Proton-conductive materials formed by coumarin
photocrosslinked ionic liquid crystal dendrimers
Citation for published version (APA):
Concellon, A., Liang, T., Schenning, A. P. H. J., Luis Serrano, J., Romero, P., & Marcos, M. (2018). Proton-
conductive materials formed by coumarin photocrosslinked ionic liquid crystal dendrimers.
Journal of Materials
Chemistry C
,
6
(5), 1000-1007. https://doi.org/10.1039/c7tc05009g
DOI:
10.1039/c7tc05009g
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Published: 07/02/2018
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Proton-conductive materials formed by coumarin
photocrosslinked ionic liquid crystal dendrimers
Alberto Concellón,
a
Ting Liang,
b
Albertus P. H. J. Schenning,
b,c
José Luis Serrano,
d
Pilar Romero,*
a
and Mercedes Marcos*
a
In this work, we have successfully examined for first time the use of ionic dendrimers as building blocks for the preparation
of 1D and 2D proton conductive materials. For this purpose, a new family of liquid crystalline dendrimers have been
synthesized by ionic self-assembly of poly(amidoamine) (PAMAM) dendrimers bearing 4, 8, 16, 32 or 64 NH
2
terminal
groups and a coumarin-containing bifunctional dendron. The noncovalent architectures were obtained by the formation of
the ionic salts between the carboxylic acid group of the dendron and the terminal amine groups of the PAMAM dendrimer.
The liquid crystal properties have been investigated by polarized optical microscopy (POM), differential scanning
calorimetry (DSC) and X-ray diffraction (XRD). All the compounds exhibited mesogenic behavior with smectic A or
hexagonal columnar mesophases depending on the generation of the dendrimer. Coumarin photodimerization was used
as crosslinking reaction to obtain liquid crystalline polymer networks. All the materials showed good proton conductive
properties as the LC arrangement leads to the presence of ionic nanosegregated areas (formed by the ion pairs) that favor
proton conduction.
Introduction
Ion transport is an important phenomenon in biological
processes, batteries and separation technologies. The use of
ionic liquid crystals (LCs) has been found to be a versatile
approach for the development of ion transporting materials.
1-2
In fact, LC materials can self-organize into various
nanostructured phases, such as nematic, smectic or columnar.
These nanosegregated structures provide well-organized
channels for the transport of electrons, holes or ions.
3-6
Columnar and smectic arrangements may lead to the
formation of 1D and 2D channels (respectively) capable of
transporting ions. As with all LC properties, ion transport is
highly anisotropic thus, the orientation of the 1D and 2D
channels on the macroscopic scale is an important and
challenging feature. Smectic LCs can be considered as 2D ion
conductors with ion conduction in the directions within the
layer plane. However, LCs showing columnar mesophases can
be used to create 1D ion conductors, with ion conduction
taking place in the direction of the columnar axes.
7-8
Therefore, LC materials have potential as new functional
electrolytes for electrochemical devices; for example, in
lithium-ion batteries (transport of Li
+
ion), dye-sensitized solar
cells (transport of the I
-
/I
3
-
redox couple) or fuel cells (proton
transport).
9-18
Nanostructured LC phases can be stabilized by
photopolymerization to maintain the anisotropic ion transport
over a longer period of time. Crosslinking of polymerizable LC
monomers in their mesophase can yield nanostructured,
thermally and mechanically stable membrane materials with
permanent pathways for ion transport.
19-23
Over the last few years, we have been working on LC
dendrimers with the aim of combining the inherent properties
of the dendrimer scaffold with the anisotropic properties
provided by the LC state.
24-25
LC dendrimers are generally
prepared by the introduction of promesogenic units at the
periphery of a preformed dendrimer. However, it is possible
the design of LC dendrimers without any promesogenic unit.
Ionic LC dendrimers are the most interesting example,
nanosegregation between polar and apolar regions was the
driving force for the formation of the observed mesophases.
26-
31
In addition, the structural versatility of dendrimers allows
the introduction of different functional units on the periphery,
obtaining materials with potential applications in targeted
drug-delivery, optoelectronics, light harvesting and sensors.
32-
35
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Figure 1. Schematic representation of the ionic self-assembly process to prepare the ionic LC dendrimers and the nomenclature of the ionic dendrimers.
To date, proton conductivity has been reported with low-
molecular-weight mesogenic compounds that were stabilized
by photocrosslinking to maintain the ionic conductivity over a
longer period of time. In the present study, to develop 1D and
2D proton-conductive materials, we have examined the
supramolecular LC organization of ionic dendrimers. A new
family of ionic hybrid dendrimers were synthesized from
poly(amidoamine) (PAMAM) dendrimer generations 0 to 4
(bearing 4, 8, 16, 32 or 64 NH
2
terminal groups) (
Figure 1
).
PAMAM was surrounded by carboxylic acid dendrons
bifunctionalized with a promesogenic unit (cholesteryl
hemisuccinate) and coumarin moieties. Coumarin derivatives
have been widely used as fluorophores in material science.
36-42
In this work coumarin was chosen as reactive group for the
crosslinking reaction. Upon UV irradiation, coumarins undergo
[2+2] cycloaddition to yield cyclobutane dimers. It doesn’t
require an initiator or catalyst and side reactions may be
avoided.
Results and discussion
Synthesis and Characterization of Ionic dendrimers
The carboxylic acid dendron (
Ac-ChCou
,
Figure 1
) was
prepared by the synthetic route and the experimental details
given in the Supporting Information.
Ionic dendrimers were prepared by mixing a tetrahydrofuran
(THF) solution of
Ac-ChCou
with a solution of the
corresponding generation of PAMAM dendrimer in the
stoichiometry necessary to functionalize all terminal amine
groups. The mixture was ultrasonicated for 5 min, then the
THF was slowly evaporated at room temperature and the
sample was dried in vacuum at 40 ᵒC until the weight
remained constant. The formation of ionic interactions
between the PAMAM dendrimer and the dendron acids was
studied by infrared spectroscopy (IR) and by nuclear magnetic
resonance (NMR).
As a representative example that demonstrates the
formation of the ionic salts, the FTIR spectra of
Ac-ChCou
,
PAMAM16
and the corresponding ionic dendrimer is shown in
Figure 2
. In the spectrum of
Ac-ChCou
, three C=O stretching
bands appeared at 1683, 1730 and 1741 cm
-1
. The band at
1730 cm
-1
is assigned to the ester groups, whereas the bands
at 1686 and 1741 cm
-1
correspond to the dimeric and free
form of the carboxylic acid group, respectively. In the
spectrum of
PAMAM16-ChCou
the signals at 1686 and 1741
were replaced by two new bands at around 1550 and 1400 cm
-
1
due to the asymmetric and symmetric stretching modes of
the carboxylate group.
Figure 2.
FTIR spectra (C=O st. region) of
PAMAM16
(black line),
Ac-
ChCou
(blue line), and
PAMAM16-ChCou
(red line). (See
Figure S1
for
the FTIR spectra in the complete frequency range)
1900 1800 1700 1600 1500 1400 1300
Absorbance (a.u.)
Wavenumber (cm
-1
)
ester
acid (free)
acid (dimer)
carboxylate
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Figure 3.
1
H (left) and
13
C (right) NMR spectra in CDCl
3
solution at 25 ᵒC of: (a) Ac-ChCou, (b) ionic dendrimer PAMAM32-ChCou, and (c) PAMAM32.
The
1
H NMR spectra recorded in CDCl
3
clearly show the
formation of the ionic assemblies. As a representative example
the
1
H NMR spectra of the dendron
Ac-ChCou
, the third
generation PAMAM dendrimer (
PAMAM32
) and the ionic
dendrimer
PAMAM32-ChCou
complex are shown in
Figure 3
.
In the initial dendron, the acid proton signal was very broad
and barely visible in the
1
H spectrum, thus this signal could not
be used to determine the formation of the salt. The protons
close to the ionic pairs experienced the highest chemical shifts.
For instance, the proton signals of the diastereotopic
methylene (H
P
and H
P’
) moved to higher field after the
formation of the salts. In the same way, quantitative
protonation of terminal amine groups of PAMAM dendrimer
was confirmed by the absence of the NH
2
proton signal at 7.91
ppm and the appearance of the NH
3
+
broad signal at 5.20-4.00
ppm. The absence of the CH
2
CH
2
-NH
2
(H
α
, δ= 2.77 ppm) and
CH
2
CH
2
-NH
2
(H
β
, δ= 3.22 ppm) signals and the appearance of
the CH
2
CH
2
-NH
3
+
(H
α
, δ= 3.13 ppm) and CH
2
CH
2
-NH
3
+
(H
β
, δ=
3.52 ppm) signals, also confirms the quantitative protonation
of terminal amine groups.
1
H-
1
H NOESY experiments were also employed to study the
formation of these ionic dendrimers in solution. The main
feature of NOESY is their ability to provide in a single
experiment all the correlations between nuclei which are
physically close in space, thus making it a very valuable tool for
determining whether supramolecular interactions were
established between
Ac-ChCou
dendron and
PAMAM
dendrimer. The
1
H-
1
H NOESY spectrum of
PAMAM32-ChCou
is
shown in
Figure S2
(Supporting Information). Significant cross-
peaks were observed between the diastereotopic protons of
Ac-ChCou
(H
P
and H
P’
) and H
α
and H
β
protons of the terminal
branches of PAMAM, indicating that these groups were close
in space because of the ionic pair formation. Besides of this, H
P
and H
P’
docked closely with the terminal NH
3
+
groups of
PAMAM.
In the
13
C NMR spectra (
Figure 3
) the carboxyl group signal
(C
S
) of the acid shifts from 176.98 to 178.18, indicating the
formation of the carboxylate (COO
‒
). Likewise, the
deprotonation of the carboxylic acid was also corroborated by
the displacement of the methylic carbon (C
Q
), the methyl (C
R
)
carbon and the methylene carbons (C
P
and C
P’
) to lower field.
In addition, when terminal amine groups of PAMAM are
protonated, the methylene carbons (C
α
and C
β
) move from
41.6/42.4 to 39.7/37.7, respectively (data confirmed by
1
H-
13
C
HSQC experiments,
Figure S3
).
Figure 4.
13
C CPMAS NMR spectra of:
(a)
Ac-ChCou
,
(b)
ionic dendrimer
PAMAM16-ChCou
, and
(c)
ionic dendrimer
PAMAM16-ChCou
after
photodimerization.
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