Synthesis, characterization, and mesomorphic investigations of calamitic liquid crystals - Methyl 4-(4′-(alkoxy)-2-hydroxybenzylideneamino) benzoates and their copper(ii) and nickel(ii) complexes

Article abstract of DOI:10.1080/15421406.2013.803872

A new series of Schiff's base mesogenic ligands with a polar methyl ester as an end group, methyl 4-(4′-(alkoxy)-2 hydroxybenzylideneamino) benzoates, C_nLH (n = 6, 8, 10, 12, 14, 16) and their copper(II) and nickel(II) complexes have been synth

Full text of DOI:10.1080/15421406.2013.803872

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Synthesis, Characterization, and
Mesomorphic Investigations of Calamitic
Liquid Crystals – methyl 4-(4′-(alkoxy)-2-
hydroxybenzylideneamino) benzoates
and Their Copper(II) and Nickel(II)
Complexes
Hemant Kumar Singh a , Sachin Kumar Singh a , Vijay Kumar a &
Bachcha Singh a
a Department of Chemistry (Center of Advance Study) , Banaras
Hindu University , Varanasi , Uttar Pradesh , India
Published online: 16 Dec 2013.
To cite this article: Hemant Kumar Singh , Sachin Kumar Singh , Vijay Kumar & Bachcha Singh (2013)
Synthesis, Characterization, and Mesomorphic Investigations of Calamitic Liquid Crystals – methyl 4-
(4′-(alkoxy)-2-hydroxybenzylideneamino) benzoates and Their Copper(II) and Nickel(II) Complexes,
Molecular Crystals and Liquid Crystals, 587:1, 1-17, DOI: 10.1080/15421406.2013.803872
To link to this article: http://dx.doi.org/10.1080/15421406.2013.803872
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Mol. Cryst. Liq. Cryst., Vol. 587: pp. 1–17, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.803872
Synthesis, Characterization, and Mesomorphic
Investigations of Calamitic Liquid Crystals – methyl
4-(4ꢀ-(alkoxy)-2-hydroxybenzylideneamino)
benzoates and Their Copper(II)
and Nickel(II) Complexes
HEMANT KUMAR SINGH, SACHIN KUMAR SINGH,
VIJAY KUMAR, AND BACHCHA SINGH∗
Department of Chemistry (Center of Advance Study), Banaras Hindu University,
Varanasi, Uttar Pradesh, India
A new series of Schiff’s base mesogenic ligands with a polar methyl ester as an end
group, methyl 4-(4ꢀ-(alkoxy)-2 hydroxybenzylideneamino) benzoates, CnLH (n = 6, 8,
10, 12, 14, 16) and their copper(II) and nickel(II) complexes have been synthesized.
They were characterized by elemental analyses, Fourier transform infrared (FT-IR),
1H and 13C nuclear magnetic resonance, and UV-VIS spectroscopy. The mesomorphic
properties of these compounds were investigated by differential scanning calorimetry
(DSC) and polarizing optical microscopy (POM). The ligands exhibit an enantiotropic
SmA mesophase with a high-temperature range and thermal stability. The square planar
copper(II) complexes show enantiotropic SmA mesophase, but the nickel(II) complexes
are nonmesogenic in nature. The thermal stability of the compounds was also determined
by thermo gravimetric analyses. Density functional theory (DFT) calculations were
performed using GAUSSIAN-03 program at B3LYP level to obtain the stable electronic
structure of the ligand and their metal complexes.
Keywords Density functional theory; differential scanning calorimetry; enantiotropic
mesophase; metallomesogen; polarizing optical microscopy
1. Introduction
Liquid crystals containing transition metals, called as metallomesogens, have gained atten-
tion because of their unusual geometries and novel properties such as color, paramagnetism,
fluidity, large birefringence, and polarizability. In addition to functional properties such as
spin cross-over, ferroelectricity, photorefractivity, nonlinear optical characteristics, large
magnetic anisotropy, and luminescent mesophases. Since the metallomesogens are achieved
through changes of molecular conformation, shape, and structure, their physico-chemical
properties can be tuned by the choice of metal ions, substituents, and position of substituents
on core moieties [1–3]. Schiff base ligands provide a wide range of ways to modify liq-
uid crystalline properties [4, 5]. The organic liquid crystalline species whose rigid core is
∗Address correspondence to Bachcha Singh, Department of Chemistry (Center of Advance
Study), Banaras Hindu University, Varanasi-221005, Uttar Pradesh, India. E-mail: bsinghbhu@
rediffmail.com
1

2
H. K. Singh et al.
salicylaldimine [6] has been found convenient starting material for the preparation of metal-
lomesogens. Copper (II), nickel (II), vanadyl (IV), palladium (II) [7–12], and iron (III) [13]
complexes of salicylideneamine derivatives have been reported to show liquid crystalline
properties. In 1984, Ovchinnikov and coworkers were first to exploit the coordination pos-
sibilities of salicylideneanilines and they reported the first imino-derived metallomesogens
[14]. As was first observed by Ovchinnikov, N-(alkoxyphenyl)-4-alkoxysalicylaldimines
mainly show smectic mesomorphism when coordinated to copper(II) [15–18]. When both
chains in the ligands are short the compounds are not liquid crystalline. If one of the tails
is elongated, a smectic A phase is observed. In addition, a smectic C phase is observed
for cases where both alkoxy tails are long. The molecular shape of these complexes is
considered to be two roughly parallel rodlike moieties mutually head to tail oriented and
connected by the metals. They are classified as lateral–lateral fused mesogens, the wider
class of nondiscotic metallomesogens [19]. In such molecules, the metallic core actually
acts as a spacer between the two elongated frameworks. In this paper, we describe the
synthesis and mesomorphic properties of a series of new salicylaldimines and their cop-
per(II) and nickel(II) complexes. We have replaced one chain by –CO2CH3 group. It would
be interesting as the polar nature of the carbonyl group plays a vital role in ferroelectric
liquid crystals and this polarity can affect the mesomorphic and physicochemical prop-
erties of metal complexes in an unpredictable manner. Their thermotropic LC properties
were examined by using various experimental techniques with the aim of understanding
the structure-property relationship and developing novel materials based on this class of
compounds.
2. Experimental Details
2.1. Materials
All reagents were purchased from commercial sources and were used as received: 2,4-
dihydroxybenzaldehyde, methyl 4-aminobenzoate,1-bromoalkanes, and all metal acetates
are from Aldrich Chemicals, USA. All other solvents and reagents were purchased from
Merck. The solvents were dried using standard methods when required [20].
2.2. Techniques
Elemental analyses were performed on a CE-440 Exeter Analytical CHN analyzer. IR spec-
tra (4000–400 cm−1) were recorded on a Varian 3100 FT-IR Excalibur series spectropho-
tometer. 1H and 13C NMR spectra were obtained on a JEOL FT-NMR AL 300 MHz spec-
trometer using tetramethylsilane as the internal standard. Electronic spectra were recorded
on an (UV)-1700 Pharma Spec. Shimadzu UV-VIS spectrophotometer. Room tempera-
ture magnetic susceptibility measurements were performed on a Cahn Faraday balance
using Co[Hg(SCN)4] as standard. The magnetic susceptibility was corrected for diamag-
netism using Pascal’s constants. Differential scanning calorimetry (DSC) thermograms
were recorded with a Mettler Toledo TC 15 TA differential scanning calorimeter at the
rate of 5.0 K min−1 under nitrogen atmosphere using spec pure grade indium as standard
and the samples were taken in close-lid aluminum pans. The transition temperatures from
DSC thermograms have been determined with an accuracy of 0.1 K. The mesophase type
was identified by visual comparison with known phase standards using an HT 30.01 NTT
268 Lomo polarizing optical microscope (POM) fitted with a hot stage having temperature

Synthesis, Characterization and Mesomorphic Investigations of Calamitic
3
controlling accuracy of 0.1 K. Thermogravimetric analysis (TGA) was performed using
a PerkinElmer-STA 6000 apparatus under high-purity nitrogen. The samples were heated
from room temperature to 800◦C at 10◦C/min and the weight data were collected contin-
uously. The onset of decomposition was defined as the decomposition temperature (Td).
Quantum chemical calculations were carried out using density functional theory (DFT) as
implemented in GAUSSIAN-03 package.
2.3. Synthesis
2.3.1. Synthesis of 4-(decyloxy)-2-hydroxybenzaldehyde. Potassium bicarbonate (55
mmol, 5.51 g) was added to a solution of equimolar amounts of 2,4-dihydroxy benzaldehyde
(50 mmol, 6.91 g) and 1-bromodecane (50 mmol, 10.4 mL) in dry 2-butanone(100 mL).
The mixture was refluxed for 30 h in the presence of KI (0.1–0.2 g) as a catalyst and filtered
while hot to remove the insoluble solids. Dilute hydrochloric acid (6 N) was added to the
filtrate until the point of neutralization and the product was then extracted twice with CHCl3
(100 mL portions). The chloroform extract was concentrated to obtain a yellow oil, which
was purified by column chromatography over SiO2 eluting first with n-hexane and then with
a mixture of n-hexane and chloroform (v/v, 1/1); evaporation of this purified extract finally
yielded 4-(decyloxy)-2-hydroxybenzaldehyde as a pale yellow oil; yield: 62% (8.63 g).
All other members of the homologous series 4-hexyloxy-2-hydroxybenzaldehyde,
4-octyloxy-2-hydroxybenzaldehyde, 4-dodecyloxy-2-hydroxybenzaldehyde, 4-tetradecy
loxy-2-hydroxybenzaldehyde, and 4-hexadecyloxy-2-hydroxybenzaldehyde were prepared
using the above procedure.
IR (KBr, cm−1): 3450 (–OH), 2939, 2858 (aliphatic C–H), 1690 (C O), 1606, 1560
1
(Ph), 1317, 1248 (–OPh). H NMR (300 MHz, CDCl3, 25◦C): δH 11.24 (s, 1H), 9.88 (s,
1H), 7.62 (d, 1H) 6.98 (d, 1H), 6.92 (s, 1H), 4.07 (t, 2H), 1.85–1.26 (m, 16H), 0.90 (t, 3H).
13C NMR (75 MHz, CDCl3, 25◦C): δc 195.47, 163.92, 163.19, 134.91, 132.46, 118.59,
110.88, 68.41, 31.90, 29.60, 25.96, 22.68, 14.10.
2.3.2. Synthesis of methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoate, C10LH
(1c). 4-(decyloxy)-2-hydroxybenzaldehyde (2.78 g, 10 mmol) and methyl 4-amino
benzoate (1.51 g, 10 mmol) were mixed together in ethanol (10 mL). To this, 6–8 drops
of glacial acetic acid were added as a catalyst. The reaction mixture was refluxed for 4 h.
The color of solution becomes yellow. The yellow precipitate that formed on cooling was
filtered off, washed with cold ethanol, and recrystallized from ethanol-chloroform mixture
(1/1, v/v).
IR (KBr, cm−1): 3416 (–OH), 2955, 2927, 2856 (aliphatic C–H), 1723 (ester, C O),
1628 (C N), 1600, 1569 (Ph), 1284, 1236 (OPh). 1H NMR (300 MHz, CDCl3, 25◦C): δH
13.41(s, 1H, –OH), 8.53 (s, 1H, –CH N), 8.09 (d, J1 (H, H) = 8.4 Hz, 2H, –C6H4), 7.28
(m, 3H, –C6H3), 6.50 (d, J1 (H, H) = 8.2 Hz, 2H, –C6H3), 4.02–3.98 (t, 2H, –OCH2), 3.93
(s, 3H, COOCH3), 1.85 (m, 8H, –OCH2CH2), 1.55–1.27 (m, 8H), 0.89 (t, 3H, –CH3); 13
C
NMR (75 MHz, CDCl3, 25◦C): δc 166.65, 164.04, 162.79, 152.58, 133.87, 131.01, 127.73,
120.96, 112.75, 108.00, 101.51, 77.42, 76.57, 68.33, 52.12, 31.88, 29.53, 25.96, 22.66.
14.10; Elemental analyses: calculated for C25H33NO4 (%) – C, 73.02; H, 8.09; N, 3.40;
Found C, 72.91; H, 8.20; N, 3.48.
2.3.3. Synthesis of methyl 4-(4ꢀ-(hexyloxy)-2-hydroxybenzylideneamino)benzoate, C6LH
(1a). IR (KBr, cm−1): 3417 (OH), 2955, 2927, 2857 (aliphatic C–H), 1723 (ester, C O),
1628 (C N), 1600, 1573 (Ph), 1284, 1243 (OPh). 1H NMR (300 MHz, CDCl3, 25◦C): δH

4
H. K. Singh et al.
13.41 (s, 1H, –OH), 8.53 (s, 1H, –CH N), 8.09 (d, J1 (H, H) = 8.4 Hz, 2H, –C6H4), 7.29
(m, 3H, –C6H3), 6.50 (d, J1 (H, H) = 8.2 Hz, 2H, –C6H3), 4.02–4.00 (t, 2H, –OCH2), 3.92
(s, 3H, COOCH3) 1.82–1.34 (m, 8H, –[CH2]4), 0.91 (t, 3H, –CH3); 13C NMR (75 MHz,
CDCl3, 25◦C): δc 166.63, 164.16, 162.79, 152.62, 133.92, 131.06, 127.73, 120.96, 112.74,
107.98, 101.52, 77.43, 76.59, 68.32, 52.11, 31.91, 29.74, 25.73, 22.81, 14.17; Elemental
analyses: calculated for C21H25NO4(%) – C, 71.01; H, 7.09; N, 3.94; Found C, 71.21; H,
7.25; N, 3.86.
2.3.4. Synthesis of Methyl 4-(4ꢀ-(octyloxy)-2-hydroxybenzylideneamino)benzoate, C8LH
(1b). IR (KBr, cm−1): 3419 (–OH), 2955, 2926, 2857 (aliphatic C–H), 1723 (ester, C O),
1629(C N), 1601, 1572 (Ph), 1283, 1236 (OPh). 1H NMR (300 MHz, CDCl3, 25◦C): δH
13.42 (s, 1H, –OH), 8.52 (s, 1H, –CH N), 8.09 (d, J1 (H, H) = 8.4 Hz, 2H, –C6H4), 7.29 (m,
3H, –C6H3), 6.50 (d, J1 (H, H) = 8.2 Hz, 2H, –C6H3), 4.02–4.00 (t, 2H, –OCH2), 3.92 (s, 3H,
COOCH3) 1.82–1.34 (m, 12H, –[CH2]6), 0.91 (t, 3H, –CH3); 13C NMR (75 MHz, CDCl3,
25◦C): δc 166.64, 164.15, 162.79, 152.62, 133.93, 131.06, 127.73, 120.97, 112.74, 107.98,
101.52, 77.43, 76.59, 68.32, 52.11, 31.91, 29.74, 29.42, 26.01, 22.81, 14.17; Elemental
analyses: calculated for C23H29NO4(%) – C, 72.08; H, 7.63; N, 3.66; Found C, 72.23; H,
7.71; N, 3.75.
2.3.5. Synthesis of Methyl 4-(4ꢀ-(dodecyloxy)-2-hydroxybenzylideneamino)benzoate,
C12LH (1d). IR (KBr, cm−1): 3420 (–OH), 2955, 2919, 2849 (aliphatic C–H), 1723 (ester,
1
C O), 1628 (C N), 1601, 1573 (Ph), 1284, 1236 (OPh). H NMR (300 MHz, CDCl3,
25◦C): δH 13.41 (s, 1H, –OH), 8.52 (s, 1H, –CH N), 8.09 (d, J1 (H, H) = 8.4 Hz, 2H,
–C6H4), 7.29 (m, 3H, –C6H3), 6.50 (d, J1 (H, H) = 8.2 Hz, 2H, –C6H3), 4.02–4.00 (t,
2H, –OCH2), 3.92 (s, 3H, COOCH3) 1.82–1.34 (m, 20H, –[CH2]10), 0.91 (t, 3H, –CH3);
13C NMR (75 MHz, CDCl3, 25◦C): δc 166.63, 164.168 162.73, 152.62, 133.91, 131.06,
127.73, 120.96, 112.74, 107.97, 101.52, 77.43, 76.59, 68.32, 52.11, 31.91, 29.74, 29.42,
26.01, 22.81, 14.17; Elemental analyses: calculated for C27H37NO4(%) – C, 73.83; H, 8.49;
N, 3.19; Found C, 73.96; H, 8.20; N, 3.02.
2.3.6. Synthesis of Methyl 4-(4ꢀ-(tetradecyloxy)-2-hydroxybenzylideneamino)benzoate,
C14LH (1e). IR (KBr, cm−1): 3421 (–OH), 2955, 2918, 2849 (aliphatic C–H), 1724 (ester,
1
C O), 1628 (C N), 1601, 1573 (Ph), 1284, 1236 (OPh). H NMR (300 MHz, CDCl3,
25◦C): δH 13.41 (s, 1H, –OH), 8.52 (s, 1H, –CH N), 8.09 (d, J1 (H, H) = 8.4 Hz, 2H,
–C6H4), 7.29 (m, 3H, –C6H3), 6.50 (d, J1 (H, H) = 8.2 Hz, 2H, –C6H3), 4.02–4.00 (t, 2H,
–OCH2), 3.92 (s, 3H, COOCH3) 1.82–1.26 (m, 24H, –[CH2]12), 0.88 (t, 3H, –CH3); 13C
NMR (75 MHz, CDCl3, 25◦C): δc 166.62, 164.19, 162.78, 152.62, 133.92, 131.08, 127.73,
120.96, 112.75, 107.98, 101.52, 77.43, 76.59, 68.32, 52.11, 31.91, 29.74, 29.42, 26.01,
22.81, 14.17; Elemental analyses: calculated for C29H41NO4(%) – C, 74.54; H, 8.84; N,
2.99; Found C, 74.32; H, 8.60; N, 2.90.
2.3.7. Synthesis of Methyl 4-(4ꢀ-(hexadecyloxy)-2-hydroxybenzylideneamino)benzoate,
C16LH (1f). IR (KBr, cm−1): 3419 (–OH), 2955, 2918, 2849 (aliphatic C–H), 1724 (ester,
1
C O), 1628 (C N), 1601, 1574 (Ph), 1284, 1236 (OPh). H NMR (300 MHz, CDCl3,
25◦C): δH 13.41 (s, 1H, –OH), 8.53 (s, 1H, –CH N), 8.09 (d, J1 (H, H) = 8.4 Hz, 2H,
–C6H4), 7.29 (m, 3H, –C6H3), 6.49 (d, J1 (H, H) = 8.2 Hz, 2H, –C6H3), 4.02–3.98 (t, 2H,
–OCH2), 3.92 (s, 3H, COOCH3) 1.82–1.26 (m, 28H, –[CH2]14), 0.86 (t, 3H, –CH3); 13C
NMR (75 MHz, CDCl3, 25◦C): δc 166.61, 164.16, 162.79, 152.65, 133.92, 131.06, 127.74,
120.96, 112.73, 107.98, 101.52, 77.42, 76.59, 68.32, 52.11, 31.91, 29.74, 29.42, 26.01,

Synthesis, Characterization and Mesomorphic Investigations of Calamitic
5
22.81, 14.17; Elemental analyses: calculated for C31H45NO4(%) – C, 75.17; H, 9.16; N,
2.83; Found C, 75.65; H, 8.96; N, 3.01.
Procedure for preparation of nickel and copper complexes
To a solution (20 mL) of methyl 4-(4ꢀ-(alkoxy)-2-hydroxybenzylideneamino)benzoate (2
eq) in hot methanol/chloroform (1/1,v/v) stirred at room temperature was added metal
acetate (1 eq) in methanol (10 mL). The mixture was stirred at room temperature for 2 h.
The resulting compound was filtered off, washed with methanol and recrystallized from
chloroform/ethanol (1/1,v/v). All the copper(II) and nickel(II) complexes are synthesized
by the above method.
2.3.8. Bis[methyl 4-(4ꢀ-(hexyloxy)-2-hydroxybenzylideneamino)benzoato]copper(II), (C6
L)2Cu (2a). Yield: 71% (brown colored solid). IR (KBr, cm−1): 2924, 2854 (aliphatic
C–H), 1720 (ester, C O), 1609 (C N), 1594, 1534 (Ph), 1290, 1242 (OPh). Elemental
analyses: calculated for (C21H24NO4)2Cu (%) – C, 65.31; H, 6.26; N, 3.63, Cu, 8.23; Found
C, 65.58; H, 6.46; N, 3.76, Cu, 8.42.
2.3.9. Bis[methyl 4-(4ꢀ-(octyloxy)-2-hydroxybenzylideneamino)benzoato]copper(II), (C8
L)2Cu (2b). Yield: 70% (brown colored solid). IR (KBr, cm−1): 2924, 2854 (aliphatic
C–H), 1720 (ester, C O), 1611 (C N), 1594, 1534 (Ph), 1290, 1242 (OPh). Elemental
analyses: calculated for (C23H28NO4)2Cu (%) – C, 66.69; H, 6.81; N, 3.38, Cu, 7.67; Found
C, 66.46; H, 6.66; N, 3.56, Cu, 7.42.
2.3.10. Bis[methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoato]copper(II),
(C10L)2Cu (2c). Yield: 70% (brown colored solid). IR (KBr, cm−1): 2924, 2854 (aliphatic
C–H), 1720 (ester, C O), 1610 (C N), 1594, 1534 (Ph), 1290, 1242 (OPh). Elemental
analyses: calculated for (C25H32NO4)2Cu (%) – C, 67.89; H, 7.29; N, 3.17, Cu, 7.18; Found
C, 67.96; H, 6.99; N, 3.36, Cu, 7.35.
2.3.11. Bis[methyl 4-(4ꢀ-(dodecyloxy)-2-hydroxybenzylideneamino)benzoato]copper(II),
(C12L)2Cu (2d). Yield: 72% (brown colored solid). IR (KBr, cm−1): 2922, 2849 (aliphatic
C–H), 1712 (ester, C O), 1609 (C N), 1594, 1534 (Ph), 1290, 1242 (OPh). Elemental
analyses: calculated for (C27H36NO4)2Cu (%) – C, 68.95; H, 7.72; N, 2.98, Cu, 6.76; Found
C, 69.16; H, 7.58; N, 3.16, Cu, 7.05.
2.3.12. Bis[methyl 4-(4ꢀ-(tetradecyloxy)-2-hydroxybenzylideneamino)benzoato]copper
(II), (C14L)2Cu (2e). Yield: 69% (brown colored solid). IR (KBr, cm−1): 2918, 2850
(aliphatic C–H), 1719 (ester, C O), 1611 (C N), 1594, 1534 (Ph), 1290, 1242 (OPh).
Elemental analyses: calculated for (C29H40NO4)2Cu (%) – C, 69.89; H, 8.09; N, 2.81, Cu,
6.38; Found C, 69.56; H, 7.98; N, 3.06, Cu, 6.79.
2.3.13. Bis[methyl 4-(4ꢀ-(hexadecyloxy)-2-hydroxybenzylideneamino)benzoato]copper
(II), (C16L)2Cu (2f). Yield: 70% (brown colored solid). IR (KBr, cm−1): 2920, 2850
(aliphatic C–H), 1719 (ester, C O), 1611 (C N), 1594, 1534 (Ph), 1290, 1242 (OPh).
Elemental analyses: calculated for (C31H44NO4)2Cu (%) - C, 70.72; H, 8.42; N, 2.66, Cu,
6.03; Found C, 70.43; H, 8.08; N, 3.01, Cu, 6.42.
2.3.14. Bis[methyl 4-(4ꢀ-(hexyloxy)-2-hydroxybenzylideneamino)benzoato]nickel(II) (3a).
Yield: 68% (orange solid), IR (KBr, cm−1): 2928, 2862 (aliphatic C–H), 1723 (ester, C O),

6
H. K. Singh et al.
1612 (C N), 1574, 1512 (Ph), 1290, 1244 (OPh). 1H NMR (300 MHz, CDCl3, 25◦C): δH
8.24 (s, 2H, –CH N), 7.89 (d, J1 (H, H) = 8.4 Hz, 4H, –C6H4), 7.08 (d, J1 (H, H) = 8.4 Hz,
6H, –C6H4), 6.35 (s, 2H, –C6H3), 6.30 (d, J1 (H, H) = 9.0 Hz, 2H, –C6H3), 3.98–3.91 (t,
4H, –OCH2), 3.85 (s, 6H, COOCH3), 1.78–1.26 (m, 32H, –[CH2]16), 0.88 (t, 6H, –CH3);
13C NMR (75 MHz, CDCl3, 25◦C): δc 167.67, 167.12, 166.44, 152.82, 133.31, 131.31
127.76, 121.10, 113.02, 107.90, 104.80, 77.42, 76.58, 68.20, 52.16, 31.96, 29.67, 25.95,
22.68, 14.15. Elemental analyses: calculated for (C21H24NO4)2Ni (%) – C, 65.72; H, 6.30;
N, 3.65; Ni, 7.65; Found C, 65.46; H, 6.43; N, 3.39; Ni, 7.33.
2.3.15. Bis[methyl 4-(4ꢀ-(octyloxy)-2-hydroxybenzylideneamino)benzoato]nickel(II) (3b).
Yield: 65% (orange solid), IR (KBr, cm−1): 2928, 2862 (aliphatic C–H), 1721 (ester, C O),
1613 (C N), 1574, 1512 (Ph), 1290, 1245 (OPh). 1H NMR (300 MHz, CDCl3, 25◦C): δH
8.24 (s, 2H, –CH N), 7.89 (d, J1 (H, H) = 8.4 Hz, 4H, –C6H4), 7.07 (d, J1 (H, H) = 8.4 Hz,
6H, –C6H4), 6.35 (s, 2H, –C6H3), 6.30 (d, J1 (H, H) = 9.0 Hz, 2H, –C6H3), 3.98–3.91 (t,
4H, –OCH2), 3.84 (s, 6H, COOCH3), 1.78–1.26 (m, 32H, –[CH2]16), 0.88 (t, 6H, –CH3);
13C NMR (75 MHz, CDCl3, 25◦C): δc 167.65, 167.14, 166.43, 152.80, 133.31, 131.33
127.76, 121.10, 113.04, 107.91, 104.81, 77.45, 76.58, 68.20, 52.16, 31.96, 29.68, 25.95,
22.68, 14.15. Elemental analyses: calculated for (C23H28NO4)2Ni (%) – C, 67.08; H, 6.85;
N, 3.40; Ni, 7.13; Found C, 66.86; H, 6.51; N, 3.13; Ni, 7.33.
2.3.16. Bis[methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoato]nickel(II) (3c).
Yield: 66% (orange solid), IR (KBr, cm−1): 2928, 2862 (aliphatic C–H), 1720 (ester, C O),
1611 (C N), 1574, 1514 (Ph), 1292, 1244 (OPh). 1H NMR (300 MHz, CDCl3, 25◦C): δH
8.24 (s, 2H, –CH N), 7.89 (d, J1 (H, H) = 8.4 Hz, 4H, –C6H4), 7.08 (d, J1 (H, H) = 8.4 Hz,
6H, –C6H4), 6.36 (s, 2H, –C6H3), 6.30 (d, J1 (H, H) = 9.0 Hz, 2H, –C6H3), 3.98–3.92 (t,
4H, –OCH2), 3.85 (s, 6H, COOCH3), 1.78–1.27 (m, 32H, –[CH2]16), 0.87 (t, 6H, –CH3);
13C NMR (75 MHz, CDCl3, 25◦C): δc 167.67, 167.16, 166.46, 152.82, 133.31, 131.31
127.76, 121.10, 113.02, 107.90, 104.81, 77.42, 76.59, 68.20, 52.16, 31.96, 29.67, 25.95,
22.68, 14.15. Elemental analyses: calculated for (C25H32NO4)2Ni (%) – C, 68.26; H, 7.33;
N, 3.18; Ni, 6.67; Found C, 67.96; H, 6.99; N, 3.03; Ni, 6.33.
2.3.17. Bis[methyl 4-(4ꢀ-(dodecyloxy)-2-hydroxybenzylideneamino)benzoato]nickel(II)
(3d). Yield: 69% (orange solid), IR (KBr, cm−1): 2928, 2862 (aliphatic C–H), 1723 (ester,
1
C O), 1613 (C N), 1573, 1512 (Ph), 1291, 1243 (OPh). H NMR (300 MHz, CDCl3,
25◦C): δH 8.24 (s, 2H, –CH N), 7.88 (d, J1 (H, H) = 8.4 Hz, 4H, –C6H4), 7.08 (d, J1 (H,
H) = 8.4 Hz, 6H, –C6H4), 6.36 (s, 2H, –C6H3), 6.31 (d, J1 (H, H) = 9.0 Hz, 2H, –C6H3),
3.97–3.91 (t, 4H, –OCH2), 3.85 (s, 6H, COOCH3), 1.77–1.26 (m, 32H, –[CH2]16), 0.89 (t,
6H, –CH3); 13C NMR (75 MHz, CDCl3, 25◦C): δc 167.68, 167.13, 166.43, 152.83, 133.31,
131.32, 127.75, 121.10, 113.03, 107.90, 104.81, 77.43, 76.58, 68.20, 52.16, 31.96, 29.67,
25.95, 22.69, 14.13. Elemental analyses: calculated for (C27H36NO4)2Ni (%) – C, 69.30;
H, 7.76; N, 2.99; Ni, 6.27; Found C, 68.98; H, 7.41; N, 3.13; Ni, 6.37.
2.3.18. Bis[methyl4-(4ꢀ-(tetradecyloxy)-2-hydroxybenzylideneamino)benzoato]nickel(II)
(3e). Yield: 65% (orange solid), IR (KBr, cm−1): 2928, 2862 (aliphatic C–H), 1722 (ester,
1
C O), 1612 (C N), 1574, 1514 (Ph), 1290, 1244 (OPh). H NMR (300 MHz, CDCl3,
25◦C): δH 8.24 (s, 2H, –CH N), 7.89 (d, J1 (H, H) = 8.4 Hz, 4H, –C6H4), 7.08 (d, J1 (H,
H) = 8.4 Hz, 6H, –C6H4), 6.35 (s, 2H, –C6H3), 6.30 (d, J1 (H, H) = 9.0 Hz, 2H, –C6H3),
3.98–3.91 (t, 4H, –OCH2), 3.85 (s, 6H, COOCH3), 1.78–1.26 (m, 32H, –[CH2]16), 0.88 (t,
6H, –CH3); 13C NMR (75 MHz, CDCl3, 25◦C): δc 167.67, 167.12, 166.44, 152.82, 133.31,

Synthesis, Characterization and Mesomorphic Investigations of Calamitic
7
131.31 127.76, 121.10, 113.02, 107.90, 104.80, 77.42, 76.58, 68.20, 52.16, 31.96, 29.67,
25.95, 22.68, 14.15. Elemental analyses: calculated for (C29H40NO4)2Ni (%) – C, 70.22;
H, 8.13; N, 2.82; Ni, 5.91; Found C, 69.96; H, 7.97; N, 3.03; Ni, 6.21.
2.3.19. Bis[methyl4-(4ꢀ-(hexadecyloxy)-2-hydroxybenzylideneamino)benzoato]nickel(II)
(3f). Yield: 68% (orange solid), IR (KBr, cm−1): 2928, 2862 (aliphatic C–H), 1722 (ester,
1
C O), 1611 (C N), 1575, 1512 (Ph), 1293, 1244 (OPh). H NMR (300 MHz, CDCl3,
25◦C): δH 8.24 (s, 2H, –CH N), 7.89 (d, J1 (H, H) = 8.4 Hz, 4H, –C6H4), 7.08 (d, J1 (H,
H) = 8.4 Hz, 6H, –C6H4), 6.34 (s, 2H, –C6H3), 6.31 (d, J1 (H, H) = 9.0 Hz, 2H, –C6H3),
3.98–3.92 (t, 4H, –OCH2), 3.85 (s, 6H, COOCH3), 1.78–1.26 (m, 32H, –[CH2]16), 0.88 (t,
6H, –CH3); 13C NMR (75 MHz, CDCl3, 25◦C): δc 167.64, 167.17, 166.43, 152.82, 133.33,
131.31 127.76, 121.12, 113.02, 107.90, 104.80, 77.42, 76.58, 68.20, 52.16, 31.96, 29.67,
25.95, 22.68, 14.15. Elemental analyses: calculated for (C31H44NO4)2Ni (%) – C, 71.05;
H, 8.46; N, 2.67; Ni, 5.60; Found C, 69.86; H, 8.08; N, 2.98; Ni, 5.92.
3. Results and Discussion
3.1. Synthesis and Characterization
The synthetic route for preparation of Schiff base ligands methyl 4-(4ꢀ-(decyloxy)-2-
hydroxybenzylideneamino)benzoate and their metal complexes is outlined in Scheme 1.
Alkylation of 2,4-dihydroxybenzaldehyde with decyl bromide in presence of strong
base in dry 2-butanone allowed us to obtain a reasonable yield of 4-(decyloxy)-
2-hydroxybenzaldehyde. The Schiff base was obtained by the condensation of
methyl 4-aminobenzoate with 4-(decyloxy)-2-hydroxybenzaldehyde in refluxing ethanol.
The metal complexes were prepared by reaction of methyl 4-(4ꢀ-(decyloxy)-2-
hydroxybenzylideneamino)benzoate with corresponding metal acetate in refluxing ethanol.
The copper and nickel complexes were then isolated as brown and orange solid, respectively,
by recrystallization from absolute ethanol in good yield (65%–72%). The ligands and their
metal complexes were characterized by elemental analyses and standard spectroscopic
techniques. The IR spectrum of 4-(decyloxy)-2-hydroxybenzaldehyde shows absorption
bands at 3450, (2939, 2858), 1690, (1606, 1560), (1317, 1248) cm−1 assigned to ν (OH), ν
(aliphatic C–H stretch), ν (C O aldehyde), ν (Ph), and ν (OPh), respectively. The IR spec-
trum of methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoate shows absorption
bands at 3416, (2955, 2856), 1723, 1628, (1600,1569), (1284,1236) cm−1 assigned to ν
(OH), ν (aliphatic C–H stretch), ν (C O) of phenyl ester, ν (C N), ν (Ph), and ν (OPh)
modes, respectively. The disappearance of the band at 1690 cm−1 due to ν (CH O) and ap-
pearance of a new band at 1628 cm−1 due to ν (C N) indicate condensation of aldehyde with
amine forming Schiff base. The proton NMR of 4-(decyloxy)-2-hydroxybenzaldehyde ex-
hibit peaks δ 11.24(s), 9.88(s), 7.61–6.92 (m), 4.07(t), 1.85–1.26 (m), and0.90(t) ppmwhich
are assigned to –OH, –CHO, ring, –OCH2, –[CH2]n, and –CH3 protons, respectively. The
proton NMR spectrum of methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoate
exhibits peaks at δ 13.41(s), 8.53(s), 8.09–6.50(m), 4.02(t), 3.93(s), 1.84–1.27(m), 0.88(t)
ppm which are assigned to –OH, –CH N, ring, –OCH2, –COOCH3, –[CH2]n, and –CH3
protons, respectively. The signals due to Schiff base formation is observed at 8.64 ppm
which corresponds to o-hydroxy benzylidene. The length of alkyl chain has no significant
effect on the position of the signals due to –OH, –CH N, ring, –OCH2, –[CH2]n, and
–CH3 protons. The IR spectrum of copper (II) complexes shows absorption bands at (2924,
2854), 1720, 1609, (1594, 1534), and (1290, 1242) cm−1 which are assigned to ν (aliphatic

8
H. K. Singh et al.
Scheme 1. Reactions and reagents: (a) potassium bicarbonate(1.1 equiv.), refluxing in acetone,
24 h, 60%–67%; (b) acetic acid (3 drops), refluxing in EtOH, 4 h, 92%; (c) Cu(OAc)2.H2O or
Ni(OAc)2.H2O (0.5 equiv.), refluxing in MeOH/CHCl3 (1:1), 4 h, 89%. R = C6H13, C8H17, C10H21,
C12H25, C14H29, C16H33.
C–H), ν (C O) of phenyl ester, ν (–C N), ν (Ph) and ν (OPh) modes, respectively. The
disappearance of the band 3416 cm−1 due to ν (OH) and shift of ν (–C N) band to a lower
frequency (1609 cm−1), indicate bonding through azomethine nitrogen. The lowering in
the ν (–C N) band caused by the coordination of nitrogen to the metal center is in agree-
ment with the result obtained for similar other complexes. The IR spectrum of nickel(II)
complexes show absorption bands at (2928, 2862), 1720, 1611, (1574, 1514), and (1292,
1244) cm−1 which are assigned to ν (aliphatic C–H), ν (C O) of phenyl ester, ν (–C N),
ν (Ph), and ν (OPh) modes, respectively. The disappearance of the band 3416 cm−1 due to
ν (OH) and shift of ν (–C N) band to a lower frequency (1609 cm−1), indicate bonding
through azomethine nitrogen. The proton NMR spectrum of bis[methyl 4-(4ꢀ-(decyloxy)-2-
hydroxybenzylideneamino)benzoato] nickel(II) exhibits peaks at δ 8.24(s), 7.89–6.30(m),
3.98–3.92(t), 3.85(s), 1.78–1.27(m), and 0.87(t) ppm, which are assigned to –CH N, ring,
–OCH2, –COOCH3, –[CH2]n, and –CH3 protons, respectively. The disappearance of the
signal at 13.41 ppm (coordination via enolic oxygen atom) and the shift in the peak position

Synthesis, Characterization and Mesomorphic Investigations of Calamitic
9
of the –CH N signal in the complex (8.24 ppm) as compared to ligand (8.53 ppm), caused
by decreased electron density due to donation of lone pair of electron from nitrogen atom
to metal center, suggest coordination of ligand through the azomethine nitrogen.
The bands observed at 394, 311, and 257 nm in the electronic spectrum of methyl 4-
(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoate are assigned to n–π∗ and π–π∗ tran-
sitions. In the spectrum of the bis[methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)
benzoato]copper(II) complex, the bands are observed at 418, 378, 327, 304, and 249 nm.
The first band is characteristic of square planar coordination around the metal center. The
remaining bands are attributed to the π–π∗ and n–π∗ transitions of the ligand. The elec-
tronic spectrum of the orange colored nickel (II) complex shows transitions at 407, 392,
362, 311, and 278 nm. The first band is characteristic of square planar coordination around
the metal centre and the remaining bands are attributed to the π–π∗ and n–π∗ transitions
of the ligand.
3.2. DFT Study
The GAUSSIAN-03 program [21] package was employed to carry out DFT [22–27] cal-
culations at the Becke’s three parameter functional and Lee, Yang, and Parr correlation
functional (B3LYP) level [28–30] of calculation and the 6–31G(d) basis set [31] was used
for ground state geometry optimization and frequency calculations of the ligand. The in-
ternal coordinates of the system, which is used as input for GAUSSIAN-03 program was
generated by the GAUSS VIEW 4.1 program [32]. The optimized structure of the ligand
and its metal complex is given in Fig. 1.
Figure 1. DFT optimized structure (ball and stick model) of (A) methyl 4-(4ꢀ-(decyloxy)-
2
hydroxybenzylideneamino)benzoate; (B) Bis[methyl 4-(4ꢀ-(decyloxy)-2 hydroxybenzylide-
neamino)benzoato]copper(II) complex.

10
H. K. Singh et al.
Table 1. Selected bond lengths and bond angles of Cu-complex of
methyl 4-(4ꢀ-(decyloxy)-2 hydroxybenzylideneamino)benzoate in DFT
optimized structure
Structure parameter
Cu-complexa
Cu–O(32)
Cu–O(87)
1.920
1.920
Cu–N(12)
2.036
Cu–N(77)
2.037
O(32)– Cu–O(87)
N(12)– Cu–N(77)
N(12)–Cu–O(87)
N(77)–Cu–O(32)
N(12)–Cu–O(32)
N(77)–Cu–O(87)
156.834
153.20
93.57
93.56
91.770
91.760
aBond lengths are reported in angstrom (Å) and bond angles in degrees (◦).
The absence of any imaginary frequencies in the calculated vibrational frequencies
ensures that the optimized geometry corresponds to a true energy minimum. The length
of fully extended ligand is found to be 27.47 Å, from DFT calculations. Some of the
selective geometric parameters of optimized Cu(II) complex, evaluated by DFT calculation
at B3LYP level, 631G(d) for ligand and LanL2DZ for metal are shown in Table 1.
The average Cu–O32 and Cu–N12 bond lengths of the complex are 1.92 and 2.036 Å,
respectively, implying the presence of regular σ and dative bonding, respectively. The bond
angles 156.83◦ and 153.20◦ for O32–Cu–O87 and N12–Cu–N77 respectively are around the
copper atom. The bond angles are commensurate with square planar geometry around the
metal ion.
3.3. Mesomorphic Behavior
The mesomorphic properties of methyl 4-(4ꢀ-(alkoxy)-2-hydroxybenzylideneamino)
benzoate and their copper(II) and nickel (II) complexes were characterized and investi-
gated by DSC and POM. The phase transition temperature and thermodynamic data of the
compounds are listed in Table 2.
The compound 1a showed endothermic peaks at 132.6◦C, 140.2◦C, and 145.9◦C fol-
lowed by the appearance of exothermic peaks at 142.13◦C and 94.37◦C. The three endother-
mic peaks observed for this compound define the crystal-to-crystal, crystal-to-mesophase,
and mesophase-to-isotropic phase transition. The two exothermic peaks reveal an isotropic-
to-mesophase and a mesophase-to-crystal phase transition. A Smectic A mesophase with
focal conic texture is revealed for this compound in the heating and cooling cycle on the
basis of POM. The texture of the mesophase is characteristic of a SmA mesophase, having
a fluid property with higher viscosity and having stratified structures with well defined
interlayer spacings. The interlayer attractions are weak and the layers are able to slide over
one another relatively easily. The flexibility of the layers leads to distortions, which give
rise to beautiful optical patterns known as the focal-conic texture (Fig. 2).
The molecules are aligned parallel to the layer normal, maintaining long range orien-
tational ordering and short-range positional ordering, which lead to the SmA mesophase. A

Synthesis, Characterization and Mesomorphic Investigations of Calamitic
11
Table 2. Transition temperature (T), transition enthalpy (ꢀH), and transition entropy (ꢀS)
of methyl 4-(4ꢀ-(alkoxy)-2-hydroxybenzylideneamino)benzoate and its copper(II) complex
T, ◦C [ꢀH, kJ/mol; ꢀS, J/(mol K)]
No Compounds
Heating
Cooling
1
1.a
1.b
1.c
1.d
1.e
1.f
kꢀ 132.6 (18.79; 46.30) kꢀꢀ 140.2
(24.9; 60.23) SmA 145.9 (4.01;
9.56) i
i 142.13 (−3.24; −7.80) SmA
94.37 (−24.21; −65.87) k
2
kꢀ 110.4 (12.19; 31.78) kꢀꢀ 122.3
(31.69; 80.01) SmA 141.9 (3.71;
8.93) i
i 138.91 (−3.08; −7.74) SmA
97.79 (−27.69; −74.65) k
3
kꢀ 92.63 (8.79; 24.03) kꢀꢀ 114.8
(27.69; 71.37) SmA 138.98 (2.76;
6.69) i
i 135.23 (−2.68; −6.56) SmA
99.25 (−29.48; −79.16) kꢀ
86.44 (8.76; 24.44) kꢀꢀ
i 137.55 (−5.58; −13.58) SmA
104.93 (−40.02; −105.85) kꢀ
99.69 (15.24; 40.87) kꢀꢀ
4
kꢀ 106.14 (15.25; 40.20) kꢀꢀ 115.39
(41.20; 106.03) SmA 139.35
(5.71; 13.84) i
5
k 114.78 (50.81; 130.97) SmA
136.25 (3.19; 7.79) i
i 133.65 (−4.43; −10.88) SmA
106.48 (−53.45; −140.60) kꢀ
104.56 (− −) kꢀꢀ
6
k 122.34 (32.81; 81.36) SmA 132.1
(2.98; 7.35) i
i 128.59 (−4.01; −9.98) SmA
112.2 (−30.09; −78.08) kꢀ
106.15 (−14.78; −38.96) kꢀꢀ
i 220.17 (−1.86; −3.77) SmA
202.46 (−32.61; −68.56) k
7
2.a
2.b
2.c
2.d
2.e
2.f
kꢀ 126.34 (12.81; 32.06) kꢀꢀ 225.12
(32.85; 65.92) SmA 232.14 (4.98;
9.80) i
8
kꢀ 124.18 (16.79; 42.25) kꢀꢀ 216.73
(42.85; 87.47) SmA 223.81 (5.59;
11.24) i
i 215.63 (−3.21; −6.57) SmA
196.20 (−39.94; −85.09) k
9
kꢀ 118.60 (3.12; 26.30) kꢀꢀ 191.92
(20.85; 108.63) SmA 208.21
(0.93; 1.93) i
i 204.21 (−0.86; −4.21) SmA
161.85 (−18.66; −115.29) kꢀ
117.18 (−1.41; −12.03) kꢀꢀ
i 213.25 (−2.11; −4.33) SmA
163.91 (−21.28; −48.68) kꢀ
116.26 (−10.45; −26.83) kꢀꢀ
i 208.71 (−5.01; −10.39) SmA
161.48 (−23.46; −53.97) kꢀ
110.78 (−6.97; −18.15) kꢀꢀ
i 200.1 (−5.79; −12.23) SmA
197.10 (−27.19; −57.82) kꢀ
118.90 (−5.37; −13.69)kꢀꢀ
10
11
12
kꢀ 123.09 (17.43; 43.98) kꢀꢀ 203.27
(33.01; 69.28) SmA 224.23 (8.76;
17.61) i
kꢀ 119.09 (24.07; 61.36) kꢀꢀ 200.61
(34.39; 72.60) SmA 217.63
(11.49; 23.41) i
kꢀ 128.23 (22.81; 56.82) kꢀꢀ
198.31(29.21; 61.95) SmA 205.87
(6.85; 14.30) i
similar phase transition with a similar textural pattern is shown by compound 1b. The com-
pound 1c showed endothermic peaks at 92.63◦C, 114.8◦C, and 138.98◦C followed by the
appearance of exothermic peaks at 135.23◦C, 99.25◦C, and 86.44◦C. The three endothermic
peaks observed for this compound define the crystal-to-crystal, crystal-to-mesophase, and
mesophase-to-isotropic phase transition. The three exothermic peaks reveal an isotropic-
to-mesophase transition, mesophase-to-crystal followed by the next crystal phase in the
cooling cycle (Fig. 3).

12
H. K. Singh et al.
Figure 2. Optical textures of methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoate show-
ing SmA mesophase under POM. (A) at 120◦C (B) at 108◦C and Optical textures of Bis[methyl
4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino) benzoato]copper(II) showing SmA mesophase (C) at
180◦C (D) at 200◦C.
Figure 3. DSC thermogram of methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoate.

Synthesis, Characterization and Mesomorphic Investigations of Calamitic
13
Figure 4. DSC thermogram of methyl 4-(4ꢀ-(tetradecyloxy)-2-hydroxybenzylideneamino)benzoate.
The similar phase behavior was obtained for the compound 1d. The compound 1e
showed endothermic peaks at 114.78◦C and 136.25◦C followed by the appearance of
exothermic peaks at 133.65◦C, 106.48◦C, and 104.56◦C. The two endothermic peaks
observed for this compound define crystal-to-mesophase and mesophase-to-isotropic
phase transition. The three exothermic peaks reveal an isotropic-to-mesophase transition,
mesophase-to-crystal followed by the next crystal phase in the cooling cycle (Fig. 4).
A similar phase transitions with a similar textural pattern is shown by compound 1f.
All the ligands (1a–f) show enantiotropic SmA mesophase. Polymorphism based on the
crystal-to-crystal phase transitions, is exhibited by compounds 1a–d in the heating cycle
and 1c–f in cooling cycle.
The copper complexes of the series also exhibit SmA mesophase, but at higher tempera-
ture (above 190◦C) in comparison to the parent ligands. The optical texture of the (C10L)2Cu
has been shown in Fig. 2. All the copper(II) complexes (2a–2e) show enantiotropic SmA
mesophase. The compound 2a showed endothermic peaks at 126.34◦C, 225.12◦C, and
232.14◦C followed by the appearance of exothermic peaks at 220.17◦C and 202.46◦C. The
three endothermic peaks observed for this compound define the crystal-to-crystal, crystal-
to-mesophase, and mesophase-to-isotropic phase transition. The two exothermic peaks re-
veal an isotropic-to-mesophase transition and mesophase-to-crystal transition. The similar
phase behavior was obtained for the compound 2b. The compound 2c showed endothermic
peaks at 118.60◦C, 191.92◦C, and 208.21◦C followed by exothermic peaks at 204.21◦C,
161.85◦C, and 117.18◦C. The three endothermic peaks observed for this compound define

14
H. K. Singh et al.
Figure 5. DSC thermogram of Bis[methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino) ben-
zoato]copper(II) complex.
the crystal-to-crystal, crystal-to-mesophase, and mesophase-to-isotropic phase transition.
The three exothermic peaks reveal an isotropic-to-mesophase and mesophase-to-crystal
followed by the next crystal phase in the cooling cycle (Fig. 5).
A similar phase transition with a similar textural pattern is shown by compound 2d–f
(Fig. 6). Polymorphism, based on the crystal-to-crystal phase transitions, is exhibited for
compound 2a–f in the heating cycle and 2c–f in cooling cycle.
The nickel(II) complexes of methyl 4-(4ꢀ-(alkoxy)-2-hydroxybenzylideneamino)
benzoate 3a–f were found to be nonmesogenic, with almost the same clearing temper-
ature independent of the carbon chain length. The nonmesogenic nature of the nickel(II)
complexes may be due to symmetrical and rigid geometry of the nickel complexes along
with the stronger interaction of the potential donor (C N) with the metal ion in the neigh-
boring layers. This results in the decomposition of the molecules at higher temperature.
3.4. Thermo Gravimetric Analysis
Thermo gravimetric analyses of the mesogens indicate about their thermal stability, and for
metal complexes along with thermal stability it also indicates about entrapping of solvents
and coordination of water or solvents with metal ion as a coligand. The TGA of the mesogen
methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoate shows only 5% weight loss

Synthesis, Characterization and Mesomorphic Investigations of Calamitic
15
Figure 6. DSC thermogram of Bis[methyl 4-(4ꢀ-(tetradecyloxy)-2-hydroxybenzylideneamino) ben-
zoato]copper(II) complex.
Figure 7. Thermal gravimetric analysis of mesogenic ligand methyl 4-(4ꢀ-(decyloxy)-2-
hydroxybenzylideneamino)benzoate (black line) and its copper(II) (brown line) and nickel(II) com-
plex (orange line).

16
H. K. Singh et al.
from 100◦C to 300◦C and indicates good thermal stability of the mesogens. The TGA of the
copper(II) complex shows no weight loss up to 300◦C which indicates very good thermal
stability of the complex. The TGA of the nickel(II) complexes shows no weight loss up to
200◦C and decomposition temperature is found to be 342.5◦C indicates that the nickel(II)
complexes are also thermally stable. The TGA analysis indicates that no water or solvent
is entrapped in the lattice or coordinated to the metal ion (Fig. 7).
4. Conclusions
A new series of mesogenic Schiff bases having a terminal alkoxy chain and a methyl
benzoate as a polar end group and their copper(II) and nickel(II) complexes, have been
synthesized and characterized. All the ligands and its copper (II) complexes show an
enatiotropic SmA mesophase, but the nickel(II) complexes are nonmesogenic in nature.
The methyl 4-(4ꢀ-(decyloxy)-2-hydroxybenzylideneamino)benzoate and its copper(II) and
nickel(II) complexes show a high-thermal stability (above 300◦C).
Acknowledgments
Hemant Kumar Singh (Junior Research Fellow), Sachin Kumar Singh (Senior Research Fel-
low), and Vijay Kumar (Junior Research Fellow project) are grateful to Council of Scientific
and Industrial Research (CSIR), New Delhi, India, for financial support. Bachcha Singh is
grateful to CSIR (Project No. 01(2550)12/EMR-II) for providing financial assistance.
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