Azobenzene liquid crystals with high birefringence and their effect of trans-cis photoisomerization on selective reflection

Article abstract of DOI:10.1080/15421406.2013.807209

A series of four novel azobenzene liquid crystals (LCs) with high birefringence were synthesized by coupling of intermediate molecules with a phenylacetylene fragment. The chemical structures of synthetic intermediates and compounds were confirmed by FT-I

Full text of DOI:10.1080/15421406.2013.807209

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Azobenzene Liquid Crystals with High
Birefringence and Their Effect of Trans-
Cis Photoisomerization on Selective
Reflection
Zong-Cheng Miao a , Yong-Ming Zhang a , Yu-Zhen Zhao b & Dong
Wang b
a Xijing University , Xi’an , Shaanxi Province , PR China
b Department of Materials Physics and Chemistry, School of Materials
Science and Engineering , University of Science and Technology ,
Beijing , PR China
Published online: 03 Oct 2013.
To cite this article: Zong-Cheng Miao , Yong-Ming Zhang , Yu-Zhen Zhao & Dong Wang
(2013) Azobenzene Liquid Crystals with High Birefringence and Their Effect of Trans-Cis
Photoisomerization on Selective Reflection, Molecular Crystals and Liquid Crystals, 582:1, 98-106,
DOI: 10.1080/15421406.2013.807209
To link to this article: http://dx.doi.org/10.1080/15421406.2013.807209
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Mol. Cryst. Liq. Cryst., Vol. 582: pp. 98–106, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.807209
Azobenzene Liquid Crystals with High
Birefringence and Their Effect of Trans-Cis
Photoisomerization on Selective Reflection
ZONG-CHENG MIAO,1 YONG-MING ZHANG,1 YU-ZHEN
ZHAO,2 AND DONG WANG2,∗
1Xijing 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, PR China
A series of four novel azobenzene liquid crystals (LCs) with high birefringence were
synthesized by coupling of intermediate molecules with a phenylacetylene fragment.
The chemical structures of synthetic intermediates and compounds were confirmed by
1
FT-IR and H-NMR. The LC behavior including transition temperatures and phase
sequences were investigated by differential scanning calorimetry (DSC) and polarizing
optical microscope (POM). As a very important parameter, the birefringence was also
measured using polarizing light interferometry method. Moreover, the characteristics of
selective reflection which was associated with the birefringence of the N∗-LC dropped
with azobenzene LCs and the effect of trans-cis photoisomerization of these compounds
on selective reflection were studied in detail.
Keywords Azobenzene; birefringence; isomerization; liquid crystal; selective
reflection
1. Introduction
Owing to the rich photochemistry, azobenzene liquid crystals (LCs) are promising materials
[1]. They have been extensively studied because of their applicability for photoswitching
devices [2, 3], nonlinear optics [4], holography [5], and optical memory [6]. One well-
known feature of an azobenzene-containing species was that polarized light could induce
reorientation of azobenzene groups through photochemically induced isomerization [7]. In
addition, liquid-crystalline polymers with the azobenzene in the side chain have attracted
much attention because a photoinduced change in birefringence ꢀn was large when com-
pared with amorphous counterparts [8, 9]. By contrast, related small molecular compounds
have been less explored. The latter have the advantage, among other things, of relatively
simple synthesis, being of higher purity, resulting in well-defined structures and correspond-
ing physicochemical properties and providing the option of homogeneously incorporating
them into polymeric matrixes [10, 11]. The azobenzene moiety in a trans-form, which is
∗Address correspondence to Dong Wang, Department of Materials Physics and Chemistry,
School of Materials Science and Engineering, University of Science and Technology, Beijing 100871,
PR China. Tel.: 86-10-62333969; Fax: 86-10-62333969. E-mail: ustbwangd@163.com
98

Azobenzene LCs with High Birefringence
99
rod-like, stabilizes the phase structure of LCs. On the other hand, bent cis-isomers disor-
ganize the phase structure of the LCs [12]. Consequently, trans-cis photoisomerization of
the azobenzene moiety in LC compounds led to a large change in ꢀn by disorganizing the
alignment [13].
The ꢀn is a very important parameter of LCs. LCs with high birefringence at room tem-
perature are useful not only in conventional display devices, such as supertwisted nematic
LC displays [14], but also in scattering-type polymer-dispersed LC displays as reflective LC
displays and in spatial light modulators [15]. Many high ꢀn materials have been obtained
by increasing the molecular π-electron conjugation length [16] and introducing highly
polarizable end groups [17]. Tolane moieties are known as a highly birefringent meso-
gen because of large π-electron conjugation length and anisotropy of polarizability [18].
Therefore, to obtain larger values of ꢀn, we synthesized a series of novel azobenzene LCs
with tolane or phenyldiacetylene moieties. In addition, we investigated the photoinduced
birefringence of the LCs upon exposure to linearly polarized light.
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
0.1 K (Linkam LK-600PM) and then confirmed using differential scanning calorimetry
(DSC, Perkin Elmer Pyris 6). 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 [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 matrix LC, chiral
dopant (specific content of 12 wt%) and one of the synthesized compounds. As usual, the
matrix nematic LC, SLC1717 and the chiral dopant, and R811 (Merck Co., Ltd.) were
used [20]. The ꢀ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 proposed on the design of azobenzene-based structures, which consist
of three main units: the azobenzene group, the polar terminal substituent, and the π-
conjugated tolane or phenyldiacetylene. The polar terminal substituent could both engen-
der the asymmetric structures and influence the intramolecular electron density [20], and
the π-conjugated groups could easily extend intramolecular π-conjugation, so as to im-
prove the values of ꢀn of the LCs. Here, one special design was that the trifluoromethyl
group as one polar terminal substituent provided the higher ꢀn and lower melting point

100
Z.-C. Miao et al.
H2N
I
O
N
I
F3C
NH2
F3C
F3C
N
F3C
N
(b)
(a)
m
N
R
R
N
(c)
1, R=
2, R=
C3H11
C5H11
3, R=
4, R=
C5H11
C5H11
Scheme 1. Reagents and conditions: (a) Oxone, CH2Cl2, H2O, r. t., 15 h; (b) AcOH, r. t., 15 h; (c)
TAE, THF, PdCl2(PPh3)2, CuI, PPh3, 30◦C, 10 h.
[21]. On the basis of the idea mentioned above, a series of azobenzene derivatives with
trifluoromethyl groups as terminal substituent have been synthesized that connected with
alkylbenzene and alkylphenylacetylenes or alkyl biphenyls linkers. The synthesis of the
target compounds was accomplished by 3-step reactions: oxidation procedure (a), azoben-
zene generation procedure (b), and Hagihara-Sonogashira cross-coupling procedures (c)
as showed in Scheme 1. 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.
3.2. Photophysical Properties
The absorption spectra of the synthesized compounds 2–4 in dilute CH2Cl2 solution at
room temperature, depicted in Fig. 1, provides some insight into the electronic structure
of the systems. All the spectra display a characteristic pattern of two absorption bands
between 280 nm and 350 nm. The data were summarized in Table 1. Compound 1 and 2
show the same absorption spectra for the similar molecular structure. Compared with the
synthesized compound 2, compound 4 revealed slight red shifts (∼10 nm) and compound 3
revealed significant blue shifts (∼20 nm) of the maxima absorption wavelengths. However,
the absorption bands at about 290 nm decreased in the order of 3, 4, and 2. One generally
approved explanation was that the maxima absorption wavelengths of azobenzene shifted,
when azobenzene was connected with phenylacetylene group in 2 or phenyldiacetylene
group in 3 or diphenylacetylene group in 4.
3.3. Phase Transitions
DSC was used to determine the phase transition temperatures during cooling at a scanning
rate of 10◦C · min−1, the phase transition temperatures of the compounds were listed
in Table 1. It was shown that the compounds 1–4 all exhibit mesomorphism. Moreover,
compared with the synthesized compound 2, compound 1 exhibits a higher melting point

Azobenzene LCs with High Birefringence
101
Figure 1. UV-Vis absorption spectra of 2–4 in CH2Cl2 under room temperature.
and a narrower temperature range of mesophase, this is because the alkyl carbon number in
compound 1 was less than compound 2. The shorter alkyl chain leads to higher crystallinity.
However, in compound 3 and compound 4, the increasing of π-electron conjugation length
was contributed to increase melting point or clearing point and to widen mesophase. It
is usually believed that phenyl group possesses richer electron density than alkynyl group
[22], so the π-electron conjugation degree of compound 4 are higher than that of compound
3, that caused an increase the melting point, clearing point, and the temperature range of
mesophase.
3.4. Mesomorphic Properties
The mesomorphic properties of the synthesized compounds 1–4 were investigated primarily
by POM. The phases were identified through the comparison of the observed textures with
reference textures [23] from POM. Figure 2 shows POM photographs of compounds 1, 2, 3,
and 4 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 2, compound 1 and 4 tend to form typical smectic textures for
the shorter alkyl chain (compound 1) and higher electron density (compound 4).
Table 1. Phase transition temperatures and ꢀn of the compounds. Cr = crystal, Sm =
smectic phase, N = nematic phase, I = isotropic
Phase transition temperatures
Compounds
λ
abs(nm)
(◦C)
ꢀn
1
2
3
4
362
362
348
369
Cr 151 Sm 178 I
Cr 117 Sm 190 I
Cr 124 Sm 203 I
Cr 153 Sm 246 I
0.469
0.428
0.531
0.553

102
Z.-C. Miao et al.
Figure 2. Polarized optical microscope images of the compounds 1–4 in mesophase, cooling rate:
10◦C · min−1
.
3.5. Birefringence
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 [19].
The ꢀn data of the compounds were collected in Table 1. Among compound 1 and 2,
the length of flexible chain is important evaluation factor for ꢀn. Longer carbon chain
decreases π-electron conjugation slightly, the ꢀn value decreased correspondingly [24].
Then, ꢀn was increased by exchanging a benzene ring for the biphenyl or phenylacetylene
group, the ꢀn of compound 3 or 4 is higher than compound 1. However, it was well known
that although the LC compounds possess lower viscosity and melting point when a phenyl
group was replaced by alkynyl group, the ꢀn values would reduce with the decreasing
of the π-electron conjugation. For all the above reasons, the highest ꢀn in the series was
belonged to compound 4, the values was 0.553.
3.6. Selective Reflection
In order to investigate the effect of trans-cis photoisomerization of the synthesized azoben-
zene LC compounds on the reflection behaviors of the N∗-LC, the synthesized compounds
doped in N∗-LC were studied with the same content of 10 wt%. The composition and
content of the N∗-LC were shown in Table 2.

Azobenzene LCs with High Birefringence
103
Table 2. The composition and content of the N∗-LC
Sample
Compound
Weight ratioa
A
B
C
D
E
—
1
2
3
4
0.0/78.0/12.0
10.0/68.0/12.0
10.0/68.0/12.0
10.0/68.0/12.0
10.0/68.0/12.0
aWeight ratio of Compound/SLC1717/S811.
Due to the law of selective reflection, the reflection bandwidth (ꢀλ) was related to ꢀn
only when the concentration of the chiral dopant was changeless [25].
Figure 3 showed the dependence of the spectra of the N∗-LC samples on the different
compounds of 1, 2, 3, and 4, respectively. In Fig. 3, ꢀλ was about 140 nm for sample A,
whereas it was 234, 222, 252, and 260 nm for B, C, D, and E, respectively. It was clearly
seen that the reflection band remarkably become more broad with the increasing in ꢀn of
the compounds of 1–4. Whereas, when samples B–E exposed to UV light (λ = 365 nm,
6.5 mW cm−2) for 30 s at room temperature, the ꢀn dropped as a result the trans-cis
isomerization-induced destabilization of aligned LC molecules [26, 27]. The comparison
spectra of B–E before and after UV light were shown in Fig. 4. The ꢀλs of samples B–E
would changed with the decreasing of ꢀn according to ꢀλ = ꢀn × P.
4. Synthesis
4.1. General Procedure for the Synthesis of Intermediate m
The 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
[28, 29]. After separation of the layers, the aqueous layer was extracted with CH2Cl2 (3×).
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
A
B
C
D
E
750
900
1050
1200
1350
150
Wavelength / nm
Figure 3. Dependence of the reflection band on the series compounds.

104
Z.-C. Miao et al.
B
D
C
E
Wavelength (a. u.)
Figure 4. The comparison spectra of B–E before and after UV light.
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.
(4-Iodo-phenyl)-(4-trifluoromethyl-phenyl)-diazene (m). 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, 810.
4.2. General Procedure for the Synthesis of the Azobenzene Liquid Crystals
with High Birefringence
The m (3.76 g, 10.0 mmol) and the terminal alkyne compound (10.0 mmol) were dissolved
in the mixed solvent of 10 mL TEA and 10 mL THF in a round bottom flask [30, 31]. The
solution was purged for 30 min with bubbling Ar followed by addition of Pd (PPh3)3Cl2
(0.211 g, 0.300 mmol), CuI (0.029 g, 0.150 mmol), and PPh3 (0.040 g, 0.150 mmol). The
reaction mixture 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 the crude material was purified by chromatography
of silica gel (2:1, hexanes: CH2Cl2).
[4-(4-Propyl-phenylethynyl)-phenyl]-(4-trifluoromethyl-phenyl)-diazene (1). Yellow
1
solid, yield: 72.8%, 2.89 g. H-NMR (400 MHz, CDCl3) (δ, ppm): δ = 0.94 (t, 3H),
1.65 (m, 2H), 2.60 (t, 2H), 7.17 (d, 2H, J = 7.2 Hz), 7.46 (d, 2H, J = 7.2 Hz), 7.66
(d, 2H, J = 7.6 Hz), 7.70 (d, 2H, J = 8.0 Hz), 7.92 (d, 2H, J = 7.6 Hz), 7.99 (d, 2H,
J = 7.6 Hz). IR (cm−1): 3029, 2964, 2849, 2202, 1920, 1607, 1530, 1460, 1333, 1263,
1129, 1110, 1065, 1014, 836, 810.
[4-(4-Pentyl-phenylethynyl)-phenyl]-(4-trifluoromethyl-phenyl)-diazene (2). Yellow
1
solid, yield: 74.5%, 3.13 g. H-NMR (400 MHz, CDCl3) (δ, ppm): δ = 0.88 (t, 3H),
1.31 (m, 4H), 1.60 (m, 2H), 2.66 (t, 2H), 7.17 (d, 2H, J = 7.2 Hz), 7.46 (d, 2H, J = 7.2 Hz),
7.66 (d, 2H, J = 7.6 Hz), 7.77 (d, 2H, J = 7.2 Hz), 7.92 (d, 2H, J = 7.6 Hz), 7.99 (d, 2H,
J = 7.6 Hz). IR (cm−1): 3030, 2965, 2860, 2212, 1920, 1594, 1511, 1467, 1320, 1161,
1129, 1066, 1015, 856, 806.

Azobenzene LCs with High Birefringence
105
[4-(4-Pentyl-phenylbiethynyl)-phenyl]-(4-trifluoromethyl-phenyl)-diazene (3). Yel-
1
low solid, yield: 52.6%, 2.34 g. H-NMR (400 MHz, CDCl3) (δ, ppm): δ = 0.88 (t,
3H), 1.31 (m, 4H), 1.61 (m, 2H), 2.61 (t, 2H), 7.17 (d, 2H, J = 7.2 Hz), 7.45 (d, 2H, J =
7.2 Hz), 7.67 (d, 2H, J = 7.2 Hz), 7.78 (d, 2H, J = 7.6 Hz), 7.92 (d, 2H, J = 7.6 Hz), 7.98
(d, 2H, J = 7.6 Hz). IR (cm−1): 3026, 2960, 2861, 2212, 1919, 1594, 1598, 1467, 1320,
1269, 1097, 1021, 862, 804.
[4-(4ꢀ-Pentyl-biphenylethynyl)-phenyl]-(4-trifluoromethyl-phenyl)-diazene (4). Yel-
low solid, yield: 54.6%, 2.71 g. 1H-NMR (400 MHz, CDCl3) (δ, ppm): δ = 0.89 (t, 3H),
1.34 (m, 4H), 1.64 (m, 2H), 2.63 (t, 2H), 7.20 (d, 2H, J = 6.4 Hz), 7.49 (d, 2H, J = 6.4 Hz),
7.56 (d, 2H, J = 6.8 Hz), 7.59 (d, 2H, J = 6.4 Hz), 7.69 (d, 2H, J = 6.8 Hz), 7.77 (d, 2H,
J = 6.4 Hz), 7.93 (d, 2H, J = 6.8 Hz), 7.99 (d, 2H, J = 6.8 Hz). IR (cm−1): 3027, 2957,
2848, 2250, 1940, 1575, 1505, 1480, 1333, 1161, 1136, 1072, 1008, 856, 820.
5. Conclusions
A series of azobenzene LCs were designed and synthesized as potential new mesogens
having large birefringence values, and the effect of these replaced modifications on LC
properties was obtained. The shorter alkyl chain and the π-electron conjugation strength,
which were contributed to increased melting point or clearing point and to widen the
mesophase, were confirmed to have higher ꢀn values. The study on selective reflection
behavior of the compounds proved that the higher ꢀn had the more broad reflection band
of the N∗-LC and the trans-cis isomerization produced important influence on the selective
reflection behavior according to ꢀn change on the isomerization.
Acknowledgments
This work was supported by National Natural Science Foundation (Grant No. 50973010);
Specialized Research Fund for Doctoral Program of Higher Education of China (Grant
No. 20110006120002); and Scholastic Science Research Foundation of Xijing University
(Grant No. XJ120230).
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