Synthesis, characterization, and mesomorphic investigations of diester-substituted salicylaldimines and their copper (II) complexes

Article abstract of DOI:10.1080/15421406.2015.1044196

A novel homologous series of N-[4-[4′-n-alkoxy)benzoyloxy-2-hydroxybenzylidene)-4-carbethoxy anilines, H_2n+1C_nOC₆H₄C(O)OC₆H₃(OH)C(H)NC₆H₄COOC₂H₅ (n = 6, 8, 10, 12, 14, 16) and their copper(II) complexes have been synthesized. All these compounds have been characterized by suitable spectroscopic techniques. The mesomorphic properties of these compounds were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). The ligands exhibit wide range of enantiotropic smectic A and nematic phases as confirmed by their typical optical texture under polarizing microscope. The square planar copper(II) complexes of the ligands show only an isotropic phase at higher temperature and no mesogenic nature is observed. DFT calculations have been performed using GAUSSIAN-03 program at B3LYP level to obtain the stable electronic structure of the ligand with decyloxy chain length and its copper(II) complex.

Full text of DOI:10.1080/15421406.2015.1044196

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis, characterization, and mesomorphic
investigations of diester-substituted
salicylaldimines and their copper (II) complexes
Sachin Kumar Singh, Madan Kumar Singh, Hemant Kumar Singh & Bachcha
Singh
To cite this article: Sachin Kumar Singh, Madan Kumar Singh, Hemant Kumar Singh &
Bachcha Singh (2016) Synthesis, characterization, and mesomorphic investigations of diester-
substituted salicylaldimines and their copper (II) complexes, Molecular Crystals and Liquid
Crystals, 624:1, 77-90, DOI: 10.1080/15421406.2015.1044196
To link to this article: http://dx.doi.org/10.1080/15421406.2015.1044196
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Date: 08 April 2016, At: 15:59

MOL. CRYST. LIQ. CRYST.
, VOL. , NO. , –
http://dx.doi.org/./..
Synthesis, characterization, and mesomorphic investigations of
diester-substituted salicylaldimines and their copper (II)
complexes
Sachin Kumar Singh, Madan Kumar Singh, Hemant Kumar Singh, and Bachcha Singh
Department of Chemistry (Centre of Advanced Studies), Faculty of Science, Banaras Hindu University, Varanasi,
India
ABSTRACT
A
KEYWORDS
novel homologous series of N-[4-[4ꢀ-n-alkoxy)benzoyloxy-2-
Copper complexes; DFT
study; mesophase;
salicylaldimines;
structure–function
relationship; thermotropic
liquid crystal
hydroxybenzylidene)-4-carbethoxy anilines, H2n+1CnOC6H4C(O)OC6H3
=
(OH)C(H) NC6H4COOC2H5 (n = 6, 8, 10, 12, 14, 16) and their copper(II)
complexes have been synthesized. All these compounds have been
characterized by suitable spectroscopic techniques. The mesomorphic
properties of these compounds were investigated by differential
scanning calorimetry (DSC) and polarizing optical microscopy (POM).
The ligands exhibit wide range of enantiotropic smectic A and nematic
phases as confirmed by their typical optical texture under polarizing
microscope. The square planar copper(II) complexes of the ligands
show only an isotropic phase at higher temperature and no mesogenic
nature is observed. DFT calculations have been performed using
GAUSSIAN-03 program at B3LYP level to obtain the stable electronic
structure of the ligand with decyloxy chain length and its copper(II)
complex.
1. Introduction
Liquid crystals (mesogens) are fascinating functional materials and are important in the field
of advanced materials such as electro-optical devices, laser addressed devices, passive block
filters, and devices based on thermochromism such as fever indicators, gadgets, reflective type
LCDꢀs, field effect transistors and sensors [1]. They are indispensable in biological systems
(e.g. membranes) and are becoming part of high-tech processes [2]. The mesomorphic (liq-
uid crystalline) behavior of an organic compound is basically dependent on its molecular
architecture in which a slight change in the molecular geometry brings about considerable
change in its mesomorphic properties. Detailed studies on liquid crystals have led to empir-
ical rules which include the effect of chemical constitution in the nematogenic and smec-
togenic mesophases [3]. Most of these studies have been focused on Schiff bases since the
discovery of 4-methoxybenzylidene-4ꢀ-butylaniline (MBBA) which exhibits a room temper-
ature nematic phase [4]. In these organic liquid crystalline, Schiff base species whose rigid
CONTACT Bachcha Singh
bsinghbhu@rediffmail.com, bsingh@bhu.ac.in
Advanced Studies), Faculty of Science, Banaras Hindu University, Varanasi , India.
Department of Chemistry (Centre of
Present address for Madan Kumar Singh: Aditya Birla Science & Technology Company Ltd., MIDC Taloja, Dist. Raigad-, Navi
Mumbai, Maharashtra, India.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

78
S. K. SINGH ET AL.
core is salicylaldimines have been proved to be convenient starting materials for the prepara-
tion of metallomesogens [5]. The copper (II) complexes of salicylaldimine have been studied
to establish structure–property relationship.
Ovchinnikov and coworkers [6] were the first to described the liquid crystalline proper-
ties of the complexes of the type bis [4-X-N-(4-Y-phenyl) salicylaldiminato] copper (II) with
X = Y = alkoxy and studied their mesogenic properties and reported them to be smecto-
genic. Since then either four long alkoxy [7–10] chains or two alkoxy (= X) and two alkyl
(= Y) chains 11–14] have been employed. Over the past few decades, low molar mass Schiff
base esters have been investigated extensively. In these studies, the influence of a terminal
alkyl chain upon the liquid crystalline properties [15,16] and the possibility of enhancing the
rigidity of the Schiff base core system through metal complex formation [17,18] have been
claimed as favorable pathways to improve the mesogenic properties (Fig. 1).
In order to explore further the factors which govern the thermal stability of liquid crys-
tals with diester containing salicylaldimine core, and the relationship with its molecular
structures, we carry out the investigations in a more comprehensive manner on wherein a
alkoxy benzoate ester group into the aldehyde fragment of benzylideneaniline compound
and ethoxybenzoate group as terminal fragment are introduced through the preparation of a
homologous series of mesogenic ligands and their copper(II) complexes. The molecular struc-
tures of the title compounds were characterized by various spectroscopic techniques and their
liquid crystalline behavior were investigated by differential scanning calorimetry (DSC) and
polarizing optical microscopy (POM).
2. Experimental details
2.1. Materials
Ethyl 4-aminobenzoate, 4-hydroxybenzoic acid, 2,4-dihydroxy benzaldehyde, bromoalkanes,
thionyl chloride, copper(II) acetate monohydrate purchased from Aldrich Chemicals, USA
were used as received. All other solvents and reagents were purchased from Merck. The sol-
vents were dried using standard methods [19] when required.
Figure . Mesogenic copper(II) complexes of salicylideneamine[, , , ].

MOL. CRYST. LIQ. CRYST.
79
2.2. Techniques
Elemental analyses were performed on a CE-440 Exeter Analytical CHN analyser. IR spectra
(4000–100 cm−1) were recorded on a Varian 3100 FT-IR Excalibur series spectrophotome-
1
ter. H NMR spectra were obtained on a JEOL FT-NMR AL 300 MHz spectrometer using
tetramethylsilane as the internal standard. UV-visible spectra were recorded on an (UV)-1700
Pharma Spec. Shimadzu UV–visible spectrophotometer. Room temperature magnetic sus-
ceptibility measurements were performed on a Cahn Faraday balance using Co[Hg(SCN)4]
as standard. The magnetic susceptibility was corrected for diamagnetism using Pascal’s con-
stants. Differential scanning calorimetry thermograms were recorded with a Mettler Toledo
TC 15 TA differential scanning calorimeter at the rate of 5.0 K min−1 under a nitrogen atmo-
sphere using spec grade pure indium as standard by taking sample 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 stan-
dards using an HT 30.01 NTT 268 Lomo polarizing optical microscope fitted with a hot stage
with temperature controlling accuracy of 0.1 K. Quantum chemical calculations were carried
out using density functional theory (DFT) as implemented in GAUSSIAN-03 package.
2.3. Synthesis
The starting materials 4-alkoxybenzoic acids and 4-(4ꢀ-alkoxy)benzoyloxy-2-hydroxy ben-
zaldehydes were prepared according to literature procedure [20].
... Synthesis of N-(-(ꢀ-n-decyloxy)benzoyloxy--hydroxybenzylidene)--carbethoxy
aniline, CLH
4-(4ꢀ-Decyloxy)benzoyloxy-2-hydroxybenzaldehyde (1.99 g, 5 mmol) and ethyl-4-
aminobenzoate (0.97 g, 5 mmol) were mixed together in ethanol (20 mL) followed by
addition of 8–10 drops of glacial acetic acid. The reaction mixture was refluxed for 4 h. The
yellow precipitate that formed on cooling was filtered off, washed with cold ethanol and
recrystallized from ethanol-chloroform (1/1. v/v) mixture.
All the other members of the series were prepared in a similar manner. The yield, IR, NMR,
UV-Vis and elemental data for the compounds are summarized as follows.
... N-(-(ꢀ-n-hexyloxy)benzoyloxy--hydroxybenzylidene)--carbethoxyaniline,
CLH
−1
=
Yield: 92%; IR (KBr, cm ): 3460(OH), 2918–2857(aliphatic C-H), 1724(ester, C( O)OPh),
1712(ester, C( O)OEt), 1628( C N), 1599, 1510(Ph), 1275, 1251(OPh); H NMR (CDCl3,
1
=
– =
–
=
TMS) δH (ppm): 13.53(s,1H,-OH), 8.87(s, 1H, CH N), 8.53–6.73(m, 11H, ArH), 4.53(q, 2H,
COOCH2-), 4.20(t, 2H, -OCH2), 2.20–1.20(m, 11H„-[CH2]n), 0.93(t, 3H, - CH3).; 13C NMR
(75 MHz, CDCl3, 25°C): δc 166.63, 164.14, 162.77, 152.57, 133.87, 131.01, 127.73, 120.97,
112.77, 108.01, 101.52, 77.42, 76.56, 68.32, 52.12, 31.88, 29.53, 25.96, 22.66. 14.11; UV-visible
(CHCl3): λmax = 350, 317, 275, 259 nm; Elemental analyses: calculated for C29H31NO6 (%),
C, 71.15; H, 6.38; N, 2.92; Found, C, 71.21; H, 6.45; N, 2.80.
... N-(-(ꢀ-n-octylo−x1y)benzoyloxy--hydroxybenzylidene)--carbethoxyaniline, CLH
=
Yield: 87%; IR (KBr, cm ): 3420(OH), 2918–2857(aliphatic C-H), 1724(ester, C( O)OPh),
1
=
– =
1711(ester, C( O)OEt), 1628( C N), 1599–1510(Ph), 1275, 1251(OPh); H NMR (CDCl3,
–
=
TMS) δH (ppm): 13.53(s,1H,-OH), 8.83(s, 1H, CH N), 8.53–6.73(m, 11H, ArH), 4.53(q, 2H,

80
S. K. SINGH ET AL.
COOCH2-), 4.20(t, 2H, -OCH2), 2.20–1.20(m, 15H„-[CH2]n), 0.93(t, 3H, - CH3).; 13C 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; UV-visible
(CHCl3): max = 367, 329, 277, 270 nm; Elemental analyses: calculated for C31H35NO6 (%), C,
71.93; H, 6.81; N, 2.70; Found, C, 71.85; H, 6.87; N, 2.74.
... N-(-(ꢀ-n-decyloxy)benzoyloxy--hydroxybenzylidene)--carbethoxyaniline,
CLH
−1
=
Yield: 87%; IR (KBr, cm ): 3470(OH), 2906–2857(aliphatic C-H), 1724(ester, C( O)OPh),
1711(ester, C( O)OEt), 1628( C N), 1599–1510(Ph), 1275, 1253(OPh); H NMR (CDCl3,
1
=
– =
–
=
TMS) δH (ppm): 13.53(s,1H,-OH), 8.83(s, 1H, CH N), 8.53–6.73(m, 11H, ArH), 4.53(q, 2H,
COOCH2-), 4.20(t, 2H, -OCH2), 2.20–1.20(m, 19H„-[CH2]n), 0.93(t, 3H, - CH3).; 13C NMR
(75 MHz, CDCl3, 25°C): δc 166.64, 164.05, 162.79, 152.58, 133.88, 131.03, 127.74, 120.96,
112.74, 108.01, 101.53, 77.42, 76.57, 68.34, 52.12, 31.89, 29.53, 25.96, 22.67. 14.10; UV-visible
(CHCl3): λmax = 357, 327, 278, 265 nm; Elemental analyses: calculated for C33H39NO6 (%),
C, 72.63; H, 7.20; N, 2.57; Found, C, 72.55; H, 7.17; N, 2.55.
... N-(-(ꢀ-n-dodecyloxy)benzoyloxy--hydroxybenzylidene)--carbethoxyaniline,
CLH
−1
=
Yield: 87%; IR (KBr, cm ): 3472(OH), 2918–2852(aliphatic C-H), 1724(ester, C( O)OPh),
1712(ester, C( O)OEt), 1633( C N), 1602–1510(Ph), 1271, 1250(OPh); H NMR (CDCl3,
1
=
– =
–
=
TMS) δH (ppm): 13.53(s,1H,-OH), 8.83(s, 1H, CH N), 8.53–6.73(m, 11H, ArH), 4.53(q, 2H,
COOCH2-), 4.20(t, 2H, -OCH2), 2.20–1.20(m, 23H„-[CH2]n), 0.93(t, 3H, - CH3).; 13C NMR
(75 MHz, CDCl3, 25°C): δc 166.75, 164.14, 162.89, 152.59, 133.77, 131.08, 127.13, 120.96,
112.79, 108.00, 101.51, 77.42, 76.57, 68.33, 52.12, 31.89, 29.53, 25.97, 22.66. 14.10; UV-visible
(CHCl3): λmax = 367, 347, 280, 263 nm; Elemental analyses: calculated for C35H43NO6 (%),
C, 73.27; H, 7.55; N, 2.49; Found, C, 73.33; H, 7.51; N, 2.53.
... N-(-(ꢀ-n-tetradecyloxy)benzoyloxy--hydroxybenzylidene)--carbethoxyaniline,
CLH
−1
=
Yield: 87%; IR (KBr, cm ): 3474(OH), 2919–2851(aliphatic C-H), 1724(ester, C( O)OPh),
1710(ester, C( O)OEt), 1636( C N), 1602–1510(Ph), 1274, 1250(OPh); H NMR (CDCl3,
1
=
– =
–
=
TMS) δH (ppm): 13.53(s,1H,-OH), 8.83(s, 1H, CH N), 8.53–6.73(m, 11H, ArH), 4.53(q, 2H,
COOCH2-), 4.20(t, 2H, -OCH2), 2.20–1.20(m, 27H„-[CH2]n), 0.93(t, 3H, - CH3).; 13C NMR
(75 MHz, CDCl3, 25°C): δc 166.67, 164.24, 162.76, 152.59, 133.77, 131.09, 127.83, 120.86,
112.74, 108.01, 101.56, 77.52, 76.59, 68.36, 52.22, 31.88, 29.53, 25.96, 22.66. 14.10; UV-visible
(CHCl3): λmax = 377, 337, 276, 260 nm; Elemental analyses: calculated for C35H43NO6 (%),
C, 73.27; H, 7.55; N, 2.49; Found, C, 73.33; H, 7.51; N, 2.53.
... N-(-(ꢀ-n-hexadecyloxy)benzoyloxy--hydroxybenzylidene)--carbethoxyaniline,
CLH
−1
=
Yield: 87%; IR (KBr, cm ): 3474(OH), 2919–2851(aliphatic C-H), 1724(ester, C( O)OPh),
1710(ester, C( O)OEt), 1636( C N), 1602–1510(Ph), 1274, 1250(OPh); H NMR (CDCl3,
TMS) δH (ppm): 13.53(s,1H,-OH), 8.83(s, 1H, CH N), 8.53–6.73(m, 11H, ArH), 4.53(q, 2H,
COOCH2-), 4.20(t, 2H, -OCH2), 2.20–1.20(m, 31H„-[CH2]n), 0.93(t, 3H, - CH3).; 13C NMR
(75 MHz, CDCl3, 25°C): δc 165.65, 164.07, 162.78, 152.59, 133.77, 131.11, 127.73, 120.95,
112.76, 108.01, 101.52, 77.42, 76.58, 68.32, 52.12, 31.88, 29.53, 25.97, 22.66. 14.11; UV-visible
1
=
– =
–
=

MOL. CRYST. LIQ. CRYST.
81
(CHCl3): λmax = 358, 329, 276, 267 nm; Elemental analyses: calculated for C39H51NO6 (%),
C, 74.37; H, 8.16; N, 2.22; Found, C, 74.31; H, 8.06; N, 2.28.
General procedures for preparation of copper complexes, Cu(CnL)2:
... Synthesis of copper(II) complex of N-(-(ꢀ-n-decyloxy)benzoyloxy--
hydroxybenzy-lidene)--carbethoxy aniline, Cu(CL)
To a solution (20 mL) of N-(4-(4ꢀ-n-decyloxy)benzoyloxy-2-hydroxybenzylidene)-4-
carbethoxy aniline (1.14 g, 2 mmol) in hot methanol/chloroform (1/1, v/v,) stirred at
room temperature was added copper acetate (0.2 g, 1 mmol) in methanol (10 mL). The
mixture was stirred at room temperature for 4 h. The resulting brown solid was filtered off,
washed with methanol and recrystallized from chloroform-ethanol (1/1, v/v).
All the other members of the series were prepared in a similar manner. The yield, IR, UV-
Vis and elemental data for the compounds are summarized as follows.
... Copper(II) complex of N-(-(ꢀ-n-hexyloxy)benzoyloxy--hydroxybenzylidene)-
-carbethoxy aniline, Cu(CL)
−1
=
Yield: 78%; IR (KBr, cm ): 2920–2857(aliphatic C-H), 1724(ester, C( O)OPh), 1712(ester,
=
– =
C( O)OEt), 1608( C N), 1599–1510(Ph), 1280(OPh); UV–visible: (nujol): λmax = 456, 379,
280, 267, 212; Elemental analyses: calculated for C58H60N2O12Cu (%), C, 66.94; H, 5.81; N,
2.69; Cu, 6.10; Found, C, 66.99; H, 5.78; N, 2.73; Cu, 6.20.
... Copper(II) complex of N-(-(ꢀ-n-octyloxy)benzoyloxy--hydroxybenzylidene)-
-carbethoxy aniline, Cu(CL)
−1
=
Yield: 89%; IR (KBr, cm ): 2922–2852(aliphatic C-H), 1724(ester, C( O)OPh), 1712(ester,
=
– =
C( O)OEt), 1608( C N), 1599–1510(Ph), 1280(OPh); UV–visible: (nujol): λmax = 463, 386,
280, 222; Elemental analyses: calculated for C62H68N2O12Cu (%), C, 67.90; H, 6.25; N, 2.55;
Cu, 5.80; Found, C, 67.88; H, 6.20; N, 2.58; Cu, 5.70.
... Copper (II) complex of N-(-(ꢀ-n-decyloxy)benzoyloxy--hydroxybenzylidene)-
-carbethoxy aniline, Cu(CL)
−1
=
Yield: 89%; IR (KBr, cm ): 2922–2852(aliphatic C-H), 1724(ester, C( O)OPh), 1712(ester,
=
– =
C( O)OEt), 1608( C N), 1599–1510(Ph), 1280(OPh);UV–visible: (nujol): λmax = 453, 384,
278, 212; Elemental analyses: calculated for C62H68N2O12Cu (%), C, 67.90; H, 6.25; N, 2.55;
Cu, 5.80; Found, C, 67.88; H, 6.20; N, 2.58; Cu, 5.70
... Copper (II) complex of N-(-(ꢀ-n-dodecyloxy)benzoyloxy--hydroxybenzylidene)-
-carbethoxy aniline, Cu(CL)
−1
=
Yield: 89%; IR (KBr, cm ): 2922–2852(aliphatic C-H), 1724(ester, C( O)OPh), 1712(ester,
=
– =
C( O)OEt), 1608( C N), 1599–1510(Ph), 1280(OPh);UV–visible: (nujol): λmax = 454, 380,
279, 210; Elemental analyses: calculated for C70H84N2O12Cu (%), C, 69.54; H, 7.00; N, 2.32;
Cu, 5.30; Found, C, 69.55; H, 7.06; N, 2.30; Cu, 5.20
... Copper (II) complex of N-(-(ꢀ-n-tetradecyloxy)benzoyloxy--hydroxybenzylide)-
-carbethoxy aniline, Cu(CL)
−1
=
Yield: 89%; IR (KBr, cm ): 2922–2852(aliphatic C-H), 1724(ester, C( O)OPh), 1712(ester,
=
– =
C( O)OEt), 1608( C N), 1599–1510(Ph), 1280(OPh);UV–visible: (nujol): λmax = 450, 387,
270, 213; Elemental analyses: calculated for C74H92N2O12Cu (%), C, 70.24; H, 7.33; N, 2.22;
Cu, 5.03; Found, C, 70.18; H, 7.20; N, 2.18; Cu, 4.98.

82
S. K. SINGH ET AL.
... Copper (II) complex of N-(-(ꢀ-n-hexadecyloxy)benzoyloxy--
hydroxybenzylidene)--carbethoxy aniline, Cu(CL)
−1
=
Yield: 89%; IR (KBr, cm ): 2922–2852(aliphatic C-H), 1724(ester, C( O)OPh), 1712(ester,
=
– =
C( O)OEt), 1608( C N), 1599–1510(Ph), 1280(OPh);UV–visible: (nujol): λmax = 457, 380,
273, 222; Elemental analyses: calculated for C78H100N2O12Cu (%), C, 70.91; H, 7.63; N, 2.12;
Cu, 4.80; Found, C, 70.95; H, 7.66; N, 2.18; Cu, 4.74.
3. Results and discussion
3.1. Synthesis and characterization
The synthetic route for preparation of Schiff base N-4-(4ꢀ-n-alkoxy)benzoyloxy-2-
hydroxybenzylidene)4-carbethoxy anilines and their copper (II) complexes is outlined
in Scheme 1.
Scheme . Reactionsandreagents: (a) CnHn+Br (. equiv.), KOH (. equiv.), refluxing in EtOH,  h, (b) SOCl
(. equiv.), refluxing in dry chloroform,  h (c) , dihydroxybenzaldehyde (. equiv.), pyridine (catalyst),
refluxing in dry DCM. (d) acetic acid ( drops), refluxing in EtOH,  h. (e) Cu(OAc).HO (. equiv.), refluxing
in EtOH/CHCl (:),  h.
Alkylation of 4-hydroxybenzoic acid with alkyl bromide in presence of strong base in
ethanol allowed us to obtain a reasonable yield of 4-alkoxybenzoic acid, which on fur-
ther esterification with 2, 4-dihydroxy benzaldehyde gave 4-(4ꢀ-alkoxy)benzoyloxy-2-hydroxy
benzaldehyde. The Schiff base was obtained by the condensation of ethyl 4-aminobenzoate
with 4-(4ꢀ-alkoxy) benzoyloxy-2-hydroxybenzaldehyde in refluxing ethanol.

MOL. CRYST. LIQ. CRYST.
83
The copper (II) complex was prepared by reaction of ethyl-(4-(4ꢀ-alkoxy) benzoyloxy-2-
hydroxy-benzylideneamino)-benzoate with Cu(OAc)2.H2O in stirring ethanol. The copper
complex was then isolated as brown crystals by recrystallization from absolute ethanol in good
yield (66–70%).
The ligands and their copper (II) complexes were characterized by elemental analyses
and standard spectroscopic techniques. The IR spectrum of 4-(4ꢀ-decyloxy) benzoyloxy-
2-hydroxybenzaldehyde shows absorption bands at 3408, (2925, 2853), 1714, 1678, (1605,
1512), (1304, 1255) cm−1 assigned to ν (OH), ν (aliphatic C-H stretch), ν (C O phenyl
=
ꢀ
=
ester), ν (C O aldehyde), ν (Ph), and ν (OPh), respectively. The IR spectrum of N-[4-[4 -n-
decyloxy)benzoyloxy-2-hydroxybenzylidene)-4-carbethoxy aniline shows absorption bands
at 3470, (2906, 2857), 1724, 1711, 1628, (1599, 1510) and (1275, 1253) which are assigned to
=
=
=
ν (OH), ν (aliphatic C-H), ν (C O) of phenyl ester, ν (C O) of ethyl ester, ν (C N), ν (Ph)
and ν (OPh) modes, respectively. The disappearance of the band at 1678 cm−1 due to ν (CH
= O) and appearance 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-(4ꢀ-decyloxy) benzoyloxy-2-
hydroxybenzaldehyde exhibit peaks δ 11.23 (s), 9.88 (s), 8.12–6.87 (m), 4.06–4.02 (m), 1.84–
1.26 (m), and 0.87 (t) ppm which are assigned to -OH, -CHO, ring, -OCH2, -[CH2]n and
-CH3 protons, respectively. The proton NMR spectrum of N-(4-(4ꢀ-n-decyloxy)benzoyloxy-
2-hydroxybenzylidene)-4-carbethoxy anilines exhibits peaks at δ 13.53(s, 1 H, -OH), 8.83(s,
1H, -CH = N), 8.53–6.73(m, 11H, ring), 4.53(q, 2H, COOCH2), 4.20(t, 2H, O CH2-), 2.2–
1.2(m, 19H,-[CH2]n) and 0.93 (t, 3H, - CH3) ppm, respectively. The signals due to Schiff base
formation is observed at 8.83 ppm which corresponds to o-hydroxy benzylidene. The signals
due to three phenyl protons overlap and are observed at δ 8.53–6.63. 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) complex Cu(C10L)2, shows absorption bands at (2922,
2852), 1724, 1712, 1608, (1599, 1510), and (1304, 1280) which are assigned to ν (aliphatic
=
=
=
C-H), ν (C O) of phenyl ester, ν (C O) of ethyl ester, ν (C N), ν (Ph) and ν (OPh) modes,
respectively. The disappearance of the band 3470 cm−1 due to ν (OH) and considerable low-
=
ering in ν (C N) indicates deprotonation of phenolic propton and coordination of ligand to
copper(II) ion through the phenoxy oxygen and azomethine nitrogen.
The electronic-spectrum of the brown color copper (II) complex shows transitions at 453,
384, 278, and 212 nm. The first two bands correspond to square planar environment of the
metal [21]. The remaining bands are attributed to the π- π∗ and n- π∗ transitions of the
ligand.
3.2. DFT study
The GAUSSIAN-03 program [22] package was employed to carry out DFT [23–28] calcula-
tions at the Becke’s three parameter functional and Lee, Yang and Parr correlation functional
(B3LYP) level [28–30] of calculation. 6–311++G (d,p) basis set [31] was used for ground
state geometry optimization and frequency calculations of the ligand (C10LH) while for metal
complex (Cu(C10L)2) mixed basis set [32] 6–311++G (d,p) for C, H, N atoms and LANL2DZ
for copper metal is used. The internal coordinates of the system, which is used as input for
GAUSSIAN-03 program was generated by the GAUSS VIEW 4.1 program [33]. The optimized
structures of the ligand, C10LH and its copper complex, Cu(C10L)2 are shown in Fig. 2.

84
S. K. SINGH ET AL.
Figure . DFT optimized structures of (a) ligand, CLH and (b) its copper(II) complex (CL)Cu.
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 and copper complex are found to be 33.41, 49.99 A°, respectively, from DFT
calculations. All the three phenyl rings are twisted with respect to central ring in the ligand and
copper complex both. For the copper complex the calculated dihedral angles between benzene
rings are as follow: C(131)-C(127)-C(137)-C(130), 132.69°; C(131)-C(27)-C(118)-C(116),
148.42; C(21)-C(17)-C(30)-C(32), 125.88; C(21)-C(17)-C(3)-C(4), 146.77°. The above cal-
culations implying twist of two outer most phenyl rings with respect to central ring.
Some of the selective geometric parameters of the optimized Cu(C10L)2 complex, evaluated
by DFT calculation at B3LYP/6–311++G (d,p) + LANL2DZ level are shown in Table 1.
From DFT study, it has been found that the complex has average Cu–O1, Cu–N1, Cu–O2,
˚
Cu–N2 bond lengths of 1.86, 2.08, 2.00 and 2.10 A respectively, and bond angles 92.350° and
87.670° for O(1)– Cu –N(1) and N(2)– Cu –O(2), respectively, which matched well with the
other related copper salicylaldimine complex [11b].
In general, the molecular shape of 4-(benzoyloxy)salicylaldimine complexes derived from
2,4-dihydroxybenzaldehyde, can be envisaged as two rod-like structures joined by a central
group and smectic C and nematic phases were identified for the copper(II) complexes of such
type [2a].
Table . Selected bond length and bond anglesa of Cu(CL), from DFT study.
Structure parameter
Distance/angles
Cu–O()
Cu –O()
.
.
Cu –N()
Cu –N()
.
.
O()– Cu –O()
N()– Cu –N()
N()– Cu –O()
O()– Cu –N()
O()– Cu –N()
N()– Cu –O()
.
.
.
.
.
.
a
˚
Bond lengths are reported in A and bond angles in degrees.

MOL. CRYST. LIQ. CRYST.
85
Table . Thermal Transitions and Corresponding Thermodynamic Parameters of CLH- CLH.
T, °C [ꢀH, kJ/mol; ꢀS, J/(mol K)]
no.

Compounds
CLH
Heating
k . (.; .) SmA .
(.; .) i
k . (.; .) SmA .
(.; .) i
Cooling
i . (−.; −.) SmA .
(−.; −.) k



CLH
CLH
CLH
i . (−.; −.) SmA .
(−.; −.) k
k . (.; .) SmA .
(.; .) i
i . (−.; −.) SmA .
(−.; −.) k
k . (.; .) k . (.;
.) SmA . (.; .) N
. (.; .) i
i . (.; .) N . (.;
.) SmA . (.; .) k

CLH
CLH
k . (.; .) k . (.;
.) SmA . (.; .) N
. (.; .) i
i . (.; .) N . (.;
.) SmA . (.; .) k

k . (.; .) k . (.;
.) SmA . (.; .) N
. (.; .) i
i . (.; .) N . (.;
.) SmA . (.; .) k
k = crystal; SmA = smectic A; N = nematic; i = isotropic liquid phases. The transition temperatures (°C), enthalpies, and
entropies are determined by DSC at the scan rate of °C/min.
3.3. Mesomorphic behavior
The liquid crystalline properties of N-(4-(4ꢀ-n-alkoxy)benzoyloxy-2-hydroxybenzylidene)-4-
carbethoxy anilines (C6LH→ C16LH) and their copper (II) complexes were investigated by
polarizing optical microscope (POM) using a heating and cooling stage. The phase transition
temperatures and enthalpies were measured by carrying out DSC thermal analysis. The phase
transition temperatures, enthalpy and entropy values for the azines are summarized in Table 2.
The symbols k, SmA, N and i denote crystalline, smectic A, nematic and isotropic phase,
respectively.
The DSC thermograms of the hexyloxy derivatives of the series, N-(4-(4ꢀ-n-
hexyloxy)benzoyloxy-2-hydroxybenzylidene)-4-carbethoxy anilines (C6LH) shown in
Figure 3 show two endothermic peaks at 103.4 (ꢀH = 32.8 kJmol−1) and 139.7°C (ꢀH =
0.4 kJmol−1) in heating cycle and two exothermic peaks at 116.4 (ꢀH = 0.1 kJmol−1) and
Figure . DSC thermogram of CLH.

86
S. K. SINGH ET AL.
Figure . Microphotographs of the compounds CnLH in heating cycle (a) broken fan texture of SmA
mesophase of CLH at .°C (b) SmA to nematic phase transition of CLH at .°C (c) schlieren texture of
nematic mesophase of CLH at .°C (d) nematic to isotropic phase transition of CLH at .°C.
61.9°C (ꢀH = 19.9 kJmol−1) in cooling cycle. The two endothermic peaks observed for this
compound define crystal → mesophase and mesophase → isotropic (I) phase transitions
respectively.
The exothermic peaks reveal an isotropic → mesophase transition and mesophase → crys-
tal phase transition, respectively. The texture of the mesophase seen through polarizing micro-
scope is signature of SmA mesophase, which has fluid property with higher viscosity and strat-
ified structures with well-defined interlayer spacing. 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. The
molecules are aligned parallel to the layer normal, maintaining long-range orientational order
and short-range positional order, which lead to the SmA mesophase. The molecular packings
are found to be random for this SmA phase. A focal-conic texture is seen for this compound
through optical microscopy (Fig. 4a). The appearance of smectic A phase may be arising due
to the additional intramolecular forces between the phenolic OH group and nitrogen atom
in the azines. Thus focal –conic texture is seen for this compound under optical microscopy,
which confirms the smectic A mesophase. The similar phase transitions with similar textural
pattern are shown by the next two higher members of the series with octyloxy and decyloxy
chain length (C8LH and C10LH).
In the first heating scan of C12LH, four sharp peaks appear at 61.4, 100.5, 157.5, and
187.0°C. The DSC thermogram recorded in the cooling scan exhibits three separate peaks
at 185.7, 150.3 and 60.1°C (Fig. 5). POM observations reveal that upon heating from crys-
tal state to 100.5°C, focal conic textures corresponding to SmA mesophase emerges. Further
heating of the compound at 157.5°C leads to formation of nematic phase (Fig. 4b). The tex-
ture for compound C12LH between crossed polarizer resulting from surface orientation of

MOL. CRYST. LIQ. CRYST.
87
Figure . DSC thermogram of CLH.
the director of the boundaries of the sample and by defects formed in the sample shows a net-
work 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 produc-
ing schlieren optical texture in nearly vertical orientation whose projection are seen as dark
points with 2–4 emerging dark stripes or brushes (Fig. 4c). These black brushes are located
in region where the director lies in the plane of polarization of the polarizer or analyzer. Fur-
ther, heating of the compound at 187°C leads to clearing (isotropization) of the compound
(Fig. 4d). It thus becomes clear that the peak at 61.4°C observed in DSC heating scan is due
to crystal transformation from one state to another (k–k transition); such polymorphism has
often been observed in Schiff’s base containing compounds [34]. In cooling cycle, the isotropic
phase changes to nematic phase at 185.7°C, which further transits to a stratified structure with
well defined inter layer spacing SmA mesophase with ordered hexagonal structure at 150.3°C
and finally transits to solid crystalline phase at 60.1°C. Thus, this compound exhibits enan-
tiotropic smectic A to nematic mesophase transition. Similar phase transitions and smectic A
to nematic mesophase transitions in both heating and cooling cycle showing similar textural
patterns are observed for the compounds C14LH and C16LH also.
The isotropization temperature (Ti) of these ester series of compounds is found to be con-
tinuously increasing with increase in alkoxy chain length (C6LH →C16LH) but melting tem-
perature (Tm) is found to be almost constant, resulting a continuous increase in mesophase
thermal stability along with higher members of the series. (Fig. 6)
The thermotropic smectic and nematic mesophase pattern based on the structural corre-
lation of these compounds reveals a calamitic (rod like) shape 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 [2]. The
study based on N-salicylidene with ether bridging moiety has revealed to the emergence of
nematic and smectic A mesophases [7–14]. The substitution of ether linkage with the ester

88
S. K. SINGH ET AL.
Figure . Phase diagram of the mesogenic ligands, CnLH.
moiety along with the incorporation of aromatic ring with long alkoxy chain results in sup-
pression of smectic A mesophase and leads to the occurrence of nematic phase in higher
homologs of the series. Thus, it may be concluded that by the replacement of –OR with –
OOC–C6H4–OR, the mesophase thermal stability of the system increases but order of the
system decreases.
The copper(II) complexes of N-(4-(4ꢀ-n-alkoxy)benzoyloxy-2-hydroxybenzylidene)-4-
carbethoxy anilines were found to be non-mesogenic with almost same clearing temperature
independent of the carbon chain length. Thus in the metal complexes, chain length plays no
role on isotropic temperature and polymorphism. This is probably due to fact that presence
of tilted ethoxy-benzoate group in imine moiety considerably broadens the molecular width
(Fig. 3). Moreover, the ester containing benzylidene ring is not coplanar with the central ring
containing metal ion as depicted by DFT studies. This broadening of molecular width and the
non coplanarity of the imine ring is less favorable for exhibiting liquid crystal properties of
copper(II) complexes. Therefore terminal alkoxy ester bond adversely affects the mesophase
formation in such ester substituted salicyldimine metal complexes. The copper complexes of
substituted imine derivatives without alkoxy ester end group of the same core (salicyldimine)
and same alkoxy chain length (n = 2) are showing SmA mesophase with high thermal stability
[11b].
4. Conclusions
A new series of ester containing Schiff base derivatives having terminal alkoxy chains (n =
6–16) and their copper (II) complexes has been synthesized. The lower member of the series
(n = 6–10) shows enatiotropic SmA mesophase while higher members enatiotropic SmA to
nematic mesophase transition with good thermal stabilities. The copper(II) complexes are
nonmesogenic in nature. Thus, addition of diester bond at salicyldeneaniline ring helps in
inducing mesogenic property in the ligands and also their thermal stability but it is unfavor-
able for mesomorphic properties of the metal complexes due to non-coplanarity of the ester

MOL. CRYST. LIQ. CRYST.
89
benzoate ring and also ethoxy ester end group which broadens the molecular width leading
to transition of the molecule in higher order state i.e. solid state.
Acknowledgments
HKS is grateful to Council of Scientific and Industrial Research (CSIR), New Delhi, India,
for providing research fellowship. Financial assistance provided by DRDO, India (Project No.
ERIP/ER/1203053/M/01/1542) is gratefully acknowledged.
References
[1] (a) Venkatesan, K., & Muller, P. (2008). J. Mat. Chem., 18, 400. (b) Seredyuk, M., Gasper, A. B., &
Reiman, S. (2006). Chem. Mater., 18, 2513. (c) Liao, C. T., & Wang, Y. J. (2007). Tetrahedron, 63,
12437. (d) Sago, K. K., & Fujita, A. (2007). Mol. Cryst. Liq. Cryst., 479, 151. (e) Chulson, K., Park,
D. J., & Kim, Y. B. (2007). Mol. Cryst. Liq. Cryst., 479, 81. (f) Bhattacharya, S. S., Rahman, M., &
Mukherjee, A. (2008). Appl. Phys. Lett., 92, 122909. (g) Vasconcelos, U. B., & Schrader, A. (2008).
Tetrahedron, 64, 4619. (h) Lee, J. H., Mehoic, S., & Pate, B. D. (2006). J. Mater. Chem., 16, 2785.
[2] (a) Serrano, J. L. (1996). Metallomesogens; Synthesis, Properties, and Applications, VCH: New York.
(b) Kumar, S. (2010). Chemistry of Discotic Liquid Crystals: From Monomers to Polymers, CRC
Press: New York. (c) Stewart, G. T. (2003). Liq. Cryst., 30, 541. (d) Stewart, G. T. (2004). Liquid
Cryst., 31, 443.
[3] (a) Gray, G. W. (1962). Molecular Structure and the Properties of Liquid Crystals, Academic Press:
London. (b) Singh, S. K., Vikram, K., & Singh, B. (2011). Liq Cryst., 38 (9), 1117.
[4] Kelker, H., & Scheurle, B. (1969). Angew. Chem. Int. Edn., 81, 903.
[5] Hoshino, N. (1998). Coord. Chem. Rev., 174, 77.
[6] Ovchinnikov, I. V., Galyametdinov, Y. G., Ivanova, G. I., & Yagfarova, L. M. (1984). Dokl. Akad.
Nauk SSR, 276, 126.
[7] (a) Ovchinnikov, I. V., Galyametdinov, Yu. G., & Bikchantaev, I. O. (1989). Izv. Akad. Nuuk SSSR
Ser. Fiz., 53, 1870. (b) Bikchantaev, I. G., Galyametdinov, Yu. G., & Ovchinnikov, I. V. (1987). Zh.
Strukt. Khim., 28, 61. (c) Ovchinnikov, I. V., Bikchantaev, I. G., Galyametdinov, Yu. G., & Galimov,
R. M. (1988). 24th Ampere congress, Poznan.
[8] Caruso, U., Roviello, A., & Sirigu, A. (1988). Liq. Cryst., 3, 1515.
[9] Carfagna, C., Caruso, U., Roviello, A., & Sirigu, A. (1987). Macromol. Chem. Rapid Commun., 8,
345.
[10] Alonso, P. J. et al. (1993). Liq. Cryst., 13, 585.
[11] (a) Ghedini, M., Morrone, S., Gatteschi, D., & Zanchini, C.Chem. Mater., 3, 752. (b) Hoshino, N.,
Murakami, H., Matsumaza, Y., Inabe, T. I., & Maruyama, Y. (1990). Inorg. Chem., 29, 1177.
[12] Ghedini, M. et al. (1988). J. Mol. Liq., 38, 207.
[13] Ghedini, M. et al. (1987). Mol. Cryst. Liq. Cryst., 151, 75.
[14] Torquati, G. et al. (1990). II Nuovo Cimento, 12D, 1363.
[15] Dave, J. S., & Kurian, G. (1977). Mol. Cryst. liq. Cryst., 42, 175.
[16] Matsunaga, Y., & Miyajima, N. (1990). Mol. Cryst. Liq. Cryst., 178, 157.
[17] (a) Singh, B., Singh, M. K., Dhar, R., & Srivastava, A. K. (2004). Mol. Cryst. Liq. Cryst., 411, 29.
(b) Caruso, U., Roviello, A., & Sirigu, A. (1988). Liq. Cryst., 3, 1515.
[18] (a) Hoshino, N., Takashi, K., Sekiuchi, T., Tanaka, H., & Matsunaga, Y. (1998). Inorg. Chem., 37,
882. (b) Tian, Y. et al. (1996). Liq. Cryst., 20, 139.
[19] Riddick, J. A., Bunger, N. B., & Sanako, T. K., (1986). Organic Solvents, Physical Properties and
Methods of Purification (Techniques of Chemistry), 4th ed. Wiley–Interscience: New York.
[20] Singh, M. K., Singh, B., Dhar, R., & Agrawal, V. K. (2005). Phase Transitions, 78(6), 495.
[21] Nicholls, D. (1973). In: Comprehensive Inorganic Chemistry, Chapter 42, Pergamon Press: Oxford,
31983, 1109.
[22] Frisch, M. J., et al. (2003). Gaussian 03, Revision A.1, Gaussian, Inc.: Pittsburgh.
[23] Hohenberg, P., & Kohn, W. (1964). Phys. Rev. B., 136, 864.
[24] Kohn, W., & Sham, L. J. (1965). Phys. Rev. A., 140, 1133.

90
S. K. SINGH ET AL.
[25] Becke, A. D. (1988). Phys. Rev. A., 38, 3098.
[26] Becke, A. D. (1992). J. Chem. Phys., 96, 2155.
[27] Becke, A. D. (1992). J. Chem. Phys., 97, 9173.
[28] Becke, A. D. (1993). J. Chem. Phys., 98, 5648.
[29] Lee, C., Yang, W., & Parr, R. G. (1988). Phys. Rev. B., 37, 785.
[30] Ditchfield, R., Hehre, W. J., & Pople, J. A. (1971). J. Chem. Phys., 54, 724.
[31] Hehre, W. J., Ditchfield, R., & Pople, J. A. (1972). J. Chem. Phys., 56, 2257.
[32] Dziembowska, T. et al. (2003). J. Spectrochim. Acta Part A, 59, 2175.
[33] Dennington II, R. et al. (2003). Gauss View 03, Semichem, Inc.: Shawnee Mission, KS.
[34] (a) Collings, P. J., & Hird, M. (1998). Introduction to Liquid Crystals Chemistry and Physics, Taylor
and Francis: London. (b) Galewaki, Z. (1994). Mol. Cryst. Liq. Cryst., 249, 43.