Synthesis and mesomorphism of new chiral imines and copper(II) complexes

Article abstract of DOI:10.1016/j.molstruc.2013.02.007

The synthesis, characterization and mesomorphic properties of a series of N-(2-hydroxy-4-n-alkyloxybenzylidene) 4′-amino-2-methylbutylcinnamate and their copper(II) complexes are reported. The imines exhibited smectic A (SmA) and chiral smectic C (SmC^·) phases while their copper(II) complexes displayed SmA-SmC phase variant. The compounds have been characterized by elemental analysis and spectroscopic studies. The phase transition temperatures were detected by differential scanning calorimetry (DSC) analysis and the phases are confirmed by polarizing optical microscopy (POM). The chiral phase (SmC^·) of the ligand disappeared on complexation. The unwinding of the helical superstructure of SmC^· phase under the influence of electric field and the observed optical textures of SmC ^· phase are also discussed.

Full text of DOI:10.1016/j.molstruc.2013.02.007

Journal of Molecular Structure 1039 (2013) 219–226
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis and mesomorphism of new chiral imines and copper(II) complexes
a
a
b
a
Manoj K. Paul a, , Gayatri Kalita , Atiqur Rahman Laskar , Trirup D. Choudhury , Nandiraju V.S. Rao
⇑
a Chemistry Department, Assam University, Silchar 788 011, Assam, India
b Department of Chemistry, Karimganj College, Karimganj 788 710, Assam, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2012
Received in revised form 14 January 2013
Accepted 6 February 2013
Available online 20 February 2013
The synthesis, characterization and mesomorphic properties of a series of N-(2-hydroxy-4-n-alkyloxyb-
enzylidene) 40-amino-2-methylbutylcinnamate and their copper(II) complexes are reported. The imines
exhibited smectic A (SmA) and chiral smectic C (SmCꢀ) phases while their copper(II) complexes displayed
SmA–SmC phase variant. The compounds have been characterized by elemental analysis and spectro-
scopic studies. The phase transition temperatures were detected by differential scanning calorimetry
(DSC) analysis and the phases are confirmed by polarizing optical microscopy (POM). The chiral phase
(SmCꢀ) of the ligand disappeared on complexation. The unwinding of the helical superstructure of SmCꢀ
phase under the influence of electric field and the observed optical textures of SmCꢀ phase are also
discussed.
Keywords:
Liquid crystals
Chirality
Metallomesogens
Schiff base and copper(II) complexes
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
core because of intramolecular hydrogen bonding. Sakurai et al. re-
ported [11] that the introduction of ortho hydroxyl group to imine
Materials exhibiting different functional properties can be
derived by a combination of organic moieties with soft material
properties and metal compounds possessing inherent rich electro-
magnetic characteristics. Ferroelectric liquid crystals possess un-
ique combination of polar symmetry, spontaneous polarization
and fluidity, which make them as potential candidate for modern
technological application [1,2]. Meyer et al. [3,4] discovered ferro-
electricity in the chiral smectic C (SmCꢀ) phase of liquid crystals
derived from molecules possessing chiral center which lift the cen-
tro-symmetry of the molecule. Several chiral smectic liquid crys-
talline compounds showing ferroelectric and anti-ferroelectric
[5–7] properties have been synthesized and well studied. The li-
quid crystals derived from benzylidene cores offer multifaceted
advantages, e.g. ease of synthesis, variety of liquid crystalline
phases and at ambient temperatures, availability of materials for
different experimental investigations as model compounds. The
presence of ortho hydroxyl group in benzylidene moiety enhances
the stability of imines through intra-molecular hydrogen bonding
and contributes to the rigidity and polarizability of the mesogenic
nucleus. The introduction of the polar hydroxyl group in a lateral
position of the molecule promoted the SmC/SmCꢀ temperature
range. For example, the resorcylidene aniline core [8–10] present
in calamitic ferroelectric liquid crystals such as in 2-meth-
ylbutylresorcylidene aniline (MBRA) and 2-methyloctylresorcylid-
ene aniline (MORA) series exhibits superior mesogenic behavior
and more stability towards hydrolysis than benzylidene aniline
linkage in a chiral Schiff base increases the mesomorphic range of
chiral SmCꢀ phase. During the last three decades the metallomeso-
gens (metal-containing liquid crystalline molecules) of d- and f-
block metals have attracted the attention of several research
groups [12–25] because of their unusual geometries, and func-
tional properties viz., ferro-electricity, non-linear optical property,
etc. Ferroelectric properties had been realized in Schiff bases viz.,
[N(4-n-decyloxy salicylidene) 40-substituted alkyl aniline] when
the end alkyl moiety is replaced with a chiral tail [26]. Baena
et al. reported [27,28] the first examples of paramagnetic and
ferroelectric copper(II) and vanadium(IV) complexes (Fig. 1) of chi-
ral Schiff bases. Wide thermal range of SmCꢀ phase is realized in
copper(II), vanadium(IV) and palladium(II) complexes [29] of lat-
eral-lateral fused twin ferroelectric metal–organic liquid crystals.
Recently, a new class of low molar mass chiral metallomesogens
containing cholesterol based N-(n-alkyl)salicylideneimines dis-
played chiral nematic (Nꢀ) and smectic A (SmAꢀ) phases [30]. In
this paper, we report the synthesis, spectroscopic characterization,
photo-physical and mesomorphic properties of a new series of cop-
per(II) complexes derived from chiral imines.
2. Experimental
2.1. Physical measurements
UV–Visible absorption spectra of the compounds in CHCl3 were
recorded on a Shimadzu UV-1601PC spectrophotometer. Infrared
spectra were recorded on a Perkin Elmer L 120-000A spectrometer
using a KBr disk. The 1H NMR spectra were recorded on a Bruker
⇑
Corresponding author. Tel.: +91 3842270848.
E-mail address: paulmanojaus@gmail.com (M.K. Paul).
0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2013.02.007

220
M.K. Paul et al. / Journal of Molecular Structure 1039 (2013) 219–226
4-n-pentyloxysalicylaldehyde dissolved in hot ethanol and an
equimolar ethanolic solution of (+)-2-methylbutyl-4-amino cinna-
mate (230 mg, 1 mmol) added drop wise with a few drops of gla-
cial acetic acid as catalyst and refluxed for 3 h to yield the yellow
Schiff base. The precipitate was collected by filtration and recrys-
tallized several times from absolute ethanol to give a pure com-
pound. Yield = 260 mg (61%).
IR (
and CH2) and 2855(
C@N, imine), 1241 (
1H NMR (300 MHz, CDCl3): d = 13.54 (s, 1H, OH), 8.54 (s, 1H,
CH@N), 7.68 (d, 1H, J = 16.2 Hz, AC@CHA, proton), 7.57–6.48
m
max, cmꢁ1, KBr): 3442 (
s (CAH) CH3 and CH2), 1702 (
CAO).
m
OH, H bonded), 2925(mas (CAH) CH3
m
m
C@O, ester), 1629
(m
m
a
(m, 7H, ArAH), 6.44 (d, 1H, J = 16.2 Hz, AC@CHA, b proton), 4.24
(t, J@CH2ACOOA), 4.11 (s, 1H, tertiary CAH), 4.00 (t, J = 6.9 Hz,
AOCH2A), 1.78–1.17 (m, 10H, A(CH2)A), 0.97 (t, 9H, J = 6.6 Hz,
CH3). 13C NMR (CDCl3): d = 11.4, 14.1, 16.6, 22.7, 25.8, 26.3, 29.0,
29.1, 31.7, 34.4, 68.8, 69.4, 101.2, 102.0, 108.9, 115.8, 118.4,
121.6, 129.5, 130.0, 134.5, 135.4, 143.6, 164.7, 167.9, 178.6. Analy-
sis Calc. for C26H33NO4: C, 73.73; H, 7.85; N, 3.31. Found: C, 73.56;
H, 7.53; N, 3.01.
Fig. 1. Molecular structure of the paramagnetic Schiff base complexes reported
[27,28].
DPX400 MHz spectrometer in CDCl3 (chemical shift in d) solution
with TMS as internal standard. Molar conductance of a representa-
tive ligand and its complex were determined in dimethylformam-
ide (ca. 10ꢁ3 mol Lꢁ1) at room temperature using Systronics CM304
conductivity meter. The optical textures to characterize the differ-
ent mesophases of the compounds were observed using a polariz-
ing microscope (Nikon Optiphot2pol), equipped with hot stage
(Instec hot- and cold-stage HCS302 with STC200 temperature con-
troller). The accuracy in temperature measurement is 0.1 °C. The
phase transition temperatures and associated enthalpies of the
compounds were recorded using differential scanning calorimeter
(DSC Perkin–Elmer Pyris1 system) at a heating or cooling rate of
5 °C minꢁ1. The electric field experiments were performed using
20 Mz function generator (33220A Agilent). The optical studies
on free standing film were studied by an indigenously assembled
depolarized reflected light microscopy (DRLM).
All the homologues (2a–2i) were synthesized following the
same procedure. The FTIR, 1H NMR and CHN data of Schiff bases
(2a–2i) are presented in ESI 1E.
2.5. Synthesis of Bis[N-(4-n-hexyloxysalicylidene)-2-methylbutyl-40-
aminocinnamate]-copper(II) complex (3a)
The ligand N-(4-n-pentyloxy-2-hydroxybenzylidene)-2-meth-
ylbutyl-40-aminocinnamate (840 mg, 2 mmol) was dissolved in
minimum volume of absolute ethanol and refluxed for 30 min. To
this refluxing solution an ethanolic solution containing copper ace-
tate monohydrate Cu(CH3COO)2ꢂH2O (200 mg, 1 mmol) was added
drop wise and refluxed it for another 3 h. The color of the solution
changed from yellow to brown. The brown precipitate appeared on
cooling to room temperature was filtered off and washed with eth-
anol and then repeatedly recrystallized from chloroform/ethanol
(1:3). Yield = 289 mg (61%).
2.2. Materials
IR (
CAH for CH3 and CH2), 1711 (
CAO).
Experimental analysis. Calc. for C52H64CuN2O8: C, 68.74; H,
m
max, cmꢁ1, KBr): 2959(
mas CAH for CH3 and CH2) and 2853(ms
The organic reagents required for the synthesis of chiral imines
and their copper(II) complexes were procured from Tokyo Kasei
(Japan), E. Merck (Germany), Aldrich (USA), BDH (India), Frinton
Chemicals (USA), Eastman Kodak Company (USA) and have been
used without further purification. The solvents used (viz. ethanol,
chloroform, n-hexane) are purified by the standard procedures
well documented in literature. Silica (60–120 mesh) from Spectro-
chem was used for chromatographic separation and silica gel G (E.
Merck, India) for TLC.
m
C@O, ester), 1610 ( C@N, imine), 1141
m
(m
7.10; N, 3.08. Found C, 68.70; H, 7.05; N, 3.01.
Similar procedure was adopted for the preparation of other
complexes (3b–i). The FTIR CHN data of Cu(II) complexes (3a–3i)
are presented in ESI 2E.
3. Results and discussion
The synthetic routes used for the preparation of chiral imines
and their mononuclear Cu(II) complexes:
The composition of chiral imines (2a–i) and corresponding
Cu(II) complexes (3a–i) were confirmed by elemental analysis.
The molecular structures were characterized by various spectro-
scopic methods, e.g. FTIR, 1H NMR and UV–Visible spectra. The
spectroscopic data confirmed the proposed molecular structures.
The liquid crystalline property was established by polarizing
microscopy and DSC study. The optical studies on free standing
film experiment were performed in one of the compound 2c under
electric field. The electric field study was carried out on a thin film
of chiral ligand 2a to investigate the effect of the electric field on
helicity.
The synthesis of the materials was carried out following the
procedures well documented in literature with little modification
[31–34]. The synthetic route for the preparation of imines and
mono nuclear copper(II) complexes are shown in Scheme 1.
2.3. Synthesis of 4-n-alkyloxysalicyldehyde (1)
4-n-Alkyloxysalicylaldehydes (1a–i) were synthesized and puri-
fied following the procedures described previously [32–34]. All the
compounds are obtained in good yield.
2.4. Synthesis of N-(4-n-pentyloxy-2-hydroxybenzylidene)-2-
methylbutyl-40-aminocinnamate (2a)
3.1. FTIR Study
All the chiral ligands were synthesized by condensing appropri-
ate 4-n-alkoxy salicylaldehyde with (+)-2-methylbutyl-4-amino
cinnamate in an ethanolic solution. About 200 mg (1 mmol) of
The IR spectra of the ligands (2a–i) showed the ACH@NA
stretching at the expected values of ꢃ1627 cmꢁ1, which has been
shifted to a lower frequency by ca. 10–12 cmꢁ1 as compared to that

M.K. Paul et al. / Journal of Molecular Structure 1039 (2013) 219–226
221
Scheme 1. The synthetic route for the preparation of chiral imines and its mono nuclear copper(II) complexes. Reagent and conditions: (i) KHCO3, dry acetone, KI (catalytic
amount), reflux for 24 h; (ii) Absolute ethanol, glacial acetic acid (2–3 drops), reflux for 3 h; (iii) Cu(CH3COO)2ꢂH2O, ethanol, reflux for 4 h.
0.8
0.6
1. Hexane
2. Benzene
3. Alcohol
4. Ethyl acetate
5. Chloroform
3f
1
2f
2
3
0.6
0.4
0.2
0.0
3f
0.4
0.2
0.0
4
5
300
350
400
450
Wavelength (nm)
300
350
400
450
Wavelength (nm)
Fig. 2. UV–Visible absorption spectrum of chiral Schiff base ligand (2f) and Cu(II)
complex (3f) recorded in chloroform at same concentration (c = 10ꢁ5 mol Lꢁ1).
Fig. 3. UV–Visible absorption spectrum of Cu(II) complex (3f) recorded in various
solvent at same concentration (c = 10ꢁ5 mol Lꢁ1).
of the free imine upon complexation with copper(II). This shift in
frequency confirmed the formation of Cu(II) complexes [35,36].
The stretching band of the C@C bond of cinnamate moieties of
imine ligands remain unchanged after complexation with the me-
tal ion. The observed stretching band at 1702–1710 cmꢁ1 region
was assigned to ester group (ACOOA) of cinnamate moieties of
the ligands (2a–i) and complexes (3a–i). The CAO stretching

222
M.K. Paul et al. / Journal of Molecular Structure 1039 (2013) 219–226
Table 1
Mesomorphic phase transition temperatures (T °C), associated enthalpies (D D
H italics, kJ molꢁ1) and entropy ( S in italics J molꢁ1 Kꢁ1) for the chiral imines (2a–2f) are presented in
parenthesis.
Chiral imines
Heating cycle
Cr
Cooling cycle
Iso
SmCꢀ
SmA
Iso
SmA
SmCꢀ
Cr
tm
tm
2a
2b
2c
2d
2e
2f
2g
2h
2i
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
78.6 [17.9, 50.9]
94.9 [22.3, 60.4 ]
94.9 [23.7, 64.6]
95.2 [24.7, 67.2]
88.5 [21.8, 60.3]
77.2 [19.2, 50.9]
76.2 [21.1, 60.4]
67.0 [46.7, 137.5]
68.1 [43.5, 127.5]
ꢅ
ꢅ
–
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
-
105.4tm
109.6
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
144.7 [4.8, 11.4]
143.5 [4.4, 10.3]
142.9 [4.7, 11.3]
141.8 [5.4, 12.7]
140.6 [5.5, 13.0]
139.3 [5.7, 13.7]
138.6 [4.7, 11.4]
137.5 [5.9, 14.3]
128.8 [2.5, 6.3]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
143.3 [4.7, 11.3 ]
143.0 [3.8, 8.9]
142.1 [4.4, 10.4]
141.3 [5.3, 12.5]
140.3 [5.4, 13.0]
139.0 [5.5, 13.2]
137.4 [3.8, 9.2]
136.4 [5.6, 13.6]
126.7 [3.2, 8.0]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
104.8
104.2
ꢅ
ꢅ
–
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
59.1 [15.6, 46.9]
82.3 [20.1, 56.6]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
tm
82.2 [22.3, 62.9]
115.9 [0.17, 0.44]
112.9 [0.16, 0.42]
117.0 [0.02, 0.05]
113.8 [0.03, 0.09]
66.4 [16.3, 18.2]
65.3 [20.2, 59.7]
59.1 [19.4, 58.5]
54.9 [17.4, 52.9]
53.2 [32.0, 98.1]
55.8 [47.8, 145.3]
tm
tm
95.5
83.6
80.1
75.5
89.3
56.7
tm
tm
tm
tm
101.5
tm
tm Indicates POM observed values. Here, Cr = Crystalline phase, SmA = smectic A phase, SmCꢀ = chiral smectic C phase. Heating and cooling rates are 5 °C minꢁ1 for the melting
and clearing processes.
Table 2
Mesomorphic phase transition temperatures (T °C), associated enthalpies (D D
H in italics kJ molꢁ1) and entropy ( S in italics J molꢁ1 Kꢁ1) for Copper(II) complexes are presented in
parenthesis.
Complexes
Heating cycle
Cr
Cooling cycle
Iso
SmC
SmA
Iso
SmA
SmC
Cr
3a
3d
3e
3f
3g
3h
3i
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
145.8 [24.7, 59.1]
141.1 [21.0, 50.8]
163.6 [36.5, 83.4]
145.8 [145.8, 26.1]
141.3 [26.1, 63.0]
129.1 [11.0, 27.3]
135.2 [23.9, 58.6]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
–
160.9 [0.11, 0.26]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
199.5 [6.8, 14.5]
193.8 [6.8, 14.6]
203.9 [7.6, 16.0]
198.6 [7.3, 15.4]
194.3 [8.0, 17.2]
182.3 [7.4, 16.3]
183.7 [7.5, 16.5]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
196.7 [6.9, 14.7]
187.8 [4.1, 9.0]
199.3 [5.4, 11.5]
196.6 [7.2, 15.4]
188.1 [6.1, 13.3]
179.7 [7.5, 16.6]
182.6 [2.1, 4.7]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
156.9 [0.15, 0.35]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
117.5 [20.9, 53.6]
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
ꢅ
165.6tm
160.4tm
111.6 [4.0, 10.1]
123.7 [13.0, 32.8]
117.1 [24.0, 61.7]
107.8 [21.6, 56.7]
100.1 [16.9, 45.4]
110.4 [22.7, 59.3]
170.7 [0.08, 0.18]
162.8 [0.18, 0.42]
tm
tm
139.7
132.2
151.9 [0.11, 0.27]
141.7 [0.21, 0.52]
tm
tm
176.9
174.6
181.5 [2.4, 5.3]
tm Indicates POM observed values. Here, Cr = Crystalline phase, SmA = smectic A phase, SmC = smectic C phase. Heating and cooling rates are 5 °C minꢁ1 for the melting and
clearing processes.
Fig. 4. (a) Focal conic fan texture of the SmA phase of 2a at 140.5 °C on the cooling the sample and (b) equidistant chiral lines running across fan-shaped texture of SmCꢀ
phase at 104.7 °C during phase transition of 2a.
frequency which appeared at ꢃ1242 cmꢁ1 for the imine ligands
(2a–i) is shifted to lower frequency ꢃ1142 cmꢁ1 for the complexes
(3a–i) indicating the coordination of the metal through phenolic
oxygen.
3.3. Optical absorption studies
The UV–Visible spectra of both chiral imines and its mononu-
clear copper(II) complexes are recorded in chloroform at concen-
tration 1 ꢄ 10ꢁ5 mol Lꢁ1 (chloroform was chosen because of
maximum solubility of the compounds). The UV-spectra of the li-
gands exhibited (Fig. 2) a characteristic absorption of ACH@NA
3.2. 1H NMR Study
bond at 364 nm (e p–
= 46,700 L molꢁ1 cmꢁ1) due to pꢀ transitions
The 1H NMR data was found to be in good agreement with the
proposed structures of the ligand. 1H NMR spectra of the ligands
(2a–2i) show a signal at d = 13.4–13.8 ppm, corresponding to the
proton of the hydroxyl (AOH) group. The imine proton appeared
at d = 8.50 ppm for ligands and confirmed the formation of the
Schiff base [35,36]. All other aromatic protons appeared in the
range of d = 7.57–6.48 ppm. The two protons of the C@C of the cin-
namate appeared at d = 7.68 and 6.44. The proton attached to the
chiral carbon appeared as a singlet at d = 4.11 ppm. The NMR
spectra of the complexes are not recorded because of paramagnetic
nature of the Cu(II) atom.
of imine linkage. This band shifted (ꢃ39 nm) towards higher en-
ergy region upon complexation and appeared at 325 nm
(e
= 56,400 L molꢁ1 cmꢁ1) in copper(II) complexes. For copper(II)
complexes another broad peak appeared at lower energy region
401 nm (
e
= 39,700 L molꢁ1 cmꢁ1), and corresponds to ligand to
metal or metal to ligand charge transfer. Solvatochromic effect
was also observed in copper(II) complexes; the absorption maxima
of both
p–
pꢀ band and charge transfer band shifted on changing
the polarity of the solvent. The shift in absorption maxima
occurred in an irregular manner, with a change in polarity of the

M.K. Paul et al. / Journal of Molecular Structure 1039 (2013) 219–226
223
1.4
X
ꢁ1 cm2 molꢁ1 respectively. The conductance values are very
low compared to 1:1 electrolyte [37], which indicates that the chi-
ral imine and the metal complexes are not ionized in solution. The
low conductivity values infer that imine and complex are consid-
ered to be neutral and non-electrolytic in nature.
40
30
20
10
2a
3.5. Thermal microscopy and differential scanning calorimetric
studies: mesomorphic behavior
3.5.1. Ligands
The phase transition temperatures and associated enthalpies of
chiral imines and copper(II) complexes are summarized in Tables 1
and 2 respectively.
All the chiral imines exhibit enantiotropic chiral smectic C
(SmCꢀ) and smectic A (SmA) phases with the lone exception 2c.
On slow cooling the samples, small batonnets grows from isotropic
(Iso) melt which developed to a focal conic fan texture of SmA
phase as shown in Fig. 4a. The Iso-SmA transition was detected
60
80
100
120
140
160
180
Temperature (oC)
Fig. 5. Representative DSC thermogram of ligand 2a in the second heating and
cooling cycles at 5 °C/min.
in DSC for 2a at 143.3 °C with an enthalpy change of 4.74 kJ molꢁ1
.
On further cooling arcs appeared on focal conic fan texture of SmA
with a typical helical stripe, characteristic of chiral SmCꢀ (Fig. 4b).
The SmA–SmCꢀ transition was not detected by DSC only in some
samples. The SmCꢀ transformed to crystalline solid on further cool-
ing, and this transition was detected by DSC for 2a at 59.1 °C with
an enthalpy change of 15.59 kJ molꢁ1. The representative, DSC
thermograms of 2a in the second heating and cooling cycles of chi-
ral ligand recorded at a rate of 5 °C minꢁ1 is shown in Fig. 5.
The clearing temperature of the ligands gradually decreases
with the increasing chain length. The mesomorphic SmCꢀ phase
range do not follow any systematic change as a function of chain
length. The phase transition temperatures as a function of chain
length is presented in Fig. 6. The appearance of the equidistant line
pattern which is a measure of pitch length of SmCꢀ phase and re-
flects the helical superstructure. The lines are parallel to the smec-
tic layer planes and the distance between two adjacent dark lines
gives the pitch of the helix. The helix axis is perpendicular to the
lines. But the helical line disappeared when the thickness of the
film decreased. This was clearly observed when the sample was
Iso
140
120
100
80
SmA
SmC*
60
Cr
8
4
5
6
7
9 10 11 12 13 14 15 16 17 18 19
Chain length (n)
Fig. 6. Variation of phase transition temperatures as a function of alkyl chain length
(n) of chiral imines.
melted in a ITO coated cell (thickness 5 lm) treated for homoge-
neous alignment (Fig. 7). This indicates the unwinding of the helix.
In a thin cell, the SmCꢀ phase have book-shelf structure of layers
[38]. The unwinding of the helix is driven by the competition be-
tween anchoring forces and the natural twisting tendency of the
molecules. This competition gives rise to an unwinding transition
if the cell thickness is below a critical threshold roughly equal to
the helical pitch [5]. Therefore, it is difficult to observe helical lines
solvent (Fig. 3). However no such effect was observed in the chiral
ligand. Solvatochromism is usually indicative of nonlinear optical
(NLO) property of the compound and it arises due to changes in di-
pole moments (Dl) between the ground and excited states upon
excitation in complexes.
in the SmCꢀ phase of a planar aligned cell with a thickness of 5
lm.
3.4. Conductivity measurements
Moreover, the helical lines are distinctly observed when sand-
wiched in untreated glass plate and coverslip because the surface
interactions remain relatively weak and the helical structure is
preserved.
The molar conductance of a representative ligand (2f) and cop-
per complex (3f) in dimethylformamide were found to be 1.9 and
Fig. 7. (a) Batonnets texture of the SmA phase at 147.5 °C and (b) smooth focal conic fan shaped texture at 98 °C of 2a in planar aligned ITO coated cell of thickness 5 lm.

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M.K. Paul et al. / Journal of Molecular Structure 1039 (2013) 219–226
Fig. 8. (a) Focal conic fan texture of the SmA phase of 3a at 192 °C on the cooling the sample from isotropic phase and (b) broken fan-shaped texture of SmC phase at 152 °C.
32
Iso
200
180
160
140
120
100
3a
30
28
26
24
22
20
18
SmA
SmC
14
Cr
10
2
4
6
8
12
16
18
Chain length (n)
100
120
140
160
180
200
Temperature (oC)
Fig. 10. Variation of phase transition temperatures as a function of alkyl chain
length (n) of Cu(II) complexes.
Fig. 9. Representative DSC thermogram of copper(II) complex 3a in the second
heating and cooling cycles at 5 °C/min.
tematic change as a function of chain length. The compounds with
longer chain length (3h and 3i) exhibited wide range of SmC phase.
3.5.2. Complexes
All the copper(II) complexes (3a–i) exhibit enantiotropic meso-
morphism of SmA and SmC phases. The compounds (3a–i) are
microcrystalline solids at room temperature and upon heating
and/or cooling from the isotropic liquid, display smectic meso-
phase. The enthalpies associated with the transitions are depicted
in Table 2. The mesophases shown by the complexes were identi-
fied by polarizing microscopy as SmA and SmC by their optical tex-
tures as shown in Fig. 8a and b. DSC thermogram of complex 3a
(5 °C minꢁ1) is shown in Fig. 9, which are in agreement with the
phase transitions observed by POM. We did not observe any helical
optical texture in SmC phase of any complex indicating the disap-
pearance of chiral phases exhibited by the ligands. The coordina-
tion of the metal ion with the ligand leads to a rigid molecular
structure. In the rigid molecular structure the molecular director
in axial direction cannot rotate across the layer plane. The broken
focal conic fan texture appears on cooling with the absence of
stripes confirmed SmC phase [39]. Similar observations of suppres-
sion of chirality upon complexation is reported in Cu(II) complexes
of chiral cholesterol based dimesogenic ligands [40]. We still do
not know any special reasons for the suppression of chirality upon
complexation only in some compounds and not in some other
compounds [30]. Further work is to be carried out to ascertain
the reasons for such suppression of chiral phases upon complexa-
tion. The clearing and melting temperatures of the complexes (3a–
i) are significantly higher than corresponding chiral imines (2a–i).
It indicated higher thermal stability of the complexes compared to
ligands. The phase diagram as function of chain length is presented
in Fig. 10. The SmA and SmC phase ranges did not follow any sys-
3.6. Electric field study
The helical superstructure of the SmCꢀ phase can be unwound
or distorted under the influence of external electric field which de-
rives the reorientation of the molecular long axis along the field
direction [41]. The helical structure of SmCꢀ phase linearly distorts
and the helical pitch increases, this process is called the distorted-
helix ferroelectric effect [42]. If the electric field is increased above
a critical threshold, the transition from a helical SmCꢀ to the homo-
geneous structure SmC occurs, the helical pitch diverges and the
helix is unwounded [43–45]. Preliminary investigations of electric
field influence on the molecular alignment and helical structure
was carried out using a polyimide coated (PI) cell of ꢃ100
lm
and applying the electric field of 0–100 V at 50 Hz. The helical pitch
unwinding and the reorientation of the molecules with the direc-
tion of the electric field or switching phenomena is observed in
(2a) in SmCꢀ phase. The observed photographs illustrating the
switching phenomena, i.e. reorientation of molecules under the
influence of electric field (V = 0–100 V, thickness = 100 lm), at
50 Hz are presented (see Fig. 11). The helical structures of SmCꢀ
phase when unwound, the equidistant arced lines across the fans
disappeared with the increase in the strength of electric field.
3.7. Free standing film
A free standing film of the ligand (2c) was prepared in a
temperature regulated two stage oven filled with argon at

M.K. Paul et al. / Journal of Molecular Structure 1039 (2013) 219–226
225
Fig. 11. Changes in optical texture (40ꢄ) of 2a in SmCꢀ phase at 104 °C as a function of electric field, in a PI coated cell thickness ꢃ 100
lm (a) fan-shaped texture with
equidistant lines of SmCꢀ phase at 0 V; (b) increasing distance between striations reflecting the changes in pitch at 10 V; (c) 20 V and (d) the helical super structure completely
unwound at 100 V.
Fig. 12. Optical textures of the free standing film of the ligand 2c under uncrossed polarizer(ꢃ5°): (a) SmA phase at 122 °C; (b) SmCꢀ phase at 94.5 °C.
0.7 atm to minimize the sample degradation. The films were pulled
across a circular hole (4 mm diameter) in the center of a glass cover
slip in the smectic A phase (T = 147 °C) of the ligand (2c). Two elec-
trodes of 5 mm in length were placed on the bottom side of the
cover slip and on the opposite side of the hole. The DRLM tech-
nique was used to study the optical texture of the free standing
indicating them as NLO active materials. The molar conductivity
studies confirmed the non-ionic nature of these compounds. The li-
gand (2a–i) exhibits enantiotropic SmA–SmCꢀ phase variants. Cop-
per(II) complexes (3a–i) display enantiotropic SmA–SmC phase
variant with an increased thermal stability than the corresponding
ligands. The metal complexation of a chiral ligand suppresses the
chiral phase and further work is in progress to ascertain the rea-
sons for such behavior.
film. It was observed that the defect lines (2
p
wall) appears on
wall origi-
the film and moves on the surface of the film. The 2
p
nates due to fact that the c-director rotates in different direction
with electric field [46]. The optical texture of the freestanding film
in SmA and SmCꢀ phase is presented in Fig. 12. The film responses
with electric field when it reached to a chiral SmCꢀ regime indi-
cated by switching phenomenon confirms the ferroelectric phase.
Acknowledgements
Authors acknowledge Department of Science and Technology,
New Delhi for financial assistance and C.C. Huang, School of Phys-
ics and Astronomy, University of Minnesota for providing DRLM
instrumental facility.
4. Conclusions
The homologous series of chiral imines, N-(4-n-alkyloxy-2-
hydroxybenzylidene)-2-methylbutyl-40-aminocinnamates (2a–i)
and their Cu(II) complexes (3a–i) had been synthesized and char-
acterized. The copper(II) complexes exhibited Solvatochromism
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