A new type of symmetrical banana-shaped material based on N-methyldiphenylamine as a core moiety exhibiting an Ad→A2 transition

Article abstract of DOI:10.1016/j.tetlet.2008.09.161

The synthesis and characterization of a homologous series of Schiff bases consisting of an N-methyldiphenylamine moiety as the central core is reported, which exhibits smectic A phase and transitions from the partial bilayer SmA_d phase to the bilayer SmA₂ phase as the temperature is lowered.

Full text of DOI:10.1016/j.tetlet.2008.09.161

Tetrahedron Letters 49 (2008) 7149–7152
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Tetrahedron Letters
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A new type of symmetrical banana-shaped material based on
N-methyldiphenylamine as a core moiety exhibiting an Ad?A2 transition
*
Krishna C. Majumdar , Buddhadeb Chattopadhyay, Santanu Chakravorty, Nilasish Pal, Randhir Kumar Sinha
Department of Chemistry, University of Kalyani, Kalyani 741 235, West Bengal, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of a homologous series of Schiff bases consisting of an N-methyldi-
phenylamine moiety as the central core is reported, which exhibits smectic A phase and transitions from
the partial bilayer SmAd phase to the bilayer SmA2 phase as the temperature is lowered.
Ó 2008 Elsevier Ltd. All rights reserved.
Received 15 August 2008
Revised 24 September 2008
Accepted 27 September 2008
Available online 17 October 2008
Keywords:
Schiff base
Bent shape
DPA
OLEDS
‘Banana-shaped’1–4 liquid crystals occupy one of the most excit-
ing areas of research in both mesogenic materials and supramole-
cular chemistry. These materials form new smectic and two-
dimensional phases, which are unlike to those obtained from
normal calamitic molecules.5,6 Such ‘banana’ compounds have
attracted considerable interest because of their unusual electro-
optical properties.7,8 Indeed, as a result of the bent shape, a polar
order within the smectic layers can be induced and some phases
exhibit ferro- or antiferroelectric properties although the mole-
cules are non-chiral.9,10 Several compounds with bent core (BC)
molecules are known to exhibit the smectic A (SmA) phase but
NMR study shows that in such molecules, the angle between the
two arms of the bent core is about 160 °C, which means that devi-
ation from a rod-like structure is quite small. On the other hand, if
amine-based material. Herein, we report the synthesis of banana-
shaped molecules with a new architecture based on the DPA core.
The synthesis of a series of banana-shaped materials 7a–e is
depicted in Scheme 1. N-Methyldiphenylamine 2 was formylated
under Vilsmeier-Haack condition to access the core entity 3 for
the synthesis of the banana-shaped molecules. 4-Nitrophenyl-4-
(alkyloxy)benzoate 5 was prepared by esterification of 4-(alkyl-
oxy)benzoic acid 4 with p-nitrophenol in dry DCM in the presence
of triethylamine and DMAP as a catalyst. 4-Nitrophenyl-4-(alkyl-
oxy)benzoate derivatives 5 were then reduced to 4-aminophenyl-
4-(alkyloxy)benzoates 6 by catalytic hydrogenation. The diformyl-
ated derivative 3 was heated under reflux with 4-aminophenyl
4-(alkyloxy)benzoate 6 in absolute ethanol in the presence of a cat-
alytic amount of glacial acetic acid to afford the homologous series
7a–e. Compounds 7a–e were characterized from their 1H NMR and
IR spectral data. The presence of a two-proton singlet at 8.39 ppm
in the 1H NMR is suggestive of the formation of a symmetrical
Schiff base. The IR peak at around 1625 cmꢀ1 further supports
the formation of the symmetrical product.
The liquid crystalline behavior of compounds 7a–e was investi-
gated initially with the help of polarizing optical microscopy
(POM) and differential scanning calorimetry (DSC). Transition tem-
peratures and associated enthalpies obtained from DSC thermo-
grams of compounds 7a–e are shown in Table 1, Figure 1a and b.
The DSC thermograms of the higher homologues of the series
(7a and 7b) show three types of phase transition in both the
heating and cooling cycles with reasonable enthalpy changes.
Compound 7a,16 when placed in a thin cell with a cell gap of
the molecules have a highly bent core, the compounds are known
11
to exhibit B-phases. Reddy and Sadashiva
also synthesized an
unsymmetrical bent core molecule in which an alkoxy chain was
attached to only one of the arms of the bent core, while the other
arm terminates in a highly polar cyano group and observed
SmAd?SmAdb transitions. Low molecular weight triphenylamine
(TPA) and diphenylamine (DPA) derivatives have been employed
as hole-transporting materials in organic light-emitting devices
(OLEDS).12–14 Diphenylamine derivatives are also used in two-pho-
ton absorbing molecules with large non-linear molecular absorp-
tivity to generate high-energy excited states with relative low
laser energy.15 Owing to the wide applicability of diphenyl-
d = 5 0.2
lm under homogeneous planar boundary conditions,
* Corresponding author. Tel.: +91 33 2582 7521; fax: +91 33 2582 8282.
E-mail address: kcm_ku@yahoo.co.in (K. C. Majumdar).
gave the textures which is shown in Figure 2a–c. A deformed
0040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.161

7150
K. C. Majumdar et al. / Tetrahedron Letters 49 (2008) 7149–7152
Me
N
Me
N
H
N
(ii)
OHC
(i)
CHO
R'O
77 %
NO2
1
2
3
O
O
(iii)
(iv)
O
O
R'O
R'O
COOH
(v)
5
NH2
4
6
Me
N
(vi)
+
3
6
7a. R' = -C18H37, 98%
7b. R' = -C16H33, 98%
7c. R' = -C14H29, 97%
7d. R' = -C12H25, 96%
7e. R' = -C10H21, 98%
N
N
O
O
7a-e
O
O
R'O
OR'
Scheme 1. Reagents and conditions: (i) CH3I, dry acetone, anhydrous K2CO3, NaI, reflux; (ii) POCl3, DMF, reflux; (iii) SOCl2, reflux; (iv) p-nitrophenol, CH2Cl2, NEt3, DMAP (cat),
rt; (v) ethyl acetate, H2/Pd, rt; (vi) dry ethanol, AcOH, reflux.
Table 1
120.0
(68.6)
203.4
(27.1)
201.6
(25.9)
132.1
(65.6)
156.6
(14.6)
153.5
(12.6)
Ad
Ad
Ad
A2
A2
I
7a
Cr
A2
A2
I
Cr
Cr
156.8
(12.9)
118.8
(60.1)
205.3
(26.5)
207.0
(27.2)
132.5
(57.8)
159.0
(13.9)
I
Ad
Ad
Cr
7b
7c
131.3
(26.3)
207.9
133.8
(21.1)
162.51
(14.0)
209.7
(26.6)
A
Ad
Cr
Unknown Phase
2
(26.2)
160.41
(
12.8)
114.9
(49.6)
Cr
I
A2
209.5
(25.8)
126.5
(19.0)
166.6
(14.4)
211.4
(26.2)
138.8
(21.1)
Cr
Ad
A2
Ad
7d
Unknown Phase
164.4
(
13.6)
111.6
(32.5)
A2
Cr
smectic leaf (Fig. 2a) appears from the isotropic melt in the cooling
cycle with a large amount of homeotropy, which on further cooling
grows slowly, and fills up the whole field of view with elliptical
domains in different sites (Fig. 2b). On further cooling, the texture
gradually changes at around 150 °C to a more order twisted
or coiled-like structure (Fig. 2c) without any homeotropic behav-
ior; on further cooling the sample solidified. Compound 7b also
shows a similar type of phase behavior on POM studies but the
clearing temperature slightly increases in this case. Compounds
7c and 7d show an extra enantiotropic transition during heating
cycles in DSC thermograms, but in cooling cycles they show the
similar three transitions as for their higher homologues. Com-
Powder X-ray diffraction of 7a shows a sharp peak in the low
angle region implying the formation of a layered structure and a
broad diffuse peak in the wide angle region centered at d-spacing
0
of‘ꢁ4.4 ÅA, indicating the liquid-like arrangement of the molecules
within the layers (Fig. 3). The d-spacing at two different temper-
atures in the low angle region is around 49 Å and 56 Å, respec-
tively, which is sufficiently larger than the calculated molecular
length (L ꢁ 29 Å) values from molecular models. The d-value at
140 °C is approximately double the calculated molecular length
of the compound 7a, and at 200 °C, it is approximately 1.5 times
the molecular length of the compound 7a. It is well known that
when the reflection within the smectic A layer corresponds to
d ꢁ 2L, the smectic A layer is called a bilayer (A2) and when the
layer-spacing is intermediate between L and 2L then it is called a
partially bilayer (Ad) phase.17,18 Therefore, we can designate the
phase at higher temperature as Ad and at lower temperature as A2.
Usually, for symmetric and non-polar molecules, SmA layer-
spacing is approximately equal to L; however, in molecules with
a strong longitudinal dipole moment near-neighbour antiparallel
correlations exist that can result in subtle changes in the structure,
pound 7d, when placed in
a thin cell with a cell gap of
d = 5 0.2 m under homogeneous planar boundary conditions,
l
gave the textures represented in (Fig. 2d–f) during the heating
cycles. Figure 2d corresponds to an unknown phase transition,
and the other two transitions were very similar to phase transi-
tions reported for their higher homologues. On the other hand,
the compound 7e shows three similar transitions during both the
heating and cooling cycles as for higher homologues.

K. C. Majumdar et al. / Tetrahedron Letters 49 (2008) 7149–7152
7151
Figure 1. (a) DSC thermograms in heating cycles of 7a-d. (b) DSC thermograms in cooling cycles of 7a–d.
Figure 2.
which can exhibit more than one form of SmA phase. The appear-
ance of a SmAd phase at higher temperature may be due to the
overlap of two neighboring bent-shaped molecules in antiparallel
orientation as in the case of highly polar rod-like molecules,19,20
and at lower temperatures the molecules stack up in layers with-
out overlapping with the electric dipoles concentrated at the
boundary between every other layer resulting in the SmA2 phase.
Conoscopic studies of 7a were conducted by inserting Bertrand
lens between the analyzer and ocular lens, observed between
crossed polarizers, set at 45° to the direction of the electric field,
show uniaxial interference patterns for the SmAd phase at 196 °C
and SmA2 phase at 140 °C (Figs. 4 and 5). In this case, the conoscop-
ic interference pattern is not dark-like Maltese cross but is instead
rather bright.
In our earlier Letter regarding a liquid crystalline material based
on a TPA core, we reported21 that trisubstitution on the TPA core
with a Schiff base linkage resulted in excellent columnar meso-
phase behavior. On the other hand, Wang et al.22 reported a similar
type of trisubstitution with a C@C linkage, which exhibited non-
mesophase behavior. This may be due to the more rigid sp2-hybrid
orbital in C@C bonds instead of a more flexible C@N bond. They
also reported that disubstitution on the TPA core with a CH2NH

7152
K. C. Majumdar et al. / Tetrahedron Letters 49 (2008) 7149–7152
In this DPA homologous series including both short- and long-
chained members, two smectic phases are evident: smectic Ad
and smectic A2 with temperatures of approximately 50 and
30 °C, respectively. Compounds 7c and 7d exhibit an unknown
phase before the smectic A2 phase on heating from solid or cooling
from the smectic A2 phase. From molecular models, it has been ob-
served that the molecule deviates from the usual bent shape and
approaches a rod-like configuration with symmetrical chains fac-
ing opposite directions. Initially, we assumed that the molecules
should have a bent structure and must give banana phases; how-
ever, X-ray profiles of these compounds led to a remarkable obser-
vation in which only one small angle diffraction peak must be for
the smectic fluctuation and the d-value larger than the molecular
length indicates Ad and A2 phases rather than a banana phase.
We conclude that this is
a remarkable observation as DPA
possesses numerous properties and also exhibits a smectic liquid
crystalline phase.
Acknowledgments
Figure 3. XRD diagram of 7a.
We thank the DST (New Delhi) for financial assistance and the
Material Science Department, IACS, Kolkata for HRXRD. Three of
us (B.C., S.C., and R.K.S.) are grateful to the CSIR (New Delhi) for
the fellowships.
References and notes
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Figure 4. Conoscopic interference pattern observed for 7a at 140 °C.
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16. Typical procedure for the synthesis of compound 7a: A mixture of compound 3
(100 mg, 0.418 mmol) and 4-aminophenyl-4-(octadecyloxy)benzoate (403 mg,
0.836 mmol) was refluxed in absolute ethanol in the presence of a catalytic
amount of glacial acetic acid for 2 h. The Schiff base 7a was precipitated out
from the hot reaction mixture. It was collected, washed with hot ethanol
repeatedly, and dried in vacuum.
Compound 7a: Yield 98%; IR (KBr): 2921, 1730, 1600, 1507 cmꢀ1 1H NMR
;
(400 MHz, CDCl3): dH 0.86–1.86 (m, 70H), 3.46 (s, 3H), 4.02 (t, 4H, J = 6.4 Hz),
6.94 (d, 4H, J = 8.8 Hz), 7.14 (d, 4H, J = 8.4 Hz), 7.19–7.23 (m, 8H), 7.82 (d, 4H,
J = 8.4 Hz), 8.11 (d, 4H, J = 8.8 Hz), 8.39 (s, 2H). 13C NMR (125 MHz, CDCl3): dC
14.5, 23.1, 26.4, 29.5, 29.7, 29.8, 29.9, 29.9, 30.0, 30.1, 32.3, 40.5, 68.7, 114.7,
120.7, 121.9, 122.2, 122.8, 130.3, 130.6, 132.6, 149.4, 150.2, 151.0, 159.8, 163.9,
165.4. Anal. Calcd for C77H103N3O6: C, 79.27; H, 8.90, N, 3.60. Found: C, 79.57;
H, 8.99; N, 3.27.
Figure 5. Conoscopic interference pattern observed for 7a at 196 °C.
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linkage resulted in similar non-mesophase behavior. At the present
time, we have studied a disubstituted N-MeDPA core instead of the
TPA core and notably, all the homologous compounds exhibit a
SmA phase, which show transitions from the partial bilayer SmAd
phase to the bilayer SmA2 phase as the temperature is lowered.
22. Wang, Y.-J.; Sheu, H.-S.; Lai, C. K. Tetrahedron 2007, 63, 1695.