Synthesis and characterization of mesogenic compounds possessing bithiophene and benzoxazole units

Article abstract of DOI:10.1080/15421406.2014.949593

A series of bithiophene-based compounds, 2-(5′-alkyl-2,2′-bithiophene-5-yl)-benzoxazole derivatives (nTBx), bearing different substituents (H, CH₃, Cl, NO₂) at the 5-position, were prepared and characterized. They exhibit enantiotrop

Full text of DOI:10.1080/15421406.2014.949593

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Synthesis and Characterization of
Mesogenic Compounds Possessing
Bithiophene and Benzoxazole Units
Kun Hua, Pei Chena, Xinbing Chena & Zhongwei Anab
a Key Laboratory of Applied Surface and Colloid Chemistry (MOE),
School of Materials Science and Engineering, Shaanxi Normal
University, Xi’an, P R China
b Xi’an Modern Chemistry Research Institute, Xi’an, P R China
Published online: 03 Mar 2015.
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To cite this article: Kun Hu, Pei Chen, Xinbing Chen & Zhongwei An (2015) Synthesis and
Characterization of Mesogenic Compounds Possessing Bithiophene and Benzoxazole Units, Molecular
Crystals and Liquid Crystals, 608:1, 25-37, DOI: 10.1080/15421406.2014.949593
To link to this article: http://dx.doi.org/10.1080/15421406.2014.949593
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Mol. Cryst. Liq. Cryst., Vol. 608: pp. 25–37, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.949593
Synthesis and Characterization of Mesogenic
Compounds Possessing Bithiophene
and Benzoxazole Units
KUN HU,1 PEI CHEN,1 XINBING CHEN,1,∗
AND ZHONGWEI AN1,2
1Key Laboratory of Applied Surface and Colloid Chemistry (MOE), School of
Materials Science and Engineering, Shaanxi Normal University, Xi’an, P R
China
2Xi’an Modern Chemistry Research Institute, Xi’an, P R China
A
series of bithiophene-based compounds, 2-(5ꢀ-alkyl-2,2ꢀ-bithiophene-5-yl)-
benzoxazole derivatives (nTBx), bearing different substituents (H, CH3, Cl, NO2) at the
5-position, were prepared and characterized. They exhibit enantiotropic mesophases
with temperature ranges of 3◦C–61◦C and 4◦C–71◦C during heating and cooling, and
nTBx with electron-withdrawing substituents (NO2, Cl) display higher clearing tem-
peratures and wider mesophase ranges than those of corresponding homologues with
electron donating substituents. Compared to biphenyl-based analogs (nB-x), nTBx
show much larger red-shifted absorption bands (374–388 nm) and photoluminescence
emission peaks (453–458 nm), thereby illustrating the impact of the bithiophene core
on electronic properties originating from the π-molecular orbitals.
Keywords Benzoxazole compounds; bithiophene derivative; mesophase; photophysi-
cal property
1. Introduction
With the change of the molecular structure of calamitic liquid crystals, their mesomorphic
properties differ significantly. To design novel thermotropic liquid crystals, the molecular
architecture, such as suitable selection of core fragment, linking group, and terminal func-
tionality, is considered to be primary. In general, a 1,4-disubstituted phenyl ring serves as
a major mesogenic core unit for the known calamitic liquid crystals. However, with the
discovery of banana type liquid crystals, attentations has been changed to the incorporation
of nonlinear units in the mesogenic core. Among the nonlinear fragments, thiophene has
attracted the interest of many research groups [1–10]. The incorporation of thiophene in the
mesogenic core will create a bent molecular structure that substantially affects the transition
temperatures, dielectric anisotropy, optic, and other properties of mesogens, in comparision
with the benzenoid analogues.
∗Address correspondence to X. Chen, Key Laboratory of Applied Surface and Colloid Chemistry
(MOE), School of Materials Science and Engineering, Shaanxi Normal University, Xi’an 710062,
P R China. Email: chenxinbing@snnu.edu.cn
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
25

26
K. Hu et al.
On the other hand, benzoxazoles are a group of heterocyclic compounds that have been
used as central cores to generate novel compounds exhibiting unique mesophases. There
are some examples of benzoxazole-based mesogenic and metallo-mesogenic materials
[11–21], and the formation of a mesophase in benzoxazole-based mesogens has been
found to be greatly affected by polar substituents, polarisability, and intramolecular or/and
intermolecular force.
Recently, we have studied series of mesogenic 2-arylbenzoxazole derivatives (nPB-x
and nB-x, as shown in Fig. 1) with different polar substituents at 5-position of terminal
benzoxazole unit, where the substituents with stronger electron-withdrawing properties
generally give a wider mesomorphic temperature domain [18, 19]. Furthermore, via intro-
ducing two lateral fluoro substituents to biphenyl unit, the nematic mesophase as well as
low melting point are achieved [20, 21].
The above mentioned results inspired us to perform a broader study of benzoxazole-
based calamitic mesogens and design novel liquid crystal materials possessing bithiophene
and benzoxazole units. Here, we describe the synthesis and properties of 2-(5ꢀ-alkyl-2,2ꢀ-
bithiophene-5-yl)-benzoxazole derivatives (nTBx) containing bithiophene and benzoxa-
zole moieties depicted in Figure 1. We focus our attentation on the relationship between
molecular structure and the properties of these molecules, as well as the corresponding
comparision among nTBx, nPB-x and nB-x.
2. Results and Discussion
2.1 Synthesis and Characterization
A synthetic route towards nTBx is illustrated in Scheme 1, in which alkylation of bithio-
phene, Vilsmeier-Haack formylation, condensation of 2-aminophenols with the correspond-
ing bithiophene aldehydes and subsequent intramolecular cyclization were carried out in
turn. It was noted that, both 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) and iodoben-
zene diacetate (IBD) were chosen as oxidative reagent in the intramolecular cyclization
to generate benzoxazole ring skeletons, while the latter gave satisfactory results. The
Figure 1. Structures of nPB-x,·nB-x, and nTBx.

Bithiophene and Benzoxazole Liquid Crystals
27
bithiophene-based compounds nTBx, with carbon atoms in the alkyl chain from 7 to 10,
and possessing polar substituents on benzoxazole moiety, were obtained in purities greater
than 98% [high performance liquid chromatography (HPLC) or gas chromatography (GC)]
and with overall yields of 40%–46%.
Scheme 1. Synthesis of nTBx.
The structures of the products were characterized by infrared (IR), proton nuclear
magnetic resonance (1H-NMR), GC with electron impact-mass spectrometry (GC/EI-MS),
and elemental analysis (EA). From the IR data of compound 10TBN, the peaks at 1617,
1577, and 1501 cm−1 could be attributed to vibrations of the aromatic ring skeleton. It
was noted that the absorption band of C N in the benzoxazole moiety overlapped with
the vibrations of the aromatic ring skeleton at 1617 cm−1, and a similar phenomenon was
found with compounds nTBH, nTBM, and nTBC. The bands at 1547 and 1344 cm−1 were
assigned to the anti-symmetric and symmetric N O stretching vibrations of the nitro group.
In 1H-NMR spectrum of 10TBN, the peaks at 8.55, 8.28, and 7.61 ppm were assigned to
the protons of phenyl ring in nitro substituted benzoxazole group. The peaks at 7.82, 7.14,
and 6.74 ppm were attributed to the protons of the bithiophene ring. The proton in CH2
group adjacent to bithiophene was assigned at 2.81 ppm, and protons in the alkyl group
appeared at 0.75–2.00 ppm. In GC/EI-MS spectrum of 10BTN, the peak of the positively
charged molecular ion appeared at m/z 468.17 with a relative intensity of 26%, consistent
with the theoretical value (468.63). EA result of 10BTN (C 63.77, H 5.71, and N 5.90)
was in accordance with the calculated data (C 64.07, H 6.02, and N 5.98). All these results
confirmed the proposed structures of compounds in the nBTx series.
2.2 Mesomorphic Properties
Mesomorphic properties of nTBx were determined by differential scanning calorimetry
(DSC) and polarizing optical microscopy (POM). DSC curves obtained under similar
conditions overlapped one another, confirming the reproducibility of the measurements.
The phase transition temperatures reported in the present paper were the peak values of the
transition on the DSC curves. Phase identification was made by comparing the observed
textures with those reported in the literature. Clear-cut transition temperatures and textures
were obtained from DSC and POM observations for all the compounds, and were in good
agreement with each other for the multiple heating/cooling cycles. The phase transition
temperatures, the associate enthalpy changes and mesophase textures of the nTBx are
summarized in Table 1. Due to the fact that the sliver-coated cell was corrupted during DSC

28
K. Hu et al.
Table 1. Types of phase transition, temperatures and corresponding enthalpies obtained
by POM and DSC methods for compounds nTBH, nTBM, nTBC, and nTBN
Transition temperature, ◦C (enthalpy change/kJ mol−1
)
Compounds
Heating process
Cooling process
7TBH
8TBH
10TBH
7TBM
Cr 105.8 (23.6) I
Cr 105.5 (35.3) I
Cr 105.4 (30.4) I
I 86.8 (–24.3) Cr
I 87.4 (–36.9) Cr
I 85.6 (–33.0) Cr
Cr 87.7 (20.2) N 93.9 (0.2) I
I 90.8 (–0.4) N 76.9 (–1.4) SmC 60.8
(–16.7) Cr
8TBM
Cr 88.5 (14.1) SmC 92.6 (7.5) I
I 87.7 (–0.7) N 82.1 (–3.5) SmC 64.3
(–22.0) Cr
10TBM
7TBC∗
8TBC∗
10TBC∗
7TBN
Cr 81.7 (19.9) SmC 95.7 (3.7) I
Cr 73.5 SmC 134.0 I
I 91.8 (–4.2) SmC 62.4 (–18.0) Cr
I 133.0 SmC 67.0 Cr
Cr 94.0 SmC 135.0 I
Cr 91.4 SmC 134.5 I
Cr 145.0 (9.2) SmC 147.9 (1.0) I
Cr 145.3 (8.5) SmC 150.3 (1.7) I
Cr 142.5 (9.6) SmC 152.5 (2.9) I
I 134.7 SmC 63.5 Cr
I 134.2 SmC 66.6 Cr
I 144.9 (–2.5) SmC 141.0 (–12.6) Cr
I 146.3 (–2.4) SmC 140.3 (–9.5) Cr
I 148.4 (–2.9) SmC 137.8 (–10.0) Cr
8TBN
10TBN
Note: ∗, data obtained by POM; Cr: crystal; SmC: smectic C mesophase; N: nematic mesophase;
I: isotropic liquid.
measurement on samples in nTBC series, the phase transition temperatures and mesophase
textures were in these cases determined only by POM.
As shown in Table 1, the compounds nTBx exhibited enantiotropic smectic mesophases
for alky chain lengths of 7 to 10 carbons with exception of nTBH. The phases of 7TBM,
7TBC, and 10TBN were assigned from the typical focal conic textures and Schlieren
textures during both heating and cooling, respectively, as shown in Fig. 2. Compared to
other series, nTBH showed no mesophase at all whether with short or long terminal alkyl
chain during both heating and cooling processes, which indicated that the substituents
(NO2, CH3, and Cl) at benzoxazole moiety were helpful to increase the mesophase stability
of bithiophene-based compounds. This was in accordance with the reported results for
compounds nPB-x and nB-x [18, 19].
Compared to biphenyl-based analogs nB-x, bithiophene groups in compounds nTBx
were observed to lower the corresponding melting point and the stability of mesophase, as
shown in Table 1 and Fig. 3, which was attributed to its nonlinear conformations resulting
from the bithiophene structure. It is obvious that, compared to nPB-x, bithiophene-based
analogs nTBx exhibited slightly higher the melting point due to its longer mesogenic
core.
2.3 Thermal Stability
As shown in Fig. 4a, the TGA curves of the samples 10TBx series were recorded at
30◦C–600◦C under a nitrogen atmosphere. It can be seen that compounds 10TBH, 10TBM,
and 10TBC showed the onset of decomposition at 250◦C, 235◦C, and 260◦C, respectively,

Bithiophene and Benzoxazole Liquid Crystals
29
Figure 2. DSC traces and photomicrographs (×200) of 10TBN (a, b, c), 7TBM (d, e, f, g), and
7TBC (h, i); focal conic texture of the smectic mesophase at 146◦C (b), 120◦C (h) on heating and
145◦C (c), 70◦C (g), 111◦C (i) on cooling; Schlieren texture of the nematic mesophase at 90◦C (e) on
heating and 87◦C (f) on cooling.
and became almost completely decomposed until 360◦C. On the other hand, 10TBN ex-
hibited a one-step decomposition starting at 260◦C, and more than 70 wt% had decom-
posed within the temperature range 260–480◦C. This indicated that the strongly polar
nitro-substituent in nTBN increased the dipole–dipole interaction between the molecules,
giving greater thermal stability.
Figure 3. The transition temperatures of series 7TBx, 7B-x, and 7PB-x.

30
K. Hu et al.
10TBM
10B-M
10PB-M
100
80
60
40
20
0
10TBH
100
80
60
40
20
0
10TBM
10TBC
10TBN
(a)
(b)
0
100
200
300
400
500
0
100
200
300
400
500
600
Temperature (oC)
Temperature (oC)
Figure 4. TGA curves of compounds 10TBx (a), series 10TBM, 10B-M, and 10PB-M (b).
Among the analogs nTBx, nB-x, and nPB-x, nTBx, and nB-x exhibited comparable
high thermal stability, whereas nPB-x gave the lowest thermal stability. As shown in Fig.
4b, 10TBM and 10B-M exhibited the high onset of decomposition at 235◦C, which was
attributed to their high π-π conjugation between the aromatic rings. 10PB-M displayed
the lowest onset of decomposition at 185◦C due to its lowest π-π conjugation derived from
the smallest mesogenic core.
2.4 The Effects of Alkyl Chain and Polar Substituents on Liquid Crystalline Properties
In general, the length of the alkyl chain had an influence, not only on the nature of the
mesophases but also on the mesomorphic temperature ranges. An increase in terminal length
resulted in enhanced dipole–dipole interaction between the terminal chains, leading to an
increase in stability of the mesophases in rod-like mesogens. Figure 5 and Table 1 illustrate
the transition temperatures of compounds nTBx. It was found that, with the increase of the
carbon numbers (n) in the alkyl chain, the mesomorphic temperature domains varied from
4◦C to 14◦C and 23◦C to 30◦Cduringheatingand cooling for nTBM, 41◦C to 61◦Cand66◦C
Figure 5. The transition temperatures of compounds nTBx during heating.

Bithiophene and Benzoxazole Liquid Crystals
31
0.50
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
7TBH
7TBC
7B-C
7PB-C
(a)
(b)
0.45
0.40
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
7TBM
7TBC
7TBN
220
270
320
370
420
470
520
220
270
320
370
420
470
520
Wavelength (nm)
Wavelength (nm)
Figure 6. UV spectra of compounds 7TBx (a) and series 7TBC, 7B-C, and 7PB-C (b) in CH2Cl2
(1.0 × 10−5 mol L−1).
to 71◦C for nTBC, 3◦C to 10◦C and 4◦C to 11◦C for nTBN, respectively. Both the melting
points and clearing points of nTBx displayed slightly increasing/decreasing tendency in
ascending homologues. Compared to biphenyl-based nB-x, the narrower mesophase range
of nTBx was attributed to its non-linear conformations resulting from the bithiophene
structure.
The polar substitutes had an influence, not only on the mesophase ranges but also on
the clearing points. Compounds nTBx with electron-withdrawing substituents (NO2, Cl)
displayed higher clearing temperatures (Tcl) than those of corresponding homologues with
electron donating substituents (Fig. 5 and Table 1), and the higher Tcl being attributed to en-
hanced conjugative interaction, which was consistent with the literature results [16, 18–20].
Substituents with a stronger electron-withdrawing effect led to a wider mesomorphic tem-
perature domain with benzoxazole-based liquid crystals, due to the stronger dipole–dipole
interaction between the molecules. In the case of bithiophene-based compounds nTBx, the
effect of substituents on the mesophase range varied in the order of Cl > NO2 > CH3 > H,
which was slightly different to that in nB-x and nPB-x (NO2 > Cl > CH3 > H).
2.5 Photophysical Properties
The absorption spectra of compounds 7TBx recorded in methylene chloride are shown
in Fig. 6a. All the compounds exhibited a broad absorption band (λmax = 374 nm for
400
200
175
150
125
100
75
7TBH
7TBM
7TBC
7TBN
7TBC
7B-C
7PB-C
(a)
(b)
350
300
250
200
150
100
50
50
25
0
0
380
430
480
530
580
630
ꢀꢁꢂ
ꢀꢃꢂ
ꢄꢁꢂ
ꢄꢃꢂ
ꢅꢁꢂ
ꢅꢃꢂ
ꢃꢁꢂ
Wavelength (nm)
Wavelength (nm)
Figure 7. Emission spectra of compounds 7TBx (a) and series 7TBC, 7B-C, and 7PB-C (b) in
CH2Cl2 (2.0 × 10−7 mol L−1).

32
K. Hu et al.
7TBH, 374 nm for 7TBM, 378 nm for 7TBC, and 388 nm for 7TBN) attributable to the
electronic transition originating from the π-molecular orbitals. The absorption bands of the
substituted compounds (7TBM, 7TBC, and 7TBN) were red-shifted compared with that
of the non-substituted compound 7TBH, illustrating the impact of substituent groups on
electronic properties.
Compared to nB-x and nPB-x, the bithiophene-based analogs nTBx showed a much
larger red-shifted absorption band, as shown in Fig. 6b, which were attributed to the increase
in the strength of π-conjugation resulting from the more stable planar conformation of the
bithiophene structure. It is known that adjacent phenyl groups will adapt a nonplanar
twisted conformation [22], so the molecular structures containing bithiophene instead of
biphenyl groups will result in planar conformation, giving rise to extensive intramolecular
conjugation [23, 24].
The fluorescence spectra of compounds 7TBx are shown in Fig. 7a, where the nitro
substituent (7TBN) let to a significantly decrease in emission, a common phenomenon in
nitro-substituted fluorophores. Apart of 7TBN, these compounds exhibited intense pho-
toluminescence emission at low concentration (2.0 × 10−7 molL−1) in CH2Cl2 solutions
when they were excited at their absorption maxima, in which the emission bands appeared
at 453 nm (7FBH), 455 nm (7FBM), and 458 nm (7FBC), respectively. Compared to nB-x
and nPB-x, the corresponding bithiophene-based analogs nTBx showed much larger red-
shifted emission peaks, as shown in Fig. 7b, which was consistent with the trend observed
in the absorption spectra (Fig. 6b).
3. Conclusions
In summary, 2-(5ꢀ-alkyl-2,2ꢀ-bithiophene-5-yl)-benzoxazole derivatives nTBx were de-
signed and synthesized. Among the nTBx series, nTBN, nTBM, and nTBC exhibited
smectic mesomorphic phases, whereas nTBH gave no mesophase at all. The bithiophene-
based analogs nTBx showed comparable mesophase range compared with nPB-x, and com-
parable thermal stability but lower melting points compared with nB-x. The substituents
had an obvious effect on the UV absorption bands of nTBx in the sequence of NO2 >
Cl > CH3 > H. Compared to nPB-x and nB-x, nTBx showed much larger red-shifted
absorption bands and photoluminescence emission peaks, which was attributed to the in-
crease in the strength of π-conjugation resulting from the more stable planar conformation
of the bithiophene structure.
4. Experimental
4.1 Materials
Bithiophene, alkyl bromide, ammonium chloride, butyl lithium, phosphorus oxychloride,
IBD, 2-aminophenol, 2-amino-4-nitrophenol, 2-amino-4-chlorophenol, and 2-amino-4 -
methylphenol were purchased from Aladdin-reagent Co. (Shanghai, China) and used as
received. Tetrahydrofuran, silica gel, N,N-dimethylformamide (DMF), petroleum ether
(PE), ethyl acetate (EA), hexane, absolute ethanol, chloroform, and anhydrous potassium
carbonate were purchased from Sinopharm Chemical Reagent Co. (Shanghai, China).
Chloroform was dehydrated by pre-dried 4Å molecular sieves which were activated at
350◦C for 4 h prior to use.

Bithiophene and Benzoxazole Liquid Crystals
4.2 Characterization and Measurements
33
The structures of the final products and intermediates were confirmed by a variety of spectral
methods. IR spectra were recorded on a Nicolet Avatar360E spectrometer (Thermo Electron
1
Corporation, Madison, Wis., USA). H-NMR spectra, with tetramethylsilane (TMS) as
internal standard, were recorded on a Bruker AV 300 (300 MHz) instruments (Bruker
Corporation, Karlsruhe, Germany). The mass spectra were obtained by GC/EI-MS Thermo
DSQ II (Thermo Finnigan, CA, USA) with m/z 50 to 650. EA was performed for C,
H, and N in an Elementar Vario EL III instrument (Elementar Analysensysteme GmbH,
Hanau, Germany). The phase transition temperatures were measured by DSC, Shimatsu
DSC-60 (Shimadzu Corporation, Kyoto, Japan) in nitrogen at a heating and cooling rate of
5◦C min−1, and POM, LEICA DM2500P (Leica Microsystems GmbH, Wetzlar, Germany)
with Linkam THMS600 hot-stage (Linkam Scientific Instruments Ltd., Tadworth, UK)
and control unit at a heating and cooling rate of 0.5◦C min−1. Thermogravimetric analysis
(TGA) was carried out with a TA Q50 (TA Instruments, New Castle, DE, USA) in nitrogen
(flow rate: 100 cm3 min−1) at a heating rate of 10◦C min−1. UV-vis absorption spectra were
recorded using a Hitachi U-3900UV spectrometer (Hitachi High-Technologies Corporation,
Tokyo, Japan). The emission spectra were determined using a Hitachi F-7000 spectrometer
(Varian Ltd., Madrid, Spain).
4.3 Synthesis
As an example, the preparation procedure of 2-(5ꢀ-decyl-2,2ꢀ-bithiophene-5-yl)-5-
nitrobenzoxazole (10TBN) is described below.
4.3.1 Synthesis of 5-decyl-2,2ꢀ-bithiophene. Under nitrogen protection, 2.5 M butyl lithium
(26 mL, 66 mmol) was injected into a cooled (−78◦C), stirred solution of bithophene
(60 mmol) in THF (150 mL). After addition, the reaction temperature was maintained at
−78◦C for 1 h, followed by the addition of solution of n-decyl bromide (66 mmol) in
THF (25 mL). Then, the system was kept at −78◦C for 1 h after addition, and allowed
to warm to room temperature overnight. After addition of 150 mL of saturated solution
of ammonium chloride, EA was added for three extraction cycles. The combined organic
extracts were washed with water (100 mL), dried with magnesium sulfate, and then the
solvent was removed in vacuo. The residue was purified via column chromatography on
silica gel using hexane as eluent to give purity above 98% for GC measurement. Yellow
liquid was obtained with yield of 65%.
4.3.2 Synthesis of 5-decyl-2,2ꢀ-bithiophene-5ꢀ-carboxaldehyde. Under nitrogen protec-
tion, 12.24 g phosphorus oxychloride (80 mmol) was dropped into a cooled (0◦C–5◦C),
stirred solution of DMF (80 mL). After addition, the reaction temperature was maintained
below 5◦C for 0.5 h, followed by the addition of solution of 5-decyl-2,2ꢀ-bithiophene
(20 mmol) in DMF (20 mL). Then, the system was stirred at 80◦C for 5 h. After the mix-
ture was cooled to room temperature, the system was diluted with water. A 10% aqueous
solution of potassium carbonate (100 mL) was added to adjust ph value to neutral, and the
mixture was extracted with EA for three times. The combined organic extracts were washed
with water (100 mL), dried over magnesium sulfate and the solvent was removed in vacuo.
The residue was purified via column chromatography on silica gel using PE/EA (8/1) as
eluent to give purity above 98% for GC measurement. Yellow crystals were obtained with
yield of 85%; m.p. 44.9◦C−45.4◦C.

34
K. Hu et al.
4.3.3 Synthesis of 2-(((5ꢀ-decyl-2,2ꢀ-bithiophene-5-yl)methylene)amino)-4-nitrophenol
(10TSN). To a 100 mL, round-bottom flask equipped with an overhead stirrer and
condenser, 0.74 g of 5-decyl-2,2ꢀ-bithiophene-5ꢀ-carboxaldehyde (2.2 mmol), 0.51 g of
2-amino-4-nitrophenol (3.3 mmol) and 25 mL of ethanol were added. The reaction system
was stirred at reflux for 6 h. After the mixture was cooled to room temperature, it was
filtrated through Celite. The solid was washed with 15 mL of ethanol for several times to
give purity above 98% (HPLC). Pale yellow crystals were obtained with yield of 90%; m.p.
141.2◦C −141.6◦C.
4.3.4 Synthesis of 2-(5ꢀ-decyl-2,2ꢀ-bithiophene-5-yl)-5-nitrobenzoxazole (10TBN). To a
100 mL round-bottom flask equipped with an overhead stirrer and condenser, 0.50 g of
10TSN (1.07 mmol), 0.43 g of IBD (1.32 mmol), and 50 mL of anhydrous chloroform. The
reaction system was stirred at reflux for 6 h. After completion of the reaction, the mixture
was diluted with water and extracted with chloroform for three times. The combined organic
phase was dried over magnesium sulfate. After removal of the solvent in vacuo, the residue
was purified via column chromatography on silica gel using PE/EA (8/1) as eluent to give
purity above 98% for HPLC measurement. Pale yellow crystals were obtained with yield
of 76%; m.p. 143◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 8.55 (d, J = 2.1 Hz, 1H), 8.28 (q, 3J =
4
9.0 Hz, J = 2.1 Hz, 1H), 7.82 (d, J = 3.9 Hz, 1H), 7.61 (d, J = 8.7 Hz, 1H), 7.14 (t,
J = 3.0 Hz, 2H), 6.74 (d, J = 3.3 Hz, 1H), 2.81 (t, J = 7.5 Hz, 2H), 1.69 (m, 2H), 1.27 (m,
14H), 0.88 (t, J = 6.6 Hz, 3H).
IR (KBr, pellet, cm−1): 3100, 3071, 3037, 2954, 2918, 2850, 1617, 1577, 1547, 1528,
1501, 1467, 1438, 1344, 1309, 1267, 1235, 1062, 997, 882, 831, 799, 738, 716, 690.
EI-MS m/z (rel. int.): 468.17 (M+, 26), 341.01 (100), 295.05 (34).
EA: Calc. for C25H28N2O3S2: C 64.07, H 6.02, N 5.98; Found: C 63.77, H 5.71,
N 5.90.
The other bithiophene-based compounds (nTBx) were prepared using a similar
procedure.
7TBH: Pale yellow crystals, yield 74%; m.p. 106◦C.
1H-NMR (300MHz, CDCl3, TMS): δ (ppm) 7.77 (d, J = 3.9 Hz, 1H), 7.71 (m, 1H),
7.52 (m, 1H), 7.32 (m, 2H), 7.12 (q, 3J = 6.3 Hz, 4J = 3.9 Hz, 2H), 6.72 (d, J = 3.3 Hz,
1H), 2.80 (t, J = 7.5 Hz, 2H), 1.69 (m, 2H), 1.34–1.29 (m, 8H), 0.89 (t, J = 6.6 Hz, 3H).
IR (KBr, pellet, cm−1): 3071, 2957, 2921, 2850, 1614, 1574, 1550, 1507, 1447, 1239,
1071, 1007, 873, 810, 792, 743, 715, 502.
EI-MS m/z (rel. int.): 381.11 (M+, 31), 296.01 (100), 176.97 (3), 147.98 (2).
EA: Calc. for C22H23NOS2: C 69.25, H 6.08, N 3.67; Found: C 69.06, H 5.79, N 3.68.
8TBH: Pale yellow crystals, yield 73%; m.p. 105◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 7.78 (d, J = 3.6 Hz, 1H), 7.72 (m, 1H),
7.52 (m, 1H), 7.34 (m, 2H), 7.12 (q, 3J = 6.3 Hz, 4J = 3.6 Hz, 2H), 6.73 (t, J = 3.3 Hz,
1H), 2.83 (t, J = 7.5 Hz, 2H), 1.69 (m, 2H), 1.28 (m, 10H), 0.88 (t, J = 6.6 Hz, 3H).
IR (KBr, pellet, cm−1): 3072, 2955, 2921, 2850, 1614, 1575, 1508, 1446, 1240, 1073,
1009, 796, 744.
EI-MS m/z (rel. int.): 395.11 (M+, 30), 296.00 (100), 177.01 (5), 147.92 (3).
EA: Calc. for C23H25NOS2: C 69.83, H 6.37, N 3.54; Found: C 69.76, H 6.36, N 3.54.
10TBH: Pale yellow crystals, yield 73%; m.p. 105◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 7.77 (d, J = 3.9 Hz, 1H), 7.71 (m, 1H),
7.52 (m, 1H), 7.33 (m, 2H), 7.12 (q, 3J = 6.3 Hz, 4J = 3.9 Hz, 2H), 6.72 (d, J = 3.3 Hz,
1H), 2.80 (t, J = 7.5 Hz, 2H), 1.69 (m, 2H), 1.27 (m, 14H), 0.88 (t, J = 6.3 Hz, 3H).

Bithiophene and Benzoxazole Liquid Crystals
35
IR (KBr, pellet, cm−1): 3090, 3073, 2954, 2920, 2849, 1614, 1575, 1552, 1508, 1469,
1452, 1444, 1384, 1240, 1073, 1009, 999, 876, 811, 796, 759, 743.
EI-MS m/z (rel. int.): 423.18 (M+, 43), 296.00 (100), 176.99 (3), 148.09 (3).
EA: Calc. for C25H29NOS2: C 70.88, H 6.90, N 3.31; Found: C 70.51, H 6.59, N 3.31.
7TBM: Pale yellow crystals, yield 75%; m.p. 88◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 7.74 (d, J = 3.6 Hz, 1H), 7.49(s, 1H),
7.39 (d, J = 8.4 Hz, 1H), 7.11 (m, 3H), 6.72 (d, J = 3.3 Hz, 1H), 2.80 (t, J = 7.5 Hz, 2H),
2.46 (s, 3H), 1.69 (m, 2H), 1.34 – 1.29 (m, 8H), 0.89 (t, J = 6.9 Hz, 3H).
IR (KBr, pellet, cm−1): 2954, 2924, 2849, 1612, 1572, 1547, 1511, 1468, 1438, 1425,
1257, 1193, 1058, 1017, 875, 821, 797, 786, 709.
EI-MS m/z (rel. int.): 395.08 (M+, 34), 310.04 (100), 177.03 (6).
EA: Calc. for C23H25NOS2: C 69.83, H 6.37, N 3.54; Found: C 69.68, H 6.18,
N 3.55.
8TBM: Pale yellow crystals, yield 75%; m.p. 88◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 7.74 (d, J = 3.9 Hz, 1H), 7.48 (s, 1H),
7.39 (d, J = 8.1 Hz, 1H), 7.11 (m, 3H), 6.72 (d, J = 3.6 Hz, 1H), 2.80 (t, J = 7.5 Hz, 2H),
2.46 (s, 3H), 1.68 (m, 2H), 1.28 (m, 10H), 0.88 (t, J = 6.6 Hz, 3H).
IR (KBr, pellet, cm−1): 2953, 2921, 2851, 1618, 1579, 1547, 1505, 1468, 1443, 1262,
1069, 1000, 874, 801, 783, 717.
EI-MS m/z (rel. int.): 409.13 (M+, 49), 310.00 (100), 177.00 (5).
EA: Calc. for C24H27NOS2: C 70.37, H 6.64, N 3.42; Found: C 70.09, H 6.40, N 3.41.
10TBM: Pale yellow crystals, yield 76%; m.p. 82◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 7.74 (d, J = 3.9 Hz, 1H), 7.49 (s, 1H),
7.39 (d, J = 8.4Hz, 1H), 7.11 (m, 3H), 6.72 (d, J = 3.3 Hz, 1H), 2.80 (t, J = 7.5 Hz, 2H),
2.46 (s, 3H), 1.68 (m, 2H), 1.26 (m, 14H), 0.88 (t, J = 6.3 Hz, 3H).
IR (KBr, pellet, cm−1): 2953, 2920, 2850, 1617, 1579, 1547, 1505, 1481, 1468, 1443,
1429, 1262, 1068, 1001, 874, 801, 716.
EI-MS m/z (rel. int.): 437.17 (M+, 73), 310.03 (100), 176.97 (3).
EA: Calc. for C26H31NOS2: C 71.35, H 7.14, N 3.20; Found: C 71.05, H 6.90, N 3.16.
7TBC: Pale yellow crystals, yield 71%; m.p. 74◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 7.76 (d, J = 3.9 Hz, 1H), 7.67 (d, J =
1.8 Hz, 1H), 7.44 (d, J = 8.4 Hz, 1H), 7.29 (q, 3J = 8.4 Hz, 4J = 1.8 Hz, 1H), 7.13 (t, J =
3.9 Hz, 2H), 6.72 (d, J = 3.6 Hz, 1H), 2.80 (t, J = 7.5 Hz, 2H), 1.69 (m, 2H), 1.34 – 1.29
(m, 8H), 0.89 (t, J = 6.6 Hz, 3H).
IR (KBr, pellet, cm−1): 2956, 2924, 2849, 1605, 1566, 1545, 1509, 1469, 1449, 1422,
1384, 1258, 1192, 1060, 1017, 808, 796, 708.
EI-MS m/z (rel. int.): 415.09 (M+, 39), 329.96 (100), 295.11 (2), 176.99 (6).
EA: Calc. for C22H22ClNOS2: C 63.52, H 5.33, N 3.37; Found: C 63.51, H 5.18,
N 3.37.
8TBC: Pale yellow crystals, yield 73%; m.p. 94◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 7.75 (d, J = 3.9 Hz, 1H), 7.66 (d, J =
3
4
2.1 Hz, 1H), 7.43 (d, J = 8.7 Hz, 1H), 7.28 (q, J = 8.7 Hz, J = 2.1 Hz, 1H), 7.11 (t,
J = 3.6 Hz, 2H), 6.72 (d, J = 3.6 Hz, 1H), 2.80 (t, J = 7.5 Hz, 2H), 1.69 (m, 2H), 1.28 (m,
10H), 0.88 (t, J = 6.9 Hz, 3H).
IR (KBr, pellet, cm−1): 2956, 2921, 2850, 1612, 1572, 1547, 1504, 1467, 1445, 1423,
1255, 1070, 1054, 1000, 920, 871, 802, 781, 715.
EI-MS m/z (rel. int.): 429.11 (M+, 42), 329.96 (100), 295.03 (3), 177.02 (4).
EA: Calc. for C23H24ClNOS2: C 64.24, H 5.63, N 3.26; Found: C 63.78, H 5.41,
N 3.25.

36
K. Hu et al.
10TBC: Pale yellow crystals, yield 74%; m.p. 91◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 7.75 (d, J = 3.9 Hz, 1H), 7.66 (d, J =
3
4
1.8 Hz, 1H), 7.43 (d, J = 8.4 Hz, 1H), 7.28 (q, J = 8.4 Hz, J = 1.8 Hz, 1H), 7.11 (t,
J = 3.6 Hz, 2H), 6.72 (d, J = 3.6 Hz, 1H), 2.80 (t, J = 7.5 Hz, 2H), 1.68 (m, 2H), 1.26 (m,
14H), 0.88 (t, J = 6.9 Hz, 3H).
IR (KBr, pellet, cm−1): 2957, 2920, 2849, 1611, 1572, 1547, 1504, 1468, 1444, 1424,
1256, 1070, 1051, 1001, 920, 871, 803, 783, 716, 702.
EI-MS m/z (rel. int.): 457.17 (M+, 38), 329.95 (100), 294.99 (3), 177.02 (5).
EA: Calc. for C25H28ClNOS2: C 65.55, H 6.16, N 3.06; Found: C 65.57, H 5.97, N
3.07.
7TBN: Pale yellow crystals, yield 75%; m.p. 145◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 8.55 (d, J = 1.8 Hz, 1H), 8.27 (q, 3J =
8.7 Hz, 4J = 2.1 Hz, 1H), 7.82 (d, J = 3.9 Hz, 1H), 7.61 (d, J = 8.7 Hz, 1H), 7.15 (d, J =
3.3 Hz, 2H), 6.74 (d, J = 3.3 Hz, 1H), 2.81 (t, J = 7.5 Hz, 2H), 1.70 (m, 2H), 1.35–1.30
(m, 8H), 0.89 (t, J = 6.6 Hz, 3H).
IR (KBr, pellet, cm−1): 3099, 3070, 2955, 2925, 2849, 1616, 1577, 1546, 1527, 1500,
1467, 1438, 1343, 1309, 1268, 1237, 1059, 995, 883, 806, 792, 739.
EI-MS m/z (rel. int.): 426.13 (M+, 49), 340.96 (100), 295.00 (32).
EA: Calc. for C22H22N2O3S2: C 61.95, H 5.20, N 6.57; Found: C 61.70, H 5.01, N
6.48.
8TBN: Pale yellow crystals, yield 73%; m.p. 145◦C.
1H-NMR (300 MHz, CDCl3, TMS): δ (ppm) 8.54 (d, J = 2.1 Hz, 1H), 8.27 (q, 3J =
4
8.7 Hz, J = 1.5 Hz, 1H), 7.81 (d, J = 3.9 Hz, 1H), 7.60 (d, J = 8.7 Hz, 1H), 7.14 (d,
J = 2.1 Hz, 2H), 6.73 (d, J = 3.0 Hz, 1H), 2.81 (t, J = 7.2 Hz, 2H), 1.69 (m, 2H), 1.28 (m,
10H), 0.88 (t, J = 6.3 Hz, 3H).
IR (KBr, pellet, cm−1): 3014, 2956, 2924, 2853, 1618, 1577, 1526, 1502, 1466, 1439,
1402, 1345, 1258, 1059, 996, 881, 808, 793, 735.
EI-MS m/z (rel. int.): 440.16 (M+, 30), 341.00 (100), 295.00 (44).
EA: Calc. for C23H24N2O3S2: C 62.70, H 5.49, N 6.36; Found: C 62.63, H 5.24, N
6.31.
Funding
The authors would like to thank the National Science Foundation Committee of China
(50802058, 51373092), Key Technologies R&D Program of Shaanxi Province (2012K08-
09, 2014K10-06), Natural Science Foundation of Shaanxi Province (2012JM2002,
2014JM7270), Specialized Research Fund for the Doctoral Program of Higher Educa-
tion (20130202120010), Program for Changjiang Scholars and Innovative Research Team
in University (IRT1070) and the Fundamental Research Funds for the Central Universities
(GK201302036, GK201302037) for financial support of this work.
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