Liquid crystal of ethyl and propyl aromatic aldehyde with azo core and photosensitivity in mesophase

Article abstract of DOI:10.14233/ajchem.2015.18367

Eight new stable rod-like aromatic aldehyde liquid crystalline molecules with azo bridge have been prepared, in which single or double six-membered carbon ring carboxylic acid mesogenic cores with shorter alkyl chain of ethyl, n-propyl were condensed with hydroxyl azo benzaldehyde. These compounds have been characterized by their spectral data, DSC and HS-POM. These molecules were expected to exhibit liquid crystal phase so that the influence of UV-light on their textures of mesophase could be detected. The results showed that all these target compounds have the temperature range of mesophase between 101 and 145°C. After irradiating under UV-light, they exihibited photo-sensitivity not only in methanol but also in mesophase.

Full text of DOI:10.14233/ajchem.2015.18367

Asian Journal of Chemistry; Vol. 27, No. 5 (2015), 1883-1888
A
SIAN
J
OURNAL OF HEMISTRY
C
http://dx.doi.org/10.14233/ajchem.2015.18367
Liquid Crystal of Ethyl and Propyl Aromatic Aldehyde
with Azo Core and Photosensitivity in Mesophase
*
YONG-SHENG WEI , MIN-YAN ZHENG, WEI GENG, SHAN WANG andYONG-HUI SHANG
School of Chemistry and Chemical Engineering, Xianyang Normal University, Xianyang, Shaanxi Province, P.R. China
*Corresponding author: Fax: +86 29 33720371; Tel: +86 29 33720704; E-mail: wys420@126.com
Received: 23 June 2014; Accepted: 11 September 2014; Published online: 20 February 2015;
AJC-16913
Eight new stable rod-like aromatic aldehyde liquid crystalline molecules with azo bridge have been prepared, in which single or double
six-membered carbon ring carboxylic acid mesogenic cores with shorter alkyl chain of ethyl, n-propyl were condensed with hydroxyl azo
benzaldehyde. These compounds have been characterized by their spectral data, DSC and HS-POM. These molecules were expected to
exhibit liquid crystal phase so that the influence of UV-light on their textures of mesophase could be detected. The results showed that all
these target compounds have the temperature range of mesophase between 101 and 145 °C. After irradiating under UV-light, they
exihibited photo-sensitivity not only in methanol but also in mesophase.
Keywords: Liquid crystals, Photosensitivity, Azo compound, Aromatic aldehyde.
synthesized 8 rod-like liquid crystalline molecules consisted
of shorter terminal alkyl chain of C2 and C3, aldehyde and
azo bridge bonding, by changing the terminal single six-
membered carbon ring from cyclohexyl to phenyl or double six-
membered carbon rings from cyclohexyl benzene to biphenyl
in order to obtain the effect of these variations on their liquid
crystalline property in the mean time to study their photo-
sensitivity when cis-trans isomerization takes place and azo
molecules change their shapes. (their structure and synthesis
pathway is shown as Scheme-I).
INTRODUCTION
Functionalizing molecular structures to make them
response to outer stimuli such as magnetism, thermal and light
is an important way to innovate new materials . Great interest
is recently attracted to molecules that have two or more forms
so that the interconversion between them can triggered by
1
2
-4
external stimulus . Some literature studied the effect of
5
UV-light on solid state of substance , but little report involved
influence of UV-light on mesophase of micromolecules.
It's well known that the molecular architecture is an
important factor for a molecule to exhibit its mesophase. Azo
liquid crystals reported in literature usually made up of terminal
EXPERIMENTAL
6
,7
longer alkoxy chain , which is easy to be synthesized by
converting hydroxy group into different ethers in one-step
reaction. Being more stable than alkoxy group, terminal alkyl
chains usually made by multi-step reactions is seldom used in
designning molecules of liquid crystals. Some publication
reported study on the effect of a polar end group on the
All initial intermediates used in the synthesis were prepared
in our laboratory with purity higher than 99 % and charac-
1
terized by IR, GC-MS and H NMR methods. Other reagents
were obtained from commercial sources and used without
further purification. 4′-Hydroxybenzene-azo-4-benzaldehyde
13
were synthesised according to literature .
8
mesomorphic property , but few of them involved aldehyde
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). The irradiation of samples
was carried out by hand-held UV-lamp ZF-7A (Shanghai
Jiapeng Technology Co. Ltd.). UV-spectra were determined
by Agilent 8453 Spectrometer (Agilent Technologies). IR
(KBr) spectra were recorded on aVertex 70 spectrophotometer
(BRUKER). Mass data were recorded on a GCMS-QP2010
9
as an end group. Previous papers have designed some series
liquid crystals with a polar end group of CHO, which exhibit
an mesophase even with shorter alkyl chain (n ≥ 2). These
compounds have thermostability and wider temperature range
of mesophase than other common functional groups, for
example alkyls, alkoxyls or cyanogroup. In addition, aromatic
aldehyde is easier to convert into other groups, such as aromatic
10
11
12
acid , Schiff base or other functional molecules . We have

1
884 Wei et al.
Asian J. Chem.
HO
N
N
R
CHO
OH
3
H C
NO
2
H
2
N
CHO
SOCl
Toluene
CH Cl , R.T
2 2
NaNO
2
, HCl ,0-5°C
2
R
A
COOH
A
2
COCl
1
O
O
A
R
N
N
CHO
3
a: R=ethyl, A=trans-cyclohexyl;
b
: R=
n
-propyl, A=trans-cyclohexyl c: R=ethyl, A= phenyl;
: R= -propyl, A=trans-cyclohexyl phenyl
: R=ethyl, A=biphenyl; h: R=
Scheme-I: Synthesis process of resulting compounds
d
: R=
n
-propyl, A= phenyl
e: R=ethyl, A= trans-cyclohexyl phenyl;
f
n
g
n
-propyl, A=biphenyl
1
1
(
Shimadzu) and IE was 70 ev. H NMR were obtained on a
51 (1.35), 41 (11.53); H NMR: 0.912 (t, 3H, J = 7.2 Hz, CH
3
),
BRUKER AV 500 spectrometer (500 MHz, solvent CDCl
3
).
0.979-1.039 (m, 2H, CH ), 1.212-2.528 (m, 10 H, proton of
2
The DSC experiments were carried out on MDSC Q100 instru-
ment (TA) and the determination conditions were: the sample
mass less than 5 mg; heating rate, 1°C/min and samples were
protected by nitrogen. The TGA experiments were carried out
on Q500 instrument (TA). All images of textures were taken
on a LV100POL Polarizing Optical Microscopy (Nikon) with
LTS E350 thermalplate (Linkam) and heating rate 1°C/min.
Synthesis of compound 2: Compound 2a was prepared
by heating and stirring a mixture of compound p-ethyl benzoic
cyclohexane), 7.249(m, 4H), 8.000, 8.028 (d, J = 8.4 Hz, 8.6
Hz, 2H of each), 10.108 (s, 1H, CHO).
4-[4-(4-n-Propylcyclolhexylacyloxy)phenyldiazenyl]-
benzldehyde (3b): Relative molecular mass: 378.46, the mass
fraction detected by LC is 98.7 %, orange powder, m.p. 99-100 °C;
Anal. Calcd. for C23
H
26
N
2
O : C 72.99, H 6.92 N 7.40; found C
3
–1
72.87, H 7.01, N 7.35 IR (KBr, νmax, cm ): 2954, 2929 (s, C-
H), 2858, 2747 (s, H of aldehyde), 2679, 2361, 2343, 1748
(vs, C=O), 1698 (vs, C=O), 1671, 1648 (m, N=N), 1599, 1559,
1541, 1508, 1496, 1457 (m, ArH), 1376, 1315, 1202, 1164,
acid (1a) (1.16 g, 5 mmol) and SOCl (0.59 g, 5 mmol) and
2
several drops of DMF in toluene (30 mL) at ambient tempe-
rature for 5 h, then the solvent was removed under reduced
pressure. A brown liquid (2a) was obtained and used to next
step without further purifying.
Synthesis of compounds 3: Compound 3a was obtained
by dissolving compound p-ethyl benzoyl chloride (1a) (0.78 g,
1123 (vs, C-O-C), 977, 869, 814 (w, 1, 4-Ar), 797,762, 727,
+
2
669 (w, (CH )n), 553; MS (70 ev) m/z (%): 378 (M , 9.61),
364 (0.14), 350 (0.08), 335 (0.02), 313 (0.02), 273 (0.07),
261 (0.05), 245 (0.21), 229 (0.11), 226 (50.73), 212 (0.79),
197 (0.08), 181 (0.01), 161 (0.17), 153 (11.91), 152 (3.13),
133 (0.70), 125 (61.48), 121 (28.24), 120 (0.73), 105 (3.30),
93 (12.91), 92 (0.81), 86 (0.85), 83 (45.90), 77 (1.35), 69 (100),
5
mmol) in CH
solution of 4′-hydroxy-benzene-azo-4-benzaldehyde (1.13 g,
mmol) and triethylamide (0.9 mL, 5 mmol) in CH Cl (40 mL)
2
Cl (20 mL) and dropped the mixture into a
2
1
67 (5.16), 57 (21.68), 51 (0.28), 41 (4.35), 29 (0.44); H NMR:
5
2
2
0.902 (t, 3H, J = 1.6 Hz, CH
3
), 0.954-1.049 (m, 2H, CH ),
2
and stirred at ambient temperature for 12 h, then the reacting
solvent was removed under vacuum condition. Some orange
solid was obtained, washing with 5 % (mass fraction) NaOH
and water. The residue was dried and recrystallized with THF,
then an orange powder (3a, 0.84 g) was obtained in yield of
1.182-1.236 (m, 2H, CH ), 1.525-2.555 (m, 10H, the proton
of cyclohexane), 7.256 (d, J = 8.5 Hz, 2H of each), 7.986-
8.033 (m, 6H), 11.105 (s, 1H, CHO).
4-[4-(4-Ethylbenzoyloxy)phenyldiazenyl]benzal-
dehyde (3c): Relative molecular mass: 358.39, mass fraction
detected by LC is 98.7 %, orange powder, m.p. 129-130 °C;
2
8
4 % (two steps). The compounds 3b-3h were also prepared
by the same way and their yields were 84, 83, 83, 85, 86, 83
and 87 %, respectively.
Anal. Calcd. for C22
H
18
N
2
O : C 73.73, H 5.06, N 7.82; found
3
–1
C 73.69, H 5.11, N 7.76; IR (KBr, νmax, cm ): 3067, 2969,
2931, 2875 (s, C-H), 2815, 2726 (H of aldehyde), 1731 (vs,
C=O), 1698 (vs, C=O), 1609, 1599, 1507, 1496, 1465, 1416,
1309, 1271, 1202, 1140, 1078, 1011 (vs, C-O-C), 929, 886,
4-[4-(4-Ethylcyclolhexylacyloxy)phenyldiazenyl]benzal-
dehyde (3a): Relative molecular mass: 364.44, the mass
fraction detected by LC is 98.8 %, orange powder, m.p. 85-
+
8
6 °C; Anal. Calcd. for C22
H
24
N
2
O
3
: C 72.50, H 6.64, N 7.69;
848, 760, 725, 696, 661; MS (70 ev) m/z (%): 358 (M , 1.28),
–1
found: C 72.34, H 6.69, N 7.62; IR (KBr, νmax, cm ): 2959,
257 (0.37), 241 (0.06), 226 (0.37), 211 (0.02), 167 (0.10),
150 (0.04), 141 (0.17), 132 (0.94), 133 (100), 121 (1.58), 105
(7.29), 93 (2.15), 79 (2.77), 65 (0.98), 57 (0.03), 51 (0.11), 44
2930 (s, C-H), 2852, 2744 (s, H of aldehyde), 2361, 2343,
1749 (vs, C=O), 1694 (vs, C=O), 1654, 1637 (m, N=N), 1559,
1495, 1458 (m, ArH), 1448, 1378, 1318, 1297, 1255, 1202,
1156, 1124, 1099, 1007 (vs, C-O-C), 995, 902, 855 (s, trans-
1
(0.14), 39 (0.12), 32 (0.12); H NMR: 1.300 (t, 3H, J = 7.2
Hz, CH
3
), 2.765 (q, 2H, J = 7.2 Hz, CH ), 7.364, 7.414, 8.145
2
H-N=N-R), 841, 817 (w, 1,4-Ar), 760, 725, 662 (w, (CH
MS (70 ev) m/z (%): 364 (M , 3.79), 360(0.26), 351 (0.03),
2
)n);
(d, J = 8.0 Hz, 8.4 Hz, 8.0 Hz, 2H of each), 8.069-8.086 (m,
6H, H of benzene ring), 10.114 (s, 1H, CHO).
+
269 (0.08), 257 (0.30), 242 (33.81), 224 (1.09), 215 (0.25),
197 (0.22), 185 (0.26), 168 (0.17), 157 (0.19), 147 (1.60),
139 (12.54), 121 (31.37), 111 (55.06), 93 (8.83), 91 (2.09),
84 (0.11), 69 (100), 65 (21.13), 61 (0.30), 57 (2.34), 55 (19.26),
4-[4-(4-n-Propylbenzoyloxy)phenyldiazenyl]benzal-
dehyde (3d): Relative molecular mass: 372.42, mass fraction
detected by LC is 98.8 %, orange powder, m.p. 123-124 °C;
Anal. Calcd. for C23
H
20
N
2
O : C 74.18, H 5.41, N 7.52; found
3

Vol. 27, No. 5 (2015)
Liquid Crystal of Ethyl and Propyl Aromatic Aldehyde with Azo Core 1885
–
1
C 74.03, H 5.52, N 7.46; IR (KBr, νmax, cm ): 3065, 2960,
2918, 2874 (s, C-H stretching vibration), 2849, 2734 (s, H of
aldehyde), 2361, 2342, 1730 (vs, C=O), 1699 (vs, C=O), 1654,
1637 (N=N), 1598, 1495, 1458 (m, ArH), 1418, 1399, 1280,
1203, 1137, 1081, 1007 (vs, C-O-C), 891 (s, trans-H-N=N-R),
2
1
1
1
7
3
1
930 (s, C-H), 2872, 2814, 2723 (H of aldehyde), 2361, 2342,
732 (vs, C=O), 1698 (vs, C=O), 1653, 1610, 1590, 1559,
541, 1496, 1457 (m, ArH), 1417, 1310, 1270, 1196, 1179,
143, 1100, 1079, 1011 (vs, C-O-C), 885, 846 (w, 1, 4-Ar)
830 (w, 1, 4-Ar), 779 (w, (CH )n), 760, 696; MS (70 ev) m/z
2
+
+
56, 696 (w, (CH
2
)n); MS (70 ev) m/z (%): 372 (M , 1.50),
17 (0.02), 255 (0.05), 239 (0.02), 225 (0.22), 209 (1.88),
81 (0.23), 165 (0.07), 149 (0.74), 147 (100), 131 (0.60), 121
(%): 434 (M , 3.16), 420 (0.22), 406 (0.12), 329 (0.02), 317
(0.11), 300 (0.03), 285 (0.02), 243 (0.14), 226 (0.77), 209
(100), 207 (0.88), 194 (0.10), 181 (3.30), 166 (3.36), 152
(6.83), 141 (0.43), 121 (0.93), 105 (0.70), 93 (1.05), 77 (0.48),
(
6
0.67), 119 (5.07), 105 (0.71), 93 (0.78), 91 (7.17), 77 (0.56),
1
1
5 (0.39), 51 (0.07), 41 (0.53), 32 (0.31); H NMR: 0.975 (t,
H, J = 7.2 Hz, CH ), 1.676-1.732 (m, 2H, CH ), 2.699 (t, 2H,
), 7.340, 7.411, 8.048 (d, J = 8.0 Hz, 8.4 Hz,
.0 Hz, 2H of each ), 8.138-8.156 (m, 6H), 10.111 (s, 1H,
CHO).
-[4-(4-(4-Ethylcyclohexyl)benzoyloxy)phenyldia-
65 (0.68), 57 (0.23), 44 (0.54), 32 (0.40); H NMR: 1.230 (t,
3
3
2
3H, J = 7.6 Hz, CH
3
), 2.659 (q, 2H, J = 7.6 Hz, CH ), 7.266,
2
J = 7.6Hz, CH
2
7.372, 7.534, 7.608, 8.208 (d, J = 8.0 Hz, 8.8 Hz, 8.0 Hz, 8.4
Hz, 8.4 Hz, 2H of each), 7.985 (b, 6H), 11.02 (s, 1H, CHO).
4-[4-(4-(4-n-Propylphenyl)benzoyloxy)phenyldiazenyl]-
benzaldehyde (3h): Relative molecular mass: 448.51, mass
fraction detected by LC is 98.5 %, orange powder, m.p. 174-
8
4
zenyl]benzaldehyde (3e): Relative molecular mass: 440.53;
the mass fraction detected by LC is 99.1 %, orange powder,
175 °C;Anal. Calcd. for C29
H
24
N
2
O : C 77.66, H 5.39, N 6.25;
3
–1
m.p. 128-129 °C; Anal. Calcd. for C28
H
28
2 3
N O : C 76.34, H
found C 77.73, H 5.43, N 6.19; IR (KBr, νmax, cm ): 2958,
2928 (s, C-H), 2869, 2728, 2676, 2552, 2360, 2341, 1732 (vs,
C=O), 1689 (vs, C=O), 1588, 1558, 1507, 1466 (m, ArH),
1428, 1398, 1284, 1206, 1139, 1103, 1006 (vs, C-O-C), 948,
6
.41, N 6.36; found: C 76.23, H 6.55, N 6.23; IR (KBr, νmax
,
–1
cm ): 2957, 2917 (s, C-H), 2849, 2729 (s, H of aldehyde),
361, 2342, 1741 (vs, C=O), 1703 (vs, C=O), 1648, 1636
N=N), 1598, 1559, 1541, 1508, 1495, 1458 (m, ArH), 1418,
375, 1308, 1272, 1223, 1198, 1178, 1139, 1099, 1063, 1009
vs, C-O-C), 985, 924, 884 (s, trans-R-N=N-R), 837, 819 (w,
2
(
1
(
838, 823 (w, 1, 4-Ar), 773, 730 (w, (CH )n), 660, 544; MS (70
2
+
ev) m/z (%): 448 (M , 5.64), 371 (0.07), 359 (0.06), 344 (0.03),
284 (0.02), 268 (0.04), 259 (0.06), 237 (0.81), 224 (0.35),
211 (0.23), 193 (0.03), 181 (0.19), 164 (0.20), 147 (100), 131
(0.66), 121 (1.30), 119 (3.92), 104 (0.51), 93 (1.64), 91 (7.77),
1
4
2
1
,4-Ar), 775(w, (CH )n), 761, 699, 684; MS (70 ev) m/z (%):
2
+
40 (M , 1.60), 426 (0.14), 364 (0.05), 323 (0.11), 293 (0.02),
43 (0.40), 217(1.55), 215(100), 199(0.05), 187(0.16),
71(0.05), 157 (0.14), 145 (0.30), 131 (1.14), 121 (1.20), 105
1
65 (1.82), 44 (1.37), 32 (0.32), H NMR: 0.981 (t, 3H, J = 7.2
Hz, CH
3
), 1.646-1.738 (m, 2H, CH ), 2.649 (t, 2H, J = 7.2,
2
(
(
(
3.30), 91 (2.23), 79 (0.55), 69 (0.69), 55 (0.52), 43 (0.31), 32
CH ), 7.293 (d, 6H, J = 8.0 Hz), 7.569, 7.695, 7.932, 8.167,
8.281 (d, J = 8.4 Hz, 8.8 Hz, 8.4 Hz, 8.0 Hz, 8.0 Hz, 2H of
each), 10.095 (s, 1H, CHO).
2
1
0.32); H NMR: 0.927 (t, 3H, J = 7.2 Hz, CH
m, 2H, CH ), 1.253-2.633 (m, 10 H, the proton of cyclohexane),
.372, 7.404, 8.140 (d, J = 8.4 Hz, 8.8 Hz, 8.4 Hz, 2H of
each), 8.046-8.066 (m, 6H), 10.114 (s, 1H, CHO).
-[4-(4-(4-n-Propylcyclohexyl) benzoyloxy)phenyldia-
3
), 1.253-2.623
2
7
RESULTS AND DISCUSSION
4
Photosensitivity in solution: The synthesis of all target
compounds is outlined in Scheme-I and their structural data
are shown above. The spectral values are in accordance with
the assigned structure. It is known that the azo functional group
can do isomerization from the more stable trans-isomer to the
cis- one under illumination of UV-light. The optical absorbance
of all target compounds in MeOH solution is summarized in
Table-1 and the change of UV-spectrum of representative
compound 3a is shown in Fig. 1. The solutions utilized were
kept in the dark for 2 days so that we can infer that the comp-
ounds were exclusively in the trans form. The UV-spectra were
recorded over the same time interval until photostationary
states were reached. The maximum time of isomerization were
obtained at photostationary states. The spectrum variations are
the evidence of the cis-trans isomerization of the azo chromo-
phores. Data depicted in Table-1 indicate, before or after irra-
diation, compounds with same terminal rings have same number
and similar position of peaks in UV-spectra. Every peak has a
relationship with molecular structure. For example, the sample
zenyl]benzaldehyde (3f): Relative molecular mass: 454.56,
the mass fraction detected by LC is 98.9 %, orange powder,
m.p. 140-141°C;Anal. Calcd. for C29
N 6.16; found: C 76.44, H 6.87, N 6.01; IR (KBr, νmax, cm ):
2
2
1
1
9
H
30
N
2
O : C 76.63, H 6.65,
3
–1
953, 2921 (s, C-H), 2843, 2736 (s, H of aldehyde), 2361,
343, 1734 (vs, C=O), 1696 (vs, C=O), 1654, 1637 (N=N),
589, 1559, 1542, 1494 (m, ArH), 1448, 1418, 1376, 1309,
266, 1223, 1198, 1176, 1137, 1099, 1060, 1010 (vs, C-O-C),
68, 924, 882 (s, trans-H-N=N-R), 847, 815 (w, 1, 4-Ar), 761
+
(
w, (CH
2
)n), 727, 698; MS (70 ev) m/z (%): 454 (M , 1.48),
4
2
1
40 (0.10), 337 (0.12), 321 (0.02), 307 (0.08), 257 (0.81),
43 (0.20), 229 (100), 227 (2.26), 209 (1.11), 199 (0.03),
87(0.32), 171(0.08), 157(0.17), 145(0.29), 131(1.31), 117
(
1.01), 105 (3.52), 91 (2.22), 81 (0.54), 67 (0.66), 55 (0.62),
1
4
1
2
8
6
3 (0.24), 32 (0.23); H NMR: 0.904 (t, 3H, J = 7.5 Hz, CH
3
),
.120-1.158 (m, 2H, CH ), 1.304-1.408 (m, 2H, CH ), 1.433-
2
2
. 638 (m, 10 H, the proton of cyclohexane), 7.375, 7.445,
.154 (d, J = 8.0 Hz, 8.0 Hz, 8.0 Hz, 2H of each), 8.137 (b,
H), 10.104 (s, 1H, CHO).
-
5
3a in solution with concentration of 5 × 10 mol/L (Fig. 1),
the small shoulder at ~430 nm is assigned to n-π* transition
and the maximum of the absorbance in the range of 300-
380 nm corresponds to a strong π-π* electronic transition of
trans-isomer of azo-moiety, while the peak at 230 is caused
by aromatic ring system. Before irradiation the maximum
4-[4-(4-(4-n-Ethylphenyl)benzoyloxy)phenyldiazenyl]
benzaldehyde (3g): Relative molecular mass: 434.49, mass
fraction detected by LC is 98.9 %, orange powder, m.p. 171-
1
72 °C;Anal. Calcd. for C28
H
22
N
2
O ; C 77.40, H 5.10, N 6.45;
3
–1
found C 77.27, H 5.28, N 6.26; IR (KBr, νmax, cm ): 2967,

1
886 Wei et al.
Asian J. Chem.
-
2
absorption wavelength λmax (nm) of compounds with terminal
single six-membered carbon ring are recorded at 228, 334 for
compound 3a, 230, 330 for 3b, 240, 338 for 3c, 240, 332 for
sities of 105 µW cm . The two isosbestic points, located at
nearby 295 and 393 nm in Fig. 1, confirm the existence of only
14
two compounds, the cis- and trans-isomer . Under sunlight,
compound 3a recover its original structure as Fig. 2b shown.
Owe to the longer molecular structure, it is difficult for them
to convert their shapes from one isomer to another. It almost
takes 60-70 min for their complete changing (Table-1). The
original absorbance of compound 3a is A
nm (Fig. 1a). Subsequent irradiation with sunlight for 70 min
causes the absorbance at 334 nm re-increase to A = 0.83630
(Fig. 1b). It is seen that A /A is 96 %, i.e., the azo compound
isomers do recover 96 %.
3d, respectively, while after irradiation λmax (nm) are at 228,
330 for compounds 3a, 228, 325 for 3b, 242, 342 for 3c, 243,
322 for 3d, respectively. According to these data above, we
observed that the UV-light of 365 nm causes the azo bands (π-π*
electronic transition) of compounds 3b and 3d a little bit more
hypsochromic shift than these of compounds 3a and 3c, while
the other peaks of them caused by aromatic ring system almost
stays practically unaltered. Before irradiation the λmax (nm) of
compounds with terminal double six-membered carbon rings
are recorded at 242, 332 for compounds 3e, 241, 334 for 3f,
1
= 0.87025 at 333
2
2
1
Liquid crystalline properties and photosensitivity in
3
λ
2
31 for 3g, 327 for 3h, respectively, while after irradiation,
max (nm) are at 245, 329 for compounds 3e, 245, 328 for 3f,
94 for 3g, 293 for 3h, respectively. We find that UV-light
mesophase:The thermal behaviours of all the target compounds
TABLE-1
λ
OF trans- AND cis-ISOMERS OF
TARGET MOLECULES IN UV-SPECTRA
causes the peaks of π-π* transition (the second peak) of the
four compounds a hypsochromic shift and the first bands caused
by aromatic ring system are not observed in the UV-spectra
of compounds 3g and 3h. The addition of a carbon in the alkyl
chain (from ethyl to propyl) has greater effect on the second
peaks of the first four compounds after irradiation. The spectral
data before and after irradiation of every compound indicates
the terminal alkyl chain has a bigger influence on the azo band
than on the aromatic band. It is noteworthy that the terminal
propyl chain causes the peak of azo group a bigger hypso-
chromic shift than terminal enthyl chain of corresponding
structure in the first six compounds (Table-2). The electronic
effect of alkyl group doesn't present in spectral data of compo-
unds 3g and 3h. We observe from the spectral data of compounds
max
trans-Isomer
λmax (nm)
cis-Isomer
λmax (nm)
Time (min) from
trans to cis in
solution
Compd.
I
II
I
II
3
a
228
230
240
240
242
241
–
334
333
338
332
332
334
331
327
228
228
242
243
245
245
–
330
325
342
322
329
328
294
293
70
60
60
70
60
70
60
70
3b
3
c
3d
3
3
3g
3h
e
f
–
–
1
.0
0
min
30 min
40 min
50 min
60 min
A
3g and 3h, after irradiation, the peaks of π-π* transition of
5 min
0.8
0.6
1
1
0 min
5 min
azo group are hyperchromic shifts of 37 nm (from 331 to 294)
and 34 nm (from 327 to 293), which are much more than these
shifts caused by the first six compounds during the process of
cis-trans isomerization. This phenomenon suggests that the
terminal bi-benzene causes such a great effect on peaks of π-π*
transition of azo group in the process of cis-trans isomerization
that it covers the effect resulted from terminal alkyl group. It
means when the number of terminal benzene ring is greater
than 1, we can not investigate the effect of terminal alkyl group.
The effect of terminal ring system on UV-spectra is deduced
from the data of compounds 3a, 3c, 3e and 3g (with terminal
ethyl), 3b, 3d, 3f and 3h (with terminal propyl). The spectral
data of 3a and 3e, 3b and 3d indicates that the addition of a
terminal benzene ring has greater effect on the absorption
bands, being more pronounced in the aromatic band than in
the azo band, while these of compounds 3c and 3e, 3d and 3f
suggests that the addition of a terminal cyclohexane ring, being
more pronounced in the azo band than the aromatic band. In
other words, the effect of the addition of a terminal cyclohexane
ring presents on the absorption of azo band, while that of the
addition of terminal benzene ring works on the absorption of
aromatic band. The two terminal benzene rings in compounds
0.06
0.4
0.2
0.0
0
.04
0.02
400
450
500
250
300
350
400
450
500
Wavelength (nm)
6
0 min
20 min
0
.8
B
50 min
10 min
5 min
0 min
4
3
0 min
0 min
0.6
0.07
0.06
0.05
0.04
0.03
0
0
0
.4
.2
.0
0
.02
.01
.00
0
0
400
450
500
3
c and 3d cause the first absorption disappeared in the mean
250
300
350
400
450
500
550
Wavelength (nm)
time a greater effect presents on the second peak of the azo
band after irradiation.
The maximum times of photoisomerization of all these
compounds are reached within 70 min for illumination inten-
Fig. 1. UV-visible absorption spectrum of compound 3a in methanol during
trans-to-cis isomerization; (a) under irradiation of UV-light; (b)
under irradiation of sunlight). The inset illustrates the small increase
of absorption in 430 nm

Vol. 27, No. 5 (2015)
Liquid Crystal of Ethyl and Propyl Aromatic Aldehyde with Azo Core 1887
TABLE-2
RESULTS OF DSC MEASUREMENT OF TARGET COMPOUNDS
Compounds
m.p. (°C)
H of m.p. (J/g)
3a
84.6
35.56
190.7
1.3320
106.1
3b
98.8
49.9
225.6
1.538
126.8
3c
3d
3e
3f
3g
3h
132.2
52.96
271.0
1.3597
138.8
122.5
55.25
267.9
1.3352
145.4
128.5
62.17
268.9
2.777
140.4
140.9
42.44
241.1
1.3838
100.2
170.1
23.17
275.2
1.890
105.1
174.5
40.36
276.5
1.788
102.0
∆
c.p. (°C)
∆H of c.p (J/g)
Range of mesophase (°C)
d.p. (°C)
205.1-388.0
222.4-381.4 287.2-395.0 280.1-404.3 291.8-333.2 290.6-340.1 285.1-392.5 283.1-305.8
*m.p. is melting point, c.p. is clearing point , d.p is decomposing point
were investigated by DSC, TGA and POM. The results are
summarized in Table-2. The POM micrographs of compounds
3a and 3b are shown in Fig. 2. All target compounds exhibit
the appearance of textures should belong to nematic phase
and their melting point, clear points and decomposing tempe-
rature are measured out (Table-2). The temperature ranges of
mesophase of these compounds are from 101 to 145 °C. DSC
analysis showed a typical nematic phase transition, crystal-
to-nematic-to-isotropic. These azo aromatic aldehydes with
shorter alkyl chain have wider ranges of mesophase and more
15
thermostable than other polymers of azo group .Among them,
compounds 3a and 3b with terminal cyclohexane ring have
the lowest melting point and clearing point. The m.p., c.p. and
range of mesophase of compounds 3c and 3d with terminal
benzene ring are higher than compounds 3a-3f had terminal
cyclohexane phenyl ring have lower m.p. than compounds 3h
and 3i had terminal double benzene rings.Among the last four
molecules in Table-2, compound 3e has wider range of meso-
phase, while the ranges of mesophase of the other three are
quite close. Among all the compounds 3d with propyl and
terminal single benzene ring has the widest temperature range
of mesophase. In summary, the compounds with terminal benzene
ring all have higher c.p and m.p. than those without terminal
benzene structure. In other words, terminal benzene ring in
liquid crystalline molecules can increase m.p. in the mean time
raise the c.p.
Fig. 2. Textures of some resulting compounds observed under polarizing
optic microscope during heating process; (a) the schlieren texture
of compound 3a taken at 110 °C × 300 °C; (b) the schlieren texture
of compound 3b taken at 113 °C × 300 °C
molecular shape of cis-isomer is so bent that it has no any
mesophase observed.After radiating under UV-light of 365 nm
each 10 min, the change of texture of compound 3a was
observed by POM (Fig. 3). The textures of compound 3a ob
served gradually disappeared as increasing the time of radiation
Since the compounds display photosensitivity in solution,
it is interesting to check this behavior in their mesophases.
Because of the lowest m.p., compound 3a was chose to check
photosensitivity of mesophase. It is well-known that the
Fig. 3. Texture change during UV-radiation of compounds 3a; (A) texture after 10 min radiation; (B) texture after 20 min radiation; (C) texture after 30 min
radiation; (D) texture after 40 min radiation; (E) texture after 50 min radiation; (F) texture after 60 min radiation; (G) texture after 70 min radiation

1
888 Wei et al.
Asian J. Chem.
of UV-light, indicating the structure of target compound is
converted from trans-to-cis-isomer. The result shows the azo
compound can also change their shape from in mesophase.
ACKNOWLEDGEMENTS
The authors gratefully acknowledge the financial support
from the National Natural Science Foundation of China (No.
1102121) and Natural Science Foundation of Shaanxi
Conclusion
2
Eight new azo aromatic liquid crystal molecules have been
prepared by a combination of rigid core involving azo group
and different terminal single or double six-membered rings
with polar CHO and shorter alkyls chain (C2 or C3) as structual
units. The molecular structure allows them to exhibit a phase
behavior belong to the nematic, which provides an opportunity
to investigate their photosensitivity not only in solution but also
in mesophase. All these compounds in a solution of methanol
can gradually change their shape under UV-light of 365 nm.
Except compounds with terminal bi-benzene structure, terminal
propyl group has higer electronic effect on the azo band than
terminal ethyl group in UV-spectrum. It is because the elec-
tronic effect of terminal bi-benzene structure present to mask
that from alkyl group. In other words, the electronic effect of
terminal alkyl chain on UV-spectra only presents if there is no
or only one terminal benzene rings. The azo compound can
recover its trans-form at a recovery rate of 96 % under sunlight.
We also performed photo-induce studies on mesophase of
compounds, which seldom reported until now. Results showed
that the molecules can also change their shapes in mesophase
under irradiation of UV-light, which are observed by gradually
disappearing of mesophasic texture under POM. Although
these molecules has shorter alkyl chain, they have so long rigid
core that they need long time of radiation of UV-light to change
their shape from trans- to cis-isomers compared to other azo
Province (No. 2010JM2009).
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16,17
molecules without mesophase , further work in this direction
is under active investigation.