Liquid crystallinity-embodied imidazolium-based ionic liquids and their chiral mesophases induced by axially chiral tetra-substituted binaphthyl derivatives

Article abstract of DOI:10.1039/c4tc02968b

We synthesised novel imidazolium-based ionic liquids with thermotropic liquid crystallinity by introducing phenylcyclohexyl and/or cyanobiphenyl mesogenic cores and hexyl or dodecyl methine chains into both sites of imidazolium moieties facing bromide anions. The liquid crystalline ionic liquids (LCILs) thus synthesised showed a nematic or smectic mesophase in both the heating and cooling processes, indicating the enantiotropic nature of the liquid crystallinity. The LCILs bearing the same types of double mesogenic cores [LCIL-2] showed a smectic A phase in the temperature range from 115 to 175 °C, whereas the LCILs with different types of double mesogenic cores [LCIL-3 and LCIL-4] showed nematic phases in the temperature ranges from 58 to 88 °C and 43 to 95 °C, respectively. The axially chiral binaphthyl derivatives substituted by LC groups at the 2,2′, 6,6′ positions of the binaphthyl rings were synthesised and used as chiral dopants with large helical twisting powers. The mixtures of the LCILs and the (R)- and (S)-binaphthyl derivatives exhibited induced chiral nematic phases with right-and left-handed helical senses, respectively. The ionic conductivities of the LCILs were evaluated to be 10^-7-10^-4 S cm^-1, depending on the isotropic, LC, and crystal phases. The temperature dependence of the ionic conductivities indicates that the LCILs can be regarded as semi-conducting materials. The LCILs might be used as anisotropic ionic conductors and can even serve as anisotropic solvents and electrolytes in electrochemical polymerisations. This journal is

Full text of DOI:10.1039/c4tc02968b

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PAPER
Liquid crystallinity-embodied imidazolium-based
ionic liquids and their chiral mesophases induced
by axially chiral tetra-substituted binaphthyl
derivatives†
Cite this: J. Mater. Chem. C, 2015,
3, 3960
Sangbum Ahn, Shohei Yamakawa and Kazuo Akagi*
We synthesised novel imidazolium-based ionic liquids with thermotropic liquid crystallinity by introducing
phenylcyclohexyl and/or cyanobiphenyl mesogenic cores and hexyl or dodecyl methine chains into both
sites of imidazolium moieties facing bromide anions. The liquid crystalline ionic liquids (LCILs) thus
synthesised showed a nematic or smectic mesophase in both the heating and cooling processes, indicating
the enantiotropic nature of the liquid crystallinity. The LCILs bearing the same types of double mesogenic
cores [LCIL-2] showed a smectic A phase in the temperature range from 115 to 175 1C, whereas the LCILs
with different types of double mesogenic cores [LCIL-3 and LCIL-4] showed nematic phases in the
temperature ranges from 58 to 88 1C and 43 to 95 1C, respectively. The axially chiral binaphthyl derivatives
substituted by LC groups at the 2,20, 6,60 positions of the binaphthyl rings were synthesised and used as
chiral dopants with large helical twisting powers. The mixtures of the LCILs and the (R)- and (S)-binaphthyl
derivatives exhibited induced chiral nematic phases with right-and left-handed helical senses, respectively.
The ionic conductivities of the LCILs were evaluated to be 10ꢀ7–10ꢀ4 S cmꢀ1, depending on the isotropic,
LC, and crystal phases. The temperature dependence of the ionic conductivities indicates that the LCILs
can be regarded as semi-conducting materials. The LCILs might be used as anisotropic ionic conductors
and can even serve as anisotropic solvents and electrolytes in electrochemical polymerisations.
Received 27th December 2014,
Accepted 9th March 2015
DOI: 10.1039/c4tc02968b
www.rsc.org/MaterialsC
controlling the helical sense and helical twisting power (HTP,
1. Introduction
bM) of LCs. Chiral LCs are applicable for use in asymmetric
Ionic liquids (ILs) have unique physicochemical properties such reaction fields5 due to their anisotropic properties.
as ionic conductivity, thermal stability, non-volatility, and good
Liquid crystalline ionic liquids (LCILs or ionic liquid crystals)
affinity with organic and inorganic materials.1,2 Generally, ILs are novel functional materials that display both liquid crystallinity
consist of organic cations and inorganic anions. The physical and and ionic liquidity.6 The properties of LCILs can be controlled
chemical properties of ILs such as the melting point, viscosity, by changing the ionic moieties and their counter ions and also
and solubility characteristics can be varied by the selection of by changing the mesogenic cores.7 ILs have a random structure,
cationic and anionic moieties. ILs are used in a wide range of but LCILs have an oriented and self-organised structure.8–12
applications such as synthesis, catalysis, and electrochemical Due to their anisotropic properties and ionic conductivities,
reactions.3
LCILs are expected to be useful for multifunctional anisotropic
Liquid crystals (LCs) are regarded as ordered liquids. The materials.13–15 Recently, we have synthesised several types of
types of LC phases distinguish the positional and orientational axially chiral binaphthyl derivatives and evaluated their HTPs.16
order of their molecules. The order of thermotropic LC phases The binaphthyl derivative with tetra-substituents at the 2,20,
from highest to lowest is Cr > Sm > N > I (Cr: crystal, Sm: smectic, 6,60-positions of the naphthyl rings showed a high HTP when it
N: nematic, and I: isotropic phase).4 The order of LC molecules was added as a chiral dopant to a host nematic LC.
can be modulated by external stimuli (mechanical, magnetic
In this work, we designed and synthesised novel imidazolium-
or electric forces and temperature) and by the addition of based LCILs bearing double mesogenic cores. The LCILs consist
chiral molecules. Chiral dopants are especially effective for of two mesogenic cores, flexible methylene chains, and counter
anions (Scheme 1). The LCILs showed nematic (N) and smectic
(Sm) phases with enantiotropic nature. We achieved chirality
Department of Polymer Chemistry, Kyoto University, Katsura, Kyoto 615-8510,
induction on the LCILs by adding axially chiral tetra-substituted
Japan. E-mail: akagi@fps.polym.kyoto-u.ac.jp
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4tc02968b binaphthyl derivatives as chiral dopants. The N* phases of the
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LCILs were first observed through a chirality transfer from the when chiral LCILs are formed upon addition of chiral dopants
chiral dopants to the LCILs. We evaluated ionic conductivities to LCILs.
in various phases such as the Sm, N*, N, and isotropic (I)
phases. The present novel LCILs should be useful as aniso-
tropic ionic conductors and anisotropic electrolytes in electro-
chemical polymerisation. Electrochemical polymerisations in
not only organic solvents but also liquid crystals usually require
2. Results and discussion
2.1 Synthesis of LCILs
supporting electrolytes such as tetrabutylammonium perchlo- We synthesised four types of imidazolium-based LCILs bearing
rate (TBAP) and tetrabutylammonium tetrafluoroborate (TBAF). double mesogenic cores (Scheme 1). The LCILs consist of
In contrast, it is anticipated that LCILs enable us to achieve the imidazolium moieties, counter anions, two mesogenic cores,
electrochemical polymerisation without supporting electrolytes and flexible methylene chains. The mesogenic core is either a
and even allow asymmetric electrochemical polymerisation propylphenylcyclohexyl (PCH3) or cyanobiphenyl (CB) group,
and the methylene chain is either a hexamethylene or dodeca-
methylene group. As shown in Scheme 2, the mesogenic core
linked with imidazole (PCH3-IM) was synthesised via an Ullmann
reaction between 1-bromo-4-(4-propylcyclohexyl) benzene (PCH3Br)
and imidazole using CuI and Cs2CO3 as catalysts. PCH mesogenic
groups with bromomethylene chains (PCH3-C6-Br, PCH3-C12-Br)
were synthesised via a Mitsunobu reaction between bromoalkyl
alcohols and 4-(40-propylcyclohexyl)phenol using triphenylphos-
phine (TPP) and diethyl azodicarboxylate (DEAD). CB mesogenic
groups with bromomethylene chains (CB-C6-Br, and CB-C12-Br)
were synthesised via a Williamson reaction between dibromo-
alkane and 40-hydroxyl-(1,10-biphenyl)-4-carbonitrile using K2CO3.
The imidazolium-based LCILs were synthesised between PCH3-IM
and mesogenic groups with flexible spacers at 80 1C in toluene.
[PCH3-IM+-C2n-PCH3][Brꢀ] (n = 3, 6) and [PCH3-IM+-C2n-CB][Brꢀ]
(n = 3, 6) are abbreviated as LCIL-1, LCIL-2 and LCIL-3,
LCIL-4, respectively. The details of the synthesis and the
Scheme 1 Structures of imidazolium-based liquid crystalline ionic liquids
(LCILs) bearing double mesogenic cores.
Scheme 2 Synthetic routes of the mesogenic core with imidazole, mesogenic groups with flexible spacers, and LCILs (LCIL-1–4); (a) CuI, Ce2CO3, DMF,
120 1C, 3 days, (b) TPP, DEAD, THF, 0 1C, (c) K2CO3, acetone, 60 1C, (d) toluene, 80 1C.
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properties of the LCILs are described in the Experimental
section and ESI.†
2.2 Thermal and optical properties of LCILs
The phase transition temperatures and optical textures of the
LCILs were measured by polarised optical microscopy (POM)
and differential scanning calorimetry (DSC). Fig. 1 shows POM
images of the LCILs in the cooling process. In Fig. 1a and b,
LCIL-1 showed no liquid crystallinity, LCIL-2 showed an oily
streak texture assignable to a Sm phase at 140 1C in the cooling
process, and LCIL-3 and LCIL-4 showed a sand-like texture and
a marble texture, respectively, both of which were assigned to
an N phase (Fig. 1c and d). These N phases were stable over a
Fig. 2 Phase transition temperatures of LCILs determined by DSC. Heat-
broad range of LC temperatures. The phase transition tempera- ing and cooling rates are 2 1C minꢀ1
tures of the LCILs were evaluated through POM and DSC (Fig. 2
.
and 3). LCIL-2 showed a Sm phase at temperatures from 115 to
peak of d5 (7.4 Å) corresponds to the phenylcyclohexyl moiety of
LCIL-2. The broad halo of d6 (4.1 Å) corresponds to the inter-
distance between the neighboring LCIL-2 molecules (Fig. 4a
and c). The lengths of mesogenic cores and LCILs in Fig. 4c and d
were estimated using molecular mechanics calculations using
Spartan 8 software. The LC phase of LCIL-2 is assigned to a smectic
A (SmA) phase with an interdigitated layer structure.
175 1C in the cooling process. LCIL-3 and LCIL-4 showed an
N phase at temperatures from 58 to 88 1C and from 43 to 95 1C,
respectively, in the cooling process. LCIL-4, which has a longer
methylene spacer, showed a wider LC temperature range than
LCIL-3, which has a shorter methylene spacer. The phase transition
temperature and enthalpies of all of LCILs are summarised in
Table S1 (ESI†). Enthalpy changes for N phase transition in LCIL-3
and LCIL-4 (3.7–4.0 J gꢀ1) are smaller than that for Sm phase
transition in LCIL-2 (12.4 J gꢀ1).
The packing structures of the LCILs were investigated through
X-ray diffraction (XRD) measurements at the LC temperatures
(Fig. 4). The XRD profile of LCIL-2 showed two sharp peaks in
the small angle region (2y = 2.301 and 4.661), three weak peaks in
the middle angle region (2y = 7.41, 9.41, and 12.01), and a broad
halo in the wide angle region (2y = 21.71) at 150 1C (Fig. 4a). The
peak of d1 (37.7 Å) corresponds to the distance of the LCIL-2
molecule, and those of d2 (18.9 Å), d3 (12.5 Å), and d4 (9.4 Å) are the
half, one-third, one-fourth diffraction of d1, respectively. The weak
The XRD profile of LCIL-4 showed a substantially sharp peak
in the small angle region (2y = 4.01) and a broad halo in the
wide angle region (2y = 20.21) at 83 1C in the cooling process.
The peak of d2 (4.4 Å) corresponds to the interdistance between
neighbouring LCIL-4 molecules (Fig. 4b). The peak of d1 (22.3 Å)
corresponds to the distance of LCIL-4 bearing a bent structure at
the imidazolium moiety. The d1 value is significantly smaller
than the molecular length of LCIL-4 (35.8 Å). The value is
estimated by assuming that the LCIL-4 molecule has a straight
conformation. If the molecule is regarded to have a bent
structure, the value of 22.3 Å could be close to the distance
between the gravity centres of two mesogenic cores in LCIL-4
(Fig. 4d). It should be noted that the XRD profile of LCIL-4 seems
to be similar to that of SmC, but the intensity of peak is weaker
than that of the Sm phase. It has been reported that the reflection
peak in the small angle region in the N phase is attributed to a
cybotactic nematic (Ncyb) phase.17,18 Judging from the POM
images and the value of enthalpy change, as well as the XRD
profile, the XRD profile of LCIL-4 could be attributed to the Ncyb
phase rather than the SmC phase. LCIL-3 in the N phase showed
an XRD pattern similar to that of LCIL-4 (Fig. S5, ESI†).
2.3 Chirality induction on LCILs
We tried to introduce chirality in the LCILs by adding chiral
dopants to the LCILs in the nematic phase. The chiral dopants
synthesised were axially chiral tetra-substituted binaphthyl
derivatives bearing CB and/or PCH mesogenic cores (Scheme 3).
Note that the introduction of CB and/or PCH moieties as
substituents aims to increase the miscibility of the binaphthyl
derivatives with the LCILs and also to afford strong HTPs
sufficient to induce chiral nematic (N*) phases. Here, (R)/(S)-
binaphthyl derivatives with CB groups at the 2,20, 6,60 positions
Fig. 1 POM images of LCILs (LCIL-1–4) in the cooling process (cooling
rate is 2 1C minꢀ1): (a) crystal phase of LCIL-1 at 90 1C, (b) oily streak
texture of the smectic phase of LCIL-2 at 140 1C, (c) sand-like texture of
the nematic phase of LCIL-3 at 90 1C, and (d) marble texture of the
nematic phase of LCIL-4 at 90 1C.
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Fig. 3 DSC curves of LCILs measured at heating and cooling rates of 2 1C minꢀ1. (a) LCIL-1, (b) LCIL-2, (c) LCIL-3, and (d) LCIL-4.
Fig. 4 (a) X-ray diffraction (XRD) profiles measured at LC temperature. (a) LCIL-2 in the smectic A (SmA) phase observed at 150 1C, (b) LCIL-4 in the
cybotactic nematic (Ncyb) phase observed at 83 1C. Estimated molecular lengths and packing structures of (c) LCIL-2 in the SmA phase and (d) LCIL-4 in
the Ncyb phase.
of the binaphthyl rings are abbreviated as (R)/(S)-D1, those with positions of the binaphthyl rings as (R)/(S)-D2, those with PCH
CB groups at the 2,20 positions and PCH groups at the 6,60 groups at the 2,20 positions and CB groups at the 6,60 positions
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Scheme 3 Molecular structures of tetra-substituted binaphthyl derivatives as chiral dopants [(R)/(S)-D1–D4]. D1, D2, D3, and D4 are 2,20,6,60-CB
substituted binaphthyl derivative, 2,20-CB and 6,60-PCH substituted binaphthyl derivative, 2,20-PCH substituted and 6,60-CB substituted binaphthyl
derivative, and 2,20,6,60-CB substituted binaphthyl derivative, respectively.
of the binaphthyl rings as (R)/(S)-D3, and those with PCH groups POM observations (Fig. 5). The mixtures of LCIL-4 and (R)-D1
at the 2,20, 6,60 positions of the binaphthyl rings as (R)/(S)-D4.
[100 : 1 and 100 : 5 in mole ratio] showed fingerprint textures
The HTPs of the (R)/(S)-dopants were evaluated with the Cano assignable to N*-LC phases (Fig. 5a and b). The distances
wedge cell method,19 and the helical senses of the dopants were between the striae in the fingerprint texture became shorter
determined by the contact test (Table 1). The (R)- and (S)-dopants upon increasing the mole ratio of the chiral dopant, indicating
gave rise to right- and left-handed helical senses, respectively, in a strengthening of the helicity of the N*-LC. The mixtures of
both host N-LCs, pentylcyanobiphenyl (5CB) and 4-methoxy- LCIL-4 and (R)-D2 [100 : 1 and 100 : 5 in mole ratio] showed a
benzylidene-40-butylaniline (MBBA).
fingerprint texture and a blue colour image, respectively (Fig. 5c
Next, the mixtures of the LCILs and the chiral binaphthyl and d). The latter (blue colour image) is ascribed to selective
derivatives (D1–D4) were prepared by changing the mixing reflection, in which the helical pitches of the N*-LC are in the
ratios and were examined in terms of optical textures through range of UV and visible light wavelengths, and they are too short
Table 1 Helical twisting powers (HTPs) and helical senses of N*-LCs containing nematic host LCs, 5CB or MBBA, and chiral dopants (D1–D4), and helical
senses of N*-mixtures containing LCIL-4s and chiral dopants
5CB
MBBA
Specific rotatione
Helical sense
N*-LCs f
N*-LCa or
Chiral
dopants
Helical
HTPd
Helical
HTPd
N*-mixture
of LCIL-4sg
N*-mixtureb
pitchc (nm)
(mmꢀ1
)
pitchc (nm)
(mmꢀ1
)
(deg dmꢀ1 gꢀ1 cm3)
1
2
(R)-D1
(S)-D1
425
427
473
471
849
848
237
237
ꢀ58.77
Right
Left
Right
Left
+60.68
3
4
(R)-D2
(S)-D2
344
335
584
600
744
722
270
278
ꢀ26.70
Right
Left
Right
Left
+26.91
5
6
(R)-D3
(S)-D3
425
428
473
470
712
727
282
276
ꢀ60.17
Right
Left
Right
Left
+60.95
7
8
(R)-D4
(S)-D4
320
322
628
624
679
679
296
296
ꢀ37.06
Right
Left
—
—
+37.75
a
N*-LC contains host nematic LC of 4-cyano-40-pentylbiphenyl (5CB) or 4-methoxybenzylidene-4-butylaniline (MBBA) and chiral dopants. Host
b
c
LC : dopant = 100 : 0.5 in mole ratio. N*-mixture contains LCIL-4 and chiral dopant. LCIL-4 : dopant = 100 : 1 in mole ratio. Helical pitches
of N*-LCs were measured using a Grandjean-Cano wedge cell. Helical twisting power (HTP) of the chiral dopant, i.e. its ability to convert N-LC
d
into N*-LC, was evaluated using the equation, HTP = [(1/p)/c]c-0, where p is the helical pitch in mm and c is the mole fraction of the chiral dopant
e
25
f
in the N*-mixture. Rotational strengths [a] were measured in CHCl3 at 25 1C (c = 1 g Lꢀ1). Helical senses of N*-LCs were determined by the
589
g
contact test with LC standard (cholesteryl oleyl carbonate). Helical senses of N*-mixtures were determined by the two-step contact test (see,
Scheme 4 and Fig. 6 and 7).
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Fig. 5 POM images of the mixtures of LCIL-4 and various chiral dopants, observed at 80 1C in the cooling process. (a) LCIL-4 with (R)-D1 (100 : 1 mole
ratio), (b) LCIL-4 with (R)-D1 (100 : 5 mole ratio), (c) LCIL-4 with (R)-D2 (100 : 1 mole ratio), (d) LCIL-4 with (R)-D2 (100 : 5 mole ratio), (e) LCIL-4 with
(R)-D3 (100 : 1 mole ratio), and (f) LCIL-4 with (R)-D4 (100 : 10 mole ratio).
to be observed in POM measurements. The mixture of LCIL-4
and (R)-D3 [100 : 1 in mole ratio] also showed a fingerprint
texture but with domains small in size (Fig. 5e). However, the
mixture of LCIL-4 and (R)-D4 showed no fingerprint texture
based on N*-LC but rather a Schlieren texture assignable to the
N-LC, even if the concentration of the chiral dopant was increased
up to the mole ratio of 100 :30 (Fig. 5f and Fig. S6, ESI†). It should
be noted that although (R)-D4 showed the largest HTP in CB
solvent, it afforded no chirality induction on LCIL-4, as mentioned
above. This is because (R)-D4 is immiscible with LCIL-4, which
depresses the chirality transfer of (R)-D4 to LCIL-4.
Next, the helical senses of the mixtures of the LCILs and
chiral dopants were investigated through CD measurement and
contact tests.
UV-Vis and CD spectra of the mixtures of LCIL-4 with chiral
dopants, (R)/(S)-D1 in the cast film are depicted in Fig. 6. Fig. 6
Fig. 6 UV-Vis and CD spectra of the mixtures of LCIL-4 with (R)/(S)-D1 in
also shows the UV-Vis spectrum of LCIL-4 in the cast film and the
CD spectra of (R)/(S)-D1 dissolved in CHCl3. It is evident that the
mixtures of LCIL-4 with (R)- and (S)-D1 exhibit mirror images of
bisignate-like Cotton effects with negative and positive signs,
respectively, in the range of 250ꢀ300 nm in CD spectra, and that
these CD bands are different in wavelength from those of the
chiral dopants, (R)/(S)-D1. This result indicates that the bisignate-
the cast film. (top) UV-Vis absorption and (bottom) CD spectra of the
mixtures of LCIL-4 and (R)/(S)-D1. Red, blue, and black lines indicate
the mixtures containing (R)-D1 and (S)-D1, and LCIL-4, respectively. Pink
and green dot lines indicate the CD spectra of chiral dopants, (R)-D1 and
(S)-D1, in CHCl3, respectively (c = 2.5 ꢁ 10ꢀ5 M).
like Cotton effects are due to the chiralities on LCIL-4s induced via evaluating the helical handedness of the N*-mixture of a LCIL and a
additions of the chiral dopants to LCIL-4. Namely, these CD bands chiral dopant. However, cholesteryl pelargonate is less miscible with
are ascribed to the chiral nematic (cholesteric) helices.
a LCIL because of the large difference in polarity and/or the ionic
It is evident from Fig. S14 (ESI†) that the absorption intensity nature between the former and the latter. Thus, we employed a two-
of the chiral dopant, (R)-D1 is substantially smaller than that of step contact test, as depicted in Scheme 4. We prepared the N*-LC
LCIL-4 when LCIL-4 and (R)-D1 are mixed in 100: 5 mole ratio. containing 5CB and 4-(4-propylcyclohexyl)phenyl-4-ethoxybenzoate
This result implies that the CD spectra of the mixture of LCIL-4 (PCH3E02) as host LCs and (R)-D2 as a chiral dopant and examined
and (R)-D1 may not be affected by the chiral dopant, as far as the its helical sense using cholesteryl pelargonate as a LC standard
same mole ratio mentioned above is used in CD measurements. in the contact test (Scheme 4).
Here, it is worth noting that cholesteryl pelargonate is a
Fig. 7 shows a discontinuous boundary between the N*-LC of
typical cholesteric LC with left-handedness that retains its N* 5CB, PCH3E02 and (R)-D2 [100:200:3 in mole ratio] and the chole-
liquid crystallinity even at high temperature, and hence, it could steryl pelargonate used as a LC standard (1) with left-handedness.
be a candidate for a LC standard to be used in the contact test for The result indicates that the N*-LC is right-handedness. Using this
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Scheme 4 Two-step contact test for the sample [LCIL-4 : (R)/(S)-dopants = 100 : 1 mole ratio] using LC standard (1) [cholesteryl pelargonate] and LC
standard (2) [5CB : PCH3E02 : (R)-D2 = 100 : 200 : 3 mole ratio].
N*-LC as a LC standard (2) with right-handedness, the second were measured using the alternating current (AC) impedance
contact test was conducted for the N*-mixture of LCIL-4 and method in the cooling process from their isotropic phases. The
(R)/(S)-D1 or (R)/(S)-D2. The results are shown in Fig. 8.
ionic conductivity (s) is defined by the equation s = l(RꢂA)ꢀ1
,
Fig. 8a and c show that there is no boundary in the POM where l, R, and A represent the thickness, resistance and the
image between the LC standard (2) and the N*-mixture containing area of the LCIL sample, respectively. The LCIL sample is
LCIL-4 and (R)-D1 or (R)-D2, indicating that the latter has the same sandwiched between ITO glass electrodes and a Teflon spacer
helical sense as the former bearing the right-handedness. Namely, (l) of approximately 2 ꢁ 10ꢀ2 cm (thickness). The area (A) of the
the N*-mixture containing LCIL-4 and (R)-D1 or (R)-D2 has a right- sample is 7.07 ꢁ 10ꢀ2 cm2 (Fig. 9a).
handed helical sense. Meanwhile, Fig. 8b and d show that there is a
The ionic conductivity of LCIL-2 was 2.8 ꢁ 10ꢀ8–4.1 ꢁ
distinct boundary in the POM image between LC standard (2) and 10ꢀ6 S cmꢀ1 at 91–107 1C in the crystal phase and 6.5 ꢁ
the N*-mixture containing LCIL-4 and (S)-D1 or (S)-D2, indicating 10ꢀ6–2.3 ꢁ 10ꢀ5 S cmꢀ1 at 114–134 1C in the SmA phase. The
that the latter has an opposite helical sense to that of the former ionic conductivity of LCIL-4 was 6.8 ꢁ 10ꢀ7–7.8 ꢁ 10ꢀ6 S cmꢀ1
bearing the right-handedness. That is, the N*-mixture containing at 76–96 1C in the N phase and 8.7 ꢁ 10ꢀ6–1.4 ꢁ 10ꢀ4 S cmꢀ1 at
LCIL-4 and (S)-D1 or (S)-D2 has a left-handed helical sense. It is also 97–138 1C in the isotropic phase. In addition, the ionic con-
found that the N*-mixture of LCIL-4 and D3 has the same helical ductivity of the mixture of LCIL-4 and (R)-D1 was 3.3 ꢁ 10ꢀ7
–
–
sense as the N*-mixture of LCIL-4 and D1 or D2 (Fig. S9, ESI†). These 2.7 ꢁ 10ꢀ6 S cmꢀ1 at 76–94 1C in the N* phase and 5.3 ꢁ 10ꢀ6
results are consistent with those of the CD measurements (Fig. 6).
7.4 ꢁ 10ꢀ5 S cmꢀ1 at 96–138 1C in the isotropic phase (Fig. 9b).
It is clear that the ionic conductivities of the LCILs increase
(decrease) with increasing (decreasing) temperature, indicating
2.4 Ionic conductivities of LCILs
We measured the ionic conductivities of LCIL-2, LCIL-4, and that the LCILs have a semi-conducting nature. LCIL-4 and the
the mixture of LCIL-4 and (R)-D1 and investigated their tem- mixture of LCIL-4 and (R)-D1 show higher ionic conductivities
perature dependence. The ionic conductivities of the LCILs than LCIL-2 at the same temperature. This is most likely because
Fig. 7 Contact test of LC standard (1) [cholesteryl pelargonate] and the N*-LC [5CB : PCH3E02 : (R)-D2 = 100 : 200 : 3 mole ratio] at 85 1C.
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Fig. 8 Contact test between LC standard (2) and the N*- mixture containing LCIL-4 with a chiral dopant at 85 1C. The mixing mole ratio of LCIL-4 and
the chiral dopant is 100 : 1. (a) LCIL-4 with (R)-D1, (b) LCIL-4 with (S)-D1, (c) LCIL-4 with (R)-D2, and (d) LCIL-4 with (S)-D2. LC standard (2) is composed
of 5CB, PCH3E02, and (R)-D2 with 100, 200, and 3 mole ratio and is confirmed to have right-handedness.
Fig. 9 Ionic conductivities of LCILs. (a) Schematic illustration of the cell for measuring the ionic conductivity. (b) Temperature dependence of the ionic
conductivities of LCILs in the SmA, N, N*, and isotropic phases. Black, red, and blue curves indicate the ionic conductivities of LCIL-2, LCIL-4, and LCIL-4
with (R)-D1 (100 : 5 in mole ratio), respectively. Yellowish green, yellow, and light blue boxes indicate SmA, N, and N* phases of LCIL-2, LCIL-4, and LCIL-4
with (R)-D1, respectively.
the former has a lesser-ordered structure with fluidity, such as a less-ordered and non-twisted structure. Therefore, the mixture
N- or N*-phase, than the latter with a higher-ordered structure of LCIL-4 and (R)-D1 exhibits the slightly lower ionic conduc-
with rigidity such as an SmA phase. This allows the ion tivity than LCIL-4.
(bromonium ion: Brꢀ) as a charge carrier to more easily move
in the former than in the latter.
It is anticipated that the ionic liquidity embodied by N- and
N*-liquid crystallinity, such as LCIL-4 and the mixture of LCIL-4
Furthermore, the N*-phase of the mixture of LCIL-4 and and (R)-D1, might be used as anisotropic and asymmetric
(R)-D1 has a helical structure in terms of the LCIL molecule. In solvents with an ability of supporting electrolytes in electro-
this situation, the transportation of ion should be restricted, chemical polymerisations for the synthesis of aligned and
compared with the transportation of ion in N-phase bearing a helical polymers, respectively. Studies on the electrochemical
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polymerisations using LCILs and chiral LCILs is now underway Br, 12.22%; N, 4.28%; O, 2.45%, found: C, 71.27%; H, 9.71%; Br,
and will be reported somewhere.
11.64%; N 4.54%; O, 3.33%.
[PCH3-IM+-C12-PCH3][Brꢀ](LCIL-2): compound PCH3-IM
(0.2 g, 0.74 mmol) and PCH3-C12-Br (0.38 g, 0.82 mmol) were
stirred in toluene at 90 1C for 2 days. The solvent was removed
by evaporation, and the crude was purified using a silica-gel
3. Conclusion
Liquid crystallinity-embodied imidazolium-based ionic liquids column (CHCl3 : MeOH = 9 : 1). The product was dried in vacuo,
(LCILs) were synthesised by introducing double mesogenic and afforded LCIL-2 0.44 g (yield 81%) of white crystalline
cores into both sites of imidazolium moieties directly or via powder. 1H-NMR (400 MHz, CDCl3, d from TMS, ppm) d =
flexible methylene chains. The LCILs synthesised were char- 0.88ꢀ0.92 (m, 6H, ꢀCH3), 1.00ꢀ1.09 (m, 4H, ꢀCH2ꢀ),
acterised by POM, DSC, and XRD analyses and found to have 1.20ꢀ1.42 (m, 31H, ꢀCH2ꢀ), 1.71ꢀ1.76 (m, 2H, cyclohexane
enantiotropic, nematic and smectic phases. Subsequently, four ꢀCH2ꢀ), 1.86ꢀ1.88 (m, 8H, cyclohexane ꢀCH2ꢀ), 1.96ꢀ2.00 (m,
types of tetra-substituted axially chiral binaphthyl derivatives 2H, cyclohexane ꢀCH2ꢀ), 2.36ꢀ2.42 (t, 1H, J = 12.0 and 12.4 Hz,
with large helical twisting powers were synthesised and added phenyl-cyclohexane ꢀCH2ꢀ), 2.48ꢀ2.54 (t, 1H, J = 12.0 and 12.4
as chiral dopants to the LCILs to achieve chirality induction on Hz, phenylꢀcyclohexane ꢀCH2ꢀ), 3.89ꢀ3.93 (t, 2H, J = 6.4 and
the LCILs. The newly designed two-step contact tests demon- 6.8 Hz, ArꢀOꢀCH2ꢀ), 4.56ꢀ4.59 (t, 2H, J = 7.6 and 7.2 Hz,
strated that mixtures of LCILs and (R)- and (S)-chiral dopants imidazole NꢀCH2ꢀ), 6.80ꢀ6.82 (d, 2H, J = 8.8 Hz, phenylꢀH),
exhibited induced chiral nematic phases with right- and left- 7.09ꢀ7.11 (d, 2H, J = 8.8 Hz, phenylꢀH), 7.36ꢀ7.38 (d, 2H,
handed helical senses, respectively. The ionic conductivities of
the LCILs in the isotropic, LC, and crystal phases were 10ꢀ7
J
=
8.4 Hz, phenylꢀH), 7.69 (m, 2H, phenylꢀH), 7.71
–
(s, 1H, imidazoleꢀCH), 7.75 (s, 1H, imidazoleꢀCH), 10.98
10ꢀ4 S cmꢀ1, and they decreased with decreasing temperature, (s, 1H, imidazole NꢀCHꢀimidazole N). 13C-NMR (CDCl3, 100 MHZ,
indicating a typical semi-conducting behaviour. The present d from TMS, ppm): 14.39, 14.42, 19.99, 20.03, 26.08, 26.28,
LCILs with induced chiral mesophases could be used as aniso- 29.07, 29.38, 29.39, 29.49, 29.53, 30.45, 33.32, 33.64, 34.14,
tropic semi-conductors and electrolytes for asymmetric electro- 34.60, 36.89, 37.03, 39.59, 39.74, 43.73, 44.22, 50.45, 67.98,
chemical polymerisations.
114.26, 120.65, 121.67, 122.91, 127.57, 128.91, 132.28, 135.87,
139.93, 150.43, 157.23. HRMS (FAB, m/z): Calc. 732.4593, found:
813.3769 (M+ + 2Aꢀ). Anal. calcd for C45H69BrN2O: C, 73.64%;
H, 9.48%; Br, 10.89%; N, 3.82%; O, 2.18%, found: C, 73.45%;
H, 9.74%; Br, 10.64%; N 3.80%; O, 2.38%.
4. Experimental section
4.1 Synthesis of LCILs
[PCH3-IM+-C6-CB][Brꢀ](LCIL-3): compound PCH3-IM (1.00 g,
3.70 mmol) and CB-C6-Br (1.46 g, 4.07 mmol) were stirred in
The details of synthesis of LCILs are described in the ESI.†
[PCH3-IM+-C6-PCH3][Brꢀ] (LCIL-1): compound PCH3-IM toluene at 90 1C for 2 days. The solvent was removed
(0.10 g, 0.37 mmol) and PCH3-C6-Br (0.16 g, 0.41 mmol) were by evaporation, and the crude was purified using a silica-gel
stirred in toluene at 90 1C for 2 days. The solvent was removed column (CHCl3 : MeOH = 9 : 1). The product was dried in vacuo,
by evaporation, and the crude was purified using a silica-gel and afforded LCIL-3 1.50 g (yield 65%) of white crystalline
1
column (CHCl3 : MeOH = 9 : 1). The product was dried in vacuo, powder. H-NMR (400 MHz, CDCl3, d from TMS, ppm) d = 0.92
and afforded LCIL-1 0.17 g (yield 70%) of white crystalline (m, 3H, ꢀCH3), 1.07ꢀ1.53 (m, 25H, ꢀCH2ꢀ), 1.72ꢀ1.88 (m, 8H,
powder. 1H-NMR (400 MHz, CDCl3, d from TMS, ppm) d = cyclohexane ꢀCH2ꢀ), 2.40 (t, 1H, J = 12.0 and 12.0 Hz,
0.88ꢀ0.92 (m, 6H, ꢀCH3), 1.00ꢀ1.09 (m, 4H, ꢀCH2ꢀ), phenylꢀcyclohexaneꢀ), 3.42 (t, 2H,
J = 7.2 and 6.8 Hz,
1.21ꢀ1.43 (m, 20H, ꢀCH2ꢀ), 1.74ꢀ1.77 (m, 2H, cyclohexane ArꢀOꢀCH2ꢀ), 3.93 (t, 2H, J = 6.4 and 6.8 Hz, ArꢀCH2ꢀ), 6.65 (d,
ꢀCH2ꢀ), 1.86ꢀ1.89 (m, 8H, cyclohexane ꢀCH2ꢀ), 2.02 (m, 2H, 2H, J = 8.4 Hz, phenylꢀH), 7.35 (d, 2H, J = 8.4 Hz, phenylꢀH),
cyclohexane ꢀCH2ꢀ), 2.36ꢀ2.39 (t, 1H, J = 7.8 and 7.8 Hz, 7.49 (d, 2H, J = 8.4 Hz, phenylꢀH), 7.48ꢀ7.7 (dd, 4H, J = 8.4 Hz,
phenyl-cyclohexane ꢀCH2ꢀ), 2.49ꢀ2.52 (t, 1H, J = 12.0 and phenylꢀH), 7.81 (s, 1H, imidazole H), 7.90 (s, 1H, imidazole H),
12.4 Hz, phenylꢀcyclohexane ꢀCH2ꢀ), 3.89ꢀ3.92 (t, 2H, 10.92 (s, 1H, imidazole H). 13C-NMR (CDCl3, 100 MHZ, d from TMS,
J = 6.4 and 6.0 Hz, ArꢀOꢀCH2ꢀ), 4.59ꢀ4.63 (t, 2H, J = 7.1 ppm): 14.38, 19.97, 25.50, 25.93, 28.92, 30.36, 33.27, 34.10, 36.85,
and 6.8Hz, imidazole NꢀCH2ꢀ), 6.77ꢀ6.80 (d, 2H, J = 8.6 Hz, 39.55, 44.16, 50.22, 67.81, 109.88, 115.11, 119.11, 120.79, 121.60,
phenylꢀH), 7.08ꢀ7.10 (d, 2H, J = 8.4 Hz, phenylꢀH), 7.37ꢀ7.39 123.37, 127.03, 128.29, 128.86, 131.16, 132.27, 132.53, 135.53,
(d, 2H, J = 8.4 Hz, phenylꢀH), 7.55ꢀ7.57 (d, 2H, J = 8.8 Hz, 145.19, 150.37, 159.65. HRMS (FAB, m/z): calc. 625.2668, found:
phenylꢀH), 7.65 (s, 1H, imidazoleꢀCH), 7.67 (s, 1H, imidazo- 706.1840 (M+ + 2Aꢀ). Anal. calcd for C37H44BrN3O: C, 70.91%;
leꢀCH), 11.10 (s, 1H, imidazole NꢀCHꢀimidazole N). 13C-NMR H, 7.08%; Br, 12.75%; N, 6.17%; O, 2.55%, found: C, 70.66%;
(CDCl3, 100 MHZ, d from TMS, ppm): 14.40, 14.43, 20.01, 20.05, H, 7.13%; Br, 12.48%; N 6.49%; O, 2.36%.
25.58, 25.92, 29.02, 30.38, 33.31, 33.63, 34.14, 34.60, 36.89,
[PCH3-IM+-C12-CB][Brꢀ](LCIL-4): compound PCH3-IM (1.00 g,
37.04, 39.60, 39.74, 43.72, 44.24, 50.35, 67.51, 114.24, 120.32, 3.70 mmol) and CB-C12-Br (1.80 g, 4.07 mmol) were stirred in
121.71, 122.58, 127.63, 128.99, 132.21, 136.31, 140.08, 150.60, toluene at 90 1C for 2 days. The solvent was removed by evaporation,
157.05. HRMS (FAB, m/z): Calc. 652.3967, found: 758.3163 and the crude was purified using a silica-gel column (CHCl3 :MeOH
(M+ + 2Aꢀ). Anal. calcd for C39H61BrN2O: C, 71.64%; H, 9.40%; = 9 : 1). The product was dried in vacuo, and afforded LCIL-4 1.31 g
3968 | J. Mater. Chem. C, 2015, 3, 3960--3970
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Journal of Materials Chemistry C
(yield 50%) of white crystalline powder. 1H-NMR (400 MHz, removed by rotary evaporation. Purification by silica gel column
CDCl3, d from TMS, ppm) d = 0.90 (m, 3H, ꢀCH3), 1.07ꢀ1.46 chromatography (eluents: CHCl3 - CHCl3/EtOAc = 9) afforded
(m, 24H, ꢀCH2ꢀ), 1.76ꢀ1.98 (m, 8H, cyclohexane ꢀCH2ꢀ), 2.50 (S)-D1 0.40 g (yield = 88%) of white powder. 1H-NMR (400 MHz,
(t, 1H, J = 12.4 Hz, phenylꢀcyclohexaneꢀ), 3.47 (t, 1H, J = 7.2 CDCl3, d from TMS, ppm) d = 0.99 (m, 12H, ꢀCH2ꢀ), 1.08ꢀ1.26
and 6.8 Hz, cyclohexaneꢀH), 3.99 (t, 2H, J = 6.4 and 6.8 Hz, (m, 16H, ꢀCH2ꢀ), 1.41ꢀ1.43 (m, 8H, ꢀCH2ꢀ), 1.73ꢀ1.77 (m,
ArꢀOꢀCH2ꢀ), 4.60 (t, 2H, J = 7.2 and 7.6 Hz, ArꢀCH2ꢀ), 7.00 (d, 4H, ꢀCH2ꢀ), 3.94ꢀ4.00 (m, 8H, ꢀOꢀCH2ꢀ), 6.95ꢀ6.97 (d, 4H,
2H, J = 8.8 Hz, phenylꢀH), 7.38 (d, 2H, J = 8.8 Hz, phenylꢀH), J = 8.4 Hz, ꢀOꢀphenylꢀH and naphthaleneꢀH), 7.29ꢀ7.31
7.52 (d, 2H, J = 8.8 Hz, phenylꢀH), 7.58ꢀ7.70 (m, 8H, phenylꢀH (d, 2H, J = 8.4 Hz, phenylꢀH), 7.46ꢀ7.54 (m, 8H, phenylꢀH),
and imidazole H), 11.02 (s, 1H, imidazole H). 13C-NMR (CDCl3, 7.61ꢀ7.75 (m, 20H, phenylꢀH), 7.79ꢀ7.81 (d, 4H, J = 8.4 Hz,
100 MHZ, d from TMS, ppm): 14.39, 20.00, 26.02, 26.30, 29.07, phenylꢀH), 8.01ꢀ8.03 (d, 2H, J = 9.2 Hz, naphthaleneꢀH), 8.12
29.22, 29.36, 29.40, 29.49, 29.51, 30.46, 33.31, 34.14, 36.89, 39.59, (s, 2H, naphthaleneꢀH). 13C-NMR (CDCl3, 100 MHZ, d from
44.23, 50.50, 68.20, 109.96, 115.13, 119.12, 120.43, 121.69, TMS, ppm): 26.09, 26.41, 29.55, 29.60, 29.78, 29.88, 29.93, 29.42,
122.61, 127.08, 128.32, 128.97, 131.21, 132.24, 132.55, 136.15, 68.45, 70.02, 115.31, 116.49, 125.68, 127.30, 127.71, 127.83,
145.32, 150.56, 159.84. HRMS (FAB, m/z): calc. 709.3607, found: 127.99, 128.56, 129.65, 129.82, 132.79, 132.88, 133.91, 135.17,
790.2783 (M+ + 2Aꢀ). Anal. calcd for C43H56BrN3O: C, 72.66%; 137.82, 145.36, 145.44, 155.17, 159.96. HRMS (FAB, m/z): calcd
H, 7.94%; Br, 11.24%; N, 5.19%; O, 2.25%, found: C, 72.40%; 1362.6962, found: 1363.7033 (M + H). Anal. calcd for C, H96H90N4O4:
H, 8.16%; Br, 11.00%; N 5.89%; O, 2.50%.
C, 84.55%; H, 6.65%; N, 4.11%; O, 4.69%, found: C, 84.44%; H,
25
6.67%; N, 4.00%, O, 4.70%. Specific rotation: [a] = +60.68 deg
589
dmꢀ1 gꢀ1 cm3 (1 g Lꢀ1, CHCl3). HTP: 471 mmꢀ1 (in 5CB).
4.2 Synthesis of chiral dopants
The chiral dopants [(R)/(S)-D1–4] were synthesised using a
Suzuki coupling method. The details of the mesogenic boronic
acid derivatives (1–3), precursors of chiral dopant (4–6), and
other chiral dopants [(R)/(S)-D2–4] are given in the ESI.†
Acknowledgements
Chiral dopants. (R)-D1: (R)-5 (0.77 g, 0.66 mmol), 2 (0.50 g, This work was supported by a Grant-in-Aid for Science Research
1.64 mmol), NaHCO3 (0.33 g, 3.93 mmol), and Pd(TPP)4 were (A) (No. 25246002) from the Ministry of Education, Culture,
mixed in THF and H2O. The mixture was refluxed for 48 h at Sports, Science and Technology, Japan. The authors thank Mr
70 1C and then cooled to room temperature. The crude product S. Sato, Mr Y. Moritani, Mr M. Sakai, and Mr K. Nagao for the
was extracted with CHCl3. The organic layer was washed with measurements of XRD (Rigaku Corporation) and Mr M. Ikeda
saturated NaCl solution and dried over Na2SO4. The organic for measurements of ionic conductivities (Toyo Corporation).
solvent was removed by rotary evaporation. Purification by
silica gel column chromatography (eluents: CHCl3 - CHCl3/
EtOAc = 9) afforded (R)-D1 0.82 g (yield = 91.7%) of white
References
powder. 1H-NMR (400 MHz, CDCl3, d from TMS, ppm) d = 0.98
(m, 12H, ꢀCH2ꢀ), 1.09ꢀ1.28 (m, 16H, ꢀCH2ꢀ), 1.41ꢀ1.43
(m, 8H, ꢀCH2ꢀ), 1.71ꢀ1.77 (m, 4H, ꢀCH2ꢀ), 3.94ꢀ4.02 (m,
8H, ꢀOꢀCH2ꢀ), 6.95ꢀ6.97 (d, 4H, J = 8.4 Hz, ꢀOꢀphenylꢀH
and naphthaleneꢀH), 7.29ꢀ7.31 (d, 2H, J = 8.4 Hz, phenylꢀH),
7.46ꢀ7.54 (m, 8H, phenylꢀH), 7.61ꢀ7.75 (m, 20H, phenylꢀH),
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