Smectic liquid crystals comprising triazole banana shaped achiral molecules: Synthesis and characterization

Article abstract of DOI:10.1080/15421406.2017.1289654

A series of novel liquid crystals containing [1,2,3]-triazole ring were synthesized via “click chemistry.” Their mesomorphic properties and photoelectric properties were investigated and the results indicated that most of compounds exhibited SmA over a wide temperature range (about 50°C). The threshold voltage and the response time of the mixture, T4 and LC 50100 were shorten 0.43?V and 71.5?ms with respect to those of LC 50100 respectively.

Full text of DOI:10.1080/15421406.2017.1289654

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Smectic liquid crystals comprising triazole banana shaped
achiral molecules: Synthesis and characterization
Xiaolian Li, Zongying Zhang, and Liuya
State Key Laboratory of Fine Chemicals, Dalian University of Technology, No.  Linggong Road, High-tech District,
Dalian, China
ABSTRACT
KEYWORDS
Heterocyclic liquid crystals;
mesomorphic; threshold
voltage
A series of novel liquid crystals containing [1,2,3]-triazole ring were syn-
thesized via “click chemistry.”Their mesomorphic properties and photo-
electric properties were investigated and the results indicated that most
of compounds exhibited SmA over a wide temperature range (about
50°C). The threshold voltage and the response time of the mixture, T4
and LC 50100 were shorten 0.43 V and 71.5 ms with respect to those of
LC 50100 respectively.
1. Introduction
Bent-core or banana liquid crystals occupy one of the most exciting areas of research in both
mesogenic materials and supramolecular chemistry in recent years due to their distinct prop-
erties, such as optical biaxiality [1], ferroelectricity [2], piezoelectricity [3]. The bent-core
mesogens are the first ferroelectric and antiferroelectric liquid crystals realized without intro-
ducing chirality [4]. Molecule spontaneous polarization will be incurred because of the bent
molecule polarity accumulation occurred in smectic phase layer [5]. Generally, most bent-
core mesogens are synthesized by introducing appropriate substituents into the 1,3-phenylene
ring. While the geometrical shape of liquid crystal molecule will bend by introducing asym-
metric heterocyclic ring or non linear heterocyclic ring, such as thiophen [6], oxadiazole [7],
pyridine [8], oxadiazole [9]. So the heterocyclic ring is a significant mesogenic core in syn-
thesis of novel bent-core liquid crystal molecule.
The common methods to design and synthesize new liquid crystals are selecting appro-
priate mesogenic core, connecting group, and selecting appropriate length flexible chain [10].
The property of liquid crystalline phase will vary dramatically by changing the mesogenic core
[11]. The research of heterocyclic ring liquid crystals including oxygen heterocyclic ring liq-
uid crystals, nitrogen heterocyclic ring liquid crystals and sulfur liquid crystals is becoming
more and more prevalent because of the unsaturation and easy polarization of heteroatom.
The property including molecular polarity [12] and dielectric anisotropy [13] will change
largely by introducing heteroatom into mesogenic core.
The compound containing [1,2,3]-triazole always possess aromaticity, high dipole moment
(5.2–5.6 D), outstanding stability and a merit acting as a hydrogen-bonding acceptor
CONTACT Xiaolian Li
xlianli@dlut.edu.cn
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
Published with license by Taylor & Francis Group, LLC ©  Xiaolian Li, Zongying Zhang, and Liuya
This is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License
(http://creativecommons.org/licenses/by-nc-nd/./), which permits non-commercial re-use, distribution, and reproduction in any medium, pro-
vided the original work is properly cited, and is not altered, transformed, or built upon in any way.

396/[664]
X. LI ET AL.
Figure . Gilmar Conte’s compounds containing [,,]-triazole.
[14]. They are generally used in medicinal chemistry [15–16], organic chemistry, materi-
als chemistry [17] and other fields, but they are rarely used in liquid crystal. In the linear
molecule, a negative dielectric anisotropy can be obtained by introducing substituent group,
such as cyanogroup [18], nitro [19], fluorine [20] and chlorine [21] to the side of the molecule.
Intermolecular lateral force is weaken because of the side substituent group and this conse-
quence go against the formation of liquid crystalline phase. While [1,2,3]-triazole ring own-
ing a high dipole moment can improve the negative dielectric anisotropy in the condition
of avoiding destroying the liquid crystalline phase. So, in this work, a representative efficient
“click chemistry” reaction, the copper catalyzed azide-alkyne cycloaddition was applied in
synthesizing [1,2,3]-triazole-based core.
Indeed, the central core is a key factor when generating bent-core mesomorphism [22].
Gilmar Conte [23] found that compound A where the [1,2,3]-triazole heterocycle was placed
at the end of the rigid core doesn’t show liquid crystal phase (Figure 1). However, when the
[1,2,3]-triazole ring(B) was placed to the center of core, mesophase was observed. It indicates
the position of triazole ring play a key role in the mesomorphic behavior. So a series of liq-
uid crystals in which [1,2,3]-triazole ring was placed in the center position were synthesized.
Their liquid crystal property and photoelectricity property were also studied. The detailed
Figure . Synthesis of target compounds T0-T7.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
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synthetic route is depicted in Figure 2. The mesophase ranges depend strongly on the modifi-
cations introduced in the general structure (i.e. number of aromatic rings, incorporation of a
halogen atom, polar/apolar nature of the terminal chain or presence of azide/methylene azide
terminal unit)[22]. The esters liquid crystals are general in the liquid crystal materials for their
good stability in chemical property and physical property, together with lighter colour, lower
viscosity and rapid response speed [24–25]. In this work ester groups were introduced to both
ends of the mesogenic core thereby creating a dipole in order to get liquid crystalline property
in the compounds.
2. Experimental
Specific synthetic process is shown in figure 1.
2.1. Materials and instruments
Unless otherwise stated, all reagents were purchased from commercial suppliers and used
without further purification.1H-NMR spectra were recorded on a VARIAN INOVA-400 spec-
trometer, using TMS as an internal standard. Mass spectrometry data were obtained with a
HP1100LC/MSD mass spectrometer and a LC/Q-TOF MS spectrometer. UV-visible spec-
tra were collected on a Perkin Elmer Lambda 35 UV/VIS spectrophotometer. Fluorescence
measurements were performed on a VAEIAN CARY Eclipse fluorescence spectrophotome-
ter (Serial No. FL0812-M018). We chose commercial nematic liquid crystal MLC6237 and
80C as the host component. Polaroid was provided by WENZHOU QIAOYE ECONOMY
DEVELOPMENT CO., LTD. Nicolet-20DXB FR-IR infrared spectrometer (Nicolet, KBr pel-
lets), Netzsch DSC 204, X-6 micro melting point apparatus.
2.2 Synthesis of target compounds
(1) Synthesis of ethyl 4-aminobenzoate (II)
300 μL concentrated sulfuric acid were added dropwise to the mixture of 685 mg (5
mmol) 4-aminobenzoic acid (I) and 5 mL ethanol at the room temperature. Then the
reaction system was heated to the reflux temperature and stirred for 3h. When the
reaction was finished, cooled it to room temperature then neutralize it to pH = 7–
8 with saturated Na2CO3 aqueous solution. The precipitate was filtered followed by
stoving to give 0.284 g white solid II. Yield: 42.0%, m.p: 18.9–19.3°C.
(2) Synthesis of ethyl 4-azidobenzoate (A)
104 mg NaNO2 was dissolved in 1.5 mL water. Then the solution was added to the
mixture of 165 mg (1 mmol) ethyl 4-aminobenzoate, 1.5 mL HCl solution and 1.5 mL
H2O in a ice-bath. After the reaction system was stirred for 1h, 1.5 mL aqueous solution
of 100 mg NaN3 was added. Stirred for another 30 min. The precipitate was filtered
followed by stoving to give 70 mg pale yellow solid A. Yield: 73.3%, m.p: 95.5–97.0°C.
(3) Synthesis of ethyl 4-(4-phenyl-1H-1,2,3-triazol-1-yl)benzoate (T0)
The mixture of 54 mg (0.31 mmol) ethynylbenzene, 60 mg (0.31 mmol) 4-
azidobenzoate (A), 3 mL t-BuOH, 3 mL H2O, 78 mg CuSO4·5H2O and 184 mg sodium
ascorbate was stirred for 24 h at the room temperature. Then the reaction mixture was
poured into ice water and yellow floccules were separated out. The precipitate was
filtered followed by stoving to give 84 mg pale yellow solid. Yield: 92.0%, m.p: 179.8–
180.8°C.

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1H NMR (CDCl3, 400 Hz): δ(ppm): 8.270 (d, J = 8.8 Hz, 2 H), 8.238 (s, 1 H), 7.929
(d, J = 7.2 Hz, 2 H), 7.915 (d, J = 8.8 Hz, 2 H), 7.490 (t, J1 = 7.2 Hz, J2 = 7.6 Hz, 2
H), 7.407 (t, J1 = 7.2 Hz, J2 = 7.6 Hz, 1 H), 4.435 (m, 2 H), 1.440 (t, J1 = 7.2 Hz, J2 =
6.8 Hz, 3 H). TOF MS EI+(m/z): 293.1174, calculated value: 293.1164.
(4) Synthesis of ethyl 4-bromobenzoate (IV)
The mixture of 1.005 g (0.5 mmol) 4-bromobenzoic acid, 10 mL absolute alcohol and
catalytic amount of concentrated sulfuric acid was stirred for 2 h in the condition of
reflux. Cooled the mixture to room temperature and then poured it into 50 mL ice
water. Extracted the target compound with CH2Cl2 (2×50 mL) then washed it with
5% NaHCO3 (2×30 mL) and NaCl (2×50 mL). The solution was dried with anhydrous
magnesium sulfate. Evaporated the mother liquid and then the yellow liquid, ethyl 4-
bromobenzoate (IV) was gained. Yield: 92.8%.
(5) Synthesis of ethyl 4-(3-hydroxy-3-methylbut-1-yn-1-yl)benzoate (V)
3 mg (0.1 mol%) Dichlorobis(tripheny1phoephine) palladium was added under N2 to
a solution of 8 mg (1 mol%) Ph3P, 3 mg (0.5 mol%) CuI, 0.645 g (3 mmol) methyl
pbromobenzoate (IV), 25 mg (3 mmol) 2-methylbut-3-yn-2-ol, 6 mL dry Et3N and
4mL of dry pyridine. After being refluxed under N2 for 40 min, the mixture was cooled
to room temperature and filtered to remove the insoluble triethylamine hydrobromide.
The salt was washed with triethylamine and ethyl ether until the ether washings were
cleared off. The combined filtrates were reduced to dryness under reduced pressure.
The obtained solid was stirred twice with H2O and then with 3% HCl and again twice
with H2O. The mixture was filtered, and the residue was dried under vacuum to yield
60 mg luminous yellow V. Yield: 85%, m.p: 51.0–52.4°C.
(6) Synthesis of potassium 4-ethynylbenzoate (VI)
0.560 g potassium hydroxide was dissolved in refluxing isopropanol (12mL); 0.581 g
V was added at once, and the mixture was refluxed for 30 min. After being cooled in
an ice-bath and filtered, it dried at 60 °C under vacuum to give the potassium 4-ethy-
nylbenzoate (VI). Yield: 85%, m.p>300°C.
(7) Synthesis of alkyl 4-ethynylbenzoate (Bn n = 1–7)
0.371 g (2.01 mmol) VI in 10 mL CHCl3 was stirred at 0°C, while SOCl2 (3 mL) was
added gradually. The reaction was exothermic. Catalytic amount of DMF was then
added, and the mixture was stirred at room temperature for 5 h. The resulting orange
solution was evaporated under vacuum to dryness at about 40°C. A yellow crystalline
product(VII) was obtained. Because of the unsteadiness of 4-ethynylbenzoyl chloride,
it was used for the next step without further purification. The mixture of 0.4 mL alco-
hol, 10 mL CH2Cl2 and 0.2 mL dry pyridine was refluxed for 2 h. After further purifi-
cation, the alkyl 4-ethynylbenzoate (Bn n = 1–7) was gained.
(8) Synthesis of T1
The mixture of THF/H2O (4 mL: 4 mL), 80 mg (0.42 mmol) 4-azidobenzoate (A),
73 mg (0.42 mmol) ethyl 4-ethynylbenzoate (B1) 105 mg (0.42 mmol) CuSO4·5H2O
and 300 mg (1.5 mmol) sodium ascorbate was refluxed for 24 h. After being cooled to
room temperature, the reaction mixture was poured into ice water and brown precipi-
tate was separated out. The precipitate was filtered followed by chromatogram column
separation (CH2Cl2:Petroleum Ether = 1:1) to give white powder solid. Yield: 90%.
IRmax/cm−1(KBr): 3138, 2954, 2919, 2853, 1713, 1607, 1517, 1442, 817.1H
NMR(CDCl3): δ(ppm): 8.352(s, 1 H), 8.251(d, J = 8.8 Hz, 2 H), 8.154(d, J = 8.0 Hz, 2
H), 8.003(d, J = 8.0 Hz, 2 H), 7.914(d, J = 8.4 Hz, 2 H), 4.424(m, 4 H), 1.433(m, 6 H).
TOF MS EI+ (m/z): 365.2381, calculated value: 365.1376.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
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(9) Synthesis of T2
The synthetic method of T2 was the same as that T1. Yield: 89%.
1H NMR (CDCl3): δ(ppm): 8.348(s, 1 H), 8.249(d, J = 8.8 Hz, 2 H), 8.148(d, J = 8.0 Hz,
2 H), 8.001(d, J = 8.0 Hz, 2 H), 7.914(d, J = 8.4 Hz, 2 H), 4.436(m, 2 H), 4.358(t, J1 =
6.8 Hz, J2 = 6.4 Hz, 2 H), 1.785(m, 2 H), 1.534(m, 2 H), 1.439(t, J1 = 7.2 Hz, J2 =
7.2 Hz, 3 H), 1.002 (t, J1 = 7.2 Hz, J2 = 7.6 Hz, 3 H). TOF MS EI+ (m/z): 379.1542,
calculated value: 379.1532.
(10) Synthesis of T3
The synthetic method of T3 was the same as that T1. Yield: 73%.
1H NMR (CDCl3): δ(ppm): 8.347(s, 1 H), 8.251(d, J = 8.8 Hz, 2 H), 8.154(d, J = 8.0 Hz,
2 H), 8.002(d, J = 8.0 Hz, 2 H), 7.915(d, J = 8.4 Hz, 2 H), 4.436(m, 2 H), 4.316(t, J1 =
6.4 Hz, J2 = 6.8 Hz, 2 H), 1.817(m, 2 H), 1.439(t, J1 = 7.2 Hz, J2 = 7.2 Hz, 3 H), 1.058
(t, J1 = 7.2 Hz, J2 = 7.6 Hz, 3 H). TOF MS EI+ (m/z): 393.1684, calculated value:
393.1689.
(11) Synthesis of T4
The synthetic method of T4 was the same as that T1. Yield: 85%.
1H NMR (CDCl3): δ(ppm): 8.348(s, 1 H), 8.251(d, J = 8.8 Hz, 2 H), 8.150(d, J = 8.8 Hz,
2 H), 8.004(d, J = 8.4 Hz, 2 H), 7.915(d, J = 8.4 Hz, 2 H), 4.436(m, 2 H), 4.348(t, J1 =
6.4 Hz, J2 = 6.8 Hz, 2 H), 1.802(m, 2 H), 1.348–1.570(m, 7 H), 0.948 (t, J1 = 6.8 Hz, J2 =
7.2 Hz, 3 H). TOF MS EI+ (m/z): 407.1836, calculated value: 407.1845.
(12) Synthesis of T5
The synthetic method of T5 was the same as that T1. Yield: 93%.
1H NMR (CDCl3): δ(ppm): 8.350(s, 1 H), 8.252(d, J = 8.4 Hz, 2 H), 8.149(d, J = 8.0 Hz,
2 H), 8.004(d, J = 8.0 Hz, 2 H), 7.915(d, J = 8.4 Hz, 2 H), 4.437(m, 2 H), 4.347(t, J1 =
6.4 Hz, J2 = 6.8 Hz, 2 H), 1.794(m, 2 H), 1.444(m, 5 H), 1.369(m, 4 H), 0.917 (t, J1 =
6.8 Hz, J2 = 6.8 Hz, 3 H). TOF MS EI+ (m/z): 421.2009, calculated value: 421.2002.
(13) Synthesis of T6
The synthetic method of T6 was the same as that T1. Yield: 91%.
1H NMR (CDCl3): δ(ppm): 8.348(s, 1 H), 8.253(d, J = 8.8 Hz, 2 H), 8.151(d, J = 8.0 Hz,
2 H), 8.005(d, J = 8.4 Hz, 2 H), 7.916(d, J = 8.4 Hz, 2 H), 4.438(m, 2 H), 4.346(t, J1 =
6.4 Hz, J2 = 6.8 Hz, 2 H), 1.794(m, 2 H), 1.449(m, 5 H), 1.332(m, 8 H), 0.892 (t, J1 =
6.8 Hz, J2 = 6.8 Hz, 3 H). TOF MS EI+ (m/z): 435.2168, calculated value: 435.2158.
(14) Synthesis of T7
The synthetic method of T7 was the same as that T1. Yield: 87%.
1H NMR (CDCl3): δ(ppm): 8.352(s, 1 H), 8.253(d, J = 8.8 Hz, 2 H), 8.151(d, J = 8.4 Hz,
2 H), 8.006(d, J = 8.4 Hz, 2 H), 7.918(d, J = 8.8 Hz, 2 H), 4.437(m, 2 H), 4.345(t, J1 =
6.8 Hz, J2 = 6.4 Hz, 2 H), 1.795(m, 2 H), 1.441(m, 5 H), 1.328(m, 8 H), 0.892 (t, J1 =
6.8 Hz, J2 = 6.8 Hz, 3 H). TOF MS EI+ (m/z): 449.2342, calculated value: 449.2316.
3. Results and discussion
3.1. Liquid crystal property of Tn
liquid crystal property of T0-T7was studied by DSC and POM. The heating rate is 10°C/min.
The phase transition behaviours of compoundsT0-T7 were observed under polarized opti-
cal microscopy upon heating. It reveals that compounds T1 possess the transition from solid
state to isotropic phase directly on melting process, and no mesophase could be observed. T0
decomposed at 179.8°C directly. However, compounds T2-T7 show typical SmA phase. The

400/[668]
X. LI ET AL.
Figure . Optical photomicrograph of T5.
length of terminal group seems to be related to the appearance of mesophases.T5 mesophase
was clearly identified by optical texture as shown in Figure 3. Further, the mesomorphic
behaviours of compounds T0-T7 were studied by differential scanning calorimeter (DSC)
as shown in Figure 4. Two peaks were observed when T2-T7 were heating, which indicated
the presence of mesophase. The [1,2,3]-triazole heterocycle causes a bent angle(c.148.9°)of
the mesomorphic core and consequently a linearity deviation that seems detrimental to the
mesomorphic packing. The main factor contributing to smectic phases may be the dipolar
interactions contributed by the [1,2,3]-triazole in addition to the dipole generated by the polar
ether group[26]. From the date presented in Table 1, the melting point of these compounds is
at 160°C. The phase transition enthalpies from SmA phase to isotropy phase ꢀH>4 kJ mol−1
shows a strong interaction in the SmA phase.
The clearing point of T1-T7 is about 50°C higher than that of B(150.8°C). This phe-
nomenon may be caused by the ester linkage. The thermostability of the molecule was
Figure . DSC Thermograms of compounds T1-T7.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
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Table . Phase transition temperature (°C) and enthal-
pies (kJ mol−) of T-T.
Compound
Transition temperature, (°C) (ꢀH, kJ mol−
)
T)
T
.–.°C
Cr . (.) I
T
T
T
T
T
T
Cr . (.) SmA . (.) I
Cr . (.) SmA . (.) I
Cr . (.) SmA . (.) I
Cr . (.) SmA . (.) I
Cr . (.) SmA . (.) I
Cr . (.) SmA . (.) I
). T0 doesn’t show liquid crystal phase. m.p = .–.°C.
enhanced for the more strong conjugate action. So the phase inversion temperature also
increases.
From the Fig. 5, it reveals that the melting point of Tn vary with the carbon number
of the terminal carbonyl chain. The clearing point decreases with the increase of the car-
bon number(n) of the terminal carbonyl chain. This indicates that methylene can weaken
the stability of the mesomorphic phase. When nࣙ3, the clearing point changes inconspic-
uously and it reveals that the length of the flexible chain have little effect on mesomor-
phic phase. The melting point of these heterocycle compounds exhibits odd even effect.
The compounds which contain odd carbon atom number in the terminal group appear
higher melting point than that of those contain even carbon atom number in the terminal
group.
3.2. Photoelectric property of Tn
Fig. 6 shows the schematic diagram of setup for measuring the photoelectric properties
of VA LC cells. The text theory: put the vertical orientation liquid crystal cell between
orthogonal polaroid P1 and P2. Red laser beam I0 irradiates vertically P1. Laser beam
Transmission light passes through the cell and P2 and transmission light was texted by
detector.
Figure . Plot of transition temperatures versus the number of carbons (n) in the alkoxycarbonyl chain of
T1-T7 during heating cycle.

402/[670]
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Figure . Schematic diagram of setup for measuring the photoelectric properties of VA LC cells.
... Measurement of threshold voltage
Added 10 mg T4 in 490 mg LC 50100 and heated the mixture to make sure that T4 was com-
pletely dissolved in LC 50100. Then cooled the mixture to room temperature. Inject pure LC
50100 and the liquid crystal mixture, LC 50100+ T4 into two parallel liquid crystal cells.
Then took the measurement according to Fig. 6. When V<Vth the liquid crystal was perpen-
dicular to the plate, the transmittance was 0. While when V>Vth, the orientation of liquid
crystal changed gradually, and the transmittance increased (Fig. 7). The threshold voltage of
LC 50100 was 1.562 V, while the threshold voltage of the liquid crystal mixture (LC 50100+
T4) was 1.132 V. It indicated that T4 could reduce the threshold voltage.
... Measurement of response time
Response time is a parameter which indicates the switching speed of pixel. It affects the
dynamic image quality of display device. Fig. 8 shows the relation between drive signal and
optics response time in normally black mode [27]. Rise time (tr) is the time when bright-
ness rises from 10% to 90%, and fall time is the time when brightness falls from 90% to 10%.
Response time is the sum of rise time and fall time, t = tr + tf.
The result is shown in figure 9. The response time of LC 50100 is 290.7 ms (tr = 253.4 ms,
tf = 37.3 ms), while that of the LC mixture(50100+T4) is 219.2 ms(tr = 205.4 ms, tf = 13.8
ms). It indicates that T4 can improve the response speed.
Figure . Relative T-V curve of 50100 and 50100+T4.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
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Figure . Drive signal and optical reaction time curves.
Figure . Response time curves of LC  (A) and +T (B).
4. Conclusions
A novel series of [1,2,3]-triazole-based mesogens was synthesised and characterised. Com-
pounds exhibit a smectic A mesophase within the temperature range 153.3–201.0°C observed
for the majority of the studied compounds. The melting point of the studied compounds
presents odd even effect obviously with the length of the terminal carbon chain. The LC mix-
ture (50100+T4) shows a shorter response time comparing with 50100. The novel [1,2,3]-
triazole based liquid crystals can be used as an candidate additive in commercial liquid crys-
tals to reduce the threshold voltage and improve the response speed.
Funding
We are grateful to the National Natural Science Foundation of China (21576043), supported by The
Fundamental Research Funds for the Central Universities.
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