Covalently Trapped Triarylamine-Based Supramolecular Polymers

Article abstract of DOI:10.1002/chem.201902404

C₃-Symmetric triarylamine trisamides (TATAs), decorated with three norbornene end groups, undergo supramolecular polymerization and further gelation by π–π stacking and hydrogen bonding of their TATA cores. By using subsequent ring-opening metathesis polymerization, these physical gels are permanently crosslinked into chemical gels. Detailed comparisons of the supramolecular stacks in solution, in the physical gel, and in the chemical gel states, are performed by optical spectroscopies, electronic spectroscopies, atomic force microscopy, electronic paramagnetic resonance spectroscopy, X-ray scattering, electronic transport measurements, and rheology. The results presented here clearly evidence that the core structure of the functional supramolecular polymers can be precisely retained during the covalent capture whereas the mechanical properties of the gels are concomitantly improved, with an increase of their storage modulus by two orders of magnitude.

Full text of DOI:10.1002/chem.201902404

A Journal of
Accepted Article
Title: Covalently trapped triarylamine-based supramolecular polymers
Authors: Ting Liang, Dominique Collin, Melodie Galerne, Gad Fuks,
Andreas Vargas Jentzsch, Mounir Maaloum, Alain Carvalho,
Nicolas Giuseppone, and Emilie Moulin
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To be cited as: Chem. Eur. J. 10.1002/chem.201902404
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Covalently trapped triarylamine-based supramolecular polymers
Ting Liang,[a] Dominique Collin,[b] Melodie Galerne,[a] Gad Fuks,[a] Andreas Vargas Jentzsch,[a] Mounir
Maaloum,[a] Alain Carvalho,[b] Nicolas Giuseppone*[a] and Emilie Moulin*[a]
Abstract: C3-symmetric triarylamine trisamides (TATAs), decorated
with three norbornene end groups, undergo supramolecular
polymerization and further gelation by π-π stacking and hydrogen-
bonding of their TATA cores. By using subsequent ring opening
metathesis polymerization, these physical gels are permanently
crosslinked into chemical gels. Detailed comparisons of the
supramolecular stacks in solution, in the physical gel, and in the
chemical gel states are performed by optical spectroscopies,
electronic spectroscopies, atomic force microscopy, electronic
paramagnetic resonance spectroscopy, X-ray scattering, electronic
transport measurements, and rheology. The results presented here
clearly evidence that the core structure of the functional
supramolecular polymers can be precisely retained during the
covalent capture whereas the mechanical properties of the gels are
concomitantly improved, with an increase of their storage modulus
by two orders of magnitude.
described and reviewed[12] in the literature to cross-link low
molecular weight gelators, including photopolymerization of
methacrylate[13] and diacetylene,[14] Cu-catalysed 1,3-dipolar
cycloaddition,[15] triethoxysilane hydrolysis-condensation,[16] and
cross metathesis.[17–19] Another very attractive chemical tool is
the ring opening metathesis polymerization reaction (ROMP) of
norbornene derivatives.[20] This was used in particular to
permanently cross-link conducting supramolecular assemblies
made of stacked hexabenzocoronene (HBC) in solution.[21,22]
ROMP was also successfully used for the crosslinking of other
stacked
structures
in
solution
such
as
perylene,
oligothiophene,[23] and C3-symmetric trisubstituted benzene.[24]
However, it should be noted that, to the best of our knowledge,
ROMP has not yet been employed to freeze supramolecular
structures in the gel state. Here, we show that ROMP can be an
effective tool to generate covalent cross-links in a physical gel
while retaining
properties.
a number of its structural and functional
Introduction
Results and Discussion
Supramolecular polymers based on triarylamine motifs represent
a new and attractive class of functional materials.[1] Their local
structure rests on the collinear arrangement of their nitrogen
center which is maintained by --stacking and supplementary
hydrogen bond interactions (implemented for instance by the
addition of amide functions at the periphery of the triarylamine
core). These primary structures usually show cooperative
mechanisms of polymerization, and their columnar primary
stacks can be further self-assembled in various superstructures
including bundles of fibers and gels.[2–8] Because of their
supramolecular packing, and because triarylamines are prone to
be (photo)oxidized, these fibers can be doped to access hole
transporting nanowires and conducting materials with peculiar
electronic and optical properties.[5,9–11] Still, the supramolecular
polymerization of triarylamines usually leads to soft nano-objects
and materials which would benefit of being mechanically
strengthened. In the present study, we have explored the
possibility to chemically and permanently cross-link
supramolecular polymers of triarylamines in order to reinforce
their mechanical properties and, at the same time, to probe if
their main intrinsic structures and functionalities would be
retained. Several chemical methods have already been
The following experimental study is based on the C3-symmetric
molecule 1 which has i) a typical triarylamine trisamide (TATA)
core to undergo supramolecular polymerization, and ii) three
norbornene end groups on the side chains to trap the resulting
self-assembly by covalent capture (Scheme 1).
=
Scheme 1. Synthetic pathway used to access C3-symmetric triarylamine
trisamide (TATA)
1 decorated with norbornene end groups and its
polymerization using ROMP with Grubbs III catalyst.
The synthesis of
1 was performed in four steps from
[a]
[b]
Dr. T. Liang, M. Galerne, Dr. G. Fuks, Dr. A. Vargas Jentzsch, Prof.
Dr. M. Maaloum, Prof. Dr. N. Giuseppone, Dr. E. Moulin
SAMS Research Group, Institut Charles Sadron, CNRS - UPR 22,
University of Strasbourg, 23 rue du Loess, BP 84047, 67034
Strasbourg Cedex 2, France
E-mail: giuseppone@unistra.fr; emoulin@unistra.fr
Dr. D. Collin, A. Carvalho
Institut Charles Sadron, CNRS - UPR 22, 23 rue du Loess, BP
84047, 67034 Strasbourg Cedex 2, France
commercially available compounds as follows. Compound 1a
was first obtained following a typical aromatic nucleophilic
substitution,[5] and subsequently reduced using tin chloride in a
1:1 mixture of acetonitrile and ethanol at reflux. The
corresponding tris-amine was then acetylated with 6-
bromohexanoyl chloride to give compound 1b in reasonable
yields. Finally, nucleophilic substitution of the terminal bromine
with 5-norbonene-2-carboxylic acid provided the expected
TATA 1 as a mixture of endo/exo (2:1) compounds (see
Supporting information for this article is given via a link at the end of
the document.
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experimental section and supporting information (SI) for further
details).
We then studied the gelation properties of compound 1 in
toluene,
a solvent which strengthens the supramolecular
The spectroscopic properties of
1
were first studied in
polymerization of TATA compared to chlorinated solvents, and
which facilitates the mechanical studies of the corresponding
gels. After a heating/cooling process, compound 1 forms self-
supporting gels in toluene for concentrations higher than ~8 mM
(~7.9 mg/mL) (Figure S2). TEM and AFM imaging experiments
chloroform solutions at a concentration of 0.1 mM. Upon light
irradiation (halogen lamp, 10 W.cm-2), we observed a series of
typical features (Figure 1a-c) associated with the stacking of
TATA molecules which include: a) disappearance of the 1H NMR
resonance signals corresponding to the triarylamine core[25]
(Figure S1); b) appearance of a NIR absorption band at ~1130
nm (for shorter times of irradiation and corresponding to the
presence of intermolecular through-space radical cation charge-
transfer between stacked triarylamine cores), followed by the
appearance of an absorption band at ~830 nm (for longer times
of irradiation and corresponding to more localized
triarylammonium radical cations);[5] c) appearance of a typical
three-line pattern on the EPR spectra after irradiation confirming
the presence of an unpaired electron on the triarylamine core;[5]
and d) disappearance of the fluorescence around 500 nm after
short irradiation times which indicates the presence of
supramolecular polarons quenching the aggregation induced
emission of the stacked TATA molecules, as already described
previously in the literature.[5] We also probed the conductivity
(I/V curve) of photo-doped compound 1 at a concentration of 17
mM. By using commercially available ITO electrodes separated
by a length gap of 4 µm, the observed ohmic behavior was
found very similar to the one recorded for a reference TATA
molecule with simple C12 alkyl chains and no norbornenes
(Figure 1d).[5]
from
1 mM toluene solutions (below the onset gelation
concentration) also demonstrate the formation of micrometric
long fibers with sections presenting a slight ribbon-like character,
of about ~100 nm in width and ~20 nm in thickness (Figures 2a-
b and S5a-b). The internal organization along the main axis of
these fibers was shown to be made of single columns of TATA
with a diameter of ~2.0 nm (Figure S5c-d). Infrared spectroscopy
was then used to understand the local organization of these self-
assembled structures at the same concentration of 1 mM (Figure
S3a). The presence of well-defined bands at ~3290 and 1650
cm-1 are characteristic of hydrogen-bonded N-H and C=O
stretching bands respectively. In addition, their high symmetry
suggests a three-fold helical arrangement of the molecules with
intermolecular hydrogen bonds between adjacent TATA cores,
as previously described for such compounds in the
literature.[5,8,26,27] Above the onset gelation concentration, the
morphology of the individual fibers in the physical gel (15 mM in
toluene; hereafter referred to as sample A) was found to be
similar to those probed from solutions, as observed for instance
by SEM imaging of the corresponding aerogel (Figure 2c).
a)
b)
a)
b)
0.8
0.6
0.4
0.2
0.0
3
0
-3
5 µm
2 µm
3450 3470 3490 3510 3530 3550 3570
400
600
800
1000 1200 1400
Field (G)
50
Wavelength (nm)
c)
d)
c)
d)
0.8
0.6
0.4
0.2
0.0
25
0
-25
-50
-0.10
1 µm
450
500
550
600
650
700
-0.05
0.00
0.05
0.10
Wavelength (nm)
Voltage (V)
Figure 2. a-b) TEM (a) and AFM (b) images of a 1 mM solution of compound 1
in toluene; c) SEM image of the aerogel obtained from the cooling to room
temperature of a 15 mM solution of compound 1 in hot toluene; d) Variation of
the storage G’ and loss G’’ moduli as a function of temperature for a physical
gel made of compound 1 in pure toluene (sample A, [1] = 15 mM).
Figure 1. a) UV-Vis-NIR spectra of a 10-4 M solution of compound 1 in
chloroform upon sequential light irradiation (from 0 to 60 minutes), the arrows
indicate the evolution of the absorption bands at 832 and 1129 nm with
increasing times of irradiation; b) EPR spectra of a 10-4 M solution of
compound 1 in chloroform upon sequential light irradiation (from 0 to 60
minutes), the arrow indicates the evolution of the EPR spectra with increasing
times of irradiation; c) Evolution of the fluorescence spectra corresponding to a
10-4 M solution of compound 1 in chloroform upon sequential light irradiation
(from 0 to 5 minutes), the arrow indicates the evolution of the emission with
increasing times of irradiation; d) I-V curves recorded after a light irradiation of
50 seconds of TATA-C12 (dashed line)[5] and compound 1 (plain line) at a
concentration of 17 mM in pure tetrachloroethane.
We then examined the mechanical properties of that gel
(Sample A) in pure toluene (Figure 2d). At room temperature,
the variation of the complex shear modulus G* with frequency
shows that the storage modulus G’ is always greater than the
loss modulus G’’ and that both moduli remain quasi-constant
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over the whole range of frequencies studied (0.05 to 10 Hz,
Figure S4a). This G* behavior, of the viscoelastic type, is
characteristic of a physical gel in the hydrodynamic regime. At
20 °C and for a frequency of 1 Hz, the shear modulus rigidity of
the gel was found  260 Pa. To determine the sol to gel
temperature transition (Tgel), the variation of the complex shear
modulus was followed at a fixed frequency of 1 Hz from 95 °C to
20 °C and with an applied cooling rate of -1 K.min-1. From high
to low temperature, G* values change progressively in
agreement with a transition from a liquid to a gel, and Tgel (as
defined by the intercept between G’ and G’’) was estimated to
occur at around 82 °C. However, our instrumental set-up does
not allow a heating of the sample above Tgel for long enough
time in order to remove any possible aggregates in the liquid
phase because of partial solvent evaporation. This constraint
was overcome by decreasing the sol-gel transition temperature
using a mixture of solvents. In particular, we examined the
possibility of adding a chlorinated co-solvent because it would
concomitantly allow for a light-induced oxidation process of the
TATA molecules. We found that compound 1 at 15 mM in a
solvent mixture made of toluene (90%) and tetrachloroethane
(10%) (named hereafter as sample B) shows a Tgel around 63 °C
(Figure 3a). Interestingly, this change in solvent almost did not
affect the shear modulus rigidity of the gel (G’ = 302 Pa for a
frequency of 1 Hz and at 20 °C). Similarly to physical gels
formed in pure toluene, SEM imaging of the aerogel made of
sample B suggested the presence of dense networks of ribbon-
like bundled fibers with lengths of several microns and widths of
about 100 nm (Figure 3b-c). Considering that, upon light
irradiation, chlorinated solvents can be used as an electron
acceptor to trigger the oxidation of the fibers made of TATA
molecules,[5] we subsequently studied the influence of this
photo-doping process on the spectroscopic, conducting, and
rheological properties of sample B. We then compared the
results
toluene/tetrachloroethane mixture, with what can be obtained in
pure chlorinated solvents. Spectroscopically, continuous
obtained
in
the
gel
state
using
the
a
increase of a NIR absorption band at ~1106 nm was observed
during the first 20 minutes of irradiation, and then remained
constant for longer irradiation times (Figure 3d). Meanwhile, the
absorption band at 818 nm starts to increase for irradiation times
longer than 20 minutes but without reaching high intensities.
This observation indicates that mainly delocalized radicals in the
form of polarons within stacks of TATA units were formed. This
interpretation is reinforced by EPR experiments in which the
hyperfine splitting made of a three line pattern is clearly
superimposed with a central one-line pattern characteristic of the
presence of fully delocalized radicals with no energetic barrier to
charge transfer (Figure 3e as compared with Figure 1b).[5,28] We
also probed the conductivity of the physical gel upon light
irradiation using commercially available ITO electrodes with a
gap of 4 m (Figure 3f). While sample B shows low conductivity
in the neutral state (i.e. before light irradiation), photo-oxidation
induces a transition to a conducting state with a characteristic
ohmic behavior. Finally, from a mechanical point of view, light
irradiation of sample B induces a slight decrease of Tgel from
63 °C to 61 °C for irradiation times longer than 15 min (see
insert in Figure 3a).
a)
b)
c)
d)
0.8
0.6
0.4
0.2
0.0
400
600
800
1000 1200 1400
2 µm
500 nm
Wavelength (nm)
e)
f)
g)
h)
0.4
0.2
4
2
0.0
0
-0.2
-0.4
-2
-4
3430 3450 3470 3490 3510 3530 3550
2 µm
500 nm
-0.10
-0.05
0.00
0.05
0.10
Field (G)
Voltage (V)
Figure 3. a) Variation with temperature of the real (G’) and imaginary (G”) parts of the complex shear modulus at a frequency of 1 Hz for the physical gel made of
compound 1 in a toluene/10% tetrachloroethane mixture (sample B, [1] = 15 mM) without light irradiation (black), and with 15 minutes (red) and 60 minutes
(green) light irradiation. The applied cooling rate was -1K/min. The insert shows the shift of Tgel induced by light irradiation; b-c) SEM images of the aerogel made
of compound 1 in a toluene/10% tetrachloroethane mixture (15 mM, sample B) before light irradiation; d) UV-Vis-NIR spectra of the physical gel made of
compound 1 in a toluene/10% tetrachloroethane mixture (15 mM, sample B) upon sequential light irradiation (from 0 to 60 minutes), the arrows indicate the
evolution of the absorption bands at 818 and 1106 nm upon irradiation; e) EPR spectra of the physical gel made of compound 1 in a toluene/10%
tetrachloroethane mixture (15 mM, sample B) upon sequential light irradiation (from 0 to 60 minutes), the arrow indicates the evolution of the EPR spectra with
increasing irradiation times; f) I-V curves recorded before irradiation (black) and after 15 (red) and 60 (blue) minutes light irradiation for a physical gel made of
compound 1 in a toluene/10% tetrachloroethane mixture (15 mM, sample B); g-h) SEM images of the aerogel made of compound 1 in a toluene/10%
tetrachloroethane mixture (15 mM, sample B) after 60 minutes light irradiation.
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Following the same trend, we also measured a slight decrease
of both G’ and G’’ with light irradiation (for a frequency of 1 Hz
and at 20 °C, G’ = 226 and 152 Pa for 15 min and 60 min of
irradiation time respectively, Figures 3a and S4b). As shown by
the phase measurements (see insert Figure S4b), the decrease
in G’ and G’’ for the gel does not induce a change in the G’/G’’
ratio, which is found constant with frequency and roughly
independent of the irradiation time. As a result, all the
measurements in this frequency range belong to the
hydrodynamic regime, regardless of light irradiation. The relative
integrity of the supramolecular polymers and gel structures
therefrom upon photodoping were confirmed by SEM imaging of
an aerogel made of sample B after 60 minutes of light irradiation
(Figure 3g-h).
irradiated chemical gels, respectively). Furthermore, the final
rheological properties were similar whether the light irradiation
occurred in the hot solution (before physical gelation) or directly
in the physical gel state before chemical cross-linking by ROMP
(Figure 4b). Therefore, the comparison of G’ for the physical and
the chemical gels, with and without light irradiation, suggests
that reticulation by chemical cross-links leads to the formation of
thermodynamically stable polymer networks with an internal
structure reminiscent of their physical counterparts. This
structural interpretation by mechanical means was reinforced by
cryo-SEM imaging of the non-irradiated chemical gel and by
SEM imaging of xerogels arising from the covalently cross-linked
irradiated gels. These micrographs indeed suggest very similar
morphologies compared to the physical gels (Figure 4c-f). In
addition, Medium and Wide Angle X-Ray scattering (MAXS and
WAXS) experiments performed on these xerogels demonstrated
the presence of a smectic-type packing as already observed for
TATA-based supramolecular polymers,[5] which indicates that
their main stacked organization is preserved in the cross-linked
materials (Figure 4g). Analysis of the X-ray data indicates a pitch
distance of ~28.4 Å and a N-N distance of 4.66 Å. These
distances are very similar to the values recorded for the xerogel
in toluene (27.8 Å and 4.72 Å, respectively) and to the initial
values reported for supramolecular columns of TATA molecules
With this series of reference experiments in hands, we studied
the possibility to freeze the supramolecular dynamic of the
physical gel of sample B by permanent chemical cross-links in
order to reinforce its mechanical properties. Reticulation of the
physical network was performed via ROMP using Grubbs III
catalyst. We initially probed the efficiency of the ROMP starting
from solutions of compound 1. At a concentration of 2 mM,
either in toluene or dichloromethane, and using 0.3 equivalents
of Grubbs III catalyst (i.e. 0.1 equivalent per norbornene
unit),[24,29] only insoluble materials were recovered, thus
suggesting full polymerization. By lowering the concentration to
0.05 mM in dichloromethane, we managed to obtain a solid
(29.1
Å
and 4.85 Å, respectively).[5] Altogether, these
experiments indicate unambiguously that ROMP represents an
efficient strategy to covalently capture triarylamine-based
supramolecular polymers in order to further enhance their
mechanical robustness while preserving their local structure.
We finally probed how the spectroscopic and conducting
properties were affected by chemical cross-links. We observed
that all chemical gels display increasing absorbance at ~820 and
1095 nm upon light irradiation; albeit with different intensities
and kinetics depending on the way the corresponding physical
gel was prepared (Figure 4h). On one hand, for the chemical gel
prepared from a non-irradiated physical gel (Figure 4h, black
data), we observed that light irradiation induces mainly an
increase of the absorption band at ~820 nm, which corresponds
to the presence of localized triarylammonium radicals. On the
other hand, for the chemical gels prepared from light-irradiated
physical gels (Figure 4h, red and blue data), light irradiation
leads to a much larger increase of the absorption bands at 1095
nm, showing that these gels favor intermolecular through-space
charge-transfer processes between stacked triarylamine cores.
These observations also suggest that triarylamine units slightly
change their ability to improve the radical delocalization upon
light irradiation of the physical gel by improving their
supramolecular stacks, a dynamic property which is apparently
lost if the light irradiation is performed only after permanent
cross-linking. The optical responses of the chemical gels can
thus be modulated by the sample preparation and show a very
interesting memory effect that combine dynamic structures and
functions. We also evaluated the conductivity of the chemical
gels upon light irradiation using a home-made ITO cell (Figure
S8). However, the currents measured for all chemical gels were
4 orders of magnitude lower than those measured for the
physical gels.
1
which proved to be soluble in THF. While H NMR and infra-red
experiments confirmed the disappearance of the cyclo-olefinic
double bond (Figure S6), GPC experiments suggested the
formation of pentamers (see SI section 6). In the gel state, the
polymerization was performed on either non-irradiated or light-
irradiated sample B, at room temperature, and using 0.3
equivalents of Grubbs III catalyst. After an overnight reaction,
methanol was used extensively to remove the catalyst and
possibly remaining monomers from the cross-linked samples.
The first evidence of the efficiency of the ROMP in the gel state
was obtained by just manipulating the chemically cross-linked
samples which appeared much more robust mechanically than
its physical counterparts. The ROMP efficiency was further
verified by a) infra-red spectroscopy which shows the
disappearance of the band at 1335 cm-1 typical of the cyclo-
olefinic double bond of norbornene (Figure S3b),[21] and b)
thermogravimetric analysis (TGA) which clearly highlights the
increased thermal robustness of the cross-linked samples (with
a decomposition starting at 320°C) compared to the monomer
(decomposition at 170°C) (Figure S7). The mechanical
robustness of the cross-linked samples was then quantified by
rheological experiments using a piezorheometer (Figure 4a-
b).[30] In contrast to the previous measurements performed on
physical gels, cross-linked gels were molded before performing
shear measurements (see experimental section). For all the
chemical gels studied, the storage modulus was found constant
and the phase value close to zero, as expected for chemical
gels in the low frequency regime.[31] Compared to the physical
gel, storage moduli G’ for non-irradiated and light-irradiated
chemical gels are increased by ~2 orders of magnitude (G’ =
22.5, 10.4 and 5.0 kPa for non-irradiated, 15 min and 60 min-
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a)
b)
c)
d)
2 µm
500 nm
1
1.2
1.0
0.8
0.6
0.4
e)
f)
g)
h)
0.1
0.01
0.1
1 µm
1 µm
1
400
600
800
1000 1200 1400
q (Å-1)
Wavelength (nm)
Figure 4. a) Variation of the storage modulus as a function of frequency for chemical gels made of 1 in a toluene/10% tetrachloroethane mixture: without light
irradiation (black), with 15 minutes light irradiation (red), and with 60 minutes light irradiation (blue) in the gel phase. Experiments were recorded at 20°C. As it is
the case or the physical gels, the phase of the chemical gel (in the insert) is roughly constant and independent of the light exposure time; b) Evolution of the
storage modulus G’ with irradiation times for physical gels (blue square), chemical gels irradiated in the gel phase before chemical cross-linking (red square), and
chemical gels formed from an irradiated hot solution phase before cooling to the physical gel phase and subsequent chemical cross-linking (black dots).
Experiments were recorded at 20°C. The dashed line is only a guide for the eyes. c-d) Cryo-SEM images of a chemical gel made of compound 1 in a toluene/10%
tetrachloroethane mixture starting from a physical gel (15 mM, sample B) without irradiation; e-f) SEM images of the chemically cross-linked xerogel made of
compound 1 in a toluene/10% tetrachloroethane mixture starting from a physical gel (15 mM, sample B) after 15 minutes (e) and 60 minutes (f) of light irradiation;
g) MAXS-WAXS spectra of the chemically cross-linked aerogels made of compound 1 in a toluene/10% tetrachloroethane mixture starting from a physical gel (15
mM, sample B) without light irradiation (black), and with 15 minutes (red) and 60 minutes (blue) light irradiation in the gel phase; h) UV-Vis-NIR spectra of the
chemical gel made of compound 1 in a toluene/10% tetrachloroethane mixture starting from a physical gel (15 mM, sample B) without light irradiation (black), and
with 15 minutes (red) and 60 minutes (blue) of light irradiation in the gel phase. The dotted line spectra were recorded before further light irradiation of the
chemical gels, whereas the plain line ones were recorded after further 60 minutes light irradiation of the chemical gels.
A tentative explanation for this difference rests on the fact that
physical gels can easily recombine and rearrange their
supramolecular structure in order to better contact their
conducting core with the ITO surface. Such a dynamic being lost
in the chemical gels, the charge injection and extraction in the
active layer become more difficult to achieve. In addition, the
crosslinking of the peripheral aliphatic chains also increases the
resistance of the insulating layer around the TATA core, at both
the interface with the electrodes and between the fibers, thus
decreasing the overall conductivity of the material.
adaptable to other functional supramolecular systems. The
physical properties of these systems are systematically studied
in solution, in the physical gel, and in the chemical gel.
Interestingly, it is shown that, by choosing the proper
experimental conditions, the material can retain its main optical
and local conducting properties – as for instance indicated by
the presence of the NIR polaronic absorption band – when the
phase transition occurs, and when the physical gel is
subsequently and permanently reticulated. However, and
although the internal conducting behaviour of the
supramolecular fibers was retained after ROMP polymerization
(as proved by the optical responses), the efficiency of the long-
range conduction in the ITO device is significantly reduced for
the chemical gel. This result can be explained by the loss of
constitutional dynamics at the interface with the electrode and
the higher resistance of the hydrophobic shell around the
conducting TATA core after reticulation which limits the charge
transport in the injection, extraction, and hopping processes.
This result is however significant because it illustrates the
general asset of supramolecular polymers – as for instance well
illustrated to provide self-healing properties – by showing how
their dynamics can be instrumental in improving some of their
functional properties, including within devices. In addition, it
Conclusions
The present study shows the possibility to trap by covalent
capture low molecular weight gelators involving triarylamine-
based supramolecular polymers. To do so, ROMP is
demonstrated as a particularly efficient method to freeze the fine
supramolecular structure of the stacked TATA molecules and to
significantly improve the mechanical properties of the
corresponding chemical gel (i.e. with a typical gain of 2 orders of
magnitude of the storage modulus). It should be noted that the
use of ROMP directly in the physical gel state is novel in the
literature, and this work highlights that the mild conditions used
in the crosslinking procedure is of broad interest as potentially
challenges
our
imagination
to
enhance
mechanical
performances of soft functional materials. In that direction, a
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Scanasyst with spring constant of 0.4 N/m). During AFM imaging, the
force was reduced in order to avoid dragging of molecules by the tip. All
analyses of the images were conducted in the integrated software. TEM
imaging was performed using a CM12 Philips microscope equipped with
a MVIII (Soft Imaging System) CCD camera. Samples were analyzed in
Bright Field Mode with a LaB6 cathode and 120 kV tension. Image
treatments were performed by using analySIS (Soft Imaging System)
software. For the sample preparation, a piece of gel was dissolved in the
swelling solvent then a 5 µL drop was cast on a grid that was placed on a
Whatman paper. The grid was then left to dry under air before analysis.
SEM imaging was performed on a FEG-SEM Hitachi SU8010 at room
temperature at 1 kV. For xerogel and aerogel imaging, a small piece of
gel was deposited on a silicon wafer covered with double sided tape.
Before analysis, a piece of conductive tape was deposited on the wafer
to ensure electrical contact between the sample and the wafer.
CryoSEM: To observe chemical gels in their native state, a small piece of
gel was rapidly plunged into a nitrogen slush in the cryo preparation
chamber of the Quorum PT 3010 machine. The frozen sample was then
transferred under vacuum into the chamber attached to the microscope
and fractured with a razor blade. Etching at -90 °C was performed
followed by the deposition of a thin platinium layer. The sample is then
transferred in the FEG-cryoSEM (Hitachi SU8010) and observed at -
150 °C at 5 kV. Conductivity experiments were performed directly in
commercially available transparent ITO electrode cells, with an active
ITO area of 10x10mm, a 4 µm cell gap and no alignment layer
(S100A040uNOPI LC cells from Instec, Inc.) for solutions and physical
gel measurements. For experiments on chemically cross-linked gels, a
home-made ITO cell was built from two glass substrates coated with ITO
(Figure S8). Experiments were performed using a Keithley 2400 source
meter with sweep cycles carried out below the electro-oxidation potential
and ascend first.
supramolecular polymerization directly performed within the
device, and followed by covalent capture, emerges as a possible
way of future investigations.
Experimental Section
General methods. All reactions were performed under an atmosphere of
argon unless otherwise indicated. All reagents and solvents were
purchased at the highest commercial quality and used without further
purification unless otherwise noted. Dry solvents were obtained using a
double column SolvTech purification system. Yields refer to purified
spectroscopically (1H NMR) homogeneous materials. Thin Layer
Chromatographies were performed with TLC silica on aluminium foils
(Silica Gel/UV254, Aldrich). In most cases, irradiation using a Bioblock
VL-4C UV-Lamp (6 W, 254 nm and/or 365 nm) as well as
phosphomolybdic acid and Cerium ammonium molybdate stainings were
used for visualization. ESI-MS measurements were carried out on a
Waters SQD apparatus. 1H NMR spectra were recorded on a Bruker
Avance 400 spectrometer at 400 MHz and 13C spectra at 100 MHz in
CDCl3 or CD3CN at 25°C. The spectra were internally referenced to the
residual proton solvent signal. For 1H NMR assignments, the chemical
shifts are given in ppm. Coupling constants J are listed in Hz. The
following notation is used for the 1H NMR spectral splitting patterns:
singlet (s), doublet (d), triplet (t), multiplet (m), large (l). UV-Vis-NIR
spectra were recorded on a Cary 5000 spectrophotometer using a quartz
cuvette with 1.0 cm optical path length (for solutions), 1.0 mm optical
path length (for physical gels) or a special cell for integration sphere
measurements (chemical gels). Infrared spectra were recorded on a
Fourier transform infrared spectrometer VERTEX 70 (Bruker) by drop-
casting a solution or putting a small piece of dry gel sample on an ATR
diamond probe. Fluorescence emission spectra were recorded on a
FluoroMax-4 (Horiba) spectrofluorometer with the following settings: slit
width = 5 nm, increment = 1 nm, integration time = 0.1 s in a quartz
cuvette with a 1.0 cm optical path length. EPR experiments were
recorded on a continuous-wave EPR X-band spectrometer (EMXplus
from BrukerBiospin) equipped with a high sensitivity resonator (4119HS-
W1, BrukerBiospin GmbH, Germany). Solutions samples were
introduced into glass capillaries (Hirschm annrincap, 100 μL) and flame
sealed prior introduction into 4 mm outer diameter quartz tubes. All gel
samples were introduced directly into 4 mm outer diameter quartz tubes
(Wilmad). Conventional field-swept spectra were performed during short
intervals between stepwise illumination periods. The principal
experimental parameters values were: a modulation amplitude of 1 G, a
microwave power of ca. 2 mW, a time constant of ca. 40 ms, 120 ms
conversion time and 200 G were swept in 120 s per scan and one scan
was recorded after each illumination step. All experiments were
Synthetic protocols. Compound 1b: A solution of compound 1a (1.00 g,
2.87 mmol) and tin dichloride (7.22 g, 32.00 mmol) in a mixture of
acetonitrile (40 mL) and ethanol (32 mL) was stirred overnight at 80 oC.
After cooling down to room temperature, the solution was diluted with
ethyl acetate (200 mL). The mixture was washed with NaHCO3sat (2 ×
200 mL) and brine (150 mL). Then, the organic phase was dried over
Na2SO4 and concentrated under reduced pressure to provide N,N-bis(4-
aminophenyl)benzene-1,4-diamine, which was pure enough to be used
as such in the next step. A solution of this crude compound and
triethylamine (1.18 mL, 2.87 mmol) in dichloromethane (60 mL) was
added dropwise to a cooled solution of 6-bromohexanoyl chloride (1.30
mL, 8.58 mmol) in dichloromethane (12 mL) at 0 oC under an argon
atmosphere. The reaction mixture was heated up slowly to room
temperature for 12 hours. Afterwards, solvents were removed under
reduced pressure and the resulting crude residue was washed with
dichloromethane several times (100 mL) to afford compound 1b (920 mg,
39 %) as a white solid. 1H NMR (CD3OD:d8-toluene 5:3, 400 MHz, 25°C):
δ = 7.54 (d, 3J = 8.8 Hz, 6H), 6.97 (d, 3J = 8.8 Hz, 6H), 3.23 (t, 3J = 6.8
Hz, 6H), 2.30 (t, 3J = 7.4 Hz, 6H), 1.78-1.70 (m, 6H), 1.68-1.61 (m, 6H),
1.45-1.37 (m, 6H). 13C NMR (CD3OD:d8-toluene 5:3, 100 MHz, 25°C): δ =
173.8, 145.3, 134.9, 125.2, 122.4, 37.7, 34.1, 33.6, 28.8, 26.0. ESI-MS:
m/z calcd. for C36H45Br3N4O3: 820.10 [M]+, found: 820.15. These data are
in agreement with data reported in the literature.[10]
performed at room temperature (295
K
±
1
K). Rheological
measurements were carried out by using two different shear rheometers
according to the nature of the gels: a conventional controlled-stress
rheometer (Haake, Mars III) for physical gels and
a home-made
piezorheometer, i.e. working with piezoelectric elements, for chemical
gels. Measurement in the chemical gels were performed after swelling.
AFM images were obtained by scanning the samples using a Nanoscope
8 (Bruker) operated in Peak-Force tapping mode. Peak-Force AFM is
based on Peak force tapping technology, during which the probe is
oscillated in a similar fashion as it is in tapping mode, but at far below the
resonance frequency. Each time the tip and the sample are brought
together, a force curve is captured. These forces can be controlled at
levels much lower than contact mode and even lower than tapping mode
allowing operation on even the most delicate soft samples, as it is the
case here. Ultra-sharp silicon tips on nitride lever were used (Bruker,
Compound 1: An oven-dried flask was charged with 5-norbornene-2-
carboxylic acid (0.20 mL, 1.647 mmol) and anhydrous potassium
carbonate (0.68 g, 4.941 mmol) in dry dimethylformamide (15 mL). The
reaction mixture was stirred at room temperature for 10 min and then, a
solution of compound 1b (150 mg, 0.183 mmol) and a catalytic amount of
tetrabutylammonium iodide (20 mg, 0.054 mmol) in dimethylformamide (5
mL) was added. The solution was then stirred at room temperature for 6
hours. After that time, the solvent was evaporated under reduced
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10.1002/chem.201902404
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pressure. The crude residue was dissolved in ethyl acetate (100 mL) and
the organic phase was extracted with water (200 mL), brine (100 mL) and
dried over Na2SO4. Further evaporation under reduced pressure and
purification by column chromatography (SiO2, cyclohexane → ethyl
acetate/cyclohexane 3:2) provided compound 1 (90 mg, 50%) as a white
solid. Rf = 0.3 (ethyl acetate /cyclohexane 3/2). 1H NMR (CDCl3, 400
Acknowledgements
This work was supported by the Chinese Scholarship Council
(CSC, fellowship to T. L.). We also wish to thank the CNRS, the
Laboratory of Excellence for Complex System Chemistry (LabEx
CSC), and the University of Strasbourg. The authors
acknowledge Bertrand Vileno for EPR experiments, Guillaume
Fleith for X-ray experiments, and "les plateformes d'analyse des
polymères et de microscopie" from the Institut Charles Sadron.
3
MHz, 25°C): δ = 7.43 (s, 3H), 7.33 (d, J = 8.4 Hz, 6H), 6.93 (d, 3J = 8.4
Hz, 6H), 6.16 (dd, 3J = 5.6, 3.1 Hz, 2H), 6.13 (dd, 3J = 5.6, 2.9 Hz, 1H),
6.09 (dd, 3J = 5.5, 3.0 Hz, 1H), 5.90 (dd, 3J = 5.7, 2.8 Hz, 2H), 4.08 (t, 3J
= 6.7 Hz, 2H), 4.02 (dd, 3J = 6.6, 3.1 Hz, 4H), 2.95-2.88 (m, 6H), 2.35 (t,
3J = 7.4 Hz, 6H), 1.93-1.83 (m, 3H), 1.80-1.71 (m, 6H), 1.71-1.59 (m, 9H),
1.52-1.31 (m, 15H). 13C NMR (CDCl3, 100 MHz, 25°C): δ = 176.5, 175.0,
171.2, 138.2, 137.9, 135.9, 132.5, 64.1, 49.8, 46.8, 46.5, 45.9, 43.5, 43.4,
42.7, 41.8, 37.5, 30.5, 29.4, 28.6, 25.8, 25.4. ESI-MS: m/z calcd. for
C60H72N4O9: 993.54 [M+H]+, found: 993.68.
Keywords: supramolecular polymers • triarylamines • gels •
ROMP • rheology
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Entry for the Table of Contents
FULL PAPER
Covalent polymerization of
supramolecular triarylamine
nanofibers leads to functional
chemical gels with improved
mechanical properties
Ting Liang, Dominique Collin, Melodie
Galerne, Gad Fuks, Andreas Vargas
Jentzsch, Mounir Maaloum, Alain
Carvalho, Nicolas Giuseppone* and
Emilie Moulin*
=
ROMP
Page No. – Page No.
Covalently trapped triarylamine-
based supramolecular polymers
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