed F1 and F2. F1 is an analogue
of F3 but does not contain the O–H groups. F2 resembles the
single arm of F3. Compounds F1 and F2 could not gelate
DMF or DMSO. The results of these comparisons indicate
that the molecular structure and phenol O–H group play a
critical role in the gelation process.
addition of solid TBA salts of AcOÀ or H2PO4 also lead to
À
the DMF gel of F3 decomposed to corresponding red-colored
solutions. While, these transition need 10 min for adding
À
AcOÀ and 27 min for H2PO4 respectively. In the samÀe
À
conditions, the addition of ClÀ, BrÀ, IÀ, HSO4 and ClO4
(all used as solid TBA salts) could not lead to the gel
decomposition. These results support the assumption that the
anions such as FÀ, AcOÀ and H2PO4À, not TBA cations, are
responsible for the transformation of the organogel to solution.
On the other hand, according to the speed difference of the
gel–sol transition, we could conclude that the anion affinities
sequence of the gelator is AcOÀ 4 FÀ 4 H2PO4À c ClÀ, BrÀ,
The morphology of the organogel was investigated by
atomic force microscopy (AFM) images and cold field
emission scanning electron microscope (SEM). The AFM
images of DMSO gel (3.1 mM) show a network structure
consisting of many entangled tape-like aggregates of widths
between 30–60 nm and lengths of several mm (Fig. 3a,b). In
addition, the SEM images of DMF gel (23.3 mM) reveal a
well-defined network, tape-like structure as well. These tapes
are approximately 50–200 nm in width and several mm long
(Fig. 3c,d).
À
À
IÀ, HSO4 and ClO4
.
The Àorganogel of F3 could not only sense FÀ, AcOÀ and
H2PO4 in their solid TBA salts, but also could colori-
metrically sense these anions in their DMSO solutions. As
shown in Fig. S4 (in the ESI), when adding 0.02 mL DMSO
À
solution (10 mM) of FÀ, AcOÀ or H2PO4 (tetrabutyl-
ammonium was used as a countercation) to the small amount
gel (ca. 0.01 g), the gels showed dramatic color changes from
yellow to nacarat (for FÀ), red (for AcOÀ) or orange
(for H2PO4À) respectively. The same tests were applied to
ClÀ, BrÀ, IÀ, HSO4À and ClO4À, no obvious color change was
observed. In addition, As shown in Fig. S5, S6 (in the ESI),w
the solution of F3 could colorimetrically sense FÀ, AcOÀ or
Owing to the fact that F3 is an efficient gelator of polar
solvents, we believe that hydrogen-bonding interactions
contribute to the stabilization of the self-assembled aggregates.
To obtain further insights we measured FT-IR spectra of F3 in
the solid state, the gel phase and the sol phase (fresh solution
in DMF) (Fig. S1 in the ESIw). The FT-IR spectra of the
xerogel showed the presence of bands belonging to O–H and
N–H groups at 3433 and 3333 cmÀ1 respectively. While, in
À
H2PO4 also.
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À
H2PO4
,
the gelator was deprotonated, then, the gel
disintegrated. While when the solution of disintegrated gel
obtained proton from protic solvents, the solution re-gelated
to gel again. Due to the smart gel reported here is very stable at
room temperature, we believe the gel could act as a convenient
and efficient anion test kit.
This work was supported by the NSFC (No. 20671077).
We thank Prof. Yong-Cheng Wang for DFT studies.
Fig. 4 Partial 1H NMR spectra of compound F3 (2.5 mM) in
DMSO-d6 upon the addition of FÀ. (a) Free, (b) 0.5 equiv. of FÀ,
(c) 1 equiv of FÀ, (d) 4 equiv. of FÀ.
Notes and references
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The further anion–F3 reaction mechanism was observed
1
from H NMR titration experiments (Fig. 4, S7 in the ESI)
in DMSO-d6. Before the addition of anions, the 1H NMR
chemical shifts of the N–H, O–H and ÀHCQNÀ protons on
F3 were at d 12.66, 12.39 and 8.75 ppm, respectively. After
adding 0.5 equiv. of FÀ, the resonances for NÀH and OÀH
protons disappeared, and a weak broad signal appeared at d
12.44 ppm, which indicated the formation of N–HÁ Á ÁFÀ and
O–HÁ Á ÁFÀ hydrogen bonds. With continuous addition of
FÀ, this signal disappeared also, which suggested that the
O–H and N–H groups perhaps underwent a deprotonation
process. Simultaneously, a new weak broad signal apÀpe1a7red at
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d 16.15 ppm, which indicated the formation of HF2
.
1
According to the H NMR titration experiments we could
presume the mechanism of anion induced gel–sol transition.
Upon the addition of FÀ, AcOÀ or H2PO4À, owing to the
deprotonation of O–H and N–H, the intermolecular hydrogen
bonds which assembled the gelator to gel were broken. Hence
the gel transitioned to solution. Meanwhile, the deprotonation
of phenol O–H group induced the color change of the gelator
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À
À
H2PO4À. Therefore ClÀ, BrÀ, IÀ, HSO4À, and ClO4 can not
lead to the phenol OH group deprotonation. So the organogel
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Since the root causes of the anion induced gel–sol transition
is deprotonation, we presume that addition of proton could
lead to re-gelation of the solution where gel disintegrated. This
presumption was confirmed by our experiments. As shown in
Fig. 1g, when 0.3 mL of HClO4 (0.1 M) was added to the
solution which was obtained from FÀ disintegrated gel, the
solution re-gelated immediately. Meanwhile, the wine-colored
solution changed to yellow gel again. Interestingly, protic
solvents such as H2O and methanol could re-gelate the gel
disintegrated solution as well (Fig. 2). The re-gelation speed is
depending on the proton concentration of the solvent. For
example, at same conditions, the sequence of re-gelation speed
is HClO4 4 H2O 4 methanol.
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In summary, a super gelator F3 bearing phenol O–H and
acylhydrazone N–H groups was synthesized. It has showed an
excellent gelation ability towards DMF and DMSO. The
organogel of F3 could allow two channels recognition for
16 (a) F. Wurthner, C. Bauer, V. Stepanenko and S. Yagai, Adv.
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À
FÀ, AcOÀ and H2PO4 through reversible gel–sol transition
and color changes. We have demonstrated that the reversible
gel–sol transition processes were controlled by proton. When
the gelator reacted with basic anions such as FÀ, AcOÀ or
17 S. O. Kang, D. Powell, V. W. Day and K. Bowman-James, Angew.
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6076 | Chem. Commun., 2009, 6074–6076