A dithienylethene-based rewritable hydrogelator

Article abstract of DOI:10.1002/chem.201304064

Dithienylethene photochromic switching units have been incorporated into a hydrogelating system based on a tripeptide motif. The resulting hybrid system provided both a photochromic response and the ability to gelate water under acidic and neutral conditi

Full text of DOI:10.1002/chem.201304064

DOI: 10.1002/chem.201304064
Full Paper
&
Hydrogels
A Dithienylethene-Based Rewritable Hydrogelator
Jochem T. van Herpt,[a] Marc C. A. Stuart,[a, b] Wesley R. Browne,*[a] and Ben L. Feringa*[a]
Abstract: Dithienylethene photochromic switching units
have been incorporated into a hydrogelating system based
on a tripeptide motif. The resulting hybrid system provided
both a photochromic response and the ability to gelate
water under acidic and neutral conditions. Fluorescence
spectroscopy shows that the dithienylethene units are in suf-
ficient proximity to each other to stack in gel fibers, with the
tripeptide unit determining solubility. TEM measurements
provided insight into the microscopic structure of the fibers
formed.
Introduction
their incorporation into a hydrophilic system is nontrivial. Sev-
eral approaches have been taken towards the production of
light-sensitive (hydro)gel systems. Molecular switches have
been added as dopants to hydrogels,[10] covalently attached to
known gelator motifs and incorporated into known gelators,
replacing parts of the original gelator.[11] Dithienylethene
switches do not show large changes in molecular geometry or
dipole moment upon switching, in contrast to azobenzenes
(Dmꢀ3D)[12] and spiropyrans (Dmꢀ12D),[13] and, therefore, the
effects of switching-dithienylethene dopants are relatively
minor. This feature has limited their use as dopants to highly
organized supramolecular structures, such as liquid crystals.[14]
Covalent attachment or incorporation of a dithienylethene
switch to a known gelator would result in a substantial in-
crease in hydrophobicity and, hence, would be detrimental to
the gelation properties. Therefore, we took the approach to re-
design a known hydrogelator that already bears an aromatic
group of similar size.[15]
The development of responsive hydrogels is of importance for
several fields of research, including controlled release of drugs,
cell growth, and adaptive materials. Light-induced transforma-
tions, in particular, are of great interest because they are non-
invasive, allow high temporal and spatial control, and have po-
tentially fast response times.[1] This is an important advantage
over other triggers, such as pH changes, chemical reactions,
and even temperature changes, which will have response
times considerably longer than the initial trigger event. Be-
cause of their tunable spectroscopic properties, high thermo-
stability, good closed to open ratios in the photostationary
state, high fatigue resistance, and the possibility to address
several distinct (chiral) states,[2] dithienylethene photoswitches
are primary candidates for incorporation as functional units
into hydrogels.[3] However, owing to their hydrophobicity, the
application of dithienylethene photoswitches in gels has, so
far, to the best of our knowledge, only been achieved in or-
ganic solvents. Nonetheless, dithienylethene switches have
seen successful application in systems that form aggregates in
water, including vesicles,[4] DNA complexes,[5,6] guest–host com-
plexes,[7] nanospheres,[8] and one-dimensional fiber-like aggre-
gates.[9] Herein, we report the incorporation of a dithienyle-
thene switch into a hydrogelator system and the characteriza-
tion of the function and structure of the resulting gels.
In recent years the ability of fluorenyl-functionalized oligo-
peptides, especially dipeptides, to gelate water, has received
attention for many hydrogel-related functionalities.[16] It has al-
ready been shown that for oligopeptide-based gelators, the
fluorenyl moiety can be replaced by other aromatic groups,
such as pyrenes,[17] naphthalenes,[18] and coumarines,[19] induc-
ing highly organized stacking and resulting in hydrogelation.[20]
Because the dithienyl switch can bare some similarity in size
and shape to the fluorenyl group, we focussed on introducing
a new maleimide-based dithienylethene switch (Figure 1).
Maleimide-based dithienyl switches were first reported by
Irie and Mohri in 1988.[3b,21] A glycine moiety provides the car-
boxylic acid functionality necessary to couple the switch to the
dipeptide, in effect, rendering the system a tripeptide. The
maleimide motif was used because it allows for functionaliza-
tion of the switch in a symmetric manner, instead of function-
alization at the thienyl groups, as would be the case for the
conventional cyclopentene-based dithienyl switches.[3] This ap-
proach results in the switch resembling the fluorene moiety
more closely, but more importantly, also increases the polarity
of the switch, thereby increasing the water solubility of the
Dithienylethene switches are amongst the more-hydropho-
bic photoswitchable moieties reported in the literature and
[a] Dr. J. T. van Herpt, Dr. M. C. A. Stuart, Prof. Dr. W. R. Browne,
Prof. Dr. B. L. Feringa
Stratingh Institute for Chemistry
University of Groningen
Nijenborgh 4, 9747 AG Groningen (NL)
E-mail: w.r.browne@rug.nl
b.l.feringa@rug.nl
[b] Dr. M. C. A. Stuart
Groningen Biomolecular Sciences and Biotechnology Institute
University of Groningen, 9747 AG Groningen (NL)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201304064.
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pound proved to be essential for the Suzuki coupling reaction
to proceed in good yields. Ester hydrolysis of compound 7 was
found to proceed best by using lithium-ion-coordinated
iodide-based dealkylation.[24] This yielded a free carboxylic acid
that was then used to couple the switch to a series of O-tert-
butyl-protected dipeptides, which were prepared in two steps
from commercially available protected amino acids. A final de-
protection step yielded the desired compounds. For synthetic
details and full characterization see the Supporting Informa-
tion.
Figure 1. Dithienylethene-functionalized tripeptide hydrogelators 1a–1h.
entire system. Because gels are a metastable phase and need
to be soluble enough to dissolve, but also insoluble enough to
aggregate anisotropically afterwards, the polarity of the mole-
cules was adapted using dipeptides attached to the switch.
Eight dipeptides were used, including apolar (phenylalanine),
neutral (glycine), and polar (lysine) amino acids, to tune the
polarity of the molecules.
The hydrogelation properties of 1a–h were tested (Table 1)
by using several different gelation methods, as described in
the Supporting Information. It was found that most com-
Table 1. Hydrogelation of 1a–h (deionized water).[a]
1a
1b
1c
1d
1e
1 f
1g
1h
cgc [mgmLÀ1
]
p
p
34
s
21
24
38
40
[a] p=precipitate, s=solution at 100 mgmLÀ1
.
Results and Discussion
Dithienylethene-based tripeptides 1a–h were synthesized ac-
cording to the route depicted in Scheme 1.
pounds formed gels in water with a critical gelation concentra-
tion (cgc) in the range of 20–40 mgmLÀ1. The incorporation of
lysine increased the cgc, and the dilysine compound (1d) re-
Maleic anhydride was brominated by using a modified litera-
ture procedure.[22] Imide formation with methyl 2-aminoacetate
was followed by transhalogenation, affording compound 4,
which was then coupled to compound 6 in a double Suzuki
coupling reaction. Compound 6 was obtained in two steps
from commercially available 2,5-dimethylthiophene.[23] Both
the use of a methoxy-protected carboxylic acid, rather than
a free carboxylic acid, and the iodination of the dibromo com-
mained in solution even at concentrations above 100 mgmLÀ1
.
Phenylalanine, as anticipated, has an opposite effect, exempli-
fied by the insolubility of the diphenyl compound (1a). The
lysine–phenylalanine compound (1b) was also found to be in-
soluble or formed precipitates, depending on the preparation
method applied. Differences in gelation behavior between
compounds with similar struc-
tures, in which only the order of
the amino acids is varied, for ex-
ample, 1b and 1c, have been re-
ported before.[15,25] This is an il-
lustrative example of the subtle
changes in interactions that can
enhance and disrupt anisotropic
aggregation.
Rheology
when the compound provided
gel. Frequency sweeps (as
was
performed
a
shown for 1h in Figure 2) re-
vealed that the gels possess
a plateau region over a wide fre-
quency range. In this region the
gels still show some frequency
dependence and G’/G’’<10 (G’=
storage modulus, G’’=loss mod-
ulus). This result classifies the
samples as weak gels or, more
precisely, viscoelastic systems
(Figure 2).[26,27] Owing to its syn-
thetic accesibility and good gela-
tion properties, 1h was used for
further analysis.
Scheme 1. Synthesis of 1a–1h (EDCI=1-ethyl-3-(3-dimethylaminopropyl)carbodiimide, DIEA=N,N-diisopropyle-
thylamine, HOBt=1-hydroxybenzotriazole, TFA=trifluoroacetic acid).
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The hydrolyzed compound was found to undergo reversible
photoswitching, with a ratio of open/closed switch, at the pho-
tostationary state (lexc =360 nm), of 62:38. Irradiation at
360 nm generated new absorbance maxima at 316 and
471 nm, whereas irradiation above 450 nm resulted in recovery
of the original spectrum (Figure 4).[28]
Figure 2. Frequency dependence of a hydrogel of 1h (40 mgmLÀ1), mea-
&
^
sured at. g=0.5% =G’(storage modulus), =G’’ (loss modulus).
The switching units were found to be stable at acidic and
neutral pH, but at pH>7 the disappearance of the original
switch and the appearance of a new species was observed by
fluorescence (Figure S3, see the Supporting Information) and
UV/Vis absorption spectroscopy (Figure 3a). FTIR and NMR
spectroscopy (see the Supporting Information) indicated that
under basic conditions (pH>7.5), hydrolysis of the imide func-
tionality occurred, yielding, for example, 12h (Figure 3b). This
base-catalyzed hydrolysis of the cyclic imide was also observed
in the synthesis of compound 8, in which attempts to hydro-
lyze the methyl ester under basic, aqueous conditions yielded
the corresponding hydrolyzed imide (amide). The newly
formed species were found to be more water soluble than the
original switch, as expected from their structure.
Figure 4. Change in UV/Vis absorbance of 12h (pH 10) upon irradiation
(360 nm).
Changes to the UV/Vis absorption spectrum of 1h below
the critical gelation concentration (0.10 mgmLÀ1), at pH 7,
were observed upon irradiation at 312 nm (Figure 5). New ab-
sorbance bands appeared at 360 and 530 nm, with a decrease
in absorbance at 290 nm.[29] An isosbestic point was main-
tained at 326 nm, indicating that side reactions, for example,
hydrolysis or degradation, did not occur. The ratio of open/
closed form at the photostationary state was determined to be
92:8 for solutions of the gelator.[30] The changes were fully re-
versed upon irradiation at longer wavelengths (>500 nm).
Figure 5. Ring closing of 1h in water (0.10 mgmLÀ1) at 312 nm, followed by
UV/Vis absorption spectroscopy.
Irradiation of 1h in the gel state at 312 nm resulted in
a change in color, turning the samples from bright yellow to
red. However, the macroscopic appearance and mechanical
properties of the gel itself remained unchanged (Figure 6).
When the gelators were dissolved in methanol and ring closing
was carried out by irradiation at 312 nm, a photostationary-
Figure 3. a) pH-dependent UV/Vis absorbance of 1h (0.04 mgmLÀ1) in
a 10 mm sodium phosphate buffer. Inset: Absorbance at 400 nm. b) Ring-
opening hydrolysis of the imide under basic aqueous conditions.
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Diffuse reflectance absorption spectroscopy was performed
on the gelators, both in the open and the photostationary
states, showing much higher closed to open ratios in the pho-
tostationary state at the glass–gel interface.[31] Comparison of
the ratio of diffuse reflectance values at the isosbestic point
and at 550 nm revealed that there is a qualitative increase in
the pss (pss=photostationary state) when the concentration
of the switch is increased (Figure S4, see the Supporting Infor-
mation). This result could be rationalized by the consideration
that at higher concentrations, higher amounts of gelator are
present in an aggregated state instead of in solution. In this
aggregated state, the switches experience a less-polar environ-
ment, which makes the photocyclization reaction more effi-
cient, as was shown in earlier reports on similar switches in or-
ganic solvents with increasing polarity.[32] The lower photocycli-
zation efficiency in polar solvents is attributed to a shift in the
equilibrium of the open switch from the planar conformer, rap-
idly undergoing photocyclization into the twisted conformer,
which cannot ring close. Aggregation of the gelators not only
provides a more apolar environment for the switches (see
below), but most likely also forces the switches into a more
planar conformation, as this would create flatter structures,
which are known to benefit the aggregation of dipeptide gela-
tors.[16]
Figure 6. Ring closing/opening in a hydrogel of 1h (44 mgmLÀ1) by using ir-
radiation at 312 nm and >500 nm, respectively.
state ratio of 52:48 (open/closed) was reached. Removal of
methanol in vacuo and attempts to form hydrogels by using
the obtained partially closed sample gave rise to suspensions,
indicating that there is an intrinsic difference in the aggregat-
ing properties of the open and closed states.
Given the low penetration depth of the irradiated light and
the clear differences in absorbance between the ring-open and
ring-closed states, it was possible to use the gel for writing by
using a mask, UV light, and visible light to address the desired
state. The gel could be “written” and erased over several cycles
without indications of instability (Figure 8).
UV/Vis absorption spectroscopy, above gelation concentra-
tions, was only possible at wavelengths longer than 500 nm. A
new absorption maximum at ꢀ530 nm appeared for all gela-
tors upon irradiation at 312 nm (Figure 7), which is similar to
the maximum observed at lower concentrations. Irradiation
with visible light (>500 nm) recovered the original spectrum,
without indication of fatigue, over several cycles (Figure 7,
inset).
Figure 8. Writing of initials “JvH” in a hydrogel by using visible light
(>500 nm) and a mask. See the Supporting Information for the complete
procedure.
All water-soluble hydrogelators examined provided similar
UV absorption spectra, as expected. The circular dichroism (CD)
spectra, however, showed significant variation between the ge-
lators. Although CD spectra could not be obtained from as-
formed gel samples because of their opacity, dilution of the
gels to approximately 2.6 mm[33] could be carried out without
dissolution of the gel fibers. In this way, CD spectra could be
obtained. As the dipeptide moieties show no absorption or CD
signal at wavelengths longer than 250 nm, signals at longer
wavelengths can be assigned to the dithienylethene switch
unit (Figure 9). The switch moieties by themselves are achiral
and do not exhibit a CD signal at concentrations at which ag-
gregation is not observed. Therefore, a CD signal at these
wavelengths is indicative of supramolecular aggregation of
Figure 7. Ring closing/opening of a hydrogel of 1h followed by UV/Vis ab-
sorption spectroscopy. Insert: Ring opening and closing followed at 525 nm
over several cycles.
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Figure 9. Gel of 1h diluted to 2.6 mm. —=original spectrum (1h open), -··-
=after irradiation at 312 nm, - - -=after subsequent irradiation with visible
light (>470 nm), ····=after heating and subsequent cooling of the 2.6 mm
solution.
a type that forces the switch preferentialy into one of the two
chiral conformations.
When the diluted gel samples were irradiated at 312 nm,
closure of the switch ocurred to some extent, manifested in
the appearance of an absorption band at 530 nm. The closing
of the switch was accompanied by a decrease in the CD signal,
indicating that the closed switch does not maintain its aggre-
gation state in the same manner as the open switch. Irradia-
tion with visible light subsequently caused the system to ring
open fully again, without further change to the CD spectrum.
Finally, heating and subsequent cooling of the sample caused
almost complete disappearance of the CD signal, owing to the
dissolution of the gelators from their aggregated state.
Most gels showed a negative Cotton effect around 410 nm
(Figure S6, see the Supporting Information), coinciding with
their UV/Vis absorption. As expected, 1d does not show a CD
signal at this wavelength because it does not form aggregates.
The lack of a CD signal for 1g is due to dissolution of the
fibers upon dilution.[34] This finding could be attributed to the
high polarity of 1g; however, 1h, containing the same amino
acids, but in a different order, remains in an aggregated state,
demonstrating the fine balance of interactions responsible for
the aggregation (see above). The lack of signal for 1e cannot
be explained by dissolution, as the samples still show turbidity
upon dilution. A possible explanation is that 1e stacks in an
achiral manner.
Figure 10. a) Fluorescence of 1d and 1h (lexc =265 nm) in ethanol at high
(4ꢁ10À3 mm) and low (4ꢁ10À5 mm) concentrations. b) Fluorescence spec-
trum of 1d and 1h in water (pH 6.5) at high (4ꢁ10À3 mm) and low
(4ꢁ10À5 mm) concentrations.
with substantial intermolecular interaction, such as stacking of
the dithienylethene units in the gel state.
Cryo-transmission electron microscopy (cryo-TEM) imaging
of the gels revealed that they do not possess a fibrilar or
ribbon-like structure, unlike the corresponding Fmoc- (9-fluore-
nylmethoxycarbonyl) functionalized gelators.[25] Instead, tube-
like structures, which appeared to be formed by curled sheets,
were observed (Figures 11 and 12).[37] This provides a rationali-
zation as to why relatively high gelator concentrations are nec-
essary to provide gelation. Formation of a three-dimensional
network with sufficient entanglement to provide rigidity is
most efficient with thin long fibers that have substantial supra-
molecular interaction with other fibers (junctions). The tube-
like structures found for the present gels, however, mainly
demonstrate intrastructural interactions, accounting for the
rolled-up-sheet shape. To have enough interaction between
these supramolecular structures, a high concentration is neces-
sary.
The fluorescence spectra of 1h, which forms a gel in water,
and 1d, which is completely soluble in water at high concen-
trations, were measured in both ethanol and water (see
Figure 10). In ethanol, switch 1h and 1d behave similarly,
showing fluorescence at 400 nm.[35] On increasing the concen-
tration, a new emission band appears at 560 nm. This emission
was red-shifted from the original emission and suggests the
formation of aggregates. In water, 1h and 1d behave very dif-
ferently. 1h shows a broad emission peak at 580 nm, even at
low concentrations, whereas the emission of 1d appears to be
completely quenched (Figure 10b).[36]
Interestingly, the structures observed for all gelators are sim-
ilar, showing that the aggregation mode is dictated by the
switch, with the dipeptide moiety functioning mainly as a solu-
bilizing group. The lack of signals assignable to hydrogen
bonding in the IR spectra (Figure S2, see the Supporting Infor-
The lack of emission from compound 1d in water is consis-
tent with quenching. By contrast, for 1h the 560 nm emission
is typical of emission from aggregates and, thus, is consistent
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Figure 11. Cryo-TEM images of hydrogels a, b) 1 f and c, d) 1h (scale bar
200 nm). For higher magnification images see Figure S8 in the Supporting
Information.
Figure 13. TEM images of a, b) 1c in a closed and c, d) 1c in an open state,
in water (scale bar 1 mm).
Figure 14. Histogram of inner and outer diameters of the tubes observed in
the cryo-TEM samples of 1h.
Figure 12. Schematic representation of curled sheets and their resulting
image when imaged from a TEM perspective.
As can be concluded from the histogram of the inner and
outer diameters of the tubes (Figure 14), the inner diameter
has a more defined size, suggesting that aggregation is proba-
bly starting from the inside of the tube and extending out-
wards, with the tubes growing to different outer diameters. It
appears that the decreased curvature required for the outer
layers of the tubes is energetically unfavorable because the
major part of the outer widths remains below 300 nm in diam-
eter.
mation), and the relative insensitivity towards changes in the
composition and concentration of the buffers used (see the
Supporting Information), support this conclusion.[38] It should
be noted, however, that both the inability of 1b to gelate
water and the structure dependence of the CD signal show
that the dipeptides also influence the stacking of the gelators
to some extent.
Aggregates of the closed (pss) compounds do not possess
this curvature, as becomes evident from the TEM images. As
a direct consequence, there is no limitation to the size of the
aggregates and, therefore, they can cover surfaces of multiple
square micrometers.
As with the mechanical properties, the microscopic structure
observed by cryo-TEM microscopy does not seem to undergo
significant change upon irradiation with UV light. Attempts to
obtain cryo-TEM images of the suspensions formed when the
closed switch was used in the gel formation were unsuccessful.
Regular TEM images did, however, reveal clear differences be-
tween the aggregates of the closed switch (Figure 13a and b)
and the open switch (imaged in the same manner for compari-
son, Figure 13c and d).[39]
Conclusion
We have shown that it is possible to incorporate dithienyle-
thene switches into hydrogels. The gelators form stable gels
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basic conditions to the corresponding amide, which forms sol-
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from large rod-like structures, formed by steeply curved
sheets.
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The high concentration of light-absorbing switch in the
sample prevents the light from penetrating the gel completely,
allowing the gelator to maintain its macroscopic and mechani-
cal properties, whereas the spectral properties can be altered
photochemically.
The gelating behavior of the compounds, as determined by
cryo-TEM microscopy, and CD and IR spectroscopy, suggests
that the switch moiety is mainly responsible for the aggrega-
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Further studies towards the nature of the aggregation, as
well as more efficient gelation, are ongoing.
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[28] It is unknown whether compound 12h undergoes ring-closing and
ring-opening, similar to compound 1h, or if the reversible switching is
due to E–Z isomerization of the double bond bridging the thienyl moi-
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[29] M. Ohsumi, M. Hazama, T. Fukaminato, M. Irie, Chem. Commun. 2008,
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[30] E. Fischer, J. Phys. Chem. 1967, 71, 3704–3706.
[31] Even though the concentration of gelator was increased by 400 times,
compared with the solution studies (Figure 5), the absorbance at
525 nm in the photostationary state only increased by a factor 30. This
can be explained by a very low penetration depth of the 312 nm light
at these concentrations, owing to high absorbance. Therefore, only the
molecules within the gel that are situated at the outer perimeter of the
sample, and are directly exposed to the light, are closed. Owing to the
lack of rapid diffusion in gel systems, over 99% of the gelator mole-
cules remain open.
Acknowledgements
The European Research Council (Advanced Investigator Grant
227897, JTvH BLF), NANONED (JTvH), the Netherlands organi-
zation for Scientific Research (NWO), the Royal Netherlands
Academy of Sciences (KNAW Academy Chair, BLF), and the
Ministry of Education Culture and Science (Gravitation program
024.001.035, WRB, BLF) are acknowledged for financial support.
Keywords: electron microscopy · gels · photochromism ·
switching
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Received: October 17, 2013
Published online on February 13, 2014
Chem. Eur. J. 2014, 20, 3077 – 3083
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3083
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