Synthesis and characterization of banana-shaped mesogens derived from a benzophenone moiety

Article abstract of DOI:10.1080/15421400903291392

A homologous series of Schiff's bases consisting of a benzophenone moiety as the central core has been synthesized. Schiff's bases were prepared starting from benzophenone by a sequence of reactions, viz., nitration followed by reduction and heating the r

Full text of DOI:10.1080/15421400903291392

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Synthesis and Characterization
of Banana-Shaped Mesogens
Derived From a Benzophenone
Moiety
K. C. Majumdar a , Santanu Chakravorty a , Randhir
Kumar Sinha a & Nilasish Pal a
a Department of Chemistry , University of Kalyani ,
Kalyani, West Bengal, India
Published online: 17 Dec 2009.
To cite this article: K. C. Majumdar , Santanu Chakravorty , Randhir Kumar Sinha &
Nilasish Pal (2009) Synthesis and Characterization of Banana-Shaped Mesogens Derived
From a Benzophenone Moiety, Molecular Crystals and Liquid Crystals, 515:1, 125-134,
DOI: 10.1080/15421400903291392
To link to this article: http://dx.doi.org/10.1080/15421400903291392
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Mol. Cryst. Liq. Cryst., Vol. 515, pp. 125–134, 2009
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400903291392
Synthesis and Characterization of Banana-Shaped
Mesogens Derived From a Benzophenone Moiety
K. C. Majumdar, Santanu Chakravorty,
Randhir Kumar Sinha, and Nilasish Pal
Department of Chemistry, University of Kalyani,
Kalyani, West Bengal, India
A homologous series of Schiff’s bases consisting of a benzophenone moiety as the
central core has been synthesized. Schiff’s bases were prepared starting from
benzophenone by a sequence of reactions, viz., nitration followed by reduction
and heating the resulting diamino derivative with 4-formyl-3-hydroxyphenyl
4-(alkyloxy)benzoate. Higher homologues of this series exhibit mesomorphism,
but the lower homologues do not seem to possess any liquid crystalline property.
The B2 mesophase was characterized by DFT study for orientation of dipole
moment. Dipolar interaction and intramolecular H-bonding (N–H) have been
considered to discuss the origin of the B2 phase in this homologous series of
benzophenone moiety derived systems.
Keywords: B2 phase; bent shape; DFT; DSC; POM; Schiff’s base
INTRODUCTION
Conventional thermotropic liquid crystals are formed by anisometric
molecules (mesogens that are either rod shaped or disc shaped).
Nematic or smectic phases are preferred by rod-like molecules
(‘‘calamatic compounds’’) [1], and the smectic phase possesses one
dimensional density modulation. Disc-like molecules (discotic
compounds) [2–4] can also show the nematic phase, but they generally
prefer to form columnar phases with a two-dimensional modulated
structure. If the shape of the molecule significantly deviates from
the classical rod- or disc-like shape, new phases as well as subphases
of well-known phases occur [5–9]. An interesting example is bent core
Address correspondence to K. C. Majumdar, Department of Chemistry, University of
Kalyani, Kalyani 741235, West Bengal, India. E-mail: kcm_ku@yahoo.co.in
125

126
K. C. Majumdar et al.
mesogens, which are the subject of extensive investigations during the
last few years in particular because of their new mesophases with
unusual properties compared to smectic phases formed by calamatic
compounds. Vorlander and Apel [10,11] first reported this type of
bent-core molecules. This type of molecule can be broadly classified
into two categories: a) V-shaped and b) banana-shaped. Disubstitution
at the 1,2- or 1,3-position of the benzene ring can generate bent-
shaped molecules. Specifically 1,2-disubstitution generates V-shaped
molecules [12] and disubstitution at 1,3-position results in banana-
shaped [13–15] molecules. Very recently we too have reported the
liquid crystalline properties of such bent-shaped molecules with a
furan-bridge at the core [16]. B-phases are found in five-, six-, and
seven-ring systems having an angle between the two molecular axes
from more than 130ꢀ to less than 100ꢀ. In this article, we present a
new class of bent-shaped molecules in which central core is different
from that reported by others, and the molecules contain six aromatic
rings. As a part of our ongoing effort to realize new materials [17–22]
to explore the limits of molecular shapes compatible with liquid crystal-
line behavior, and to study the structure property relationship, we
have undertaken a study on the synthesis and characterization of bent
mesogens based on 3,3-disubstituted benzophenone rather than
1,2-disubstituted benzene, in the core with Schiff’s base linkage. Here
we report the results.
2. EXPERIMENTAL
The methodology of synthesis of the banana-shaped materials 6a–e is
depicted in Scheme 1. Benzophenone was first converted into its
dinitro derivative (2), which was then reduced to the diamine deriva-
tive (3). The diamino derivative 3 was then heated under reflux with
4-formyl-3-hydroxyphenyl 4-(alkyloxy)benzoates in absolute ethanol
in the presence of a catalytic amount of glacial acetic acid to afford
the homologous series 6a–e. The compounds 6a–e were characterized
1
from their elemental analyses, H NMR and IR spectra.
All the chemicals were procured from either Sigma Aldrich
Chemicals Pvt. Ltd. or Spectrochem, India. Silica gel [(60–120 mesh)
was used for chromatographic separation. Silica gel G [E-Merck
(India)] was used for TLC. Petroleum ether refers to the fraction boiling
between 60ꢀC and 80ꢀC. IR spectra were recorded on a Perkin-Elmer
L 120–000A spectrometer (nmax in cmꢁ1) on KBr disks. UV absorption
spectra were recorded in CHCl3 on a Shimadzu UV-2401PC spectro-
1
photometer (kmax in nm). H NMR (300 MHz) spectra were recorded
on a Bruker DPX-500 spectrometer in CDCl3 (chemical shift in d) with

Banana-Shaped Mesogens
127
SCHEME 1 Reaction conditions and reagents: (i) fuming HNO3=Conc. H2SO4
(1:4, 10 ml=g), heat, 60ꢀC, 2 h (ii) ethyl acetate, SnCl2 ꢂ 2H2O, conc. HCI (cat),
reflux, 4 h; (iii) SOCL2, reflux, 1 h; (iv) 2,4-dihydroxy benzaldehyde, DCM,
aq K2CO3, Bu4NHSO4, 12 h; (v) dry EtOH, glacial AcOH, reflux, 4 h.
TMS as internal standard. CHN was recorded on 2400 series II CHN
analyzer Perkin Elmer. The liquid crystalline properties were estab-
lished by thermal microscopy (Nikon polarizing microscope LV100POL
attached to an Instec hot and cold stage HCS302, with STC200 tem-
perature controller configured for HCS302 and the phase transitions
were confirmed by differential scanning calorimetry (Perkin-Elmer
Diamond DSC system).
2.1.
3,30-Dinitrobenzophenone 2 and 3,30-diaminobenzophenone 3 were
prepared according to the published procedure [23].
2.2. General Procedure for the Preparation
of Compounds 6a–e
3,30-Diaminobenzophenone 3 (0.05 g, 0.02 mmol) was heated under
reflux with 4-formyl-3-hydroxyphenyl 4-(octyloxy)benzoate 5a (0.025 g,
0.048 mmol) in absolute ethanol in the presence of a catalytic amount
of glacial acetic acid to afford the desired product 6a. All the other
compounds were prepared according to the same procedure.

128
K. C. Majumdar et al.
Compound: 6a
Yield 86%, Pale yellow solid; IR (KBr) nmax: 2915, 2850, 1725, 1651,
1611 cmꢁ1; 1H-NMR (CDCl3, 400 MHz): dH 13.29 (s, 2 H), 8.69 (s, 2 H),
8.14 (d, J ¼ 8.8 Hz, 4 H), 7.70–7.73 (m, 4 H), 7.52–7.59 (m, 4 H), 7.44 (d,
J ¼ 8.4 Hz, 2 H), 6.95 (d, J ¼ 8.8 Hz, 4 H), 6.89 (d, J ¼ 1.7 Hz, 2 H), 6.86
(dd, J ¼ 8.3, 2.3 Hz, 2 H), 4.02 (t, J ¼ 6.5 Hz, 4 H), 1.79–1.83 (quin, 4 H,
J ¼ 7.6 Hz), 1.24–1.54 (m, 60 H), 0.85 (t, J ¼ 6.9 Hz, 6 H); 13C-NMR
(CDCl3, 100 MHz): 195.49, 164.31, 163.72, 163.09, 162.59, 155.26,
148.74, 138.64, 133.50, 132.39, 129.55, 128.45, 125.83, 122.02, 121.12,
116.90, 114.36, 113.37, 110.72, 68.40, 31.92, 29.69, 29.58, 29.56,
29.36, 29.07, 25.97, 22.69, 14.11; Anal Calcd. For C77H100N2O9; C,
77.22; H, 8.42; N, 2.34; Found, C, 77.38, H, 8.52, N, 2.40%.
Compound: 6b
Yield 90%, Pale yellow solid; IR (KBr) nmax: 2916, 2850, 1726, 1652,
1610 cmꢁ1; dH 13.30 (s, 2 H), 8.72 (s, 2 H), 8.10 (d, J ¼ 8.6 Hz, 4 H),
7.72–7.76 (m, 4 H), 7.50–7.56 (m, 4 H), 7.47 (d, J ¼ 8.3 Hz, 2 H), 6.94
(d, J ¼ 8.6 Hz, 4 H), 6.92 (d, J ¼ 2.0 Hz, 2 H), 6.82 (dd, J ¼ 8.3, 2.0 Hz,
2 H), 4.01 (t, J ¼ 6.4 Hz, 4 H), 1.77–1.84 (quin, 4 H, J ¼ 6.5 Hz),
1.24–1.46 (m, 52 H), 0.85 (t, J ¼ 6.9 Hz, 6 H); 13C-NMR (CDCl3,
100 MHz): 195.48, 164.24, 163.70, 163.05, 162.56, 155.25, 148.71,
138.64, 133.47, 132.35, 129.50, 128.38, 125.78, 122.00, 121.10,
116.87, 114.34, 113.33, 110.68, 68.34, 31.90, 29.67, 29.63, 29.56,
29.53, 29.33, 29.06, 25.95, 22.66, 14.09; Anal Calcd. For C73H92N2O9:
C, 76.81; H, 8.12; N, 2.45; Found, C, 76.93, H, 8.20, N, 2.52.
Compound: 6c
Yield 90%, Pale yellow solid; IR (KBr) nmax: 2916, 2850, 1726, 1652,
1610 cmꢁ1; 1H-NMR (CDCl3, 400 MHz): dH 13.32 (s, 2 H), 8.72 (s, 2 H),
8.14 (d, J ¼ 8.8 Hz, 4 H), 7.74–7.77 (m, 4 H), 7.57–7.60 (m, 4 H), 7.47 (d,
J ¼ 8.3 Hz, 2 H), 6.98 (d, J ¼ 8.8 Hz, 4 H), 6.92 (d, J ¼ 2.3 Hz, 2 H), 6.86
(dd, J ¼ 8.3, 2.3 Hz, 2H), 4.05 (t, J ¼ 6.6 Hz, 4 H), 1.81–1.88 (quin, 4 H,
J ¼ 6.3 Hz), 1.28–1.58 (m, 44 H), 0.88 (t, J ¼ 7.0 Hz, 6 H); 13C-NMR
(CDCl3, 100 MHz): 195.5, 164.30, 163.74, 163.09, 162.59, 155.29,
148.74, 138.67, 133.51, 132.39, 129.55, 128.43, 125.83, 122.04, 121.12,
116.90, 114.37, 113.38, 110.72, 68.37, 31.93, 29.67, 29.60, 29.57,
29.37, 29.10, 25.98, 22.70, 14.13; Anal Calcd. For: C69H84N2O9, C,
76.35; H, 7.80; N, 2.58; Found, C, 76.55, H, 7.92, N, 2.67.
Compound: 6d
Yield 90%, Pale yellow solid; IR (KBr) nmax: 2915, 2850, 1726, 1651,
1610 cmꢁ1; 1H-NMR (CDCl3, 400 MHz): dH 13.32 (s, 2 H), 8.72 (s, 2 H),
8.14 (d, J ¼ 9.0 Hz, 4 H), 7.74–7.77 (m, 4 H), 7.57–7.60 (m, 4 H), 7.47

Banana-Shaped Mesogens
129
(d, J ¼ 8.6 Hz, 2 H), 6.98 (d, J ¼ 9.0 Hz, 4 H), 6.92 (d, J ¼ 2.0 Hz, 2 H),
6.86 (dd, J ¼ 8.6, 2.0 Hz, 2 H), 4.05 (t, J ¼ 6.6 Hz, 4H), 1.82–1.86 (quin,
4 H, J ¼ 7.8 Hz), 1.29–1.57 (m, 36 H), 0.88 (t, J ¼ 7.0 Hz, 6 H); 13C-NMR
(CDCl3, 100 MHz): 195.56, 164.32, 163.79, 163.13, 162.65, 155.34,
148.77, 138.72, 133.57, 132.44, 129.60, 128.47, 125.86, 122.10, 121.18,
116.96, 114.43, 113.41, 110.76, 68.43, 31.98, 29.72, 29.70, 29.65,
29.62, 29.42, 29.15, 26.04, 22.75, 14.18; Anal Calcd. For: C65H76N2O9,
C, 75.85; H, 7.44; N, 2.72; Found, C, 75.99, H, 7.53, N, 2.76.
Compound: 6e
Yield 90%, Pale yellow solid; IR (KBr) nmax: 2915, 2850, 1725, 1651,
1610 cmꢁ1; 1H-NMR (CDCl3, 400 MHz): dH 13.32 (s, 2 H), 8.72 (s, 2 H),
8.14 (d, J ¼ 8.8 Hz, 4 H), 7.73–7.76 (m, 4 H), 7.55–7.62 (m, 4 H), 7.47 (d,
J ¼ 8.5 Hz, 2 H), 6.98 (d, J ¼ 8.8 Hz, 4 H), 6.92 (d, J ¼ 2.0 Hz, 2 H), 6.86
(dd, J ¼ 8.5, 2.0 Hz, 2 H), 4.05 (t, J ¼ 6.6 Hz, 4 H), 1.82–1.86 (quin, 4 H,
J ¼ 8.0 Hz), 1.51–1.86 (m, 28 H), 0.89 (t, J ¼ 7.0 Hz, 6 H); 13C-NMR
(CDCl3, 100 MHz): 195.55, 164.30, 163.74, 163.09, 162.59, 155.28,
148.72, 138.66, 133.52, 132.40, 129.55, 128.44, 125.83, 122.05, 121.12,
116.91, 114.38, 113.38, 110.72, 68.37, 31.90, 29.56, 29.37, 29.32,
29.09, 25.98, 22.69, 14.13; Anal Calcd. For: C61H68N2O9, C, 75.28; H,
7.04; N, 2.88; Found, C, 75.37, H, 7.15, N, 2.97.
3. RESULTS AND DISCUSSION
The transition temperatures and associated enthalpies of all the
compounds of the series were determined by differential scanning
calorimetry (DSC) and are represented in Table 1. In the DSC scan
we observed several solid–solid transitions (and=or crystalline smectic–
smectic) transitions for 6a and 6b with small but noticeable enthalpy
changes (<3.7 KJ=mole), which were not observed by thermal micro-
scopy and by DSC scan on the cooling cycle. We observed no such
transition for lower homologues of the series.
The solid–liquid crystal phase transition recorded in the heating
cycle is confirmed by thermal microscopy. DSC scans of higher homolo-
gues of the series (6a, 6b, 6c) showed only one liquid crystalline phase
transition, but the lower homologues (6d, 6e) of the series with a short
alkoxy chain at the terminal position exhibited no mesophase behavior.
The enthalpy recorded at the liquid crystalline phase-isotropic transi-
tion is much higher than that of the solid–liquid crystalline phase
transition, which is not uncommon in bent shaped molecules. The dif-
ference in enthalpy value from the first and subsequent heating=cooling
cycles is probably due to the ordered arrangement of molecules within
each layer which must have been driven by minimized steric packing

130
K. C. Majumdar et al.
TABLE 1 Phase Transition Temperature (ꢀC) and Associated Enthalpies (DH,
KJmoleꢁ1) of Compounds 6a–e
forces as well as minimization of coulombic free energy to an ordered
arrangement of the molecules that sets in during the solidification
process. The enthalpy change that occurred during the phase transition
is sufficiently larger for compounds 6a–e. This may be due to the pre-
sence of intramolecular H-bonding (which is confirmed by the presence
1
of a one proton singlet at dHꢃ13.3 ppm in the H NMR of compounds
6a–e). The observed optical texture of compound 6c is represented in
Fig. 1a,b. The phase generated on heating could not be identified.
However, a fan-like texture gradually appeared from the isotropic phase
FIGURE 1 (a) Fan like texture of 6c at 148ꢀC at 20X. (b) Fan like texture of
6c at 148.2ꢀC at 50X.

Banana-Shaped Mesogens
131
on slow cooling and strongly developed at 148ꢀC clearly indicating the
existence of a B2 phase, a characteristic of bent-shaped molecules.
Compounds 6a and 6b also showed similar phase behavior on POM
study. However, the other two lower homologues (6d, 6e) did not show
any liquid crystalline phase transition. In most cases, the angle between
the molecular long axes is about 120o. When the bending angle is lower
than 100o, nematic and smectic phases also appear in addition to the B2
phase [24].
Dipolar interaction plays an important role in stabilizing the liquid
crystalline phases. B2 is a tilted smectic phase having monolayer struc-
ture with polar order. The textural appearance with the molecular
structure having a long terminal chain (C14, C16, C18) suggested that
the phase should be B2 type rather than B1 or B6. It is a well-known fact
that if the terminal chain length is short, e.g., C6 the nematic (N) and
intercalated smectic phase (B6) is often observed. The elongation of
chains disfavors the formation of B1 and polar smetic phases with
monolayer structure (B2). Hence phase sequence B6-B1-B2 is often
observed with increasing chain length [25,26]. The direction of the
dipole of the compound 6c was calculated by Density Functional The-
ory (DFT), which may determine the presence of dipolar interaction
within the molecules. All calculations have been performed by the
Chem3D (version 10) [27] with GAUSSIAN 03 Interface [28]. We have
computed the DFT (B3LYP) level of theory using the basis set 6–31 G to
obtain the dipole moment and dipolar orientation of molecules. The
dipole moment of 6c have been calculated as 13.7706 D (X ¼ 9.0864,
Y ¼ ꢁ8.3206, Z ¼ 6.1509). After geometry optimization of the compound
6c (Fig. 2) by addition of atoms step by step, the optimized structure
suggests that the molecule possess a V shape.
The first optimization was done by the MM2 method followed by the
AM1 method, and finally DFT was performed as stated earlier. The
high dipole moment (13.7706 D) of 6c indicates the presence of
1
H-bonding (N–H) as also evident from IR and H NMR. If we suppose
Z-axis as molecular axis and the maxima of dipole moment is in the
positive X-axis direction (9.0864 D), the direction of net dipole moment
should be transverse to molecular axis. Near-neighbour antiparallel
correlation of molecules within the layer of B2 phase will occur in
reality if the direction of dipole is transverse=longitudinal. The bent
direction in adjacent layers should be antiparallel, so that the layer
polarization alternates from layer to layer; leading to a macroscopic
apolar antiferroelectric structure which is applicable to B2 with switch-
ing behavior. Within the layers the molecules are in most cases addi-
tionally tilted parallel to the polar direction in the case of the SmCPA
or B2 phases. However, there are a few reports where non-tilted SmAPA

132
K. C. Majumdar et al.
FIGURE 2 Optimized geometry of compound 6c by DFT (B3LYP) level of
theory using the basis set 6-31G.
structures were also observed [29]. DFT study suggests that the
near-neighbour antiparallel correlation exists and in broad-on side
position with respect to the direction of dipole within the layers. Based
on the direction of the dipole moment, calculated from DFT and con-
sidering the presence of H-bonding we have constructed a model of
supramolecular arrangement in B2 phase which is depicted in Fig. 3.
Textural appearance of 6c and DFT study for geometry optimization
suggest that the compound 6c possesses a B2 phase rather than any
other banana mesophase as the molecule is bent shaped. In general,
the achiral banana-shaped molecular systems that generate reduced
symmetry mesophases normally consist of five aromatic rings. In our
present study, the compounds 6a–e possess six-ring structures.
Additionally, in the present instance, we have used 3,30-disubstituted
benzophenone as a core with Schiff’s base linkage. It is relevant to
mention that Schiff base linkage is well known in standard calamatic
system [30–33]. This is because they are resistance to thermal and
hydrolytic degradation. Most importantly, they can be substituted with
different entities and readily coordinate with a range of metal ions to

Banana-Shaped Mesogens
133
FIGURE 3 Molecular organization within the polar smectic phase SmCP or
B2 base direction of dipole moment derived from DFT. The arrows indicate
the direction of the dipole moment.
form metallomesogens. There are also several examples of the
introduction of hydroxyl group (Schiff base linkage) in one of the arms
of banana-shaped material to stabilize the phase [22,34]. However, in
this particular case, we have used Schiff’s base linkage to stabilize
the existing liquid crystalline phase and to maintain proper geometry
of the banana-shaped material. To the best of our knowledge, there is
no report of 3,30-disubstituted benzophenone containing V-shaped
liquid crystal. We are continuing this work with the new class of
organic mesogens, and a full account will be communicated later. The
demonstration of antiferroelectricity could be the theme of future
exploration.
ACKNOWLEDGMENT
We thank the DST (New Delhi) for financial assistance. Three of us
are grateful (S. C., N. P., and R. K. S) to the CSIR (New Delhi) for
Fellowships. We are also thankful to Prof. N. V. S. Rao of Assam
University (Silchar) for POM micrograph of compound 6a.
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