Liquid crystalline properties of unsymmetrical N-(o-hydroxybenzylidene)- N′-(4-n-alkoxybenzoyloxybenzylidene)azines: Synthesis and characterization

Article abstract of DOI:10.1080/15421400903483973

A new series of mesogenic unsymmetrical azines, N-(o-hydroxybenzylidene)- N′-(4-n-alkoxybenzoyloxybenzylidene)azines, HOC₆H ₄CH=N-N=CHC₆H₄OCOC₆H₄O C_mH_2m+1 (m=7, 8, 9, 10, 11, 12, 14, and 16) have been prepared. They have been characterized by elemental analyses, FT-IR, UV-visible, ¹H, and ¹³C NMR (Nuclear Magnetic Resonance) spectra. The mesomorphic properties of these compounds were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The azines m=7, 10, and 14 exhibit features of enantiotropic nematic (N) mesophase whereas azines m=8, 11 exhibit features of a monotropic nematic phase. The azines m=9, 12, and 16 do not exhibit mesogenic nature. Copyright Taylor & Francis Group, LLC.

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Liquid Crystalline Properties
of Unsymmetrical N-(o-
hydroxybenzylidene)-N′-(4-n-
alkoxybenzoyloxybenzylidene)azines:
Synthesis and Characterization
a
a
Bachcha Singh & Ashwini Pandey
a
Department of Chemistry , Faculty of Science, Banaras Hindu
University , Varanasi, India
Published online: 01 Mar 2010.
To cite this article: Bachcha Singh & Ashwini Pandey (2010) Liquid Crystalline Properties
of Unsymmetrical N-(o-hydroxybenzylidene)-N′-(4-n-alkoxybenzoyloxybenzylidene)azines:
Synthesis and Characterization, Molecular Crystals and Liquid Crystals, 517:1, 148-158, DOI:
10.1080/15421400903483973
To link to this article: http://dx.doi.org/10.1080/15421400903483973
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Mol. Cryst. Liq. Cryst., Vol. 517: pp. 148–158, 2010
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903483973
Liquid Crystalline Properties of Unsymmetrical
N-(o-hydroxybenzylidene)-
0
N -(4-n-alkoxybenzoyloxybenzylidene)azines:
Synthesis and Characterization
BACHCHA SINGH AND ASHWINI PANDEY
Department of Chemistry, Faculty of Science, Banaras Hindu University,
Varanasi, India
0
A new series of mesogenic unsymmetrical azines, N-(o-hydroxybenzylidene)-N -(4-
n-alkoxybenzoyloxybenzylidene)azines, HOC
C H
6 4 6 4 6 4
H H OCOC H O
CH=N-N=CHC
(m ¼ 7, 8, 9, 10, 11, 12, 14, and 16) have been prepared. They have been
m
2m þ 1
1
13
characterized by elemental analyses, FT-IR, UV-visible, H, and C NMR (Nuclear
Magnetic Resonance) spectra. The mesomorphic properties of these compounds were
investigated by differential scanning calorimetry (DSC) and polarized optical
microscopy (POM). The azines m ¼ 7, 10, and 14 exhibit features of enantiotropic
nematic (N) mesophase whereas azines m ¼ 8, 11 exhibit features of a monotropic
nematic phase. The azines m ¼ 9, 12, and 16 do not exhibit mesogenic nature.
Keywords Differential scanning calorimetry; enantiotropic nematic mesophase;
monotropic nematic mesophase; polarized optical microscopy
1
. Introduction
Azines (or diimines) have been extensively studied due to their utility in a number of
interesting organic reactions and applications [1,2]. Furthermore, they have received
attention as possible nonlinear optical (NLO) materials, especially the unsymmetri-
cal azines with an electron donor on one side and an electron acceptor on the other
side [3–5]. Azines have been used for the preparation of the nematic liquid crystals
and liquid crystal display devices such as TN-LCD (twisted nematic-LCD),
STN-LCD (supertwisted nematic-LCD) and TFT-LCD (thin film transistor-LCD)
having improved electro-optical properties [6,7]. Wilfrid and Glenn [8] reported a
series of azine liquid crystals having high clearing points. Since then experimental
and theoretical studies [9–15] have been carried out on the synthesis and meso-
morphic properties of azine type liquid crystals.
Yang et al. [16] reported two series of new symmetrical azine based liquid crys-
0
tals, 4,4 -(4-alkylbenzoyloxy)-benzalazine that contain only ester groups in their
Address correspondence to Bachcha Singh, Department of Chemistry, Faculty of Science,
BHU, Varanasi-221005, India. E-mail: bsinghbhu@rediffmail.com
148

Liquid Crystalline Azines
149
linear mesogenic core and alkyl or alkoxy end groups. The end groups of the liquid
crystals had an effect on their mesomorphic properties. The series with alkoxy end
groups exhibited both smectic A and nematic mesophase whereas the series with
alkyl end groups showed only nematic phase at their lower analogues and showed
both smectic A and nematic properties at their higher analogues.
Chudgar et al. [15] reported a series of azine type liquid crystals with ester group
and lateral substituents in the rigid core and alkoxy end groups. Kelker and cowor-
0
kers [17] reported Schiff base liquid crystals 4-methoxybenzylidene-4 -butylaniline
(MBBA) which exhibits a room temperature nematic phase. Since then, Schiff bases
based on esters have attracted much attention on liquid crystal researches. Over the
past few decades, a lot of low molar mass Schiff base esters have been synthesized
and investigated extensively for liquid crystalline applications [18–23]. Centore and
Garzillo [24] reported a structural and theoretical analysis of biphenylene azines con-
taining a methyl group on the azine system and strong electron donor-acceptor
groups on the phenylene rings. Centore et al. [5] reported asymmetrical azines which
show both nematic and smectic phases.
The literature survey reveals that a few reports are available on synthesis,
characterization, and mesomorphic properties of ester based unsymmetrical azines.
The synthesis, characterization, and mesomorphic properties of hitherto
uninvestigated new series of ester based unsymmetrical azines liquid crystals
0
N-(o-hydroxybenzylidene)-N -(4-n-alkoxybenzoyloxybenzylidene)azines (6) have
been under taken. The results of these investigations are reported in this
article.
2
. Experimental
.1. Materials
Salicylaldehyde (Spectrochem), hydrazine monohydrate (Qualigens), pyridine
Qualigens), thionyl chloride (Qualigens), 4-hydroxybenzaldehyde (AcrosOrganic),
-hydroxybenzoic acid (Aldrich), and 1-bromoalkanes (Aldrich) were used as
2
(
4
received. All other solvents and reagents were purchased commercially and used
after further purification.
2
.2. Techniques
Elemental analyses were performed on CE-440 Exeter Analytical CHN analyzer.
ꢀ1
Infrared (IR) spectra (4000–100 cm ) were recorded on Varian 3100 FT-IR Excali-
1
13
bur spectrophotometer. H and C NMR spectra were obtained by using a JEOL
FT NMR AL 300 MHz spectrometer using tetramethylsilane as internal standard.
Electronic spectra were recorded on UV-1700 Pharma Spec Shimadzu UV-Visible
spectrophotometer. Differential scanning calorimetry (DSC) thermograms were
recorded on Mettler Toledo TC 15 TA differential scanning calorimeter at the rate
ꢀ
1
of 10.0 Kmin under nitrogen atmosphere using spec pure grade indium as standard
by taking samples in close lid aluminum pans. The transition temperatures from
DSC have been determined with accuracy of ꢁ0.1 K. The mesophase type was
assigned by visual comparison with known phase standards using HT-30.01 NTT
268 LOMO polarizing optical microscope fitted with a hot stage with temperature
controlling accuracy of 0.1 K.

1
50
B. Singh and A. Pandey
2
.3. Synthesis of Compounds
2.3.1. Synthesis of Salicylhydrazone 4. Salicylhydrazone was synthesized by the
procedure as mentioned elsewhere [25].
0
2
.3.2. Synthesis of 4-(4 -heptyloxybenzoyloxy)benzaldehyde 5. 4-Heptyloxybenzoic
acid (1.41 g, 6 mmol) was dissolved in dry chloroform (55 ml) and thionyl chloride
0.51 ml, 7 mmol) was added and refluxed for 8 h. The solvent was removed under
reduced pressure to give a yellow oil. It was further treated with a solution of
-hydroxybenzaldehyde (0.73 g, 6 mmol) in dry chloroform (50 ml) in the presence
(
4
of 2–3 drops of pyridine. The mixture was then refluxed for 9 h. After cooling,
light yellow solution was washed with water (50 ml), dried over anhydrous MgSO
4
,
0
and then concentrated to form white crystals of 4-(4 -heptyloxybenzoyloxy)
ꢀ
1
benzaldehyde. Yield: 72%. IR (KBr, cm ): 2924, 2854 (aliphatic CꢀH), 1776
1
(
C=O ester), 1712 (C=O aldehyde), 1605, 1511 (Ph), 1307, 1258 (OPh). H NMR
3
4
00 MHz (CDCl , TMS) dH (ppm): 10.02 (s, 1 H, CHO), 8.15–6.88 (m, 8 H, ArH),
3
.07–3.97 (m, 2 H, ꢀOCH ), 1.81–1.27 (m, 10 H, ꢀ[CH ] ), 0.90–0.86 (t, 3 H, ꢀCH ).
2
2 n
3
All other homologous members of the series were prepared following the above
procedure.
0
2.3.3. Synthesis of Unsymmetrical N-(o-hydroxybenzylidene)-N -(4-n-heptyloxy-
benzoyloxybenzylidene)azine 6a. A mixture of salicylhydrazone (0.13 g, 6 mmol)
0
and 4-(4 -heptyloxy benzoyloxy)benzaldehyde (2.38 g, 6 mmol) in 45 ml of
chloroform containing 3–4 drops of glacial acetic acid was stirred for 7 h. A
yellow solution was obtained which was evaporated under reduced pressure to
yield cream colour crystals. The product was washed with water and recrystallized
from methanol. All other homologous members of the series were prepared in a
similar manner. Their IR, analytical, NMR data are summarized as follows.
0
2
C H N O (6a). Yield: 65%. IR (KBr, cm ): 3445 (OH), 2925, 2854 (aliphatic
.3.4. N-(o-hydroxybenzylidene)-N -(4-n-heptyloxybenzoyloxybenzylidene)azine,
1
ꢀ
2
8
30
2
4
1
CꢀH), 1727 (C=O ester), 1681 (C=N), 1613 (Ph), 1259 (OPh), 1018 (NꢀN). H
NMR 300 MHz (CDCl , TMS) d (ppm): 11.37 (s, 1 H, ꢀOH), 8.71 (s, 1 H,
3
ꢀ
C H OHCH=N), 8.67 (s, 1 H, ROC H CH=N), 8.15–6.90 (m, 12 H, ArH),
6 4 6 4
4
.04–3.99 (t, 2 H, ꢀOCH ), 1.85–1.32 (m, 10 H, ꢀ[CH ] ), 0.90–0.89 (t, 3 H,
2
2 n
1
3 c 3
3
ꢀ
CH ). C NMR d (CDCl ): 164.6 (COO ester), 163.6 (CH=N), 161.8, 159.7
(
CꢀOH), 153.3, 133.4, 132.5 (ArꢀC), 132.3, 131.5, 129.7, 122.2, 121.5, 119.7,
1
3
3
6
17.2, 117.1, 114.3, 114.1, 77.4, 77.0, 76.5 (CDCl , 3 peaks), 68.3 (ꢀOCH ), 68.2,
3
2
1.7, 29.0, 25.9 (OCH CH ), 22.5, 14.0 (CH ). UV-visible (CHCl ): k
¼ 451,
2
2
3
3
max
76, 304, 269 nm. Elemental analyses: calculated (for C H N O ), C, 73.34; H,
.59; N, 6.10%, found, C, 73.02; H, 6.34; N, 6.02%.
2
8
30
2
4
0
2.3.5. N-(o-hydroxbenzylidene)-N -(4-n-octyloxybenzoyloxybenzylidene)azine,
C H N O (6b). Yield: 60%. IR (KBr, cm ): 3446 (OH), 2927, 2857 (aliphatic
ꢀ1
2
9
32
2
4
1
CꢀH), 1728 (CꢀO ester), 1682 (C=N), 1616 (Ph), 1263 (OPh), 1017 (NꢀN). H
NMR 300 MHz (CDCl , TMS) d (ppm): 11.37 (s, 1 H, ꢀOH), 8.71 (s, 1 H,
3
ꢀ
C H OHCH=N), 8.67 (s, 1 H, ROC H CH=N), 8.15–6.90 (m, 12 H, ArH),
6 4 6 4
4
.04–3.99 (t, 2 H, ꢀOCH ), 1.82–1.29 (m, 12 H, ꢀ[CH ] ), 0.89 (t, 3 H, ꢀCH ).
C 3
2
2 n
3
1
3
C NMR d (CDC1 ): 164.6 (COO ester), 163.6 (CH=N), 161.1, 159.7 (CꢀOH),
1
53.3, 133.4, 132.5 (ArꢀC), 132.3, 132.2, 131.4, 129.7, 122.2, 121.1, 119.7, 117.2,
17.1, 114.3, 114.1, 113.9, 77.4, 77.0, 76.5 (CDC1 , 3 peaks), 68.3 (ꢀOCH ), 68.2,
1
3
2

Liquid Crystalline Azines
151
3
1.7, 29.3, 29.1, 29.0, 25.9 (OCH CH ), 22.6, 14.0 (CH ). UV-visible (CHC1 ):
2
2
3
3
kmax ¼ 446, 307, 269 nm. Elemental analyses: calculated (for C H N O ), C,
29
32
2
4
7
3.70; H, 6.82; N, 5.92%, found, C, 73.82; H, 6.56; N, 5.83%.
0
2
C H N O (6c). Yield: 62%. IR (KBr, cm ): 3449 (OH), 2919, 2851 (aliphatic
.3.6. N-(o-hydroxybenzylidene)-N -(4-n-nonyloxybenzoyloxybenzylidene)azine,
ꢀ
1
3
0
34
2
4
1
CꢀH), 1736 (C=O ester), 1680 (C=N), 1606 (Ph), 1263 (OPh), 1017 (NꢀN). H
NMR 300 MHz (CDC1 , TMS) d (ppm): 11.37 (s, 1 H, ꢀOH), 8.71 (s, 1 H,
3
ꢀ
C H OHCH=N) 8.67 (s, 1 H, ROC H CH=N), 8.15–6.90 (m, 12 H, ArH),
6 4 6 4
4
.04–3.99 (t, 2 H, ꢀOCH ), 1.82–1.29 (m, 14 H, ꢀ[CH ] ), 0.89 (t, 3 H, ꢀCH ).
2
2 n
3
1
3
C NMR d (CDC1 ): 164.6 (COO ester), 163.6 (CH=N), 161.1, 159.7 (CꢀOH),
C
3
1
1
3
53.3, 133.4, 132.5 (ArꢀC), 132.3, 132.2, 131.4, 129.7, 122.2, 121.1, 119.7, 117.2,
17.1, 114.3, 114.1, 113.9, 77.4, 77.0, 76.5 (CDC1 , 3 peaks), 68.3 (ꢀOCH ), 68.2,
3
2
1.7, 29.3, 29.1, 29.0, 25.9 (OCH CH ), 22.6, 14.0 (CH ). UV-visible (CHC1 ):
2
2
3
3
kmax ¼ 444, 373, 309, 268 nm. Elemental analyses: calculated (for C H N O ), C,
30
34
2
4
7
4.04; H, 7.04; N, 5.75%, found, C, 74.12; H, 7.13; N, 5.36%.
0
2
C H N O (6d). Yield: 59%. IR (KBr, cm ): 3445 (OH), 2923, 2854 (aliphatic
.3.7. N-(o-hydroxybenzylidene)-N -(4-n-decyloxybenzoyloxybenzylidene)azine,
ꢀ
1
3
1
36
2
4
1
CꢀH), 1728 (C ¼ O ester), 1680 (C=N), 1614 (Ph), 1261 (OPh), 1019 (NꢀN). H
NMR 300 MHz (CDC1 , TMS) d (ppm): 11.37 (s, 1 H, ꢀOH), 8.71 (s, 1 H,
3
H
ꢀ
C H OHCH=N), 8.67 (s, 1 H, ROC H CH=N), 8.15–6.96 (m, 12 H, ArH),
6 4 6 4
4
.07–4.02 (t, 2 H, ꢀOCH ), 1.84–1.28 (m, 16 H, ꢀ[CH ] ), 0.88 (t, 3 H, ꢀCH ).
2
2 n
3
UV-visible (CHC1 ): k
¼ 347, 305, 273, 263 nm. Elemental analyses: calculated
3
max
(
for C H N O ), C, 74.37; H, 7.24; N, 5.59%, found, C, 74.02; H, 6.92; N, 5.12%.
31 36 2 4
0
2
C H N O (6e). Yield: 70%. IR (KBr, cm ): 3354 (OH), 2922, 2853 (aliphatic
.3.8. N-(o-hydroxybenzylidene)-N (4-n-undecyloxybenzoyloxybenzylidene)azine,
ꢀ
1
3
2
38 24
4
1
CꢀH), 1727 (C=O ester), 1678 (C=N), 1605 (Ph), 1260 (OPh), 1019 (NꢀN). H
NMR 300 MHz (CDC1 , TMS) d (ppm): 11.39 (s, 1 H, ꢀOH), 8.71 (s, 1 H,
3
H
ꢀ
C H OHCH=N), 8.67 (s, 1 H, ROC H CH=N), 8.15–6.90 (m, 12 H, ArH),
6 4 6 4
4
.04–4.00 (t, 2 H, ꢀOCH ), 1.83–1.27 (m, 18 H, ꢀ[CH ] ), 0.89 (t, 3 H, ꢀCH ).
2
2 n
3
UV-visible (CHCl ): k
¼ 451, 379, 270 nm. Elemental analyses: calculated (for
3
max
C H N O ), C, 74.68; H, 7.44; N, 5.44%, found, C, 74.52; H, 7.32; N, 5.21%.
3
2
38
2
4
0
.3.9. N-(o-hydroxybenzylidene)-N -(4-n-dodecyloxybenzoyloxybenzylidene)azine,
2
C H N O (6f). Yield: 66%. IR (KBr, cm ): 3451 (OH), 2921, 2852 (aliphatic
ꢀ1
3
3
40
2
4
1
CꢀH), 1727 (C=O, ester), 1679 (C=N), 1615 (Ph), 1261 (OPh), 1021 (NꢀN). H
NMR 300 MHz (CDC1 , TMS) d (ppm): 11.37 (s, 1 H, ꢀOH), 8.71 (s, 1 H,
3
H
ꢀ
C H OHCH=N), 8.67 (s, 1 H, ROC H CH=N), 8.15–6.90 (m, 12 H, ArH),
6 4 6 4
4
.04–3.99 (t, 2 H, ꢀOCH ), 1.82–1.29 (m, 20 H, ꢀ[CH ] ), 0.89 (t, 3 H, ꢀCH ).
2
2 n
3
1
3
C NMR dC (CDC1 ): 164.6 (COO), 163.6 (CH=N), 161.8, 159.7 (CꢀOH),
3
1
1
2
53.2, 133.2, 132.1 (ArꢀC), 132.3, 131.4, 129.6, 122.2, 121.5, 119.7, 117.2, 117.1,
14.3, 114.1, 77.4, 77.0, 76.5 (CDC1 , 3 peaks), 68.3 (ꢀOCH ), 68.2, 31.7, 29.0,
3
2
5.9 (ꢀOCH CH ), 22.5, 14.0 (CH ). UV-visible (CHC1 ): k
¼ 331, 304 nm.
2
2
3
3
max
Elemental analyses: calculated (for C H N O ), C, 74.97; H, 7.62; N, 5.29%,
3
3
40
2
4
found, C, 74.59; H, 7.81; N, 5.01%.
0
.3.10. N-(o-hydroxybenzylidene)-N -(4-n-tetradecyloxybenzoyloxybenzylidene)azine,
2
C H N O (6g). Yield: 64%. IR (KBr, cm ): 3428 (OH), 2923, 2856 (aliphatic
ꢀ1
3
5
44
2
4
1
CꢀH), 1726 (C ¼ O ester), 1682 (C=N), 1610 (Ph), 1270 (OPh), 1011 (NꢀN). H

1
52
B. Singh and A. Pandey
NMR 300 MHz (CDC1 , TMS) d (ppm): 11.39 (s, 1 H, ꢀOH), 8.71 (s, 1H,
3
H
ꢀ
C H OHCH=N) 8.67 (s, 1 H, ROC H CH=N), 8.15–6.90 (m, 12 H, ArH),
6 4 6 4
4
.04–3.99 (t, 2 H, ꢀOCH ), 1.82–1.29 (m, 24 H, ꢀ[CH ] ), 0.89 (t, 3 H, ꢀCH ).
2
2 n
3
UV-visible (CHCl ): k
¼ 439, 314, 270 nm. Elemental analyses: calculated (for
3
max
C H N O ), C, 75.50; H, 7.96; N, 5.03%, found, C, 75.91; H, 7.91; N, 5.24%.
3
5
44
2
4
0
.3.11. N-(o-hydroxybenzylidene)-N -(4-n-hexadecyloxybenzoyloxybenzylidene)azine,
2
C H N O (6h). Yield: 61%. IR (KBr, cm ): 3446 (OH), 2920, 2851 (aliphatic
ꢀ1
3
7
48
2
4
1
CꢀH), 1729 (C=O, ester), 1680 (C=N), 1615 (Ph), 1261 (OPh), 1027 (NꢀN). H
NMR 300 MHz (CDCl , TMS) d (ppm): 11.38 (s, 1 H, ꢀOH), 8.71 (s, 1 H,
3
ꢀ
C H OHCH=N) 8.67 (s, 1 H, ROC H CH=N), 8.15–6.90 (m, 12 H, ArH),
6 4 6 4
4
.04–3.99 (t, 2 H, ꢀOCH ), 1.80–1.26 (m, 28 H, ꢀ[CH ] ), 0.87 (t, 3 H, ꢀCH ).
2
2 n
3
UV-visible (CHCl ): k
¼ 445, 307, 271 nm. Elemental analyses: calculated (for
3
max
C H N O ), C, 75.99; H, 8.27; N, 4.79%, found, C, 75.82; H, 8.50; N, 4.50%.
3
7
48
2
4
3
. Results and Discussion
0
The compounds N-(o-hydroxybenzylidene)-N -(4-n-alkoxybenzoyloxybenzylidene)-
azines 6a–h have been prepared as shown in Scheme 1. The elemental data are in
agreement with the stoichiometry of the compound. The IR and NMR spectral data
are fully consistent with the structure. In the IR spectrum of salicylhydrazone the
ꢀ
1
bands appearing at 3481, 3383, 3288, 1617, 1576, and 1028 cm are attributed to
v(OH), v (NH ), v (NH ), v(Ph), and (NꢀN) modes, respectively. The IR spectrum
as
2
s
2
0
of 4-(4 -heptyloxybenzoyloxy)benzaldehyde show absorption bands at 2924, 2854,
ꢀ
1
1
776, 1712, 1605, 1511, 1307, 1258 cm assigned to v (aliphatic CꢀH stretch)
v(C=O of phenyl ester), v(C=O of aldehyde), v(Ph), and v(OPh), respectively. The
0
IR spectrum of N-(o-hydroxybenzylidene)-N -(4-n-heptyloxybenzoyloxybenzyli-
dene)-azine 6a shows absorption bands at 3445, 2925, 2854, 1727, 1681, 1613,
259, 1018 cm which are assigned to v(OH), v(aliphatic CꢀH), v(C=O) of phenyl
ester, v(C=N), v(Ph), v(OPh), and v(NꢀN) modes, respectively. The disappearance
ꢀ1
of the band 1712 cm due to v(CH=O) and appearance of a new band at 1681 cm
ꢀ1
1
ꢀ1
due to v(C=N) indicates condensation of aldehyde with amine forming Schiff base.
The proton NMR of salicylhydrazone depicts peaks at d 11.03 (s), 7.87 (s), 7.25–6.83
(
m), 5.42 (s) ppm which are attributed to ꢀOH, (CH=N), ring, and NH . The proton
2
0
NMR of 4-(4 -heptyloxybenzoyloxy)benzaldehyde exhibit peaks d 10.02 (s),
8
.15–6.88 (m), 4.07–3.97 (m), 1.81–1.27 (m), and 0.90–0.86 (t) ppm which are
assigned to ꢀOH, ring, ꢀOCH , ꢀ[CH ] , and ꢀCH protons, respectively. The pro-
2
2 n
3
0
ton NMR spectrum of N-(o-hydroxybenzylidene)-N -(4-n-heptyloxybenzoyloxy-
benzylidene)azine 6a exhibits peaks at d 11.37 (s, 1 H, ꢀOH), 8.71 (s, 1 H,
ꢀ
C H OHCH=N), 8.67 (s, 1 H, ROC H CH=N), 8.15–6.90 (m, 12 H, ring),
6 4 6 4
4
CH ) ppm, respectively. The signals due to Schiff base formation is observed at 8.71
.04–3.99 (t, 2 H, ꢀOCH ), 1.85–1.32 (m, 10 H, ꢀ[CH ] ), and 0.90–0.89 (t, 3 H,
2
2 n
3
and 8.67 ppm which correspond to o-hydroxy benzylidene and p-alkoxybenzylidene
groups, respectively. The signals of three rings overlap and are observed at d
8
.15–6.90. The length of alkyl chain has no significant effect on the position of the
signals due to ꢀOH, ꢀCH=N, ring, ꢀOCH , ꢀ[CH ] , and ꢀCH protons, respect-
2
2 n
3
0
ively. The UV-visible spectrum of N-(o-hydroxybenzylidene)-N -(4-n-heptyloxy-
benzoyloxybenzylidene)azine 6a shows bands at 451, 376, 304, 269 nm which are
ꢂ
ꢂ
assigned as the transitions due to n ! p and p ! p transitions of the ligand moiety.

Liquid Crystalline Azines
153
0
Scheme 1. Synthetic route for N-(o-hydroxybenzylidene)-N -(4-n-alkoxy benzoyloxybenzyli-
dene)azines.
4
. Optical Properties
0
The liquid crystalline character of N-(o-hydroxybenzylidene)-N -(4-n-alkoxyben-
zoyloxybenzylidene)azines 6a–h were investigated by polarizing optical microscope
(POM) observation using a heating and cooling stage. The phase transition tempera-
tures and enthalpies were measured by carrying out DSC thermal analysis. The
phase transition temperatures along with the corresponding enthalpy values for
the compounds are summarized in Table 1. The symbol K, N, and I are used to
denote crystalline, nematic, and isotropic phases, respectively. The compound
0
N-(o-hydroxybenzylidene)-N -(4-n-heptyloxybenzoyloxybenzylidene)azine 6a exhibits
ꢀ1
ꢃ
ꢃ
ꢀ1
endothermic peaks at 72.6 C (DH ¼ 22.28 kJmol ), 104.6 C (DH ¼ 0.07 kJmol ),
ꢃ
ꢀ1
ꢃ
ꢀ1
1
crystal transitions while the fourth peak refers crystal to mesophase transition
45.4 C (DH ¼ 0.07 kJmol ), 160.5 C (DH ¼ 3.27 kJmol ) due to crystal to

1
54
B. Singh and A. Pandey
Table 1. Transition temperatures, transition enthalpy (DH), and transition entropy
0
(
DS) of N-(o-hydroxybenzylidene)-N -(4-n-alkoxy benzoyloxybenzylidene)azines
ꢃ
ꢀ1
ꢀ1
ꢀ1
Compound
Transitions
T= C
DH=kJmol
DS=JK mol
1
K –K
2
3
4
6
a
72.6
104.6
145.4
160.5
192.0
150.7
109.7
101.2
49.2
72.0
85.5
111.1
121.1
109.6
81.9
59.8
64.6
220.0
93.7
41.6
53.6
56.6
64.4
69.8
22.28
0.07
0.07
3.27
0.05
0.12
0.13
15.73
4.40
7.18
4.88
1.47
2.34
0.07
12.95
6.24
0.09
25.20
0.11
1.69
0.04
0.06
0.06
0.23
1.32
0.05
6.72
1.73
0.20
0.10
10.39
2.79
14.61
2.05
4.99
0.81
0.26
0.23
0.45
0.59
0.83
19.41
0.92
13.43
64.46
0.18
0.16
7.54
0.10
0.28
0.33
42.03
13.65
20.81
13.61
3.82
5.93
0.18
36.48
18.75
0.26
51.11
0.30
5.37
0.12
0.18
0.17
0.67
3.73
0.13
17.11
4.18
0.43
0.26
28.17
8.32
45.45
5.94
13.71
2.14
0.62
0.49
0.96
1.39
2.16
56.32
2.72
36.75
2
K –K
3
K –K
4
K –N
N–I
I–N
N–K
4
4
K –K
1
K –K
2
3
4
5
6
b
2
K –K
3
K –K
4
K –K
5
K –I
I–N
N–K
5
2
5
K –K
1
K –K
6
6
c
2
K –I
I–K
1
K –K
2
3
4
5
6
7
8
9
d
2
K –K
3
K –K
4
K –K
5
K –K
6
K –K
80.8
7
K –K
104.4
119.7
139.9
183.6
109.6
95.8
62.1
48.4
71.8
90.9
104.9
145.9
187.1
191.9
150.5
109.5
71.6
8
K –K
9
K –N
N–I
I–N
N–K
9
9
K –K
1
K –K
2
3
4
5
6
6
e
2
K –K
3
K –K
4
K –K
5
K –K
6
K – K
7
7
K –I
I–N
N–K
7
7
K – K
1
K –K
2
6
f
64.5
92.4
2
K –K
3
(Continued)

Liquid Crystalline Azines
155
Table 1. Continued
Compound Transitions
ꢃ
ꢀ1
ꢀ1
ꢀ1
T= C
DH=kJmol
DS=JK mol
3
K –K
4
5
6
7
104.6
111.3
164.4
187.1
192.0
150.0
109.4
98.1
83.9
51.8
64.5
78.2
104.6
192.0
150.6
109.5
87.3
0.05
1.64
7.55
0.07
0.15
0.27
0.39
17.76
0.13
14.92
0.06
23.85
2.60
0.03
0.08
0.13
0.03
16.29
1.80
4.64
9.20
0.12
0.20
0.35
18.71
2.71
0.13
4.26
17.26
0.15
0.32
0.63
1.01
47.85
0.36
45.93
0.17
67.91
6.88
0.06
0.18
0.33
0.08
46.43
4.94
11.74
20.74
0.25
0.47
0.91
50.34
8.07
4
K –K
5
K –K
6
K –K
7
K –I
7
6
5
4
I–K
7
K –K
6
K –K
5
K –K
4
K –K
1
K –K
2
3
6
6
g
h
2
K –K
3
K –N
N–I
I–N
N–K
3
3
K –K
1
K –K
2
3
4
5
77.8
90.9
2
K –K
3
K –K
121.9
170.4
192.0
150.7
109.7
98.6
4
K –K
5
K –I
5
4
3
I–K
5
K –K
4
K –K
3
K –K
62.5
ꢃ
and the one observed at 192.0 C is assigned as mesophase to isotropic liquid
ꢃ
ꢀ1
)
transition. The exothermic peaks were observed at 150.7 C (DH ¼ 0.12 kJmol
ꢃ
ꢀ1
ꢃ
ꢀ1
and 109.7 C (DH ¼ 0.13 kJmol ) and 101.2 C (DH ¼ 15.73 kJmol ) which corre-
sponds to isotropic to mesophase and mesophase to crystal followed by the next
crystal phase. The texture of the mesophase observed under polarizing microscope
is characteristic of enantiotropic nematic mesophase appearing in both heating
and cooling cycle. The nematic droplets that are formed during transition from iso-
tropic phase to nematic mesophase exhibits a characteristic texture depending on
the droplet size and the director orientation at the isotropic nematic interface. The tex-
ture for the flat compound 6a between crossed polarizer resulting from surface orien-
tation of the director of the boundaries of the sample and by defects formed in the
sample shows a network of black brushes connecting center of points and line defects
giving rise to disclination lines which are defect in molecular orientational order.
These disclination lines are producing a schlieren optical texture in nearly vertical
orientation whose projection are seen as dark points with 2–4 emerging dark stripes
or brushes. These black brushes are located in region where the director lies in the
plane of polarization of the polarizer or analyzer. Similar phase transitions and

1
56
B. Singh and A. Pandey
enantiotropic nematic mesophase in both heating and cooling cycle showing similar
textural patterns are observed for the compounds 6d and 6g, respectively. The com-
0
pound N-(o-hydroxybenzylidene)-N -(4-n-octyloxybenzoyloxybenzylidene)azine 6b
ꢀ1
ꢃ
ꢃ
exhibits sharp endothermic peaks at 49.2 C (DH ¼ 4.40 kJmol ), 72.0 C
ꢀ1
ꢃ
ꢀ1
ꢃ
ꢀ1
(
DH ¼ 7.18 kJmol ), 85.5 C (DH ¼ 4.88 kJmol ), 111.1 C (DH ¼ 1.47 kJmol
)
corresponding to subsequent crystal to crystal phase transitions (i.e., polymorphism)
ꢃ
and the one at 121.1 C exhibit crystal to isotropic liquid phase transition. The exo-
ꢀ1
ꢃ
ꢃ
ꢀ1
thermic peaks at 109.6 C (DH ¼ 0.07 kJmol ), 81.9 C (DH ¼ 12.95 kJmol ), and
ꢃ
ꢀ1
5
9.8 C (DH ¼ 6.24 kJmol ) show isotropic to mesophase and mesophase to crystal
phase transitions which on the basis of optical microscope displays monotropic
nematic mesophase with schlieren texture with planar alignment showing defect
centers with 2–4 emerging brushes. This compound under crossed polarizer shows a
nucleus which is the center of a points or line defect from which black brushes
originate. A nucleus can indicate either the disclination line terminating at the surface
of a sample or an isolated defect. Similar phase transitions and monotropic nematic
phase with schlieren texture are seen for compound 6e.
ꢃ
ꢀ1
The compound 6f exhibits endothermic peaks at 64.5 C (DH ¼ 0.92 kJmol ),
ꢃ
ꢀ1
ꢃ
ꢀ1
ꢃ
9
1
2.4 C (DH ¼ 13.43 k Jmol ), 104.6 C (DH ¼ 0.05 kJmol ), 111.3 C (DH ¼
ꢀ ꢀ1 ꢀ1
1
ꢃ
ꢃ
.64 kJmol ), 164.4 C (DH ¼ 7.55 kJmol ), 187.1 C (DH ¼ 0.07 kJmol ), corre-
ꢃ
sponding to subsequent crystal to crystal phase transitions and the one at 192.0 C
is assigned to crystal to isotropic phase transition. The exothermic peaks for this
ꢃ
ꢀ1
ꢃ
compound are exhibited at 150.0 C (DH ¼ 0.27 kJmol ), 109.4 C (DH ¼
ꢀ1
ꢃ
ꢀ1
ꢃ
ꢀ1
0
5
.39 kJmol ), 98.1 C (DH ¼ 17.76 kJmol ), 83.9 C (DH ¼ 0.13 kJmol ), and
ꢀ1
ꢃ
1.8 C (DH ¼ 14.92 kJmol ), respectively, which are assigned as isotropic to crystal
followed by subsequent crystal to crystal phase transition with no existence of meso-
phase in both the cycles. This compound exhibits polymorphic behavior (crystal to
crystal phase transition) in both the cycle, and no mesophase was thus identified on
the basis of DSC measurements and optical microscopy. Similar phenomenon of
polymorphism showing subsequent crystal to crystal phase transition both in heating
and cooling cycle along with similar optical behavior was recorded for compound
0
6
h. The compound N-(o-hydroxybenzylidene)-N -(4-n-nonyloxybenzoyloxybenzyl-
ꢀ1
ꢃ
idene)azine 6c exhibited two endothermic peaks at 64.6 C (DH ¼ 0.09 kJmol )
and 220 C pertaining to crystal to crystal phase transition and crystal to isotropic
ꢃ
ꢃ
phase respectively followed by single exothermic peaks at 93.7 C (DH ¼
ꢀ1
0
.11 kJmol ) which defines a sudden phase transition from isotropic to crystal
phase. However, the compound failed to show optical texture as viewed under cross
polaroids. The thermotropic nematic mesophase pattern based on the structural cor-
relation of these compounds reveals a calamitic geometry for all the homologues of
the series. The presence of ester linker moiety between the two phenyl groups plays a
major role for the emergence of nematic mesophase in these series of compounds. It
is known that the presence of ester bridging group increases flexibility and mobility
of particular parts of liquid crystal molecule and hence affects the type of mesophase
and its temperature range [26]. The study based on unsymmetrical azines with ether
bridging moiety has revealed to the emergence of nematic and smectic A mesophases
[
27]. The substitution of ether linkage with the ester moiety along with the incorpor-
ation of aromatic ring with long alkoxy chain results in suppression of smectic A
mesophase and leads to the occurrence of nematic phase in some homologs of the
series. Thus, it may be concluded that by the replacement of –OR with –OOC–
C H –OR, the orderness of the system decreases.
6
4

Liquid Crystalline Azines
157
5
. Conclusions
0
new series of unsymmetrical N-(o-hydroxybenzylidene)-N -(4-n-alkoxy
benzoyloxybenzylidene)azines have been synthesized having terminal alkoxy chains
A
(
m ¼ 7–16). The chemical structures of the final products were investigated by
1
13
FT-IR, NMR, UV-visible, H, and C NMR spectra. The mesomorphic properties
and optical textures of the products were characterized by DSC and POM. The exis-
tence of nematic mesophase (schlieren texture) was confirmed by POM. It was found
that the bridging ester moiety between the aromatic rings with long alkoxy chain
leads to the occurrence of nematic phase as the replacement of –OR by –OOC–
C H –OR decreases the ordernes of the system.
6
4
Acknowledgement
The authors thank the Head, Department of Chemistry, BHU, Varanasi for
providing laboratory facilities. One of the authors (AP) thanks BHU for research
scholarship. Thanks are also due to Mr. Satish Tiwari for CHN and DSC analyses,
Mr. R. C. P. Bipin for NMR and Mr. V. N. Pandey for IR and electronic spectral
data. We are grateful to Dr. R. Dhar, Ewing Christian College, Allahabad for
suggestions regarding the texture analysis.
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