Ion and hole dual-channel transport columnar mesomorphic organic electronic materials with high anisotropic conductivities based on supramolecular discotic ionic liquid crystals

Article abstract of DOI:10.1039/d0tc04931j

Thermotropic ionic liquid crystals (ILCs) are a kind of fascinating functional organic material combining the characteristics and advantages of both liquid crystals (LC) and ionic liquids. Here in this work, a series of supramolecular discotic ILCs of Cn-

Full text of DOI:10.1039/d0tc04931j

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Materials Chemistry C
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Ion and hole dual-channel transport columnar
mesomorphic organic electronic materials with
high anisotropic conductivities based on
Cite this: J. Mater. Chem. C, 2021,
9, 3871
supramolecular discotic ionic liquid crystals†
Jian Chen,‡a Yuhao Sun,‡a Weiguang Zhao,a Jiang Liu,a Jianglin Fang,b Tianchi Xu*a
a
and Dongzhong Chen
*
Thermotropic ionic liquid crystals (ILCs) are a kind of fascinating functional organic material combining the
characteristics and advantages of both liquid crystals (LC) and ionic liquids. Here in this work, a series of
supramolecular discotic ILCs of Cn-tri-CTP have been produced through host–guest complexation
together with p–p stacking and electrostatic interactions between the readily prepared electrically neutral
tris(18-crown-6)triphenylene (tri-CTP) and the facilely available potassium trialkoxybenzenesulfonates with
different length alkyl tails of carbon number n = 8, 10, 12, 14, 16. The thus obtained ILC complexes show
enantiotropic ordered columnar mesophases, such as center-faced rectangular columnar (Colr) and
oblique columnar (Colob) mesophases with broad LC temperature ranges and excellent thermal stability. It
is very interesting and fascinating to find that after a simple alignment, such supramolecular discotic ILC
complexes in their ordered columnar structures achieve simultaneously anisotropic ionic conductivities as
Received 17th October 2020,
Accepted 3rd February 2021
high as 1.99 ꢀ 10ꢁ2 S cmꢁ1 and hole mobility reaching 4.6 ꢀ 10ꢁ2 cm2 Vꢁ1
s
ꢁ1, thus exhibiting fantastic
DOI: 10.1039/d0tc04931j
dual-channel transport properties both for ions and holes, which may develop into a kind of versatile
organic electronic material for various promising unique applications.
rsc.li/materials-c
modifications played a pivotal role such as different spacers
between the aromatic core and alkyl tails, introducing polar groups,
Introduction
Thermotropic ionic liquid crystals (ILCs) are fascinating func- hydrogen bonding and metal coordination,16–23 which increased
tional organic materials, which integrate the self-assembly the intramolecular and intermolecular order and at the same time
characteristics and molecular orientational order of liquid reduced the stacking distance. Some discotic ILCs have been
crystals (LCs) with ionic conductivity and other advantages of reported to be capable of forming characteristic columnar
ionic liquids.1–8 Various columnar organizations of discotic LC structures and combining the ionic transport and electronic
semiconductors endow them with energy/charge transport conduction potentials as a kind of fascinating versatile
characteristics along the pseudo one-dimensional (1D) columnar organic semiconductor.5–8,24–33 The exploration of covalently
direction, thus bearing broad attractive applications in nano- bonded discotic ILCs has been limited because of their usually
technology and organic optoelectronics.9–15 It showed that realizing complicated synthesis and complex purification procedures for
efficient transport and high charge carrier mobilities in discotic LC the charged discotic aromatic moieties,5–8 while noncovalent
semiconducting materials was not only closely related to the central supramolecular approaches markedly simplify the synthesis
aromatic core, but also various molecular design or structural procedure, which can also show some particular functions
and expanded applications.34,35 Kato and coworkers demon-
strated that noncovalent design exhibited significantly enhanced
a Key Laboratory of High Performance Polymer Materials and Technology of
ionic conductivities in nanosegregated columnar LCs in virtue of
Ministry of Education, Collaborative Innovation Center of Chemistry for Life
the increased mobility of the ionic moieties.8,36
Sciences, Department of Polymer Science and Engineering,
School of Chemistry and Chemical Engineering, Nanjing University,
Nanjing 210023, China. E-mail: cdz@nju.edu.cn, xtc@nju.edu.cn
b Center for Materials Analysis, Nanjing University, Nanjing 210093, China
† Electronic supplementary information (ESI) available: Synthesis and characterization
of building blocks and some supplementary figures and tables. See DOI: 10.1039/
d0tc04931j
Crown ether has been playing a key role in supramolecular
assemblies via host–guest interactions.37–39 A range of crown
ether modified disc-like derivatives have already been reported,
with alkali cations or some onium salts employed for regulating
their association behaviors in solution40–42 or self-organized
superstructures in the bulk state.43–50 However, so far in most
‡ These authors contributed equally to this work.
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of the reported cases the decorated crown ether macrocycles
were usually located in the middle part with different number
alkoxy substituents covalently linked to the attached aromatic
moieties, exhibiting applications such as sensing for various
characteristic ions.40–47 Among these crown ether involved
discotic ILCs, triphenylene (TP) is among the most widely
adopted discotic cores.43–46,50 On the other hand, tapered or
wedge-shaped benzenesulfonates have been often employed for
the construction of ILCs through nanosegregated disc-like
stacking.51–55 Very recently, we reported a rational supra-
molecular strategy for constructing discotic ILCs via crown
ether based host–guest complexation from facilely synthesized
tris(18-crown-6)triphenylene and potassium of mono- or di-alkyl
substituted benzenesulfonates.56 This is the first example from
the easily accessible electrically neutral tri-crown-ether tripheny-
lene (tri-CTP) and potassium benzenesulfonates through host–
guest interactions to show rich phases of three-dimensional (3D)
cubic solid crystals, and 2D columnar and 1D lamellar LC
mesophases and predict that the 2D ordered columnar LC
mesophases may serve as promising organic electronic materials
both for electron and ion transport.56 However, in spite of their
abundant phase behavior characteristics, most of the complexes
from tri-CTP and potassium benzenesulfonates with mono- or
di-substituted alkyl tails developed only monotropic LC meso-
phases with narrower temperature ranges.56 What’s more, such
complexes from mono- or di-alkyl substituted benzenesulfonates
exhibited poor thermal stability with partial decomposition in
each thermal cycle thus severely limiting their significance and
application possibilities, which implied the direction for our next
Fig. 1 Schematic illustration and comparison of supramolecular ILCs
based on host–guest interactions between tri-CTP and potassium benze-
nesulfonates with mono- or di-alkyl substituted tails as previously reported
by our group and by adopting the facilely available potassium trialkoxy-
benzenesulfonates with different length alkyl tails in this work.
effort and promoted our further intensified study to boost di-alkyl substituted tails were first constructed via host–guest inter-
potential unique organic electronic applications. actions as recently reported in our previous work, which exhibited
In this work, with the readily prepared electrically neutral tri-CTP only monotropic LC mesophases, showing obvious thermal
as the host, a set of supramolecular discotic ILCs have been instability and greatly limiting their application potential. In this
constructed through host–guest interactions with the facilely avail- work, based on the easily obtainable and inexpensive potassium
able potassium trialkoxybenzenesulfonate derivatives of different trialkoxy-benzenesulfonates with different length alkyl tails and
length alkyl tails. The thus obtained discotic ILCs exhibited enantio- the same electrically neutral tri-CTP, well-organized discotic ILCs
tropic ordered columnar mesophases with significantly broadened have been achieved, showing enantiotropic ordered columnar
LC temperature ranges and excellent thermal stability thanks to the mesophases with excellent thermal stability.
increase in the proportion of nonpolar flexible alkyl moieties and
The host–guest complexes were prepared just by simply solution
synergistic enhancement through host–guest recognition, electro- mixing the host tri-CTP composed of the aromatic TP core with
static interactions and p–p stacking. Moreover, three crown ether three peripheral crown ether rings and slightly excessive guest
rings surrounding the TP core constituted the ion migration channel compounds of potassium trialkoxy-benzenesulfonates with variant
and greatly reduced the 1D transport resistance of potassium cations length alkyl tails of carbon number n = 8, 10, 12, 14, 16 (denoted as
to achieve a quite high ionic conductivity meeting the critical C8, C10, C12, C14 and C16, respectively) in dichloromethane. After
requirement for solid electrolytes in battery applications. Together evaporation of the solvent at room temperature, the obtained solid
with the measured rather high hole mobilities, such supramolecular mixture was co-dissolved in methanol and precipitated in deio-
discotic ILCs with dual-channel transport capabilities both for ions nized water; the dissolution/precipitation procedure was repeated
and holes are promising to be applied as novel functional organic three to four times to completely remove the excessive guest
electronic materials in various fields.
molecule. The composition and the stoichiometric ionic LC
complexes in a host/guest ratio of 1 : 3 were confirmed by the
characteristic H NMR spectra, wherein the integral area ratio of
1
the aromatic proton signals resulting from the TP core and from
the benzene ring of the associated guest trialkoxybenzenesulfonate
was exactly 1 : 1, denoted as a at around 7.83 and g at around 7.11,
Results and discussion
Building supramolecular ILCs from host–guest complexation
As shown in Fig. 1, discotic ILCs from tri-CTP of tris(18-crown-6) respectively, for the representative complex C12-tri-CTP (Fig. 2a).
triphenylene and potassium benzenesulfonates with mono- or As compared with that from the neat host component tri-CTP,
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Thermal properties and phase behaviors
The thermal properties and phase behaviors of the prepared
ILCs were investigated and comprehensively analyzed by a
combination of thermal gravimetric analysis (TGA), differential
scanning calorimetry (DSC), polarized optical microscopy
(POM) and small/wide angle X-ray scattering (SAXS/WAXS), as
summarized in Table 1.
The supramolecular ILC complex series exhibited high
thermal stability. The temperatures of 5% weight loss (T5%
)
both for C8-tri-CTP and C10-tri-CTP were around 210 1C, with
the decomposition temperature of about 250 1C. Those with
longer alkyl tails of C12-tri-CTP, C14-tri-CTP and C16-tri-CTP
showed more enhanced thermal stability, with T5% of at least
10 K higher than that of those analogues with shorter tails.
Among them, C12-tri-CTP displayed exceptionally high thermal
stability with T5% reaching 267 1C (Fig. S11, ESI†). DSC thermal
analyses together with POM observations were employed for the
determination of phase transition temperatures. Almost all the
series of complex samples exhibited a crystal melting peak at
around 30–50 1C in the 2nd heating DSC traces and a corres-
ponding crystallization exothermic transition during the cooling
scan with some degree supercooling, and also a weak high-
temperature phase transition peak at more than 100 1C with a
smaller enthalpy change (Fig. S12, ESI†). Except that C8-tri-CTP
with the shortest alkyl tails did not show any melting peak during
DSC scans, crystallization occurring was confirmed from POM
observation, which was conducted at a slower heating/cooling
rate. For the high-temperature phase transitions, compared to the
other homologues, C12-tri-CTP showed significantly enhanced
peaks, which are related to their crystallization dominated specific
phase behaviors as discussed next. POM investigations mani-
fested that during gradual cooling from the isotropic state,
obvious birefringence with atypical mosaic-like textures were
observed for the series of complexes in their LC mesophases
(Fig. S13, ESI†), except that the C12-tri-CTP complex always
showed feathered fan-shaped or focal conical fan-like crystalline
textures even persistent to higher temperatures consistent with
those from DSC analysis, manifesting the transition between
different crystalline forms of C12-tri-CTP with polymorphism until
the isotropization into a melting liquid state showing a dark field
(Fig. S14, ESI†). Upon reaching ambient temperature in the
crystalline state, C8-tri-CTP and C10-tri-CTP exhibited needle-
like textures, while C14-tri-CTP and C16-tri-CTP exhibited spherulite
Fig. 2 (a) The typical 1H NMR spectrum of the representative complex
C12-tri-CTP in CDCl3. (b) Comparison of the TP aromatic proton chemical
shifts in the 1H NMR spectra of the series of supramolecular LC complexes
Cn-tri-CTP (n = 8, 10, 12, 14, 16) and of the neat host molecule tri-CTP.
the peaks of the aromatic TP proton (Ar–H) in the supramolecular
LC complexes slightly shifted to a higher field (Fig. 2b), which was
attributed to the shielding effect of the ionic pair formed by a
sulfonate anion and a potassium cation bounded to the crown
ether of tri-CTP.56 Moreover, a more significant shift was exhibited
by complexes C8-tri-CTP and C10-tri-CTP with relatively short
alkoxy substituents, which implied the reduced ion pair effect with
the lowered proportion of the ionic moieties for those Cn-tri-CTP
(n = 12, 14, 16) with extended length alkyl tails and thus increased
van der Waals interactions.
Table 1 Thermotropic phase behaviors and associated structures of the series of supramolecular ILCs of Cn-tri-CTP (n = 8, 10, 12, 14, 16)
Thermal transitions/1C (enthalpy changes/J gꢁ1 a
)
Complex code
Second heatingb
First coolingb
T5%/1Cc
C8-tri-CTP
C10-tri-CTP
C12-tri-CTP
C14-tri-CTP
C16-tri-CTP
Cr 52d Mx 126(35.2) Iso
Cr 56(17.4) Colr 101(2.6) Iso
Cr1 38(109.8) Cr2 149(21.0) Cr3 174(98.3) Iso
Cr 40(250.5) Colob 158(17.7) Iso
Cr 54(543.4) Colob 159(24.6) Iso
Iso 115d Mx 44d Cr
207
208
267
225
218
Iso 88d Colr 48(11.9) Cr
Iso 143(16.8) Cr3 128(71.5) Cr2 10(67.8) Cr1
Iso 103(39.2) Colob 34(201.2) Cr
Iso 107(32.8) Colob 50(504.4) Cr
a
Cr, crystal phase; Mx, undefined mesophase; Colr, center-faced rectangular columnar LC mesophase; Colob, oblique columnar LC mesophase; Iso,
isotropic state. Heating or cooling rate at 10 1C minꢁ1
. From TGA. Determined from POM observation.
b
c
d
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crystalline textures (Fig. S13, ESI†). It is worth noting that unlike the longer alkyl tails of C14-tri-CTP and C16-tri-CTP at 80 1C, both
polycrystalline C12-tri-CTP could not be sheared until near the exhibited LC mesophases of an oblique columnar structure (P2)
isotropization temperature, all other analogous complexes of with lattice parameters of a = 4.03 nm, b = 3.50 nm, and y = 601
Cn-tri-CTP (n = 8, 10, 14, 16) showed fluidity in their LC temperature and a = 5.23 nm, b = 3.62 nm, and y = 56.51, respectively (Fig. 3d, e,
ranges as demonstrated with quite uniform alignment after and Fig. S18, S19, Tables S2 and S3, ESI†). For those complexes
shearing at 70 1C, exhibiting distinctly different shear-aligned showing columnar mesophases, such as C10-tri-CTP, C14-tri-CTP
POM images (Fig. S15, ESI†) in contrast to those before shearing and C16-tri-CTP, the distance between two adjacent tri-CTP
(the left column in Fig. S13, ESI†).
cores was determined from SAXS/WAXS analysis to be 0.35 nm
(Tables S1–S3, ESI†), comparable to or even closer than the typical
p-stacking distance in the TP-based columnar structures,14,15
indicating that the introduction of guests in the complexes did
not significantly perturb the stacking of the aromatic TP cores.
Instead, such a kind of supramolecular complexation promoted
noticeably the organization and packing of columnar structures as
effective as the reported various molecular design or structural
modification strategies,16–23 which constituted the structural basis
for their electron/hole transport together with the ionic conduc-
tance of the crown/K+ channel, contributing to their dual-channel
transport properties as discussed next. It is worth mentioning that
the calculated density of 1.36 g cmꢁ3 and 1.33 g cmꢁ3, respec-
tively, for the ionic complexes C10-tri-CTP and C14-tri-CTP
(Tables S1 and S2, ESI†) seemed to be slightly higher than
the common density values for organic solid materials, while
the densities of ionic complexes of triphenylene-substituted
crown ether complexed with K+ were calculated to be higher than
Self-organized structures
Variable temperature SAXS/WAXS analysis is a powerful tool for
investigating and elucidating the organized structures. In general,
all the complexes showed typical crystalline diffraction character-
istics of multiple sharp peaks at room temperature due to the
crystallization of the peripheral alkyl tails or the complex as a
whole (Fig. S16, ESI†). As shown in Fig. 3, at higher temperatures,
all other complexes exhibited different LC columnar mesophases
with a perceptible small peak at around q = 14.5 nmꢁ1 presumably
ascribed to the partial crystallization of alkyl chains, except that
C12-tri-CTP showed the crystalline state of various polycrystalline
forms in different temperature ranges with characteristic multiple
sharp peaks until above the clearing point (here, the highest
melting temperature). Specifically, for C10-tri-CTP at 80 1C in
the LC state, all the 2D Miller indices (hk) satisfied h + k = 2m
(m = 1, 2, 3,...), which indicated a center-faced rectangular
columnar LC mesophase (Colr, C2mm) with lattice parameters
of a = 4.70 nm and b = 4.35 nm (Fig. 3b, and Fig. S17, Table S1,
ESI†). The phase behavior of C8-tri-CTP was to some extent
similar to that of C10-tri-CTP, while it was much less organized
and not sufficient to clearly determine its phase structure due to
few scattering peaks and weak signal intensities. For those with
1.2 g cm .
ꢁ3 44 Moreover, in the ionic complexes of semifluorinated
tapered monodendrons containing crown ethers reported by
Percec and coworkers in various columnar mesophases, the
measured densities were even higher than 1.4 g cmꢁ3, attributed
to ion complexation, fluorine substitution and the ordered regular
stacking.57 The 2D discotic columnar mesophases and packing
structures of the series of ionic LC complexes are illustrated in
Fig. 4.
Electron density maps (EDMs)
Electron density maps (EDMs) can provide a visualized scenario
of electron density distributions, thus corroborating the
proposed structures or to some extent amending possible
deviations of the structure assignments.58–61 As shown in Fig. 5,
EDMs were reconstructed from the measured SAXS intensities for
complexes C10-tri-CTP, C14-tri-CTP and C16-tri-CTP in their
columnar LC mesophases of Colr or Colob. EDMs showed that
the reddish orange oval regions with the highest electron density
in both Colr and Colob were concentrated in the center of the
column, with the six violet areas of the lowest electron density
discretely distributed around the periphery of the column.
A comparison of EDMs for the ILC complexes also allowed us to
locate molecular building blocks in the corresponding supra-
molecular columnar structure. As shown in Fig. 5b, the shorter
axis length of the central high-density oval part was about
0.77 nm, which was in good agreement with the contour diameter
of the TP unit of 0.74 nm as estimated from its conformation by
Chem3D software. Meanwhile, the thickness of the medium
density areas around the central ellipsoid was also in accordance
with the size of crown ether, which further corroborated its
columnar stacking structure. The longer axis of the central
Fig. 3 SAXS/WAXS profiles of the series of complexes at indicated tempera-
tures with assigned phases. (a) C8-tri-CTP assigned as an undefined meso-
phase (Mx) at 80 1C due to limited effective signals, (b) C10-tri-CTP at 80 1C in
Colr, (c) C12-tri-CTP in the crystalline state persistent to high temperatures,
(d) C14-tri-CTP at 80 1C in Colob, and (e) C16-tri-CTP at 80 1C in Colob
.
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Fig. 4 Schematic illustration of the series of ionic LC complexes with proposed packing structures of 2D discotic columnar mesophases composed of
molecule cables with a central TP-stacking electron/hole hopping conduction channel and a surrounding ion channel through a facile supramolecular
approach of host–guest complexation.
ellipsoid in all three EDMs was different and increased slightly substituted systems and the significant improvement of thermal
with the lengthening of the alkyl tails, which indicated that the stability. Moreover, the composition and proportion between the
stacking of tri-CTP cores within the column was not simply nonpolar flexible alkyl moiety and the polar core composed of
overlapped, but stacking with a certain degree of dislocation. rigid aromatic TP and host–guest K+/crown ether directly affected
Such a stacking character was consistent with the measured the phase structure and its degree of order. Though at present we
results that the ionic conductivity decreased with the increased still cannot fully understand why an ordered polycrystalline phase
alkyl tails of the ILC complexes. From the stacking structure as was always formed for C12-tri-CTP in sharp contrast to the
shown in Fig. 5b, it is clear that the mono- or di-alkyl substituted columnar LC mesophases for other analogous complexes with
systems will lead to unfavorable space packing with overmuch shorter or longer alkyl tails, it was presumbly that the tri-
vacancies, which helps us to understand the transition from the dodecyloxy tails with flexibility and nonpolarity and the core
monotropic mesophases of mono- or di-alkyl based complexes56 moieties of rigid and polar nature might be exactly in an ideal
into the enantiotropic columnar LC phases of the tri-alkyl balance for the complex of C12-tri-CTP, leading to formation of a
highly ordered structure in polycrystalline solid state.
Anisotropic ionic conductivities
We explored and determined the ionic conductivities of the
columnar ILC complexes with the alternating current impedance
method adopting interdigital gold electrodes as reported by Kato
and coworkers.8,36,58,62–64 Fig. 6a shows the ionic conductivities of
the unoriented polydomain thin films of C10-tri-CTP, C14-tri-CTP
and C16-tri-CTP as a function of temperature. For comparison,
the temperature-dependent ionic conductivities of the neat guest
potassium trialkoxybenzenesulfonates of C10-K and C14-K were
also measured (for representative electrochemical AC impedance
curves see Fig. S20 (ESI†), with variable temperature ionic con-
ductivities summarized in Table S4, ESI†). However, the effective
ionic conductivities of C8-tri-CTP and C12-tri-CTP could not be
detected as a result of C8-tri-CTP in a relatively ill-organized
mesophase and C12-tri-CTP always in its polycrystalline state
Fig. 5 The reconstructed electron density maps (EDMs) for the complexes
of (a) C10-tri-CTP, Colr, 80 1C; (b) a single supramolecular column with the
before melting into isotropic liquid, which implied either not
forming or severely blocking effective channels for ion transport.
suggested molecular stacking mode for C10-tri-CTP; (c) C14-tri-CTP, Colob
,
Similarly, in the crystalline region of C14-tri-CTP and C16-tri-CTP
at room temperature, the ionic conductivities were too low to be
80 1C; and (d) C16-tri-CTP, Colob, 80 1C, with proposed lattice parameters
based on SAXS/WAXS analyses.
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(s>) to the interdigital electrodes by applying a shearing force in
the oriented films as confirmed by the changed POM images
rotating every 451 (Fig. 6c and d). It is worth mentioning that here
the ion transport was in parallel to the columns in the perpendi-
cularly aligned (s>) thin films, and across the columns in the
parallel aligned (s8) cases. For C10-tri-CTP, as shown in Fig. 6b,
the perpendicularly aligned (s>) conductivities were
10–15 times higher than those of the parallel aligned (s8). The
conductivity gap between s> and s8 only slightly changed with
the increase of temperature until over 80 1C, and then decreased
gradually until the gap disappeared showing almost the same
conductivities for the s8 and s> aligned samples upon the
increase in temperature up to 107 1C, which was beyond
the clearing point temperature and resulted in the breakup of
the columnar ionic channels. The anisotropic behavior of
complex C14-tri-CTP was similar to that of C10-tri-CTP, with the
ionic conductivities in parallel to the columns (s>) being about 2–
6 times higher than those across the columns (s8) at temperatures
below 100 1C, and then the difference decreased upon heating,
and finally merged at temperatures higher than 150 1C with
transition into the isotropic state (Fig. S21 and S22, ESI†). Similar
anisotropic behaviors of ionic conductivity were impressively
demonstrated by Kato and coworkers from various columnar ILC
systems.8,36,58,62–64
Fig. 6 (a) Ionic conductivities as a function of temperature of the series of
ILC complexes of C10-tri-CTP, C14-tri-CTP, and C16-tri-CTP, in the
unoriented polydomain thin films with a heating rate of 5 1C minꢁ1
,
together with those of the guest compounds of C10-K and C14-K for
comparison. (b) Ionic conductivities of C10-tri-CTP thin films in uniaxial
orientation with the columns in the Colr state aligned parallel (s8) or
perpendicular (s>) to the interdigital electrodes through shearing. POM
images of the perpendicularly aligned (s>) C10-tri-CTP thin film in the
dark (c) or under brightness (d) under crossed polarizers periodically
changed upon rotating every 451, with the shearing direction (S), polarizer
(P) and analyzer (A) indicated by single or two-way arrows.
From the ionic conductivity performance comparison of
three columnar ILC complexes, it can be concluded that the
ionic conductivities of C14-tri-CTP and C16-tri-CTP in oblique
columnar mesophase Colob are markedly lower than that of
C10-tri-CTP in Colr of the center-faced rectangular columnar
structure, which is mainly ascribed to the following two points:
(1) the tri-CTP cores stacked with an increased degree of
dislocation with the length increase of alkyl tails, which
resulted in the correspondingly increased structural distortion
of assembled ionic channels and thus the decrease of ionic
conductivities, as similarly reported for various columnar
mesophase transformations and difference in conductivities
by Kato and coworkers.58,59,64 (2) Since the ionic conductivities
of organic ionic conductors increase with increased ionic
content,36,62 the decreasing tendency of the ionic conductivities
of the ILC complexes of C10-tri-CTP, C14-tri-CTP and C16-tri-CTP
is consistent with the increased proportions of the non-
conductive alkyl moieties. These results further confirmed the
efficiency of 1D ion transport channels in various self-organized
columnar mesophases of Colr and Colob, with the orientation
direction being regulated simply through a directed shearing.
It is worth pointing out that the adoption of sulfonic
counterions here is also an important factor that cannot be
ignored, because our preliminary comparative study with the
sulfonic replaced by carboxylic counterions only showed that
the conductivities of the complexes from tri-CTP and potassium
trialkoxy gallate with various alkyl tails were remarkably
reduced as low as almost impossible to be detected even at
elevated temperatures, which was mainly attributed to the
formation of much closer ion pairs between potassium and
carboxyl counterions, thus seriously hindering the ion trans-
port. It is very encouraging to notice that the measured ionic
observably detected. However, the ionic conductivity of C10-tri-CTP
at room temperature can be well measured, reaching a value of
1.32 ꢀ 10ꢁ3 S cmꢁ1. With an increase in temperature, the
conductivities of the complexes increased gradually, when entering
the temperature range of columnar LC mesophases, reaching a
quite high level beyond 10ꢁ2 S cmꢁ1 for the C10-tri-CTP complex.
The temperature dependence and the increasing tendency of the
ionic conductivities during the transition from the crystalline to LC
mesophase state were in agreement with those disclosed by Kato
et al. based on ILCs36,62–65 and by Percec et al. from the supra-
molecular cylindrical channel-like architectures based on com-
plexation of the dendron-substituted crown ether and alkali metal
salts.66 As shown in Fig. 6a, the ionic conductivities of the neat
guest C10-K were 2–3 orders of magnitude lower than those of the
corresponding complex C10-tri-CTP at the same temperature. For
the guest C14-K with longer alkyl tails, the ionic conductivities
could not be measured at room temperature because of its
excessive resistance. At higher temperatures, the ionic conduc-
tivities of C14-K were 1–2 orders of magnitude lower than those
of the C14-tri-CTP complex. Therefore, the formation of columnar
ion channels in the ILC complexes indeed played a decisive role in
remarkably enhancing the ionic conductivities, which is more
prominently judged from the anisotropic ionic conductivity test as
presented below.
Furthermore, anisotropic ionic conductivities were measured
for the macroscopically aligned thin films of C10-tri-CTP
and C14-tri-CTP in their columnar mesophases (Table S5, ESI†).
A uniaxial orientation was achieved in two directions, with the
columnar mesophases aligned either parallel (s8) or perpendicular
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conductivities of 1.32 ꢀ 10ꢁ3 S cmꢁ1 at room temperature and p–p interactions between conjugated aromatic cores in the
1.48 ꢀ 10ꢁ2 S cmꢁ1 at 100 1C for the trialkoxybenzenesulfonate solid state and slightly increased with the lengthening of alkyl
based complex of C10-tri-CTP in the unoriented polydomain tails, which implied a correlation with their charge carrier
thin films even reached 1.99 ꢀ 10ꢁ2 S cmꢁ1 at 75 1C in uniaxial mobility performance.
orientation with the columns in the Colr state aligned perpendi-
To further verify the optoelectronic properties of ILC complexes,
cular (s>) to the interdigital electrodes (Fig. 6, and Tables S4 their charge carrier mobilities were evaluated by time-of-flight
and S5, ESI†), which has met the requirements for solid state (TOF) measurements. The TOF technique possesses some advan-
electrolytes competent for applications in commercial batteries tages such as simplicity of preparation and intuitive data analysis
(10ꢁ3–10ꢁ2 S cmꢁ1).67 Recently, as a new generation of alkali reflecting the long-range carrier mobility, which is much closer to
metal batteries, potassium ion batteries (PIBs) have attracted the real environment for applications. Therefore, the TOF test
much attention due to their more abundant reserve and results are more meaningful and widely used for optoelectronic
economic advantage of the element potassium as compared to performance evalution.15,19,20,73–83 The adopted laser pulse of wave-
lithium.68,69 However, the study of solid electrolytes for PIBs is length 337 nm is near the absorption shoulder peak of the UV-vis
still in its infancy as compared to Li+ or Na+ ionic batteries,68–70 spectra for the Cn-tri-CTP (n = 10, 14, 16) complexes in thin films
because the ionic conductivity of common solid electrolytes for (Fig. S24, ESI†). The electron mobility of the TP-based ILC com-
K+ is relatively low for the much larger size of K+ as compared to plexes was too low to be detected by the TOF equipment used. With
Li+, which is one of the main factors restricting the develop- an applied reverse bias voltage of 20 V, the representative photo-
ment of PIBs. Here the supramolecular discotic ILCs, showing current curves and temperature dependence of the hole mobilities
very high ionic conductivities thanks to the potassium ion for the ILC complexes measured by TOF are shown in Fig. 7 and in
channels based on columnar mesophases, are expected to be the ESI† (Tables S6–S8). The hole mobilities of C10-tri-CTP, C14-tri-
applied as solid-state electrolytes in PIBs, and further work CTP and C16-tri-CTP at room temperature in their crystalline state
related to this is underway.
were measured to be in the range from 10ꢁ3 to 10ꢁ2 cm2 Vꢁ1 sꢁ1
.
With the increase of temperature, the ILC complexes transformed
into ordered 2D columnar structures, resulting in the better estab-
lishment of transport channels, and thus the hole mobilities
increased significantly, reaching 10ꢁ2 to 10ꢁ1 cm2 Vꢁ1 sꢁ1 for the
annealed samples in their homeotropically aligned columnar
mesophases. The temperature dependence of hole mobilities as
Electrochemical properties and charge carrier mobilities
The C10-tri-CTP, C14-tri-CTP and C16-tri-CTP complexes and
the host compound tri-CTP were first investigated with cyclic
voltammetry (CV) to determine the approximate energy values
of their HOMO frontier orbitals with ferrocene as the reference
(Fig. S23, ESI†). Optical HOMO–LUMO gap energies (Eg) were
determined by UV-Vis spectroscopy both in solution and in film
(Fig. S23, ESI†), as presented in Table 2, together with the
values obtained by CV, while the LUMO levels of all investigated
samples were calculated from HOMO energy and Eg values
measured in solution, referring to the literature method.71,72
All compounds showed quasi-reversible oxidation peaks but
no reduction peaks in the observed electrochemical window
(ꢁ2 to 2 V). The obtained HOMO, LUMO and Eg values of
tri-CTP and complexes C10-tri-CTP, C14-tri-CTP and C16-tri-
CTP were in good agreement with those reported values
for triphenylene-ether deriatives,71,72 while Eg values of the
complex series in the film reduced below 3 eV mainly due to
Table 2 Oxidation potentials determined by CV and optical HOMO–
LUMO gaps of the Cn-tri-CTP complexes (n = 10, 14, 16) and the host
compound tri-CTP
d
d
1st Ox
Eg (in
Eg (in
HOMOb/
eV
LUMOc/
eV
solution)/
eV
film)/
eV
a
Eonset
/
Compound
V
C10-tri-CTP
C14-tri-CTP
C16-tri-CTP
Tri-CTP
0.84
0.82
0.90
0.98
ꢁ5.29
ꢁ5.27
ꢁ5.35
ꢁ5.43
ꢁ1.58
ꢁ1.57
ꢁ1.63
ꢁ1.70
3.71
3.70
3.72
3.73
2.36
2.86
2.94
—
a
EOx-onset in V vs. Ag/AgCl with a glassy carbon working electrode in
Fig. 7 The representative photocurrent curves with a laser pulse at 337 nm
(indicated with the test temperature) and temperature dependence of TOF
measured hole mobilities for the ILC complexes of (a and b) C10-tri-CTP,
(c and d) C14-tri-CTP, and (e and f) C16-tri-CTP.
b
DCM. EHOMO in eV = ꢁe[EOx-onset ꢁ 0.35 V] ꢁ 4.8 eV (0.35 V is the
EOx-onset of ferrocene/ferrocenium (Fc/Fc+) versus Ag/AgCl). Calculated
c
d
from HOMO energy and optical gap in DCM solution. Optical energy
gap (Eg) in DCM solution and film.
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measured by the TOF technique of the ILC complex series was largely columnar mesophases simply by directed shearing, with the
consistent with their thermal behaviors and phase transition trends, conductivities aligned perpendicularly to the electrode (s>) being
thus precluding the possible interference or artificial signals from ion many times higher than those of the parallel aligned (s8).
migration the carrier mobility of which increased monotonously with Moreover, the hole mobilities of the ILC complexes C10-tri-CTP,
temperature, conforming to the general characteristics of hole C14-tri-CTP and C16-tri-CTP in the homeotropically aligned
transport.84–86 It should be noted that in sharp contrast to the columnar mesophases after annealing as evaluated with the
inspiring performance of annealed samples, those upon cooling TOF technique reached rather high levels in the order of 10ꢁ2 to
rapidly into the LC temperature region from the isotropic melts 10ꢁ1 cm2 Vꢁ1 ꢁ1. Therefore, the supramolecular discotic ILC
s
failed to be detected due to the lack of sufficient non-discrete complexes in ordered columnar structures after proper alignment
photocurrent signals. Therefore, adequate annealing to realize the exhibited simultaneously rather high ionic conductivities and hole
desired homeotropic alignment and establish suitable conduction mobilities, such fantastic dual-channel transporting characteristics
pathways is an indispensable key procedure for such columnar ILC of both ions and holes should be very hopeful for their development
complexes to achieve high hole transport performance.
as versatile organic electronic materials for many promising unique
Molecular packing is believed to be another key point for applications.
determining and regulating the performance of organic and
polymeric optoelectronic materials.87 Here, the improved hole
Experimental section
Instrumentation
mobilities of 10ꢁ2 to 10ꢁ1 cm2 Vꢁ1 sꢁ1 achieved by the discotic
ILC complexes with tri-CTP cores in columnar mesophases are
comparable to those reported for the corresponding modified
alkoxy TP discotic monomers or dimers,19,20,77–79 indicating
more efficient packing of the aromatic TP moieties in virtue of
the host–guest interactions between the attached crown ether
and potassium cations.40,41,44,45,57,88,89 Such readily prepared
ILC complexes with good thermal stability in their properly
aligned ordered columnar structures exhibit fantastic dual-
channel transport properties both for ions and holes, which are
very attractive and hopeful to be developed as promising organic
electronic materials satisfying some specific requirements, such
as acting as versatile solid-state electrolytes in PIBs or in dye
sensitized solar cells (DSSCs).90,91
1H NMR and 13C NMR spectra in solution were obtained using
400 MHz (Bruker DRX400) instruments. Electrospray ionization
mass spectra (ESI-MS) were obtained on a LCMS-2020 single
quadrupole mass spectrometer (Shimadzu). Differential scanning
calorimetry (DSC) thermograms were recorded on a Mettler-Toledo
DSC I calorimeter equipped with a cooling accessory and under a
N2 atmosphere. Typically, about 8 mg of the solid sample was
encapsulated in a sealed aluminum pan with an identical empty
pan as the reference. Indium was used as a calibration standard.
The polarized optical microscope equipped with a Leitz-350 heating
stage and a Nikon (D3100) digital camera was used to characterize
thermal transitions, and observe and photograph various LC or
crystalline textures. The samples were prepared by melt-pressing
and sandwiched between two glass plates. X-ray scattering experi-
ments were performed with a high-flux small-angle X-ray scattering
instrument (SAXSess mc2, Anton Paar) equipped with a Kratky
block-collimation system and temperature control units (Anton
Paar TCS 120 and TCS 300). Both small angle X-ray scattering
(SAXS) and wide angle X-ray scattering (WAXS) were simultaneously
performed on an imaging-plate (IP) which extended to the high-
angle range (the q range covered by the IP was from 0.06 to
29 nmꢁ1, q = 4psin y/l, where wavelength l is 0.1542 nm for
Cu-Ka radiation and 2y is the scattering angle) at 40 kV and 50 mA.
The powder samples were encapsulated with aluminum foil for the
measurement and the obtained X-ray analysis data were processed
with the associated SAXSquant software 3.80. UV-vis spectra of thin
film samples were recorded using a Shimadzu UV3600 UV-vis
spectrophotometer. The thin films for UV-vis tests were typically
prepared by spin-coating the sample chloroform solution (100 mg
solid in 2 mL CHCl3) on quartz substrates (2.0 cm ꢀ 2.0 cm) at
800 rpm, and then the prepared thin films were annealing at 60 1C
for 6 h before UV-vis tests.
Conclusions
In conclusion, a series of supramolecular discotic ILCs of Cn-tri-CTP
have been produced through host–guest interactions together with
p–p stacking and electrostatic interactions between the readily
prepared electrically neutral tri-CTP and the facilely available potas-
sium trialkoxybenzenesulfonates with different length alkyl tails of
carbon number n = 8, 10, 12, 14, 16. The thus obtained ILC
complexes exhibited enantiotropic ordered columnar mesophases
with broad LC temperature ranges and excellent thermal stability
as characterized by TGA, DSC, POM and SAXS/WAXS. Such as for
C10-tri-CTP at 80 1C in the LC state, a center-faced rectangular
columnar LC mesophase (Colr, C2mm) was constructed, and for the
complexes with longer alkyl tails of C14-tri-CTP and C16-tri-CTP at
80 1C, oblique columnar mesophases (Colob, P2) were generated
with some different lattice parameters.
Ionic conductivities of some selected columnar ILC complexes
have been measured via the alternating current impedance
method to show impressive high values of several orders higher
than those of the corresponding neat guest potassium trialkoxy-
benzenesulfonates, and increased with elevated temperature to
enter the columnar LC mesophases. High anisotropic ionic con-
ductivities were achieved for the macroscopically aligned ILC
complex thin films of C10-tri-CTP and C14-tri-CTP in their
Reconstruction of electron density maps (EDMs)
Two-dimensional electron density maps58–61 were reconstructed
on the basis of the general formula:
X
rðxyÞ ¼
FðhkÞ exp½i2pðhx þ kyÞꢂ
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Here F(hk) is the structure factor of a diffraction peak with index platinum wire as a counter electrode were used. Typically, 20 mg
(hk). The experimentally observed diffraction intensity is given by: of complex Cn-tri-CTP (n = 10, 14, 16) was added into 10 mL CH2Cl2
with 0.1 M tetrabutylammonium hexafluorophosphate. The Eg
I(hk) = KꢃF(hk)ꢃF*(hk) = Kꢃ|F(hk)|2
values were determined from UV-vis spectra through extrapolating
the linear region to zero absorption.71
where K is a constant related to the sample volume, incident
beam intensity etc. In this work only the relative electron
densities are concerned, hence this constant is simply set to
Charge transport measured by time of flight (TOF)
The hole mobilities m of the complexes were measured using a
TOF-401 device. LC cells (E.H.C. Company, Japan) with semi-
transparent indium tin oxide (ITO) electrodes were loaded with
the samples via the capillary effect in their isotropic melts and
then cooled very slowly or annealed in the selected temperature
to obtain highly ordered LC films. The thus-prepared cells were
then illuminated with light pulses from a N2 laser (USHO KEC
160, wavelength = 337 nm, pulse width = 1000 ps). Simulta-
neously, the transient photocurrent across the cell at an external
bias voltage supplied with a power unit (Kikusui PAN110-3A) was
amplified using a NF low-digital phosphor oscilloscope (Tektronix
TDS 3032C). The transit time tT of the photo-generated charge
carriers travelling on the LC layer was determined from the
inflection point of a double-logarithmic plot of transient photo-
current as a function of time. Then the mobility was calculated
according to:
1. Thus the electron density is calculated according to the
equation:
X
rðxyÞ ¼
sqrt½IðhkÞꢂ exp½i2pðhx þ kyÞ þ fhkꢂ
where fhk is the phase angle of the hk Bragg reflection. As the
observed diffraction intensity I(hk) is only related to the amplitude
of the structure factor |F(hk)|, the information about the phase of
F(hk), fhk, cannot be determined directly from the experimental
results. However, the problem is significantly simplified with
most space group symmetries, where the phase angle fhk of the
given (hk) reflection is limited to 0 or p.
Dynamic ionic conductivity (DIC) measurements
The ionic conductivity was measured using a VersaSTAT 3F
(aka, V3F) Princeton Electrochemical Workstation equipped
with a Leitz-350 hot stage and VersaStudio software, with
parameters of an amplitude of 0.3 V, the frequency range of
100 Hz–1 MHz and a heating rate of 5 1C minꢁ1. In order to
ensure the reliability of the electrodes, temperatures involved
in our experiments were all below 200 1C. The surface of the
interdigital electrodes was cleaned with ethanol before use.
Then the resistance of the interdigital electrodes was moni-
tored with a multimeter to guarantee the insulating char-
acteristic. The samples were evenly applied on the surface of
the interdigital electrodes and then heated slowly to the clearing
point at a rate of 10 1C minꢁ1. Then the composite was covered
with a glass slide and slightly pressed to form a homogeneous
film on the surface of the electrode without any crack or air
bubbles, followed by cooling to room temperature at a rate of
20 1C minꢁ1. The working electrode of the electrochemical
workstation was connected to one of the interdigital electrodes,
and the reference electrode was connected to the other one. The
open circuit voltage in the preparation was measured to be 0.3 V.
Subsequently, the impedance was measured using the AC impe-
dance test mode with parameters of an amplitude of 0.3 V, an
initial frequency of 100 Hz and a terminal frequency of 1 MHz.
With the whole test device placed on the hot stage, the impedance
values were measured every 5–10 1C. The testing process and
adopted method for the conductivity calculation were similar to
that reported by Kato and coworkers.62–64
m = d2/VtT = d/EtT
where d is the thickness of the charge-transport layer (ca. 9 mm),
V is the applied bias voltage, and E is the electric field strength.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (Grants No. 21875098 and 21574062)
and also partially by the Program for Changjiang Scholars
and Innovative Research Team in University (IRT_16R38), and
the Fundamental Research Funds for the Central Universities
and the Scientific Research Foundation of Graduate School of
Nanjing University (Grant 2017ZDL06). We express our sincere
gratitude to Prof. Chunxiu Zhang at the Beijing Institute of
Graphic Communication for her kind help in the TOF measure-
ments and to Dr Huanjun Lu at Soochow University for her
generous assistance in the reconstruction of EDMs.
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