Synthesis, characterization and mesomorphic properties of unsymmetrical N-[4-(alkoxy)-2-hydroxybenzylidene]-N-(4-alkoxybenzylidene) azines and their copper (II) complexes

Article abstract of DOI:10.1080/15421406.2011.607323

A new series of mesogenic unsymmetrical azines, N-[4-(alkoxy)-2- hydroxybenzylidene]-N-(4-n-alkoxybenzylidene) azines, ROC₆H ₄CHNNCHC₆H₃OHOR, where RC_mH2 _m+1 (m = 6, 8, 10, 12, and 14), and their copper (II) complexes has been synthesized. They were characterized by elemental analyses, Fourier transform infrared (FT-IR), ¹H and ¹³C nuclear magnetic resonance (NMR) spectra, ultraviolet (UV)-visible, and magnetic susceptibility measurements. The copper (II) complexes are square planar. The mesomorphic properties of these compounds were investigated by differential scanning calorimetry and polarizing microscopy. The unsymmetrical azines with m = 6, 12, and 14 exhibit monotropic nematic, enantiotropic, and monotropic smectic A phases, whereas azines with m = 8 display enantiotropic nematic and m = 10 display both enantiotropic smectic A and nematic mesophases in the heating and cooling cycles. The copper (II) complexes of the azines are nonmesomorphic in nature with high clearing temperatures and displayed polymorphism (crystal-crystal transitions) in both the heating and cooling cycles.

Full text of DOI:10.1080/15421406.2011.607323

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Synthesis, Characterization
and Mesomorphic Properties of
Unsymmetrical N-[4-(Alkoxy)-2-
Hydroxybenzylidene]-N′-(4-
Alkoxybenzylidene) Azines and Their
Copper (II) Complexes
Ashwini Pandey a & Bachcha Singh a
a Department of Chemistry (Centre of Advanced Study), Faculty of
Science, Banaras Hindu University, Varanasi, Uttar Pradesh, India
Published online: 27 Dec 2011.
To cite this article: Ashwini Pandey & Bachcha Singh (2012): Synthesis, Characterization
and Mesomorphic Properties of Unsymmetrical N-[4-(Alkoxy)-2-Hydroxybenzylidene]-N′-(4-
Alkoxybenzylidene) Azines and Their Copper (II) Complexes, Molecular Crystals and Liquid Crystals,
552:1, 123-133
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Mol. Cryst. Liq. Cryst., Vol. 552: pp. 123–133, 2012
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2011.607323
Synthesis, Characterization and Mesomorphic
Properties of Unsymmetrical N-[4-(Alkoxy)-2-
Hydroxybenzylidene]-Nꢀ-(4-Alkoxybenzylidene)
Azines and Their Copper (II) Complexes
ASHWINI PANDEY AND BACHCHA SINGH∗
Department of Chemistry (Centre of Advanced Study), Faculty of Science,
Banaras Hindu University, Varanasi, Uttar Pradesh, India
A new series of mesogenic unsymmetrical azines, N-[4-(alkoxy)-2-hydroxybenzylidene]-
Nꢀ-(4-n-alkoxybenzylidene) azines, ROC6H4CH
N N CHC6H3OHOR, where
R
CmH2m+1 (m = 6, 8, 10, 12, and 14), and their copper (II) complexes has been
synthesized. They were characterized by elemental analyses, Fourier transform
infrared (FT-IR), H and 13C nuclear magnetic resonance (NMR) spectra, ultraviolet
1
(UV)-visible, and magnetic susceptibility measurements. The copper (II) complexes are
square planar. The mesomorphic properties of these compounds were investigated by
differential scanning calorimetry and polarizing microscopy. The unsymmetrical azines
with m = 6, 12, and 14 exhibit monotropic nematic, enantiotropic, and monotropic
smectic A phases, whereas azines with m = 8 display enantiotropic nematic and m
= 10 display both enantiotropic smectic A and nematic mesophases in the heating
and cooling cycles. The copper (II) complexes of the azines are nonmesomorphic in
nature with high clearing temperatures and displayed polymorphism (crystal–crystal
transitions) in both the heating and cooling cycles.
Keywords Differential scanning calorimetry; enantiotropic nematic mesophase; enan-
tiotropic smectic A phase; monotropic nematic phase; polarizing optical microscopy
1. Introduction
Azine molecules possess interesting physicochemical properties. They have increased in-
terest for their potential in medical and biological applications [1,2], in the design of metal
complexes [3,4], and other materials applications [5,6].
Iwan et al. [7] reported liquid crystalline phases formed by symmetrical azines with
different terminal alkoxy semiperfluorinated end groups, which exhibit the smectic C
(SmC) phase, whereas azines with octadecyloxy end chains showed SmC and SmA phases.
Azines with 5ꢀꢀ-octyl-2,2ꢀ,5ꢀ,2ꢀꢀ-terthiophene groups exhibit only a nematic phase. Different
types of nematic and SmC textures were found and investigated by polarizing optical
microscopy. Besides liquid crystalline properties, azines also exhibit photoluminescence
and thermoluminescence properties.
∗Address correspondence to Bachcha Singh, Department of Chemistry (Centre of Advanced
Study), Faculty of Science, Banaras Hindu University, Varanasi 221005, Uttar Pradesh, India.
E-mail: bsinghbhu@rediffmail.com
123

124
A. Pandey and B. Singh
We have also reported a new series of mesogenic unsymmetrical azines [8], N-(o-
hydroxybenzylidene)-Nꢀ-(4-n-alkoxybenzoyloxybenzylidene) azines, with terminal alkoxy
chain lengths of 7–16. HOC6H4CH N N CHC6H4COC6H4OCmH2m+1 in which the
azines with m = 7, 10, and 14 exhibit an enantiotropic nematic (N) mesophase, whereas
azines with m = 8 and 11 exhibit a monotropic nematic phase. Ligands with m = 9 and
12 displayed no mesomorphic behavior. Furthermore, we have also reported [9] another
series of mesogenic unsymmetrical azines, N-(o-hydroxybenzylidene)-Nꢀ-(4-n alkoxyben-
zylidene) azines, HOC6H4CH N N CHC6H4OCmH2m+1 (m = 6–16) and their copper
(II) complexes. The mesomorphic properties of these compounds were investigated by
differential scanning calorimetry (DSC) and polarizing optical microscopy. All of the un-
symmetrical azines, except m = 7, exhibit mesomorphism. The azine with m = 8 shows
a monotropic nematic mesophase, whereas all other azines exhibit an SmA mesophase in
the cooling cycle. The copper (II) complexes of the azines show only an isotropic phase at
higher temperatures (∼180◦C) and no mesomorphism is observed.
It was thought worthwhile to synthesize a new series of liquid crystalline unsym-
metrical azines, N-[4-(alkoxy)-2-hydroxybenzylidene]-Nꢀ-(4-alkoxybenzylidene) azines,
because of their interesting liquid crystalline behavior over a broad thermal range of ne-
matic and smectic phases. The characterization details and the result of the investigations
based on DSC and polarizing optical microscopy are reported in this article.
2. Experimental Details
2.1 Materials
2,4-Dihydroxybenzaldehyde, 4-hydroxybenzaldehyde, 1-bromoalkanes, hydrazine mono-
hydrate, potassium carbonate, potassium iodide, and copper acetate monohydrate were
purchased from Aldrich Chemicals (St. Louis, MO, USA) and used as received. All other
solvents and reagents were purchased commercially and used after purification.
2.2 Techniques
Elemental analyses were performed on a CE440 Exeter Analytical CHN analyzer. In-
frared (IR) spectra were recorded on a Fourier transform infrared (FT-IR) Varian 3100
1
Excalibur series spectrophotometer in KBr pellets in the 4000–100 cm−1 region. H and
13C nuclear magnetic resonance (NMR) spectra were obtained on a JEOL FT-NMR AL
(300 MHz) spectrometer using tetramethyl silane as the internal standard. Electronic spectra
were recorded on an ultraviolet (UV)-1700 Pharma Spec.Shimadzu UV-visible spectropho-
tometer. Magnetic susceptibility measurements at room temperature were carried out on a
Cahn–Faraday balance using Hg[Co(NCS)4] calibrant, and the experimental magnetic sus-
ceptibilities were corrected for diamagnetism using Pascal’s constants. DSC thermograms
were recorded with a Mettler Toledo TC15 TA differential scanning calorimeter at the rate
of 10.0 K min−1 under a nitrogen atmosphere using spec pure grade zinc and indium as
standards by taking samples in closed-lid aluminum pans. The transition temperatures from
DSC have been determined with an accuracy of 0.1 K. The mesophase type exhibited by
the compounds was identified by visual comparison with known phase standards using an
HT 30.01 NTT 268 Lomo polarizing optical microscope fitted with a hot stage with a tem-
perature controlling accuracy of 0.1 K. The copper content in the complex was determined
using an atomic absorption spectrophotometer (ASH Corp.).

Benzylidene Azines and Their Copper Complexes
2.3 Synthesis of Compounds
125
2.3.1 Synthesis of 4-n-Alkoxybenzaldehydes (2). The synthesis of 4-n-alkoxy
benzaldehydes (2), namely, 4-hexyloxy-, 4-heptyloxy-, 4-decyloxy-, 4-dodecyloxy-, and
4-tetradecyloxybenzaldehydes was carried out as described earlier [10].
2.3.2 Synthesis of 4-Decyloxybenzhydrazone (3c). To an ice-cold ethanolic solution (10
ml) of hydrazine hydrate (0.49 g, 10 mmol), an ethanolic (10 ml) solution of 4-
decyloxybenzaldehyde (2.62 g, 10 mmol) was added dropwise with continuous stirring.
The reaction mixture was then immediately evaporated on a rotavapor to yield a white
residue. This was washed with water, dried, and recrystallized from chloroform. Yield:
70%. FT-IR (KBr, cm−1): 3381νa (N H), 3284 νS (N H), 2934, 2861 (aliphatic C H),
1632 (C N), 1601, 1505 (Ph), 1308, 1260 (OPh), 1031 (N N). 1H NMR (CDCl3, TMS)
δH (ppm): 7.87 (s, 1H, CH N), 7.84–6.88 (d, 4H, ArH), 5.42 (s, 2H, NH2), 4.06–4.01
(t, 2H, OCH2), 2.36–1.31 (m, 16H, [CH2]n), 0.88 (t, 3H, CH3).
All other members of the homologous series, namely, 4-hexyloxy-, 4-heptyloxy-,
4-dodecyloxy-, and 4-tetradecyloxy benzhydrazones, were prepared using the above pro-
cedure.
2.3.3 Synthesis of 4-n-Decyloxy-2-Hydroxybenzaldehyde (5c). To a solution of 2,4-
dihydroxybenzaldehyde (4) (0.138 g, 10 mmol) in 35 ml of ethanol, 1-bromodecane (4.4 ml,
22 mmol), K2CO3 (3.36 g, 22 mmol), and KI in catalytic amount were added. The reaction
mixture was heated under reflux for 36 h. After cooling, the reaction mixture was filtered
out. The filtrate was neutralized by dilute HCl and further 40 ml of distilled water was
added. The aqueous layer was extracted thrice with 30 ml portions of chloroform. The
chloroform extract was combined and dried over anhydrous MgSO4. On evaporation it
gave 2-hydroxy-4-(decyloxy)benzaldehyde as yellow oil in 53% yield. 1H NMR (CDCl3,
TMS) δH (ppm): 11.48 (s, 1H, OH), 9.69 (s, 1H, CHO), 7.80 (d, 1H, ArH), 6.51 (d, 1H,
ArH), 6.41 (s, 1H, ArH), 4.04 (t, 2H, OCH2), 2.21–1.13 (m, 16H, [CH2]n), 0.88 (t, 3H,
CH3).
All other members of the homologous series, namely, 4-(hexyloxy)-, 4-(heptyloxy)-,
4-(dodecyloxy)-, and 4-(tetradecyloxy)-2-hydroxybenzaldehydes, were prepared using the
above procedure.
2.3.4 Synthesis of N-[4-(Decyloxy)-2-Hydroxybenzylidene]-Nꢀ-(4-Decyloxybenzylidene)
Azine (6c). To a solution of 4-decyloxybenzhydrazone (1.31 g, 5 mmol) in chloroform
(25 ml), a solution of 4-(decyloxy)-2-hydroxybenzaldehyde (1.39 g, 5 mmol) in chloro-
form along with 2–3 drops of glacial acetic acid was added. It was stirred for 6 h at room
temperature. The yellow precipitate formed was filtered, washed with methanol (10 ml),
and dried under reduced pressure.
All other members of the homologous series were prepared in a similar manner. The
yield, IR, NMR, and elemental data for the compounds are summarized as follows.
2.3.5 N-[4-(Hexyloxy)-2-Hydroxybenzylidene]-Nꢀ-(4-Hexyloxybenzylidene) Azine (6a).
Yield: 75%. FT-IR (KBr, cm−1): 3433 (OH), 2921, 2852 (aliphatic C H), 1624 (C N),
1509, 1470 (Ph), 1307, 1250 (OPh), 1020 (N N). 1H NMR (CDCl3, TMS) δH (ppm): 11.72
(s, 1H, OH), 8.65 (s, 1H, ROC6H3OHCH N), 8.60 (s, 1H, ROC6H4CH N), 8.51–6.50
(m, 7H, ArH), 4.02–3.96 (t, 4H, OCH2), 1.82–1.27 (m, 16H, [CH2]n), 0.90–0.86 (t,
6H, CH3). 13C NMR δc (CDCl3): 163.5, 163.1 (CH N), 161.8, 161.7, 161.5, 161.1
160.6 (C OH), 133.2 (Ar C), 130.1, 130.0, 126.7, 114.7, 111.2, 107.6, 101.5, 77.4, 77.0,

126
A. Pandey and B. Singh
76.5 (CDCl3), 68.1 ( OCH2), 31.8, 29.5, 29.3, 29.1, 29.0, 25.9, 22.6, 14.1 (CH3). UV-
visible (CHCl3): λmax = 354, 330, 319, 304, 260 nm. Elemental analyses: calculated for
C26H36N2O3 (%): C, 73.55; H, 8.54; N, 6.59; found: C, 73.21; H, 8.32; N, 6.83.
2.3.6 N-[4-(Heptyloxy)-2-Hydroxybenzylidene]-Nꢀ-(4-Hexyloxybenzylidene) Azine (6b).
Yield: 72%. FT-IR (KBr, cm−1): 3432 (OH), 2921, 2852 (aliphatic C H), 1621 (C N),
1507, 1472 (Ph), 1307, 1251 (OPh), 1021 (N N). 1H NMR (CDCl3, TMS) δH (ppm): 11.74
(s, 1H, OH), 8.65 (s, 1H, ROC6H3OHCH N), 8.60 (s, 1H, ROC6H4CH N), 8.51–6.50
(m, 7H, ArH), 4.02–3.97 (t, 4H, OCH2), 1.82–1.27 (m, 20H, [CH2]n), 0.90–0.86 (t,
6H, CH3). 13C NMR δc (CDCl3): 163.5, 163.1 (CH N), 161.8, 161.7, 161.5, 161.1,
160.6 (C OH), 133.2 (Ar C), 130.1, 130.0, 126.7, 114.7, 111.2, 107.6, 101.5, 77.4, 77.0,
76.5 (CDCl3), 68.1 ( OCH2), 31.8, 29.5, 29.3, 29.1, 29.0, 25.9, 22.6, 14.1 (CH3). UV-
visible (CHCl3): λmax = 350, 338, 318, 308, 261 nm. Elemental analyses: calculated for
C28H40N2O3 (%): C, 74.30; H, 8.90; N, 6.18; found: C, 74.56; H, 8.81; N, 5.82.
2.3.7 N-[4-(Decyloxy)-2-Hydroxybenzylidene]-Nꢀ-(4-Decyloxybenzylidene) Azine (6c).
Yield: 70%. FT-IR (KBr, cm−1): 3433 (OH), 2921, 2852 (aliphatic C H), 1626 (C N),
1574, 1510 (Ph), 1301, 1250 (OPh), 1020 (N N). 1H NMR (CDCl3, TMS) δH (ppm): 11.72
(s, 1H, OH), 8.66 (s, 1H, ROC6H3OHCH N), 8.60 (s, 1H, ROC6H4CH N), 8.51–6.48
(m, 7H, ArH), 4.02–3.98 (t, 4H, OCH2), 1.84–1.34 (m, 32H, [CH2]n), 0.93–0.91 (t, 6H,
CH3). 13C NMR δc (CDCl3): 163.5, 163.1 (CH N), 161.8, 161.7, 161.5, 161.1, 160.6
(C OH), 133.2 (Ar C), 130.1, 130.0, 126.7, 114.7, 114.8, 111.2, 107.6, 101.5, 77.4, 77.0,
76.5 (CDCl3), 68.1 ( OCH2), 31.8, 29.5, 29.3, 29.1, 29.0, 25.9, 22.6, 14.1 (CH3). UV-
visible (CHCl3): λmax = 386, 356, 332, 305, 257, 222 nm. Elemental analyses: calculated
for C34H52N2O3 (%): C, 76.07; H, 9.76; N, 5.21; found: C, 75.68; H, 9.48; N, 5.68.
2.3.8 N-[4-(Dodecyloxy)-2-Hydroxybenzylidene]-Nꢀ-(4-Decyloxybenzylidene) Azine (6d).
Yield: 70%. FT-IR (KBr, cm−1): 3428 (OH), 2920, 2861 (aliphatic C H), 1628 (C N),
1572, 1508 (Ph), 1301, 1252 (OPh), 1021 (N N). 1H NMR (CDCl3, TMS) δH (ppm): 11.72
(s, 1H, OH), 8.66 (s, 1H, ROC6H3OHCH N), 8.60 (s, 1H, ROC6H4CH N), 8.51–6.48
(m, 7H, ArH), 4.02–3.97 (t, 4H, OCH2), 1.84–1.34 (m, 40H, [CH2]n), 0.93–0.86 (t, 6H,
CH3). 13C NMR δc (CDCl3): 163.5, 163.1 (CH N), 161.8, 161.7, 161.5, 161.1, 160.6
(C OH), 133.2 (Ar C), 130.0, 126.7, 114.7, 111.2, 107.6, 101.5, 77.4, 77.0, 76.5 (CDCl3),
68.1 ( OCH2), 31.8, 29.5, 29.3, 29.1, 29.0, 25.9, 22.6, 14.1 (CH3). UV-visible (CHCl3):
λmax = 353, 315, 302, 264, 205 nm. Elemental analyses: calculated for C38H60N2O3 (%):
C, 76.98; H, 10.19; N, 4.72; found: C, 76.52; H, 9.92; N, 4.39.
2.3.9 N-[4-(Tetradecyloxy)-2-Hydroxybenzylidene]-Nꢀ-(4-Decyloxybenzylidene) Azine
(6e). Yield: 71%. FT-IR (KBr, cm−1): 3428 (OH), 2922, 2852 (aliphatic C H), 1627
1
(C N), 1574, 1509 (Ph), 1301, 1252 (OPh), 1020 (N N). H NMR (CDCl3, TMS) δH
(ppm): 11.72 (s, 1H, OH), 8.66 (s, 1H, ROC6H3OHCH N), 8.60 (s, 1H, ROC6H4CH N),
8.51–6.48 (m, 7H, ArH), 4.02–3.98 (t, 4H, OCH2), 1.84–1.34 (m, 48H, [CH2]n),
0.93–0.86 (t, 6H, CH3). 13C NMR δc (CDCl3): 163.5, 163.1 (CH N), 161.8, 161.5,
161.1, 160.6 (C OH), 133.2 (Ar C), 130.0, 126.7, 114.7, 111.2, 107.6, 101.5, 77.4, 77.0,
76.5 (CDCl3), 68.1 ( OCH2), 31.8, 29.5, 29.3, 29.1, 29.0, 25.9, 22.6, 14.1 (CH3). UV-
visible (CHCl3): λmax = 353, 321, 279, 263, 208 nm. Elemental analyses: calculated for
C42H68N2O3 (%): C, 77.72; H, 10.56; N, 4.31; found: C, 77.49; H, 10.32; N, 4.24.

Benzylidene Azines and Their Copper Complexes
127
3. Synthesis of Copper (II) Complex of N-[4-(Decyloxy)-2-
Hydroxybenzylidene]-Nꢀ-(4-Decyloxybenzylidene) Azine (7c)
To a solution of N-[4-(decyloxy)-2-hydroxybenzylidene]-Nꢀ-(4-decyloxybenzylidene)
azine (6c) (2.14 g, 4 mmol) in chloroform (15 ml), a solution of copper (II) acetate
(0.39 g, 2 mmol) in methanol was added and a brown precipitate was formed. The reaction
mixture was stirred at room temperature for 1 h to complete the reaction. The precipitate
was filtered off and washed with chloroform and methanol, respectively. The copper (II)
complexes of all azines were prepared following the above procedure. These compounds
were characterized by IR, NMR and UV-visible spectroscopy. The magnetic moment and
analytical data are summarized below.
3.1 Bis-N-[4-(Hexyloxy)-2-Hydroxybenzylidene]-Nꢀ-(Hexyloxybenzylidene) Azine
Copper (II) Complex, C52H70N4O6Cu (7a)
Brown, yield: 65%. FT-IR (KBr, cm−1): 2928, 2854 (aliphatic C H), 1609 (C N), 1573,
1511 (Ph), 1245 (OPh), 1024 (N N). 1H NMR (CDCl3, TMS) δH (ppm): 8.60 (s, 4H,
CH N), 7.77–6.93 (m, 14H, ArH), 4.02–4.00 (t, 8H, OCH2), 1.82–1.27 (m, 32H,
[CH2]n), 0.90–0.86 (t, 12H, CH3). UV-visible (Nujol): λmax = 352, 326, 310, 298,
284, 268, 252, 210 nm. µeff = 1.84 BM. Elemental analyses: calculated for C52H70N4O6Cu
(%): C, 68.58; H, 7.74; N, 6.15; Cu, 6.97; found: C, 68.24; H, 7.38; N, 5.72; Cu, 6.71.
3.1.1 Bis-N-[4-(Heptyloxy)-2-Hydroxybenzylidene]-Nꢀ-(Heptyloxybenzylidene)
Azine
Copper (II) Complex, C56H78N4O6Cu (7b). Brown, yield: 62%. FT-IR (KBr, cm−1): 2921,
2859 (aliphatic C H), 1610 (C N), 1578, 1510 (Ph), 1249 (OPh), 1022 (N N). 1H NMR
(CDCl3, TMS) δH (ppm): 8.60 (s, 4H, CH N), 7.77–6.93 (m, 14H, ArH), 4.02–3.98
(t, 8H, OCH2), 1.82–1.27 (m, 40H, [CH2]n), 0.90–0.86 (t, 12H, CH3). UV-visible
(Nujol): λmax = 378, 346, 321, 322, 297, 272, 252, 207 nm. µeff = 1.82 BM. Elemental
analyses: calculated for C56H78N4O6Cu (%): C, 69.57; H, 8.13; N, 5.79; Cu, 6.97; found:
C, 69.81; H, 7.92; N, 5.38; Cu, 6.24.
3.1.2 Bis-N-[4-(Decyloxy)-2-Hydroxybenzylidene]-Nꢀ-(Decyloxybenzylidene) Azine Cop-
per (II) Complex, C68H102N4O6Cu (7c). Brown, yield: 67%. FT-IR (KBr, cm−1): 2921,
2852 (aliphatic C H), 1608 (C N), 1573, 1511 (Ph), 1305, 1251 (OPh), 1023 (N N). 1H
NMR (CDCl3, TMS) δH (ppm): 8.60 (s, 4H, CH N), 7.77–6.93 (m, 14H, ArH), 4.02–3.98
(t, 8H, OCH2), 1.82–1.27 (m, 64H, [CH2]n), 0.90–0.86 (t, 12H, CH3). UV-visible
(Nujol): λmax = 425, 346, 326, 298, 284, 268, 252, 210 nm. µeff = 1.78 BM. Elemental
analyses: calculated for C68H102N4O6Cu (%): C, 71.95; H, 9.05; N, 4.93; Cu, 5.59; found:
C, 71.56; H, 8.68; N, 5.12; Cu, 5.34.
3.1.3 Bis-N-[4-(Dodecyloxy)-2-Hydroxybenzylidene]-Nꢀ-(Dodecyloxybenzylidene) Azine
Copper (II) Complex, C76H118N4O6Cu (7d). Brown, yield: 62%. FT-IR (KBr, cm−1): 2931,
2849 (aliphatic C H), 1610 (C N), 1578, 1509 (Ph), 1302, 1250 (OPh), 1021 (N N). 1H
NMR (CDCl3, TMS) δH (ppm): 8.60 (s, 4H, CH N), 7.77–6.93 (m, 14H, ArH), 4.02–3.98
(t, 8H, OCH2), 1.82–1.27 (m, 80H, [CH2]n), 0.90–0.86 (t, 12H, CH3). UV-visible
(Nujol): λmax = 382, 356, 345, 315, 298, 268, 251, 212 nm. µeff = 1.74 BM. Elemental
analyses: calculated for C76H118N4O6Cu (%): C, 73.18; H, 9.53; N, 4.49; Cu, 5.09; found:
C, 73.54; H, 9.41; N, 3.95; Cu, 5.17.

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A. Pandey and B. Singh
3.1.4 Bis-N-[4-(Tetradecyloxy)-2-Hydroxybenzylidene]-Nꢀ-(Tetradecyloxybenzylidene)
Azine Copper (II) Complex, C84H134N4O6Cu (7e). Brown, yield: 69%. FT-IR (KBr,
cm−1): 2932, 2852 (aliphatic C H), 1614 (C N), 1574, 1512 (Ph), 1305, 1253 (OPh),
1024 (N N). 1H NMR (CDCl3, TMS) δH (ppm): 8.60 (s, 4H, CH N), 7.77–6.93 (m,
14H, ArH), 4.02–3.98 (t, 8H, OCH2), 1.82–1.27 (m, 96H, [CH2]n), 0.90–0.86 (t, 12H,
CH3). UV-visible (Nujol): λmax = 372, 342, 290, 284, 262, 250, 208 nm. µeff = 1.72
BM. Elemental analyses: calculated for C84H134N4O6Cu (%): C, 74.21; H, 9.93; N, 4.12;
Cu, 4.67; found: C, 73.51; H, 9.43; N, 3.98; Cu, 4.51.
4. Results and Discussion
The synthetic routes for the preparation of the compounds N-[4-(alkoxy)-2-hydroxy-
benzylidene]-N’-(alkoxybenzylidene) azines (6a–e) and their copper (II) complexes (7a–e)
are outlined in Scheme 1. The elemental analyses, FT-IR, and NMR spectra are fully
consistent with the structures. The IR spectrum of 4-decyloxybenzhydrazone (3c) exhibits
bands at 3381, 3284, 2934, 2861, 1632, 1601, 1505, 1308, 1260, and 1031 cm−1, which are
attributed to νa(N H), νS(N H), aliphatic C H, ν(CH N), ν(Ph), ν(OPh), and ν(N N)
modes, respectively. In the spectrum of N-[4-(decyloxy)-2-hydroxybenzylidene]-Nꢀ-(4-
decyloxybenzylidene) azine (6c), bands appearing at 3433, (2921, 2852), 1626, (1574,
1510), (1301, 1250), and 1020 cm−1 are assigned to the ν(OH), ν(aliphatic C H), ν(C N),
ν(Ph), ν(OPh), and ν(N N) modes, respectively. In the IR spectrum of the copper com-
plex of N-[4-(decyloxy)-2-hydroxybenzylidene]-Nꢀ-(4-decyloxybenzylidene) azine (7c),
the ν(C N) bands shift to a lower frequency (1608 cm−1), indicating bonding through
the azomethine nitrogen. The lowering in the frequency of ν(C N) band caused by the
coordination of the nitrogen to the metal center is in agreement with the result obtained
for similar other complexes [11–16]. The bonding through the nitrogen of the azine to the
Cu (II) ion is further supported by the shift of the ν(N N) bond to a higher position (1023
cm−1) compared with its position in the spectrum of the parent ligand (6c) at 1020 cm−1
.
The increase in frequency of the ν(N N) bond in the metal complex is due to a decrease
in repulsion between the lone pairs of electrons on both the nitrogens.
The proton NMR spectrum of N-[4-(decyloxy)-2-hydroxybenzylidene]-Nꢀ-(4-
decyloxybenzylidene) azine (6c) exhibits signals at δ 11.72 (s), 8.66 (s), 8.60 (s), 8.51–6.48
(m), 4.02–3.98 (t), 1.84–1.34 (m), and 0.93–0.91 (t), which are attributed to OH,
CH N of o-hydroxy-p-alkoxybenzylidene, CH N of p-alkoxybenzylidene, aromatic
ring, OCH2, [CH2]n, and CH3 protons, respectively. The major signals in the 13C
NMR spectrum of the compound (6c) are observed at δ 163.5, 160.6, 114.7, 68.1, 25.9, and
14.1, respectively, which are attributed to CH N, ortho OH, aromatic ring, OCH2,
OCH2CH2, and CH3 carbon, respectively.
The proton NMR spectrum of bis-N-[4-(decyloxy)-2-hydroxybenzylidene]-Nꢀ-
(decyloxybenzylidene) azine copper (II) complex (7c) reveals signals at δ 8.60,
7.73–6.93, 4.02–3.98, 1.82–1.27, and 0.90–0.86, which are attributed to CH N of para-
alkoxybenzylidene, aromatic ring, OCH2, [CH2]n, and CH3 protons, respectively.
The disappearance of the OH signal from the spectrum of the metal complex indicates
the deprotonation of the OH group and the resulting phenolate oxygen is bonded to the
metal. The bonding of one of the azomethine nitrogens in proximity to the phenoxy oxy-
gen would result in a downfield shift in the CH N signal and thus giving one signal
at higher positions than that in the free azine and one signal due to the CH N of alkoxy
azine. However, only one signal is observed at the upfield (δ 8.60). The downfield shift
in CH N near copper may be counterbalanced by the upfield shift in the signal due to

Benzylidene Azines and Their Copper Complexes
129
Scheme 1. Route for the synthesis of N-[4(alkoxy)-2-hydroxybenzylidene]-Nꢀ-(4-alkoxy
benzylidene) azines and their copper (II) complexes.
the delocalization of the unpaired d-electron in the chelate ring. Thus, only one signal is
observed due to both types of CH N protons. This phenomenon of the bonding effect of
the metal ion, and the appearance of the sharp signal in the metal complex without line
broadening, is similar to the unsymmetrical azines reported earlier and its reasoning has
also been discussed earlier [17,18]. Based on these observations and elemental data, it is

130
A. Pandey and B. Singh
concluded that the unsymmetrical azine is bonded to the metal ion as a bidentate (N, O)
ligand.
The
electronic
spectrum
of
N-[4-(decyloxy)-2-hydroxybenzylidene]-Nꢀ-
(decyloxybenzyilidene) azine (6c) displays bands at 386, 356, 332, 305, 257, and
222 nm, which are assigned to n-π∗ and π-π∗ transitions. In the spectrum of the
bis-N-[4-(decyloxy)-2-hydroxybenzylidene]-N’-(decyloxybenzylidene) azine copper (II)
complex (7c), the UV bands are observed at 425, 346, 326, 298, 284, 262, 252, and 210
nm. The first band is characteristic of square planar coordination around the metal center
[19]. The remaining bands are attributed to the π-π∗ and n-π∗ transitions of the ligand. The
copper compounds exhibit magnetic moments at 1.72–1.84 BM, suggesting the presence
of one unpaired electron.
5. Optical Properties
The liquid crystalline properties of N-[4-(alkoxy)-2-hydroxybenzylidene]-Nꢀ-(4-
alkoxybenzylidene) azines 6a–e and their copper (II) complexes were investigated by
polarizing optical microscopy using a heating and cooling stage. The phase transition
temperatures and enthalpies were measured by carrying out DSC thermal analyses. The
Cr, SmA, N, and I denote crystalline, smectic A, nematic, and isotropic phases, respec-
tively. The compound 6c exhibited endothermic peaks at 68.5◦C, 75.3◦C, 84.2◦C, 90.8◦C,
104.3◦C, 127.5◦C, 133.7◦C, 147.2◦C, and 186.7◦C as shown in Table 1. The endothermic
peaks observed are the characteristic of crystal-to-crystal phase transition (polymorphism)
with enthalpies of 0.4, 13.4, 3.3, 2.6, 9.8, 1.6, 0.8, 0.5, and 0.3 kJ mol−1, but the peaks cen-
tered at 133.7◦C (ꢀH = 0.8 kJ mol−1), 147.2◦C (ꢀH = 0.5 kJ mol−1), and 186.7◦C (ꢀH =
0.3 kJ mol−1) define crystal-to-SmA, SmA-to-nematic, and nematic-to-isotropic phase
transitions. The exothermic peak at 184.1◦C (ꢀH = 0.4 kJ mol−1) defines the isotropic-
to-nematic phase transition, which further drifts to SmA centered at 144.1◦C (ꢀH =
0.5 kJ mol−1), followed by a crystal phase at 128.3◦C (ꢀH = 1.2 kJ mol−1), and then
Table 1. Transition temperatures (T) of the compound N-[4-(alkoxy)-2-hydroxy
benzylidene]-Nꢀ-(4-alkoxybenzylidene) azine
Compounds
Transition temperatures (◦C)
6a
6b
6c
Cr1 138 I 105 N 100 Cr1
Cr 140 N 149 I 139 N 132 Cr
Cr1 68.5 (0.4)∗(1.1)∗∗ Cr2 75.3 (13.4) (38.4) Cr3 84.2 (3.3) (9.2) Cr4 90.8
(2.6) (7.1) Cr5 104.3 (9.8) (25.9) Cr6 127.5 (1.6) (4.1) Cr7 133.7 (0.8)
(3.9) SmA 147.2 (0.5) (1.1) N 186.7 (0.3) (0.6) I 184.1 (0.4) (0.8) N
144.1 (0.5) (1.1) SmA 128.3 (1.2) (2.6) Cr7 123.3 (0.9) (2.5) Cr6 104.4
(0.7) (1.9) Cr5 95.4 (13.5) (36.6) Cr4 78.2 (0.3) (0.8) Cr3 70.6 (3.7)
(10.4) Cr2 44.7 (17.0) (53.5) Cr1
6d
6e
Cr1 137 SmA I 135 SmA 115 Cr1 110 Cr
Cr1 117 Cr2 138 I 127 SmA 115 Cr1 110 Cr
Note: Cr signifies crystal phase, SmA signifies smectic A phase, N signifies nematic phase, and I
signifies isotropic liquid.
∗The value in parentheses denotes enthalpy in kJ mol−1
.
∗∗The value in parentheses denotes entropy in J K−1 mol−1
.

Benzylidene Azines and Their Copper Complexes
131
to successive polymorphic crystal phases. Thus, under optical microscopy, enantiotropic
SmA and nematic mesophases were obtained for this compound. In the heating cycle, a
transition from the crystal to the SmA phase appears at 133.7◦C, which is texturally char-
acterized by a focal-conic texture having a fluid property with higher viscosity exhibiting a
stratified boundary with well-defined interlayer spacings. Further heating at 147.2◦C shows
the appearance of two to four black brushes emerging from the central nuclei forming
disclination lines with a Schlieren texture showing the nematic mesophase. At 186.7◦C,
these nematic brushes disappear by forming bubbles and enter the isotropic phase of the
heating cycle. In the cooling cycle, nematic droplets again appear at 184.1◦C, which are
slowly transformed into the SmA phase at 144.1◦C and further drift to the crystal phase at
128.3◦C. The crystal phase exhibits a diffuse transition to the successive crystal phase with
characteristic polymorphism.
Under the optical microscope, compound 6d exhibits a fluid property with dense
viscosity and hexagonal pattern of an SmA phase at 137◦C in the heating cycle drifting to
the isotropic phase at 145◦C. In the cooling cycle at 135◦C, numerous droplets diffuse to
the crystal phase at 115◦C followed by the next sharp crystal phase transition at 110◦C. This
compound is characterized by a prominent enantiotropic SmA phase in both the heating
and cooling cycles. The prominence of the SmA phase is due to the molecular packing,
which is found to be random with additional intramolecular forces between the phenolic
OH group and the nitrogen atoms in the unsymmetrical azines. Thus, a hexagonal array
with a focal-conic texture is observed for this compound by optical microscopy, confirming
the SmA phase. Moreover, it seems that strong core–core intermolecular interactions are
present in the mesophase by means of hydrogen bonding between the hydroxyl groups.
The asymmetrical azines reported by Centore et al. show both nematic and smectic phases
[20,21].
Compound 6b shows sudden nematic droplets in the heating cycle with a Schlieren
texture at 140◦C, which becomes an isotropic phase at 149◦C. In the cooling cycle at
139.3◦C, nematic droplets (bubbles) appear with a characteristic Schlieren texture (black
stripes or black brushes) with surface defect centers (hedgehog type) and tilted alignment
as four brushes are seen arising from the central nuclei. The mesophase drifts to the crystal
phase at 132◦C. Thus, the enantiotropic nematic phase is observed for this compound with
the appearance of texture pattern in both the heating and cooling cycles.
Compound 6e shows a fast crystal-to-crystal transition under the optical microscope at
117◦C with a sharp transition to the isotropic phase at 138◦C. In the cooling cycle, a sharp
transition from the isotropic to the SmA phase, having a hexagonal array with focal-conic
domains formed by deformed smectic layers that fold around two confocal line defects
(preserving equidistance of structural layers everywhere except in the vicinity of the defect
lines), is seen at 127◦C. This transits slowly to an inconspicuous crystal phase at 120◦C and
finally to the next crystal phase at 113◦C. Finally, compound 6a exhibits no optical texture
in the heating cycle and reaches the clearing temperatures at 138◦C. In the cooling cycle,
some nematic droplets appear and immediately cause crystallization at 105◦C, followed by
the next crystal phase at 100◦C.
The compounds of the series N-[4-(alkoxy)-2-hydroxybenzylidene]-Nꢀ-(4-
alkoxybenzylidene) azines 6a–e exhibit both enantiotropic nematic and SmA mesophases
in which the nematic phase is dominated by the SmA phase in the higher homologs of
the series. Presumably, as strong core–core intermolecular interactions are present in
the mesophase by means of hydrogen bonding between the hydroxyl group or with the
nitrogen atom of the unsymmetrical azine in homologs with m = 10, 12, and 14, which
promotes the occurrence of SmA phase. Thus, with an increase in the alkoxy chain

132
A. Pandey and B. Singh
length, the van der Waals forces increase, which alters the molecular packing and adds
to the intermolecular forces between the hydroxyl group and the nitrogen atom of the
unsymmetrical azine favoring the SmA phase in the higher homologs of the series.
The metal complexes of N-[4-(alkoxy)-2-hydroxybenzylidene]-Nꢀ-(4-alkoxy
benzylidene) azines 7a–e were found to be nonmesogenic with prominent polymorphic
crystal phases exhibited in both cycles. The nonmesogenic property of the metal complexes
is likely due to the symmetrical and rigid geometry of the metal complexes along with the
stronger interaction of the potential donor (C N) with the metal ion in the neighboring
layers, resulting in packing of the molecules in the solid state. This result can be compared
with our previous work [22] on metal-combining unsymmetrical azines. Although the
basic unsymmetrical azines [22] displayed the dominant SmA phase, with a monotropic
nematic phase in the m = 8 homolog. However, in this article, where we observe
both enantiotropic nematic and smectic phases in some homologs of the series of the
unsymmetrical N-[4-(alkoxy)-2-hydroxybenzylidene]-Nꢀ-(4-alkoxybenzylidene) azines,
although in the higher homologs, the nematic phase is suppressed and the SmA phase
dominates.
6. Conclusions
A new series of mesogenic unsymmetrical azines, N-[4-(alkoxy)-2-hydroxybenzylidene]-
Nꢀ-(4-alkoxybenzylidene) azines, having terminal alkoxy chains (m = 6∼14) and their
copper (II) complexes have been synthesized. The structural elucidation of the final products
was investigated by FT-IR, UV-visible spectroscopy, 1H and 13C NMR spectra, and magnetic
susceptibility measurements. The transition enthalpy and entropy were characterized by
DSC. The existence of the nematic and SmA phases was confirmed by polarizing optical
microscopy. It was found that the unsymmetrical azines show mesomorphic properties,
whereas their copper (II) complexes were nonmesogenic in nature and showed high melting
points of ∼240◦C.
Acknowledgments
The authors thank the Head of the Department of Chemistry, Banaras Hindu University
(BHU), Varanasi, for providing laboratory facilities. Ashwini Pandey thanks the Council
of Scientific and Industrial Research (CSIR, New Delhi) for financial support as senior
research fellow at BHU, Varanasi. Thanks are also due to the staff of analytical division for
CHN analyses, and for 1H, 13C NMR, IR, and electronic spectra.
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