Ultralow-Molecular-Weight Stimuli-Responsive and Multifunctional Supramolecular Gels Based on Monomers and Trimers of Hydrazides

Article abstract of DOI:10.1002/asia.202001041

The simpler, the better. A series of simple, neutral and ultralow-molecular-weight (MW: 140–200) hydrazide-derived supramolecular gelators have been designed and synthesized in two straightforward steps. For non-conjugated cyclohexane-derived hydrazides, their monomers can self-assemble to form gels through intermolecular hydrogen bonds and dipole-dipole interactions. Significantly, conjugated phthalhydrazide can self-aggregate into planar and circular trimers through intermolecular hydrogen bonds and then self-assemble to form gels through intermolecular π–π stacking interactions. It is interesting that these simple gelators exhibit unusual properties, such as self-healing, multi-response fluorescence, and visual and selective recognition of chiral (R)/(S)-1,1′-binaphthalene-2,2′-diamine and S^2? through much different times of gel re-formation and blue-green color change, respectively. These results underline the importance of supramolecular gels and extend the scope of supramolecular gelators.

Full text of DOI:10.1002/asia.202001041

DOI: 10.1002/asia.202001041
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Ultralow-Molecular-Weight Stimuli-Responsive and Multifunctional
Supramolecular Gels Based on Monomers and Trimers of
Hydrazides
Dehua Wu, Jintong Song, Lang Qu, Weilan Zhou, Lei Wang, Xiangge Zhou, and
[a]
Haifeng Xiang*
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Abstract: The simpler, the better. A series of simple, neutral
and ultralow-molecular-weight (MW: 140–200) hydrazide-
derived supramolecular gelators have been designed and
synthesized in two straightforward steps. For non-conjugated
cyclohexane-derived hydrazides, their monomers can self-
assemble to form gels through intermolecular hydrogen
bonds and dipole-dipole interactions. Significantly, conju-
gated phthalhydrazide can self-aggregate into planar and
circular trimers through intermolecular hydrogen bonds and
then self-assemble to form gels through intermolecular π–π
stacking interactions. It is interesting that these simple
gelators exhibit unusual properties, such as self-healing,
multi-response fluorescence, and visual and selective recog-
2
À
nition of chiral (R)/(S)-1,1’-binaphthalene-2,2’-diamine and S
through much different times of gel re-formation and blue-
green color change, respectively. These results underline the
importance of supramolecular gels and extend the scope of
supramolecular gelators.
Introduction
dicarboxylate gelator can form stable gels in nitrobenzene
[8a]
solvent. Choline chloride/phenylacetic acid can be used as
[1]
[9a]
Low-molecular-weight gelators (LMWGs) (typically with molec-
gelation solvent for isoleucine and tryptophan. In addition,
ular weights, MW<3000) have attracted a lot of attention due
dipole-dipole/donor-acceptor interactions acting as auxiliary
[2]
to their wide applications, such as multi-responsive sensing,
forces can help neutral π-conjugated molecules to form gels in
[3]
[4]
[1h,5b,10]
self-healing,
optoelectronics.
environmental
protection,
and
normal solvents.
[1f,h,5]
The LMWGs, which self-assemble into 3D
Owing to their strong intermolecular hydrogen bonds,
hydrazides have been widely used as precursors to construct
networks through intermolecular noncovalent interactions,
including hydrogen bonds, π–π stacking, electrostatic, and van
der Waals interactions, are capable of immobilizing solvent
molecules and forming supramolecular gels. Especially,
supramolecular gels are recently used as a chiral matrix and
amplification to discern the subtle stereochemical differences
[1,11]
LMWGs.
Most of these LMWGs have complicated structures
and need tedious organic synthesis. In this study, in order to
simplify LMWGs, we combining the effects of dipole-dipole and
aggregation-induced oligomer have developed a novel and
convenient strategy to prepare a series of neutral hydrazide-
derived LMWGs (6 samples, Scheme 1). The introduction of
electron-rich hydrazide moieties that are capable of forming
hydrogen bonded assemblies is essential for gelation. Chair-
shape cyclohexane would not only increase molecular asymme-
try and dipole moment (μ) to assist to form gels but also
incorporate chirality for chiral recognition. The presence of
benzene ring would lead to a planar structure for aggregation-
induced trimers that have bigger π-conjugated systems to form
functional π-gelators through intermolecular π–π stacking
interactions. Although these LMWGs containing one
cyclohexane or benzene are very simple and their molecular
weights (MW) are within the range of 140–200, they have some
useful applications in multi-response fluorescence, visual chiral
recognition, and self-healing. First of all, to the best of our
knowledge, this is the first example of trimer-based LMWGs and
the simplest example of π-gelators.
[6]
between enantiomers for visual chiral recognition. The basic
principle of designing these functional LMWGs, such as π-
[1h]
gelators,
is the incorporation of some long side groups
including long alkyl chains, amide, urea peptides, cholesterol,
sugar, or steroidal groups into the π-conjugated cores. How-
ever, designing these LMWGs is still a daunting task and most
of them have been found by an accident rather than by design.
Therefore, simple, straightforward, and low-cost LMWGs are
highly intriguing and in great demand. It should keep in mind
that the simpler molecules always mean weaker intermolecular
noncovalent interactions, which are harmful for gelation. In
[7]
order to solve this problem, organic salts that have much
stronger charge-assisted hydrogen bonds than normal neutral
[8]
organic compounds are adopted as gelators and gelation
[9]
solvents. For example, imidazolium hydrogen cyclobutane-1,1-
[
a] D. Wu, J. Song, L. Qu, W. Zhou, L. Wang, Prof. X. Zhou, H. Xiang
College of Chemistry
Sichuan University
Chengdu, 610041 (P. R. China)
Results and Discussion
E-mail: xianghaifeng@scu.edu.cn
All hydrazides were prepared straightforwardly in two steps
with favorable yields (Scheme 1), according to our previous
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/asia.202001041
Chem Asian J. 2020, 15, 1–10
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Figure 1. Images of the hot solutions and gels of hydrazide-derived LMWGs.
The diameter of round bottles is 1.5 cm.
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[14]
(
1:1). For the purpose of the mechanism evaluation, the 15
reference substances that have similar chemical structures with
the hydrazide gelators but can’t form gels were prepared as
well.
Remarkably, some of hydrazides in this work (Scheme 1) are
organogelators and hydrogelators. The gels were prepared
through a heating (60–70°C) and then room-temperature (RT,
Scheme 1. Synthesis and structures of the compounds in this work. G
represents gelator.
2
5°C) cooling process in 2–20 minutes (Figure 1 and Table 1).
Gel formation is confirmed by the test tube inversion method;
the gels are found to withstand their own weight against
gravity. The minimum gelator concentration (MGC) of hydrazide
gelators is within the range of 0.60–5.1 wt%. All the gels are
white and opaque, except that the gels of G-Cy in toluene are
colorless and transparent and the gels of G-Ph-o in DMSO are
pale-yellow and opaque. All the sealed gels are thermo-
reversible (Figure S1) and found to be stable for several months
at RT.
[12]
reports.
The preparation method is described in detail in
Supporting information. At first, we planned to prepare o-
phthaloyldihydrazide (Ph-o, Scheme 1), but the x-ray single-
1
crystal structures, H nuclear magnetic resonance (NMR), and
mass spectra reveal that the product is not Ph-o but its
condensation product of phthalhydrazide (G-Ph-o, G represents
gelator). We also found that there is no report on Ph-o, after
searching its chemical structure by SciFinder (American Chem-
ical Society). Moreover, the literatures show that G-Ph-o would
be always generated using the similar synthetic method. On
the other hand, using the same method, m-phthaloyldihydra-
zide (G-Ph-m) and p-phthaloyldihydrazide (Ph-p) were synthe-
sized successfully. The single crystals of G-Cy (CCDC 2011913)
and G-Ph-o (CCDC 2011915) were obtained by a slow diffusion/
Solubility is one of important factors for gelation. Most of
the hydrazides are soluble in polar solvents (DMSO, DMF,
[13]
MeCN, MeOH, EtOH, and H O) but not in non-polar solvents
2
(toluene, CH Cl , CHCl , and hexane) (Table S1). A moderate
2
2
3
solubility can help to achieve the balance between dissolution
and precipitation for gelation. For example, Ph has a very good
À 1
solubility in most solvents (>1 gml ) and consequently it
evaporation of DMSO/MeOH or H O/MeOH solution at room
doesn’t form gels. On the contrary, Ph-p with a poor solubility
2
À 1
temperature during about one week. We failed to grow good-
quality single crystals of enantiopure G-(R,R)Cy-o and G-(S,S)Cy-
o, and then we had to grow single crystals of racemic G-Cy-o
(<0.0001 gml ) doesn’t either. For the hydrazide gelators,
except that G-Cy can form gels in non-polar solvent of toluene,
they can form gels only in polar solvents of DMSO, DMF, MeCN,
(CCDC 2011914) by the mixture of G-(R,R)Cy-o and G-(S,S)Cy-o
Table 1. Gelation ability of the hydrazides in different solvents in air at RT.
[
a]
[b]
[c]
Compound
MW
Solvent
MGC [wt%/M]
Time /Tgel [min/°C]
G-Cy
142.11
Toluene
MeCN
MeOH
DMSO
MeOH
DMSO
DMSO
1.3/0.081
5.1/0.30
5/50
5/50
G-(R,R)Cy-o
G-(S,S)Cy-o
200.12
200.12
0.90/0.036
3.0/0.17
0.90/0.036
3.0/0.17
3.0/0.17
10/60
10/80
10/60
10/80
20/80
4/60
G-Cy-o
G-Cy-m
G-Ph-o
200.12
200.12
162.04
H O
2
4.0/0.21
EtOH
0.60/0.029
4.2/0.30
1.1/0.065
2/70
2/75
6/70
DMSO
DMF
H O
2
G-Ph-m
194.08
2.5/0.13
10/60
À 3
[
a] M=mol/dm . [b] Cooling Time for Gelation at RT. [c] Heating Temperature for Gel Collapsing.
Chem Asian J. 2020, 15, 1–10
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MeOH, EtOH, and H O, because of their moderate solubilities
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(0.06–0.005 gml ) in these polar solvents.
It is amazing that the simple and neutral hydrazide of G-Cy
with MW of 142 can form gels in toluene and MeCN. The x-ray
single-crystal structures and packing of G-Cy are shown in
Figure 2. G-Cy molecule is double Z-type with a size of 2.880×
7
.524 Å. There is no any intramolecular hydrogen bond and the
distances of =O···H N are 2.569 and 2.710 Å (Figure. S2).
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However, there are many intermolecular hydrogen bonds (=
O···HN: 2.000; =O···H N: 2.268 and 2.318; H N···H N: 2.354 Å)
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between neighboring G-Cy molecules, leading to its tight
packing in one crystal cell (a=4.861, b=11.668, c=14.566 Å).
The molecular dipole moment of G-Cy is 2.21 Debye, calculated
by gas-phase density functional theory (DFT). Under the same
conditions, the molecular dipole moment of H O is calculated
2
to be 2.13 Debye (Figure. S3), indicating that G-Cy is highly
polar. This is in agreement with the fact that G-Cy has a good
solubility in polar solvents but not in non-polar solvents. As
depicted in Figure 2c, the orientation of blue arrow is the
direction from the center of negative charge (hydrazide) to the
center of positive charge (cyclohexane), which might be
contributed to the electron-rich nature of hydrazide. These
obvious dipole-dipole interactions would help G-Cy molecules
to self-assemble into cross head-to-head (cyclohexane) and tail-
to-tail (hydrazide) lamellar packing structure with repetitive unit
d-spacing of 14.566 Å and interlayer I-spacing of 4.861 Å.
Figure 2. a) X-ray single-crystal structures and packing of G-Cy in one crystal
cell with a view down the crystallographic a, b, and c axes. b) Intermolecular
interactions of the neighbouring molecules in G-Cy single-crystals (1: 2.000,
As expected, there are equimolar G-(R,R)Cy-o and G-(S,S)Cy-
2
: 2.268, 3: 2.354, 4: 2.318 Å). c) Optimized structure of G-Cy with the
o in the racemic G-Cy-o·H O single-crystals (Figure 3). G-Cy-o
2
direction of the molecular dipole moment.
(μ=1.18 Debye) with two hydrazide moieties is more symmetric
and consequently lower polar than G-Cy. G-Cy-o molecule is V-
type with a size of 6.807×7.319 Å. There is no any intra-
molecular hydrogen bond either. Like G-Cy, G-Cy-o molecules
exhibit cross head-to-head and tail-to-tail lamellar packing
through many intermolecular hydrogen bonds between not
only neighboring G-Cy-o molecules (=O···HN: 2.002 and 2.037;
=
O···H N: 2.366 and 2.397 Å) but also G-Cy-o and water
2
molecules (H O···H N: 2.181 and 2.193; H N···H O: 2.306 and
2
2
2
2
2
.075 Å) (crystal cell: a=8.411, b=8.407, c=33.268 Å). G-Cy-o
molecules assemble with water molecules to form repetitive
unit d-spacing of 33.268 Å and interlayer I-spacing of 8.411 Å.
Unlike G-Cy and G-Cy-o, π-conjugated G-Ph-o is planar and
in the form of enol (Figure 4). Through multiple intermolecular
hydrogen bonds (O···H: 1.319–1.340; N···H: 1.408–1.420 Å), three
G-Ph-o molecules can self-aggregate into a circular trimer,
which is equilateral triangle with side length of about 13.18 Å.
Although the molecular dipole moment of asymmetric mono-
mer (μ=3.30 Debye) is very high, that of symmetric trimer is
calculated to be near to zero (μ=0.01 Debye, Figure S4). It is
interested that the trimers undergo multiple intermolecular
O···H (2.465, 2.549, and 2.678 Å) and π–π stacking (I=3.335 Å)
interactions to self-assemble into 2D network structure, just like
[15]
graphene.
Figure 3. a) X-ray single-crystal structures and packing of G-Cy-o in one
crystal cell with a view down the crystallographic a, b, and c axes. b)
Intermolecular interactions of the neighbouring molecules in G-Cy-o single-
crystals (1: 2.037, 2: 2.366, 3: 2.181, 4: 2.306, 5: 2.193, 6: 2.075, 7: 2.397, 8:
Scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) are versatile techniques for
supramolecular science and materials science to elucidate the
[16]
2
.022 Å). c) Optimized structure of G-(S,S)Cy-o with the direction of the
microscopic structures of self-assembled systems. SEM and
TEM analyses of the hydrazide xerogels (Figure 5 and S5) reveal
molecular dipole moment.
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Figure 6. PXRD of G-Cy (1.3 wt% in toluene) G-Cy-o (3.0 wt% in DMSO), G-
(
R,R)Cy-o (0.9 wt% in MeOH), G-Cy-m (4.0 wt% in H
2
O), and G-Ph-o (0.6 wt%
in EtOH), and G-Ph-m (2.5 wt% in H O) xerogels.
2
Figure 4. a) and b) X-ray single-crystal structures and packing of G-Ph-o in
one crystal cell with a view down the crystallographic a, b, and c axes. c) 2D
molecular packing of G-Ph-o.
Cy xerogels exhibit six diffraction peaks, which are consistent
with the value observed in its single crystal simulated pattern.
The main sharp peak (2θ=6.2°) is equivalent to layer-to-layer d-
spacing of 14.25 Å, which is in good agreement with the value
observed in single-crystal structure of G-Cy (14.566 Å, (0,0,1))
(
1
Figure 2). The peaks at 6.2° (d=14.25 Å), 12.6° (d=7.02 Å),
8.9° (d=4.69 Å), 25.1° (d=3.55 Å), and 37.9°(d=2.37 Å) with d
values almost exactly in the ratio of 1:1/2:1/3:1/4:1/6,
[16,17]
suggesting a lamellar organization.
Therefore, it is clear that
G-Cy molecules can self-assemble in a similar way with its
single-crystal molecular arrangement, which plays a key for
gelation. For G-Cy-o xerogels, the small peak (2θ=5.3°) is
equivalent to d=16.66 Å, which is an half of c axis of crystal cell
(
9
33.268 Å) (Figure 3). However, the main peak (2θ=9.1°, d=
.71 Å) of G-Cy-o xerogels can’t be found in its single crystal
simulated patterns, revealing that the molecular arrangement
of gelation might be different from that of crystal. Owing to the
same reason, the racemic G-Cy-o has different diffraction
[14]
signals from enantiopure G-(R,R)Cy-o as well. On the same
time, the diffraction signals of racemic G-Cy-o are much sharper
and stronger than those of enantiopure G-(R,R)Cy-o, and thus
the former would pack closely and orderly, like left and right
band, and consequently be easy for crystal growth. For G-Ph-o
xerogels, the peak (2θ=6.7°) corresponds to d=13.18 Å that is
Figure 5. SEM and TEM images of G-Cy (1.3 wt% in toluene) G-(S,S)Cy-o
(
H
0.9 wt% in MeOH), G-(R,R)Cy-o (0.9 wt% in MeOH), G-Cy-m (4.0 wt% in
O), and G-Ph-o (0.6 wt% in EtOH), and G-Ph-m (2.5 wt% in H O) xerogels.
The last one is TEM image and the others are SEM images.
2
2
molecular size of its trimer (13.18 Å, Figure 4). The peak (2θ=
9
.4°) of G-Cy-m corresponds to d=9.40 Å that is exactly equal
to its molecular size (9.407 Å), but that (2θ=8.0°, d=11.04 Å) of
G-Ph-m xerogels is a bit bigger than its molecular size (9.972 Å,
Figure S6). Like G-Cy, all the hydrazide gelators show continu-
ous proportional changes in d values, indicating that their high
degree of crystallinity. Therefore, a model with the packing can
be proposed in which hydrazide molecules are packed into a
layered structure.
that the hydrazide molecules can self-assemble into fine thin
films by well-defined 3D nanofiber networks with breadths and
lengths of hundreds of nanometers and tens micrometers,
respectively, which could account for their high viscosity at
these low concentrations. In addition, the precipitates of Cy-p
in EtOH are sheets but not nanofibers, which is why it can’t
form gels.
1
H NMR is a powerful technique for studying supramolecular
To gain insight into how the hydrazide molecules pack into
fibrous self-assemblies, the wide-angle X-ray powder diffraction
gels on the molecular scale and it is suitable to provide a
[16]
dynamic picture of supramolecular gels.
Generally, the
(XRPD) of xerogels were examined. As shown in Figure 6, the G-
gelators incorporated and immobilized within the ‘‘solid-like’’
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networks cannot be monitored by H NMR spectroscopy due to
Among the hydrazide molecules, only G-Ph-o is fluorescent
(λem =450 nm, emission quantum yield Φ=2.5% in DMF), and
thus its photo-physical properties are studied. In a dilute DMF
solution, G-Ph-o monomer exhibits a dominant absorption
peak (λabs =301 nm), which is effectively predicted by time-
dependent DFT (TD-DFT) calculations (295 nm) (Figure 8). This
absorption peak (295 nm, oscillator strength, fosc =0.1365) is
mainly originated from the highest occupied molecular orbital
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line broadening and loss of spectral resolution, while the
gelators within the “liquid-like” solution phase have strong NMR
signals as a consequence of its molecular-scale mobility.
Compared with free hydrazide molecules in solution, the
hydrazide molecules in gels have strong intermolecular hydro-
gen bonds and consequently show broader NMR peaks (Fig-
ure 7).
[16,18]
(
(
HOMO) into the lowest unoccupied molecular orbital (LUMO)
95%) and from HOMO into LUMO+1 (3%). Their energy level
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diagram and frontier molecular orbitals reveal that the π-
conjugation system is efficiently stretched into the whole
molecule (enol form) and the absorption can be attributed to
the π–π* transitions. At high concentrations, G-Ph-o trimers
might be produced like its single crystals, but there is no
concentration effects on absorption spectra. The wavelengths
of absorption peak show little change from 10 μM to 1 mM
(
Figure S7) (Absorbance would reach saturation over 1 mM,
even although 1 mm path length quartz cuvettes are used). The
calculated absorption spectrum of trimers (306 nm, fosc =0.0798:
HOMO ! LUMO, 85%; HOMO!LUMO+1, 11%) is almost same
1
Figure 7. H NMR spectra of G-(R,R)Cy-o (gels: 3.0 wt% in DMSO-d
6
; solution:
in DMSO-d
6
) and G-Ph-o (gels: 4.2 wt% in DMSO-d
6
; solution: in DMSO-d ).
6
[12,14c]
as that of monomer.
The transition of trimers includes not
only π–π* transitions but also charge transfers (CT) between
the monomers. However, excitation spectra (λem =450–495 nm)
of G-Ph-o solutions show obvious changes from 10 μM to
1
1
00 mM (Figure 8 and S8). H NMR spectra of G-Ph-o solutions
give an obvious concentration effect (Figure 8) as well. The
chemical shift of reactive H atoms in NH groups moves from
4
changes demonstrate the possibility of trimer formation in
solution through intermolecular hydrogen bonds. Furthermore,
the emission peak of G-Ph-o gels (1.2 wt% in DMF), high-
concentration solutions, and powders would red shift into λem
4
ing” (ACQ) (Figure 8 and S8), which is a common phenomenon
for planar π-conjugated molecules and caused by the intermo-
.30 (10 mM) into downfield 7.01 (400 mM) ppm. These above
=
90–495 nm along with emission “aggregation-caused quench-
[19]
lecular π–π stacking interactions (Figure 4).
Since the driving forces of gelation mainly are derived from
intermolecular noncovalent interactions that are susceptible to
environment conditions, supramolecular gels as smart soft
materials can be widely used in multi-responsive sensing. The
responses of all the hydrazide gels upon adding common
anions and cations are listed in Table S2–S6. In general, for
cation sensing, when adding equimolar Na , Li , or K , the
hydrazide gels would not collapse. Nonetheless, transition
metal ions would lead to gel collapsing, because they can
coordinate with the hydrazide molecules. Anions also might
have strong interactions with hydrazide molecules through
[
2]
+
+
+
[12,14a,20]
hydrogen bonds
to destroy the gels. There is no uniform
rule for cation responses. G-Ph-o gels (1.2 wt% in DMF) are
emissive and then selected as example to discuss the anion/
cations sensing properties. Among many metal ions that would
lead to gel collapsing (Figure 9), only Al shows an obvious
emission enhancement (Figure 8 and S9). The similar emission
enhancement is also observed for G-Ph-o solution (Figure S10).
When Al
collapse and the emission intensity (λem =450 nm) of resultant
Figure 8. Top: Computational and experimental absorption and emission
excited at 301 nm) spectra (in DMF). Photographs of G-Ph-o solution
0.01 mM in DMF) and gels (1.2 wt% in DMF) and the gels+Al under
365 nm UV light. Middle: Normalized excitation (left) and emission (right)
spectra of G-Ph-o in DMF solution. Bottom: Energy level diagram, and
frontier molecular orbitals of G-Ph-o. H NMR spectra of G-Ph-o solutions in
3
+
(
(
3+
1
3+
is added over 0.1 equivalents, the gels would
6
DMSO-d .
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Figure 9. Photographs of G-Ph-o gels (1.2 wt% in DMF) upon adding
equimolar cations and anions.
2
solutions linearly enlarges (correlation coefficient R =0.991, n=
3
+
8
) upon adding 0.1–0.8 equivalents of Al and then reaches
saturation (Φ=7.7%). Furthermore, the emission of G-Ph-o
solution would be quenched upon adding vary anions through
hydrogen bonds (Figure S11). It is interested that only S
makes the gels to collapse along with the color change from
2
À
white into blue-green (λabs =618 nm, Figure S12), indicating
Figure 10. (a) Photographs and absorption spectra of G-Ph-o solution
(different concentrations in DMF) upon the addition of 1 equivalent of S
2
À
2À
that G-Ph-o might be used as S indicator.
(
1 mm path length quartz cuvettes). (b) MM2-optimized structures of G-Ph-
The blue-green color is very unusual for simple G-Ph-o, and
thus we firstly guessed one new substance might be produced
2
À
o trimer (À 3H)+S (get rid of three H atoms). (c) DFT-optimized structures
and frontier molecular orbitals of G-Ph-o trimer (À 3H).
2
À
after adding S . However, we tried to separate new substance
2
À
form the blue-green the mixture solution of G-Ph-o and S but
failed. The thin-layer chromatography analysis further confirms
that there is no any new substance (Figure S13). At not too low
concentrations (<1 mM), the blue-green color (λabs =618 nm)
would not be detected by eyes and even by absorption spectra
supramolecular gels and it encouraged us to attempt to use the
gels for self-healing. For this experiment, a block of gels was cut
into three pieces. The middle piece was dyed with perylene
(yellow fluorescent dye) and the pieces were put together and
allowed to self-heal. As shown by Figure 11, the pieces of gels
heal within 3 hours to form a single structure, and the whole
structure is strong enough to hold its weight when placed on
two columns separated by 2.0 cm. G-Ph-o in the crystal
structure is 2D and believed to be responsible for gel network
formation, which would be good to reconstruct G-Ph-o in the
interface and thus leads to self-healing.
(
Figure 10a), which reveals that blue-green color is originated
2
À
from the aggregation of G-Ph-o and S . In addition, the NMR
spectra (Figure S14) show that chemical shift of reactive H
atoms in NH groups of G-Ph-o shifts from 6.00 into 3.79 ppm
upon adding S , suggesting strong intermolecular hydrogen
bonds between G-Ph-o and S . Such chromatic change is
2
À
2
À
+
À
reversible upon adding H /OH (Figure S15). We proposed that
the cavity size G-Ph-o trimers is suitable to capture S , which
2À
would distort the trimers and consequently destroy the gels
To date, there have been only a limited number of
[21]
(
Figure 10b). Meanwhile, the transition of resultant twisty and
examples that have been reported for visual enantioselective
recognition through gel formation or collapsing. Since chiral G-
(R,R)Cy-o and G-(S,S)Cy-o might have intermolecular hydrogen
bonds with amino acids for visual chiral recognition through
gel collapsing, we firstly tried to use their gels for the chiral
recognition of L- and D-amino acids (Table S7). However, the
gels would not collapse when adding different equimolar L/D-
amino acids, and then some other chiral substances, including 7
pairs of (S)/(R) methyl 2-hydroxy-2-phenylacetate, L/D-tartaric
[12b,20]
asymmetric trimers would be dominated by CT
and lead to
an obvious red shift in absorption band (Figure 10c).
We found that G-Ph-o gels in DMSO (4.2 wt%) is very stable.
The mechanical strength of gels can be examined by oscillatory
rheological properties (Figure S16). G’ is higher than G’’,
demonstrating the general “solid-like” rheological property of
[
16]
gels. The gels have strong mechanical strength with G’ up to
4
1
.5×10 Pa). Such strength is remarkable for a non-polymeric
Chem Asian J. 2020, 15, 1–10
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1
50 minutes. Therefore, it provide a new method for visual
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chiral recognition.
Finally, in order to evaluate the mechanism of gelation,
some reference substances (Scheme 1) were examined. All of
them can’t form gels under the same conditions. Compared
with G-Cy, Cy-amid (μ=3.46 Debye) has a À COÀ NH group
2
rather than À COÀ NHÀ NH , revealing the greatest importance of
2
NH-based hydrogen bonds for gelation. Ph (μ=2.52 Debye) has
a too good solubility for gelation, even if it has a similar
molecular packing and multiple intermolecular hydrogen bonds
1
1
1
1
1
1
1
1
1
1
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[22]
with G-Cy (Figure S17). π-Extended system (Naph, μ=4.96
Debye) may be not a good way for these simple LMWGs. The
introduction of chiral, À OH (D/L–MM μ=4.40 Debye; D/L-TA
μ=3.08 Debye; PhOH μ=4.40 Debye), or long alkyl chain
(
Hex1 μ=2.20 Debye; Hex2 μ=3.02 Debye; Oct2 μ=3.02 De-
bye) groups would not help for gelation either. The further
simplification of chemical structures (EA μ=2.49 Debye; CA μ=
Figure 11. Photographs of self-healing behaviour for G-Ph-o gels (4.2 wt%
in DMSO). The middle piece was dyed with perylene (0.5 wt% in DMSO).
5
.36 Debye) leads to failure as well. Compared with G-(S,S)Cy-o
μ=1.18 Debye) and G-Cy-m (μ=2.76 Debye), Cy-p is much
more symmetric and its molecular dipole moment is 0 Debye
Figure S18). Similar phenomenon is observed for G-Ph-m (μ=
(
(
acid, 1,1’-Binaphthyls, and (S,S)/(R,R)1,2-diaminocyclohexane,
were examined (Table S8). Fortunately, we found that G-(R,R)
Cy-o gels (3.0 wt% in DMSO) would not collapse upon adding
both (R)1,1’-binaphthalene-2,2’-diamine ((R)BINAM) and (S)BI-
NAM, but their times of gel re-formation are much different. As
descripted in Figure 12, the transmittance (λabs =450 nm) of G-
2.28 Debye), G-Cy (μ=2.21 Debye), and Ph-p (μ=1.34 Debye).
These results indicate that, except G-Ph-O, the driving forces of
these gels mainly are derived from intermolecular hydrogen
bonds and dipole-dipole interactions. The driving forces of
trimer-based G-Ph-O gels is intermolecular hydrogen bonds
and π–π stacking interactions. Therefore, we proposed that the
hydrazide gelators can undergo the above mentioned intermo-
lecular interactions to self-assemble in three steps (Figure 13) to
form nanofiber structures during gelation.
(
R,R)Cy-o solution would decrease to zero in about 10 minutes
after heating, indicating the formation of gels. If (S)BINAM is
added, the time of gel re-formation would decrease to 200
seconds. On the contrary, for (R)BINAM, that would enlarger to
Conclusion
A series of simple cyclohexane/benzene-derived hydrazides (22
samples) have been systematically prepared and studied to
uncover the inherent relationships between their chemical
structures and gelation properties. For non-conjugated and
asymmetric cyclohexane-derived hydrazide gelators, the key
drive forces are intermolecular hydrogen bonds and dipole-
dipole interactions, but those are intermolecular hydrogen
bonds and π–π stacking interactions for conjugated and
symmetric G-Ph-o trimers. Especially, π-functional G-Ph-o gels
Figure 12. Photographs and transmittance (T) of G-(R,R)Cy-o (3.0 wt% in
DMSO) upon adding equimolar (R)/(S)BINAM. Transmittance at 450 nm for
work line (1 cm path length quartz cuvettes).
Figure 13. Proposed molecular packing diagrams of the hydrazide gelators.
Chem Asian J. 2020, 15, 1–10
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exhibit unusual self-healing and multi-response fluorescence.
Moreover, G-(R,R)Cy-o gels can achieve visual and selective
recognition of (R)/(S)-BINAM through much different times of
gel re-formation. These results underline the importance of
supramolecular gels and provide unequivocal insights for the
design of LMWGs.
Jia, C. Li, Angew. Chem. 2019, 131, 3925–3929; Angew. Chem. Int. Ed.
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This work was supported by the National Natural Science
Foundation of China (no. 21871192) and Sichuan Science and
Technology Program (no. 2018JY0559). We would like to thank Dr.
Daibing Luo and Dr. Daichuan Ma from the Analytical & Testing
Center of Sichuan University for x-ray diffraction work and single
crystal analysis.
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Keywords: low-molecular-weight gelators · hydrazides · chiral
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Manuscript received: September 1, 2020
Accepted manuscript online: September 7, 2020
Version of record online: ■■■, ■■■■
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Chem Asian J. 2020, 15, 1–10
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FULL PAPER
The simpler, the better. Simple
hydrazide LMWGs (MW: 140–200)
have been designed and synthesized
for self-healing, multi-response
D. Wu, J. Song, L. Qu, W. Zhou, L.
Wang, Prof. X. Zhou, H. Xiang*
fluorescence, and visual and selective
1 – 10
2
À
chiral recognition and S indicator.
Cyclohexane-hydrazide monomers
self-assemble to form gels through
intermolecular hydrogen bonds and
dipole-dipole interactions, but
phthalhydrazide self-aggregates into
planar and circular trimers and then
forms gels through intermolecular
hydrogen bonds and π–π stacking
interactions.
Ultralow-Molecular-Weight Stimuli-
Responsive and Multifunctional
Supramolecular Gels Based on
Monomers and Trimers of Hydra-
zides
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