Multi-arm ionic liquid crystals formed by pyridine-mesophase and copper phthalocyanine

Article abstract of DOI:10.1016/j.molliq.2019.111012

Phthalocyanine emerged as a promising molecular component for photoreceptors because of their high extinction coefficients, stability, and energy band gaps well-matched to the incident solar spectrum. The cavity in the center can be occupied by dozens of cation types to form metallo-phthalocyanines which show a number of special properties such as electrical, optical, non-linear optical and opto-electronic. A kind of novel multi-arm ionic liquid crystalline materials combined the characteristics of liquid crystals with phthalocyanine were prepared using copper phthalocyanine as a matrix, [1,1′-biphenyl]-4,4′-diyl diisonicotinate or cyclohexane-1,4-diyl diisonicotinate as a mesogen, and different ions (B-SO₃ ^?, C-SO₃ ^?, H₂PO₄ ^? and BF₄ ^?) as counterions. The mesomorphic properties of these ionic liquid crystals were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction measurements. The copper phthalocyanine-based multi-arm ionic liquid crystals showed excellent electrical conductivity (the optimum value of conductivity can reach 0.0248 S/cm) because of their ionic channels and π-π stacking structure. This materials might provide a promising application prospect for electronic or optoelectronic applications.

Full text of DOI:10.1016/j.molliq.2019.111012

Accepted Manuscript
Multi-arm ionic liquid crystals formed by pyridine-mesophase
and copper phthalocyanine
Shengwen Kong, Xiaodong Wang, Lu Bai, Ying Song, Fanbao
Meng
PII:
S0167-7322(19)30357-5
https://doi.org/10.1016/j.molliq.2019.111012
111012
DOI:
Article Number:
Reference:
MOLLIQ 111012
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
19 January 2019
16 May 2019
20 May 2019
Please cite this article as: S. Kong, X. Wang, L. Bai, et al., Multi-arm ionic liquid crystals
formed by pyridine-mesophase and copper phthalocyanine, Journal of Molecular Liquids,
https://doi.org/10.1016/j.molliq.2019.111012
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ACCEPTED MANUSCRIPT
Multi-arm ionic liquid crystals formed by pyridine-mesophase
and copper phthalocyanine
*
Shengwen Kong, Xiaodong Wang, Lu Bai, Ying Song, Fanbao Meng
College of Science, Northeastern University, Shenyang, 110819, PR China. E-mail:
mengfb@mail.neu.edu.cn
Abstract
Phthalocyanine emerged as a promising molecular component for photoreceptors
because of their high extinction coefficients, stability, and energy band gaps
well-matched to the incident solar spectrum. The cavity in the center can be occupied
by dozens of cation types to form metallo-phthalocyanines which show a number of
special properties such as electrical, optical, non-linear optical and opto-electronic. A
kind of novel multi-arm ionic liquid crystalline materials combined the characteristics
of liquid crystals with phthalocyanine were prepared using copper phthalocyanine as a
matrix,
[1,1’-biphenyl]-4,4’-diyl
diisonicotinate
or
cyclohexane-1,4-diyl
-
-
-
4
-
4
diisonicotinate as a mesogen, and different ions (B-SO , C-SO , H PO and BF ) as
3
3
2
counterions. The mesomorphic properties of these ionic liquid crystals were
investigated by differential scanning calorimetry, polarizing optical microscopy, and
X-ray diffraction measurements. The copper phthalocyanine-based multi-arm ionic
liquid crystals showed excellent electrical conductivity (the optimum value of
conductivity can reach 0.0248 S/cm) because of their ionic channels and π-π stacking
structure. This materials might provide a promising application prospect for electronic
-
1 -

ACCEPTED MANUSCRIPT
or optoelectronic applications.
Keywords: Copper phthalocyanine; ionic liquid crystals; discotic nematic phase;
conductivity
1
. Introduction
Liquid crystals (LCs) are a fascinating class of soft materials due to their combined
properties of fluidity and long-range order. These materials possess orientational order
without or with only low dimensional positional order [1]. The combination of order
and mobility in LCs is essential to achieve complex self-assembly structures or to yield
well-organized materials that are able to respond to external stimuli [2]. For a long time,
the attention about LCs has turned to the ionic liquid crystals (ILCs) that represent a
kind of smart materials for their ability to effectively combine the molecular order to
LCs with the solvent properties of ionic liquids [3, 4]. Owing to the cations and
counterions, the properties of the ILCs are different significantly from those of
conventional LCs. Especially, the intermolecular interactions between counterions and
cations are stronger because of ionic characteristics, and they play a vital role to control
the packing of structures and affect thermal properties [5, 6]. The interaction between
ions tends to form a stable lamellar liquid crystal state, but can also exhibit a rich
unconventional liquid crystal state, such as a discotic nematic phase (N ), a smectic A
D
phase, and a bicontinuous cubic etc [7, 8]. Among those states, nematic phase (N) of
discotic mesogens are not so common and much less reported than their
two-dimensional discotic columnar-forming LC analogous materials, and the
-
2 -

ACCEPTED MANUSCRIPT
alignment of discotic liquid crystals becomes a crucial point for high conductivity in
different electronic devices. The main reason for the rare observation of N phases lies
D
essentially in the difficulty of their design, as most discotic mesogens tend to stack into
columns, which eventually will aggregate into essentially planar arrays to form 2D
columnar mesophases [9-11]. Nevertheless, N phases LCs are considered now as quite
D
relevant materials due to their lower viscosity and easier alignment tendency, which is
crucial for many potential industrial applications such as negative birefringent
materials, optical sensors, detectors, optical compensation films, one-dimensional
conductors, photovoltaic solar cells, anisotropic photoconductors and so on [12-17].
Phthalocyanines consist of four isoindole heterocycles connected by nitrogen
bridges into one macrocycle with four nitrogen atoms in the center. The cavity in the
center can be occupied by dozens (more than 70) of cation types to form
phthalocyanine derivatives and metallo-phthalocyanines [18]. They emerged as a
promising molecular component for artificial photosynthetic systems such as
photoreceptors and solar cells [10, 19] because they exhibit large absorption in the
visible region [20]. They contain large and conjugated π-systems suitable for efficient
electron-transfer processes, and possess strongly reducing or oxidizing characteristics
determined by the nature of the central metals and the peripheral substituents. Apart
from their uses as dyes or pigments, metallo-phthalocyanines show a number of
special properties, such as electrical, optical, non-linear optical and opto-electronic
[
21-27]. Especially, copper phthalocyanine (CuPc) is a potential candidate for a stable
-
3 -

ACCEPTED MANUSCRIPT
hole selective material for perovskite solar cells. It can be easily prepared by vacuum
processes, and exhibits high hole mobility, chemically stable and relatively
inexpensive [28, 29]. Lots of composite structures based on phthalocyanines are being
actively developed at present for organic electronic elements such as chemical sensors
[
30], biological sensors [31], and solar cells [32] with improved characteristics.
However, more data are required to provide a comprehensive set of design principles
even for the relatively well-examined phthalocyanines materials.
In this work, we have synthesized a new type of multi-arm ionic liquid crystalline
materials that combines the thermal-orientational features of LCs with phthalocyanine.
-
-
These multi-arm pyridyl-containing ILCs with different counterions (Br , B-SO ,
3
-
-
-
C-SO , H PO , and BF ) are made using copper phthalocyanine as a matrix,
3
2
4
4
[
1,1’-biphenyl]-4,4’-diyl diisonicotinate and cyclohexane-1,4-diyl diisonicotinate as
mesogens. A lot of techniques were used to investigate LC structures and properties of
the multi-arm pyridinium salts. The effects of ionic interactions on thermal stability, LC
behavior, and electrical conductivity were studied. Compared to traditional
phthalocyanine materials, the CuPc-based multi-arm ILCs showed excellent electrical
conductivity.
2
. Experimental Section
2
4
.1 Materials
,4’-dihydroxydiphenyl, cyclohexane-1,4-diol, isonicotinic acid, potassium phosphate
monobasic, dodecylbenzenesulphonic acid sodium salt (B-SO Na), sodium
3
-
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(
+)-10-camphorsulfonate (C-SO Na) and sodium fluoborate were purchased from
3
ALADDIN Chemical Company. Copper phthalocyanine (CuPc)
and
1
,4-dibromobutane were purchased from Shenyang Chemical Reagent Company. All
the reagent-grade solvents were purchased from Beijing HWRK Chemical Company.
Tetrahydrofuran (THF) was purified by distillation over NaH, and pyridine was
purified by distillation over KOH before using. Toluene was used in the reaction over
sodium and then distilled under nitrogen.
2
.2 Techniques
FTIR spectra of the samples in solid state via the KBr method were measured by a
PerkinElmer instruments Spectrum One Spectrometer (PerkinElmer, Foster City, CA).
1
13
H-NMR (300MHz) and C-NMR (75MHz) spectra were obtained using a Varian
Gemini 300 NMR Spectrometer (Varian Associates, Palo Alto, CA). Element
analyses were performed by an Elementar Vario EL III instrument (Elementar,
Germany). Thermal properties were determined by using a differential scanning
calorimetry (DSC) 204 of NETZSCH (Netzsch, Wittelsbacherstr, Germany) at heating
-1
and cooling rates of 10°C min under a nitrogen atmosphere. Polarized optical
microscopic (POM) observation of the LC textures was collected using a Leica DMRX
(
Leica, Wetzlar, Germany) microscope with crossed polarisers and equipped with a
LinKamTHMSE-600 (Linkam, Surrey, England) hot stage. X-ray diffraction (XRD)
was measured by CuKα (λ = 1.542 Å) radiation monochromatised with a Riaku
DMAX-3A X-ray diffractometer (Rigaku, Japan). Conductivity was performed by a
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ACCEPTED MANUSCRIPT
RTS-9 four-probe meter (Guangzhou four-point probe technology, Guangzhou, China)
equipped with a hot stage.
2
.3 Synthesis of the monomers
The synthetic route of LC monomers [1,1’-Biphenyl]-4,4’-diyl diisonicotinate (BDI)
and cyclohexane-1,4-diyl diisonicotinate (HDI) are shown in Scheme 1. The detailed
synthesis and characterization of BDI were reported in the previous work [33]. The
experimental details and results of compound BDI were consistent with the report. The
compound HDI was synthesized using cyclohexane-1,4-diol and isonicotinic acid.
Isonicotinic acid (36.93g, 0.3mol) and thionyl chloride (70 ml) was added into a
round flask and stirred at room temperature for 2 h, then heated at 65°C for 25 h in a
water bath. The excess thionyl chloride was distilled out under reduced pressure to
obtain 40.6 g isonicotinoyl chloride hydrochloride. Cyclohexane-1,4-diol (8.13g,
0
.07mol) and 20 mL of pyridine were dissolved in 50mL freshly distilled THF in a
three-necked flask. Isonicotinoyl chloride hydrochloride (25g, 0.14mol) was dissolved
in 100 mL of THF and then added dropwise into the flask. Subsequently, stirred at
room temperature for 2 h, these mixed solutions reacted at reflux temperature for 24 h.
The solvent was removed under reduced pressure after the reaction finished. Then, the
mixture was poured into 1000 mL of sodium bicarbonate solution. The precipitates
were filtrated and washed with water and recrystallized in ethanol to obtain 20.5 g
-1
white crystals of compound HDI. Yield: 90%. FTIR (KBr, cm ): 2935, 2866 (-CH -);
2
1
730 (C=O); 1594 - 1399 (C=C, C=N in pyridine); 1264 (C-N in pyridine); 1176
-
6 -

ACCEPTED MANUSCRIPT
1
(
C-O-C). Anal. calc. for HDI: C, 66.21%; H, 5.50%; N, 8.52%. H-NMR (300MHz,
DMSO): δ = 1.46 - 2.04 (m, 8H, -CH -), 4.53 - 5.07 (m, 2H, -CH-O-), 7.89 (d, J = 6
2
1
3
Hz, 4H, pyridyl-H), 8.76 ppm (d, J = 9 Hz, 4H, pyridyl-H). C-NMR (75MHz,
DMSO): δ = 26.72, 26.86 (-CH -); 72.45, 72.80 (-CH-O-); 124.04, 124.74 (C=C in
2
pyridyl); 141.84, 142.07 (O=C-C=C in pyridyl); 148.66, 149.19 (C=N in pyridyl);
1
63.42, 163.59 ppm (O=C).
2
.4 Synthesis of multi-arm bromides
The synthetic route of multi-arm bromides was shown in Scheme 1.
,4-dibromobutane (10.75mL, 0.09 mol), copper phthalocyanine (CuPc) (5.76 g, 0.01
1
mol) and 60ml of N,N-dimethylformamide were dissolved in a round-bottom flask.
After stirring at room temperature for 2 h, the mixture was heated to 80°C for 36 h.
The excess solvents were removed under reduced pressure. Other byproducts were
removed by washing three times with 50 mL of ethanol. The blue solids were dried
-1
for 12 h to obtain 6.8g of [CuPc-But]-[Br]. FTIR (KBr, cm ): 2992 (-CH -);
2
1
5
612-1416 (C=C, C=N); 1057 (C-N); 635 (C-Br). Anal. Calcd for C H Br CuN : C,
36 24 2 8
4.60%; H, 3.05%; N, 14.15%. Found: C, 54.42%; H, 3.09%; N, 14.05%.
-
7 -

ACCEPTED MANUSCRIPT
Scheme 1. Synthetic routes of the multi-arm ionic liquid crystals.
2
.5 Synthesis of multi-arm ILCs with bromide counterions
As shown in Scheme 1, the multi-arm ILCs with bromide ions were synthesized using
CuPc-But]-[Br], HDI and BDI, respectively. For [CuPc-But-BDI]-[Br] and
[
-
8 -

ACCEPTED MANUSCRIPT
[
[
CuPc-But-HDI]-[Br], the synthetic experiment was similar, therefore synthesis of
CuPc-But-BDI]-[Br] was described as an example. [CuPc-But]-[Br] and BDI were
dissolved in 50mL of N,N-dimethylformamide, and the reaction solution was stirred
at room temperature under a nitrogen atmosphere. Then the reaction mixture was kept
refluxing for 48 h in anhydrous condition under nitrogen atmosphere at 80°C. After
the solvent was removed under reduced pressure, the precipitates were recrystallized
with ethanol for 2-3 times and then dried at 55°C under vacuum for 2 days to obtain
-1
blue solid [CuPc-But-BDI]-[Br]. FTIR (KBr, cm ): 3032 (C-H); 1734 (C=O);
1
599-1409 (C=C, C=N); 1279 (C-O); 1163, 1057 (C-N).
2
.6 Synthesis of multi-arm ILCs with other counterions
-
-
-
-
[
CuPc-But-BDI]-X and [CuPc-But-HDI]-X (X = C-SO , B-SO , H PO , BF ) were
3
3
2
4
4
synthesized by a counterion-exchange reaction using each compounds of
CuPc-But-BDI]-[Br] and [CuPc-But-HDI]-[Br] with sodium fluoborate, potassium
phosphate monobasic, dodecylbenzenesulphonic acid sodium salt and sodium
+)-10-camphorsulfonate, respectively, as shown in Scheme 1. The synthetic
experiments of all theses ILCs were similar, therefore synthesis of
CuPc-But-BDI]-[C-SO ] was described as a sample. [CuPc-But-BDI]-[Br] (1.5 g,
[
(
[
3
1
.26 mmol) was added into 50 mL of 1 N sodium (+)-10-camphorsulfonate aqueous
solution. The reaction mixture was kept vigorous stirring at room temperature for 3
days. The precipitates were segregated by filtration when the reaction was finished.
The anion-exchange reaction was monitored by silver nitrate indicator solution until
-
9 -

ACCEPTED MANUSCRIPT
-
-
disappearance of Br in the filtrate, indicating that Br in [CuPc-But-BDI]-[Br] should
-
be completely replaced by C-SO . The precipitates were washed three times by
3
ethanol, dried at 55°C under vacuum for 2 days to obtain 1.3 g of
-1
[
CuPc-But-BDI]-[C-SO ]. FTIR (KBr, cm ): 3066, 2952 (-CH -, -CH -); 1741(C=O),
3 3 2
1
1
633 - 1412 (C=C, C=N); 1278 (C-O), 1195 (S=O stretching in sulfonate); 1163,
057 (C-N).
3
. Results and discussion
3
.1 Confirmation of molecular structure
HDI was obtained by isonicotinic acid and 1,4-cyclohexanediol. The multi-arm ionic
liquid crystals [CuPc-But-BDI]-[Br] and [CuPc-But-HDI]-[Br] were obtained from
copper phthalocyanine, BDI and HDI with 1,4-dibromobutane, respectively. The
chemical structures were confirmed by FTIR spectra. BDI exhibited C=O, C-O and
-1
C-N stretching vibration at 1744, 1272 and 1222 cm , respectively. Compared with
the corresponding precursor BDI, the multi-arm ionic liquid crystals
[
CuPc-But-BDI]-[Br] exhibited additional peaks at 1599 - 1409, which were due to
phthalocyanine symmetric skeletal vibrations. Analogously, HDI displayed C=O and
-1
C-N stretching at 1730 and 1264 cm respectively. The multi-arm ionic liquid crystals
[
-
CuPc-But-HDI]-[Br] displayed additional phthalocyanine skeletal vibrations at 1594
-1
1399cm . For the ILCs [CuPc-But-BDI]-X and [CuPc-But-HDI]-X with other
counterions, [CuPc-But-BDI]-[C-SO ] was discussed as an example. In addition to the
3
-1
peaks discussed above, other significant peaks appeared at 1195 cm , which was
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10 -

ACCEPTED MANUSCRIPT
specified as tensile vibration in the sulfonate. These data confirmed that the structures
of the product were consistent with the molecular designs.
3
.2 LC properties of the monomers
The phase behavior of the compounds BDI and HDI were inspected particularly by
DSC, POM, and XRD. The thermal properties calculated by DSC are exhibited in
Table 1. All the phase transitions were reversible and did not change on repeated
heating and cooling cycles. Typical DSC curves are presented in Fig. 1. HDI showed a
crystalline melting endotherm (ΔH 23.68 kJ/mol) at 191.6°C, and a nematic
(
N)-isotropic phase transition (ΔH 1.97 kJ/mol) at 282.1°C on the heating cycle in the
DSC thermograms. BDI showed the melting enthalpy change (ΔH 50.93 kJ/mol) at
88.4°C, and a nematic-isotropic phase transition (ΔH 3.17 kJ/mol) at 335.9°C on the
2
heating cycle in the DSC thermograms.
Table 1. Phase transitions, thermodynamic data, XRD data for BDI, HDI and the
multi-arm liquid crystals
b
XRD
Sample
a
Phase transition temperature (°C) and enthalpy changes (kJ/mol)
(Å)
Heating cycle
Cooling cycle
I
II
BDI
HDI
Cr 288.4(50.93) N 335.9(3.17) I
Cr 191.6(23.68) N 282.1(1.97) I
I 327.2(-1.79) N 272.0(-45.71) Cr
I 244.3 (-1.38)N 181.2(-19.58) Cr
-
-
3.59
3.92
4.25
[
CuPc-But-BDI]-[Br]
Cr 283.5(35.28) N 347.6(4.86) I
I 265.9(-3.67) N 181.2(-19.10) Cr
19.85
D
D
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11 -

ACCEPTED MANUSCRIPT
[
CuPc-But-HDI]-[Br]
a
Cr 187.9 (29.62)N 310.8(2.66) I
I 214.4(-2.56)N 137.8(-18.75) Cr
19.27
4.48
D
D
Transition temperatures (°C) and enthalpies (kJ/mol) were determined on the second
heating and cooling at a scanning rate of 10°C/min. Cr, crystal; N, nematic phase; I,
b
isotropic; N , discotic nematic phase.
d-spacing in N phase measured by X-ray
D
D
diffraction experiments. Narrow reflections at low angles (I) and broad reflections at
wide angles (II).
Fig.1. DSC thermograms of (a) BDI, (b) HDI, (c) [CuPc-But-BDI]-[Br] and (d)
[CuPc-But-HDI]-[Br] on heating and cooling cycles.
The optical textures of liquid crystal state are observed by POM with hot stage,
and some typically optical textures are shown in Fig. 2. The POM image of the BDI
exhibited enantiotropic mesophase with marbled texture during heating progress and
the schlieren texture during cooling cycles, while HDI showed the same marbled
-
12 -

ACCEPTED MANUSCRIPT
textures during heating and cooling cycles. Marbled texture and schlieren texture are
the most commonly observed textures of N phase prepared between two untreated
glass substrates.
Fig.2. Optical textures of the samples: (a) marbled texture of BDI on heating at 304°C, (b)
schlieren texture of BDI on cooling at 296°C, (c) marbled texture of HDI on heating at 262°C, and
(d) marbled texture of HDI on cooling at 230°C.
3
.3 LC properties of the multi-arm ILCs with bromide ion
The optical textures and transition temperatures of multi-arm ILCs with bromide ions
were also studied by POM and DSC, and their phase transitions and thermodynamic
data are listed in Table 1. The product [CuPc-But-BDI]-[Br] was taken for an example.
It exhibited a crystalline melting endotherm (ΔH 35.28 kJ/mol) at 283.5°C, and a
mesophase-isotropic phase transition (ΔH 4.86 kJ/mol) at 347.6°C on the heating cycle
in the DSC thermograms, as shown in Fig. 1. The mesophase-isotropic phase
transition temperature (T ) increased distinctly. This fact may attribute to the different
i
-
13 -

ACCEPTED MANUSCRIPT
nature of the mesogen order because of the introduction of non-mesogenic alkyl
bromide component in the LC structure of the [CuPc-But]-[Br]. For
[
CuPc-But-BDI]-[Br], when it was heated on the hot stage under the polarizing
microscope, the field of horizon became bright, and then marble textures appeared.
o
When it was heated to 347.6 C, eyeshot became dark indicating the
anisotropic-to-isotropic phase transition. The marble texture appeared again during
the cooling cycle. For the sample [CuPc-But-HDI]-[Br], the analogous marble
textures were observed. The results reveal that introduction of non-mesogenic alkyl
bromide component and the rigid phthalocyanine nucleus can improve T effectively,
i
but it not change the type of liquid crystal textures.
In order to further elucidate the structure of the mesophase of products, XRD
measurement of the mesophase range was carried out. Fig. 3 shows the X-ray
diffraction pattern of [CuPc-But-BDI]-[Br] at 285°C and [CuPc-But-HDI]-[Br] at
2
15°C during heating process. The XRD patterns of BDI were constituted by a diffuse
halo in the wide angle region 2θ = 24.8° (with d-spacing value of 3.59 Å), indicating
the structure of the rod-like molecule makes it arranged in the most compact state [33].
[
CuPc-But-BDI]-[Br] exhibited both a narrow reflection in the small angle region and
a broad peak in the wide-angle region. These observations indicate the absence of any
positional order in the N mesophase and thus exclude the possibility of the existence
of a smectic and columnar phase structure of this rod-disc oligomer, which are
consistent with their microscopic textures [34, 35]. The calculated d-spacing of
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around 19.85 Å for the signal in the small angle region corresponds to the average
length of the CuPc core and rod-like BDI units along with alkyl chains, indicating a
molecularly mixed nematic phase. The spacing of the wide angle peak is 4.25Å which
originates from the average vertical separation of the molecules in the mesophase [34].
This pattern was consistent with the presence of an N phase. Comparing with the
D
compound [CuPc-But-HDI]-[Br] and HDI, the analogous patterns were exhibited.
Due to the addition of BDI or HDI, inconsistent with the precursor, the multi-arm
ILCs with bromide counterions exhibited a N phase.
D
Fig.3. XRD patterns of (a) [CuPc-But-BDI]-[Br] at 285°C, (b) [CuPc-But-HDI]-[Br] at 215°C.
3
.4 LC properties of the multi-arm ILCs with other counterions
-
All the synthesized ILCs [CuPc-But-BDI]-X and [CuPc-But-BDI]-X (X = C-SO ,
3
-
-
-
B-SO , H PO , BF ) exhibited N liquid-crystalline phases, which were also proved
3
2
4
4
D
by POM, DSC and XRD. The phase transitions, enthalpy changes and XRD data are
summarized in Table 2, and the plot of phase-transition temperatures is shown in Fig.4.
Due to the substitution of other counterions, T of the liquid crystals decreased
i
-
4
obviously with the increase of anion size. For example, because the ion size of BF is
-
15 -

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-
larger than that of Br , T of [CuPc-But-BDI]-[BF ] is slightly lower than that of
i
4
[
CuPc-But-BDI]-[Br]. As for [CuPc-But-BDI]-[B-SO ], because of the counterion
3
was larger and less symmetric salt, the transitions are shifted to lower temperatures.
However, there is no regularity about the enthalpy changes of mesophase-isotropic
phase transition. This phenomenon was consistent with previous documents [33,
3
6-39].
Table 2. Phase transitions, thermodynamic data and some properties of the multi-arm
liquid crystals with other counterions
b
XRD
a
Phase transition temperature (°C) and enthalpy changes (kJ/mol)
Sample
c

(Å)
-
3
(
10 S/cm)
Heating cycle
Cooling cycle
I
II
[
[
[
[
[
[
[
[
CuPc-But-BDI]-[BF4]
Cr 253.48(30.64) N 284.21(5.30) I
Cr 226.19(4.15)N 181.73(15.46) I
19.97
20.04
20.09
20.13
19.42
19.52
19.77
19.83
4.24
13.4
13.8
15.7
14.5
20.4
20.6
22.6
21.3
D
D
CuPc-But-BDI]-[H PO ]
Cr 213.37(41.21) N 283.54(4.14) I
Cr 246.74(3.31) N 182.21(35.47) I
4.26
4.27
4.27
4.45
4.49
4.49
4.48
2
4
D
D
CuPc-But-BDI]-[C-SO H]
Cr 198.77 (16.01)N 275.83(3.32) I
Cr 203.25 (2.23)N 161.12(16.58) I
3
D
D
CuPc-But-BDI]-[B-SO H]
Cr 192.97(44.62) N 270.73 (6.55)I
Cr 219.22(4.01) N 183.21 (38.84)I
3
D
D
CuPc-But-HDI]-[BF4]
Cr 251.36(41.60) N 286.91(5.20) I
Cr 238.97(4.08) N 190.85(55.66) I
D
D
CuPc-But-HDI]-[H PO ]
Cr 191.93(26.10) N 273.89(4.25) I
Cr 183.07(3.36) N 149.07(19.78) I
2
4
D
D
CuPc-But-HDI]-[C-SO H]
Cr 189.90 (28.88)N 272.32(4.81) I
Cr 220.09(3.23) N 173.91(15.39)I
3
D
D
CuPc-But-HDI]-[B-SO H]
Cr 183.20(27.31) N 270.14(3.35) I
Cr 213.95(2.80) N 167.86(12.86) I
3
D
D
a
Transition temperatures (°C) and enthalpies (kJ/mol) were determined for the second
heating at a scanning rate of 10°C/min. Cr, crystal; I, isotropic; N discotic nematic
D
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b
phase. d-spacing in N phase measured by X-ray diffraction experiments. Narrow
D
c
reflections at low angles (I) and broad reflections at wide angles (II). Measured by a
four-point method (RTS-9) conductivity material parameter test instrument at 270 °C.
Fig. 4. The plot of phase-transition temperatures recorded on the heating run for multi-arm ionic
liquid crystals with other counterions.
All the ILCs [CuPc-But-BDI]-X and [CuPc-But-HDI]-X (X = C-SO , B-SO ,
3
3
H PO , BF ) showed similiar optical textures. They all displayed marbled texture on
2
4
4
heating and cooling cycles. However, there was some subtle difference among them.
For example, [CuPc-But-BDI]-[BF ] showed marbled textures, which are analogous
4
to those of the precursor [CuPc-But-BDI]-[Br]. But [CuPc-But-HDI]-[BF ] showed
4
larger marbled textures than those of the precursor [CuPc-But-HDI]-[Br], as shown in
Fig. 5. These results indicate that the disc-like matrix should be dispersed owing to
the electrostatic attraction of ion pairs.
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Fig.5. Optical textures of the samples: (a) marbled texture of [CuPc-But-BDI]-[Br] at 286°C, (b)
marbled texture of [CuPc-But-HDI]-[Br] at 205°C, (c) marbled texture of [CuPc-But-BDI]-[BF4]
at 260°C, and (d) marbled texture of [CuPc-But-HDI]-[BF ] at 265°C.
4
3
.5 Conductivity
CuPc is known as an excellent semiconductor employed in electronic devices
such as organic light-emitting diodes by its high hole mobility. CuPc-based organic
semiconductors are advantageous for the fabrication of electronic devices because of
material variety, architectural flexibility, and environmental safety. It is necessary to
investigate the electrical properties of CuPc-based multi-arm ILCs synthesized in this
work. Fig.
6
shows the conductivities (σ) of the multi-arm ILCs
[
CuPc-But-BDI]-[BF ], [CuPc-But-HDI]-[BF ] as a function of temperature during
4
4
heating cycles. The conductivities significantly increase with temperature during the
solid phase, which has been consistent with previous report [40, 41]. For these
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CuPc-based multi-arm ILCs, the optimum value of conductivity can reach 0.0168
S/cm and 0.0248 S/cm, respectively.
Fig. 6. Ionic conductivity as a function of temperature: (a) [CuPc-But-BDI]-[BF4].
(b) [CuPc-But-HDI]-[BF4].
The dramatically improved conductive capability of CuPc-based multi-arm ILCs
might be due to the π-π stacking structure which favors the conductivity through
intermolecular π-delocalization. When the CuPc-based multi-arm ILCs are in the ND
mesophase, the discotic molecules possess full translational and rotational freedom
around their short molecular axis (disk normal). But on an average, the short
molecular axes are oriented in a preferred direction, as shown in Fig.7. In other words,
the short molecular axes of the molecules orient more or less parallel to each other,
while their centers of mass are isotropically distributed in the nematic phase [42].
Furthermore, disc-like structural units can be deformed by the shifting flow during
experiments connected with the electrical conductivity anisotropy [43, 44]. Recent
research has indicated that π-stacking is a crucial structure feature for a charge-carrier
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transport in CuPc complex semiconductors [45, 46]. For these ionic CuPc-based
multi-arm ILCs, the ionic channels formed in the N mesophase provide another
D
convenient access for the charge-carrier transport, as shown in Fig.7. Therefore it can
dramatically improve the conductive capability.
Fig. 7. Schematic representation of the anisotropic ion transport in the discotic nematic phase.
4
. Conclusion
A series of novel CuPc-based multi-arm ionic LC materials, which combined the
character of ILCs and phthalocyanine, were prepared using CuPc as a matrix, [1,1’
biphenyl]-4,4’-diyl diisonicotinate and cyclohexane-1,4-diyl diisonicotinate as a
-
-
-
-
-
mesogen, and multiple ions (Br , B-SO , C-SO , H PO , BF ) as counterions.
3
3
2
4
4
Because the mesogenic units exhibited a highly thermally stable nematic phase, the
CuPc-based multi-arm ILCs showed discotic nematic liquid crystalline phase with
typical marble optical texture. For these CuPc-based multi-arm ILCs containing
different anions, T decreased clearly with an increase of anion size. Moreover, these
i
ILCs exhibited excellent electrical conductivity. The dramatically improved
conductive capability of CuPc-based multi-arm ILCs might be mainly due to the π-π
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stacking structure of CuPc. In addition, the ionic channels formed in the ND
mesophase might provide another convenient access for the charge-carrier transport.
Therefore these CuPc-based ILCs might provide a kind of promising electronic
materials.
Acknowledgments
This work was supported by the National Natural Science Foundation of China
[
Project 51273035].
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Graphical abstract
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Highlights
•
A series of Pyridine mesophase-based ILCs have been synthesized and
characterized.
•
All the ILCs with different anions exhibit a discotic nematic (ND)
mesophase.
•
•
We have analyzed the behavior of ILCs by DSC, POM and XRD.
The ILCs revealed large ionic conductivity, and markedly increases with
temperature.
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