Synthesis of multiring azo-benzoic acid liquid crystalline molecules and their special photosensitive property

Article abstract of DOI:10.1080/15421406.2014.949591

10 new rod-like azo benzoic acid liquid crystals belonged to three series have been prepared, in which trans-cyclohexyl benzene, biphenyl or bi-trans-cyclohexyl carboxylic acid mesogenic cores with ethyl, n-propyl, n-butyl or n-pentyl substituents were connected with azo benzoic acid. Meantime, a simple method was developed in synthesizing compounds bearing both ester and carboxyl groups in one-step reaction. All these compounds have been characterized on the basis of their spectral data, differential scanning calorimeter (DSC) and hot stage polarizing optical microscope (HS-POM). All these compounds have higher thermo-stability. 8 of these compounds have liquid crystalline properties. Their temperature ranges of mesophase have not been obtained, resulting from all the compounds decomposed before their clearing points. Although these compounds are photosensitive in solution under illumination of UV light (365nm), they can't do trans-cis isomerization in their mesophses, indicating the super stabilities of the trans isomer in mesophase.

Full text of DOI:10.1080/15421406.2014.949591

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Synthesis of Multiring Azo-Benzoic Acid
Liquid Crystalline Molecules and Their
Special Photosensitive Property
a
a
a
a
Min-Yan Zheng , Yong-Sheng Wei , Wei Geng , Nai-Ni Guo & Ping
a
Zhang
a
School of Chemistry and Chemical Engineering, Xianyang Normal
University, Xianyang, Shaanxi, China
Published online: 03 Mar 2015.
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To cite this article: Min-Yan Zheng, Yong-Sheng Wei, Wei Geng, Nai-Ni Guo & Ping Zhang (2015)
Synthesis of Multiring Azo-Benzoic Acid Liquid Crystalline Molecules and Their Special Photosensitive
Property, Molecular Crystals and Liquid Crystals, 608:1, 1-13, DOI: 10.1080/15421406.2014.949591
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Mol. Cryst. Liq. Cryst., Vol. 608: pp. 1–13, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.949591
Synthesis of Multiring Azo-Benzoic Acid Liquid
Crystalline Molecules and Their Special
Photosensitive Property
∗
MIN-YAN ZHENG, YONG-SHENG WEI, WEI GENG,
NAI-NI GUO, AND PING ZHANG
School of Chemistry and Chemical Engineering, Xianyang Normal University,
Xianyang, Shaanxi, China
1
0 new rod-like azo benzoic acid liquid crystals belonged to three series have been pre-
pared, in which trans-cyclohexyl benzene, biphenyl or bi-trans-cyclohexyl carboxylic
acid mesogenic cores with ethyl, n-propyl, n-butyl or n-pentyl substituents were con-
nected with azo benzoic acid. Meantime, a simple method was developed in synthesizing
compounds bearing both ester and carboxyl groups in one-step reaction. All these com-
pounds have been characterized on the basis of their spectral data, differential scanning
calorimeter (DSC) and hot stage polarizing optical microscope (HS-POM). All these
compounds have higher thermo-stability. 8 of these compounds have liquid crystalline
properties. Their temperature ranges of mesophase have not been obtained, resulting
from all the compounds decomposed before their clearing points. Although these com-
pounds are photosensitive in solution under illumination of UV light (365nm), they
can’t do trans-cis isomerization in their mesophses, indicating the super stabilities of
the trans isomer in mesophase.
Keywords Azo compounds; benzoic acids; liquid crystals; photosensitivity
Introduction
The liquid crystal is one of the most convenient materials to operate optical properties due
to it possesses a large anisotropy in dielectric and optical properties, which are controlled
by change alignment of molecules with external stimuli such as heat and electric, elec-
tromagnetic or light fields [1–3]. Optical switching materials composed by liquid crystal
molecules need at least two optically distinct states of an inherent one and a latent one. Es-
pecially, all optical switching materials, whose optical properties can be changed selectively
and reversibly by light, have considerable potential use in photonics. These materials are
operative by monitoring the change in the optical properties [4]. The wavelengths remote
from these used for switching liquid crystalline azobenzene compound can display new
properties related to the cis-trans photoisomerization of azobenzene on UV light exposure
[5]. According to Rau [6], azo compounds are classified into three classes based on the
∗
Address correspondence to Min-Yan Zheng, School of Chemistry and Chemical Engineering,
Xianyang Normal University, Xianyang, Shaanxi 712000, China. Tel.: +86-29-33720704; Fax: +86-
2
9-33720371. E-mail: zmy71@126.com
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
1

2
M.-Y. Zheng et al.
∗
∗
relative energetic order of their (n, π ) and (π, π ) states: molecules of the azobenzene
type, molecules of the amino azobenzene type, and molecules of pseudo-stilbenes type.
∗
The azobenzene type molecules are characterized spectroscopically by a low-intensity n-π
∗
band in the visible region of the spectrum and a high intensity π-π band in the UV light
[7]. Azo functionalized materials are of great interest due to their photo-sensitive nature,
which can be exploited for optical and optoelectronic devices[8]. Many researches focused
on synthesizing of liquid crystal with core of azo group, most of them involved the synthesis
of such kind of liquid crystal of polymers [9, 10] or preparation of other compounds [11,
12], but little report about design such kind of liquid crystal with smaller molecular weight,
especially with terminal groups of strong polarity.
Liquid crystals with azo group were always designed with terminal longer alkoxy
chain [13, 14], which is easier to be formed by terminal hydroxy reacting with different
haloalkanes. Althoughbeingmorestablethanalkoxychain, the terminal alkyl chains located
in liquid crystalline structure is difficult to form and hence seldom use in constructing liquid
crystalline molecules. Some literature involved the effect of some polar end groups on the
mesomorphic property [15], but few of them took carboxyl acid as an end group. Our former
publication has synthesized liquid crystalline molecules with terminal alkyls and a polar
end group of carboxyl acid, which exhibited mesophase and thermostability [16]. Beside
properties of mesophase, aromatic acids are often used as intermediates of other reactions.
As a kind of new compounds, it is worth to obtaining their properties of photosensitivity
and liquid crystalline. Based on the idea above, we have synthesized here 10 rod-like
liquid crystalline molecules of terminal alkyl chain and COOH with azo bridge bonding,
by changing the terminal double rings to cyclohexyl benzene or biphenyl or bicyclohexane
in order to research the effect of these variations on their liquid crystalline property and
their photosensitivities.
Experimental Section
Materials
All initial intermediates used in the synthesis were prepared in our laboratory with purity
1
higher than 99%, and characterized by IR, GC-MS and H NMR methods. Other reagents
from commercial sources and used without further purification. 4-(4-Hydroxyphenylazo)
benzoic acid was prepared using the method reported elsewhere [17].
Instruments and Conditions
The purities of compounds were detected by LC-10A (Shimadzu) instrument with methanol
as eluent and flowing rate was 1 mL/min. Elemental analyses were conducted by PE-
2400 analyzer (Perkin Elmer). UV spectra were determined by Agilent 8453 Spectrometer
(
(
(
(
Agilent Technologies). IR (KBr) spectra were recorded on a Vertex 70 spectrophotometer
BRUKER). The irradiation of samples was carried out by hand-held UV lamp ZF-7A
Shanghai Jiapeng Technology Co. Ltd.). Mass data were recorded on a GCMS-QP2010
1
Shimadzu) and IE was 70 ev. H NMR spectra were obtained on a BRUKER Avance
500 spectrometer (500 MHz, solvent CDCl3). The DSC experiments were carried out on
MDSC Q100 instrument (TA) and the determination conditions were: the sample mass less
◦
than 5 mg; heating rate, 10 C/min and samples were protected by nitrogen. All images
of textures were taken on a LV100POL Polarizing Optical Microscopy (Nikon) with LTS
◦
E350 hot-stage (Linkam) and heating rate 1 C/min.

Synthesis of Multiring Azo-Benzoic Acid Liquid Crystalline Molecules
Synthesis
3
Synthesis of Compound 2
a was prepared by heating and stirring a mixture of 4-ethyl(4-trans-cyclohexyl) benzoic
2
acid(1a) (1.16 g, 5 mmol) and SOCl2 (0.60 g, 5 mmol) and several drops of DMF in
toluene (25 mL) at ambient temperature for 5 hr. The solvent was then removed under
vaccum condition. A brown liquid (2a) was obtained and used to next step without further
purifying.
Synthesis of Compound 3
3
a was obtained by dissolving 2a into CH2Cl2 (20 mL) and the mixture was dropped into
a solution of 4’-hydroxylazo-4-benzoic acid (1.21 g, 5 mmol) and triethylamine (0.9 mL,
mmol) in CH2Cl2 (50 mL) and stirred at ambient temperature for 12 hr. The reacting
5
solvent was removed under vacuum condition. Some orange solid was obtained, washed
by 5% (mass fraction) NaOH and water. The residue was dried and recystalized with THF.
An orange powder (3a, 1.82 g) was obtained in yield of 80% (two steps). The 3b, 3c, 3d,
3e, 3f, 3g, 3h, 3i and 3j were also prepared by the same way, and their yields were 85%,
84%, 80%, 81%, 85%, 82%, 81%, 85%, 82%, 81%, 79% and 83%, respectively.
4-(4-(4-(4-ethylcyclohexyl)benzoyloxy)phenyldiazenyl) benzoic acid (3a), Relative
molecular mass: 456.53, the mass fraction detected by LC is 98.5%, yield: 80%, orange pow-
◦
der, m.p.181-182 C. Anal. Calcd for C28H28N2O4: C 73.66, H 6.18, N 6.14, found: C 73.54,
H 6.02, N 5.97; IR(KBr)ν: 3100-2500(m, OH), 2960, 2914, 2848(s, C H), 1734(vs,C O),
1
683(vs, C O), 1608(m, N N), 1594, 1506(m, ArH), 1406, 1315, 1284, 1180, 1128,
−1
1016(vs, C O C), 880(s, trans H N N R), 807(w, 1,4-Ar), 776(w, (CH2)n)cm ;
MS(70 ev)m/z(%): 456(M+, 14), 388(0.62), 360(0.66), 274(0.70), 257(18.17), 242(6.74),
2
7
6
7
8
29(0.96), 215(100), 185(0.74), 147(17.86), 119(28.99), 105(10.44), 91(22.12), 81(4.91),
1
7(11.84), 69(14.30), 66(29.72), 55(10.79), 44(12.35), 41(8.08). H NMR: 0.925(t, 3H, J =
.4Hz, CH3), 1.062–1.117(m, 2H, CH2), 1.276–2.618(m, 10H, the proton of cyclohexane),
.378, 7.412, 8.048, 8.118, 8.149, 8.317(d, J = 8.0 Hz, 8.8 Hz, 8.0 Hz, 8.0 Hz, 8.0 Hz,
.4 Hz, 2H of each), 12.79(s, 1H, COOH).
4
-(4-(4-(4-n-propylcyclohexyl)benzoyloxy)phenyldiazenyl) benzoic acid (3b), Rela-
tive molecular mass:470.56, the mass fraction detected by LC is 98.0%, yield: 85%, or-
◦
ange powder, m.p. 223-224 C, Anal. Calcd for C29H30N2O4: C 74.02, H 6.43, N 5.95;
found C 73.86, H 6.23, N 6.05; IR(KBr)ν: 3100-2500(m, C H), 2955, 2921, 2849(s,
C H), 1734(vs, C O), 1687(vs, C O), 1602(m, N N), 1597, 1502, 1495(m, ArH),
1
4
3
1
5
1
8
424, 1266, 1221, 1140, 1065(vs, C O C), 881(s, trans-R N N R), 866, 817(w, 1,
−1
-Ar), 764, 736(w, (CH2)n)cm ; MS(70ev) m/z(%): 470(M, 3.5), 456(1.48), 388(0.34),
60(1.14), 346(0.29), 274(0.33), 257(24.46), 229(100), 215(38.15), 147(4.09), 131(14.02),
19(17.31), 105(9.46), 91(16.97), 481(4.76), 77(11.34), 69(7.88), 65(23.20), 55(7.81),
1
1(2.11), 41(5.89). H NMR: 0.907(t, 3H, J = 7.2 Hz, CH3), 1.017-1.102(m, 2H, CH2),
.211–1.248(m, 2H, CH2), 1.310–2.554(m, 10H, the proton of cyclohexane), 7.308, 7.344,
.020, 8.061, 8.147, 8.316(d, J = 8.0 Hz, 8.2 Hz, 8.0 Hz, 8.2 Hz, 8.2 Hz, 8.2 Hz, 2H of
each), 12.83(s, 1H, COOH).
4
-(4-(4-(4-n-butylcyclohexyl)benzoyloxy)phenyldiazenyl) benzoic acid(3c), Rela-
tive molecular mass:484.59; the mass fraction detected by LC is 98.0%, yield:
84%, orange powder, m.p. undetected; Anal. Calcd for C30H32N2O4: C 74.36, H
6.66, N 5.78; found C 74.54, H 6.49, N 5.57; IR(KBr)ν: 3000–2500( OH), 2921,

4
M.-Y. Zheng et al.
2
1
4
3
1
6
1
850(s, C H), 1735(vs, C O), 1684(vs, C O), 1595, 1495(m, ArH), 1414(m),
267, 1223, 1198, 1178, 1141(vs, C O C), 880(s, trans-R N N R), 815(w, 1,
−1
-Ar), 719(w, (CH2)n)cm ; MS(70 ev)m/z(%): 484(M’, 2.66), 470(0.52), 456(0.78),
60(0.73), 274(0.35), 257(25.09), 243(200), 229(22.45), 215(17.53), 185(0.98), 157(1.28),
47(12.16), 135(4.01), 131(12.63), 119(11.39), 105(9.22), 91(15.60), 81(4.72), 77(2.12),
1
9(5.11), 65(2.11), 55(7.11), 51(1.86), 41(6.20). H NMR: 0.899(t, 3H, J = 4.8 Hz, CH3),
.041-1.094(m, 2H, CH2), 1.216-1.249(m, 4H, CH2CH2), 1.251–2.565(m, 12H, the proton
of cyclohexane), 7.366, 7.516, 8.000, 8.048, 8.139, 8.250(d, J = 8.4 Hz, 8.0 Hz, 8.0 Hz,
.4 Hz, 8.0 Hz, 8.0 Hz, 2H of each), 12.03(s, 1H, COOH).
-(4-(4-(4-n-pentylcyclohexyl)benzoyloxy)phenyldiazenyl) benzoic acid (3d), Rel-
8
4
ative molecular mass: 498.61, mass fraction detected by LC is 98.5%, yield: 80%,
◦
orange powder, m.p. 195-196 C; Anal. Calcd for C31H34N2O4: C 74.67, H 6.87,
N 5.62; found C 74.57, H 6.78, N 5.53; IR(KBr)ν: 3100–2500(s,OH), 2954, 2922,
2850(s, C Hstretching vibration), 1743(vs, C O), 1687(vs, C O), 1595, 1506(m,
ArH), 1264, 1177, 1059(vs, C O C), 880(s, trans H N N R), 838, 812(w, 1,4-Ar),
−
1
7
3
1
6
2
19(w, (CH2)n)cm ; MS(70ev)m/z(%): 498(M+, 2.89), 484(0.46), 70(0.18), 456(0.20),
88(0.15), 360(0.18), 346(0.10), 274(0.89), 257(100), 243(17.24), 229(6.32), 215(6.61),
99(0.21), 185(0.90), 147(5.48), 131(10.92), 121(7.00), 105(6.39), 91(10.67), 77(6.88),
1
9(0.94), 65(12.23), 55(5.48), 41(4.19). H NMR: 0.898(t, 3H, J = 7.2 Hz, CH3), 1.075(m,
H, CH2), 1.256-1.311(m, 6H, CH2CH2CH2), 1.458–2.610(m, 10H, the proton of cyclo-
hexane), 7.378,7.481, 8.011, 8.061, 8.136, 8.398(d, J = 8.4 Hz, 8 Hz, 8.4 Hz, 8.0 Hz,
.4 Hz, 8.4 Hz, 2H of each), 11.02(s, 1H, COOH).
-(4-(4-(4-ethylphenyl)benzoyloxy)phenyldiazenyl)benzoic acid (3e), Relative molec-
8
4
ular mass: 450.49, mass fraction detected by LC is 98.1%, yield: 81%, orange pow-
der, m.p. undetected; Anal. Calcd for C28H22N2O4: C 74.65, H 4.92, N 6.22; found
C 74.49, H 4.77, N 6.32; IR(KBr)ν: 3100-2500( OH), 2966, 2931 (s, C H), 1732,
1
687(vs, C O), 1603(m, N N), 1559, 1541, 1495(m, ArH), 1400, 1110, 1071, 1007
−
1
(
vs, C O C), 883(s, trans H N N R ), 827(w, 1,4-Ar), 776(w, (CH2)n) cm
;
MS(70ev)m/z(%): 450(M+, 0.20), 429(0.08), 414(0.35), 396(0.19), 382(0.09), 369(0.05),
3
2
9
2
55(0.20), 341(0.09), 329(0.14), 317(0.31), 270(0.92), 254(5.96), 239(2.87), 226(72.54),
11(100), 198(1.81), 181(10.69), 165(38.55), 152(28.08), 139(5.91), 115(7.35), 105(3.01),
1
7(1.65), 86(8.85), 65(6.99), 44(6.72), 32(8.60). H NMR: 1.243(t, 3H, J = 7.6 Hz, CH3),
.673(q, 2H, J = 7.6 Hz, CH2), 7.287, 7.402, 7.639, 7.636, 8.001, 8.106, 8.234, 8.297(d, J
=
8.0 Hz, 8.4 Hz, 8.0 Hz, 8.0 Hz, 8.4 Hz, 8.0 Hz, 8.4 Hz, 8.4 Hz, 2H of each), 12.780(s,
1H, COOH).
4
-(4-(4-(4-n-propylphenyl)benzoyloxy)phenyldiazenyl)benzoic acid (3f), Relative
molecular mass: 464.51; the mass fraction detected by LC is 98.2%, yield: 85%, orange pow-
◦
der, m.p. 190–191 C; Anal. Calcd for C29H24N2O4: C 74.98, H 5.21, N 6.03; found C 74.88,
H 5.16, N 6.08; IR(KBr)ν: 3000–2500( OH), 2956, 2928, 2869(s, C H), 1737, 1686(vs,
C O), 1604(m, N N), 1496, 1429(m, ArH), 1401, 1272, 1222, 1139, 1070, 1003(vs,
C O C), 881(s, trans H N N R ), 838, 814(w, 1, 4-Ar), 741(w, (CH2)n), 691,
−
1
5
3
2
1
3
2
49 cm ; MS(70ev)m/z(%): 464(M+, 2.06), 450(3.24), 435(0.06), 422(0.10), 405(0.03),
88(0.11), 360(0.16), 43(0.05), 331(0.29), 317(0.61), 301(0.09), 286(0.16), 268(1.38),
55(0.37), 251(8.40), 240(10.98), 223(76.54), 209(100), 194(1.69), 181(5.58), 165(36.46),
52(39.70), 139(5.54), 121(13.72), 119(3.82), 115(8.38), 93(9.07), 77(5.60), 65(27.36),
1
9(8.89), 29(10.5). H NMR: 0.979(t, 3H, J = 7.2Hz, CH3), 1.662–1.728(m, 2H, CH2),
.662(t, 2H, J = 7.2 Hz, CH2), 7.260, 7.313, 7.451, 7.759, 8.048, 8.160, 8.282, 8.345(d, J
=
8.0 Hz, 8.4 Hz, 8.0 Hz, 8.0 Hz, 8.0 Hz, 8.0 Hz, 8.4 Hz, 8.4 Hz, 2H of each), 12.972(s,
1
H, COOH).

Synthesis of Multiring Azo-Benzoic Acid Liquid Crystalline Molecules
5
4
-(4-(4-(4-n-butylphenyl)benzoyloxy)phenyldiazenyl)benzoic acid (3g), Relative
molecular mass: 478.54; the mass fraction detected by LC is 98.0%, yield: 82%, orange
◦
powder, m.p. 226–227 C; Anal. Calcd for C30H26N2O4: C 74.98, H 5.21, N 6.03; found
C 74.81, H 5.16, N 6.18; IR(KBr)ν: 3000-2500( OH), 2958, 2926, 2856(s, C H), 1779,
1
1
5
3
1
8
7
7
8
737, 1688(vs, C O), 1604(m, N N), 1495, 1463(m, ArH), 1400, 1272, 1222, 1140, 1070,
003(vs, C O C), 882(s, trans H N N R ), 821(w, 1, 4-Ar), 712(w, (CH2)n), 691,
−
1
47 cm ; MS(70 ev)m/z(%): 478(M+, 0.11), 450(0.04), 429(0.07), 403(0.04), 355(0.11),
31(0.08), 317(0.03), 301(0.09), 272(0.39), 254(42.94), 237(4.52), 224(1.02), 211(100),
91(0.17), 178(3.67), 165(3.18), 152(17.21), 139(4.27), 128(2.74), 115(5.81), 93(2.62),
1
6(10.33), 77(3.32), 65(4.27), 51(1.93), 41(5.93), 29(7.57), H NMR: 0.958(t, 3H, J =
.2 Hz, CH3), 1.375-1.672(m, 4H, CH2CH2), 2.678(t, 2H, J = 7.2 Hz, CH2), 7.317, 7.455,
.563, 7.759, 8.092, 8.148, 8.237, 8.349(d, J = 8.0 Hz, 8.8 Hz, 8.0 Hz, 8.0 Hz, 8.4 Hz,
.4 Hz, 8.4 Hz, 8.4 Hz, 2H of each), 12.079(s, 1H, COOH).
4
-(4-(4-(4-n-pentylphenyl)benzoyloxy)phenyldiazenyl)benzoic acid (3h), Relative
molecular mass: 492.57; the mass fraction detected by LC is 98.5%, yield: 81%, orange pow-
◦
der, m.p. 220–221 C; Anal. Calcd for C31H28N2O4: C 75.59, H 5.73, N 5.69; found C 75.32,
H 5.66, N 5.57; IR(KBr)ν: 3000-2500( OH), 2954, 2923, 2854(s, C H), 1738, 1687(vs,
C O), 1642, 1631, 1603(m, N N), 1593, 1495,(m, ArH), 1428, 1271, 1225, 1198, 1178,
1
142, 1072, 1009(vs, C O C), 881(s, trans H N N R ), 812(w, 1, 4-Ar), 713(w,
−
1
(CH2)n), 690, 549 cm MS(70ev)m/z(%): 492(M, 0.11), 470(0.14), 456(0.28), 435(0.18),
3
2
1
88(0.10), 360(0.23), 346(0.04), 329(0.07), 315(0.12), 286(0.10), 268(0.89), 257(45.93),
51(100), 243(12.51), 229(4.14), 215(6.7), 211(2.21), 194(1.27), 178(0.11), 165(6.25),
52(5.9), 121(9.86), 105(3.85), 91(.86), 77(5.41), 65(17.27), 55(3.66), 43(7.16), 32(1.31).
1
H NMR: 0.910(t, 3H, J = 7.4 Hz, CH3), 1.256-1.658(m, 6H, CH2CH2CH2), 2.660(t, 2H,
J = 7.2 Hz, CH2), 7.458, 7.562, 7.687, 7.761, 8.148, 8.238, 8.403(d, J = 8.0 Hz, 8.0 Hz,
8
.0 Hz, 8.0 Hz, 8.0 Hz, 8.4 Hz, 8.0 Hz, 8.4 Hz, 2H of each), 12.069(s, 1H, COOH).
4
-(4-(4-(4-n-propylcyclohexyl)cyclohexylacyloxy)phenyldiazenyl)benzoic acid (3i),
Relative molecular mass: 476.61; the mass fraction detected by LC is 98.3%, yield:
◦
7
9%, orange powder, m.p. 200–201 C; Anal. Calcd for C29H36N2O4: C 73.08 H
7.61, N 5.88; found C 73.02, H 7.56, N 5.97; IR(KBr)ν: 2913, 2851, 2853(s,
C H), 1744, 1684(vs, C O), 1541,1507,1457(m, ArH), 1308, 1291, 1262, 1125,
−1
1
064, 1010(vs, C O C), 988, 948, 867, 849, 806(w, 1, 4-Ar), 765(w, (CH2)n)cm .
MS(70ev)m/z(%): 476(M, 3.26), 464(0.41), 450(1.19), 435(0.16), 404(0.03), 388(0.06),
3
2
1
5
4
8
71(0.09), 345(0.41), 331(0.09), 317(0.21), 301(0.13), 251(6.19), 242(94.90), 237(78.42),
23(16.43), 209(37.37), 194(1.73), 178(3.01), 165(14.45), 152(23.63), 137(20.27),
25(29.54), 121(66.83), 111(45.04), 109(39.13), 93(38.12), 83(69.41), 69(100), 65(49.67),
1
5(80.78), 43(15.98), 41(36.55). H NMR: 0.868(t, 3H, J = 7.8 Hz, CH3), 0.944-1.148(m,
H, CH2CH2), 1.256-2.525(m, 20H, the proton of cyclohexane), 7.319, 7.461, 8.087,
.384(d, J = 8.0 Hz, 8.4 Hz, 8.4 Hz, 8.4 Hz 2H of each), 12.41(s, 1H, COOH).
4
-(4-(4-(4-n-pentylcyclohexyl)cyclohexylacyloxy)phenyldiazenyl)benzoic acid (3j),
Relative molecular mass: 504.66; the mass fraction detected by LC is 98.2%, yield:
◦
8
3%, orange powder, m.p 208–209 C; Anal. Calcd for C31H40N2O4: C 73.78, H 7.99,
N 5.55; found C 73.62, H 8.04, N 5.44; IR(KBr)ν: 2953, 2921, 2850 (s, C H),
1
1
734, 1685 (vs, C O), 1605(m, N N), 1507, 1457, 1494(m, ArH), 1418, 1267,
221, 1197, 1141, 1066, 1010(vs, C O C), 951, 883, 810(w, 1, 4-Ar), 776, 719((w,
−1
(CH2)n), 670, 547, 531cm ; MS(70ev)m/z(%):504(M, 0.11), 478(0.15), 460(0.10),
3
2
1
71(0.22), 327(0.10), 280(0.36), 269(1.11), 262(4.64), 251(3.56), 242(100), 235(9.93),
24(3.37), 211(7.02), 198(1.48), 179(2.58), 165(4.44), 153(11.61), 139(26.72), 121(37.26),
1
09(35.37), 97(44.84), 83(47.85), 69(15.66), 55(9.78), 44(8.44), 29(1.85). H NMR:

6
M.-Y. Zheng et al.
0
.828(t, 3H, J = 7.2 Hz, CH3), 0.895-0.001(m, 4H, CH2CH2), 1.076–1.101(m, 4H,
CH2CH2), 1.076-2.197(m, 20H, the proton of cyclohexane), 7.3884, 7.971, 8.162, 8.332(d,
J = 9.2 Hz, 8.8 Hz, 8.4 Hz, 8.4 Hz 2H of each), 12.725(s, 1H, COOH).
Results and Discussion
The Pathway of Synthesis of Target Products
4-(4-Hydroxyphenylazo)benzoic acid, as an initial material with double functional groups,
is used to synthesize different photo-sensitive materials. It is usually applied to form an
ester of carboxyl acid and an ether of the hydroxyl group [17, 18], or sometimes esterify
the carboxyl acid with the hydroxyl unchanged [19], or etherify hydroxyl with the carboxyl
acid group keeping constant [20–21]. There is little report of a method to esterify the
hydroxyl of the molecule with the carboxyl acid unchanged in one-step reaction. Generally
speaking, both carboxyl acid and hydroxyl groups can be easily transformed into the
corresponding ester groups so that it is difficult to only esterify hydroxyl in molecules
containing carboxyl acid unit in one-step reaction. Through a great deal of attempts, we
found esterification of hydroxyl can be done at room temperature, while esterification of
carboxyl can be carried out at heated condition. Accordingly, the reaction we expected can
be done selectively between both carboxyl acid and hydroxyl groups with triethylamine
and CH2Cl2 as reactants at room temperature with yields more than 79%. Furthermore,
there was no other byproduct found in the reaction solution. The pathway gives us a chance
to obtain compounds bearing both ester and carboxyl acid in one-step reaction and further
to synthesize compounds of asymmetrical bi-ester with other hydroxy benzoic acids.
Scheme 1. The synthesis process of resulting compounds.

Synthesis of Multiring Azo-Benzoic Acid Liquid Crystalline Molecules
7
Determination of Photo-sensitivities in Solution
Target compounds were synthesized according to Scheme 1, their structural data were
shown above. The spectral values are in accordance with the assigned structure. Generally
speaking, the azo functional group may be isomerized from the more stable trans isomer
to the cis one under illumination of UV light. The respective solutions utilized were kept
in the dark for 2 days in order to we can infer that the compounds were almost in the trans
form. The UV spectra were recorded over the same time interval until photostationary states
were reached. The maximum time of photoisomerization were obtained at photostationary
states. Under exposure on UV-light, the optical absorbances of all target compounds and
the maximum time of isomerization in methanol solution are summarized in Table 1 and
the change of UV spectrum of 3b is shown in Fig. 1. The result shows that these compounds
can do cis-trans isomerization in a solution of methanol. The highest photoisomerization
of target compounds is reached between 40 and 80 min. for illumination intensities of
−2
1
05 μWcm . The observation of two isosbestic points at nearby 300 nm and 400 nm in
Fig. 1 indicate the existence of only two compounds, the trans and cis isomer[22]. Table 1
tells us that, before irradiation, the maximum absorption wavelength (λmax) of 3a, 3b, 3c
and 3d(with structure of terminal cyclohexyl benzene ring) recorded at 237, 331; 241, 334;
239, 338; 239, 341nm, and λmax of 3e, 3f, 3g and 3h (with structures of terminal double
benzoic rings) at 296, 325; 298, 320; 296, 324; 299, 323 nm and 3i, 3g(with structures of
terminal double trans cyclohexane rings) at 230, 333; 230, 333 nm; respectively, while after
irradiation, all maximum absorption wavelength (λmax) change their previous positions,
indicating all the compounds are photosensitive in solution. Figure 1 is the absorption
−5
spectral change of 3b (its concentration is 5×10 mol/L) in methanol by irradiation
with 365 nm light for the same time interval of 10min. Every peak in UV spectra has a
relationship with molecular structure. From the sample of 3b in solution (Fig. 1), we observe
∗
that the small shoulder at ∼430 nm is assigned to n-π transition and the maximum of the
∗
absorbance in the range of 334 nm corresponds to a strong π-π electronic transition of
azo-moiety, while the peak at 241nm is caused by aromatic ring system. We can see from
Table 1 that, before irradiation by UV-light, these compounds all have two main absorption
Table 1. The UV spectral data of target compounds
Trans isomer Cis isomer
λmax(nm)
λmax(nm)
Time(min) of cis-trans isomerization
−
Series
1
Compd.
I
II
I
II
(5×10 5 mol/L)
3a
237
241
239
239
296
298
296
299
230
230
331 240
334 246
338 244
341 241
325 293
320 294
324 293
323 295
333 232
333 232
328
328
335
337
327
325
326
326
330
327
50
60
70
40
40
60
40
40
80
80
3b
3c
3d
2
3
3e
3f
3g
3h
3i
3g

8
M.-Y. Zheng et al.
Figure 1. The UV-Visible absorption spectrum of 3b in methanol during cis-trans isomerization.
The inset illustrates the small increase of absorption in 430 nm.
peaks. After irradiation by UV-light, the compounds with terminal double benzene rings just
have one peak left in the UV spectra, while the other two series still had two peaks for each
compound. According to the data presented in Table 1, we find that terminal ring system
promote two effects on the λmax of the compounds. The first is a shift of the absorption
bands, having the same effect on the azo band and on the aromatic ring. Under exposure
to UV light, the first peaks of compounds in the series 1 promotes a small bathochromic
shift in the range of 2∼5 nm, while the second peak of benzene absorption band produces
a small hypsochromic shift between 3 and 6 nm. The first peak of compounds in the series
2
promotes a small hypsohromic shift in the range of 3∼4 nm, while the second peak
produces a small bathochromic shift between 2 and 5 nm. The first peak of the compounds
in the series 3 yields a small bathochromic shift of 2 nm, while the second peak of them
presents a small hypsochromic shift of 2 nm. The second effect is that, after irradiation,
compounds with terminal double benzene rings make the first peak a small hypsochromic
shift and the second one a bathochromic shift, while compounds with one or two terminal
cyclohexane rings make the first peak a small bathochromic shift and the second one a small
hypsochromic shift, suggesting the terminal rings have effects on the λmax of peaks of these
compounds during cis-trans isomerization process. Compared the period of each compound
need to change their structure from trans to cis isomer, compounds with terminal double
benzene rings are easiest to transfer their structures, while those with terminal double
cyclohexane rings are the most difficult to change.
Liquid Crystal Properties
The transition temperatures, associated enthalpy values obtained from DSC studies and the
ranges of decomposing from TG for the target compounds are summarized in Table 2. DSC
◦
thermograms were measured at the rate of 10 C/min for heating process. Among them, 8
compounds exhibit the appearance of textures should belong to nematic. The mesophase
of these molecules were observed by POM. The POM micrographs are shown in Fig. 2.

9

10
M.-Y. Zheng et al.
Figure 2. The textures of resulting compounds observed under polarizing optic microscope during
◦
heating process. (a) The schlieren texture of 3a taken at 281 C; (b) the schlieren texture of 3b taken
◦
◦
◦
at 240 C; (c) the schlieren texture of 3d taken at 254 C; (d) the schlieren texture of 3f taken at 270 C;
◦
◦
(
e) the schlieren texture of 3g taken at 310 C; (f) the schlieren texture of 3h taken at 275 C; (g) the
schlieren texture of 3i taken at 289 C; (h) the schlieren texture of 3j taken at 238 C.
◦
◦
Except 3c and 3e, which decompose before melt, the melting points of other compounds
◦
◦
are detected. the 8 compounds have higher decomposing point from 257 C to 323 C, while
◦
◦
melting points from 181 C to 227 C, indicating these compounds have thermostablity.
◦
Figure 3. The texture change during UV radiation of 3b at 260 C. (a) The texture taken after 10 min
UV radiation; (b) the texture taken after 180 min UV radiation.

Synthesis of Multiring Azo-Benzoic Acid Liquid Crystalline Molecules
11
Figure 4. Dimers of one of the target compounds in mesophase (drawn by chembioDraw 3D ultra
2.0 and minized molecular energy).
1
The 3j(with terminal pentyl bicyclohexane rings) is the least thermostable one, while the
d(with terminal pentyl cyclohexyl phenyl rings) is the most thermostable. There has no
3
clear trend of change of their melting point, indicating melting points of these compounds
has great relationship with the carboxyl acid group other than terminal alkyls and terminal
rings in other compounds [23]. Although the 8 compounds show no any clear point during
determination of DSC, the mesophases of them are still observed. Unlike azo benzaldehyde
compounds we have prepared [24], their mesophases remained unchanged under radiation
of UV light, suggesting these compounds are difficult to change their structures form trans
isomer to cis one in mesophase under the radiation of UV light. The maximum absorpotion
∗
wavelengths of π-π electronic transition of azo-moiety in cis isomer of 3i and 3j calculated
by Gaussian 3.0 at 332 nm and 328 nm are in good agreement with determining results
of these cis isomers, showing these trans compounds almost convert into cis isomer after
enough radiation of UV light.
Determination of Photo Sensitivities in Liquid Crystal Phase
After radiating under UV light of 365 nm from 10 min to 180 min, the textures of all
compounds were observed by POM (represent compound 3b shown as Fig. 3). Figure 3
shows that the textures kept unchanged as the increasing time of UV light radiation,
indicating although at higher temperature, these azo benzoic acids do not change their
shapes in mesophase by the radiation of UV light. As it is known to all, conventional
azobenzenes can do cis-trans isomerization at higher temperature. But these azo compounds
in the paper remain their trans structures in mesophase, suggesting they must have formed
some stable structures in mesophase to resist transferring their structures by radiation of
UV-light under heating condition. It is because the carboxyl acid group is easy to form a
dimer in solid and liquid crystalline phase (as Fig. 4 shown)
Conclusions
We have discussed the synthesis, mesomorphic properties, and photo sensitive property of
asymmetric azo benzoic acid liquid crystals. 10 target molecules synthesized are consisted
by asymmetric four-ring molecule with variation terminal alkyl chains (n = 2, 3, 4 and
5) on one end and the carboxyl acid group on the other end. A simple one-step reaction
was applied to synthesize the target compound bearing both groups of ester and carboxyl
acid. 8 compounds appear the mesophase before their decomposing points. The existence
of phase is confirmed by POM investigation. The mesophase of them belongs to nematic.
The photo-switching properties of these compounds were investigated by using UV–Vis
spectroscopy under illumination with UV light (365 nm). Photoisomerization processes
in solution revealed all compounds exhibited cis-trans isomerization occurs around 40 to
80 min. The compound with terminal double hexane rings took the longest average time to
transfer from trans isomer to cis one, while the compounds with terminal double benzene

12
M.-Y. Zheng et al.
rings took the least mean time to change their configuration. All the compounds can not
do cis-trans isomerization in mesophase, suggesting these compounds form some stable
structure to resist the change of configuration, such as dimmer.
Funding
We gratefully acknowledge the financial support from the National Natural Science Foun-
dation of China (No. 21102121), Natural Science Foundation of Shaanxi Province (No.
2010JM2009) and Natural Science Foundation of Shaanxi Province Educational Depart-
ment (No. 2010JK902).
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