Ionic Conduction in Poly(ethylene glycol)-Functionalized Hexa-peri-hexabenzocoronene Amphiphiles

Article abstract of DOI:10.1021/acs.macromol.7b00224

Discotic liquid crystals based on hexa-peri-hexabenzocoronenes (HBCs) symmetrically substituted with six poly(ethylene glycol) (PEG) chains and further doped with LiCF₃SO₃ (LiTf) salt at different [EG]:[Li⁺] ratios nanophase-separate in domains composed from HBC columns and PEG chains. These model amphiphiles behave as viscoelastic solids with a shear modulus of 5 × 10⁶ Pa and an ionic conductivity of 10^-5 S/cm at 373 K. At temperatures below 333 K an ionic superstructure is formed of higher shear modulus (10⁸ Pa) that surrounds the HBC columns and follows the disks in their rotational motion. However, the ionic superstructure impedes ion transport. Substituting the HBC core with two PEG chains breaks the symmetry of the ionic superstructure and increases ionic conductivity by an order of magnitude while retaining a high shear modulus and a viscoelastic response. These findings demonstrate that PEG-functionalized HBCs have great potential as new electrolytes because they combine ionic conductivity with mechanical stability. Moreover, when combined with electronic conduction within the HBC columns, this design can result in structures that exhibit simultaneous electronic and ionic transport.

Full text of DOI:10.1021/acs.macromol.7b00224

Article
pubs.acs.org/Macromolecules
Ionic Conduction in Poly(ethylene glycol)-Functionalized Hexa-peri-
hexabenzocoronene Amphiphiles
Achilleas Pipertzis,† George Zardalidis,† Katrin Wunderlich,‡ Markus Klapper,‡ Klaus Mullen,‡
̈
and George Floudas*,†
†Department of Physics, University of Ioannina, P.O. Box 1186, 451 10 Ioannina, Greece
‡Max Planck Institute for Polymer Research, 55128 Mainz, Germany
S
* Supporting Information
ABSTRACT: Discotic liquid crystals based on hexa-peri-
hexabenzocoronenes (HBCs) symmetrically substituted with
six poly(ethylene glycol) (PEG) chains and further doped with
LiCF3SO3 (LiTf) salt at different [EG]:[Li+] ratios nanophase-
separate in domains composed from HBC columns and PEG
chains. These model amphiphiles behave as viscoelastic solids
with a shear modulus of 5 × 106 Pa and an ionic conductivity of
10−5 S/cm at 373 K. At temperatures below 333 K an ionic
superstructure is formed of higher shear modulus (108 Pa) that
surrounds the HBC columns and follows the disks in their
rotational motion. However, the ionic superstructure impedes ion transport. Substituting the HBC core with two PEG chains
breaks the symmetry of the ionic superstructure and increases ionic conductivity by an order of magnitude while retaining a high
shear modulus and a viscoelastic response. These findings demonstrate that PEG-functionalized HBCs have great potential as
new electrolytes because they combine ionic conductivity with mechanical stability. Moreover, when combined with electronic
conduction within the HBC columns, this design can result in structures that exhibit simultaneous electronic and ionic transport.
temperatures composed of columns of tilted disks (i.e.,
“herringbones”) in a monoclinic unit cell. Extensive struc-
tural,19,20 kinetic,21,22 and dynamic23,24 experiments in a series
of dipole-functionalized HBCs revealed that the two phases
possess not only distinctly different unit cells but also different
dipolar and viscoelastic signatures. Furthermore, confinement
within rigid nanopores furnished arrays of 1D supramolecular
wires with uniform columnar orientation on a macroscopic
scale suitable to generate supramolecular 1D wires for
electronic charge transport.25
Coaxial structures based on HBC cores decorated with short
PEG chains26,27,18 when doped with electronic charge and
lithium ions, respectively, offer a unique possibility for
electronic and ionic conducting pathways. The long-term aim
of our research is the design of coaxial structures composed of
electronically conducting, yet ionically insulating, columnar
structures (HBC) surrounded by an electronically insulating,
yet ionically conducting phase (PEG). Herein we explore
mainly ionic conduction based on doping of HBC-PEG with
LiCF3SO3 (LiTf) and provide some preliminary results on
electronic conduction in the same systems. The choice of PEG
as the ion-conducting pathway is straightforward: PEG is a
flexible polymer with high solvating capability for a number of
lithium salts and at the same time is biodegradable and
I. INTRODUCTION
In recent years there has been considerable interest in materials
which conduct both electronic charge and ions on the
nanometer length scales. Earlier studies concentrated on
inorganic1,2 and more recent studies on organic materials.3−6
The latter were based on mixtures of lamella-forming poly(3-
hexylthiophene)-b-poly(ethylene oxide) (P3HT-b-PEO) block
copolymers with a lithium salt.5,6 In this case, the lithium salt
was found in both P3HT and PEO nanophases and ionic
conductivity was some orders of magnitude higher than the
electronic conductivity. Other possible candidates for such
materials are based on liquid crystals (LC). Recent studies
explored the possibility of using liquid crystal ion-conducting
materials for energy storage devices such as lithium
batteries.7−10 Discotic liquid crystals (DLCs), in particular,
constitute a class of organic semiconductors with applications
in organic field-effect transistors and photovoltaic cells.
Applications of DLCs in nanoscale conductive devices rely on
the optimal stacking of the aromatic cores that allows for charge
carrier mobility along the columnar axis (molecular wires).11
Hexabenzocoronenes (HBCs) with the large conjugated core
are an important class of DLCs.12−18 Within their columnar
phases, molecules are stacked one on top of each other forming
columns that are further organized in a two-dimensional lattice.
X-ray scattering revealed two main columnar structures in
HBCs: First, a liquid crystalline phase (Colh) at higher
temperatures composed of columns that are further organized
in a hexagonal lattice. Second, a crystalline phase (Cr) at lower
Received: January 31, 2017
Revised: February 23, 2017
© XXXX American Chemical Society
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Figure 1. Synthetic scheme for HBC-TEG6 (4): (a) TsO(CH2CH2O)3CH3, K2CO3, DMF, 80 °C, 24 h; (b) TMS-acetylene, PdCl2(PPh3)2, CuI,
DBU, benzene, H2O, 60 °C, 24 h; (c) [Co2(CO)8], dioxane, reflux; 12 h (58%); (d) FeCl3, CH3NO2, DCM, 20 °C, 5 h (95%).
environmentally friendly.28 However, PEG is lacking the
necessary mechanical stability and high elastic modulus29
necessary to prevent dendritic growth at the electrodes, a key
issue in battery applications.30 This fact has initiated the pursue
of effective scaffolds for PEO electrolytes based on more stiff
macromolecules. Hence, PEG has been “combined” with stiffer
macromolecules, as for example in block copolymers with
poly(styrene-b-ethylene oxide) (PS-b-PEO) being the most
studied copolymer. Here the hard nanophase (PS) provides the
required elastic modulus and the soft phase (PEG) supports ion
conduction.31−39
a broad temperature range. Our results demonstrate that LiTf is
located exclusively in the PEG nanophase and has an ionic
conductivity of 10−5 S/cm at 373 K. We explore the role of ions
(a) in stabilizing a certain unit cell and (b) in creating an ionic
superstructure that surrounds the HBC columns. The role of
the latter regarding the mechanical stability and ionic
conductivity is discussed. Furthermore, we explore the effect
of asymmetric PEG functionalization of the HBC core on the
self-assembly and ionic conductivity. We find that asymmetric
substitution results in a further increase of ionic conductivity by
an order of magnitude.
We have recently investigated Li ion conduction in propeller-
shaped hexaphenylbenzenes (HPB) substituted with PEG
chains and doped with LiTf.40 A unique feature of the latter
system was the Vogel−Fulcher−Tammann temperature
dependence of dc conductivity with values that were
comparable to the archetypical polymer electrolyte PEG/
LiTf.29 However, HPBs cannot be employed as efficient
scaffolds as they lack the required mechanical stability. In the
present work the scaffold for the PEG electrolytes is provided
by the rigid HBC columnar structures. To our knowledge, this
is the first time that HBCs are used as scaffolds for Li ion
conductivity. HBC-PEG/LiTf with a range of electrolyte
concentrations (with ratio [EO]:[Li+] = 3:1, 8:1, and 12:1)
was investigated with respect to the thermal properties, self-
assembly, mechanical stability, and Li-ion conductivity. These
model amphiphiles are viscoelastic solids with a high shear
modulus (with values in the range from 5 × 106 to 108 Pa) over
II. EXPERIMENTAL SECTION
Materials and Methods. Unless otherwise noted, all starting
materials were purchased from Sigma-Aldrich, Acros, ABCR, and Alfa
Aesar and used as received without further purification. Synthesis was
carried out under inert atmosphere using anhydrous solvents. For
reactions performed at higher temperatures, the given temperature
refers to the temperature of the oil bath used for heating. Column
chromatography for purification purposes was performed with silica
gel (particle size 0.063−0.200 mm from Merck) and for analytic thin
layer chromatography on silica-coated aluminum sheets with
fluorescence indicator from Macherey-Nagel. Synthesis of compounds
1 and 2 has been reported in the literature18 as well as the preparation
of compound 6.41
Measurements. 1H and 13C NMR spectroscopies were carried out
with Bruker AMX 300 (300 MHz) and Bruker DRX 700 (700 MHz).
MALDI-TOF was performed on a Bruker Daltonic Reflex machine
that was calibrated using a fullerene mixture (Sigma-Aldrich, CAS
131159-39-2) with dichloromethane as solvent and dithranol as matrix
B
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Figure 2. Synthesis of HBC containing two PEG chains (8): (a) K2CO3, acetone, 18 h, 80 °C; (b) 2,3-dichloro-5,6-dicyano-p-benzoquinone
(DDQ), trifluoromethanesulfonic acid, dichloromethane, 2 h, RT, 95%; (c) [Pd(PPh3)4], tetraethylammonium hydroxide, toluene, 90 °C, 16 h.
or solvent-free and tetracyanoquinodimethane (TCNQ) as matrix.
Elemental composition (C, H, N) was analyzed with a Foss
HeraeusVario EL machine (Analytic Laboratory, Institute of Organic
Chemistry, Johannes Gutenberg University Mainz). The optical
absorption measurements were performed at ambient temperature
using a UV/vis/NIR PerkinElmer Lambda 9 spectrometer. The
molecular weights and molecular weight distributions were determined
by means of gel permeation chromatography (GPC) with THF as
eluent (1 mL/min) at 30 °C. The instrument was equipped with a
refractive index (RI) detector (ERCRI-101 differential refractometer
detector) and fitted with a PSS-SDV 300 × 8 mm, particle size 10 μm,
and pore size 500, 105, and 106 Å. The molecular weights were
determined from a calibration curve based on linear poly(styrene)
standards.
Synthesis of HBC-TEG6 (4). The synthesis of HBC-TEG6 is
displayed in Figure 1. The synthesis of HBC-TEG6 (4) was performed
using etherification chemistry and continuing with a Sonogashira
reaction to generate a diphenylacetylene derivative (2). Subsequent
cyclotrimerization42 of , catalyzed by [Co2(CO)8] afforded compound
3. The amphiphilic discotic molecule HBC-TEG6 (4) was obtained by
oxidative cyclo-dehydrogenation with a solution of FeCl3 in nitro-
methane.
Synthesis of HBC with Two PEG Chains (8). HBC with two PEG
chains (8) was prepared as shown in Figure 2. 4-(4,4,5,5-Tetramethyl-
1,3,2-dioxaborolan-2-yl)phenol was first subjected to etherification
with α-methoxy-ω-bromo-PEG (Mn(PEG) = 750 g/mol, Mw/Mn =
1.01). Oxidative cyclo-dehydrogenation of 4,4″-dibromo-3′,4′,5′,6′-
tetraphenyl-1,1′:2′,1″-terphenyl (6) led to 2,5-dibromohexabenzo-
[bc,ef,hi,kl,no,qr]coronene (7). Then HBC with two PEG chains (8)
was synthesized with a palladium-catalyzed Suzuki coupling of
compounds 5 and 7. To remove the Pd catalyst, HBC with two
PEG chains was washed several times with water and extracted with
dichloromethane. After size exclusion chromatography, 8 was obtained
as a pure yellow oil.
were preparedand used as reference points for the conductivityas
described in the Supporting Information. Further details on the
synthesis and characterization are provided in the Supporting
Information (Figures S1−S7).
Differential Scanning Calorimetry. The thermodynamic state of
the neat compounds 4 and 8 as well as after doping with LiTf was
studied by differential scanning calorimetry (DSC) with a Q2000 (TA
Instruments). The instrument was calibrated for best performance in
the specific temperature range and heating/cooling rate. The
calibration sequence included a baseline calibration for the
determination of the time constants and capacitances of the sample
and reference sensor using a sapphire standard, an enthalpy and
temperature calibration for the correction of thermal resistance using
indium as standard (ΔH = 28.71 J/g, Tm = 428.8 K), and a heat
capacity calibration with sapphire standard. Measurements were made
on cooling and subsequent heating at a rate of 5 K/min. The DSC
traces of compounds 4 and 8 from the second heating scans are shown
in Figures 3 and 12, respectively. The traces reveal a very different
thermodynamic state in the two compounds.
Polarizing Optical Microscopy (POM). A Zeiss Axioskop 40
equipped with a video camera and a fast frame grabber was used to
follow the structural changes of the two samples. The samples were
prepared between two Linkam glass microscopy slides with a distance
of 25 μm maintained by Teflon spacers. A Linkam temperature control
unit (THMS600), equipped with a TMS94 temperature programmer,
was employed for the temperature-dependent studies. Images were
recorded following slow cooling (1 °C/min) from the isotropic state.
X-ray Scattering. Wide-angle X-ray scattering (WAXS) measure-
ments were made using Cu Kα radiation from a RigakuMicroMax 007
X-ray generator (λ = 1.541 84 nm), using Osmic Confocal Max-Flux
curved multilayer optics. Samples in the form of fibers were prepared
with a mini-extruder at 303 K. Temperature-dependent WAXS
measurements were performed by inserting the fibers into glass
capillaries (1 mm diameter) at temperatures in the range from 298 to
393 K in steps of 10 K. A waiting (equilibration) time of 1800 s and a
measurement time of 3600 s were set in the temperature program.
Diffraction patterns were obtained by radial averaging of the data
recorded by a 2D detector (Mar345 Image Plate).
According to GPC and MALDI-TOF analysis, a small amount of
PEG and byproducts such as HBC with one PEG chain were formed
after the Suzuki reaction. The removal of these impurities was achieved
by preparative GPC. The resulting molecules (5−8) were charac-
1
terized by H and 13C NMR spectroscopy and MALDI-TOF mass
Dielectric Spectroscopy. Dielectric spectroscopy (DS) measure-
ments were performed with a Novocontrol Alpha frequency analyzer
as a function of temperature. Measurements were recorded at different
temperatures in the range from 203 to 413 K in steps of 5 K for
spectrometry. Molecules 5 and 8 were further characterized by GPC.
In addition to the HBC-TEG6/LiTf mixtures, triethylene glycol
dimethyl ether with lithium triflate mixtures at the same compositions
C
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unit cellas in bulkbut associates with the formation/
melting of an ionic superstructure.
Self-Assembly. Information on the morphology is obtained
by WAXS. Figures 4 and 5 refer to the self-assembly in neat
Figure 3. DSC traces of neat HBC-TEG6 (4) (blue) and of the same
HBC-TEG6 doped with LiCF3SO3 at different salt concentrations
[EO]:[Li+] = 12:1 (red), 8:1 (green), and 3:1 (magenta). Traces are
obtained during the second cooling (left) and the second heating run
(right) with a rate of 5 K/min. Traces are shifted vertically for clarity.
frequencies in the range from 10−2 to 107 Hz. The complex dielectric
permittivity ε* = ε′ − iε″, where ε′ is the real and ε″ is the imaginary
part, has been obtained as a function of frequency ω and temperature
T i.e., ε*(T,ω).43−45 The analysis of the T-dependent experiments was
made using the empirical equation of Havriliak and Negami (HN):46
σ0(T)
iεfω
Δε(T)
*
ε
HN(ω, T) = ε∞(T) +
+
[1 + (iωτHN(T))m]n
(1)
Figure 4. WAXS patterns of neat HBC-TEG6 at two temperatures, at
323 K (top) and at 293 K (middle) during heating. 2D-WAXS images
from extruded HBC-TEG6-neat fibers at the respective temperatures
are also depicted. (bottom) WAXS intensity contour plots during
heating are also depicted. Blue and green arrows indicate the main
equatorial and meridional reflections, respectively. Dashed lines give
the position and (hkl) indices of the Bragg reflections from the
hexagonal lattice (top) and rhombohedral lattice (middle). The
horizontal white dashed lines depicted in the contour plot indicates the
change of unit cell.
where ε∞(T) is the high-frequency permittivity, τHN(T) is the
characteristic relaxation time in this equation, Δε(T) = ε0(T) −
ε∞(T) is the relaxation strength, m and n (with limits 0 < m, mn ≤ 1)
describe respectively the symmetrical and asymmetrical broadening of
the distribution of relaxation times, σ0 is the dc conductivity, and εf is
the permittivity of free space. From τHN, the relaxation time at
maximum loss, τmax, is obtained analytically following
⎛
⎜
⎞
⎟
⎛
⎜
⎞
⎟
πm
πmn
τmax = τHN sin−1/m
sin1/m
⎝ 2(1 + n)⎠
⎝ 2(1 + n)⎠
(2)
HBC-TEG6 and in HBC-TEG6 doped with LiTf at [EO]:[Li+]
= 8:1, respectively. The WAXS pattern of the neat compound 4
at 293 K reveals a set of equatorial reflections that correspond
to the (100), (110), (1−10), (200), (210), and (2−10)
reflections belonging to a rhombohedral unit cell with
interplanar spacings, dhkl, of the (hkl) lattice planes given by
In addition to the measured ε″ spectra, the derivative of ε′ (dε′/d ln ω
∼ −(2/π)ε″) has been used in the analysis of the dynamic behavior.
The characteristic time of ion mobility is obtained from the crossing of
the real and imaginary parts of ε* or, equivalently, of the modulus
(M*) representation (ε* = 1/M*).
III. RESULTS AND DISCUSSION
1
dhkl
=
Thermodynamic State. The thermodynamic transitions of
the neat compound HBC-TEG6 (compound 4) are depicted in
the DSC traces of Figure 3. The thermogram depicts two
exothermic peaks on cooling at ∼300 and ∼200 K, suggesting
two first-order phase transitions. On heating, there is hysteresis
and the low-temperature endothermic peak now appears at
∼230 K, while the high-temperature peak at 323 K. The low-
temperature feature most likely involves a phase transition from
the crystalline phase to a liquid crystalline phase. The transition
at higher temperatures involves a change in the unit cell from
rhombohedral to hexagonal within the liquid crystalline phase
(see below with respect to the WAXS investigation). In HBC-
TEG6 doped with LiTf with salt concentrations [EO]:[Li+] =
12:1, 8:1, and 3:1, the low-temperature first-order transition
remains intact. However, there are differences in the high-
temperature transition which on cooling now appears at ∼320
K, e.g., 20 K higher than in the neat compound 4. As will be
seen below, this transition does not relate to a change in the
2
(h2 + k2 + l2) sin2 γ + 2(hk + kl + hl)(cos2 γ − cos γ)
α2(1 − 3 cos2 γ + 2 cos3 γ)
(3)
The corresponding unit cell parameters are a = b = 2.66 nm
and γ = 85.2°. At higher temperatures, the equatorial reflections
have ratios of 1:31/2:41/2 relative to the main peak and are
associated with the (100), (110), and (200) reflections from a
hexagonal unit cell. The spacings, dhkl, now are related with the
unit cell parameter a (=2.92 nm) as
(h2 + hk + k2)
1
dhkl
4
3
=
2
a2
(4)
Furthermore, the transition temperatures from the rhombohe-
dral to the hexagonal unit cells (indicated with the dashed
horizontal lines in the intensity contour plots of Figure 4) are in
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Figure 6. (left) Temperature dependence of intracolumnar spacing for
neat HBC-TEG6 (squares) and HBC-TEG6/LiTf with [EO]:[Li+]
ratio 8:1 (triangles). Red and blue symbols indicate data obtained on
heating and cooling, respectively. (right) Temperature dependence of
domain spacing along the intercolumnar direction for neat HBC-
TEG6 (blue squares); HBC-TEG6/LiTf at three concentrations:
[EO]:[Li+] = 12:1 (red circles), 8:1 (green up triangles), and 3:1
(magenta down-triangles) on cooling.
Figure 5. WAXS patterns of HBC-TEG6/LiTf with [EO]:[Li+] = 8:1
at two temperatures, at 373 K (top) and at 303 K (middle) during
heating. (bottom) WAXS intensity contour plot during heating are
also depicted. The blue and green arrows indicate the main equatorial
reflections and the main meridional reflections, respectively. Dashed
lines give the position and (hkl) indices of the Bragg reflections from
the hexagonal lattice. The horizontal white dashed lines in the contour
plot indicates the melting of the ionic superstructure.
Figure 6 depicts only a minor change of the intracolumnar
spacing and a major increase of intercolumnar distances with
the addition of salt. Within the hexagonal lattice, intercolumnar
distances increase from 2.9 nm in the neat compound 4 to ∼3.5
nm in the electrolyte with the higher salt concentration. This
reveals that ions are incorporated exclusively within the
intercolumnar space which is a prerequisite for a good ionic
conductor. In addition, an ionic superstructure is formed as
indicated by the weak equatorial reflection (corresponding
distance of ∼5 nm) and weak meridional reflections
(corresponding distances of 0.41 and 0.37 nm). Since ionic
correlations exceed the intercolumnar spacing, they could
suggest an ionic superstructure that surrounds the columns in
the form of a helix with correlation distances of ∼5 and 0.41 nm
between and along the columns, respectively. This is schemati-
cally depicted in Figure 7. The lack of sufficient reflections from
the ionic superstructure precludes a more definite structural
assignment. Nevertheless, ions are spatially correlated in a
superstructure that surrounds the HBC columns and further
stabilize the hexagonal lattice from higher temperatures. Below
it will be explored how this ionic superstructure affects the ionic
conductivity.
Ionic Conductivity. The dc ionic conductivity was
extracted from the plateau in σ′ both for the neat compound
4 and for the HBC-TEG6 doped with LiTf with salt
concentrations [EO]:[Li+] = 12:1, 8:1, and 3:1. In the neat
compound 4, conductivity is some 2 orders of magnitude lower
than in the doped systems (Figure S8). This reflects the
presence of some ionic impurities of in the neat compound
(despite efforts to remove ions with water). The temperature
dependence of the dc conductivity in the doped system is
plotted in Figure 8 on cooling and subsequent heating. On
cooling, and for temperatures below (above) the formation of
the ionic superstructure, σ′dc(T) exhibits a Vogel−Fulcher−
Tammann (Arrhenius) T dependence. Instead, during heating,
two VFT dependencies are required: one at temperatures
below and one above the melting temperature of the
superstructure. In addition, in samples with salt concentrations
[EO]:[Li+] = 12:1 and 8:1, there is a discontinuous increase of
agreement with the ones from DSC. Discotic liquid crystals
within their columnar arrangement have anisotropic inter-
actions: van der Waals between the columns and π−π within
the columns. This anisotropy in molecular interactions gives
rise to anisotropic thermodynamic properties, as for example, in
thermal expansion which now depends on directionality.20
From the temperature dependence of the first equatorial and of
the more intense meridional reflection the inter- and
intramolecular distances can be extracted associated respec-
tively with inter- and intracolumnar correlations. In the bulk,
the thermal expansion coefficients associated with the
intercolumnar correlations within the hexagonal and rhombo-
hedral lattices correspond to αH
= 3.06 × 10−4 K−1 and
inter
αRinter = 1.38 × 10−4 K−1, respectively. From the intracolumnar
correlations, a weaker temperature dependence is obtained with
αH = 4.9 × 10−5 K−1 and αR = 3.3 × 10−5 K−1 in accord
inra
intra
with the smaller thermal expansion coefficient found in graphite
(∼2.5 × 10−5 K−1).20
The self-assembly in the presence of salt is distinctly different
and is discussed with respect to Figures 5 and 6. The WAXS
patterns (Figure 5) at low and higher temperatures indicate
peaks associated with the (100), (110), and (200) reflections
from a hexagonal unit cell. In addition, a weak equatorial
reflection and additional meridional reflections appear at lower
temperatures (T = 303 K in Figure 5). Addition of salt has two
effects; first, it stabilizes the high-temperature hexagonal
structure found in the bulk and, second, induces a reversible
transition at temperatures around 333 K associated with an
ionic superstructure (see below). These effects are discussed
with respect to the temperature dependence of the intra- and
intercolumnar spacings.
E
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Figure 7. Schematic representations of (a) neat HBC-TEG6, (b) its constituents, (c) the columnar arrangement of neat HBC-TEG6, and (d) the
HBC-TEG6 columnar organization in the presence of salt with salt concentration [EO]:[Li+] = 3:1.
effect, we compare the dc conductivities of the linear TEG
chains with the ones in HBC-TEG6 doped with LiTf at the
same salt concentrations. The comparison is shown in Figure
S9. In HBC-TEG6/LiTf, the normalized dc conductivity with
respect to the TEG/LiTf content is reduced by 1−2 orders of
magnitude, suggesting that the HBC columnar structures
perturb the conformation of TEG chains and effectively
increase the viscosity for ion transport. Therefore, there are
two factors that control ionic conductivity in the doped HBCs:
the HBC columnar structures that on one hand provide the
required rigidity (see below) but reduce ionic conductivity, on
the other, and the ionic superstructure that further reduces the
ionic conductivity. Evidently, both structural features lead to a
higher effective viscosity (see below with respect to the
viscoelastic properties). Nevertheless, columnar structures are
an important element of electronic conduction. Below we will
consider ways to circumvent formation of the ionic super-
structure.
Figure 8. The dc conductivity of HBC-TEG6/LiTf at different [EO]:
[Li+] ratios: 12:1 (spheres), 8:1 (up triangles), and 3:1 (down
triangles) during cooling (left) and during heating (right). The gray
areas indicate the transition region. The black arrow indicates the
melting of the ionic superstructure. The solid and dash-dotted lines
represent fits to the Arrhenius and VFT equations, respectively.
Local Dynamics. Dielectric spectroscopy, apart from
affording the ionic conductivity, provides access to the local
dipolar dynamics in the bulk and doped system. Some
representative dielectric loss curves for the neat and doped
compound 4 are shown in Figure 9. In 4, three dielectrically
active processes exist, that one at lower temperatures
corresponds to the local γ- and β-processes and at higher
temperatures to the α-process that effectively carries most of
the dielectric strength. At lower frequencies/higher temper-
atures the mechanism of ionic conductivity originating from
ionic impurities is also apparent from the steep rise of dielectric
loss.
The origin of these dynamics in DLCs has been the subject
of combined efforts by dielectric spectroscopy with site-specific
NMR techniques.23,24 It was shown that the α-process reflects
the in- and out-of-plane reorientation of the disks within the
columnar structures. This dynamic process is cooperative in
nature as reflected in the distribution of relaxation times (with
low- and high-frequency shape parameters of m ∼ 0.5 and mn ∼
σ′dc(T) by about an order of magnitude at the melting of the
ionic superstructures. This fact demonstrates the detrimental
role of the ionic superstructure on ion conduction. There is
some analogy of the ionic superstructure in doped HBCs with
the complex crystal formed in the archetypical polymer
electrolyte composed of linear PEG chains doped with
LiTf.29,37−39 In both cases, complexation impedes ion transport
and reduces the dc conductivity. A recent study demonstrated a
switching of ionic conductivity in wedge-shaped liquid
crystalline ammonium salts.10 In this case the conductivity in
the hexagonal columnar phase was several orders of magnitude
higher than in the rectangular columnar phase.
In addition to the ionic superstructure, the HBC columnar
organization may also impede ionic motion. To explore this
F
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Here, τ0 (=10−12 s) is the limiting relaxation time at very high
temperatures, B (=1400 20 K) is the activation parameter,
and T0 (=183.2
0.7 K) is the “ideal” glass temperature,
located below the glass temperature, Tg (=227 1 K) the latter
defined as the temperature where the α-relaxation times is at
100 s. On the contrary, the τ(T) dependence for the β- and γ-
processes is Arrhenius-like with similar activation energies of
∼39.6 0.1 kJ/mol. These processes reflect local fluctuations
of ethylene glycol within the glassy state.
Doping with LiTf alters significantly the local dynamics as
indicated in the dielectric loss curves of HBC-TEG6/LiTf with
salt concentration [EO]:[Li+] = 8:1 (Figure 9) (the dielectric
loss spectra are compared in Figure S10). Figure 9 depicts
similar local dynamics (β and γ) below the glass temperature
associated with the ethylene oxide dipoles as in bulk compound
4 but distinctly different slower dynamics. Now the α-process is
coupled to the motion of ions. This is shown in Figure 10,
where the relaxation times for the α-process are compared with
the characteristic times associated with the mechanism of ionic
conduction. First, both mechanisms have a VFT temperature
dependence. Second, the two mechanisms are coupled,
meaning that the motion of ions within the ionic superstructure
and the reorientation of disks are concerted.19,23,24 This highly
cooperative process is also reflected in the broader distribution
of relaxation times in the doped systems.
Viscoelastic Properties. Application of materials as
alternatives to solid polymer electrolytes requires a high
modulus as to prevent dendritic growth at the electrodes.30
The viscoelastic properties of the HBC-TEG6 doped with LiTf
at salt concentration [EO]:[Li+] = 3:1 was studied with
rheology, and the storage and loss shear moduli are shown in
Figure 11 as a function of temperature under isochronal
Figure 9. (top) Dielectric loss curves of neat HBC-TEG6 at some
selected temperatures as indicated. The lines represent the result of a
fit using two HN functions and the conductivity contribution. The
different dynamic processes are indicated as α, β, and γ. (bottom)
Dielectric loss curves of HBC-TEG6/LiTf with salt concentration
[EO]:[Li+] = 8:1 at some selected temperatures as indicated. The lines
are fits to a summation of HN functions together with the conductivity
contribution.
0.24) and in the temperature dependence of the associated
relaxation times (Figure 10). The τ(T) dependence conforms
to a VFT dependence
⎛
⎜
⎝
⎞
⎟
⎠
B
τ = τ exp
0
T − T
(5)
0
Figure 11. Temperature dependence of storage (filled symbols) and
loss (open symbols) moduli for HBC-TEG6/LiTf with [EO]:[Li+] =
3:1 obtained during heating at a frequency of 10 rad/s with rate of 2
K/min. Inset: storage (filled symbols) and loss (open symbols) moduli
at 293 and 403 K. At both temperatures the sample exhibits
viscoelastic behavior.
Figure 10. Temperature dependence of the relaxation times of the α-
process (filled symbols), β-process (open symbols), and γ-process
(filled symbols) on inverse temperature of HBC-TEG6 neat (squares),
HBC-TEG6/LiTf with salt concentration [EO]:[Li+] = 12:1 (circles),
HBC-TEG6/LiTf with [EO]:[Li+] = 8:1 (up-triangles), and HBC-
TEG6/LiTf with [EO]:[Li+] = 3:1 (down-triangles). The dashed and
solid lines represent fits to the VFT and Arrhenius equations,
respectively. The vertical solid, dashed, and dash-dotted lines indicate
the melting temperatures from DSC.
conditions. They depict an elastic response at temperatures
below and above melting of the ionic superstructure, with
respective values of storage moduli of ∼108 and ∼106 Pa. In
particular, the high value of the storage modulus at room
temperature (G′ ∼ 108 Pa) reflects the presence of the
columnar structure and of the ionic superstructure.
G
DOI: 10.1021/acs.macromol.7b00224
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At higher temperatures, melting of the ionic superstructure
gives a viscoelastic solid with a modulus of (G′ ∼ 106 Pa) that is
constant over a broad temperature range. At such temperatures
ionic conductivity is maximized. More information on the
viscoelastic state is provided by frequency sweeps performed at
different temperatures (293 and 403 K) corresponding to
temperatures below and above melting of the ionic super-
structure. At both temperatures they indicate a viscoelastic solid
with relatively high moduli, a prerequisite for battery
applications. At this point it would be interesting to explore
means of further increasing dc conductivity by e.g. suppressing
the propensity for an ionic superstructure, while maintaining a
high modulus.
Effect of Asymmetric PEG Substitution (Compound 8). A
way to explore this possibility is by desymmetrization of the
model compound 4, e.g., by employing the same HBC building
block but substituted with only two PEG chains (compound 8).
In this case, PEG with a molecular weight of 750 g/mol was
used due to solubility reasons. Attachment of only two PEG
chains is expected to break the symmetry of the ionic
superstructure. This conjecture is confirmed by comparing
the DSC traces of the neat compound 8 with the same
compound doped with LiTf (Figure 12). The DSC traces of
Figure 13. The dc conductivity of compound 8 at different [EO]:[Li+]
ratios 12:1 (red squares), 8:1 (green squares), and 3:1 (blue squares).
The vertical line indicates the melting temperature of the complex for
the 3:1 ratio. The solid lines represent fits to the VFT equation. The
vertical dashed line gives the melting of the weak structure in [EO]:
[Li+] = 3:1. Vertical arrows give the respective glass temperatures. The
inset gives the normalized conductivities relative to the corresponding
linear PEG chains.
tration [EO]:[Li+] = 12:1 being about an order of magnitude
higher than in compound 4 with the same salt concentration.
Furthermore, the σ(T) dependence displays a VFT temperature
dependence over a broad temperature associated with the
amorphous nature of the systems. Lastly, the higher ionic
conductivity is more evident by plotting the normalized ionic
conductivity with respect to the corresponding linear PEG
chains. In the inset to Figure 13, the normalized conductivity is
less than a decade lower as compared to the linear PEG chains.
This shows that desymmetrization suppresses the propensity
for formation of an ionic superstructure and facilitates ionic
conduction.
In addition to ionic conductivity, electronic conductivity
measurements of compound 4 are currently in progress.
Figure 12. DSC traces of neat compound 8 (black) and of the same
compound doped with LiTf at different salt concentrations:[EO]:[Li+]
= 12:1 (red), 8:1 (green), and 3:1 (blue). Traces are obtained during
the second cooling (left) and the second heating run (right) with a
rate of 10 K/min. Traces are shifted vertically for clarity.
IV. CONCLUSION
Ion conductive liquid crystals show great potential as new
electrolytes because they provide efficient conductive pathways
for ion transport. Furthermore, liquid crystals composed of flat
hydrocarbon cores with enhanced π-stacking and hydrophilic
chains that can solvate lithium salts and are mobile at ambient
temperature are possible candidates of materials that combine
electronic and ionic transport. Discotic liquid crystals based on
HBCs substituted with short PEG chains were synthesized and
doped with LiTf. Structural analysis revealed nanophase-
separated domains composed from HBC cores organized in
columns surrounded by PEG chains. The columns were further
organized in rhombohedral and hexagonal lattices at lower and
higher temperatures, respectively. In analogy with the
crystalline complex formed in the archetypical PEG/LiTf
electrolyte, ions in PEG-functionalized HBCs are spatially
correlated forming an ionic superstructure. Structural analysis
demonstrated that ions are incorporated exclusively within the
PEG domain, surround the HBC columns, and follow the disks
in their rotational motion. Furthermore, ions stabilize the
hexagonal lattice from higher temperatures. Because of the
presence of the columnar structures and the ionic super-
neat 8 indicate a crystallization/melting peak associated with
the crystalline structure of PEG. This structure as well as any
other superstructure are absent in the traces of the doped
compound with [EO]:[Li+] = 12:1 and 8:1 salt concentrations.
The presence of a step in the heat flow at the respective glass
temperature further confirms that these systems are completely
amorphous.
Moreover, there is an increasing glass temperature with
increasing LiTf content (at 231, 240, and 309 K for [EO]:[Li+]
salt concentrations of 12:1, 8:1, and 3:1, respectively). Only in
[EO]:[Li+] = 3:1 salt concentration there exists a small
endothermic peak at 372 K (ΔH ∼ 0.4 J/g). This situation in
the doped compound 8 is distinctly different from compound 4
with the strong ionic superstructure. (The self-assembly of the
neat compound 8 is discussed with respect to Figure S11.)
The effect of desymmetrization on the ionic conductivity is
discussed with respect to Figure 13. The figure depicts dc
conductivities for the doped compound 8 with salt concen-
H
DOI: 10.1021/acs.macromol.7b00224
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(6) Patel, S. N.; Javier, A. E.; Stone, G. M.; Mullin, S. A.; Balsara, N.
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structure, the electrolyte behaves as a viscoelastic solid with a
high shear modulus (G′ ∼ 108 Pa) at ambient temperature.
However, the presence of the ionic superstructure impedes ion
motion. At temperatures higher than the melting of the ionic
superstructure, ionic conductivity increases by an order of
magnitude to a value of 10−5 S/cm. As a means of suppressing
the ionic superstructure we have explored the desymmetriza-
tion effect on HBC functionalization e.g. with only two PEG
chains in the disk periphery. Asymmetric PEG substitution
breaks the symmetry of the ionic superstructure and increases
ionic conductivity by another order of magnitude while
retaining a high shear modulus (G′ ∼ 5 × 106 Pa) and a
viscoelastic response. These findings demonstrate that PEG-
functionalized HBCs have great potential as new electrolytes
because they combine high ionic conductivity with mechanical
stability through the mobile PEG phase and HBC columnar
structures, respectively.
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Zwitterionic Liquid Crystals as 1D and 3D Lithium Ion Transport
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Ungar, G.; Kato, T. Ionic Switch Induced by a Rectangular-Hexagonal
Phase Transition in Benzenammonium Columnar Liquid Crystals. J.
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Ionic conduction can be combined with electronic
conduction. Efforts in this direction could pave the way for
the construction of molecular devices based on HBC-PEG
coaxial structures that best combine electronic and ionic
pathways.
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dimensional Electronic Conductivity in Discotic Liquid Crystals. J.
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(12) van de Craats, A. M.; Warman, J. M.; Fechtenkotter, A.; Brand,
̈
J. D.; Harbison, M. A.; Mullen, K. Record Charge Carrier Mobility in a
̈
ASSOCIATED CONTENT
* Supporting Information
Room-Temperature Discotic Liquid-Crystalline Derivative of Hex-
abenzocoronene. Adv. Mater. 1999, 11, 1469−1472.
(13) van de Craats, A. M.; Warman, J. M. The Core-size Effect on the
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■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.7b00224.
(14) van de Craats, A. M.; Warman, J. M. Charge Mobility in Discotic
Materials Studied by PR-TRMC. Mol. Mol. Cryst. Liq. Cryst. 2003, 396,
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Sample preparation methods, ionic conductivity, struc-
tural relaxation times and structure analysis of compound
8 (PDF)
(15) Pisula, W.; Kastler, M.; Wasserfallen, D.; Mondeshki, M.; Piris,
J.; Schnell, I.; Mullen, K. Relation between Supramolecular Order and
̈
Charge Carrier Mobility of Branched Alkyl Hexa-peri-hexabenzocor-
onenes. Chem. Mater. 2006, 18, 3634−3640.
AUTHOR INFORMATION
Corresponding Author
*(G.F.) E-mail gfloudas@uoi.gr.
■
(16) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb, U.;
Tracz, A.; Sirringhaus, H.; Pakula, T.; Mullen, K. A Zone-Casting
̈
Technique for Device Fabrication of Field-Effect Transistors Based on
Discotic Hexa-peri-hexabenzoeoronene. Adv. Mater. 2005, 17, 684−
689.
ORCID
Klaus Mullen: 0000-0001-6630-8786
̈
George Floudas: 0000-0003-4629-3817
(17) Feng, X.; Marcon, V.; Pisula, W.; Hansen, M. R.; Kirkpatrick, J.;
Notes
Grozema, F.; Andrienko, D.; Kremer, K.; Mullen, K. Towards High
̈
Charge-Carrier Mobilities by Rational Design of the Shape and
Periphery of Discotics. Nat. Mater. 2009, 8, 421−426.
(18) Hill, J. P.; Jin, W.; Kosaka, A.; Fukushima, T.; Ichihara, H.;
Shimomura, T.; Ito, K.; Hashizume, T.; Ishii, N.; Aida, T. Self-
Assembled Hexa-peri-hexabenzocoronene Graphitic Nanotube. Science
2004, 304, 1481−1483.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The current work was supported by the Research unit on
Dynamics and Thermodynamics of the UoI cofinanced by the
European Union and the Greek state under NSRF 2007-2013
(Region of Epirus, call 18).
(19) Haase, N.; Grigoriadis, C.; Butt, H.-J.; Mullen, K.; Floudas, G.
̈
Effect of Dipole Functionalization on the Thermodynamis and
Dynamics of Discotic Liquid Crystals. J. Phys. Chem. B 2011, 115,
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