The first examples of optically active tris(N-salicylideneaniline)s: Manifestation of chirality from molecules to fluid columnar phases

Article abstract of DOI:10.1039/b708432c

Tris(N-salicylideneaniline)s comprising chiral tails in varying numbers self-organize into a room temperature helical columnar mesophase resulting from the chiral stacking of the constituent molecules differing in their rotational symmetry and ratio as ev

Full text of DOI:10.1039/b708432c

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The first examples of optically active tris(N-salicylideneaniline)s:
manifestation of chirality from molecules to fluid columnar phases{
Channabasaveshwar V. Yelamaggad,* Ammathnadu S. Achalkumar, Doddamane S. Shankar Rao and
Subbarao Krishna Prasad
Received 4th June 2007, Accepted 23rd August 2007
First published as an Advance Article on the web 5th September 2007
DOI: 10.1039/b708432c
Tris(N-salicylideneaniline)s comprising chiral tails in varying numbers self-organize into a room
temperature helical columnar mesophase resulting from the chiral stacking of the constituent
molecules differing in their rotational symmetry and ratio as evidenced by optical, chiroptical,
calorimetric and X-ray studies.
mixture of two keto-enamine forms with C3h and Cs rotational
symmetries (Fig. 1: I–IV), display Col phases.8 These discogens
Introduction
The thorough understanding and/or mimicking Nature’s art of
are rather unique as their central core is a flat electron-
expressing and augmenting chirality from microscopic to
accepting (n-type) entity featuring multiple strong intramole-
cular H-bondings. Lee and co-workers9c have shown that
mesoscopic levels remains elusive. However, the ubiquitous
bio-molecular self-assembly into helical structures has inspired
sterically hindered TSANs could be used as mechanically
chemists to generate novel materials not only to reveal the
coupled biconcave molecules, which can release the clathrated
guest molecules. Subsequently, MacLachlan and co-workers7a
structure–property correlation but also to explore their usage
in diverse applications such as biosensors, lithography and
drug delivery.1 The foremost objective of such studies has been
groups, TSANs may form H-bonded capsules, clefts and
revealed that, upon appending with additional functional
the design and synthesis of chiral molecular systems capable of
yielding complex large-scale helical structures originating from
the manifestation of chirality of the constituent molecules
through noncovalent interactions such as hydrogen (H)-
bonding, p–p stacking and van der Waals forces.2 Such
investigations are becoming increasingly fascinating given the
fact that many chiral molecules do not generate helical
aggregates implying that the intramolecular rigidity (the
distinct shape of molecules) and intermolecular interactions,
although not directly related to molecular chirality, are also
imperative.1–3 This is especially noteworthy in the case of
chiral discotic liquid crystals (LCs) exhibiting columnar (Col)
phases characterized by indefinitely long molecular columns
aggregating into two-dimensional (2D) lattices having different
symmetries.3 For instance, the interactions among peripheral
chiral tails of a discotic core are generally ineffective in
inducing helical order at the mesoscopic level and thus the
majority of the self-assembled fluid columns do not possess
handedness. Nonetheless, few chiral columns are known to be
formed if the discotic core3–5 or metallomesogen6 features an
intrinsic shape that favors packing with a preferred rotation.
Recently, tris(N-salicylideneaniline)s [TSANs] have been
demonstrated to be versatile compounds in which both
electronic as well as novel molecular material features like
self-organizing ability, molecular recognition etc. can be
imbued.7–9 For example, we have discovered that novel
discotics derived from TSANs existing in an inseparable
chains. Apparently, the induction of molecular chirality in
TSANs and the consequent effects on their self-assembly have
not been reported hitherto. In continuation of our investiga-
tion on LC TSANs we became interested in realizing their
optically active analogues. Since Col phases of TSANs
comprise two keto-enamine forms in varying proportions, it
may be reasoned that the presence of chiral tails at the
peripheral region influence their self-assembly. Here we report
on the synthesis and thermal behavior of the first examples of
chiral TSANs exhibiting helical Col phases via the transfer of
the exterior-chain chirality into supramolecular structure.
Thus, another remarkable feature of TSANs has been
disclosed.
Results and discussion
Synthesis
The molecular structures of different chiral TSANs prepared
(Schemes 1 to 3) in the present investigation are shown in
Fig. 1. For a comparative study, the central cyclohexane-1,3,5-
trione core has been appended with either 1,2-bis(alkoxy)
benzenes (where both alkoxy tails are chiral: 1) or
1,2,3-tris(alkoxy)benzenes [wherein one tail (2a) or two tails
(2b) or all the three tails (2c) are chiral]. The literature
reports3,5 and our earlier investigation8b directed us to employ
the (3S)-3,7-dimethyloctyloxy entity as a chiral tail. The target
chiral TSANs 1 and 2a–c were obtained by a 3-fold reaction of
chiral 3,4-dialkoxyaniline 6a and 3,4,5-trialkoxyanlines 6b,
13a, 13b with 1,3,5-triformylphloroglucinol, respectively
(Scheme 3). In turn, anilines were prepared, starting from
catechol and pyrogallol.10 Catechol or pyrogallol upon
O-alkylation with (S)-(+)-citronellyl bromide following
Centre for Liquid Crystal Research, Jalahalli, Bangalore, 560 013,
India. E-mail: Yelamaggad@gmail.com
{ Electronic supplementary information (ESI) available: Detailed
synthetic procedures and characterization data. See DOI: 10.1039/
b708432c
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Fig. 1 Molecular structures of achiral and chiral TSANs, pertaining to the previous and present studies.
Williamson’s ether synthesis protocol yielded alkoxybenzenes
3a and 3b. These compounds on catalytic hydrogenation
furnished the intermediates 4a and 4b, in which the olefinic
bond is reduced. The alkoxybenzenes 4a and 4b obtained were
subjected to the two-phase nitration using an aqueous solution
of nitric acid (70%) and sodium nitrite in dichloromethane to
achieve the corresponding 3,4-dialkoxynitrobenzene 5a and
3,4,5-trialkoxynitrobenzene 5b. The catalytic hydrogenation of
these nitro derivatives furnished the key 3,4-dialkoxyaniline 6a
and 3,4,5-trialkoxyanline 6b in almost quantitative yields. As
shown in Scheme 3, by treating 1,3,5-triformylphloroglucinol
with 6a and 6b in refluxing ethanol furnished target molecules
1 and 2c respectively. Similarly, the target compounds 2a and
2b were prepared starting from pyrogallol. To begin with, two
vicinal hydroxyl groups of pyrogallol were protected using
˚
triethylorthoformate in the presence of 4 A molecular sieves
(as a water scavenger). The ethoxymethylene acetal 7 was then
O-alkylated with either (S)-(+)-citronellyl bromide or n-octyl-
bromide using Williamson’s ether synthesis protocol to obtain
intermediates 8a and 8b. These mono alkylated compounds
were subjected to deprotection under mild acidic conditions to
give dihydroxy compounds 9a and 9b. These products were
O-alkylated with either n-octylbromide or (S)-(+)-citronellyl
bromide using a mild base. The double bonds of the tails of
trialkoxybenzenes 10a and 10b were reduced by catalytic
hydrogenation to give 11a and 11b respectively. The above-
mentioned two-phase nitration of these trialkoxybenzenes
followed by reduction yielded nitro compounds 12a–b and
anilines 13a–b respectively. The anilines 13a and 13b thus
obtained were condensed with 1,3,5-triformylphloroglucinol to
obtain the target molecules 2a and 2b respectively. The
molecular structures of all the target compounds and inter-
mediates were confirmed by standard spectroscopic methods
(see Experimental section) and elemental analysis. All the
target molecules existed as
a mixture of keto-enamine
tautomers of C3h and Cs symmetry as evidenced by 1H (see
1
Table 1) and H–1H COSY NMR spectra.10
Thermal behavior
The mesomorphism of optically active TSANs was probed
with the help of polarized optical microscope (POM),
differential scanning calorimeter (DSC), and X-ray diffraction
(XRD) studies. Table 1 summarizes the phase behavior of
these compounds as determined by these investigations. As can
be seen, all the TSANs show enantiotropic Col mesophases.
Scheme 1 Synthetic route for the compounds 6a–b. Experimental
conditions: (i) (S)-(+)-citronellyl bromide, anhyd. K2CO3, DMF,
80 uC, 12 h (3a: 78%; 3b: 74%); (ii) H2 (1 atm, balloon), 10% PdC,
THF, 12 h; (iii) 70% HNO3, NaNO2, CH2Cl2, rt, 1 h (5a: 56%; 5b: 70%);
(iv) H2 (1 atm, balloon), 10% PdC, THF, 12 h (6a: 90%; 5b: 88%).
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˚
Scheme 2 Synthetic route for the compounds 13a–b. Experimental conditions: (i) triethylorthoformate, toluene, 4 A molecular sieves, 100 uC, 1 h;
(ii) (S)-(+)-citronellyl bromide or n-bromooctane, anhyd. K2CO3, butanone, reflux, 17 h; (iii) PTSA, methanol–water (9 : 1), rt, 2 h (9a: 68%; 9b:
70%); (iv) n-bromooctane or (S)-(+)-citronellyl bromide, anhyd. K2CO3, DMF, 80 uC, 12 h (10a: 74%; 10b: 76%); (v) H2 (1 atm, balloon), 10% PdC,
THF, rt, 12 h; (vi) 70% HNO3, NaNO2, CH2Cl2, rt, 1 h (12a: 70%; 12b: 72%); (vii) H2 (1 atm, balloon), 10% PdC, THF, rt, 6 h (13a: 74%; 13b:
76%).
The optical textures showed fan-shaped defects, homeotropic
digitated stars, fernlike domains, a pattern with tiny needles
and a grainy pattern (Fig. 2).10 Some of these textures are
unusual, while their XRD patterns originate from Col phases.
DSC thermograms showed first-order transitions at the phase
transition temperatures in compliance with the optical
observations. In general, the powder X-ray diffractograms
obtained for Col phases (Table 2) showed a broad diffuse
˚
scattering centered around 4.6–4.9 A at wide angles (2h # 20u)
which relates to the liquid like order of the peripheral tails. In
some cases, another relatively sharp reflection corresponding
˚
to a distance of y3.5 A can be attributed to the stacking
distance of the discs within the same column (core–core
separation). In the small-angle region, sharp Bragg reflections
observed could be indexed to hexagonal11 or rectangular 2D
lattices of the Col phases. Thus, analysis of the results of these
Scheme 3 Synthetic route for the chiral TSANs. Experimental
conditions: (i) 6a, ethanol, reflux, 6 h (60%); (ii) 13a, ethanol, reflux,
6 h (72%); (iii) 13b, ethanol, reflux, 6 h (74%); (iv) 6b, ethanol, reflux,
6 h (72%).
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Table 1 Phase transition temperaturesa (uC) and corresponding
enthalpies (J g21) of optically active TSANs. Cr: crystalline state;
Colr, Colr1, Colr2: rectangular columnar phase; Colh: hexagonal
columnar phase; I: isotropic liquid state
Table 2 The results of indexation of XRD profiles of chiral TSANs
TSANs
a
˚
Phase
(T/uC)
dobs
˚
/
dcal
˚
/
Lattice Miller
parameters/A indices hkl
˚
(D/A)
A
A
Phase sequence
Heating/Cooling
1 (27.6)
Colh (50)
26.66 26.66 a: 30.78
100
Compound (C3h : Cs)
1 (1 : 2.3)
4.8
3.46
c: 3.46
001
100
Colh 117.2 [2.9] I
2a (31.6) Colh (144) 24.79 24.79 a: 28.63
4.7
I 113.4 [1.2]b Colh
2a (1 : 2.5)
2b (1 : 1.3)
2c (1 : 1.1)
a
Cr 64.3 [5.9] Colr 136c Colh 160.8 [4.8] I
I 158.7 [4.9] Colh 133.5c Colr 221.6 [1.6] Cr
Colr 133.3c Colh 144.4 [4.1] I
I 142.3 [4.4] Colh 129.1b,c Colr
Colr2 53.9 [2.1] Colr1 108.2 [2.8] I
I 102.7 [3.0] Colr1 44.1 [2.9]b Colr2
Colr (30)
27.5
27.5
a: 44.5
110
200
220
22.25 22.25 b: 34.97
13.73 13.75
4.6
2b (31.7) Colh (136) 24.87 24.87 a: 28.72
4.8
100
Colr (50)
27.64 27.62 a: 45.86
22.93 22.93 b: 34.6
13.78 13.81
9.3
4.7
110
200
220
330
Peak temperatures in the DSC thermograms obtained during the
first heating and cooling cycles at 5 uC min21
not crystallize up to 260 uC as evidenced by DSC. The phase
transition observed with a POM was too weak to be detected by
DSC.
.
The mesophase did
c
b
9.21
2c (29.2) Colr1 (90)
25.82 25.82 a: 46.62
23.31 23.31 b: 31.04
110
200
4.9
3.65
c: 3.65
3.65
001
110
020
030
320
230
complementary studies indicate that TSANs 1, 2b and 2c
bearing multiple chiral tails, stabilize hexagonal Col (Colh) or
rectangular Col (Colr) phases over a wide thermal range well
below and above room temperature (RT). Of special sig-
nificance, 2c shows a transition from one rectangular (Colr1)
phase to another (Colr2) which must originate from the
increase in the intercolumnar distance but a closer packing of
the discs within the same column (Fig. 3, Table 2). Compound
2a, with one chiral and two achiral tails, which is crystalline at
Colr2 (30)
28.34 28.34 a: 43.37
18.72 18.72 b: 37.44
12.29 12.48 c: 3.42
11.44 11.44
10.69 10.82
4.7
3.42
3.42
001
a
The diameter (D) of the disk (estimated from Chem3D Ultra 9.0
molecular model software).
RT, stabilizes Colr and Colh phases, among which Colr phase
supercools well below ambient temperature.
Circular dichroism studies
In view of the fact that TSANs
1 and 2a–c possess
chromophores and molecular chirality, their fluid columnar
as well as isotropic states were probed for electronic
absorption and chiroptical properties. UV-Vis spectroscopy
was used to measure the wavelength and intensity of ordinary
light absorption by the chromophores of the constituent
TSANs of the two states. On the other hand, the circular
dichroism (CD) spectroscopic technique was employed to
know if the molecular chirality has facilitated molecular
organization in a helical fashion in the LC and isotropic
phases. The substances held between two quartz plates were
heated to their isotropic phase and cooled slowly. To have a
uniform sample the plates were mechanical sheared repeatedly
while maintaining the material in the isotropic state and
subsequently cooled slowly. Owing to the small thickness of
the sample required to keep the sample within the upper limit
of measurement, no prefabricated cells were used and thus the
thickness was not fixed. Hence, the optical (absorption and
CD) experiments performed using such cells are rather
qualitative.
As shown in the lower panels of Fig. 4(a–d), irrespective of
the difference in the molecular structures and the temperatures
at which measurements were carried out, UV-Vis spectra of all
the TSANs showed two absorption maxima centered around
335 nm and 416 nm due to p–p* and n–p* transitions,
Fig. 2 Photomicrographs of the textures obtained for (a) Colh phase
of 2a at 135 uC; (b) Colr phase of 2a at 129 uC; (c) Colh phase of 2b at
135 uC; (d) Colr phase of 2b at 123 uC; (e) Colr1 phase of 2c at 101 uC;
(f) Colr2 phase of 2c at 42 uC.
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occurrence of CD bands in the mesophase corresponds to
chromophores that are organized into
a specific chiral
orientation. That is, CD activity originates from intermole-
cular interactions between two or more electronic transition
dipoles arranged in a helical order.
On the other hand, the CD spectrum of the isotropic phase
of TSAN 1, unlike others, showed an intense negative signal
implying the existence of helical aggregates made of chromo-
phores (TSANs) due to the favorable stacking effects.11a
Obviously, CD activity was also observed in the Col phase.
Thus, the molecular chirality of 1 is well expressed in the
isotropic state and LC phase. It can bee seen that there are
certain similarities in the CD spectral patterns10 of Col phases
of 1 [Fig. 4a] and 2a [Fig. 4b], and 2b [Fig. 4c] and 2c [Fig. 4d].
Among these, the spectral pattern of 2a is much simpler
exhibiting a strong negative Cotton effect (lmax = 496 nm),
where the CD signal increases on lowering the temperature. In
the case of 1, the CD spectra of lower temperatures (25–60 uC)
consisted of a positive first Cotton effect (lmax = 492 nm), a
negative second Cotton effect (475 nm) and a positive third
Cotton effect (371 nm) [see blue trace in Fig. 4(a) and ref. 10].
At higher temperatures, the first positive band disappears. In
contrast, the spectra of 2b obtained in the temperature interval
of 20 uC to 120 uC showed a first negative (481 nm) followed
by two positive (443 and 358 nm) Cotton effects. At higher
temperatures (above 120 uC), the first negative Cotton effect
almost merges with the base line and concomitantly the
intensity of the two positive bands decreases. Whereas, for
TSAN 2c the higher temperature (70 uC to 100 uC) CD spectra
showed a negative band (491 nm), besides a positive (464 nm)
and two negative (381 nm, 310 nm) Cotton effects. The
intensity of all the CD signals subsides on lowering the
temperature, which is opposite to the behavior of 2b. These
results suggest the possibility of different chiral dispositions of
the Col phase, which seem to be dependant on the
temperature. Importantly, the bisignate nature of the CD
bands indicates closer packing of TSANs within the column.
Importantly, the manifestation of chirality seems to be
independent of the number of chiral tails in the peripheral
region of the TSANs.
Fig. 3 (a) DSC thermograms obtained during the first heating (red
trace)–cooling (blue trace) cycles for TSAN 2c; (b) and (c) show the
X-ray 1D profiles obtained for the Colr1 (at 90 uC) and Colr2 (at 30 uC)
phases of 2c.
respectively. The CD spectra were recorded over the entire
thermal range of Col phases10 at 10 uC temperature intervals.
However, for comparison, the CD spectra at three tempera-
tures, for which UV-Vis data are also available, are shown in
the top panels of Fig. 4(a–d). It is important to mention here
that the origin of these CD peaks in the mesophase of all the
TSANs was authenticated by recording the spectra at three
different temperatures by rotating (in-plane) the sample cells
through 90u; the spectra were found to be nearly identical to
those obtained for the original orientation of the samples. This
clearly indicated that the signal is not arising from the possible
linear dichroism (LD) of the sample.3d In fact, the instrument
used is capable of detecting the LD signal directly; in the
measurements performed no LD signal was found within the
instrument limits.10 Notably, all the Col structures displayed
intense peaks in the CD spectra signifying large chiral
correlations in the molecular packing.3 For compounds 2a–c
the optical (CD) activity was not observed in the isotropic state
[see red traces of Fig. 4(b–d)] indicating that the chromophores
are not influenced by the molecular chirality. Thus, the
Given the fact that chiral Col phases can be ferroelectric,
electrical switching studies were performed on all of the above-
mentioned mesophases. For this purpose, samples contained
between two indium tin oxide (ITO) coated glass plates were
cooled slowly from the isotropic phase. After transition to the
Col phase, a low frequency AC triangular wave electric field
was applied and increased gradually. Surprisingly, even with
quite large voltages, no switching was observed. Therefore, it
may be assumed that the columns are upright. In fact, XRD
data support the view that the molecular director is along the
columnar axis. As is well known various helical structures have
been suggested for the columnar phases in the literature.3 In
particular, two different helices are reported3e,f to exist along
the column axis if the discotic core (with peripheral chiral tails)
features an intrinsic shape that favors packing with a preferred
rotation: (i) the spiral staircase-like helix where the flat cores
are displaced perpendicular to the Col axis and (ii) the helix
where the constituent discs are tilted along the columnar axis
and rotate around the columnar axis. Thus, all the optically
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Fig. 4 CD (top panels) and UV-Vis spectra (bottom panels) obtained in the isotropic and columnar phases for chiral TSANs 1 (a), 2a (b), 2b (c)
and 2c (d).
active TSANs form the chiral Col phases where either the
position of the molecules or the orientation of the molecular
director may spiral within the column. In order to postulate
which of these chiral organizations is favored in TSANs, the
lattice parameter a and diameter of the disks (estimated from
Chem3D Ultra 9) may be taken into account. As can be seen in
Table 2, the lattice parameter is either comparable to or higher
than the estimated diameter of the molecules. Therefore, we
suggest that the helix could be of the form shown in Fig. 5,
where the constituent discs are slightly tilted and rotate around
the columnar axis.3d,g
TSAN 2c were obtained in both solution and the mesophase
states. Indeed, the THF solution absorption spectra were
Absorbance and fluorescence studies
Tris(N-salicylideneaniline)s, owing to the presence of multiple
chromophores and possible p-conjugation between the per-
ipheral region and the central core, are expected to exhibit
promising photophysical properties, which is evident from
earlier reports.8b,9 Thus, fluorescence as well as UV-Vis
absorption spectra (for comparison) for a representative
Fig. 5 Schematic representation of the self-assembly of C3h and Cs
rotational isomers of TSANs into a helical Col phase where the
constituent discs are slightly tilted and rotate around the columnar
axis.
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recorded for all the compounds and their patterns looked
alike, as expected. For studying photophysical properies of the
mesophase, a thin film was employed. As shown in the upper
and lower panels of Fig. 6, the UV-Vis spectra (blue trace) of
chiral TSAN 2c in solution and the mesophase, respectively,
showed two absorption maxima centered around 333 nm and
413 nm. Upon irradiating the solution with one of the
absorption maxima viz., 413 nm, a fluorescence maximum
was observed at around 481 nm with a negligible shoulder
(orange traces in the upper panel of Fig. 6). The Stokes shift,
which is the difference between the spectral positions of the
band maxima of absorption and florescence arising from
the same electronic transition, was found to be 68 nm. The
occurrence of luminescence at a longer wavelength than the
absorption is in agreement with the general observations. As
shown in the inset, the green light is visually perceivable in the
emissive state. The lower panel of Fig. 6 shows the absorption
and the emission spectra of the fluid Col phase. As can be seen,
the Col structure of 2c shows two absorption bands analogous
to the solution state. The apparent small red shift in the two
absorption maxima of the Col structure is indicative of the
intimate overlap of the fluorophores in such states. When
excited at 413 nm, two broad bands with fluorescence maxima
at around 540 and 610 nm (see red trace of the lower panel of
Fig. 6) were observed.
thermally activated radiationless processes.12 Notably, when
compared with the emission behavior of the solution, the films
of neat Col phase exhibit a strong bathochromic (red) shift
that can be attributed to the strong co-facial proximity of C3h
and Cs isomers within the Col structure as well as the
formation of excimers/aggregates. In general, the excimers are
considered as molecular dimers or stoichiometric complexes
with associated excited electronic states, dissociative ground
states, and structureless emission spectra.13 Thus, it is apparent
that the TSANs realized in the present investigation are
capable of generating light in both solution and mesophase
states. This is especially remarkable in the latter situation
wherein the 2D order with short inter-core distances facilitates
motion of the charges with the concomitant freezing of the
ionic impurities14 with the preserved luminescence.
Nonetheless, detailed investigations involving the annealing
of films, film morphology, fluorescence efficiency, solvent
effects, etc. are required to further elucidate the photophysical
properties of these molecules quantitatively.
Conclusion
In summary, novel optically active TSANs are synthesized and
shown to self-assemble into room temperature helical fluid Col
phases. The preferential rotation of the constituent shape-
persistent C3h and Cs keto-enamine tautomers enables the
chirality present in the side-chains to be expressed into the
helical conformation of the columns that is reflected by Cotton
effects in the CD. Thus, another remarkable feature of TSANs
is reported.
Further, the fluorescence spectra of the thin film at different
temperatures (above room temperature) for the above-men-
tioned excitation wavelength were measured and are shown in
the right hand side of lower panel of Fig. 6. As can be seen, the
intensity of the emission peak decreases progressively with
the increase in the temperature, that can be attributed to the
breaking of larger columnar stacks into smaller ones and
Experimental
General methods
Chemicals were obtained from Fluka, Aldrich, Lancaster and
local chemical companies, and were used without any
purification; solvents were dried following the standard
procedures. Chromatography was performed using either
silica gel (60–120, 100–200 and 230–400 mesh) or neutral
aluminiun oxide. For thin layer chromatography, aluminium
sheets pre-coated with silica gel (Merck, Kieselgel 60, F254
)
were employed. IR spectra were recorded on a Perkin-Elmer
Spectrum 1000 FT-IR spectrometer. The spectral positions are
given in wave number (cm21) units. NMR spectra were
recorded using either a Bruker DRX-500 (500 MHz) or Bruker
AMX-400 (400 MHz) spectrometer or Bruker Aveance series
1
DPX-200 (200 MHz) spectrometer. For H NMR spectra, the
chemical shifts are reported in ppm relative to TMS as an
internal standard. Coupling constants are in Hz.
Microanalyses were performed using a Euro EA3000 elemental
analyzer. Mass spectra were determined on a JEOL JMS-600H
spectrometer in FAB+ mode using 3-nitrobenzyl alcohol as a
liquid matrix. The molecular length is calculated from the
energy minimized structure deduced from the CS Chem 3D
version 9 programme. X-Ray diffraction studies were carried
out on powder samples in Lindemann capillaries with Cu Ka
radiation using an Image Plate Detector (MAC Science,
Japan) equipped with a double mirror focusing optics. The
compounds were investigated for their liquid crystalline
Fig. 6 Absorption and emission spectra in THF solution (upper
panel) and in the thin film state at different temperature intervals
(lower panel) of 2c. The inset in the upper panel shows emission in
THF solution on irradiation with 365 nm light.
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behavior employing an optical polarizing microscope (Leitz
DMRXP or Leica DMLP) equipped with a programmable hot
stage (Mettler FP90) and differential scanning calorimeter
(Perkin Elmer DSC7); these were also used to determine the
melting points of non-mesomorphic compounds. Circular
dichroism spectra were recorded using JASCO J-810 spectro-
polarimeter. The specific rotation of the target molecules was
measured using the same instrument. Emission spectra in
solution were recorded with a luminescence spectrometer
(Perkin Elmer LS 50B), while emission spectra as a function
of temperature were obtained by using a Fluorolog spectro-
fluorimeter (SPEX) in conjunction with a programmable
hotstage (Linkam).
LCNH), 12.87 (d, J = 12.9 Hz, LCNH), 8.66–8.7 (m, 3H,
LCHN), 6.48 (s, 6H, Ar), 3.96–4.0 (m, 18H, 9 6 OCH2), 0.87–
1.92 (m, 159H, 12 6 CH, 42 6 CH2, 21 6 CH3); MS(FAB+):
m/z for
C111H191N3O12 (M+2), calculated: 1758.4,
found:1758.2; anal. calcd for C111H189N3O12: C, 75.85; H,
10.84; N, 2.39. Found: C, 75.6; H, 11; N, 2.4.
2c. Yield: 72%; Rf = 0.70 (30% EtOAc–hexanes); a brown
gummy solid, [a]D = 28.5 (c = 1; CHCl3, temperature =
22.6 uC); UV-Vis: lmax = 413.08 nm, e =6.6967 6 104
L
mol21cm21; IR (KBr pellet): nmax in cm21 2953, 2926, 2869,
1618, 1585, 1550, 1509, 1459, 1383, 1293, 1233, 1308, 1118,
1
1050, 989, 835, 810, 754, 665, 624 and 436; H NMR (CDCl3,
400 MHz): d 13.48 (d, J = 13.0 Hz, LCNH), 13.29 (d, J =
13.6 Hz, LCNH), 13.01 (d, J = 14.0 Hz LCNH), 12.87 (d, J =
14.4 Hz, LCNH), 8.63–8.8 (m, 3H, LCHN), 6.49 (s, 6H, Ar),
3.92–4.07 (m, 18H, 9 6 OCH2), 0.92–1.92 (m, 171H, 18 6
Materials
General procedure for the synthesis of 1, 2a, 2b and 2c. A
mixture of triformylphloroglucinol (0.4 mmol, 1 equiv.) and
alkoxy aniline (1.36 mmol, 3.4 equiv.) in absolute ethanol
(15 ml) was heated to reflux under inert an atmosphere for 2 h
with vigorous stirring. The solvent was removed in vaccuo
from the reaction mixture to give the target molecules as a
gummy mass. The crude products were purified by repeatedly
dissolving them in dichloromethane and re-precipitating
the LC mass by the addition of a large excess of ethanol at
10–15 uC.
CH, 36
6 CH2, 27 6 CH3); MS (FAB+): m/z for
C117H201N3O12, calculated: 1840.5, found: 1840.3; anal. calcd
for C117H201N3O12: C, 76.3; H, 10.99; N, 2.28. Found: C, 76.4;
H, 10.7; N, 2.5.
Acknowledgements
Prof. Suresh Das, Dr Shibu Abraham, RRL, Trivendrum and
Prof. Uday Maitra, Mr. Nonappa, IISc, Bangalore are
greatfully acknowledged for the luminescence experiments.
Mrs Sandya Hombal is acknowledged for recording the CD
spectra.
1. Yield = 60%; Rf = 0.73 (30% EtOAc–hexanes); a brown
gummy mass; [a]D = 217.7 (c = 1; CHCl3, temperature =
22.6 uC); UV-Vis: lmax = 416.84 nm, e = 9.3157 6 104
L
mol21cm21; IR (KBr pellet): nmax in cm21 3423, 2953, 2925,
References
2868, 1621, 1594, 1544, 1519, 1460, 1383, 1306, 1262, 1136,
1
1018, 828 and 756; H NMR (CDCl3, 400 MHz): d 13.48 (d,
1 (a) J. J. L. M. Cornelissen, A. E. Rowan, R. J. M. Nolte and N. A.
J. M. Sommerdijk, Chem. Rev., 2001, 101, 4039; (b) L. Brunsveld,
B. J. B. Folmer, E. W. Meijer and R. P. Sijbesma, Chem. Rev.,
2001, 101, 4071; (c) T. Kato, N. Mizoshita and K. Kishimoto,
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2 (a) J. H. Fuhrhop and W. Helfrich, Chem. Rev., 1993, 93, 1565; (b)
J. M. Schnur, Science, 1993, 262, 1669; (c) M. S. Spector, J.
V. Selinger and J. M. Schnur, Materials-Chirality, ed. M. M.
Green, R. J. M. Nolte and E. W. Meijer, 2003, John Wiley and
Sons, Inc., New Jersey, Vol. 24, Ch. 5: Chiral Molecular Self-
Assembly, p. 281.
J = 13.1 Hz, LCNH), 13.37 (d, J = 14.4 Hz, LCNH), 13.01 (d,
J = 11.6 Hz, LCNH), 12.96 (d, J = 16.4 Hz, LCNH), 8.63–8.77
(m, 3H, LCHN), 6.82–6.9 (m, 9H, Ar), 4.29 (m, 12H, 6 6
OCH2), 0.86–1.88 (m, 114H, 12 6 CH, 246 CH2, 18 6 CH3);
MS (FAB+): m/z for C87H141N3O9, calculated: 1372.1, found:
1372.7; anal. calcd for C87H141N3O9: C, 76.1; H, 10.35; N,
3.06. Found: C, 76.3; H, 10.2; N, 3.2.
2a. Yield: 72%; Rf = 0.72 (30% EtOAc–hexanes); a brown
solid, [a]D = 23.6 (c = 1; CHCl3, temperature = 22.6 uC); UV-
Vis: lmax = 415.50 nm, e =7.4893 6 104 L mol21cm21; IR
(KBr pellet): nmax in cm21 2925, 2855, 1619, 1587, 1509, 1459,
1384, 1296, 1252, 1118, 1040, 991, 835, 758 and 624; 1H NMR
(CDCl3, 400 MHz): d 13.46 (d, J = 13.6 Hz, LCNH),13.27 (d,
J = 13.2 Hz, LCNH), 12.99 (d, J = 13.0 Hz, LCNH), 12.87 (d,
J = 13.3 Hz, LCNH), 8.62–8.77 (m, 3H, LCHN), 6.48 (s, 6H,
Ar), 3.91–4.06 (m, 18H, 9 6 OCH2), 0.86–1.88 (m, 147H, 6 6
3 (a) C. F. Van Nostrum, A. W. Bosman, G. H. Gelinck, S. J. Picken,
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P. M. Kentgens, M. A. C. Devillers, A. Meijerink, S. J. Picken,
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CH, 48
6 CH2, 15 6 CH3); MS (FAB+): m/z for
C105H178N3O12 (M+1), calculated: 1673.3, found: 1673.2; anal.
calcd for C105H177N3O12: C, 75.36; H, 10.66; N, 2.51. Found:
C, 75.7; H, 10.7; N, 2.6.
2b. Yield: 74%; Rf = 0.71 (30% EtOAc–hexanes); a brown
gummy solid, [a]D = 29.7 (c = 1; CHCl3, temperature =
22.6 uC); UV-Vis: lmax = 416.74 nm, e =7.3582 6 104
L
mol21cm21; IR (KBr pellet): nmax in cm21 2956, 2362, 1620,
1588, 1509, 1459, 1384, 1295, 1232, 1117, 1050, 991, 834, 773,
and 623; 1H NMR (CDCl3, 400 MHz): d 13.5 (d, J = 12.0 Hz,
LCNH),13.3 (d, J = 12.0 Hz, LCNH), 13.0 (d, J = 13.3 Hz
4528 | J. Mater. Chem., 2007, 17, 4521–4529
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