Synthesis and characterization of supramolecular optically active bisamides derived from amino acids

Article abstract of DOI:10.1016/j.tet.2012.05.070

The synthesis and phase transitional behaviour of three pairs of enantiomeric supramolecular hexacatenar liquid crystals (LCs) derived from natural α-amino acids such as l/d-alanine, l/d-leucine and l/d-valine are described. Their preparation with high enantiomeric purity was accomplished by condensing optically active (amino acid residue containing) trialkoxy amines with a 3,4,5-trialkoxy cinnamic acid core using a peptide coupling reagent namely, 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyl-uronium hexafluorophosphate (HBTU). The mesomorphic behaviour of these self-complementing mesogens was ascertained by polarising optical microscopy, differential scanning calorimetry and X-ray diffraction. The compounds exhibit columnar (Col) phase over a wide thermal range. Particularly, a pair of enantiomers derived from l/d-leucine residues notably stabilize hexagonal Col (Col_h) phase over a wide temperature range of -5 °C to 180 °C. Circular dichroism (CD) and FTIR studies suggest the chiral (helical) organization of mesogens within the columns through intermolecular hydrogen bonding; thus, these enantiomers represent one of the rarely reported examples of LCs exhibiting supramolecular Col_h phase at room temperature. The gelation studies reveal the ability of these bisamides to form stable supramolecular gels in ethanol caused through H-bonding interactions.

Full text of DOI:10.1016/j.tet.2012.05.070

Tetrahedron 68 (2012) 6528e6534
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Synthesis and characterization of supramolecular optically active bisamides
derived from amino acids
y
*
G. Shanker , D.S. Shankar Rao, S. Krishna Prasad, C.V. Yelamaggad
Centre for Soft Matter Research, P.B. No. 1329, Jalahalli, Bangalore 560013, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 April 2012
Received in revised form 17 May 2012
Accepted 20 May 2012
Available online 26 May 2012
The synthesis and phase transitional behaviour of three pairs of enantiomeric supramolecular
hexacatenar liquid crystals (LCs) derived from natural
-valine are described. Their preparation with high enantiomeric purity was accomplished by
condensing optically active (amino acid residue containing) trialkoxy amines with a 3,4,5-trialkoxy
cinnamic acid core using peptide coupling reagent namely, 2-(1H-benzotriazol-1-yl)-
a-amino acids such as L/D-alanine, L/D-leucine and
L/D
a
1,1,3,3-tetramethyl-uronium hexafluorophosphate (HBTU). The mesomorphic behaviour of these
self-complementing mesogens was ascertained by polarising optical microscopy, differential scanning
calorimetry and X-ray diffraction. The compounds exhibit columnar (Col) phase over a wide thermal
range. Particularly, a pair of enantiomers derived from L/D-leucine residues notably stabilize hexagonal
Col (Colh) phase over a wide temperature range of ꢀ5 ꢁC to 180 ꢁC. Circular dichroism (CD) and FTIR
studies suggest the chiral (helical) organization of mesogens within the columns through intermolecular
hydrogen bonding; thus, these enantiomers represent one of the rarely reported examples of LCs
exhibiting supramolecular Colh phase at room temperature. The gelation studies reveal the ability of
these bisamides to form stable supramolecular gels in ethanol caused through H-bonding interactions.
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
enforcing the self-assembly of complementary mesogenic/non-
mesogenic molecules in generating a range of LC phases, called
The past two decades have witnessed
a
remarkable de-
supramolecular LCs, is especially noteworthy. Supramolecular H-
bonded LCs, primarily being anisotropic fluids, are attracting
enormous attention as they additionally possess a unique combi-
nation of properties as a consequence of the presence of H-bonds,
such as moderate bonding energy, directionality, selectivity and
reversibility.2,11e13,16 In fact, significant advances have been made in
developing multifunctional columnar (Col) LC phases where the H-
bonding plays an important role.2,8e14,16e36 As is well-known, Col
phases, which are generated by the spontaneous self-assembly of
velopment in supramolecular chemistry where the properties of
chemical species, resulting from the noncovalent interactions
among simple molecules or supramolecules (self-complementing
motifs) or supermolecules, are rationally investigated to demon-
strate their significance in materials and biological sciences.1e5 The
studies encompassing molecular engineering and synthesis of
novel materials have particularly established that noncovalent in-
teractions, such as hydrogen (H) bonding, electrostastic in-
teractions,
p
e
p
stacking interactions and van der Waals or
disk-like (discotic)
stacks, are highly anisotropic and ordered structures with extensive
-orbital overlap leading to high degree of uniaxial charge-carrier
p-conjugated molecules into fluid columnar
dispersive or hydrophobic forces, greatly facilitate the generation of
novel structures featuring a high degree of complexity and func-
tion.2 Among these, the H-bond interaction is proven to be very
effective in generating numerous interesting structures such as
dimers, multimers, ionophores, capsules (molecular containers),
chiral capsules, liquid crystal (LC) phases, self-assembled nano-
tubes and macromolecular assemblies.1e16 The ability of H-bonds
p
mobilities.37 Thus, Col phases hold immense promise in elec-
tronic devices such as photovoltaic cells,38 field effect transistors39
and organic light emitting devices (OLEDs).40 In recent times it has
also been demonstrated that by the rational design and synthesis of
H-bonded supramolecular materials, especially nondiscoid (non-
conventional) motifs, many other important functional featur-
esdattractive from the view points of both life-science and
technologydcan be incorporated in Col LC phases.16e36,41e43
One area of interest that we have been focussing on for some
time is the design and synthesis of nondiscoid materials such as
* Corresponding author. E-mail addresses: Yelamaggad@gmail.com, Yelamaggad@
csmr.res.in (C.V. Yelamaggad).
y
Present
address:
Institute
of
Chemistry,
Organic
Chemistry,
Martin-Luther-University Halle-Wittenberg, Kurt Mothes Str. 2, D-06120 Halle/
Saale, Germany.
hexacatenars, in which naturally occurring a-amino acids are used
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2012.05.070

G. Shanker et al. / Tetrahedron 68 (2012) 6528e6534
6529
as one of the important building blocks.41e43 Precisely, we have
realized several chiral supramolecules, which can be regarded as
intermediates between the polycatenars and taper-shaped am-
phiphiles as they possess two lipophilic (half-disk shaped) seg-
ments interlinked through a central linking segment derived from
one42 or two41 amino acid residues. These compounds exhibit
a range of important properties such as macroscopic chirality,41,42
ferroelectric behaviour41 and gelating ability.41e43 For example,
these self-complementing motifs with multiple H-donating and H-
accepting sites self-organize into the chiral Col phase through the
intermolecular H-bond interaction; this means that the molecular
chirality of the materials is very well manifested in the fluid col-
umns. Furthermore, the nature of the central linking segment in the
molecules appears to dictate their packing, which in turn reflects
on the magnitude of out-of-plane fluctuation of the lattice plane of
2D (Col) structure. Besides, they form stable organogels in
ethanol41e43 and our investigations reveal the occurrence of 3D
network comprising helical fibres in the gel.41 Most importantly,
some of these compounds, especially the dipeptides derived from
appropriate Na-Fmoc-amino acids with aniline 6 in the presence of
NMM and IBC-Cl gave protected amines 7aeb, 8aeb and 9aeb;
deprotection of these compounds using a mixture of (C2H5)2NH
and CH2Cl2 furnished the key amines 10aeb, 11aeb and 12aeb.41
Finally, the target bisamides, the CHBA series of compounds,
were obtained in 60e65% yield by coupling cinnamic acid 4 with
these amines (10aeb, 11aeb and 12aeb) using HBTU in dry THF. All
the synthesized hexacaternars and their intermediates were char-
acterized by spectroscopic methods and elemental analysis (see
Supplementary data for details).
2.2. Evaluation of liquid crystal behaviour
The mesomorphic behaviour of the synthesized hexacatenars,
CHBA-1 to CHBA-6, was investigated by three complementary ex-
perimental techniques of polarizing optical microscopy (POM),
differential scanning calorimetry (DSC) and X-ray diffraction (XRD).
The results of these investigations are summarized in Table 1 and
discussed in detail in the following sections.
Le
L-/D
eD
-alanine combinations, exhibit
a
polar (ferroelectric)
The enantiomeric pairs CHBA-1/CHBA-2, CHBA-3/CHBA-4 and
oblique columnar (Colob) phase with a high spontaneous polari-
zation value41 and are thus the first examples of supramolecular
columnar LCs showing ferroelectric switching characteristics.
These observations motivated us to prepare the variants of above-
mentioned supramolecular hexacatenars comprising a new func-
tional moiety. In the molecular design suitable for our objectives,
we opted to replace one of the taper-shaped units with other li-
CHBA-5/CHBA-6, respectively, derived from
L-/D-alanine, L-/D-leu-
cine and -/ -valine display identical mesomorphic property
L
D
exhibiting enantiotropic columnar behaviour with an excellent
thermal stability as ascertained by optical and calorimetric studies.
Notably, the enantiomeric bisamides CHBA-3 and CHBA-4 stabilize
the Col phase well below and above the room temperature (rt). In
fact, the occurrence of Col phase at rt was immediately apparent as
they were isolated, after purification, as sticky mass, which could be
readily spread by mechanical shearing. When viewed under mi-
croscope, the spread thin films showed a fluid, highly birefringent
nonspecific textural pattern. However, sandwiched between
a clean glass slide and a coverslip, and on slow cooling from the
isotropic liquid, the compounds exhibit birefringent dendritic tex-
tural pattern (Fig. 1a) emanating from the dark background of
isotropic liquid. This pattern is well-known to be an important
signature and conclusive evidence for the occurrence of Col phase.
On lowering the samples temperature further, neither any other
textural change (Fig. 1b) nor crystallization was seen till rt implying
that the Col phase once formed upon cooling from the isotropic
phase, crystallizes by no means instead it remains unaltered. These
phase transitions were found to be reproducible for any number of
heatingecooling cycles.
pophilic core featuring extended
p-conjugation. We especially
chose to incorporate a lipophilic (half-disc like) core derived from
3,4,5-tris(dodecyloxy)cinnamic acid as it not only reduces the
symmetry of the resulting hexacatenars but also accounts for
electronic properties.44,45 In fact, it is well documented that cin-
namic acid or its derivatives (cinnamates) comprising flexible tails
exhibit a range of smectic and/or nematic mesophases.45 Further,
owing to the presence of a trans double bond in conjugation with
benzene ring and carbonyl group, the cinnamoyl moiety imparts
photophysical properties as well as a particular geometry (shape) to
the systems.
Herein, we present the synthesis and characterization of hex-
acatenar bisamides in which 3,4,5-tris(dodecyloxy)cinnamic acid
and 3,4,5-tris(dodecyloxy)aniline cores are interlinked through an
amide spacer derived from amino acids such as
L/D-alanine, L/D-
leucine and -valine. The general molecular structure of the
L
/
D
As shown in Fig. 2, the DSC thermograms obtained during the
first and second heating-cooling cycles, corroborate the optical
observation. That is, as can be seen in Fig. 2, no exothermic or en-
dothermic peaks exist between the temperature range 0e180 ꢁC
and thus, supporting the optical observation that Col phase exists
over a wide thermal range covering rt. The other two pairs of en-
synthesized bisamides, hereafter referred to as CHBA series (where
CHBA stands for Cinnamoyl-based Hexacatenar BisAmides), is
shown in Scheme 1.
2. Experimental
2.1. Synthesis
antiomers CHBA-1/CHBA-2 and CHBA-5/CHBA-6 derived from
L-/D-
alanine and -/ -valine, respectively, exhibit analogous behaviour
L
D
with the exception that they show Col mesomorphism at elevated
temperatures (see Table 1).
In order to ascertain the structure of the mesophase formed by
bisamides CHBA-3 and CHBA-4, X-ray diffraction (XRD) measure-
The synthetic pathway for preparing the target hexacatenar
molecules is depicted in Scheme 1. Ethyl (3,4,5-tris-dodecyloxy)
benzoate (1) was prepared by esterification of gallic acid with
ethanol followed by O-alkylation using Williamson’s ether syn-
thesis protocol. This material was then subjected to reduction using
LAH in dry THF to obtain 3,4,5-tris(dodecyloxy) benzylalcohol (2) in
98% yield; the oxidation of this primary alcohol using PCC-silica in
dry CH2Cl2 yielded 3,4,5-tris(dodecyloxy)benzaldehyde (3). The
Knoevenagel condensation of benzaldehyde 3 with malonic acid
using catalytic amount of piperidine in pyridine delivered the key
intermediate 3,4,5-tris(dodecyloxy)cinnamic acid (4) in 81% yield.
1,2,3-Tris-dodecyloxy-5-nitrobenzene (5), prepared by nitrating
1,2,3-tris-dodecyl-oxybenzene using fuming HNO3 in the presence
of NaNO2, was subjected to catalytic hydrogenation to obtain 3,4,5-
tri(dodecyloxy)aniline (6).24 Upon treating, readily available,
ments were carried out on powder (unaligned) samples. These
ꢀ
experiments were performed using Cu K
a
radiations (
l
¼1.5418 A)
from a fine focus sealed-tube generator (Enraf Nonius FR590), in
conjunction with double mirror focussing optics. The detector
employed was an image plate detector (MAC Science, Japan; Model
DIP 1030). The mesogens CHBA-3 and CHBA-4 were filled in
a Lindemann capillary (1 mm diameter) tube in the isotropic phase
and both the ends of tube were flame sealed. The diffraction pat-
terns obtained at two different temperatures viz., rt (25 ꢁC) and
high temperature (HT) (170 ꢁC), were collected in the cooling mode.
The XRD patterns obtained for both the compounds at rt and HT
were found to be virtually indistinguishable, as expected. As

6530
G. Shanker et al. / Tetrahedron 68 (2012) 6528e6534
OC12H25
OC12H25
OC12H25
C12H25O
C12H25O
OEt
OC12H25
NO2
5
1
O
(iv)
(i)
OC12H25
OC12H25
OC12H25
C12H25O
C12H25O
OH
OC12H25
NH2
6
2
(v)
(ii)
OC12H25
OC12H25
OC12H25
C12H25O
C12H25O
O
O
FmocHN
*
N
OC12H25
3
R1
H
(iii)
7a : R = (S)-Methyl 8a : R = (S)-Isobutyl
9a : R1 = (S)-Isopropyl
9b : R1 = (R)-Isopropyl
1
1
7b : R = (R)-Methyl8b : R = (R)-Isobutyl
1
1
(vi)
OC12H25
OC12H25
C12H25O
C12H25O
OC12H25
O
OH
H2N
*
N
H
OC12H25
4
O
R1
11a : R = (S)-Isobutyl
10a : R1 = (S)-Methyl
10b : R1 = (R)-Methyl
12a : R1 = (S)-Isopropyl
12b : R1 = (R)-Isopropyl
1
11b : R = (R)-Isobutyl
1
(vii)
OC12H25
OC12H25
OC12H25
C12H25O
H
N
O
*
C12H25O
N
H
OC12H25
O
R1
CHAB-5 : R1 = (S)-Isopropyl
CHAB-6 : R1 = (R)-Isopropyl
CHAB-1 : R1 = (S)-Methyl
CHAB-2 : R1 = (R)-Methyl
CHAB-3 : R1 = (S)-Isobutyl
CHAB-4 : R1 = (R)-Isobutyl
Scheme 1. Reagents and conditions. (i) LAH, THF, 0 ꢁC, 2 h (98%); (ii) PCC, DCM, rt, 1 h (85%); (iii) Malonic acid, pyridine, piperidine (catalyst), reflux, 10 h (81%); (iv) H2 (1 atm,
balloon), 10% PdeC, THF, rt, 4 h (88%); (v) Fmoc-L/D-alanine or Fmoc-L/D-leucine or Fmoc-L/D-valine, N-methylmorpholine, isobutoxycarbonyl chloride, THF, 0 ꢁC to rt, 10 h, (74e78%);
(vi) diethylamineedichloromethane (1/1), rt, 2 h (not isolated); (vii) 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyl-uroniumhexafluorophosphate (HBTU), THF, N-ethyldiisopropyl-
amine (DIEA), rt, 10 h (60e65%).
ꢀ
ꢀ
ꢀ
representative cases, the 1D intensity versus 2
q
profiles deduced
observed; the spacings (d) being 24 A and 14.3 A at HT, and 23.5 A,
ꢀ
from the powder 2D patterns of the sample CHBA-3 is shown in
Fig. 3. The results of indexing these profiles of the phases are
summarized in Table 2.As seen in Fig. 3, the wide angle region of
both HT and rt profiles comprises a diffuse reflection with a spacing
and 13.7 A at rt. These two reflections of Col phase with the spacing
ratio of 1:O3 can be assigned to (10) and (11) reflections from
a quasi 2D hexagonal lattice with a lattice dimension 27.8 A at HT
and 27.1 A for rt. Thus, the textural observation coupled with XRD
data suggests that these two compounds stabilize a fluid columnar
phase possessing D6h (hexagonal) symmetry; that is, they exhibit
columnar hexagonal (Colh) phase. It may be recalled here that all
ꢀ
ꢀ
ꢀ
(d) of about 5 A (Table 2), that can be attributed to the average
distance between the molecules within column. In the low angle
region (0<2q
<5ꢁ) of the diffractograms, two sharp reflections were

G. Shanker et al. / Tetrahedron 68 (2012) 6528e6534
6531
Table 1
the six hexacatenars synthesized exhibit a textural pattern, that is,
indistinguishable from that for CHBA-3 and CHBA-4 and therefore,
it is reasonable to assume that the Col phase stabilized by other four
compounds, CHBA-1, CHBA-2, CHBA-5 and CHBA-6, as well possess
hexagonal lattice. In fact, the mesophase of CHBA-3 and CHBA-4
could be well-mixed with the LC phase formed by other four
compounds, clearly indicating that all of them show an identical
mesophase.
Phase transition temperatures (ꢁC)a and enthalpies [kJ molꢀ1 b of hexacatenars
]
Compound
Phase sequence
Heating
Cooling
CHBA-1
CHBA-2
CHBA-3
CHBA-4
CHBA-5
CHBA-6
Cr 67 [61.2] Colc 184 I
Cr 66 [57.4] Colc 183 I
Cr ꢀ5 [30] Colh 184 [6.6] I
Cr ꢀ5 [25.6] Colh 185 [5.9] I
Cr 66 [70.6] Colc 180 I
Cr 65 [73.7] Colc 180 I
I 183 Colc 55 [ꢀ12.5] Cr
I 182 Colc 54 [ꢀ10.3] Cr
I 183 [ꢀ6.4] Colh ꢀ11 [ꢀ26.4] Cr
I 184 [ꢀ5.8] Colh ꢀ10 [ꢀ17.7] Cr
I 178 Colc 51 [ꢀ7.4] Cr
I 178 Colc 68 [ꢀ4.6] Cr
2.3. Determination of hydrogen bonding
Abbreviations: Colh¼hexagonal columnar phase; I¼isotropic phase; Cr¼crystal.
a
Peak temperatures in the DSC thermograms obtained during the first heating
and cooling cycles at 5 ꢁC/min.
As is well-known, molecular interactions and conformations
have a remarkable effect on the wavenumber, intensity and shape
of the infrared (IR) absorption bands.46 Likewise, H-bonding is one
of the best-known factors that alters such parameters of IR bands.
b
Transition enthalpies were obtained from the DSC thermograms.
The ColeI or IeCol phase transition was observed under optical polarizing mi-
c
croscope and too weak [<0.01 kJ molꢀ1] to get recognized in DSC.
Fig. 1. Microphotograph of the optical texture seen for the Colh phase of compounds (a) CHBA-3 at 179 ꢁC. (b) CHBA-4 at 30 ꢁC.
Thus, FTIR technique is being employed as an important tool to
characterize H-bonding, especially in mesomorphic amides.32,33,36
To ascertain the self-assembly through intermolecular H-bonding
leading to the formation Col phase, all the six compounds synthe-
sized were investigated with the help of FTIR study. They displayed
very similar IR spectral patterns, as expected, which ascertained the
presence of H-bonding in the Col phase. However, we discuss the
experimental details and results of the hexacatenars CHBA-3, as
a representative case. Compound CHBA-3 sandwiched between
a pair of KBr plates, was cooled from the isotropic phase to rt, and
the FTIR spectra were recorded as a function of decreasing tem-
perature. Two important regions of the spectra are shown in Fig. 4.
The occurrence of bands due to stretching vibrations of NeH and
C]O groups at around 3270 cmꢀ1 and 1615 cmꢀ1, respectively,
indicate the presence of H-bonded amide groups in the mesophase.
Indeed, the observed band values are less than 3440 cmꢀ1 and
1680 cmꢀ1 usually observed for the free NeH and C]O groups,
respectively. As can be seen in Fig. 4, the bands due to NH and C]O
stretching vibrations become stronger (increase in the intensity)
Cr
I
II
I
I
194 °
II
I
Heating
Cooling
Cr
-50
0
50
100
150
T (°C)
Fig. 2. DSC traces of the first and second heating-cooling cycles recorded at a rate of
ꢁC/min. for the compound CHBA-3. Notice that the thermograms of both heating and
cooling cycles are identical suggesting the high thermal stability of the compound.
5
Fig. 3. 1D intensity versus 2q
profiles obtained for the Colh phase (a) at 170 ꢁC (b) at 25 ꢁC for the compound CHBA-3.

6532
G. Shanker et al. / Tetrahedron 68 (2012) 6528e6534
Table 2
using sample cells prepared for CD measurements (see below for
details). It may be noted here that the absorption maxima of these
compounds shift dramatically to higher wavelength when com-
pared to other hexacatenar reported earlier;41 needless to say, this
The result of indexation of XRD profiles of CHBA-3 and CHBA-4 at a given temper-
ature (T/ꢁC) of mesophase
Compound Phase T/ꢁC d Observed d Calculated Miller
Lattice
index, hk parameters (A)
ꢀ
ꢀ
ꢀ
(A)
(A)
shift can be attributed to the presence of extended
cinnamoyl moiety.
p-conjugation in
CHBA-3
CHBA-4
Colh
170 24
24
13.9
10
11
a¼27.8
14.3
It is well documented that circular dichroism (CD) spectroscopy
is employed as one important optical techniques to examine if the
molecular chirality of the mesogens (discotics) has facilitated mo-
lecular organization in chiral (helical) fashion in the columnar and
isotropic phases.47 The appearance of exciton-split Cotton effect in
the CD spectrum of the samples especially indicates the manifes-
tations of molecular chirality of discotics into the fluid columnar
structure featuring handedness; here, the constituent molecules
within the column are twisted with respect to each other and it is
the precession of the twist that gives rise to the macroscopic helical
structure. It may be noted here that the CD activity of the Col phase
may also arise if the optically active discotics are stacked on top of
each other randomly and thus, molecules are not correlated.
However, if the CD signals (intensities) are strong enough, it may be
reasonable to infer that the constituent mesogens are arranged in
a helical fashion within the column.41,47 In order to find out
whether the molecular chirality of the molecules, has caused such
effects in the Colh phase, all the three pairs of enantiomers were
probed for chiroptical properties with the aid of CD spectroscopic
studies.
4.8
25 23.5
13.7
23.5
13.5
10
11
a¼27.1
4.5
Colh
170 24.1
14.4
4.8
25 23.4
13.7
24.1
13.9
10
11
a¼27.7
a¼27.1
23.4
13.6
10
11
4.5
Abbreviation: Colh¼Hexagonal columnar phase.
Samples held between two quartz plates were heated to their
isotropic liquid state using programmable hot stage, which gives an
accuracy of about ꢂ0.6 ꢁC variations and cooled slowly. The hea-
tingecooling cycle along with the mechanical shearing was per-
formed repeatedly to spread the sample uniformly. It must be
mentioned here that very thin films of samples (mesophase) were
required for the experiment to work within the upper limits of
measurement. Consequently, usage of prefabricated cells with the
known thickness was precluded; thus, our experiments are rather
qualitative. The CD spectra were recorded carefully48 as a function
of decreasing temperature from isotropic liquid state to rt, covering
the entire thermal range of Col phase. None of the samples showed
CD activity in isotropic state whereas the mesophases (Colh phases)
occurring just below the isotropic phase produce signals indicating
chiral induction at the isotropic liquideColh phase transition. This
observation clearly suggests that the optical activity of these hex-
acatenars does not originate from the presence molecular chirality
but from the chiral induction in the columnar structure. Most im-
portantly, each of the enantiomeric pairs exhibited mirror image
CD curves, as expected. The intensity of CD signals increases with
decrease in temperature implying that the supramolecular order,
handedness, caused by cooling into Colh phase, enhances CD signals
remarkably. These features, for example, can be seen in Fig. 6a and b
where enantiomers CHBA-3 and CHBA-4 exhibit mirror image
cotton effect. That is, enantiomers CHBA-3 and CHBA-4 display
Fig. 4. The regions of the FTIR spectra recorded at different temperature intervals
during cooling the sample CHBA-3 from the isotropic phase.
and get pronounced near rt, as expected. These features clearly
indicate the correlation between NH and CO groups through in-
termolecular H-bonds.32,33,36 In addition, the disordering of n-alk-
oxy tails present on either side of the hexacatenar was evidenced
based on the observation of bands of asymmetric and symmetric
CeH stretching at 2937 cmꢀ1 and 2858 cmꢀ1, respectively.32,33,36
2.4. UVevisible and circular dichroism (CD) studies
The photophysical properties of these compounds were in-
vestigated with the aid of UVevis absorption and CD spectroscopic
studies. The dichloromethane solution UVevis spectra of all the
hexacatenar bisamides exhibit almost identical pattern with ab-
sorption maxima centred around 310 nm; as representative cases
the spectra obtained for bisamides CHBA-3 and CHBA-4 are shown
in Fig. 5a and b, respectively. In fact, identical UV spectral patterns
were obtained when thin films of the mesophase were studied
(a)
0.10
(b)
0.15
0.10
0.05
0.00
0.08
0.06
0.04
0.02
0.00
260
280
300
320
λ
340
(nm)
360
380
400
260
280
300
320
λ
340
(nm)
360
380
400
Fig. 5. UVevis spectra of bisamides CHBA-3 (a) and CHBA-4 (b) dissolved in CH2Cl2.

G. Shanker et al. / Tetrahedron 68 (2012) 6528e6534
6533
Fig. 6. The CD spectra obtained in the Col phases as a function of temperature for the enantiomeric pairs (a) CHBA-3 and (b) CHBA-4. Notice that they exhibit mirror image CD
curves in the mesophase.
a negative and a positive exciton splittings, respectively (Table S1),
centred at the wavelength of the transition (350e360 nm).
In fact, the exciton coupling process is an outcome of through-
space interaction between two or more chromophores exhibiting
was no response from the Col phase indicating the absence of po-
larity. On the other hand, these kinds of bisamides have tendency to
gel with various solvents, hence a representative sample tested for
gelation property. Compound CHBA-3 forms gel with 5% ethanol as
confirmed by test tube inversion method (Fig. S1a) and IR in-
pep
*
bands due to the
pep transition. This interaction through-space
*
divides the excited state into two energy levels. For the occur-
rence of exciton-splitting phenomenon, interacting electric transi-
tion moments should not be parallel as it is a vectorial product.47b,e
The occurrence of exciton splittings in the CD spectra of the Colh
phase of the present compounds, should be resulting from the
twisted (nonparallel) organization of the hexacatenars within the
columns. In essence, Cotton effect observed in the CD spectra of the
samples indicates the expression of chirality of the molecules into
the helical conformation of the columns.
vestigation shows peak at nNeH¼3269 cmꢀ1
,
nCeH(as)¼2920 cmꢀ1
;
nCeH(s)¼2853 cmꢀ1
,
nCO¼1633 cmꢀ1 indicating H-bonding in the
gel. Scanning electron microscopy (SEM) images showed the
presence of entangled networks (Fig. S1bed) at different parts of
the sample.
3. Summary
Summarizing, optically active cinnamoyl-based bisamides
These molecules are arranged in a helical fashion through in-
termolecular hydrogen bonding as evidenced by CD and IR studies.
Perhaps, the substituent (viz., methyl, isobutyl and isopropyl
groups) present in the supramolecular central spacer enforces the
molecules to twist with respect to each other within the column.
Such an arrangement of molecules causes a dramatic reduction
of the steric repulsions while maintaining the intermolecular H-
bonding through NH and C]O groups. Based on these facts,
a model for the Colh phase where the molecules within the columns
are arranged in helical manner is shown in Fig. 7.
comprising an amino acid residue such as
L/D-alanine or L/D-leucine
or -valine have been synthesized and characterized. All the
L/D
compounds display enantiotropic columnar mesophase; XRD study
establishes hexagonal (D6h) symmetry for the columnar lattice.
Notably, bisamides derived from leucine exhibit Col phase over
a wide thermal range through rt. The occurrence of CD signals
perhaps indicates the chiral organization of the molecules within
the column facilitated by intermolecular H-bonding.
Supplementary data
Supplementary data related to this article can be found online at
doi:10.1016/j.tet.2012.05.070.
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