Synthesis and phase transitional behavior of dimer-like optically active liquid crystals

Article abstract of DOI:10.1021/jp206224k

The results of our detailed studies pertaining to a relatively new class of low molar mass mesogens are presented. Four homologous series of optically active dimer-like mesogens, comprising cholesterol as the conventional pro-mesogenic core covalently tet

Full text of DOI:10.1021/jp206224k

ARTICLE
pubs.acs.org/JPCB
Synthesis and Phase Transitional Behavior of Dimer-like Optically
Active Liquid Crystals
Govindaswamy Shanker† and Channabasaveshwar V. Yelamaggad*
Centre for Soft Matter Research, Prof. U. R. Rao Road, P. B. No. 1329, Jalahalli, Bangalore 560 013, India
S Supporting Information
b
ABSTRACT: The results of our detailed studies pertaining to a relatively
new class of low molar mass mesogens are presented. Four homologous
series of optically active dimer-like mesogens, comprising cholesterol as the
conventional pro-mesogenic core covalently tethered to a nonmesogenic
salicylaldimine segment through a flexible spacer of varying length and
parity, have been synthesized and evaluated for their thermal, electrical
switching, electrochemical, and gelation properties. The thermal behavior,
being the prime focus of this study, has been characterized by optical,
calorimetric, X-ray diffraction, and electrical switching studies. In each
series, the length of the even/odd-parity spacer is held constant, while the length of the terminal N-alkyl tail attached to
nonmesogenic salicylaldimine core has been varied to gain insight into the fundamental correlation between structure and phase
transitional properties. These compounds, with a few exceptions, exhibit liquid crystal phase(s); the identified phases are chiral
nematic (N*), twist grain boundary (TGB), chiral smectic A (SmA), and chiral smectic C (SmC*) phases. The selective reflection
property of the N* phase and ferroelectric behavior of the SmC* phase have been ascertained for some selected members. In general,
the phase behavior shows a dependence on the length and parity of the central spacer as well as the length of the terminal tail. The
oddÀeven effect has been prominently found in the clearing temperatures; the even-parity dimer-like compounds belonging to
three different series exhibit higher values when compared to members of an odd-parity series. Thus, our study demonstrates that
these new class of low molar mass materials behave analogous to liquid crystal dimers. Electrochemical behavior and gelation ability
have been demonstrated for some selected materials.
even-parity spacer exhibit higher clearing temperature than that
of odd-parity members.4,5
I. INTRODUCTION
Liquid crystal (LC) phases, owing to their special structural
features and unique properties, have been attracting a great deal
of attention in both fields of basic research and practical
applications.1 Specifically, LC phases formed by rod-like and
disk-shaped organic molecules, called conventional mesogens,
have been studied intensively, and, as a result, they have been
used in different technologies. However, over the years, several
new mesogenic motifs differing in their shape-anisotropy from
conventional LCs have been designed, prepared, and shown to
exhibit mesomorphism with improved characteristics and desired
functionality.2À6 LC dimers, which are made by covalently
tethering two identical (symmetrical) or nonidentical (non-
symmetrical) conventional mesogens through a flexible spacer
of varying length and parity, are among the large variety of
nonconventional LCs reported hitherto.4À6 These compounds,
which were initially regarded as model systems forLC polymers,5aÀc
are of significant interest in their own right as they display
different thermal behavior than conventional LCs. One of the
most striking characteristics of this class of low molar mass LCs is
the critical dependence of their transitional behavior on the
length and parity (oddÀeven) of the flexible spacer.4 The most
immediately apparent effect is the prominent alternations in
clearing temperature and associated entropy change upon
varying the parity of the spacer; for example, the dimers with an
In fact, another class of low molar mass mesogens made by
covalently joining a typical mesogenic (two-ring) core with a
bulky nonmesogenic segment, such as one-ring aliphatic/
aromatic cores, via a flexible spacer is also known since the early
1970s.4,6À20 Notably, their thermal behavior closely resembles
that observed in LC dimers. For example, the spacer parity
drastically influences their transitional property; in particular,
clearing temperatures exhibit a dramatic oddÀeven effect as the
parity of the spacer is varied. They have been regarded as the
structural intermediates between classical low molar mass meso-
gens (monomers) and LC dimers; thus, they can be referred to as
“dimer-like” mesogens. Hitherto, several series of both achiral
and chiral dimer-like mesogens have been reported, and the
studies indicate their thermal behavior dependence on the
chemical nature and/or size of the nonmesogenic segment
attached to the conventional mesogen via a flexible spacer. The
chiral dimer-like compounds, formed by tethering cholesterol
and benzene core (with or without substitution) via a spacer,
especially attracted our attention as they display monotropic
Received: July 1, 2011
Revised:
August 5, 2011
Published: August 10, 2011
r
2011 American Chemical Society
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Scheme 1
Reagents and conditions(i) 2,4-Dihydroxybenzaldehyde, anhyd. K2CO3, butanone, reflux, 12 h (78À85%)(ii) n-Alkylamines, dry EtOH, AcOH
(catalytic), reflux, 1 h (68À80%)
Table 1. Phase Transition Temperatures (°C)c and Associated Enthalpies [JgÀ1]d of Dimer-like Mesogens Belonging to the DM-
3,R Seriesa
Phase sequence
Compound
Heating
Cr 98.3 [32.1] N* 124.1 [2.4] I
Cooling
DMÀ3,6
DMÀ3,7
DMÀ3,8
DMÀ3,9
DMÀ3,10b
DMÀ3,11
DMÀ3,12
DMÀ3,16
I 123.6 [2.4] N* 97 TGB 95.5 [0.8]e SmA 57.9 [7.1] Cr
I 111.6 [3.5] N* 88.2e TGB 86.5f SmA 62.4e SmC* 40.5 Crg
I 119.9 [2.4] N* 106.3 TGB 104.6 [1]e SmA 71.2f SmC* 40.8f Cr
I 120.4 [2.8] N* 111.2 TGB 109.5 [0.8]e SmA 87.9e SmC*, f 60.6 [11.9] Cr
I 118.4 [2.9] N* 112.8 TGB 111.3 [1.4]e SmA 91.1f SmC* 58.4 [10] Cr
I 118.3 [2.6] N* 113.8 TGB 111.6[1.3]e SmA 95.4f SmC* 59.2 Crg
I 115.8 [3] N* 114.3 [1.1] SmA 89.2f SmC* 57.9 Crg
Cr 104.1 [56.7] N* 114 [3.6] I
Cr 87.4 [46.8] SmA 105.1 TGB 106.9 [0.9]e N* 120.5 [2.5] I
Cr 94.3 [47.3] SmA 110.2 TGB 111.6 [0.9]e N* 121.1 [3.1] I
Cr 99.1 [46.9] SmA 111.9 TGB 113.3 [1.6]e N* 119 [3.1] I
Cr 96.5 [39.3] SmA 112.8 TGB 114.4 [2.6]e N* 118.8 [2.8] I
Cr 91.6 [41.2] SmA 115 [1.6] N* 116.5 [3.1] I
Cr 74.3 [18.9] SmA 109.1 [4] I
I 106.2 [3.9] SmA 71.4f SmC* 45.4 Crg
a Cr = Crystal; SmC* = Chiral smectic C phase; SmA = Smectic A phase; TGB = Twist grain boundary phase with either SmA or SmC blocks (SmC =
Smectic C phase); TGBC* = Twist grain boundary phase with SmC* blocks; N* = Chiral nematic phase; I = Isotropic phase. b Known compound but
included for the sake of comparison and completeness (ref 21). c Phase transition temperatures were determined by both POM and DSC studies: peak
temperatures in the DSC thermograms obtained during the first heating and cooling cycles (at 5 °C minÀ1) coupled with optically measured
temperatures are given. d Transition enthalpies were obtained from the DSC thermograms. e Although N*ÀTGB/TGBÀN* and TGBÀSmA/
SmAÀTGB phase transitions were observed in POM, they were not resolved in DSC trace; hence the enthalpy value represents the combined enthalpy
f
for N*ÀTGB/TGBÀN* and TGBÀSmA/SmAÀTGB transitions. Phase transition was observed under POM; too weak to be detected by DSC.
g Crystallization was observed under POM only.
mesophase/s with a strong to moderate oddÀeven effect in the
clearing temperatures. This is notable given the fact that they
possess pro-mesogenic cholesterol and moreover, the analogous
LC dimers, as demonstrated by the pioneering work of Jin
and co-workers6 as well as other groups,5d exhibit fascinating
phase transitional behavior, with the mesophase being generally
enantiotropic in nature. However, we recently observed that
four dimer-like mesogens formed by covalently linking the
pro-mesogenic cholesterol entity with the N-(n-decyl)salicylaldimine
core through either an even-parity or an odd-parity spacer exhibit
enantiotropic mesomorphism convincingly;21 this behavior
can be attributed primarily to the presence of a N-(n-decyl)-
salicylaldimine core that enhances the shape-anisotropy, reduces
the melting point and maximizes the probability of mesophase
formation. These observations inspired us to prepare the lower
and higher homologues of the four mesogens mentioned above.
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Figure 1. Photomicrographs of the textures observed for different mesophase of DM-3,8: (a) planar textures of the N* phase (110 °C), (b) greyish
planar texture of the TGB phase (105.5 °C), (c) focal-conic texture observed for the homogeneously aligned SmA phase (80 °C), (d) focal conics with
striations seen for the homogeneously aligned SmC* phase (68 °C), and (e) filamentary texture seen for the homeotropically aligned TGB phase
(105 °C).
were reacted with 2,4-dihydroxybenzaldehyde in the presence of
a mild base in butanone to obtain cholesteryl ω-(3-hydroxy-4-
formylphenoxy)alkanoates (2aÀd). The acid catalyzed conden-
sation of the salicyladehydes 2aÀd with n-alkylamines in reflux-
ing ethanol gave the target molecules in reasonably good
yields. The detailed synthetic procedures and molecular struc-
tural characterization data of all the compounds are given in
the Supporting Information under the heading Experimental
Section.
II.b. Evaluation of Thermal Behavior. The phase transitional
behavior of the four series of dimer-like mesogens was estab-
lished with the help of a polarizing optical microscope (POM),
differential scanning calorimeter (DSC), and X-ray diffraction
(XRD). Clean glass slides were used to examine the LC
(the birefringence and fluidity) property of the samples. For con-
firmation of the mesophase assignment, the glass substrates
Figure 2. DSC thermograms of the first and second heating/cooling
cycles recorded at a rate of 5 °C/min for the compound DM-3,8.
treated for either planar or homeotropic alignments were em-
Herein, we report the synthesis and characterization of
ployed. Peak temperatures in the DSC traces due to phase
four series of dimer-like mesogens where the pro-mesogenic
transitions of the samples were found to be in agreement with
cholesterol segment covalently combines with N-(n-alkyl)salicy-
laldimine through a flexible spacer. These four series of com-
pounds, namely, cholesteryl ω-(3-hydroxy-4-(n-alkyliminomethyl)
phenoxy)alkanoates, primarily differ in the number of carbon atoms
those of the optical experiments. The transition temperatures
obtained from calorimetric measurements of the first heating and
cooling cycles at a rate of 5 °C/min are presented; in the cases
where the signatures are not observed in DSC traces, the
of the spacer; they comprise 4-oxybutanoyloxy (C4; even-parity),
transition temperatures have been taken from the microscopic
5-oxypentanoyloxy (C5; odd-parity), 6-oxyhexanoyloxy (C6; even-
observations. The details of these studies with analysis of the
parity), and 8-oxyocatnoyloxy (C8; even-parity) spacers. In each
results are presented in the following sections.
II.b.1. Microscopic and Calorimetric Studies. DM-3,R Series.
series, the number of carbon atoms in the terminal tail attached to
the salicylideneamine core have been varied from C6 (n-hexyl) to
C12 (n-dodecyl); this includes the C16 (n-hexadecyl) tail also. For
convenience, we denote these four series (I to IV) of compounds
by mnemonics DM-3,R, DM-4,R, DM-5,R, and DM-7,R, where
DM stands for Dimer-like Mesogens, and R indicates the length
of the terminal n-alkyl tail.
The phase sequence, transition temperatures and the associated
enthalpies of the DM-3,R series of compounds, in which the two
rigid segments are separated by an 4-oxybutanoyloxy (C4; even-
parity) spacer, are summarized in Table 1. As can be seen, all the
compounds exhibit enantiotropic mesomorphism. Particularly,
the lower homologues DM-3,6 and DM-3,7 show enantiotropic
chiral nematic (N*) phase, the medium members DM-3,8, DM-
3,9, DM-3,11, and DM-3,12, show enantiotropic N*, twist grain
boundary (TGB) and smectic A (SmA) phases, and the higher
homologue DM-3,16 shows enantiotropic SmA phase only. This
is in agreement with the fact that in many homologous series of
calamitic LCs, the lower members are nematic, the medium
members show nematic and smectic behavior, and the higher
members are smectic only.1b,d However, these compounds
display a rich transitional behavior during the cooling mode.
II. RESULTS AND DISCUSSION
IIa. Synthesis and Molecular Structural Characterization.
The new dimer-like compounds belonging to series (IÀIV) DM-
3,R, DM-4,R, DM-5,R, and DM-7,R were prepared by synthetic
steps identical to those reported earlier as shown in Scheme 1.21
Cholesteryl ω-bromoalkanoates 1aÀd, readily prepared by
known methods starting from commercially available cholesterol,22
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Table 2. Phase Transition Temperatures (°C)c and Asso-
ciated Enthalpies [JgÀ1]d of Chiral Dimer-Like Compounds
Belonging to the DMÀ4,R Seriesa
phase sequence
compound
heating
cooling
I 75.6 [32.3] Cr
Figure 3. Energy minimized structures of dimer-like compound DM-
4,7. Notice that the two cores are inclined with each other.
DM-4,6
DM-4,7
DM-4,8
DM-4,9
Cr 99.5 [32.4] I
Cr 100.4 [50.1] I
Cr 100.7 [42.9] I
Cr 94.8 [26.6] Cr1
97.8 [11.4] I
I 76.3 [37.2] Cr
I 72.4 [32.7] Cr
reproducible; as a representative case, the DSC traces obtained
during the first and second heating/cooling scans are shown in
Figure 2.
I 75.3 [2.9] SmA 72.5 [24.8] Cr
DM-4,10b Cr 90 [39.1] I
DM-4,11 Cr 78 [38.1] Cr1
81.3 [5] I
I 76.9 [7.7] N* 64.1 [18.5] Cr
I 80.4 [5.1] SmA 68.6e
SmC* 62f Cr
I 78.7 [9.3] SmA 72.4e
SmC* 63.5f Cr
DM-4,R Series. Table 2 summarizes the thermal behavior of
the DM-4,R series of dimer-like compounds. In this series,
the cholesterol moiety and N-(n-alkyl)salicylaldimine core are
tethered together through an 5-oxypentanoyloxy (odd-parity)
spacer. It is immediately apparent from Table 2 that these
compounds do not favor the mesomorphism convincingly. First
three homologues DM-4,6, DM-4,7, and DM-4,8 are nonme-
somorphic. The loss of mesomorphism in these compounds can
be suitably interpreted in terms of pronounced bent molecular
conformation (Figure 3) and thus, reduced shape anisotropy.
The middle members DM-4,9, DM-4,11, and DM-4,12 exhibit
monotropic LC behavior. More clearly, compound DM-4,9
displays the monotropic SmA phase; the SmA phase was
identified based on the microscopic observation of a typical
focal-conic texture using slides treated for planar orientation, and
a dark field of view with slides treated for homeotropic or-
ientation. The next two dimers, viz., DM-4,11 and DM-4,12,
stabilize metastable SmA and SmC* phases. These compounds
placed between two glass slides treated for homeotropic or-
ientation and cooled slowly from the isotropic liquid showed a
pseudoisotropic pattern typical of the SmA phase. On cooling the
sample further, the SmC* phase appears with a cloudy texture. As
expected, in slides treated for planar orientation, the SmA phase
showed a focal-conic texture, whereas for the SmC* phase
dechiralization lines appear on the top of the focal-conic texture.
Notably, the higher member DM-4,16 exhibits enatiotropic
SmC* phase. Subjecting the sample to planar boundary condi-
tion, the SmC* phase from the isotropic phase appears as
nucleating batonnets superimposed by the equidistant line
pattern as shown in Figure 4a; on further cooling, the pattern
fills the field of view. The DSC thermograms of the two heating
and cooling cycles obtained for this compound are shown in
Figure 4b; these traces corroborate the optical observation and
also indicate the thermal stability of the compound.
DM-5,R Series. Table 3 gives the phase sequence, transition
temperatures, and associated enthalpies for the compounds of
the DM-5,R series wherein the cholesterol and N-(n-alkyl)-
salicylaldimine moieties are connected through a 6-oxyhexanoyloxy
(even-parity) spacer. Unlike the previous series of compounds,
all the homologues exhibit enantiotropic mesomorphism. The
first member DM-5,6 exhibits SmC*, SmA, TGB, and N*phases;
of these, the SmC* phase is metastable. The second homologue
DM-5,7 stabilizes an identical phase sequence where only the N*
phase is enantiotropic in nature. The middle members DM-
5,8ÀDM-5,12 exhibit enantiotropic SmA, TGB, and N* meso-
phases in addition to a monotropic SmC* phase. The higher
member DM-5,16 displays an enantiotropic SmA phase in
addition to a metastable SmC* phase. Needless to say, all the
mesophases were identified based on the observation of their
DM-4,12 Cr 65.5 [10.2] Cr1
78.2 [48.9] I
DM-4,16 Cr 75.2 [57.6] Cr1 81.5
I 82.7 [10.1] SmC*
[4.1] SmC* 83.7 [10.1] I
58.1 [29.2] Cr
a Cr = Cr1 = crystal. b Known compound but included for the sake of
comparison and completeness (ref 21). c Phase transition temperatures
were determined by both POM and DSC studies: peak temperatures in
the DSC thermograms obtained during the first heating and cooling
cycles (at 5 °C minÀ1) coupled with optically measured temperatures
are given. d Transition enthalpies were obtained from the DSC thermo-
grams. e Phase transition was observed under POM; too weak to be
detected by DSC. f Crystallization was observed under POM only.
For example, DM-3,6 stabilizes monotropic TGB and SmA phases,
while DM-3,7 exhibits a metastable SmC* phase additionally.
The middle and the higher members stabilize a metastable SmC*
phase. The occurrence of SmC*, SmA, TGB, and N* phases was
evidenced with the aid of polarizing microscope mainly.
A phase sequence, viz., N*ÀTGBÀSmAÀSmC* recognized
for the compound DM-3,8, as a representative case, is discussed
as follows. The sample was placed between a pair of ordinary
glass slides, and upon cooling from its isotropic phase, the
N* phase appears with a focal conic texture that upon being
subjected to mechanical stress yields a planar texture having oily
streaks as shown in Figure 1a. Upon cooling further from the
planar texture of the N* phase, a transition to the TGB phase
occurs where the oily streaks disappear to give a gray blurry
planar texture (Figure 1b). On lowering the temperature further,
the SmA phase appears with a characteristic focal-conic texture,
as shown in Figure 1c. Of course, in some regions of the slide, a
pseudoisotropic region typical of the SmA phase was observed.
When this phase was cooled further, the SmC* phase appears
with a focal-conic fan texture featuring dechiraliztaion lines
(Figure 1d). The appearance of an arced equidistant line pattern
superimposed on the fans indicates that the pitch of the helix of
the phase is on the order of several micrometers. In the home-
otropic regions, the SmC* phase showed a cloudy greyish
pattern, as expected. The TGB phase shows a typical filamentary
textural pattern (Figure 1e) if it is obtained by heating the
pseudoisotropic pattern of the SmA phase. Thus, the usage of
ordinary slides made it possible to visualize the optical textural
patterns arising due to both planar and homeotropic alignment of
the molecules. Needless to mention, when slides treated for
planar or homeotropic alignments were used, these mesophases
showed the expected textural patterns. It is worth mentioning
here that the above-mentioned phase sequences are highly
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Figure 4. (a) Photomicrograph of the texture seen for the planar SmC* phase appearing just below the isotropic phase of compound DM-4,16. (b) DSC
traces of first and second heating/cooling cycles obtained for DM-4,16 at a rate of 5 °C minÀ1
.
Table 3. Phase Transition Temperatures (°C)b and Associated Enthalpies [JgÀ1]c of Chiral Dimer-like Mesogens of the DM-5,R Series
Phase sequence
Compound
Heating
Cooling
DM-5,6
DM-5,7
DM-5,8
DM-5,9
DM-5,10a
DM-5,11
DM-5,12
DM-5,16
Cr 83.4 [40.3] SmA 92.4 TGB 93.4 [0.8]d N* 116.6 [2.8] I
Cr 103.2 [68.4] N* 115.8 [3.5] I
I 116.2 [2.5] N* 92.9 TGB 90.8 [0.7]d SmA 44.4e SmC*, f
I 115.3 [3.2] N* 99 TGB 97.3 [1.1]d SmA 63.3e SmC*, f
I 111.7 [3.1] N* 101.2 TGB 95.2 [1.4]d SmA 66.9e SmC*, f
I 111.7 [3.8] N* 103.9 TGB 101.8 [1.5]d SmA 78e SmC*, f
I 109.3 [3.9] N* 104.6 TGB 102.9 [1.8]d SmA 81.1e SmC*, f
I 109.3 [4] N* 106.1 TGB 104.6 [2]d SmA 86.5e SmC* 29.8 Crg
I 107.5 [2.8] N* 106.6 [1.3] SmA 88.1e SmC* 60.8 Crg
I 105.4 [11.8] SmA 79e SmC* 51.8 Crg
Cr 83.9 [48.4] SmA 96.5 TGB 101.5 [1.5]d N* 112.2 [3.2] I
Cr 94.7 [55.5] SmA 102.1 TGB 104.3 [1.6]d N* 112.2 [4] I
Cr 97.5 [53.6] SmA 103.3 TGB 104.9 [2]d N* 109.8[3.9] I
Cr 95.8 [52.1] SmA 104.8 TGB 106.5 [2]d N* 110 [4.6] I
Cr 93.4 [51.4] SmA 107.2 [1.8] N* 108.1 [3.4] I
Cr 81.2 [39.4] SmA 106.6 [11.8] I
a Known compound but included for the sake of comparison and completeness (ref 21). b Phase transition temperatures were determined by both POM
and DSC studies: peak temperatures in the DSC thermograms obtained during the first heating and cooling cycles (at 5 °C minÀ1) coupled with optically
measured temperatures are given. c Transition enthalpies were obtained from the DSC thermograms. d Although N*ÀTGB/TGBÀN* and TGBÀSmA/
SmAÀTGB phase transitions were observed in POM, they were not resolved in the DSC trace. Hence the enthalpy value represents the combined
e
enthalpy for N*ÀTGB/TGBÀN* and TGBÀSmA/SmAÀTGB transitions. Phase transition was observed under POM; too weak to be detected
by DSC. f No crystallization was observed until the limitation of the DSC (À60 °C) instrument. g Crystallization was observed under POM only.
Table 4. Phase Transition Temperatures (°C)c and Associated Enthalpies [JgÀ1]d of Chiral Dimer-Like Mesogens Belonging to
the DM-7,R Seriesa
Phase sequence
Compounds
Heating
Cre 88.3 [32.6] N* 107.1 [3] I
Cooling
DM-7,6
DM-7,7
DM-7,8
DM-7,9
DM-7,10b
DM-7,11
DM-7,12
DM-7,16
I 106.3 [2.9] N* 72f TGB 65.3f SmAg
Cr 95.5 [62.1] N* 106.1 [3.5] I
Cr 79.9 [50.4] SmA 83.9 TGB 87.2 [0.8]h N* 103.5 [3.4] I
I 105.4 [3.4] N* 77.8f TGB 53.7f TGBC* À8.6 [3.8] M
I 103 [3.2] N* 86.6 TGB 82 [0.6]h SmA 61.1f SmC* À7.1 [1.8] M
I 102.2 [3.8] N* 86f TGB 84.5f SmA 68.1f SmC* À3.6 [2.3] M
I 100.6 [3.6] N* 91.4f TGB 90f SmA 72.9f SmC* 14.6 [10.2] M
I 95.1 [2.7] N*, f
I 97.6 [4.2] N* 93.5 TGB 92.8 [1.3]h SmA 78.7f SmC* 41 [30.1] Cr
I 93.7 [3.8] N* 90.3 TGB 86.4[0.5]h SmA 72.4f SmC* 49.4 [2.4] Cr
Cr 89.1 [52.7] N* 102.6 [3.8] I
Cr 92.2 [55.9] N* 101 [3.9] I
Cr 85.2 [20.2] N* 95.6 [3] I
Cr 86.6 [47.6] SmA 93 TGB 94.1 [1.3]h N* 98.4 [4.4] I
Cr 74.2 [22.2] SmA 88.2 TGB 91.9 [0.6]h N* 94.3 [4.4] I
a TGBC* = twist grain boundary phase with SmC* blocks. b Known compound but included for the sake of comparison and completeness (see ref 21).
c Phase transition temperatures were determined by both POM and DSC studies: peak temperatures in the DSC thermograms obtained during the first
heating and cooling cycles (at 5 °C minÀ1) coupled with optically measured temperatures are given. d Transition enthalpies were obtained from the DSC
thermograms. e Crystal to crystal transition was observed for DM-7,6 at 78.1 [12.5]. f Phase transition was observed under POM; too weak to be detected
by DSC. g No crystallization was observed until the limitation of the DSC (À60 °C) instrument. h Although N*ÀTGB/TGBÀN* and TGBÀSmA/
SmAÀTGB phase transitions was observed in POM, it was not resolved in DSC trace. Hence the enthalpy value represents the combined enthalpy for
N*ÀTGB/TGBÀN* and TGBÀSmA/SmAÀTGB transitions.
characteristic textural patterns. For example, for compounds
DM-5,8ÀDM-5,12, the microscopic examination revealed a
focal-conic texture for the N* mesophase when untreated slides
were used; indeed, the focal-conic texture changes into an oily
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streak texture on slight shear. On further cooling, the N* phase
passes through a transient TGB phase to the SmA phase. The
TGB phase showed a filament texture in the homeotropic
alignment, as expected. The smectic A phase was recognized
based on the observation of focal-conic texture in slides treated
for planar orientation and a dark field of view in slides treated of
homeotropic alignment. In slides treated for planar orientation
the SmC* phase shows dechiralization lines on top of the focal-
conic fan texture, while with the surfaces treated for homeotropic
alignment a cloudy texture was observed. It must be mentioned
here that the phase transitions of all these compounds are highly
reproducible for any number of heating and cooling cycles. The
DSC thermograms of the first/subsequent heating and cooling
scans corroborated the microscopic observations and further
supported the repeatability of the transitions.
DM-7,R Series. The phase transitional behavior of the DM-7,
R series of compounds is shown in Table 4. All the members of
the series display enantiotropic mesomorphism. The first mem-
ber DM-7,6 displays only N* phase during heating, and while
cooling it shows TGB and SmA phases additionally. The N*
phase showed characteristic focal-concic texture, which on slight
shearing gave Grandjean planar texture. For the TGB and
SmA phases, planar/filamentary pattern and pseudoisotropic/
focal-conic texture are observed when slides treated for homogeneous/
homeotropic alignments were used. The second member
DM-7,7 behaves almost identical to the first member except for
the stabilization of the TGB phase having SmC* blocks (denoted
as the TGBC* phase) additionally. Notably, the TGBC* phase
supercools until À8.6 °C, at which it transforms into another
unknown phase, hereafter referred to as the M phase as indicated
by the DSC traces (see Table 4). The nature of the M phase could
not be ascertained under POM owing to the temperature
limitation of the hot-stage of our laboratory. On cooling the
isotropic phase of compound DM-7,7, a focal-conic texture of
the N* phase appears sharply, which on mechanical shear
furnishes a Grandjean planar texture. The planar texture of the
N* phase on further cooling transforms into a TGB phase (which
may have either SmA or SmC blocks) as evidenced by the
observation of a gray planar texture (Figure 5a). On lowering
the temperature, a square grid pattern piercingly appears over the
gray planar texture indicating the presence of a TGBC* phase
(Figure 5b). When the glass plates were treated to give a
homeotropic alignment, filament pattern (Figure 5c) and an
undulated filament texture (Figure 5d) were observed for the
TGB and TGBC* phase respectively. The next members DM-7,8
and DM-7,9 while cooling from the isotropic phase show an
identical phase sequence: N*ÀTGBÀSmAÀSmC*ÀM where
Figure 5. Photomicrographs of the textures of mesophase exhibited
by compound DM-7,7: (a) planar texture of the TGB phase
(72 °C), (b) square grid pattern of the TGBC* phase emerging from
the planar texture of the TGB phase (48 °C), (c) filamentary texture of
the homeotropically aligned TGB phase (75 °C), and (d) undulated
filamentary texture of the homeotropically aligned TGBC* phase
(48 °C).
Figure 7. The influence of the number of carbon atoms in the
ω-oxyalkanoyloxy spacer on the clearing (isotropic liquid-mesophase)
transition temperatures of four series of compounds. In all the plots,
dashed lines joining points are suggestive of the general trend; it may be
noted that experimental data do not cover for n = 7.
Figure 6. (a) DSC thermograms obtained during the first and second heating/cooling cycles at a rate of 5 °C/min for DM-7,11. Note that the N* phase
supercools well below the RT. (b) Plot showing the dependence of selective reflection wavelength of the N* phase on the temperature of DM-7,11.
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Figure 8. Intensity versus 2θ profile extracted from the 2D XRD pattern of the (a) SmA phase at 95 °C and (b) SmC* phase at 65 °C of dimer-like
compound DM-3,8.
Table 5. The Spacings (d1 and d2) Corresponding to the Low-Angle and Wide-Angle Reflections of XRD Pattern of Different
Mesophases of Compounds DM-3,8, DM-5,10, and DM-7,6a
compound (L /Å)
DM-3,8 (38)
temperature /°C
d1/Å (intensity)
d2/Å (intensity)
d1/L
mesophase
95
65
90
75
69
60
37.9 (s)
34.8 (s)
43.9 (s)
39.2 (s)
41.5 (s)
41.2 (s)
5.2 (d)
5.1 (d)
5.1 (d)
5 (d)
0.99
0.91
0.98
0.87
0.98
0.98
monolayer SmA (SmA1)
SmC* (tilt angle =23.3 o)
SmA1
SmC* (tilt angle =30.9 o)
TGB with SmA1 blocks
SmA1
DM-5,10 (45)b
DM-7,6 (42)
5.1 (d)
5.1 (d)
a The estimated all-trans molecular length (L) and d1/L ratio and structural assignment of the mesophases are also given. s: strong; d: diffuse. b Known
compound; see ref 21.
pitch length of the N* phase is on the order of the wavelength of
incident visible light. The helical pitch is temperature-dependent
and thus, the color of the reflected light varies with temperature, a
feature that has been well used in temperature sensors. Thus, the
temperature dependence of selective reflection was measured
with the aid of ultravioletÀvisibleÀnear-infrared (UVÀvisÀ
NIR) scanning spectrophotometer coupled with a programma-
ble hot stage. The sample DM-7,11 placed between two quartz
plates was heated to its isotropic phase, and cooled slowly. The
N* phase formed was sheared to obtain Grandjean (planar)
texture in which the helix of the phase becomes normal to the
quartz plates and the structure reflects the incident light. After
attaining this geometry, selective reflection wavelength as a
function of temperature was recorded and during each scan,
the temperature of the sample was kept constant. Figure 6b
Figure 9. Current response peak obtained on application of a
triangular wave field (∼ 8 V/μm, 16 Hz) in the SmC* phase
of compound DM-3,16 at 65 °C. Note that the measured Ps is
24 nC cmÀ2
.
shows the thermal dependence of selective reflection (λmax) in
the entire thermal range (up to 20 °C) of the N* phase; a strong
variation in the pitch with the variation in the temperature is
apparent. The λmax shifts to longer wavelengths (redshift) as the
temperature of the phase is lowered. However, as can be seen in
Figure 6b, the pitch of the phase decreases and then increases in
the temperature region between 35 and 60 °C, and this may be
arising due to slow freezing of the N* phase in the glassy state.
Thus, the ability of this molecular design to change the pitch
length and hence the N* color over the visible region as a
function of temperature is demonstrated.
The results of the thermal behavior of the four homologous
series of dimer-like compounds formed by covalently linking
cholesterol with N-(n-alkyl)salicylaldimine through either 4-oxy-
butanoyloxy or 5-oxypentanoyloxy or 6-oxyhexanoyloxy or
8-oxyoctanoyloxy spacer is surmised here. Within each series,
the length of the flexible terminal tail varies from n-hexyl to
M is an unknown phase existing well below room temperature
(RT). A similar phase sequence but without the M phase was
evidenced for the homologues DM-7,12 and DM-7,16.
Compound DM-7,11 stabilizes enantiotropic N* phase solely,
which is rather surprising given the observation that its immedi-
ate lower and higher members show a rich phase behavior.
Notably, once the N* phase formed upon melting of the sample,
no sign of crystallization was noticed even after keeping the
samples at RT for more than 6 days. However, repeated
mechanical disturbances induced the phase to freeze in a glassy
state. The DSC traces obtained (Figure 6a) corroborate the
observation. Particularly, in the cooling cycle traces, only a
signature due to the IÀN* phase is seen. The N* phase showed
vivid colors as the temperature was varied, indicating that the
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Figure 10. (a) Current response peaks obtained on application of a triangular wave field (5.04 VμmÀ1, 12 Hz) in the SmC* phase at different
temperatures of compound DM-5,10. (b) The dependence of the Ps value on the temperature of the SmC* phase of compound DM-5,10.
Table 6. Electrochemicala Activities of Dimer-like Compounds
two lengths are comparable.5d Therefore, it is interesting to
examine whether the structure of the SmA phase is governed by
the relative lengths of spacer and terminal tail in these cholester-
ol-based dimer-like compounds. XRD study was carried out on
DM-3,8 and DM-7,6 as representative cases; for the comparison,
the known compound DM-5,1021 was also investigated. These
samples were individually filled in a Lindemann capillary (1 mm
diameter) tube in the isotropic phase, and both the ends of the
tube were flame-sealed. The diffraction patterns obtained as a
function of temperature in different mesophases were collected
in the cooling mode. The XRD patterns obtained for the
mesophases were found to be nearly identical; they consisted
of a diffuse peak and a sharp peak in the wide angle and low angle
regions, respectively. As a representative case, the 1D intensity
versus 2θ profiles derived from two-dimensional (2D) XRD
patterns of the SmA and SmC* phases of DM-3,8 are shown in
Figure 8. The spacings (d1) derived from the low angle Bragg
reflections of the mesophases in each compound are indicative of
the layered structure. The measured spacings (d2) of wide angle
diffuse reflections correspond to liquid-like order within the
layers. These spacings and the ratio d1/L, where L is the
estimated all-trans molecular length derived from a Chem3D
molecular model, are presented in Table 5.
As can be seen in Table 5, the d1-values obtained for the SmA
phases of dimer-like compounds DM-3,8, DM-5,10, and DM-
7,6 are 37.9, 43.9, and 41.5 Å, respectively; in fact, these values
are nearly equivalent to the estimated all-trans molecular lengths
38, 45, and 42 Å for DM-3,8, DM-5,10, and DM-7,6, respec-
tively. Thus, the d1/L ratio is one suggesting the monolayered
arrangement of molecules in the SmA phase. The formation of
monolayer SmA (SmA1) phase by DM-3,8 and DM-5,10 is in
agreement with the general observation that the cholesterol-
based dimers5d having the spacer length less than the length of
the terminal tail stabilize SmA1 phase. The collected data in the
SmC* phase for DM-3,8 and DM-5,10 reveals that the layer
spacing d1 is much less than the molecular length L, and thus the
d1/L ratio is less than 1. This is expected considering the fact that
in the SmC* phase, the molecules tilt with respect to the layer
normal direction.
b
b
c
c
c,d
c,e
compound EOX
ERED
EHOMO
ELUMO
ΔECV
ΔEUV
DM-3,6
DM-3,8
DM-3,16
DM-5,7
DM-5,9
DM-5,10
DM-7,6
DM-7,8
DM-7,10
DM-7,11
1.20
À1.30
À1.29
À1.28
À1.33
À1.29
À1.24
À1.30
À1.23
À1.27
À1.29
5.90
5.91
5.89
5.85
5.84
5.89
5.84
5.89
5.89
5.84
3.40
3.41
3.42
3.37
3.41
3.46
3.40
3.47
3.43
3.41
2.50
2.50
2.47
2.48
2.43
2.43
2.44
2.42
2.46
2.43
2.79
2.77
2.82
2.86
2.92
2.90
2.85
2.85
2.84
2.83
1.21
1.19
1.15
1.14
1.19
1.14
1.19
1.19
1.14
a Experimental conditions: Pt-disk working electrode, Pt-wire counter
electrode, Ag/Ag+ClÀ in saturated LiCl solution as reference electrode,
(Bu)4NPF6 (0.1 M), RT. b In volts (V). c In electron volts (eV). d Estimated
band gap from the onset oxidation and reduction potential (vs Ag/Ag+)
plus 4.7 (see refs 27d and 27e). e Band gap determined from the
red edge of the longest wavelength in the UVÀvis absorption of
compounds.
n-dodecyl and n-hexadecyl, whereas the nature of the spacer
obviously remained unaltered. Within each series, the variation of
the length of the terminal tail seems to influence the phase
transitional behavior. In some of the series, a general trend seems
to be followed, especially during the heating cycle, where the
lower members are chiral nematics (N*), the medium members
show chiral nematic and smectic (SmA and SmC*) behavior with
frustrated phases (TGB), and the higher members are smectics.
In each series, the clearing temperature decreases with the
increase in the terminal tail length. In comparison with
an even-parity spacer containing three series of compounds,
the homologous series comprising an odd-parity spacer show
lower clearing temperatures, as summarized in Figure 7; such a
behavior can be explained in terms of their overall molecular
shape governed by the geometry and flexibility of the spacer.
II.b.2. XRD Study. As discussed in a recent review article,5d
the SmA phase formed by the cholesterol-based dimer, shows
fascinating structural variations (periodicities) that largely de-
pend upon the relative lengths of the spacer and terminal chains
as well as, to an extent, the nature of the aromatic core. For
example, the monolayer or partially bilayered SmA phase is seen
if the spacer length is less than the length of the terminal tail. The
intercalated SmA phase is stabilized if the spacer is longer than
the terminal chain and the smectic phase disappears when the
II.b.3. Electrical Switching Studies. It may be reiterated here
that, except for a few, all the dimer-like compounds synthesized
stabilize the SmC* phase; notably, in the majority of the cases, it
exists over a fairly wide thermal range. Most importantly, the
electric switching behavior of the phase is known to be useful
in fabricating display devices.1 Thus, the SmC* phase of these
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Figure 11. (a) Cyclic voltammogram of DM-5,7 (10À3 M/CH2Cl2) at a scan rate of 100 mVsÀ1. (b) UV absorption spectrum (10À3 M/CH2Cl2)
obtained for DM-5,7. (c) Energy gaps expressed in electron volts deduced by both CV and UV experiments.
Figure 12. (a) Vial comprising a hot clear solution of DM-7,16 in 2% (w/v) ethanol. (b) Vial is inverted to show the formation of stable gel at RT. Also
shown are the SEM images (cÀf) of DM-7,16 gel in 2% ethanol having different morphologies of micrometer size.
dimer-like compounds was investigated for its switching prop-
erty, and the study revealed their ferroelectric switching behavior.
For the purpose of this study, cells (5.01 μm thick) were built
using indium tin oxide (ITO)-coated glass plates pretreated with
a polyimide solution, which enables the molecules to align
homogeneously. The samples in their isotropic phase were filled
into the cell by capillary action and cooled slowly. After attaining
the SmC* phase, a triangular-wave field was applied where the
samples showed the current response curves. As a representative
case, the current response profile obtained for DM-3,16
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on application of a low frequency (16 Hz) triangular-wave
field (∼ 8 V/μm) at 65 °C is illustrated in Figure 9. As can be
seen, the current passing through the sample shows one peak
per half period of the applied field, indicating the ferro-
electric behavior of the SmC* phase. The area under the peak
trace is a direct measure of Ps and it was found to be 24
nC cmÀ2. Seemingly, the Ps value is small but increases upon
lowering the temperature, as expected. In some cases, the
value of Ps was found to be much smaller; this is especially
notable in one of four known compounds, DM-5,10 studied.
Figure 10a illustrates the electrical response profiles ob-
tained on applying a triangular wave electrical field of 5 V
μmÀ1 (12 Hz) at different temperatures of the SmC* phase.
The temperature dependence of Ps obtained by integrating
the area under the peaks is shown in Figure 10b. It can be
seen that the Ps value is quite small but increases with the
decrease in temperature, as expected. The smaller magnitude
of spontaneous polarization found in these compound can be
attributed to the weak intramolecular transfer of the bulky
chirality from the cholesterol unit to the other mesogens.
sensing and external stimuli responsive motifs, light harvesting
systems, templates for the transcription of nanofibrous materials,
and so forth.28,29 Given the fact that the synthesized dimer-like
molecules possess a cholesterol component, we became inter-
ested in examining their gelation ability and the morphology of
the gel formed. Thus, a qualitative investigation on the gelation
ability of all the synthesized dimer-like compounds was carried
out. A series of common organic solvents were employed to
reveal the gelation behavior of the compounds. Our study shows
that these compounds readily dissolve in dichloromethane,
chloroform, diethylether, and tetrahydrofuran but are insoluble
in hexanes as well as in protic solvents such as methanol and
ethanol. However, these compounds were found to be soluble in
hot ethanol and reappear upon cooling in the form of soft mass,
indicating their ability to gelatinize. Thus, the minimum gelation
concentration (MGC) necessary for gel formation in ethanol was
investigated; the study revealed the formation of gel with an
MGC of 2 g/100 mL. For example, a mixture of DM-7,16 (20 mg)
and absolute ethanol (1 mL) was warmed in a screw-capped
vial until the compound dissolved (Figure 12a). The homoge-
neous solution thus formed was cooled to RT yielding gel; this
was ascertained by the test tube inversion method as illustrated in
Figure 12b. Of course, the sol to gel phase was found to be
thermally reversible. A scanning electron microscope (SEM) was
used to evaluate the gel morphology. The SEM images obtained
are illustrated in Figures 12cÀf. These images indicate the self-
assembly of these chiral molecules into different morphologies at
a supramolecular scale of micrometer size.
III. ELECTROCHEMICAL BEHAVIOR
In recent times, cholesterol-based oligomesogens have been
shown to be multifunctional.23À25 For example, their antimicro-
bial activity23 and electrochemical behavior24,25 have been re-
vealed recently. Inspired by these findings, we intended to probe
these synthesized dimer-like compounds for their electrochemi-
cal behavior. Thus, the oxidationÀreduction potentials were
measured by cyclic voltametry (CV) at a scanning rate of 100
mV sÀ1 for the millimolar solutions of several randomly selected
compounds in dichloromethane;26 the results deduced from this
study are summarized in Table 6. Tetrabutylammonium hexa-
fluorophosphate (0.1 M) in dichloromethane was used as a sup-
porting electrolyte (buffer). A conventional three-electrode cell,
consisting of a 1 mm diameter platinum disk as a working
electrode, silverÀsilver chloride as a reference electrode, and a
platinum wire as a counter electrode, were used. The reference
electrode consisted of a Ag/Ag+ClÀ electrolyte in saturated
lithium chloride solution. The electrochemical potential was
calibrated with respect to a ferrocene/ferrocenium (Fc/Fc+)
couple. As a representative case, the cyclic voltammogram
obtained for the compound DM-5,7 is shown in Figure 11a.
The IP (EHOMO) and EA (ELUMO) were determined by adding
4.7 eV (vs Ag/Ag+ClÀ) to the first oxidation and first reduction
potentials, respectively, and the difference between these furn-
ished the energy gaps (ΔECV).26,27 The energy gap (ΔEUV) was
also deduced from the red edge of the longest wavelength
absorption (Figure 11b), using the expression E = hυ (where
υ = c/λ). Seemingly, ΔE values deduced from both the methods
are nearly comparable, indicating that these compounds genu-
inely possess electrochemical activity. The schematic representa-
tion of energy gap deduced by both CV and UV experiments is
shown in the Figure 11c.
V. SUMMARY
It is demonstrated that the thermal behavior and other
functional characteristics of a relatively new class LCs, called
dimer-like mesogens, are similar to that of well-known LC
dimers. Thereby, the synthesis and various properties character-
ization of 28 new optically active dimer-like compounds belong-
ing to four series are reported. They are made by covalently
tethering cholesterol with a salicylideneamine core through an ω-
oxyalkanoyloxy spacer of varying length and parity. These four
series of compounds primarily differ in the number of carbon
atoms (length and parity) of the spacer. With a few exceptions in
the series having odd-parity spacers, all the compounds exhibit
enantiotropic mesomorphism. In all the three even-parity spacer
series, the variation in the length of the terminal tail influences
the thermal property. For example, the lower members display
the N* phase, the medium members show the N* and smectic
phases intervened by the TGB phase, and the higher members
are purely smectics. Notably, in some cases, the N* and SmC*
phases exist over a wide thermal range; the latter phase exhibits
ferroelectric switching behavior. A dramatic oddÀeven effect
in the clearing temperatures occurs wherein the even-parity
dimers possess higher values. Thus, the thermal behavior of
these dimer-like compounds is analogous to that of LC dimers.
Most importantly, this study has demonstrated that the terminal
phenyl ring of dimer-like compounds should have an n-alky tail to
stabilize thermodynamically stable mesophases. UVÀvis study
demonstrated the temperature dependences of the wavelength
selective reflection (λmax) of the phase. The redox behavior of
some representative samples has been probed by CV experi-
ments, and the energy gaps are estimated by both CV and UV
experiments. The results of these measurements are comparable,
which emphasize that they genuinely possess electrochemical
IV. GELATION STUDY
Gels derived from low molecular mass compounds have been
attracting a great deal of attention as they hold promise for
a number of biological and materials applications.28,29 This
is especially notable in the case of compounds/mesogens (gelators)
derived from cholesterol as they form stable organic gels that
have been regarded as novel molecular recognition systems,
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activity. Notably, these compounds have the tendency to form
stable organic gel in ethanol where the gelators self-assemble into
chiral aggregates with different morphologies of micrometer size.
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’ AUTHOR INFORMATION
Corresponding Author
*Tel: 91-80-28381119. Fax: 91-80-28382044. E-mail: yelamaggad@
csmr.res.in or yelamaggad@gmail.com.
Present Addresses
†Institute of Chemistry, Organic Chemistry, Martin-Luther-University,
Halle-Wittenberg, Kurt Mothes Str. 2, D-06120 Halle/Saale, Germany.
(16) Cowling, S. J.; Hall, A. W.; Goodby, J. W. Liq. Cryst. 2005, 32,
1483–1498.
’ ACKNOWLEDGMENT
(17) Cowling, S. J.; Hall, A. W.; Goodby, J. W. Chem. Commun.
2005, 1546–1548.
(18) Cowling, S. J.; Hall, A. W.; Goodby, J. W. Adv. Mater. 2005, 17,
The authors express their gratitude to Dr. D. S. Shankar Rao
for providing the X-ray diffraction data. Dr. S. Krishna Prasad is
gratefully acknowledged for helpful discussions. The authors
wish to thank Ms. Sandhya D. Hombal (CSMR) for her technical
assistance; Dr. Gurumurthy Hegde for helping in carrying out
electrical switching experiments; Dr. M. Pandurangappa and Mr.
T. Ramakrishnappa, Central College, Bangalore, for their help in
carrying out cyclic voltammetry experiments; and Prof. G. U.
Kulkarni, JNCSR, for providing SEM images.
1077–1080.
(19) Cowling, S. J.; Goodby, J. W. Chem. Commun. 2006, 4107–4109.
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J. W. Liq. Cryst. 2005, 32, 1045–1051.
(21) Yelamaggad, C. V.; Rashmi, Prabhu; Shanker, G.; Bruce, D. W.
Liq. Cryst. 2009, 36, 247–255.
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(c) Hamley, I. W.; Castelletto, V.; Parras, P.; Lu, Z. B.; Imrie, C. T.; Itoh,
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