Light-Responsive Arylazopyrazole Gelators: From Organic to Aqueous Media and from Supramolecular to Dynamic Covalent Chemistry

Article abstract of DOI:10.1002/chem.201806042

Versatile photoresponsive gels based on tripodal low molecular weight gelators (LMWGs) are reported. A cyclohexane-1,3,5-tricarboxamide (CTA) core provides face-to-face hydrogen bonding and a planar conformation, inducing the self-assembly of supramolecular polymers. The CTA core was substituted with three arylazopyrazole (AAP) arms. AAP is a molecular photoswitch that isomerizes reversibly under alternating UV and green light irradiation. The E isomer of AAP is planar, favoring the self-assembly, whereas the Z isomer has a twisted structure, leading to a disassembly of the supramolecular polymers. By using tailor-made molecular design of the tripodal gelator, light-responsive organogels and hydrogels were obtained. Additionally, in the case of the hydrogels, AAP was coupled to the core through hydrazones, so that the hydrogelator and, hence, the photoresponsive hydrogel could also be assembled and disassembled by using dynamic covalent chemistry.

Full text of DOI:10.1002/chem.201806042

A Journal of
Accepted Article
Title: Light-responsive Arylazopyrazole Gelators: From Organic to
Aqueous Media and From Supramolecular to Dynamic Covalent
Chemistry
Authors: Chih-Wei Chu, Lucas Stricker, Thomas Kirse, Matthias
Hayduk, and Bart Jan Ravoo
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Light-responsive Arylazopyrazole Gelators: From Organic to
Aqueous Media and From Supramolecular to Dynamic Covalent
Chemistry
Chih-Wei Chu,‡ Lucas Stricker,‡ Thomas M. Kirse, Matthias Hayduk and Bart Jan Ravoo*[a]
Abstract: We report versatile photo-responsive gels based on
tripodal low molecular weight gelators (LMWGs). A cyclohexane-
1,3,5-tricarboxamide (CTA) core provides face-to-face hydrogen
bonding and a planar conformation, inducing the self-assembly of
supramolecular polymers. The CTA core was substituted with three
arylazopyrazole (AAP) arms. AAP is a molecular photoswitch that
isomerizes reversibly under alternating UV and green light irradiation.
The E-isomer of AAP is planar, favoring the self-assembly, whereas
the Z-isomer has a twisted structure, leading to a disassembly of the
supramolecular polymers. Using tailor-made molecular design of the
tripodal gelator, light-responsive organogels as well as hydrogels
were obtained. Additionally, in case of the hydrogels, AAP was
coupled to the core through hydrazones, so that the hydrogelator and
hence the photo-responsive hydrogel could also be assembled and
disassembled using dynamic covalent chemistry.
Materials stimulated by light are of particular interest, since light
is a non-invasive stimulus. Also, light irradiation can spatially and
temporally target a specific area or spot on a surface or inside a
material.[39,40] Photo-responsive gelators have gained increasing
attention in recent years.[41] LMWGs comprising coumarins,[42,43]
spiropyrans,[44]
stilbenes,[45,46]
azobenzenes[47,48]
and
dithienylethenes[49,50] were reported. Among them, azobenzenes
are without doubt the most investigated class of photoswitches.
However, it is known that the photo-isomerization of azobenzenes
results in an equilibrium, the photostationary state (PSS), which
is typically around 80% in both directions E to Z and Z to E.[51] This
phenomenon of incomplete photo-isomerization can be even
worse in solid phase materials due to the reduced dynamics
required for isomerization of the azobenzene.[52,53] To address this
limitation, arylazopyrazoles (AAPs) were recently introduced as
improved light-responsive molecular switches.[54–56] In further
studies, we and others investigated the properties of AAPs[57] as
molecular switches and developed several supramolecular
systems ranging from colloidal host-guest complexes,[58,59]
foams,[60] adhesives[61] and DNA complexes.[62,63] Recently, also a
light-responsive peptide hydrogel featuring host-guest
interactions between AAP peptides and cyclodextrin vesicles was
introduced by our group.[64]
Introduction
Low molecular weight gelators (LMWGs) represent a viable
alternative for the development of soft materials.[1–4] They can
self-assemble via non-covalent interactions, such as hydrogen
bonding,[5,6] host-guest (e.g. crown ether-cation),[7] metal-
ligand,[8–11] and π-π interactions,[12,13] forming extended and
entangled supramolecular polymers.[14–16] Depending on the
solvents used in the gels, they can be classified as organogels
(organic solvent) or hydrogels (aqueous media). Gelation of
LMWGs typically occurs from the highest soluble state, e.g. high
temperature and ionized form. Owing to the dynamic properties of
LMWGs, their formation can be triggered by pH,[17,18] solvent,[19]
temperature,[20] enzyme,[21,22] light[23,24] or addition of salt.[25,26] As
a consequence, the solubility of the gelator is gradually reduced.
Nucleation occurs and the growth of nanofibrils is initiated. Thus,
self-assembly of LMWGs is most often a kinetic trapping
A particularly versatile class of supramolecular polymers
assembles from tripodal “core-arm” small molecules. In most
cases, these supramolecular polymers are based on
cyclohexane[65,66] or benzene[67,68] cores. Benzene-1,3,5-
tricarboxamide (BTA) derivatives established by Meijer and co-
workers are distinguished examples of tripodal monomers with
benzene as core and variable functionalities connected to the
core by amide bonds.[68] The intermolecular hydrogen bonding
provided by the amides initiates the self-assembly. By appending
different arms to the BTA core, diverse soft materials and
applications, including hydrogels,[69,70] protein recognition,[71,72]
imaging[67] and catalysis[73] were developed. Similar to BTA,
process.[27,28] Recent applications of LMWGs include tissue
cyclohexane-1,3,5-tricarboxamide
(CTA)
also
affords
[31–33]
engineering,[29,30] optoelectronic device,
actuators.[37,38]
sensors[34–36] and
intermolecular face-to-face hydrogen bonding and in addition,
displays conformational flexibility. The flexible cyclohexane core
permits the optimal stacking of the LMWGs and forms an
extended supramolecular polymer. More interestingly, if
hydrazones are applied to connect the arms and the core,
dynamic and catalytic controlled in situ formation of hydrogels can
be performed.[74–76] Under this circumstance, a hydrazide core
and aldehyde arms are combined.[77] A detailed study of acid-
catalyzed dynamic covalent gels was investigated by van Esch,
Eelkema and co-workers with negatively charged liposomes[78]
and patterned sulfonic acid surfaces.[79] The introduction of
[a]
C.-W. Chu, L. Stricker, T. M. Kirse, M. Hayduk and B. J. Ravoo
Organic Chemistry Institute and Center for Soft Nanoscience (SoN)
Westfälische Wilhelms-Universität Münster
Corrensstrasse 40, 48149 Münster, Germany
E-mail: b.j.ravoo@uni-muenster.de
Supporting information for this article is given via a link at the end of
the document.
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dynamic covalent bonds such as hydrazones can not only Results and Discussion
facilitate the synthesis, but also broaden the possible application
of dynamic molecular systems and materials, by simply mixing
hydrazide core and aldehyde arms with diverse functions.[80–83]
Zhou et al. reported photo-responsive organogels based on a
CTA core and arms comprising azobenzene units.[65] In their study,
the light-induced sol-gel transition and thermal stability of the
organogelator were explored. However, a rheological study of the
organogel was not reported. Also, the overlapping UV/vis
absorption of E- and Z-azobenzene potentially limits the
reversibility of the sol-gel transition. Very recently, Venkataramani
and co-workers reported tripodal AAP derivatives comprising a
BTA core and demonstrated light-controlled rewritable imaging in
the solid state.[67] From these preceding studies, we conclude that
the combination of a CTA core and AAP arms is highly promising
to develop versatile light-responsive LMWGs for both organic
solvents and aqueous solutions.
Molecular design and synthesis
light-responsive organogelator (G1, Figure 1A) was
A
synthesized using cis,cis-cyclohexane-1,3,5-tricarboxylic acid as
a core allowing optimal stacking. An amine functionalized AAP
was selected as arm. In addition, the AAP was equipped with a
long alkyl chain at the end of each arm. This design offers
excellent gelation properties by three kinds of interactions: (1) The
amide link between core and arms forms hydrogen bonds and
allows planar orientation of the AAP arms. (2) The AAP unit
contributes π-π interactions of the aromatic rings and, more
interestingly, provides light-responsive control of the stacking
properties of the LMWG. (3) The aliphatic chain at the end of the
arms creates an apolar exterior of the LMWG, yielding good
solubility in organic solvents and additional dense packing by van
der Waals interactions.
Herein, we report a family of light-responsive tripodal LMWGs,
in which the molecular design was pursued by the core-arm
approach. The general design and the functions of each
molecular component are described in Scheme 1. First of all, the
CTA core allows optimal stacking by intermolecular hydrogen
bonding. Secondly, the AAP moieties in the arms stabilize the
self-assembly via π-π interactions and provide photo-response.
Thirdly, the functionalities at the end of the arms increase
solubility in the desired solvent and potentially give rise to higher-
ordered self-assembly, such as bundles, enhancing gelation.
Finally, the connections between the core and the arms are
designed to be either permanent (amide) or dynamic covalent
(hydrazone). The dynamic covalent approach can not only
facilitate the in situ synthesis of hydrogelator, but also be used as
a chemical stimulus for gel formation and disassembly. Light-
response, rheology and dynamic properties of tripodal AAP based
LMWGs are explored in this article.
To enable biomimetic applications, hydrogels rather than
organogels are desirable and hence the enhancement of water
solubility is required. One straightforward approach is to maintain
the molecular design of the organogelator G1, while attaching the
amino acid alanine at the end of the arms (G2, Figure 1B). The
terminal amino acid not only improves water solubility, but also
presents additional hydrogen bonding between supramolecular
polymers. Furthermore, using a closely similar molecular design,
it was possible to introduce the dynamic covalent properties of
hydrazones in the hydrogelator. For this purpose,
a
cyclohexanetrishydrazide (CTH) core and an aldehyde
terminated AAP peptide (AAP-CHO) were prepared (Figure 1C).
The hydrazone can be assembled by hydrazide and aldehyde
under acidic (pH 4-6) or catalytic (e.g. aniline) conditions.[74] By
exploiting the hydrolysis of glucono-δ-lactone (GdL) to generate
acidic conditions, the hydrazone-linked hydrogelator (G3, Figure
1C) should potentially form in situ. Experimental details of the
preparation of the precursors and gelators can be found in the
supporting information.
Photo-isomerization of gelators
The photo-isomerization of gelators G1-G3 was investigated by
UV/vis spectroscopy. It should be noted that the UV/vis
Figure 1. Molecular structure of amide-linked (A) organogelator (G1) and (B)
hydrogelator (G2) and the in situ formation of (C) hydrazone-linked
hydrogelator (G3) from CTH and AAP-CHO triggered by the hydrolysis of GdL.
Scheme 1. Molecular design of tripodal cyclohexane-1,3,5-tricarboxamide
(CTA) based gelators and the function of each molecular component to form
supramolecular polymers and gels.
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measurements were carried out far below the critical gelation
concentrations (see below). Thus, the spectrum of a solution of
G1 (6.7 μM, as prepared) in toluene was measured (Figure 2A).
Subsequently, the solution was irradiated with UV light
(λ = 365 nm). Upon irradiation the typical changes in the
absorbance spectra for the E-Z isomerization of AAPs could be
observed.[54] The strong absorbance at 340 nm corresponding to
the π-π* transition is reduced and blue-shifted, whereas the
absorbance around 440 nm (n-π* transition) increases in intensity
and shows a characteristic red-shift (Figure 2A). Upon irradiation
with green light (λ = 520 nm), the original spectrum was observed
with slightly higher intensity at 340 nm, indicating that in gelator
G1 as prepared by chemical synthesis a small percentage of Z-
isomer is present (Figure 2A). The photo-isomerization of G1 is
reversible over at least four cycles without any sign of loss of
efficiency (Figure 2B).
Quantification of the photoisomerization was carried out
determining the PSS by NMR spectroscopy (Figure 2C). Due to
the intermolecular stacking, G1 showed rather broad signals in
the proton NMR. Therefore, a one-armed reference molecule G1-
ref was used for NMR experiments. (Structure and analysis of G1-
ref are provided in the supporting information). By integration of
the proton signals in the aromatic region, the PSS for both the
photo-isomerization from E to Z and from Z to E could be
determined. It was found that in the solution of G1-ref as prepared
the isomer ratio was 91:9 (E:Z), which changed drastically to 2:98
(E:Z) after irradiation with UV light (λ = 365 nm). After irradiation
with green light (λ = 520 nm), the E-isomer was reobtained with
94:6 (E:Z). Again, this value is higher compared to the percentage
of E-isomer in the initial spectrum, which is in a good agreement
to the results obtained by UV/vis spectroscopy. In summary, G1
showed excellent light-response identical to the AAP derivatives
reported previously by our group.[54]
Similar characteristics were found for the hydrogelators via the
same approach using a combination of UV/vis and NMR
measurements. Both G2 and G3 exhibited typical AAP UV/vis-
spectra and showed fully reversible photoswitchability for at least
five cycles (Figure S1 and S2). Besides, PSS measurements
confirmed the excellent photoisomerization efficiency of both
hydrogelators. For G2, it showed PSS of 93% for E to Z and 94%
for Z to E (Figure S3). In the case of hydrazone based gelator G3,
cyclohexane monohydrazide (CMH) was applied instaed of CTH
to give G3-ref to avoid broad signals due to stacking in D2O. After
mixing CMH and AAP-CHO in presence of GdL overnight, PSS of
90% for E to Z and 89% for Z to E were observed (Figure S4).
The spectroscopic data and PSS of gelators G1-G3 are
summarized in Table 1.
Table 1. Absorbance maxima (λmax) and photostationary state PSS of
gelators G1-G3.
π-π* λmax (nm)
n-π* λmax (nm)
PSSE-Z
(%)
PSSZ-E
(%)
E
Z
E
Z
isomer
isomer
isomer
isomer
G1
336
303
427
437
98[a]
94[a]
G2
G3
347
346
314
321
419
421
437
434
93[b]
90[c]
94[b]
89[c]
[a] NMR using G1-ref in THF-d8. [b] NMR in DMSO-d6. [c] NMR using G3-
ref in D2O.
Photoresponsive organogels
Gelation tests for organogelator G1 were performed using a range
of organic solvents and increasing concentrations of G1 to
determine the critical gelation concentration (CGC) (Table 2). A
heating-cooling sequence was applied to obtain homogenous
organogels. In each case, a known amount of G1 was added to 1
mL of solvent and heated until it was fully dissolved. Afterwards,
the solution was allowed to cool down to room temperature and
the gel formation was checked by a reverse-vial test. If a solution
was obtained, the amount of G1 was increased stepwise until a
gel was achieved. The results displayed in Table 2 document the
formation of a transparent orange organogel (G) in 9 out of 17
tested solvents with rather low CGC (< 1 wt%). In most other
solvents, the gelator did not show any significant solubility at room
temperature and precipitated (P). An exception to this is
chloroform, in which even at rather high amount of G1 (20 mg/mL)
a clear solution was obtained (S). The formation of a chloroform
gel might be possible at higher concentration of G1 but was not
further investigated in this study. Table 2 shows that G1 gelates
rather non-polar and aprotic solvents with exception of n-alkanes.
Figure 2. Photoisomerization of G1 confirmed by (A) UV/vis spectroscopy
(6.7 μM in toluene). (B) Reversible E-Z-photoisomerization for four cycles. (C)
Quantification of the E-Z-photoisomerization under UV and visible light
irradiation. PSS measurements were carried out using G1-ref in THF-d8
(500 μM).
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Table 2. Solvent tests for gelation of G1 and CGC in each solvent (G: gel, P: precipitate, S: solution).
Solvent
State
CGC (mg/mL)
Solvent
State
CGC (mg/mL)
n-Decane
n-Hexane
P
P
―
―
7
THF
CHCl3
DCM
G
S
G
P
P
P
P
P
14
―
4
1-Bromohexane
Cyclohexane
Me-cyclohexane
Xylene
G
G
G
G
G
G
G
6
EtOAc
Acetone
DMF
―
―
―
―
―
14
8
Toluene
8
DMSO
ACN
Bromobenzene
Dioxane
6
7
twisted Z-isomer.53-55 The resulting twisted configuration of Z-G1
prevents intermolecular stacking, resulting in a clear solution.
Upon green light irradiation, the planar E-isomer is re-obtained,
the intermolecular interactions are restored, and an organogel is
reformed that passes the reverse-vial test. As
a control
experiment a sample of the organogel was irradiated for 2.5 h with
infrared light (980 nm), which does not induce isomerization of the
AAP unit. As expected, in this experiment no gel-to-sol transition
is observed. Thus, it can be concluded that the observed
macroscopic changes are caused by the photo-isomerization of
the AAP units and cannot be attributed to e.g. heating upon
irradiation (Figure S10). The microstructures of the obtained gels
were visualized by scanning electron microscopy (SEM) of a cast-
dried organogel sample. In Figure 3B and 3C, it shows the
formation of fibers in the scale of micrometers that entangle to a
three-dimensional network typical for LMWG.
Upon close inspection of Figure 3A it can be observed that in
comparison to the organogel of E-G1, the liquid state of Z-G1 has
a more reddish orange color that is due to the increased and red-
shifted absorbance of the n-π*-transition. Thus, the initials “WWU”
of our university were written on the surface of the gel by UV
irradiation through a positive photomask (Figure 3D). This
observation indicates that these organogels can be applied for
light-responsive patterning using a photomask, similar to what
has been recently described by Venkataramani and co-workers
for tripodal AAP derivatives in the solid state.64
Rheological measurements confirmed the viscoelasticity and
the photo-response of the organogel prepared in toluene. Two
different concentrations of the organogel, 10 and 15 mg/mL, were
tested by frequency sweep measurements (Figure 4A). Both
organogels displayed one order of magnitude higher storage
modulus (G’) than loss modulus (G”), indicating the formation of
viscoelastic organogels that are fully stable for the entire
frequency sweep. Also, a concentration dependency to the
stiffness was observed, which a 15 mg/mL sample exhibited
Figure 3. (A) Schematic representation of the sol-gel transition of G1 in under
UV and green light irradiation in toluene. SEM images of 10 mg/mL G1 in
toluene at (B) 5 k and (C) 25 k magnification (scale bars: (B) 2 μm and (C)
400 nm). (D) “WWU” was written in an organogel of 10 mg/mL G1 in toluene
by UV light using a positive photomask.
No gelation is observed in polar solvents and solvents that can
function as hydrogen bond donors.
Next, the light-response of the organogel were investigated in
more detail. The organogels showed a distinct light-induced sol-
gel transition (Figure 3A). For example, a heated solution of the
planar E-isomer of G1 in toluene (10 mg/mL) forms an organogel
upon heating then cooling due to intermolecular stacking of G1 as
a result of hydrogen bonding, π-π- and van der Waals interactions.
This gel fully liquefies upon irradiation with UV light due to the
photo-isomerization of AAP from the planar E-isomer to the
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A
B
C
After After After After After After
UV Vis UV Vis UV Vis
Figure 4. Rheological studies of G1 in toluene. (A) Frequency sweep for increasing gelator concentrations (10 and 15 mg/mL). (B) Oscillatory rheological
measurements for light-responsive sol-gel transition of 15 mg/mL G1. (C) Reversible sol-gel transition for three cycles.
higher moduli, both G’ and G”, to the 10 mg/mL one. Then, the
photo-response of the organogel was further investigated (Figure
4B). In the oscillatory rheological measurements, the original gel
was first recorded. After irradiation with UV light for 5 min, another
measurement was carried out. It showed a drastically reduced
stiffness with almost three orders of magnitude. Moreover,
randomly fluctuating points were observed for both moduli and
even sometimes G” was over G’, suggesting that a liquid-like
material was obtained. As described above, we attribute this
observation to disruption of intermolecular stacking by the twisted
configuration of the Z-isomer of G1. After irradiation with green
light for 5 min, over two orders of magnitude of G’ was retrieved
and again, stable moduli with G’ over G” were observed. This
observation is in accordance with the restoration of intermolecular
stacking of the planar E-isomer of G1. Since the initial G’ is
retrieved, it can be excluded that the initial drop of G’ can be
attributed to mechanical disruption or heating of the gel during the
rheological measurements. Two more irradiation cycles were
carried out and a similar trend was observed, with high and stable
moduli for the E-isomer and low and unstable moduli for the Z-
isomer (Figure 4C).
peptide-based hydrogelators, tuning the pH is a popular and
useful trigger to make hydrogels. In particular, making use of the
slow hydrolysis of GdL to generate an acidic environment, a more
homogenous hydrogel can be obtained.[84] Thus, G2 (2 or 5
mg/mL) was first dissolved in a “highly soluble state” (fully
deprotonated at pH ~ 8). Upon the addition of GdL (5 or 10
mg/mL), the hydrogelator was gradually transferred into a “poorly
soluble state” (partially protonated at pH ~ 5), since the apparent
pKa of alanine is around 5. Indeed, after standing in ambient
conditions overnight, hydrogels were obtained that pass the
reverse-vial test. Again, irradiation of this sample with infrared
light (980 nm) does not lead to any softening of the hydrogel
(Figure S10) The microstructure of the self-supporting hydrogels
was confirmed by SEM (Figure S5).
The viscoelastic properties of the hydrogels were investigated
by rheological measurements (Figure 5). Firstly, frequency-sweep
oscillation experiments were performed. Similar to organogels of
G1, almost one order of magnitude higher G’ over G” were found
for hydrogels of G2, indicating the success of hydrogel
preparation. In general, the stiffness of the gel is proportional to
both the gelator concentration and the amount of GdL. The higher
the concentration of hydrogelator, the more and longer nanofibers
are obtained. This leads to an increasingly cross-linked network
and results in a stiffer gel. The amount of GdL determines the
acidity of the gel solution. This is important for C-terminal peptide
amphiphiles, since they show higher stiffness in more acidic
Photoresponsive hydrogels
Since the formation of hydrogels from LMWGs is a kinetically
controlled process, a smooth and homogeneous transition from
the soluble and insoluble state of the gelator is critical. For
Figure 5. Rheological studies of G2. (A) Frequency sweep for increasing G2 and GdL concentrations. (B) Light-induced stiffness modulation under UV and
visible light irradiation (2 mg/mL G2 and 10 mg/mL GdL). (C) Reversible storage modulus response to alternating UV and vis light irradiation for three cycles.
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condition.[85] As shown in Figure 5A, the highest G’ was found in
5 mg/mL G2 and 10 mg/mL GdL (solid triangles), while the lowest
storage modulus was measured in the case of 2 mg/mL G2 and
5 mg/mL GdL (solid squares). By comparing the same
hydrogelator concentration (2 mg/mL) with different amount of
GdL, the hydrogel had almost 6-fold higher stiffness by increasing
the GdL concentration from 5 mg/mL to 10 mg/mL. This
observation is due to the higher degree of protonation of G2
obtained in a more concentrated GdL solution. Thus, more and
longer nanofibers form, which make a stiffer hydrogel.
Next, the light-responsive modulation of stiffness for hydrogels
prepared from G2 was examined (Figure 5B and C). The hydrogel
formed with 2 mg/mL G2 and 10 mg/mL GdL was prepared one
day before. The measurement started without any irradiation and
the storage modulus showed a plateau value. Upon UV irradiation
(λ = 365 nm), G’ dropped by over 50% (Figure 5B). We attribute
this to the photo-isomerization of the hydrogelator. Similar to G1,
E-G2 is expected to provide a stiffer material due to the face-to-
face hydrogen bonding and the π-π interactions between the
gelators. On the contrary, the Z-G2 loses the additional π-π
interactions between the AAPs due to their twisted structure.53-55
Thus, a diminished elastic modulus (G’) was observed. During the
visible light irradiation (λ = 520 nm), the storage modulus was
restored. This can be explained by the recovery of the planar
structure of AAP in E-G2, which allows the restoration of π-π
interactions (Figure 5B). Interestingly, during UV irradiation, the G’
reached a plateau faster, whereas upon visible light irradiation, a
plateau was reached rather slowly. A reasonable explanation for
this observation is that upon UV irradiation, the hydrogel softens,
and the oscillation enhances this process. In contrast, when the
material stiffens during the green light irradiation, the oscillation
disturbs the recovery of the fibers and the network. Nevertheless,
Figure 7. (A) In situ formation of G3 from CTH (2.5 mM) and AAP-CHO
(15 mM) triggered by GdL (5 mg/mL). (B) Control experiment in absence of
CTH (AAP-CHO: 15 mM and GdL: 5 mg/mL). (C) SEM image of 5 mM G3
(scale bar 200 nm).
the photo-isomerization of G2 induced a modulation of stiffness
that can be repeated for at least three cycles (Figure 5C). It is
noteworthy that during irradiation with either UV or green light the
loss modulus remained almost constant. Additional photo-
rheology experiments for different concentrations of samples (2
and 5 mg/mL G2 with 5 and 10 mg/mL GdL) can be found in the
supporting information (Figure S6).
Figure 6. (A) GdL triggered formation of hydrazane G3-ref from CMH and AAP-CHO was carried out using proton NMR in D2O (CMH: 500 μM, AAP-CHO:
500 μM and GdL: 1 mg/mL). (B) Formation of hydrazone in time based on the signal at 8.27 ppm.
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of magnitude higher G’ over G” were found, indicating the success
of hydrogel preparation. Again, the stiffness of the gel is
proportional to both gelator concentration and the amount of GdL.
The concentration of G3 refers to the concentration of CTH since
an excess of AAP-CHO was applied. The highest G’ was
observed for 5 mM G3 with 10 mg/mL GdL and the lowest for
2.5 mM G3 with 5 mg/mL GdL (Figure S8). Then, oscillatory
rheological measurements were performed to examine the light-
response of the stiffness of G3. The hydrazone-linked hydrogel
formed with 5 mM CTH, 30 mM AAP-CHO and 5 mg/mL GdL was
prepared 24 h in advance. Similar to G1 and G2, E-G3 is
expected to give a stiffer gel, whereas Z-G3 should give a softer
gel. The measurements were carried out in the same fashion by
recording firstly a non-irradiated sample and subsequently
irradiating with UV and visible light in an alternating fashion for
three cycles. Indeed, a light-responsive modulation of stiffness
was observed similar to G2, i.e., higher G’ for E-G3 and lower G’
for Z-G3. The same explanation can be applied to these findings:
planar configuration and additional π-π interactions for the E-
isomer; twisted configuration and loss of π-π interactions for Z-
isomer. After the first cycle, around 30% loss of storage modulus
was found upon UV irradiation and in total three cycles of stiffness
modulation were demonstrated (Figure 8A and B). We conclude
that the introduction of amides (G2) or hydrazones (G3) to
conjugate the core to the AAP arms does not significantly affect
the formation of hydrogels nor their photo-response.
Figure 8. (A) Light-induced stiffness modulation of a hydrogel of 2.5 mM G3
triggered by 5 mg/mL GdL under UV and visible light. (B) Storage modulus
response to light.
Photoresponsive and dynamic covalent hydrogels
Hydrogelator G3 containing dynamic covalent hydrazone bonds
was synthesized in situ by employing GdL in the same fashion as
describes above for G2, since the hydrazone formation is favored
at pH 4-6, which is close to the apparent pKa of alanine. To ensure
that a tripodal gelator will form, excess arms to core (6:1) were
employed in this study.[77] AAP-CHO (15 mM) and CTH (2.5 mM)
were dissolved at pH ~ 8 and subsequently, GdL was added.
After gentle mixing, the solution was left overnight. A self-
supporting hydrogel was obtained and
a
schematic
representation of the self-assembly is depicted in Figure 6A.
Since the arms can be also considered as peptide amphiphiles
(with AAP as hydrophobic tail and alanine as hydrophilic head
group), a control experiment was carried out to ensure that the
obtained hydrogels were achieved by the tripodal core-arm based
hydrogelator G3. Indeed, when CTH was absent, a turbid solution
with yellow precipitates of AAP-CHO was observed (Figure 6B).
This finding supports our claim that the hydrogel is formed by the
tripodal core-arm based hydrogelator G3. The self-assembled
structures of the hydrogels were examined by SEM. Again, typical
fibrous and cross-linked structures were found (Figure 6C).
To gain more information about the formation of G3, a kinetic
study of the hydrazone formation was carried out. As a
The formation of G3 involves a dynamic reaction between
hydrazide core and aldehyde arms. This means that when other
hydrazides (or aldehydes) are added, they can be used to tune
the self-assembly of the gelator and the gel formation. Here,
competitive experiments involving two different hydrazides were
conducted. In the first assay,
a neutral component, 4-
hydroxylbutyric acid hydrazide was applied (Figure 9A). The
addition of neutral hydrazide (NHy) suppresses the formation of
G3 (prepared from 2.5 mM CTH, 15 mM AAP-CHO and 5 mg/mL
GdL), since the probability for AAP-CHO to form a gelator (with
CTH) or a non-gelator (with NHy) are equal. Thus, after one day,
a self-supporting hydrogel was present in absence of the
competitor, whereas a liquid remained in presence of NHy (5 or
25 mM). These findings are in agreement with our previous
suggestions that the three-armed structure is required for the
hydrogel formation. Interestingly, if the concentration of NHy is
rather low (5 mM), a hydrogel is obtained after two days
(Figure 9B). On the contrary, if the concentration of NHy is high
enough (25 mM), the sample remains liquid-like even after five
days (Figure 9C). This can be explained from the competing
reactions of AAP-CHO with NHy and CTH and the consideration
that the self-assembly of G3 is thermodynamically favoured. Only
if sufficient three-armed gelator G3 is formed, the self-assembly
will take place and as a result, a gel will form. When NHy is less
concentrated, the formation of G3 is initially suppressed, but
eventually wins out since the gel acts as a thermodynamic sink
for G3. Also, a limited number of NHy arms will probably not
disturb the self-assembly process. Instead, if NHy is more
concentrated, the formation of G3 is suppressed to such an extent
that no nucleation points for the formation of self-assembled
nanofibrils are available, and no gelation occurs even after 5 days.
proof-of-principle, the formation of hydrazone between
a
hydrazide and an AAP-CHO in presence of GdL was tracked by
proton NMR (Figure 7A). Similar to the determination of PSS for
G1, in this study, a cyclohexane monohydrazide (CMH) was
applied to avoid broadening proton signals due to intermolecular
stacking of G3 in aqueous media. The decreasing signal of the
aldehyde proton of AAP-CHO (at ~ 10 ppm) as well as the
increasing signal of the hydrazone proton (red circle, 8.27 ppm)
reveals the hydrazone formation. Besides, the large upfield shift
of the ortho-protons at the benzyl ring of AAP supports our claim
for the formation of a hydrazone. The more downfield shifted
proton (blue rhombus, 8.12 ppm) is attributed to the presence of
a strong electron withdrawing formyl group at the ortho position,
instead, the more upfield shifted proton (purple pentagon,
7.86 ppm) is due to a more electron donating functional group,
hydrazone. Integrating the hydrazone signal (red circle,
8.27 ppm) in 1 h intervals offers a quantitative measure of the
hydrazone formation (Figure 7 and Figure S7). Experimental
details are provided in the Experimental Section.
The viscoelastic properties of the hydrogels of G3 were also
investigated by rheology. Similar to G2, frequency-sweep
oscillation experiments were performed. Again, almost one order
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Figure 9. (A) Schematic representation of the dynamic properties of G3 in presence of a neutral hydrazide. Macroscopic pictures of G3 (2.5 mM CTH, 15 mM
AAP-CHO and 5 mg/mL GdL) in presence of (B) 5 mM and (C) 25 mM of NHy.
In a final experiment, if a cationic hydrazide (CatHy, 5 mM),
betaine hydrazide, was applied, gel formation was completely
suppressed even at low concentration (Figure S9). This
observation can be explained by considering the formation of
cationic arms when CatHy reacts with CTH. The repulsion of the
cationic arms disturbs the self-assembly of G3 and hence it
persists in a liquid state.
demonstrate that molecular design of gelators with embedded
photoswitches can lead to versatile light-responsive and dynamic
soft materials. The stiffness of these materials can be modulated
by light, which is especially important when considering
biomedical applications such as tissue engineering.[86]
Experimental Section
Organogels. A certain amount of G1 was added to 1 mL of organic solvent
and heated until it was fully dissolved. Afterwards, the solution was allowed
to cool down to rt and the gel formation was checked by a reverse-vial test.
If a solution was obtained, the amount of G1 was increased until a gel was
achieved. The results displayed in Table 1 showed the formation of a
transparent orange organogel (G) in 9 out of 17 tested organic solvents
with different CGCs. Appearance of results was presented by gel (G),
precipitate (P) and solution (S).
Conclusions
In this study, we demonstrated that a family of light-responsive
LMWGs can form gels in organic and aqueous media, and that
the hydrogelator can have permanent (amide) or dynamic
covalent (hydrazone) linkage. Tripodal LMWGs G1-G3,
comprising a CTA core and three AAP arms, were successfully
synthesized
and
fully
characterized.
The
excellent
photoisomerization efficiency of all gelators were confirmed by
UV/vis and NMR spectroscopy. Organogelator G1 forms
organogels in nine organic solvents with rather low CGC (<
Hydrogels. Stock solutions of 10, 5 and 2 mg/mL of G2 were prepared in
ddH2O and NaOH (1 M) was used to adjust the pH until the hydrogelators
were fully dissolved (pH ~ 9). In a glass vial containing 2, 5, 10, 20 mg/mL
of GdL, 200 μL of different concentrations of the stock solution was added.
After no crystals were observed, the solution was left overnight for gelation.
Samples were defined as gels if a reverse vial test was passed. Critical
gelation concentration was found in 2 mg/mL and the minimum required
GdL was 5 mg/mL in this concentration.
1 wt%) and exhibits
a reversible sol-gel transition under
alternating UV and visible light irradiation. Homogeneous
hydrogels prepared from G2 and G3 were obtained by tuning the
pH through the hydrolysis of GdL. UV and visible irradiation was
utilized to modulate the stiffness of the hydrogels. In both cases,
the E-conformation of the hydrogelators represents the more
stable and planar structure, which shows a higher storage
modulus. On the contrary, the Z-isomer of both G2 and G3 disrupt
the planarity and the additional π-π interactions contributed by
AAPs, resulting in a lower storage modulus and a softer gel.
Finally, the dynamic formation of G3 and its influence on the
hydrogel properties was explored. Dynamically linked G3 was
obtained in situ from a trihydrazide core (CTH) and AAP aldehyde
arms (AAP-CHO). Neutral and cationic hydrazides were added to
reverse the gel formation by hydrazone exchange. Our results
Dynamic covalent hydrogels. Stock solutions of 5 and 10 mM of CTH
and the correspondingly 30 and 60 mM of AAP-CHO were prepared. In a
glass vial containing 2, 5, 10, 20 mg/mL of GdL, the core (CTH) and arm
solutions (AAP-CHO) were mixed with a 1:1 fashion. After a clear solution
was obtained, it was then left untouched for gelation overnight. Critical
gelation concentration was found in presence of 2.5 mM of CTH and
minimum required GdL was 5 mg/mL in this concentration.
Rheology
and
photoswitching
experiments.
Rheological
measurements were performed by using a shear rheometer (MCR 102,
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Chih-Wei Chu,‡ Lucas Stricker,‡
Thomas M. Kirse, Matthias Hayduk
and Bart Jan Ravoo*
Page No. – Page No.
Light-responsive Arylazopyrazole
Gelators: From Organic to Aqueous
Media and From Supramolecular to
Dynamic Covalent Chemistry
Programmable gelators: Tripodal core-arm gelators can be tailor-made by covalent
and dynamic covalent linkages to form organo- and hydrogels. By incorporating
arylazopyrazole as photoswiches, the stiffness of the gels can be modulated by light.
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