Asymmetrical azobenzene liquid crystals with high birefringence prepared via click chemistry

Article abstract of DOI:10.1080/15421406.2013.827842

Asymmetrical azobenzene liquid crystals with high birefringence were prepared via the click chemistry method. The chemical structures of the synthetic intermediates and compounds were confirmed by FT-IR and ¹H-NMR. The liquid crystal behavior,

Full text of DOI:10.1080/15421406.2013.827842

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Asymmetrical Azobenzene Liquid
Crystals with High Birefringence
Prepared via Click Chemistry
Zongcheng Miaoa, Yongming Zhanga, Yuzhen Zhaob, Zhixue Wanga &
Dong Wangb
a Department of Foundation, Xijing University, Xi’an, PR China,
Shaanxi Province
b Department of Materials Physics and Chemistry, School of Materials
Science and Engineering, University of Science and Technology
Beijing, Beijing, PR China
Published online: 08 Apr 2014.
To cite this article: Zongcheng Miao, Yongming Zhang, Yuzhen Zhao, Zhixue Wang & Dong Wang
(2014) Asymmetrical Azobenzene Liquid Crystals with High Birefringence Prepared via Click Chemistry,
Molecular Crystals and Liquid Crystals, 591:1, 10-18, DOI: 10.1080/15421406.2013.827842
To link to this article: http://dx.doi.org/10.1080/15421406.2013.827842
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Mol. Cryst. Liq. Cryst., Vol. 591: pp. 10–18, 2014
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.827842
Asymmetrical Azobenzene Liquid Crystals with
High Birefringence Prepared via Click Chemistry
ZONGCHENG MIAO,1 YONGMING ZHANG,1 YUZHEN
ZHAO,2 ZHIXUE WANG,1 AND DONG WANG2,∗
1Department of Foundation, Xijing University, Xi’an, Shaanxi Province, PR
China
2Department of Materials Physics and Chemistry, School of Materials Science
and Engineering, University of Science and Technology Beijing, Beijing,
PR China
Asymmetrical azobenzene liquid crystals with high birefringence were prepared via
the click chemistry method. The chemical structures of the synthetic intermediates
and compounds were confirmed by FT-IR and 1H-NMR. The liquid crystal behavior,
including transition temperatures and phase sequences, were investigated by DSC and
POM. The effect of the alkyl substituents on liquid crystal behavior is discussed in detail.
Then, the birefringence values of the liquid crystals were measured using polarizing
light interferometry method. Moreover, the characteristics of selective reflection, which
was associated with the value of birefringence, were studied.
Keywords Azobenzene; birefringence; click chemistry; liquid crystal; selective
reflection
1. Introduction
Owing to the rich photochemistry of azobenzene chromophores, a fundamental research as-
pect of materials science and optical technologies [1], has received much attention because
of applications in information storage [2], optical processing [3,4], holographic grating
[5,6], liquid crystal alignment [7], and so on. These interesting applications cannot be sep-
arated from the photochemical trans-cis isomerization induced by UV and visible light [8],
which is the most important and well-known property of azobenzene-containing species,
and is usually accompanied by the structural change, as a result of the changes of dipole
moment and geometry [9].
Presently, various photoresponsive systems including small-molecular azobenzene liq-
uid crystals have been exploited [10,11]. Among them, azobenzene liquid crystals with high
birefringence (ꢀn) were especially concerned for use as smart light-responsive materials
in various potential applications [12]. Furthermore, Huisgen [3+2] cycloaddition reaction
(“click chemistry” CC) plays an important role in the preparation of novel liquid crys-
tal compounds [13]. CC, which takes place between terminal alkynes and high-energy
azides under thermal condition, gives two isomer products, the 1,4- and 1,5-disubstituted
∗Address correspondence to Dong Wang, Department of Materials Physics and Chemistry,
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing,
100083, PR China. Tel.: +86-10-62333969; Fax: 86-10-62333969. E-mail: ustbwangd@163.com
10

Asymmetrical Azobenzene Liquid Crystals
11
1,2,3-triazoles [14,15]. In 2001, Kolb, Finn, and Sharpless published a landmark review
describing a new strategy for CC. They used Cu(I) salt as the catalyst, and obtained the
1,5-disubstituted 1,2, 3-triazoles selectively [16]. Since then CC has been broadly used
in the areas of biological, materials, and medicinal chemistry, gives high yields, tolerates
many functional groups, and the reaction conditions are simple [17,18].
In the present work, our objective was to synthesize a series of azobenzene liquid
crystals molecules, through CC reactions, with high ꢀn values. The synthesis and meso-
morphic properties are presented for a variety of functional groups that were made in order
to investigate the effect of these modifications.
2. Experimental
Reagents were purchased from commercial sources (Aldrich) and used without further
purification. Triethylamine (TEA) and tetrahydrofuran (THF) were distilled and purged
with argon before use. 1H-NMR spectra of the samples were recorded with a Bruker DMS-
400 spectrometer instrument. Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer
Spectrum One) was also measured.
Transition temperatures and phase sequences were measured using a polarizing optical
microscope (POM, Olympus BX51) equipped with a hot stage calibrated to an accuracy of
0.1K (Linkam LK-600PM) and then conformed using differential scanning calorimetry
(DSC, Perkin Elmer Pyris 6). ꢀ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 [19].
The reflectance characteristics were investigated by unpolarized UV/VIS/NIR spec-
trophotometer (JASCO V-570) in transmission mode at normal incidence. The transmittance
of the blank cell was normalized as 100%. The N∗ LC was composed of a matrix LC, chiral
dopant (specific content of 12.0 wt%), and one of the synthesized compounds (specific
content of 10.0 wt%). The matrix nematic LC, SLC1717 and the chiral dopant, R811
(Merck Co., Ltd.) were used [20]. ꢀλs were measured from the spectrum by considering
respectively the peak bandwidth at half-height.
3. Results and Discussion
3.1. Synthesis
A strategy was planned for the design of azobenzene-based structures, which consist of
four main units: the azobenzene group, the triazole group, the polar terminal substituent,
and the alkyl terminal substituent. The polar terminal substituent could both engender
the asymmetric structures and influence the intramolecular electron density [21], and the
triazole could be easily obtained through Huisgen 3+2 cycloaddition reaction between
phenylacetylene and RN3 [16]. On the basis of the above methodology, a series of azoben-
zene derivatives with trifluoromethyl groups as terminal substituents were synthesized that
connected with alkyl groups or chiral alkyl groups. The synthesis of the target compounds
was accomplished mainly by three-step reactions as shown in Scheme 1: azobenzene gen-
eration procedure (a and b), phenylacetylene derivatives generation procedure (c and d)
and Huisgen 3+2 cycloaddition procedures (e). All of above compounds were obtained in
good yields and high purities. Their molecular structures and purities were assigned using
1H-NMR and FT-IR.

12
Z. Miao et al.
H2N
I
O
N
I
F3C
N
F3C
N
F3C
NH2
(a)
(b)
m1
TMSA
N
F3C
N
(d)
(c)
m2
N
N
N
N
R N3
(e)
R
F3C
N
∗
D1: R = C4H9
; D2: R = C8H17
; D3: R =
;
∗
D4: R =
; D5: R = C12H25
Scheme 1. Reagents and conditions: (a) Oxone, CH2Cl2, H2O, r. t., 15 h; (b) AcOH, r. t., 15 h;
(c) TEA, THF, CuI, PPh3, PdCl2(PPh3)2, 30◦C, 10 h; (d) MeOH, THF, K2CO3, r. t., 2 h; (e) CuI,
DMF, 100◦C, 15 h.
3.2. FT-IR Spectra
The FT-IR spectra of the synthesized compounds D1-D5, which were taken into KBr
pellets, were shown in Fig. 1. It was indicated that the absorption peaks of D1-D5 were
very similar according to Fig. 1, because there were the similar molecular major structures
except for the terminal alkyl substituents [22]. The position (cm−1) of IR absorption peaks
were in the order of 3132, 2969, 2916, 2847, 1931, 1731, 1604, 1470, 1336, 1224, 1102,
1071, 1016, 977, and 858.
3.2. Phase Transitions
Differential scanning calorimetry (DSC) was used to determine the phase transition temper-
atures during heating at a scanning rate of 10◦C · min−1; the phase transition temperatures
D1
D2
D3
D4
D5
3500 3000 2500 2000 1500 1000 500
Wavenumber/cm-1
Figure 1. FT-IR spectra of D1-D5.

Asymmetrical Azobenzene Liquid Crystals
13
D1
D2
D3
D4
D5
50
75
100
125
150
175
200
Temperature / oC
Figure 2. DSC curves of compounds D1-D5 during heating with a scanning rate of 10◦C · min−1
,
under a N2 atmosphere.
of the compounds were shown in Fig. 2; and the associated data are listed in Table 1. It was
observed that all compounds were crystals at room temperature, D2 and D5 formed a meso-
morphic phase after melting [23]. Two endothermic peaks appeared when the compounds
D2 and D5 were heated, which corresponded to the transformation from the crystalline (Cr)
phase to an LC phase and that from the LC to the isotropic (I) liquid, respectively. However,
there was only one endothermic peak during heating for the compounds D1, D3, and D4,
which corresponded to the melting and crystallisation of the compounds. In Fig. 2, the
melting points decreased with the increasing of carbon numbers of the alkyl substituents,
while as to the LC monomers of D2 and D5, clearing temperatures decreased and liquid
crystal temperature range increased accordingly.
3.3. Mesomorphic Properties
In order to further confirm the above DSC results, the textures of the LC phase of each
compound were observed using polarizing optical microscope (POM) (results shown in
Fig. 3). The phases were identified through the comparison of the observed textures with
reference textures [24] from POM. D2 and D5 developed the LC state at the first-phase
transition temperature shown in the DSC curve, and a bright, colourful, and smectic phase
texture could be observed. Upon further heating, the compounds entered into the I state and
the field of view finally turned dark. When cooled from the I state, D2 and D5 developed
a liquid Cr state again and then started to crystallize. The observations above indicated
that D2 and D5 showed enantiotropic phase behavior with the existence of a smectic
phase between the crystal and the I liquid, which were consistent with the DSC results.
Table 1. Phase transition temperatures and ꢀn of D2 and D5. Cr =
crystal, Sm = smectic phase, I = isotropic
Phase transition
Compounds
temperature (◦C)
ꢀn
D2
D5
Cr 153 Sm 183 I
Cr 126 Sm 165 I
0.426
0.412

14
Z. Miao et al.
Figure 3. Polarised optical microscope images of D2 and D5 in mesophase. Cooling rate: 5◦C ·
min−1
.
Compounds D1, D3, and D4 could not form an LC phase during heating and cooling. This
phenomenon indicated that the structure of the alkyl substituent played an important role
for the azobenzene derivates to form LC phase, shorter (D1) and irregular (D3, D4) alkyl
groups were not beneficial to the formation of LC phases.
3.4. Optical Anisotropy
After confirming the LC phase of D2 and D5, we turned our attention to their birefringent
(ꢀn) properties, because there was a certain number of π-electron conjugation in D2 and
D5. ꢀn was evaluated as extrapolated values from mixtures containing 10 wt% of each
test compound in SLC 1717. ꢀn data of the compounds were collected in Table 1. Among
compound D2 and D5, the length of alkyl chain was important evaluation factor for ꢀn.
Longer carbon chain decreased electron cloud density slightly, the ꢀn value decreased
correspondingly [24].
3.5. Photophysical Properties
D2 and D5 showed the similar absorption spectra for the same rigid segment structure. The
photochemical responses to UV or visible light exposure were monitored by the unpolarized
0 min
2 min
5 min
200 300 400 500 600 700 800
Wavelength (nm)
Figure 4. UV-Vis absorption spectra of D2 in CH2Cl2 exposed to UV (8.5 mW·cm−2) for 0 min,
2 min, and 5 min at room temperature.

Asymmetrical Azobenzene Liquid Crystals
15
Table 2. The composition and content of the N∗-LC
Compound
Weight ratioa
Black sample
D2
D5
0.0/78.0/12.0
10.0/68.0/12.0
10.0/68.0/12.0
aWeight ratio of Compound/SLC1717/S811.
D2
D5
Blank sample
750
900
1050 1200 1350
Wavelength / nm
Figure 5. Dependence of the reflection band on D2 and D5.
UV/VIS/NIR spectrophotometer (JASCO V-570) in absorption mode at normal incidence.
In this paper, D2 was used to study the photophysical properties of the azobenzene derivates
in dilute CH2Cl2 solution at room temperature, the results were depicted in Fig. 4. All
displayed a characteristic pattern of two absorption bands between 280 and 350 nm before
UV lighting. When UV light (λ = 365 nm, 8.5 mW·cm−2) was applied to the sample for
2 min and 5 min at room temperature, the absorption at 350 nm dropped as a result the
trans-cis isomerization, and revealed significant blue shifts, that was about 6 nm for lighting
2 min and 15 nm for 5 min. However, when visible light (λ = 440 nm, 4.5 mW·cm−2
)
was turned on (UV turned off) to induce the reverse cis-trans isomerization, the absorption
returned to the initial state again.
3.6. Selective Reflection
In order to investigate the effect of D2 and D5 on the reflection behaviors of the N∗-LC,
the spectra of N∗-LC samples with D2 or D5 were shown in Fig. 5. The composition and
content of the N∗-LC were shown in Table 2. In Fig. 5, ꢀλ was about 140 nm for Blank
sample, whereas it was 196, 210 nm for D2 and D5, respectively. It was clearly seen that the
reflection band remarkably become more broad with the increasing in ꢀn of the synthetic
compound.
4. Conclusions
The compounds of asymmetrical azobenzene liquid crystals with high birefringence were
designed and synthesized as potential new mesogens via click chemistry. The effect of these
replaced modifications had been presented, that shorter and irregular alkyl groups weren’t

16
Z. Miao et al.
benefit to the formation of LC phases. From the ꢀn values of the series, it was shown that
the length of alkyl chain was important evaluation factor for ꢀn, longer side chain decreased
electron cloud density conjugation slightly, the ꢀn value decreased correspondingly. The
compound with higher ꢀn was usually benefit to increase the broad reflection band of the
N∗-LC.
5. Synthesis
5.1. General Procedure for the Synthesis of Intermediate m1
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 h under an N2 atmosphere.
After separation of the layers, the aqueous layer was extracted with CH2Cl2 (3 × 50 mL).
The combined 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 condensation step without further purification. To the nitrosoarene dissolved in
200 mL of acetic acid was added 4-Iodo-aniline (21.9 g 0.100 mol). The resulting mixture
was stirred at room temperature for 15 h. 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 h [25,26].
(4-Iodo-phenyl)-(4-trifluoromethyl-phenyl)-diazene (m1). Yellow solid, yield: 72.3%,
27.2 g. 1H-NMR (400 MHz, CDCl3) (δ, ppm): 7.66 (d, 2H, J = 8.8 Hz), 7.76 (d, 2H, J =
8.4 Hz), 7.88 (d, 2H, J = 8.8 Hz), 7.98 (d, 2H, J = 8.4 Hz). IR (cm−1): 2109, 1932, 1600,
1492, 1409, 1326, 1174, 1139, 1066, 856.
5.2. General Procedure for the Synthesis of Intermediate m2
m1 (3.76 g, 10 mmol) was dissolved in the mixed solvent of 100 mL TEA and 100 mL THF
in a round-bottom flask. The solution was purged for 30 min with bubbling Ar followed
by addition of Pd(PPh3)3Cl2 (0.211 g, 0.300 mmol) and CuI (0.033 g, 0.300 mmol). The
addition of TMSA (1.96 g, 20 mol) occurred via injection after Ar purge. The reaction
mixture was then stirred at 30◦C for 10 h under an Ar atmosphere. Upon completion, the
mixture was concentrated, rediluted with CH2Cl2, and filtered through a plug of silica gel.
The solvent was removed in vacuo and then dissolved in THF: MeOH (7: 3, 100 mL), added
K2CO3 (3 equiv per silyl group). The reaction mixture was stirred at 20◦C for 3 h. Upon
completion, the mixture was diluted with Et2O, washed with aq. NH4Cl and H2O, dried
(MgSO4) [27,28]. The solvent was removed in vacuo and the crude product was purified
by chromatography of silica gel (3:1, hexanes: CH2Cl2).
(4-Ethynyl-phenyl)-(4-trifluoromethyl-phenyl)-diazene (m2). Yellow solid, yield:
1
91.4%, 2.51 g. H-NMR (400 MHz, CDCl3) (δ, ppm): δ = 3.12 (s, 1H), 7.66 (d, 2H,
J = 8.8 Hz), 7.76 (d, 2H, J = 8.8 Hz), 7.88 (d, 2H, J = 8.8 Hz), 7.98 (d, 2H, J = 8.4 Hz).
IR (cm−1): 3307, 2142, 2906, 1594, 1415, 1333, 1174, 1136, 1059, and 856 [27].
5.3. General Procedure for the Synthesis of R-N3
R-Br (10 mmol) and NaN3 (3.25 g, 50 mmol) were dissolved in dry DMF) (50 mL). The mix-
ture was stirred at 80◦C for 20 h. After cooling, the solution was poured into H2O (150 mL).

Asymmetrical Azobenzene Liquid Crystals
17
Theresulting mixturewas extracted withEt2O (3 × 20 mL) and thecombinedorganicphases
were washed three times with saturated aqueous NaCl solution (3 × 50 mL). The combined
organic layers were dried over Na2SO4 and concentrated in vacuo [28,29]. The obtained
oil was diluted with CH2Cl2, and the solution was filtered with a short silica gel column.
Concentration of the filtered solution gave the corresponding azides in 85%–92% yield.
5.4. General Procedure for the Synthesis of the Azobenzene Liquid Crystals via Click
Chemistry
To a 100 mL round-bottomed flask were charged m2 (1.37 g, 5 mmol), R-N3 (10 mmol),
CuI (57.0 mg, 0.3 mmol), and DMF (50 mL). The mixture was heated to 100◦C for 24 h,
and then cooled to room temperature. The solution was poured into H2O (150 mL) and
then extracted with Et2O (3 × 20 mL). The combined organic layers were washed with
brine, dried over Na2SO4, and concentrated in vacuo. Upon completion, the mixture was
rediluted with CH2Cl2, and filtered with a short silica gel column [30,31]. The solvent was
removed in vacuo and the crude material was purified by chromatography of silica gel (1:1,
hexanes: CH2Cl2).
[4-(1-Butyl-1H-[1,2,3]triazol-4-yl)-phenyl]-(4-trifluoromethyl-phenyl)-diazene (D1).
Yellow solid, yield: 68.2%, 1.27 g. 1H-NMR (400 MHz, CDCl3) (δ, ppm): δ = 0.98 (t, 3H),
1.46 (m, 2H), 1.75 (m, 2H), 4.43 (t, 2H), 7.77 (d, 2H, J = 8.4 Hz), 7.85 (s, 1H), 7.99 (d,
2H, J = 8.0 Hz), 8.01 (s, 4H). IR (cm−1): 3132, 2969, 2916, 2847, 1931, 1731, 1604, 1470,
1336, 1224, 1102, 1071, 1016, 977, and 858.
[4-(1-Octyl-1H-[1,2,3]triazol-4-yl)-phenyl]-(4-trifluoromethyl-phenyl)-diazene (D2).
1
Yellow solid, yield: 74.9%, 1.61 g. H-NMR (400 MHz, CDCl3) (δ, ppm): δ = 0.88 (t,
3H), 1.26 (m, 2H), 1.33 (m, 8H), 1.94 (m, 2H), 4.34 (t, 2H), 7.77 (d, 2H, J = 8.4 Hz), 7.84
(s, 1H), 7.99 (d, 2H, J = 8.0 Hz), 8.01 (s, 4H). IR (cm−1): 3132, 2969, 2916, 2847, 1931,
1731, 1604, 1470, 1336, 1224, 1102, 1071, 1016, 977, and 858.
{4-[1-(1-Methyl-heptyl)-1H-[1,2,3]triazol-4-yl]-phenyl}-(4-trifluoromethyl-phenyl)-
diazene (D3). Yellow solid, yield: 42.3%, 0.908 g. 1H-NMR (400 MHz, CDCl3) (δ, ppm):
δ = 0.84 (t, 3H), 1.23 (m, 6H), 1.28 (m, 2H), 1.60 (d, 3H), 1.90 (m, 2H), 4.72 (m, 1H),
7.77 (d, 2H, J = 8.4 Hz), 7.84 (s, 1H), 7.99 (d, 2H, J = 8.0 Hz), 8.02 (s, 4H). IR (cm−1):
3132, 2969, 2916, 2847, 1931, 1731, 1604, 1470, 1336, 1224, 1102, 1071, 1016, 977, and
858.
{4-[1-(1-Ethyl-hexyl)-1H-[1,2,3]triazol-4-yl]-phenyl}-(4-trifluoromethyl-phenyl)-dia
zene (D4). Yellow solid, yield: 56.2%, 1.21 g. 1H-NMR (400 MHz, CDCl3) (δ, ppm): δ =
0.87 (t, 3H), 0.94 (t, 3H), 1.27 (m, 4H), 1.30 (m, 2H), 1.60 (d, 3H), 1.94 (m, 2H), 4.33 (m,
1H), 7.77 (d, 2H, J = 8.4 Hz), 7.82 (s, 1H), 7.99 (d, 2H, J = 8.0 Hz), 8.01 (s, 4H). IR
(cm−1): 3132, 2969, 2916, 2847, 1931, 1731, 1604, 1470, 1336, 1224, 1102, 1071, 1016,
977, and 858.
[4-(1-Dodecyl-1H-[1,2,3]triazol-4-yl)-phenyl]-(4-trifluoromethyl-phenyl)-diazene
(D5). Yellow solid, yield: 76.5%, 1.86 g 1H-NMR (400 MHz, CDCl3) (δ, ppm): δ = 0.85
(t, 3H), 1.23 (m, 16H), 1.33 (m, 2H), 1.95 (m, 2H), 4.41 (t, 2H), 7.77 (d, 2H, J = 8.4 Hz),
7.84 (s, 1H), 7.99 (d, 2H, J = 8.0 Hz), 8.01 (s, 4H). IR (cm−1): 3132, 2969, 2916, 2847,
1931, 1731, 1604, 1470, 1336, 1224, 1102, 1071, 1016, 977, and 858.
Acknowledgments
This work was supported by Natural Science Basic Research Plan in Shaanxi Province
of China (Grant No. 2013JQ8043); Postdoctoral Science Foundation of China (Grant No.

18
Z. Miao et al.
2013M542323); Postdoctoral Science Foundation of Shaanxi Province, China; Scholastic
Science Research Foundation of Xijing University (Grant No. XJ120230 and XJ120233).
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