Regulating mesogenic properties of ionic liquid crystals by preparing binary or multi-component systems

Article abstract of DOI:10.1039/c1jm14790k

In this study, a series of binary systems based ionic liquid crystals (ILCs) C₁₂MIm(BF₄)_yI_x (x + y = 1) are prepared by mixing ILCs 1-dodecyl-3-methylimidazolium iodide (C ₁₂MImI) and 1-dodecyl-3-methylimidazolium tetrafluoroborate (C ₁₂MImBF₄), aiming to combine the properties of both C ₁₂MImBF₄ and C₁₂MImI. Their mesogenic properties are characterized by thermal analysis, polarized optical microscopy, X-ray diffraction as well as anisotropic ionic conductivity measurements. It is found that the smectic A phase with interdigitated bilayer structure is observed in the C₁₂MIm(BF₄)_yI_x series. The binary system C₁₂MIm(BF₄)_yI_x has a larger mesophase temperature range and interdigitated layer spacing than pure C₁₂MImBF₄. And the anisotropic ionic conductivity of the binary system, including parallel and perpendicular to smectic layer, is higher than pure C₁₂MImI. Moreover, in binary system C ₁₂MIm(BF₄)_yI_x, with the increasing content of C₁₂MImI the ability of the anion to form three dimensional hydrogen bonds increases, resulting in the increase of the mesophase temperature range, interdigitated layer spacing and the decrease of anisotropic ionic conductivity. Furthermore, multi-component system based ILCs are obtained by introducing iodine to the binary system, and the mesophase temperature range is lowered, which is associated with the weak ability of I₃ ^- to form hydrogen bond with cations. All these results suggest that we can regulate the mesogenic properties of ILCs as we need by making binary or even multi-component systems.

Full text of DOI:10.1039/c1jm14790k

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PAPER
Regulating mesogenic properties of ionic liquid crystals by preparing binary or
multi-component systems†
Meng Wang, Xu Pan, Shangfeng Xiao, Changneng Zhang, Wenxin Li and Songyuan Dai*
Received 26th September 2011, Accepted 3rd November 2011
DOI: 10.1039/c1jm14790k
In this study, a series of binary systems based ionic liquid crystals (ILCs) C MIm(BF ) I (x + y ¼ 1)
1
2
4 y x
are prepared by mixing ILCs 1-dodecyl-3-methylimidazolium iodide (C MImI) and 1-dodecyl-3-
1
2
methylimidazolium tetrafluoroborate (C12MImBF
4
), aiming to combine the properties of both
C12MImBF and C12MImI. Their mesogenic properties are characterized by thermal analysis,
4
polarized optical microscopy, X-ray diffraction as well as anisotropic ionic conductivity measurements.
It is found that the smectic A phase with interdigitated bilayer structure is observed in the C12MIm
(
4 y x 4 y x
BF ) I series. The binary system C12MIm(BF ) I has a larger mesophase temperature range and
interdigitated layer spacing than pure C MImBF . And the anisotropic ionic conductivity of the
1
2
4
binary system, including parallel and perpendicular to smectic layer, is higher than pure C MImI.
1
2
Moreover, in binary system C MIm(BF ) I , with the increasing content of C MImI the ability of the
12
1
2
4 y x
anion to form three dimensional hydrogen bonds increases, resulting in the increase of the mesophase
temperature range, interdigitated layer spacing and the decrease of anisotropic ionic conductivity.
Furthermore, multi-component system based ILCs are obtained by introducing iodine to the binary
system, and the mesophase temperature range is lowered, which is associated with the weak ability of
ꢀ
I
3
to form hydrogen bond with cations. All these results suggest that we can regulate the mesogenic
properties of ILCs as we need by making binary or even multi-component systems.
1
. Introduction
in the smectic A and columnar phases compared to that in the
5
isotropic phase. Thus, they have exhibited interesting potential
Ionic liquids (ILs) are molten salts with melting points at or
below ambient temperature, which have attracted great interest
due to their unique properties, including nonvolatility,
nonflammability, good thermal stability, and high ionic
applications as ion conductive materials in electrochemical
6
devices, as organized reaction media for the selectivity for
7,8
organic reactions, and as templating agents for functional
9
nanostructured materials in materials science.
1–3
conductivity.
Liquid crystals (LCs) are considered as the
Because of the widespread application of imidazolium-based
10–12
ILs, imidazolium-based ILCs have been frequently studied.
‘‘fourth state of matter’’, whose properties are intermediate
between that of the crystalline solid state and that of the liquid
state. The merger of ILs and LCs has yielded a new class of
materials called ionic liquid crystals (ILCs). ILCs can be
considered as substances that combine the properties of both ILs
and LCs. Some properties of ILCs differ significantly from not
only those of conventional neutral organic LCs, but also
conventional ionic liquids because of their anisotropic character
caused by the self-organized orientational ordering at the
Many studies are focused on the 1-alkyl-3-methylimidazolium
salts based ILCs. The smectic phase with lamellar structure is
commonly obtained for imidazolium-based ILCs. Generally, with
1
the increase of the long alkyl chain attached to the N -position of
the imidazolium ring, the mesophase temperature range increases
13
drastically, while the anisotropic ionic conductivity decreases.
Additionally, some groups have also studied counterion effects on
mesogenic properties. Examples of liquid crystalline 1-alkyl-3-
methylimidazolium salts containing different anions such
4
nanometre level. For instance, ionic conductivity was enhanced
1
4
15
10
as chloride, bromide, hexafluorophosphate and tetra-
11
fluoroborate have been reported, which shows that the coun-
terion has a great effect on the mesogenic properties of ILCs. All
these efforts have been made to explore a kind of ILCs with a large
mesophase temperature range and a high anisotropic ionic
conductivity for further application. However, a phenomenon is
usually observed that one material has a relatively larger meso-
phase temperature range but lower anisotropic ionic conductivity,
while another material has larger anisotropic ionic conductivity
Key Laboratory of Novel Thin Film Solar Cells, Institute of Plasma
Physics, Chinese Academy of Sciences, P. O. Box 1126, Hefei, Anhui,
2
30031, P. R. China. E-mail: sydai@ipp.ac.cn; Fax: +86-551-5591377;
Tel: +86-551-5591377
Electronic supplementary information (ESI) available: details about
†
1
H-NMR data, DSC traces of
interdigitated bilayer structure of
measurements of anisotropic ionic conductivity. See DOI:
0.1039/c1jm14790k
C
4 y x
12MIm(BF ) I , POM textures,
4 0.4 3 0.15
C12MIm(BF ) I0.45(I ) ,
1
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but a lower mesophase temperature range. For example, imida-
zolium salts with fluorinated anions tend to have higher aniso-
tropic ionic conductivity than compounds with nonfluorinated
different molar ratios of C MImI and C MImBF were dis-
1
2
12
4
solved in vigorously stirred solvent (acetone). Then the solvent
was removed in vacuo to yield the binary system based ILCs. The
multi-component system was prepared by introducing iodine to
a certain binary system followed by heating and re-cooling, then
repeated at least three times, by which stage it was assumed that
a homogeneous multi-component system had been made.
13,16
17
anions,
but a lower mesophase temperature range. Thus, this
triggered our interest to explore a new type of ILCs to combine the
advantages of two different ILCs.
Herein, a series of binary systems based ILCs were successfully
prepared by mixing ILCs 1-dodecyl-3-methylimidazolium
iodide (C12MImI) and 1-dodecyl-3-methylimidazolium tetra-
2.3 Analytical measurements
fluoroborate (C12MImBF
4
), aiming to combine the advantages
Differential scanning calorimetry (DSC) was performed with
a computer controlled thermal analyzer (DSC Q2000, TA
Instruments, America) with nitrogen as protection gas. The
samples were placed in aluminium pans which were cold-sealed
under nitrogen. Experimental data are displayed in such a way
that exothermic peaks occur at positive heat flow and endo-
thermic peaks at negative heat flow. Heating and cooling rates
of both C MImI and C MImBF . The reason why ILCs with
1
2
12
4
the same cations are chosen is that the imidazolium cation plays
the most important role on the mesogenic properties. Besides,
incorporation of C MImBF into the binary systems based ionic
1
2
4
liquid crystals C12MIm(BF
4
)
y
I
x
(x + y ¼ 1) may lead to high
anisotropic ionic conductivity, since fluoroborate ionic liquids
1
8
exhibit high conductivity. The mesogenic properties of binary
ꢀ
1
were set as 10 K min .
4 y x
systems C12MIm(BF ) I were characterized by several tech-
Polarized optical microscopy (POM) was performed using an
Olympus BX-51 microscope (Japan) equipped with a hot stage
under cross-polarized light at 100ꢂ magnification. The samples
were placed between two cover slips and inserted into the hot
niques, including differential scanning calorimetry (DSC),
polarized optical microscopy (POM), and small-angle X-ray
diffraction (SAXD). The anisotropic ionic conductivity of
C MIm(BF ) I was also measured for the liquid crystalline
1
2
4 y x
ꢁ
stage. The temperatures of all samples were fixed at 35 C.
mesophase. Furthermore, multi-component system based ILCs
were prepared by introducing iodine to the binary system. The
The small angle X-ray powder diffraction patterns of imida-
zolium salts were determined using a TTRAX3 diffractometer
mesogenic properties of a multi-component system as well as
ꢀ
ꢀ
with Cu-Ka X-rays, l ¼ 1.542 A (Rigaku, Japan). Data were
diffusion of I
3
in the multi-component system were also studied.
ꢁ
ꢁ
recorded from between 1 and 10 in steps of 0.02 at 300 K.
The anisotropic ionic conductivity was measured by the
impedance technique using an Im6e workstation (Zahner-Elek-
2
. Experimental section
2
.1 Materials
trick, German). Ionic conductivities, including parallel (s
k
) and
perpendicular to smectic layers (s
t
5,13,19,20
), were measured according
All reagents are purchased from Acros and used as received.
to the previously reported method.
A pair of Cu electrodes
was used to measure s (Fig. S2† and S3†). A Cu electrode (about
k
50 mm in thickness) was placed on a glass substrate. The sample
2
.2 Synthesis
1
was placed in the region between two Cu electrodes and covered
with a piece of glass substrate to help the formation of the
alignment. A pair of FTO (F-doped tin oxide) glass electrodes
Synthesis of 1-dodecyl-3-methylimidazolium iodide (C12MImI).
C
12MImI was synthesized by the quaternization of 1-methyl-
imidazole and 1-iododecane. The mixture of 1-methylimidazole
and 1-iodododecane was dissolved in acetone. Then the solution
was placed into a 58 mL Teflon lined, stainless steel autoclave
21
was used to measure st, (Fig. S4† and S5†). The sample was
sandwiched with two FTO glass electrodes and fixed with
a Teflon spacer (300 mm in thickness). The cell constants of these
cells were calibrated with a standard KCl aqueous solution at
room temperature and the change of cell constant with increasing
temperature is neglected.
ꢁ
and heated at 100 C for 12 h. The crude product was washed
with diethyl ether several times and the pale yellow viscous liquid
C MImI was obtained with a yield of 90.5%. The purity of
1
2
1
C MImI was identified by H-NMR (see ESI†).
1
2
The steady-state voltammetry of the multi-component system
was adopted in a conventional electrochemical cell equipped with
a 10 mm platinum ultra-microelectrode (CHI107) as working
electrode, and with a 1 mm radius platinum disk electrode
Synthesis of 1-dodecyl-3-methylimidazolium tetrafluoroborate
12MImBF ). A solution of 1-dodecyl-3-methylimidazolium
iodide(C12MImI)(3.378 g, 10mmol)inacetone(50mL)wasadded
slowly to a rapidly stirred solution of AgBF (2.04 g, 10.5 mol) in
acetone (20 mL) and stirring continued until no residual C12MImI
was detected with the use of AgNO in acetone. The solvent of the
(
C
4
(
CHI102) as counter electrode and reference electrode. A slow
4
ꢀ
1
scan rate (5 mV s ) was used in order to obtain steady-state
current–voltage curves. This work was carried out at 300 K on an
electrochemical workstation (CHI660A, CH Instruments Inc.,
Austin, TX).
3
filtrate was removed by rotary evaporation, then the crude product
was dissolved in dichloromethane (50 mL) and washed with water
(
3 ꢂ 40 mL). The organic layer was collected, dried over MgSO ,
4
filtered and the solvent was removed in vacuo to yield the
3. Results and discussion
C MImBF colourless liquid with a yield of 88.5%. The purity of
2
1
4
3
.1 Binary systems
1
C MImBF was identified by H-NMR as well (see ESI†).
1
2
4
3
.1.1 Thermal behavior of C12MIm(BF
traces (Fig. S1†), it can be seen that a mesophase is observed for
4 y x
12MImI, C12MImBF and binary systems C12MIm(BF ) I
4 y x
) I . From the DSC
Preparations of binary and multi-component systems. To make
the mixture of C12MImI and C12MImBF homogeneous,
4
C
4
2
300 | J. Mater. Chem., 2012, 22, 2299–2305
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(
x + y ¼ 1). The DSC data obtained from the DSC curves (phase
transition temperatures and phase transition enthalpies, DH) are
listed in Table 1 and Fig. 1. Obviously, C MImI has a relatively
1
2
larger mesophase temperature range than C12MImBF
increasing x from 0.2 to 0.8, the clearing point of C12MIm
4 0.8
BF increases significantly (except C12MIm(BF ) I0.2),
4
. With the
(
4
)
y
I
x
while the melting point varies little. As a result, the mesophase
temperature range increases with increasing x in C12MIm
(
4 y x
BF ) I , which suggests that the mesophase temperature range
can be regulated by making a binary system.
As reported, hydrogen bonding between anions and protons of
imidazolium cations plays an important part in the formation and
1
6,22
ꢀ
the stability of a mesophase.
ion and composed of the central atom (B) and co-ordinate ions
F). Herein, the hydrogen bonds between anions and protons of
The anion BF4 is a coordination
(
imidazolium cations in tetrafluoroborate saltsare formedbetween
F and protons. Charge density (r) is used to characterize the
hydrogen bond acceptor abilities of halides in this type of anions,
Fig. 1 The phase transition of C12MIm(BF
4
)
y
I
x
in (a) the heating
process and (b) the cooling process.
10,23
and r can be calculated by the following equation,
2
z
mesophase temperature range of pure C MImI is larger than
12
r ¼
(1)
n
that of pure C MIm BF . As for binary system C MIm
4
1
2
12
(
BF ) I , the ability of the anions to form a three dimensional
4 y x
Where, z is the overall charge on the ion and n is the number of
halide atoms in the complex. When r < 1, it will drastically
weaken the hydrogen bond between halide and cations. For
hydrogen bond will obey the following relationship,
ꢀ
ꢀ
ꢀ
BF4 < (BF ) I < I
4 y x
(3)
BF4 , the charge density of F is <1. As a result, compared with
iodide, the ability of the anions to form a three dimensional
hydrogen bond will obey the following relationship,
Thus, the mesophase temperature ranges of binary system
C MIm(BF ) I are intermediate between pure C MImBF
4
1
2
4 y x
12
ꢀ
ꢀ
and C MImI. Combining eqns (1), (2) and (3), we can see that
12
BF4 < I
(2)
ꢀ
with the increase of x, the ability of [(BF ) I ] to form three
4 y x
dimensional hydrogen bonds with imidazolium cations increases,
resulting in the increase of the mesophase temperature range of
Generally, the hydrogen bond between anions and protons of
imidazolium cations exerts a positive effect on the formation of
a mesophase. Therefore, it is easily understood that the
C
12MIm(BF
4
)
y
I
x
. Thus, it is suggested that the mesophase
4 y x
Table 1 A summary of the thermal analysis for the C12MIm(BF ) I series
Heating
Cooling
ꢁ
ꢀ1
ꢁ
ꢀ1
C
C
12MIm(BF
4
)
y
I
x
T/( C)
DH/(kJ mol
)
T/( C)
DH/(kJ mol
)
Phase transition
a
12MImBF
12MIm(BF
12MIm(BF
4
48.38
2
0.09
11.99
ꢀ3.17
0.29
31.73
ꢀ15.39
0.37
45.61
7.47
2.3
47.17
0.23
—
52.42
ꢀ0.86
—
ꢀ0.07
ꢀ4.91
ꢀ0.81
ꢀ0.26
ꢀ15.25
—
ꢀ0.27
ꢀ14.51
—
Lc–L
S–Lc
S1–S
1.93
b
ꢀ
14.9
48.54
0.3
12.64
53.77
0.73
8.04
5.65
57.32
1.26
6.9
12.73
65.58
4.42
6.85
C
C
4
)
)
0.8
0.6
I
0.2
0.4
Lc–L
S–Lc
S1–S
Lc–L
S–Lc
S1–S
2
ꢀ
4
I
2
1
ꢀ
2.95
1.73
b
ꢀ11.16
—
—
S2–S1
C
C
C
12MIm(BF
12MIm(BF
12MImI
4
)
)
0.4
0.2
I
0.6
0.8
0.30
2.03
55.87
ꢀ3.28
—
ꢀ0.28
ꢀ14.51
—
Lc–L
S–Lc
S1–S
S2–S1
Lc–L
S–Lc
S1–S
S2–S1
Lc–L
S–Lc
S1-S
2
1
ꢀ
12.21
ꢀ11.91
0.40
—
—
4
I
64.61
ꢀ4.27
—
ꢀ0.32
ꢀ14.91
—
2
1
ꢀ
3.50
11.05
ꢀ12.86
0.42
10.58
68.01
6.4
8.1
—
—
66.84
ꢀ8.03
—
ꢀ0.31
ꢀ13.68
—
2
1
ꢀ
1.66
12.73
ꢀ10.91
The symbols S, Lc, and L denote crytstal, liquid crystal and isotropic liquid. ‘‘S2–S1’’ and ‘‘S1-S’’ denote the phase transition from solid to solid.
3.28
—
—
S2–S1
a
b
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1
3
temperature range of C MIm(BF ) I can be regulated by
1
the long alkyl chain. With the increasing x, the peak shifts to
the lower angle region slightly, implying that the proportion
of the composition in C MIm(BF ) I would exert a slight effect
2
4 y x
altering the proportion of the composition, namely, the driving
forces for the formation of the mesophase.
1
2
4 y x
Additionally, it should also be noted that the melting transi-
on the structure of the SmA meophase. Fig. 4 shows layer
spacings (d) of C12MIm(BF plotted against x. It is seen that
d of C12MImBF is smaller than that of C12MImI and the layer
spacing d of the C12MIm(BF series is intermediate that of
pure C12MImI and pure C12MImBF . With the increase of x,
tion for C12MIm(BF
transition as observed for pure C12MImBF
several degrees. This phenomenon is reflected by the broad
peaks for C12MIm(BF 0.6, C12MIm(BF 0.8 and C12MIm
BF 0.2 (Fig. S1†), which may be associated with the exis-
tence of C12MImI in C12MIm(BF . While in the cooling
4
)
y
I
x
(x from 0.4 to 0.8) isn’t a sharp
4 y x
) I
4
, but is spread
4
4 y x
) I
4
)
0.4
I
4
)
0.6
I
4
(
4
)
0.8
I
d values gradually increases. Besides, the layer spacing d is found
to satisfy l < d < 2l, where l is the fully extended length of the
4 y x
) I
25–27
process, the peak is sharp at the phase transition from liquid
crystalline to solid, which is different from that in heating
process. Furthermore, in the cooling process, both the clearing
point and freezing point of C MIm(BF ) I can also be regu-
cation and is roughly estimated from crystallographic data.
This result indicates an interdigitated bilayer structure is formed
17
in the SmA liquid crystalline mesophase for C MIm(BF ) I
4 y x
1
2
as shown in Fig. 5.
1
2
4 y x
lated by altering the proportion of the composition, and the
mesophase for C12MIm(BF obtained in the cooling process
4 y x
As the C12MIm(BF ) I series has the same cation, the
4
)
y
I
x
difference in layer spacings d is mainly associated with the
17
is larger than that observed in the heating process due to the
extensive supercooling before crystallization (Fig. 1b).
anions. Bradley and co-workers have systematically investi-
gated the influence of anions on the structure of the SmA liquid
crystalline mesophase by using the small-angle X-ray scattering
technique. They found that the ability of anions to form a three
dimensional hydrogen bond would exert a significant effect on
the layer spacing d. Generally, anions forming a strong three
dimensional hydrogen bond with cations will help enlarge the
3
4 y x
.1.2 Structural properties C12MIm(BF ) I . The smectic
phase with lamellar structure is commonly obtained for four
types of ILCs (ammonium, phosphonium, imidazolium and
1
1
pyridinium salts), and there is no exception for C MImBF
4
1
2
6,24
and C MImI.
2
Since the cation dominates formation of the
layer spacing d. According to eqn (3), with the increase of x, the
ꢀ
1
10,14,17
smectic layer
12MImBF , it can be inferred that the mesophase of binary
systems C12MIm(BF are also smectic A phase (SmA). Fig. 2
shows the polarized optical microscopic (POM) texture of
12MIm(BF 0.6, which is typical of the C12MIm(BF
and C MImI has the same cations as
ability of [(BF ) I ] to form a three dimensional hydrogen bond
4 y x
1
2
C
4
with imidazolium cations increases, therefore, the layer spacing
4
)
y
I
x
d of C12MIm(BF
12MImI in the mixture. This will influence the anisotropic ionic
conductivity of the C12MIm(BF series as described below.
4 y x
) I increases with the increasing content of
C
C
4
)
0.4
I
4
)
y
I
x
4 y x
) I
series. It is obvious that focal conic fan textures with dark
homeotropic areas are observed. The dark homeotropic areas are
caused by rod-like molecules aligned parallel to the light source.
The focal conic fan textures originate from the formation of
layered structure. This result of the POM suggests that the liquid
crystalline mesophase of C MIm(BF ) I is assigned to the SmA
3.1.3 Anisotropic ionic conductivity of C12MIm(BF
anisotropic ionic conductivity of C12MIm(BF , including
parallel (s ) and perpendicular (s ) to the smectic layer, were
4 y x
) I . The
4 y x
) I
k
t
measured. The cell using Cu and FTO glass electrode was used
for this measurement. The formation of homeotropic alignment
of C MIm(BF ) I in the cell was obtained by exerting pressure
1
2
4 y x
mesophase and a lamellar structure which has an average
direction perpendicular to the layer surface is proposed.
1
2
4 y x
on the sample using a piece of glass (Fig. S6†), which has been
13
To study the structural properties of C12MIm(BF
detail, small-angle X-ray diffraction (SAXD) is performed for
12MIm(BF at 300 K in which the C12MIm(BF series,
including pure C12MImI and C12MImBF , are in SmA phase
Fig. 3). Only one peak is observed for all samples, suggesting the
loss of positional ordering in the layer plane due to melting of
4
)
y
I
x
in
reported previously. Fig. 6 shows the temperature dependence
of s and s for C12MIm(BF . For the C12MIm(BF
series, s is at least five times higher than s , suggesting the high
ion mobility in the ionic layer shown in Fig. 5. It is seen that the
conductivity of pure C12MImBF , including s and s , is higher
k
t
4
)
y
I
x
4 y x
) I
C
4
)
y
I
x
4
)
y
I
x
k
t
4
(
4
k
t
than that of pure C12MImI and the anisotropic ionic conduc-
tivities of binary systems C MIm(BF ) I are intermediate
1
2
4 y x
between that of pure C MImI and C MImBF . With the
1
2
12
4
4 0.4 0.6
Fig. 2 The polarized optical microscopic texture of C12MIm(BF ) I
at 308 K.
Fig. 3 SAXD patterns of the C12MIm(BF
4
)
I
y x
series at 300 K.
2
302 | J. Mater. Chem., 2012, 22, 2299–2305
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carriers and/or in the mobility will result in changing the total
conductivity value. As mentioned above, with the increasing x,
the layer spacing d of C MIm(BF ) I keeps increasing, result-
1
2
4 y x
ing in the reduction of the number of ion-conductive layers per
unit volume, namely, nanion. On the other hand, manion is expected
to relate to the hydrogen bond between the anions and protons of
30
imidazolium cations. According to eqn (2), we may obtain that
ꢀ
ꢀ
m of BF
12MImBF
ꢀ
4 y x
3), with the increase of x, the ability of [(BF ) I ] to form
4
is larger than that of I . Therefore, s
k
t
and s of
C
4
are larger than that of C12MImI. According to eqn
(
a three dimensional hydrogen bond with imidazolium cations
increases, which would result in the decrease of the mobility of
the anions. As a result, combining the changing of nanion with x,
the anisotropic ionic conductivity of C MIm(BF ) I decreases
Fig. 4 Layer spacings of C12MIm(BF
4
)
y
I
x
(x ¼ 0.2, 0.4, 0.6, 0.8) in the
liquid crystalline SmA mesophase (300 K).
1
2
4 y x
with the increasing content of C12MImI.
3
.2 Multi-component system
3
.2.1 Mesogenic properties of C12MIm(BF
ꢀ
4
)
0.4
I
0.45(I
3
)
0.15. It
ꢀ
is known that I can easily react with I
2
and form I
3
. Herein,
a certain content of I
BF 0.6 ([C12MImI] : [I
nent system I , BF
2
is introduced to the binary system C12MIm
(
4
)
0.4
I
2
] ¼ 4 : 1), and in the multi-compo-
ꢀ ꢀ
ꢀ
ꢀ
4
3 3
as well as I co-exist. The effects of I on
the mesophase temperature range of C12MIm(BF
shown in Fig. 7 and Table 2. It is found that a mesophase is also
formed in multi-component system C MIm(BF ) (I )
4 0.4
) I0.6 are
I .
4 0.4 0.45 3 0.15
1
2
Compared with binary system C MIm(BF )
2
I , both the
1
4 0.4 0.6
Fig. 5 A schematic illustration of the interdigitated bilayer structure of
12MIm(BF in the SmA liquid crystalline phase.
melting point and clearing point of C MIm(BF ) I (I )
12 4 0.4 0.45 3 0.15
C
4
)
y
I
x
decrease, which results in a relatively lower mesophase temper-
ꢀ
ature range. I
3
can be seen as a coordination ion which is
increase of x, both s
indicating that the anisotropic ionic conductivity of C12MIm
BF ) I can also be regulated by varying the proportion of the
k
t 4 y x
and s of C12MIm(BF ) I decrease,
composed of a central atom (I) and two co-ordinate ions (I).
According to eqn (1) the charge density of the co-ordinate ion I is
also <1. Therefore, the ability of anions to form a three dimen-
sional hydrogen bond will have the following relation,
(
4
y x
composition.
For ILCs, the cations are almost immobilized because they are
much larger than anion and are linked to the adjacent cation by
Van der Waals interactions. Therefore, the dominant charge
ꢀ
ꢀ
[
(BF
4
)
0.4
I
0.45(I
3
)
0.15] < [(BF
4
)
0.4
I
0.6
]
(5)
13
carriers is thought to be anions. As suggested by previous
28,29
reports,
the total conductivity is given by the relation,
4 0.4 0.45 3 0.15
Thus, the mesophase stability of C MIm(BF ) I (I )
1
2
decreases. As for the melting point, it may be associated with the
2
7,31
s ¼ nqm z nanionqmanion
(4)
interaction energy.
ꢀ
Due to the relative lower charge density of
ꢀ
I in I
3
and the larger radius of I
ꢀ
3
, coulombic interactions
Where n is the number of dissociated charge carriers per unit
volume, q is the charge carried by them, and m is the mobility of
the carriers. Therefore, any change in the number of charge
between I
3
ꢀ
and cations will be weakened compared with those
between I and cations. Consequently, the melting point of
C MIm(BF ) (I ) decreases. Additionally, since the
I
4 0.4 0.45 3 0.15
1
2
4 0.4
Fig. 7 DSC heating and cooling traces of C12MIm(BF ) I0.6 and
ꢀ
Fig. 6 Anisotropic ionic conductivities for C12MIm(BF
4
)
y x
I .
C
12MIm(BF
4
)
0.4
I0.45(I
3
)
0.15
.
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J. Mater. Chem., 2012, 22, 2299–2305 | 2303

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ꢀ
4 0.4 3 0.15
0.6 and C12MIm(BF ) I0.45(I )
Table 2 A summary of the thermal analysis for C12MIm(BF
4
)
0.4
I
Heating
Cooling
ꢁ
ꢀ1
ꢁ
ꢀ1
Ionic liquid crystals
T/( C)
DH/(kJ mol
)
T/( C)
DH/(kJ mol
)
Phase transition
C
12MIm(BF
4
)
0.4
I
0.6
57.32
0.30
2.03
12.21
ꢀ11.91
0.19
27.68
ꢀ1.38
55.87
ꢀ0.28
ꢀ14.51
—
Lc–L
S–Lc
S1–S
2
1
1.26
6.90
—ꢀ3.28
—
34.08
ꢀ8.30
ꢀ28.58
a
a
ꢀ12.73
38.70
—
S2–S1
C
12MIm(BF
4
)
0.4
I0.45(I
3
)
0.15
ꢀ0.18
ꢀ14.01
ꢀ0.07
Lc–L
S–Lc
S1–S
1
5.16
14.73
ꢀ
a
‘
‘S2–S1’’ and ‘‘S1–S’’ denote the phase transition from solid to solid.
cation in C12MIm(BF
12MIm(BF 0.6, the structure of C12MIm(BF
resembles that of C12MIm(BF 0.6, that is, an interdigitated
4 0.4 3 0.15
bilayer structure is formed in 12MIm(BF ) I0.45(I )
4 0.4 3
) I0.45(I )0.15 is the same as that in
C
4
)
0.4
I
4 0.4 3 0.15
) I0.45(I )
4 0.4
) I
C
(
Fig. S7† and S8†).
3
.2.2 Diffusion
of
iodide
species
in
C MIm
12
(
BF4)
I
0.4 0.45
(I3)0.15. As mentioned above, the existence of the
smectic layer would benefit the ionic conductivity, because the
SmA phase provides a two-dimensional conductive path in the
interlayer regions and consequently facilitates the electric
conductivity. Besides, it may also benefit the diffusion of iodide
species. The diffusion of iodide species in ILC C12MIm
Fig. 8 Steady-state voltammograms of ILC C12MIm(BF
4 0.4 3 0.15
) I0.45(I )
4 3
and IL [(C12MImBF )0.4(C11MIm)0.6I0.45(I )0.15] at 300 K.
(
BF
technique. An IL composed of C12MImBF
methylimidazolium iodide (C MImI) and I is used as the
4
)
0.4
I
0.45(I
3
)
0.15 was measured by the ultra-microelectrode
32
4
, 1-undecyl-3-
iodide species are located between the interdigitated layers,
which increases the collision frequency of iodide species and
1
1
2
reference and the IL is denoted [(C MImBF ) -
4 0.4
1
2
33
consequently the diffusion based on the exchange reaction.
ꢀ
(
C MIm)
11
I
0.6 0.45 3 0.15
(I ) ]. The molar ratio of C MImBF ,
12
4
ꢀ
As we know, I /I
electrolyte used in dye-sensitized solar cells (DSCs) as discovered
3
are frequently used as charge carriers in the
C MImI and I in [(C MImBF ) (C MIm) I (I ) ] is
0.6 0.45 3 0.15
1
1
3
12
4 0.4 11
0.45(I
3
0.4(C11MIm)0.6I0.45(I )
the same as in C12MIm(BF
observed in [(C12MImBF
shows the steady-state voltammograms of
4
)
0.4
I
3
)
0.15, and no mesophase is
0.15]. Fig. 8
12MIm
0.45(I
34
by Gr €a tzel, and the diffusion of iodide species plays a crucial
role on the photovoltaic performance of DSCs. Thus, ILCs may
be used to replace ILs as the electrolyte for DSCs due to their
contribution to the exchange reaction based diffusion (Fig. S9†).
4
)
C
(
BF
4
)
0.4
I0.45(I
3
)
0.15 and [(C12MImBF
4
)
0.4(C11MIm)0.6
I
3 0.15
) ]
ꢀ
at 300 K. The diffusion flux of I
3
can be obtained from the
32
following equation,
Conclusions
I
s
D -c - ¼
(6)
I
I
3
3
4
nFr
In summary, a series of binary systems based ionic liquid crystals
ꢀ
Where Is is steady-state current of I3 , DI3
ꢀ
ꢀ
is the apparent
ꢀ
(ILCs) C MIm(BF ) I (x + y ¼ 1) are prepared by mixing
12
4 y x
diffusion coefficient of I3 , cI
ꢀ
is the bulk concentration of I3 , n
the ILCs 1-dodecyl-3-methylimidazolium tetrafluoroborate
(C MImBF ) and 1-dodecyl-3-methylimidazolium iodide
3
is the electron transfer number per molecule, F is the Faraday
1
2
4
constant, and r is the radius of ultra-microelectrode.
ꢀ
(C12MImI). The effect of the molar proportion of C12MImBF
4 y x
and C12MImI on the mesogenic properties of C12MIm(BF ) I ,
4
From Fig. 8, it can be seen the I
BF
s
of I
3
in ILC C12MIm
(
4
)
0.4
0.4(C11MIm)0.6
12MIm(BF
0.4(C11MIm)0.6
I
0.45(I
3
)
0.15 is higher than that in IL [(C12MIm-
0.15], indicating that the diffusion of
0.15 is faster than that in
0.15] according to eqn (6).
including mesophase temperature range, layer spacings d as well
as anisotropic ionic conductivity are systematically investigated.
It is found that a SmA phase with interdigitated layers is formed
BF
ꢀ
4
)
I
0.45(I
3
)
I
3
in
C
4
)
0.4
I
0.45(I
3
)
[
(C12MImBF
However, the viscosity of ILC C12MIm(BF
larger than that of IL [(C MImBF ) (C MIm)
4
)
I
0.45(I
3
)
4 y x 4
in the C12MIm(BF ) I series the same as in pure C12MImBF
4
)
0.4
I
0.45(I
I
3
)
0.15 is
and C12MImI. With the increasing content of C12MImI in
ꢀ
(I ) ],
C MIm(BF ) I , the ability of [(BF ) I ] to form a three
12
1
2
4 0.4 11
0.6 0.45 3 0.15
4 y x
4 y x
which means the physical diffusion in C MIm(BF ) I (I )
4 0.4 0.45 3 0.15
dimensional hydrogen bond with cations increases, which results
in the increase of the mesophase temperature range and layer
spacing d as well as the decrease of the anisotropic ionic
1
2
is lower than that in [(C MImBF ) (C MIm)
4 0.4 11
I (I ) ].
0.6 0.45 3 0.15
1
2
This would lead to an idea that the exchange reaction based
diffusion in C12MIm(BF
(C12MImBF
4
)
0.4
I
0.45(I
3
)
0.15 is higher than that in
conductivity of C12MIm(BF
perpendicular (s ) to the smectic layer. It is suggested that the
binary systems C12MIm(BF combine the advantages of pure
4
)
y
I
x
, including parallel (s ) and
k
[
4
)
0.4(C11MIm)0.6
I
0.45(I
3
)
0.15]. Because of the exis-
t
tence of the smectic layer in C12MIm(BF
4
)
0.4
I
0.45(I
3
)
0.15, the
4 y x
) I
2
304 | J. Mater. Chem., 2012, 22, 2299–2305
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View Article Online
C MImBF and C MImI, and show a relatively larger meso-
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12
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2
4
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1
2
2
multi-component system based ILC C12MIm(BF
4
)
0.4
I
0.45(I
3
)
0.15
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12MIm(BF
4
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I
0.6
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I
4 0.4 0.45 3 0.15
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the National High Technology Research and Development
Program of China under Grant No. 2009AA050603, National
High Technology Research and Development Program of China
under Grant No. 2011AA050510, Funds of the Chinese
Academy of Sciences for Key Topics in Innovation Engineering
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