Synthesis and mesomorphic properties of novel high birefringence azobenzene liquid crystals with lateral fluorosubstituent

Article abstract of DOI:10.1080/15421406.2016.1272798

In order to obtain liquid crystals with high birefringence (Δn), four azobenzene compounds based on lateral fluorinated bistolane moieties with trifluoromethyl terminal groups were synthesized with Hagihara–Sonogashira cross-coupling reaction. The molecul

Full text of DOI:10.1080/15421406.2016.1272798

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis and mesomorphic properties of novel
high birefringence azobenzene liquid crystals with
lateral fluorosubstituent
Yuzhen Zhao, Kexuan Li, Yongming Zhang, Yang Zhao & Zongcheng Miao
To cite this article: Yuzhen Zhao, Kexuan Li, Yongming Zhang, Yang Zhao & Zongcheng Miao
(2017) Synthesis and mesomorphic properties of novel high birefringence azobenzene liquid
crystals with lateral fluorosubstituent, Molecular Crystals and Liquid Crystals, 650:1, 7-12, DOI:
10.1080/15421406.2016.1272798
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1272798
Published online: 23 Aug 2017.
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Date: 23 August 2017, At: 23:17

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
https://doi.org/./..
Synthesis and mesomorphic properties of novel high
birefringence azobenzene liquid crystals with lateral
fluorosubstituent
Yuzhen Zhaoa,b, Kexuan Lia, Yongming Zhanga, Yang Zhaoa, and Zongcheng Miaoa
aDepartment of Applied Statistics and Sciences, Xijing University, Xi’an, China; bDepartment of Materials Science
and Engineering, College of Engineering, Peking University, Beijing, China
ABSTRACT
KEYWORDS
Azobenzene; birefringence;
lateral fluorosubstituent
In order to obtain liquid crystals with high birefringence (ꢀn), four
azobenzene compounds based on lateral fluorinated bistolane moieties
with trifluoromethyl terminal groups were synthesized with Hagihara–
Sonogashira cross-coupling reaction. The molecular structures of the
1
compounds were confirmed by Fourier transform infrared, H-nuclear
magnetic resonance. The transition temperatures, optical textures, and
optical anisotropies were determined by differential scanning calorime-
try, polarizing optical microscopy. The effect of the alkyl substituents
on liquid crystal behavior is discussed in detail. The compounds have
showed higher birefringence values.
Introduction
High birefringence (ꢀn) liquid crystals have attracted much attention in the past few
years because they are useful not only in conventional display devices, such as super-
twisted nematic liquid crystal displays (STNLCDs), but also in scattering-type polymer-
dispersed liquid crystal displays (PDLCDs) as reflective liquid crystal displays (LCDs)
and in spatial light modulators [1–4]. In addition, these materials are thought to be appli-
cable to laser beam steering, infrared spatial modulators and optical communication
[5–6].
Turning to materials, from the single-band model, the ꢀn of a liquid crystalline compound
is determined mainly by π-electron conjugation, order parameter and differential oscillator
strength. Molecules that contain highly polarizable groups with high electron density, such
as acetylene linking groups or benzene rings, will therefore have large optical anisotropies
[7–12]. In addition, liquid-crystalline polymers with the azobenzene in the side chain have
attracted much attention because a photo-induced change in birefringence ꢀn was large when
compared with amorphous counterparts [13, 14]. Azobenzene liquid crystals (LCs) have been
extensively studied because of their applicability for photoswitching devices [15, 16], optical
memory [17], nonlinear optics [18], and holography [19]. By contrast, related small molecular
compounds have been less explored.
CONTACT Zongcheng Miao
versity, Xi’an , China.
miaozongcheng@xijing.edu.cn
Department of Applied Statistics and Sciences, Xijing Uni-
©  Taylor & Francis Group, LLC

8
Y. ZHAO ET AL.
In this article, we focus our attention on a series of compounds containing azobenzene core
with trifluoromethyl polarizable groups, which were highly conjugated along the molecular
long axis to yield materials with high ꢀn values.
Experimental
Reagents were purchased from commercial sources (Aldrich), and used without further
purification. Tetrahydrofuran (THF) and triethylamine (TEA) were distilled and purged with
argon before use. Reactions were carried out in an inert atmosphere (dry Ar or N2). The
preparative routes for the azobenzene liquid crystals compounds are shown in Scheme 1.
The structures of the final compounds and various synthetic intermediates were character-
ized by 1H NMR spectroscopy (Bruker DMS-400 spectrometer instrument). All spectra were
recorded in CDCl3 with TMS as internal standard. Fourier transform infrared spectroscopy
(FT-IR, Perkin Elmer Spectrum One) was also measured.
Scheme . Reagents and conditions () Oxone, CHCl, HO, r. t.,  h; () AcOH, r. t.,  h; () TAE, THF,
PdCl(PPh), CuI, °C,  hr.
The phase transition temperatures of the target compounds were measured by differential
scanning calorimetry (DSC, Perkin Elmer Pyris 6). The optical textures were measured using
a polarizing optical microscope (POM, Olympus BX51) equipped with a hot stage calibrated
to an accuracy of 0.1 K (Linkam LK-600PM). The ꢀn was evaluated using the guest-host
method from mixtures containing 10 wt% of each test compound in SLC 1717 (Slichem Liquid
Crystal Material Co., Ltd.) and a polarized UV/VIS/NIR spectrophotometer (JASCO V-570)
was used to measure the birefringence of the mixtures [20].
Synthesis
General procedure for the synthesis of intermediate m
4-Trifluoromethyl-aniline (16.1 g, 0.100 mol) dissolved in 200 mL of CH2Cl2 and Oxone
(123.0 g, 0.200 mol) dissolved in 200 mL of water were added in a round-bottom flask, the
reaction mixture then stirred at room temperature for 15 hr under an N2 atmosphere. After
separation of the layers, the aqueous layer was extracted with CH2Cl2 (3 × 50 mL). The com-
bined organic layers were washed with 1 N HCl, saturated sodium bicarbonate solution, water,
brine, and dried with MgSO4. After filtration, removal of the solvent from the filtrate in vacuo
yielded the corresponding labile nitrosoarene, which was submitted to the next condensa-
tion step without further purification. To the nitrosoarene dissolved in 200 mL of acetic acid
was added 4-bromo-2-fluoro-aniline (19.0 g, 0.100 mol). The resulting mixture was stirred at
room temperature for 15 hr. The precipitate was separated by filtration and the collected solid
was washed with acetic acid and water and dried in a desiccator over P2O5 under reduced
pressure for 24 hr.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
9
(4-Bromo-2-fluoro -phenyl)- (4-trifluoromethyl-phenyl)-diazene (m1). Yellow solid, yield:
71.2%, 24.8g.1H-NMR (400 MHz, CDCl3) (δ, ppm): 7.34 (s, 1H), 7.42 (d, 1H, J = 8.8 Hz), 7.66
(d, 2H, J = 8.8 Hz), 7.82 (d, 1H, J = 8.4 Hz), 7.88 (d, 2H, J = 8.8 Hz). IR (cm−1): 2109, 1932,
1600, 1492, 1409, 1326, 1174, 1139, 1066, 856,652.
General procedure for the synthesis of the azobenzene liquid crystals
(4-Bromo-2-fluoro -phenyl)- (4-trifluoromethyl-phenyl)-diazene (17.4 g, 0.05 mol) and
1-Ethynyl-4-R-benzene (0.06 mol) dissolved in 500 ml dry Et3N:THF (1:1). Add Pd(PPh3)4
(1.4 g, 1.2 mmol)) and CuI (0.45 g, 2.4 mmol) The reaction mixture was then stirred at room
temperature for 8 hr under an Ar atmosphere. Upon completion, the solvent was removed in
vacuo and the crude material purified by chromatography on silica gel (2:1, hexanes: CH2Cl2)
to give target compound as a Yellow solid.
[4-(4-Ethyl-phenylethyny)-2-fluoro-phenyl]-(4-trifluoromethyl-phenyl)-diazene (a). yie-
ld: 74%, Yellow solid. 1H-NMR (400 MHz, CDCl3) (δ, ppm): 1H-NMR (400 MHz, CDCl3) (δ,
ppm):1.24(t, 3H), 2.60(m, 2H), 7.08 (d, 2H, J = 8.8 Hz), 7.34 (s, 1H), 7.40 (d, 2H, J = 8.8 Hz),
7.42 (d, 1H, J = 8.8 Hz),7.66 (d, 2H, J = 8.8 Hz), 7.82 (d, 1H, J = 8.4 Hz), 7.88 (d, 2H, J =
8.8 Hz). IR (cm−1): 2109, 1932, 1600, 1492, 1409, 1326, 1174, 1139, 1066, 856,652.
[4-(4-Propyl-phenylethyny)-2-fluoro-phenyl]-(4-trifluoromethyl-phenyl)-diazene (b). yi-
1
eld:73%, Yellow solid. H-NMR (400 MHz, CDCl3) (δ, ppm): 0.98(t, 3H), 1.66((m, 2H)),
2.56(t, 2H), 7.08 (d, 2H, J = 8.8 Hz), 7.34 (s, 1H), 7.40 (d, 2H, J = 8.8 Hz), 7.42 (d, 1H,
J = 8.8 Hz),7.66 (d, 2H, J = 8.8 Hz), 7.82 (d, 1H, J = 8.4 Hz), 7.88 (d, 2H, J = 8.8 Hz). IR
(cm−1): 2109, 1932, 1600, 1492, 1409, 1326, 1174, 1139, 1066, 856,652.
[4-(4-butyl-phenylethyny)-2-fluoro-phenyl]- (4-trifluoromethyl-phenyl)-diazene (c).yie-
ld:75%, Yellow solid. 1H-NMR (400 MHz, CDCl3) (δ, ppm): 0.96(t, 3H), 1.32(m, 2H), 1.64(m,
2H), 2.56(t, 2H), 7.08 (d, 2H, J = 8.8 Hz), 7.34 (s, 1H), 7.40 (d, 2H, J = 8.8 Hz), 7.42 (d, 1H,
J = 8.8 Hz),7.66 (d, 2H, J = 8.8 Hz), 7.82 (d, 1H, J = 8.4 Hz), 7.88 (d, 2H, J = 8.8 Hz). IR
(cm−1): 2109, 1932, 1600, 1492, 1409, 1326, 1174, 1139, 1066, 856,652.
[4-(4-amyl-phenylethyny)-2-fluoro-phenyl]- (4-trifluoromethyl-phenyl)-diazene (d).yie-
ld:76%, Yellow solid. 1H-NMR (400 MHz, CDCl3) (δ, ppm): 0.96(t, 3H), 1.28(m, 2H), 1.33(m,
2H), 1.62(m, 2H), 2.56(t, 2H), 7.08 (d, 2H, J = 8.8 Hz), 7.34 (s, 1H), 7.40 (d, 2H, J = 8.8 Hz),
7.42 (d, 1H, J = 8.8 Hz),7.66 (d, 2H, J = 8.8 Hz), 7.82 (d, 1H, J = 8.4 Hz), 7.88 (d, 2H, J =
8.8 Hz). IR (cm−1): 2109, 1932, 1600, 1492, 1409, 1326, 1174, 1139, 1066, 856,652.
Results and discussion
Phase transitions
DSC was used to determine the phase transition temperatures during cooling at a scanning
rate of 10°C min−1, the DSC curves of these compounds are shown in Fig. 1 and the associated
data are listed in Table 1. The phase transition temperatures of the compounds were listed in
Table . Phase transition temperatures and ꢀn of the compounds. Cr = crystal, Sm = smectic phase, I =
isotropic.
Compounds
Phase transition temperatures (°C)
ꢀn
a
b
c
Cr . Sm . I
Cr .
Cr . Sm . I
Cr 
.
/
.
/
d

10
Y. ZHAO ET AL.
Figure . DSC curves of compounds a–d.
Table 1. It was shown that only the compound a and compound c are exhibit mesomorphism,
that is related to the length of carbon chain in the mols, and present the odd–even effect to car-
bon atom nos. Moreover, compared with the synthesized compound a, compound c exhibits
a higher melting point and a narrower temperature range of mesophase, this is because the
alkyl carbon number in compound a was less than compound c. The Low molecular weight
resulted in a lower melting point.
Optical textures
The optical textures of the synthesized compounds a and c were investigated primarily by
POM. The phases were identified through the comparison of the observed textures with ref-
erence textures [21] from POM. Figure 2 shows POM photographs of compounds a and c
observed in their LC phases in the course of cooling. The observations above indicate that the
compounds show enantiotropic phase behavior with the existence of mesophase between the
crystal and the isotropic liquid, which are consistent with the DSC results. Compared with
compound c, compound a tend to form typical smectic textures for the shorter alkyl chain
and higher electron density.
Figure . Microphotographs of the liquid crystalline phases of compounds a and c.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
11
Figure . UV–vis absorption spectra of compound c in CHCl.
Optical anisotropy
Having demonstrated the LC properties of the azobenzene derivatives, we focused our
attention on their ꢀn properties. The ꢀn was evaluated from mixtures containing 10 wt%
of each test compound in SLC 1717 using polarizing light interferometry method [20].
The ꢀn data of the compounds were collected in Table 1. Among compound a and
c, the length of flexible chain is important evaluation factor for ꢀn. Longer carbon
chain decreases π-electron conjugation slightly, the ꢀn value decreased correspondingly
[7].
Photophysical properties
For provides some insight into the electronic structure of the systems, the absorption spectra
of the synthesized compound c in dilute CH2Cl2 solution at room temperature, depicted in
Fig. 3. The spectra display a characteristic pattern of two absorption bands at 334 nm and
357 nm. The change of absorption spectra under UV irradiation was tested, the absorption
intensity of the absorption band at 357 nm become weaker gradually with the UV irradiation
and the absorption band revealed slight blue shifts (∼3 nm). One generally approved expla-
nation was that trans–cis photoisomerization of the azobenzene moiety in LC compound by
disorganizing the alignment.
Conclusions
We have synthesized and evaluated the properties of a series of azobenzene liquid crystals
with lateral fluorosubstituent. The effect of the alkyl substituents on liquid crystal behavior
was obtained, and present the odd–even effect to carbon atom nos, compounds with even
carbon atom nos showed good liquid-crystalline behavior.
Funding
This work was supported by the National Natural Science Foundation of China (No: 51673157).

12
Y. ZHAO ET AL.
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