Optically Biaxial, Re-entrant and Frustrated Mesophases in Chiral, Non-symmetric Liquid Crystal Dimers and Binary Mixtures

Article abstract of DOI:10.1002/asia.201600918

Sixteen optically active, non-symmetric dimers, in which cyanobiphenyl and salicylaldimine mesogens are interlinked by a flexible spacer, were synthesized and characterized. While the terminal chiral tail, in the form of either (R)-2-octyloxy or (S)-2-oct

Full text of DOI:10.1002/asia.201600918

DOI: 10.1002/asia.201600918
Full Paper
Soft Matter
Optically Biaxial, Re-entrant and Frustrated Mesophases in Chiral,
Non-symmetric Liquid Crystal Dimers and Binary Mixtures
Vediappen Padmini,[a, b] Palakurthy Nani Babu,[a] Geetha G. Nair,[a] D. S. Shankar Rao,[a] and
Channabasaveshwar V. Yelamaggad*[a]
Abstract: Sixteen optically active, non-symmetric dimers, in
which cyanobiphenyl and salicylaldimine mesogens are in-
terlinked by a flexible spacer, were synthesized and charac-
terized. While the terminal chiral tail, in the form of either
(R)-2-octyloxy or (S)-2-octyloxy chain attached to salicylaldi-
mine core, was held constant, the number of methylene
units in the spacer was varied from 3 to 10 affording eight
pairs of (R & S) enantiomers. They were probed for their
thermal properties with the aid of orthoscopy, conoscopy,
differential scanning calorimetry and X-ray powder diffrac-
tion. In addition, the binary mixture study was carried out
using chiral and achiral dimers with the intensions of stabi-
lizing optically biaxial phase/s, re-entrant phases and impor-
tant phase sequences. Notably, one of the chiral dimers as
well as some mixtures exhibited a biaxial smectic A (SmAb)
phase appearing between a uniaxial SmA and a re-entrant
uniaxial SmA phases. The mesophases such as chiral nematic
(N*) and frustrated phases viz., blue phases (BPs) and twist
grain boundary (TGB) phases, were also found to occur in
most of the dimers and mixtures. X-ray diffraction studies re-
vealed that the dimers possessing oxybutoxy and oxypen-
toxy spacers show interdigitated (SmAd) phase where smec-
tic periodicity is over 1.4 times the molecular length; where-
as in the intercalated SmA (SmAc) phase formed by a dimer
having oxydecoxy spacer the periodicity was found to be
approximately half the molecular length. The handedness of
the helical structure of the N* phases formed by two enan-
tiomers was examined with the aid of CD measurements; as
expected, these enantiomers showed optical activities of
equal magnitudes but with opposite signs. Overall, it ap-
pears that the chiral dimers and mixtures presented herein
may serve as model systems in design and developing novel
materials exhibiting the apolar SmAb phase possessing D2h
symmetry and nematic-type biaxiality.
Introduction
self-complementing (H-bonding) cores. In fact, these represent
a rich family of LC materials as they have provided strategies
to incorporate the functional molecular components of choice.
They, being multifunctional hybrid materials, have the ability
to revolutionize materials science,[1–9] the medical field[12,32–38]
and applied research,[11,13,19,23] as they self-assemble and self-or-
ganize inherently into novel hierarchical fluid structures with
exceptionally improved and easily tuneable/addressable char-
acteristics; moreover, they respond well to external stimuli.
It is well demonstrated that liquid-crystal morphologies of
these systems can be modulated to a large extent by varying
the numbers, polarity, compatibility, size, shape, etc. of rigid
cores as well as the length and parity of the spacers.[1–30] For
example, the influence of the latter parameter is prominently
seen in LC dimers in which two mesogens are covalently
bound to each other in an end-to-end manner via a flexible
spacer.[28–31,39–57] In particular, their phase transitional properties
are governed by the spacer parity as it determines the confor-
mation (geometry/overall molecular shape) of the dimeric
mesogens. The dimers with an even-parity spacer, existing in
its all-trans form, possess a zig-zag shape wherein the constitu-
ent mesogens remain parallel to each other. On the contrary,
the dimers comprising an odd-parity spacer have a bent-con-
formation/banana-shape in which the mesogenic cores incline
Over the past two decades or so, tremendous progress has
been made towards the development of non-conventional
liquid crystals (LCs) through the rational molecular design and
bottom-up synthesis.[1–31] These hybrid mesogens differ in their
shape-anisotropy and phase transitional behavior from the
conventional LCs. They comprise molecular entities such as
flexible units, that is, tails and spacers (hydrocarbons/fluorocar-
bons/hydrophobic–hydrophilic
units/carbisilanes/polysilanes
etc.) and functional rigid mesogenic and/or non-mesogenic/
[a] Dr. V. Padmini, Dr. P. N. Babu, Dr. G. G. Nair, Dr. D. S. S. Rao,
Dr. C. V. Yelamaggad
Centre for Nano and Soft Matter Sciences
P. B. No. 1329, Prof. U. R. Rao Road, Jalahalli, Bengaluru 560013 (India)
Fax: (+91)80-28382044
E-mail: yelamaggad@cens.res.in
yelamaggad@gmail.com
[b] Dr. V. Padmini
Department of Organic Chemistry
School of Chemistry, Madurai Kamaraj University
Madurai 625021, Tamil Nadu (India)
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
asia.201600918.
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with respect to one another.[28–31] They can be broadly classi-
fied into two categories: (a) symmetric dimers, wherein two
mesogenic cores are chemically similar and (b) non-symmetric
dimers, which comprise two chemically different meso-
gens.[28–30] Symmetric dimers are known to show a rich smectic
polymorphism; remarkably, the existence of a twist-bend nem-
atic (Ntb) phase signifying a structural correlation between the
uniaxial nematic (N) and the chiral nematic N* phases has
been reported in an achiral symmetric dimer having an odd-
numbered spacer.[47] Non-symmetric dimers, unlike their sym-
metric counterparts, show intercalated and interdigitated
phases. Molecular chirality profoundly influences the phase
transitional properties of both symmetric and non-symmetric
dimers.[28–31,52] A wide variety of both types of chiral dimers
have been studied to primarily figure out the influence of the
number of carbon atoms of the central flexible spacer, besides
other structural parameters, on the chirality of the LC
phases.[28–31,52]
a heptamethylene spacer exhibits a new phase sequence,
namely, blue phase (BP)–chiral nematic (N*)–twist grain boun-
dary (TGB)–uniaxial smectic A (SmA)–biaxial smectic A (SmAb)–
uniaxial smectic A (SmA) phases; the SmA phase exhibiting re-
entrance is believed to stem from dipolar and steric factors.[54]
Inspired by these findings, we intended to undertake a system-
atic investigation on the variants of this dimer so as to under-
stand the correlation between molecular structure and LC
properties. Herein, we report on the synthesis and characteri-
zation of sixteen non-symmetric dimers (eight enantiomeric
pairs); in this study, the aforesaid dimer is also included for the
sake of comparison and completeness. As shown in Scheme 1,
each pair of enantiomers comprise (R)-2-octyloxy and (S)-2-oc-
tyloxy chiral tails and the number of methylene units in the
flexible spacer has been varied from 3 to 10. Besides, we have
prepared binary systems by mixing one of the (R)-enantiomers,
I(R)-7, and a known analogous achiral dimer[58] in varying pro-
portions.
We have been actively involved in the design, synthesis and
characterization of such chiral oligomers primarily to under-
stand the structure–property correlations; in particular, our Results and Discussion
work has largely focused on optically active non-symmetric
Synthesis and molecular structural characterization
dimers as they exhibit new mesophases, unprecedented se-
quences, frustrated phases, etc.[52,54,55] In 2009, for example, we
observed that a chiral non-symmetric LC dimer made by cova-
lently tethering cyanobiphenyl and salicyladimine cores via
Scheme 2 shows the synthetic steps followed to prepare the
eight pairs of enantiomers. To begin with, 4’-hydroxy-4-cyano-
biphenyl was alkylated with a,w-dibromoalkanes in the pres-
Scheme 1. Molecular structures of fifteen optically active, nonsymmetric dimers synthesized and characterized (a) and the known dimers, I(R)-7 [54] and I-5
[58] used for binary mixture studies (b).
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Scheme 2. Synthesis of optically active, non-symmetric dimers. Reagents and conditions: (i) Dibromoalkane, K2CO3, KI (cat.), butanone, reflux, 12 h; (ii) 2,4-dihy-
droxybenzaldehyde, K2CO3, KI (cat.), butanone, reflux, 12 h; (iii) (S)-(+)-2-octanol, Ph3P, DIAD,THF, rt, 18 h; (iv) (R)-(-)-2-octanol, Ph3P, DIAD,THF, rt, 18 h; (v) H2, (1
atm, balloon), 10 % Pd-C, THF, rt, 4 h; (vi) ethanol, acetic acid (cat.), reflux, 4 h.
ence of anhydrous K2CO3 in butanone to obtain 4’-((w-bro-
moalkyl)oxy)-4-cyanobiphenyls (1a–h) in good yields.[59–61] The
reaction of 2,4-dihydroxybenzaldehyde with cyanobiphenyls
(1a–h) in the presence of a mild base in butanone yielded the
respective O-alkylated products, namely, 4’-((w-(4-formyl-3-hy-
droxyphenoxy)alkyl)oxy)-4-cyanobiphenyls (2a–h).[58–62] Under
Mitsunobu reaction conditions,[63] 4-nitrophenol was O-alkylat-
ed with the (S)- and (R)-2-octanol, to obtain (R)-4-(octan-2-ylox-
y)nitrobenzene (3) and (S)-4-(octan-2-yloxy)nitrobenzenes
(5);[64–68] these nitro materials were catalytically hydrogenated
using H2 (1 atm, balloon) and Pd/C (10%) in THF, to obtain the
respective (R)-4-(octan-2-yloxy)aniline (4) and (S)-4-(octan-2-
yloxy)aniline (6) in notable yields.[64–68] In the final step, anilines
4 and 6 were condensed with aldehydes 2a–h in the presence
of a weak acid in ethanol to obtain the targeted optically
active, non-symmetric dimers in excellent yields. The synthetic
procedure for the target dimers and their molecular structural
characterization data are given in the Supporting Information.
LC phases were identified based on the observation of fluid bi-
refringent textures under POM; specific LC phase assignment
and phase sequences were determined based on the observa-
tion of characteristic textures and their pattern transformation
when undergoing phase transitions.[68] The peak temperatures
recorded in DSC thermograms due to phase transitions were
found to be consistent with those of POM studies. The transi-
tion temperatures and the corresponding enthalpy values ob-
tained from DSC measurements are given in Table 1.
From the accumulated results (Table 1) it is apparent that all
the dimers with an even-numbered methylene spacer show,
unlike the corresponding odd-members, enantiotropic meso-
morphism. This is expected given the fact that dimers with an
even-membered spacer attain an overall linear conformation,
which favors the parallel arrangement of the mesogenic cores
resulting in their self-assembly into the LC state. As a represen-
tative case, an energy-minimized space-filling model of an
even-member I(R)-4 with a zig-zag shape is shown in Figure 1a
where the mesogenic cores lie along the long molecular axis
of the dimer. Both the POM and DSC studies confirmed that
the enantiomers I(R)-4 and I(S)-4, wherein the two mesogenic
groups are separated by an oxybutoxy moiety, exhibit analo-
gous enantiotropic LC behavior, as expected. DSC traces re-
corded for both the samples are shown in Figure 2. When thin
films of the samples I(R)-4 and I(S)-4 were held between clean
(untreated) glass plates and cooled from the isotropic phase,
they display a fan-like texture of the N* phase (Figure 3a) at
~1798C (DH=5.3 Jgꢀ1); here, within individual fans the helix
orients uniformly. As expected, the oily streaks characteristic of
Thermal properties
Eight pairs of enantiomers prepared were examined for their
thermal behavior with the aid of polarizing optical microscopy
(POM), differential scanning calorimetry (DSC) and powder X-
ray diffraction (XRD). It may be mentioned here that POM and
DSC techniques were used as prime and complementary tools
for determining the thermal characteristics of all dimers,
whereas XRD was used to measure the layer spacings (d) of
the smectic phases of a few chosen, representative materials.
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Table 1. Phase sequence, transition temperatures (oC)[a] and the corresponding enthalpies (Jgꢀ1) of chiral dimers belonging to II-n,R and III-n,S series. Cr=
Crystal; SmA=uniaxial smectic A phase; SmAb =biaxial smectic A phase; TGB=twist grain boundary phase; N*=chiral nematic phase; and BP=blue
phase.
Dimer
Phase sequence
Heating
Cooling
I(R)-3
I(S)-3
I(R)-4
I(S)-4
I(R)-5
I(S)-5
I(R)-6
I(S)-6
I(R)-7*
I(S)-7
I(R)-8
I(S)-8
I(R)-9
I(S)-9
I(R)-10
I(S)-10
Cr 102.4 (68.1) I
Cr 102.3 (64.3) I
Cr 133.9 (56.9) SmA 174.9[b] TGBc-N* 179.9 (5.4) I
Cr 134.2 (45.2) SmA 174.6[b] TGB[c]-N* 179.9 (5.4) I
Cr 109.7 I
–
–
I 178.7 (5.3) N*-TGB 173.8 SmA 116.4 (51.2) Cr
I 179 (5.3) N*-TGB 173.9 SmA 116.4 (51.5) Cr
I 91.1 (0.4) BP 83.5 N*-TGB[c] 81.7[b] SmA 44.1 (12.4) Cr
I 91.8 (0.4) BP 83.7 N*-TGB[c] 81.5[b] SmA 44 (19.2) Cr
I 155.5 (4.9) N* 95.8 (35.8) Cr
Cr 109.3 I
Cr 113.7 (62.4) N* 156.3 (5.1) I
Cr 116.6 (57.8) N* 156.6 (5.1) I
Cr 74.2 (50.6) N* 100.1 (0.4) I
Cr 73.7 (49.3) N* 100 (0.4) I
Cr 114 (68.2) N* 140.7 (5.2) I
Cr 112.7 (53.5) N* 140.1 (5.1) I
Cr 105.7 (49.6) I
Cr 105 (45.6) I
Cr 115.3 (56.3) SmA 129.9 (1.9) N* 138.8 (4.8) I
Cr 113.1 (55) SmA 129.7 (1.8) N* 138.7 (4.9) I
I 155.7 (5) N* 92.9 (25.5) Cr
I 99.2 (0.4) BP 90 N*-TGB[c] 74[b] SmA 53.9[b] SmAb 51.2[b] SmA (re-entrant) 29.1[b] Cr
I 99.4 (0.4) BP 89.8 N*-TGB[c] 73.8[b] SmA 53.6[b] SmAb 51.1[b] SmA (re-entrant) 35[b] Cr
I 139.7 (5.1) N* 95.9 (42.6) Cr
I 138.7 (5.3) N* 98.4 (56.4) Cr
I 103.4 (0.4) BP 94.4 N* 56.9 (31.6) Cr
I 103.6 (0.5) BP 94.2 N* 56.7 (25.1) Cr
I 137.6 (4.6) N* 127.8 (1.8) SmA 94.4 (49.60 Cr
I 136.9 (4.8) N* 127.6 (1.8) SmA 88.9 (47.9) Cr
* Known dimer, but included in this study for the sake of comparison and completeness. [a] Peak temperatures in the DSC thermograms obtained during
the first heating and cooling cycles at a rate of 58Cminꢀ1. [b] The phase transition was observed under a polarizing microscope and too weak to get recog-
nized in DSC. [c] A transient phase.
Figure 1. Space-filing energy minimized (all-trans) models of an even-mem-
bered dimer I(R)-4 (a) and an odd-membered dimer I(R)-3 (b). Note that
owing to the presence of four methylene units in the spacer, the dimer I(R)-
4 attains nearly a linear geometry, whereas I(R)-3 with three methylene
groups gains a bent-shape.
the Grandjean plane texture (Figure 3b) were observed when
the fan-like texture was subjected to a mechanical stress. On
Figure 2. DSC thermograms of the first heating–cooling cycles recorded at
cooling the samples further, the N* phase transforms to the
SmA phase through a fleeting TGB phase. The presence of the
both TGB and SmA phases was clearly identified from the opti-
cal textures obtained on slow cooling from the N* phase. The
SmA phase showed the co-occurrence of both focal-conic fan
and homeotropic domains. When the samples were subjected
to homeotropic boundary conditions, the SmA phase displayed
a pseudo-isotropic texture exclusively; by contrast, it showed
a focal-conic texture solely when tested in glass substrates
treated for planar orientation. The TGB phase showed a striking
filament texture when the pseudo-isotropic texture of the low-
temperature SmA phase was heated slowly. In fact, all the
three phases, that is, N*, TGB and SmA phases, existed togeth-
er for a short while; this was evidenced by the observation of
textures corresponding to these three LC phases appearing
a rate of 58Cminꢀ1 for dimers I(R)-4 (blue traces) and I(S)-4 (red traces).
Note that these dimers, being enantiomers, behave identically, and thus
peak positions due to their phase transitions are exactly matching.
concurrently (Figure 3c). The next two enantiomeric pairs,
namely, I(R)-6&I(S)-6, and I(R)-8&I(S)-8, show an identical mes-
omorphic behavior stabilizing an enantiotropic N* phase only;
the presence of this phase was established, as earlier, by polar-
izing microscopy. The dimers I(R)-10 and I(S)-10, wherein
mesogenic cores are interconnected through the decamethy-
lene spacer, show thermodynamically stable N* and SmA
phases that was evidenced by textural observation.
Powder XRD studies have shown that the SmA phase
formed by polar mesogens (i.e., with a terminal cyano group)
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generally consist of antiparallel pairs.[28,61,69–71] The pairing of
polar mesogens results from the strong intermolecular longitu-
dinal association so as to minimize the dipolar energy contri-
bution effectively. Such organization of mesogenic cores gives
rise to either a partially bilayered SmA (SmAd) phase where the
layer spacing (d) is greater than the molecular length (L), or
a bilayer SmA (SmA2) phase with the layer thickness dꢁ2L.
Indeed, dimeric molecules, depending upon the lengths of ter-
minal tail and spacer, may also adopt an intercalate SmA struc-
ture wherein the spacers and terminal chains are mixed ran-
domly; in this case, the layer spacing is approximately half the
molecular length (d=1= L). Thus, the SmA phase, which can be
2
regarded as an one-dimensional (1D) mass density wave along
the director, was examined by powder XRD. In particular, XRD
measurements were performed on the unaligned SmA phase
of dimers I(R)-4, I(S)-5 and I(R)-10 as a representative cases.
The samples were placed in Lindemann glass capillaries of
1 mm diameter by capillary action in the isotropic state. Subse-
quently, both ends of the capillaries were flame sealed. The
samples were heated to their isotropic state and cooled
slowly; the XRD data were acquired as a function of tempera-
ture in the SmA phase region.
The profiles acquired at different temperatures of both sam-
ples were found to be nearly identical; specifically, the 1D in-
tensity versus diffraction (2q) profiles obtained at different
temperatures were found to be nearly the same. Figure 4a rep-
resents such profiles obtained at 1608C, 708C and 1258C for
dimers I(R)-4, I(S)-5 and I(R)-10, respectively. The temperature
dependence of the layer spacing profiles derived from the
low-angle Bragg reflections is shown in Figure 4b. Each profile
consists of a diffuse peak in the wide-angle area (2qꢁ208) and
a sharp, intense (first-order) reflection, in the small-angle
region (0<2q<58), evidently establishing the layer structure.
In fact, the presence of only the first-order layer reflection sug-
gests that the density profile along the layer normal is close to
sinusoidal. In Table 2, the spacings related to the low-angle re-
flection (d), wide-angle peak positions and the molecular
length (L) of the dimers in their all-trans geometry and the d/L
ratio are presented. The wide-angle reflection with the spacing
value centered at 4.5 ꢁ is characteristic of the liquid-like ar-
rangement of the mesogens within the layers. As expected,
the wide-angle diffuse maximum remains unaltered on passing
Figure 4. (a) The 1D intensity vs. 2q profiles obtained for the SmA phase of
dimers I(R)-4, I(S)-5 and I(R)-10 at 1608C, 708C and 1258C, respectively;
note that each trace comprises a sharp reflection in the low-angle region
and a diffuse peak in the wide-angle region. (b) The layer spacing (d) ob-
tained as a function of temperature in the SmA phase of I(R)-4, I(S)-5 and
I(R)-10.
from higher to lower temperature of the SmA phase implying
that the short-range remains liquid-like. It can be seen from
Table 1 that for dimers I(R)-4 and I(S)-5 the spacing (d) corre-
sponding to the wave vector of the mass density wave is
longer than the estimated all-trans length of the molecule with
the d/L ratio being in the range of 1.5 to 1.7 ꢁ, indicating a par-
tial bilayer structure for the SmA phase where, due to the mi-
crophase segregation, chemically compatible (like) parts of the
Figure 3. Microphotographs of the optical textural patterns of the LC phases stabilized by even-membered dimeric molecule I(R)-4: (a) the fan-like texture of
the N* phase obtained at 1758C; (b) the oily streaks characteristic of the Grandjean plane texture of the N* phase observed when the fan-like pattern-(a) was
sheared; (c) the texture seen across the N*-SmA transition at about 1748C; note that the textures of the SmA (pseudo-isotropic texture), TGB (filaments) and
N* (left portion: non-specific) are co-occurring.
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Table 2. XRD and related data of the SmA of dimers I(R)-4, I(S)-5 and
I(S)-10: The estimated all-trans molecular length (L) together with the
spacings (d) of the low-angle reflection, the wide-angle peak positions
and d/L ratio are given.
Mesogen
[L/ꢁ]
T [8C]
Layer spacing - d
(low-angle peak
position) [ꢁ]
Wide-angle peak
position [ꢁ]
d/L
160
155
150
145
140
135
130
125
70
55.77
55.40
55.12
54.89
54.67
54.54
54.49
54.34
56.61
56.58
56.582
22.44
22.45
22.46
22.48
22.49
22.50
22.30
22.31
22.318
22.32
22.31
22.30
22.28
22.30
22.29
22.29
22.27
22.28
1.467
1.457
1.450
1.444
1.462
1.435
1.433
1.430
I(R)-4
(38)
4.6
4.5
1.715
1.714
1.714
I(S)-5
(33)
Figure 5. (a) A sketch of a partial bilayer SmA (SmAd) phase formed by the
dimer having an oxytetramethyleneoxy (even) spacer: I(R)-4; L=38 ꢁ. Note
that antiparallel pairs (dotted square) self-assemble to yield SmAd phase; the
measured spacing (d) is longer than the estimated all-trans length of the
dimer and thus the d/L ratio is ꢁ1.4 ꢁ. (b) A schematic representation of an
intercalated SmA (SmAc) phase of dimer I(R)-10 where, within the chain
region, random mixing of terminal tails and the spacers occurs. Here, the
layer periodicity is equal to half the all-trans molecular length of I(R)-10
(45 ꢁ); thus, d/L ratio is ꢁ0.49 ꢁ.
65
60
125
120
115
110
105
100
95
90
85
80
75
70
65
60
55
50
45
40
0.4987
0.4991
0.4992
0.4997
0.4998
0.5000
0.4957
0.4958
0.4959
0.4960
0.4958
0.4956
0.4952
0.4955
0.4954
0.4954
0.4950
0.4951
I(R)-10
(45)
4.6
cules, by and large, disfavor the LC behavior that can be as-
cribed to the bent-conformation (Figure 1b) and thus reduced
shape-anisotropy of the mesogens. The enantiomers I(R)-3 and
I(S)-3, where the mesogenic cores are interconnected by an
oxytrimethyleneoxy spacer, were found to be non-mesomor-
phic showing a direct transition from crystal to isotropic liquid
state. The next pair of dimers, I(R)-5 and I(S)-5, showed mono-
tropic LC behavior; upon heating, they transformed directly
from the crystalline state to isotropic liquid while upon cool-
ing, they stabilized BP, N*, TGB (transient) and SmA phases; the
presence of the latter three mesophases was clearly estab-
lished by POM study as described before. Compound I(R)-9
and I(S)-9 behaved nearly in the same way with the exemption
of the TGB phase. The occurrence of BP, over a thermal range
of about seven degrees, was evidenced by optical textural ob-
servation. These dimers, placed between untreated glass slides
and cooled slowly (0.18Cminꢀ1) from the isotropic phase, show
a striking, characteristic texture of the BP (BP-I/BP-II) composed
of micron-sized bright platelets of different color. The bright
color of platelets indicates selective reflection of the periodic
structure. Indeed, the platelet pattern transforms in to a Grand-
jean plane texture when the top glass substrate was sheared
gently. It is generally observed that blue phases occur only in
strongly chiral LC systems with short helical pitch. Chirality,
being purely a symmetry property (absence of inversion sym-
metry), can be either be present or absent. However, quantita-
tive measures of chirality have been made by considering the
pitch of the helix.[52] In the case of N* phase, the structure
which is always chiral, is termed strongly or weakly chiral by
considering whether the pitch is shorter or longer, respectively.
Thus, the pitch of the N* phase appearing below the BPs
formed by these strongly chiral dimers must be short.
molecules overlap. Thus, the dimers I(R)-4 and I(S)-5 show the
interdigitated smectic A (SmAd) phase resulting from self-as-
sembly of antiparallel pairs of mesogens (Figure 5a) formed by
electrostatic interaction among the polar cyanobiphenyl
cores.[28,71] It can be seen from the data presented in Table 1
that the d-values obtained as function of temperature in the
SmA phase of dimer I(R)-10 are significantly smaller than the
estimated all-trans molecular length, and thus the d/L ratio is
~0.49 ꢁ. Indeed, this ratio is characteristic of an intercalated
SmA (SmAc) phase where, within a layer, different entities of
molecules overlap (Figure 5b). It is well known that if the
length of the terminal tail is shorter than, or comparable to,
the length of the central spacer, an intercalated structure is
formed. This type of structure is considered to be favored not
only by the electrostatic quadrupolar interaction among the
groups having quadrupole moments of opposite signs but
also due to entropy gain.[28,71] Thus, X-ray diffraction experi-
ments reveal the formation of SmAd and SmAc phases by the
dimers which is governed by the relative lengths of the spacer
and terminal tail.
Now we describe and discuss the thermal properties of four
pairs of enantiomers having an odd-numbered methylene
spacer. The data accumulated in Table 1 clearly reveal that
these odd-members, unlike even-membered dimeric mole-
The (S)-enantiomer I(S)-7 with an oxyheptoxy (7) spacer ex-
hibits, analogous to the (R)-enantiomer I(R)-7,[54] a very rich
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phase sequence. It shows BP, N*, TGB, SmA, SmAb and SmA
phases with the uniaxial smectic A phase showing re-occur-
rence. Before discussing the aforesaid phase sequence, we first
comment on the chiroptical properties of the N* phase of
both the enantiomers, I(R)-7 and I(S)-7, investigated by circular
dichroism (CD) spectroscopy. CD activity is generally observed
when the optically active medium absorbs left- and right-
handed circular polarized light differently. The N* phase, due
to its helical superstructure where the constituent nematic
layers twist continuously with respect to each other, exhibits
some unique optical phenomena such as circular dichroism
(CD) and Bragg-like light reflection.[72a–d] In a homogeneously
aligned N* phase, the mesogenic molecules adopt a macro-
scopic helical order; that is, the director rotates in a helical
fashion around a perpendicular axis. The helical sense of the
N* phase, being a chirality-dependent issue, can be right- or
left-handed; in other words, it indicates the director’s rotation
from layer to layer. Needless to say, the configuration of the
chiral element(s) of the constituent mesogens dictates the
handedness to the structure. The dimers I(R)-7 and I(S)-7,
being enantiomers of a chiral compound, should show optical
activities of equal magnitudes but with opposite
signs. Therefore, the helical screw sense of the N*
phase of dimers I(R)-7 and I(S)-7 must to be opposite
to each other. To investigate this, a CD study was un-
dertaken as the circularly polarized light (CPL), being
chiral, interacts differently with the N* phase formed
by enantiomers I(R)-7 and I(S)-7. To begin with, pre-
fabricated cells of known thickness were used to
quantify the measurements; however, the CD peaks
(saturated) obtained were found to be out of the
measurable range of the equipment. This problem
was circumvented by using thin films of the samples;
thus, the CD experiments carried out are purely quali-
tative measures. Enantiomers I(R)-7 and I(S)-7, sand-
wiched between two quartz plates, were heated
slightly above their isotropic temperatures and press-
ed to obtain thin films where the samples are uni-
formly spread. Next, the samples were cooled to the
N* phase and mechanically sheared to obtain
a planar alignment of the molecules. While cooling
the sample, the temperature-dependent CD spectra
were recorded in the entire temperature range of the
N* phase. It may be mentioned here that in the iso-
tropic phases, negligibly weak CD signals, when com-
pared to those of the N* phase, were seen. This im-
plies that the CD activity stems from the chiral induc-
tion in the LC structure but not from the molecular
chirality.
genuinely due to the macroscopic helical structure of the N*
phase but not because of LD. Moreover, the CD spectrometer
used by us also measures LD directly and hence the samples
were subjected to such a measurement; no LD activity was
found. Figure 6a and 6b, respectively, show the CD spectra ob-
tained for dimers I(R)-7 and I(S)-7; they comprise an intense
CD band in the wavelength (l) range of 490–480 nm with
a small shoulder at ~435 nm. The intense CD band is arising
due to the n–p* transition of the chromophores present in the
chiral environment; in other words, the chromophores of
dimers show optically active electronic transitions in the visible
region of the CD spectrum. The presence of a positive CD
band has been generally attributed to a right-handed screw
sense for the N* phase derived from lyotropic systems and
cholesterol-based mesogens;[52,72b–d] we therefore presume that
the helices in the N* phases of enantiomer I(R)-7 and I(S)-7, ex-
hibiting positive and negative CD bands, have a right- and left-
twisted structure, respectively.
We now discuss the thermal behavior of dimer I(S)-7. When
a thin film of the compound held between a clean glass slide
and a coverslip was gradually cooled (0.18Cminꢀ1) from the
It is well known that, in the absorption region, the
visible CD band may combine linear dichroism (LD)
and birefringence. However, this possibility was ruled
out by recording the temperature-dependent CD
spectra of the samples subjected to 908 in-plane ro-
tation. The recorded spectra were found to be virtu-
ally identical to those obtained in the original posi-
Figure 6. CD spectra recorded as a function of temperature in the N* phase formed by
tion of the samples, suggesting that CD activity is the mirror-image isomers I(R)-7 (a) and I(S)-7 (b)
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isotropic phase, a texture appears sharply manifesting optically
in the form of a misty blue texture (without any structure in it)
(Figure 7a) characteristic of the blue phase III (BPIII). Subse-
quently, bright platelets of different colors begin to appear on
top of the foggy pattern (Figure 7b) and eventually fill the
field of view implying the occurrence of another BP; the pres-
ence of crinkled lines between the plates is indicative of BPI.
As expected, the Grandjean plane pattern appears if the BPI is
subjected to mechanical stress. On cooling the sample from
BPI, the N* phase, through a transient TGB phase, comes into
existence at ~738C with an uncharacteristic optical texture
which upon shearing placidly changes to oily streaks typical of
the Grandjean plane texture (Figure 7c) where the helical axis
lies perpendicular to the glass plates (along the viewing direc-
tion). Upon lowering the temperature of the sample further,
a change to a focal-conic fan coexistent with pseudo-isotropic
domains occurs. The focal-conic fan texture supports a layered
structure, whereas homeotropic regions imply an orthogonal
organization of the molecules with respect to the layer planes.
These textural patterns hint towards the existence of the SmA
phase having uniaxial symmetry; the presence of this phase
was confirmed indubitably by the observation of a dark field
of view in slides treated for homeotropic anchoring conditions
and a focal-conic fan texture (Figure 7d) in glass substrates
treated to induce homogeneous orientation. The presence of
a short-lived TGB phase was determined by the observation of
filamentary texture when the homeotropically aligned SmA
phase was slowly heated towards N* phase (Figure 7e); even-
tually, the filamentary texture coalesces to give a short-lived
non-specific texture.
Further cooling of the sample caused a transition to another
mesophase sharply at 53.68C with a notable change in the op-
tical textural pattern; as shown in Figure 7 f, the homeotropic
(pseudo-isotropic) region and homogeneous (focal-conic) pat-
tern of the preceding SmA phase transform to a low birefrin-
gence schlieren texture and a stripped focal-conic texture, re-
spectively. The schlieren texture in the homeotropic orientation
is suggestive of the director field in the plane of the cells.
These optical textural features are typical of an orthogonal
biaxial SmA (SmAb) phase[53,54,58,73–84] which possesses an addi-
tional director (m) in the plane of the layers (see a later section
for details). For the sake of confirmation, the sample was ex-
amined in cells treated for homeotropic and homogeneous an-
choring. As expected, a focal-conic texture having faint stripes
on top of fans was observed (Figure 7g) when the sample was
studied using substrates treated for homogeneous configura-
Figure 7. Photomicrographs of the textures observed under POM for the mesophases of odd-membered dimeric molecule I(S)-7: (a) the misty blue texture of
the BP-III coming into view immediately below the isotropic phase; (b) the platelet texture of the BP-I appearing over the blue fog of BP-III; (c) the oily streaks
typical of the Grandjean plane texture of the N* phase (858C); (d) the focal-conic fan texture of the planarly aligned SmA phase (658C); (e) filamentary texture
of the TGB phase originating from the homeotropic domains of the SmA phase; (f) texture of the SmAb phase (538C) comprising both schlieren pattern (with
vigorous flickering) and focal conics (having fine striations across the fans), respectively, stemming from the homeotropic and homogeneous alignment of the
dimer; (g) focal-conic texture of the planarly aligned SmAb phase where faint bars can be observed (52.58C) (see the marked area); (h)-(i) the schlieren texture
with two- and four-brush defects (see the circled area) and the stripped pattern seen for the homeotropically oriented SmAb phase (52.58C).
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tion. Subjecting the sample to homeotropic boundary condi-
tions yielded a schlieren texture comprising both two-brush
1
(strength s= ꢂ = ) and four-brush (s= ꢂ1) defects with
2
a strong director fluctuation (Figure 7h); besides, in some re-
gions of the cell, a striped texture over the schlieren texture
with alternate bright and dark bands moving across the
sample also existed (Figure 7i). The occurrence of both integer
1
and half-integer defects, particularly of the s= ꢂ = defect,
2
rules out the LC phase being a smectic C (SmC) phase where
the tilted organization of mesogens allows the disclination of
integer strength exclusively. Using a tilting plate compensator,
the quantitative in-plane birefringence (Dm) was measured,
which gave a value of 0.002 (52.58C) which is smaller than that
measured for the dimer I(R)-7.[54] As the temperature is low-
ered further, the sample exhibits, at 51.28C, an optical texture
where the schlieren and the striped patterns disappear sharply
leaving a dark (pseudo-isotropic) field of view; in the planarly
aligned regime, a focal-conic texture typical of an uniaxial SmA
phase was observed where transition bars across the fans
(seen for the SmAb phase) vanish totally; upon shearing gently,
a pseudo-isotropic pattern was observed solely, implying a com-
plete homeotropic alignment of the mesogens. The existence
of the SmA phase below the SmAb phase was substantiated
based on the observation of a focal-conic pattern and dark tex-
ture when the sample was examined, respectively, in between
glass slides treated for homogeneous and homeotropic an-
choring. Thus, the POM study reveals that dimer I(S)-7 exhibits
a re-entrant phase sequence, SmA-SmAb-SmA, below the BP-
N*-TGB phases.
Figure 8. Photomicrographs of the orthoscopic (a–c) and conoscopic (d–f)
patterns observed for the LC phases of a homeotropically aligned dimer
I(S)-7: the SmA phase at 578C (a,d); the SmAb phase at 53.58C (b,e); re-en-
trant SmA phase 508C (c,f). Note that in the conoscopic pattern (e) of the
SmAb phase isogyres are separated.
Optical conoscopy is an important tool that greatly helps in
distinguishing uniaxial and biaxial LC phases. In this technique,
the sample is observed between crossed polarizers with the
optical axis remaining along the direction of light propagation.
Experiments were carried out on a homeotropically anchored
sample wherein the molecules are uniformly oriented with the
director (n) normal to the substrate. A cell was fabricated
using an indium tin oxide (ITO)-coated glass plate and an ordi-
nary glass plate; the inner surfaces of these substrates were
treated with octadecyltriethoxysilane for homeotropic orienta-
tion of the molecules. The cell comprised two aluminium
10 mm-thick strips separated by ~0.5 mm. The sample was
heated to its isotropic state and filled into the cell by capillary
action. After cooling slowly from the isotropic phase to the
higher temperature of SmA phase, conoscopic observation
was made between the crossed polarizers set to 458 to the di-
rection of the in-plane electric field (sine-wave field, 310 Vpp,
90 Hz); as expected, a typical crosslike uniaxial interference
pattern (Maltese cross) was observed. As the temperature was
lowered further, a biaxial pattern in which the isogyres split
was obtained at ~538C and the gap between the isogyres re-
mained unaltered until about 51.58C. On reducing the temper-
ature further, the arms of the cross (isogyres) begin to come
closer and merge at ~518C to give a Maltese cross indicating
that the lower temperature phase is indeed unaxial. The ortho-
scopic and conoscopic patterns seen for the SmA phase (at
578C), SmAb phase (53.58C) and re-entrant SmA (50.58C) are
respectively shown in Figure 8a–c and 8d–f. While the well-
separated isogyres are seen for the SmAb phase (Figure 8e),
the SmA (Figure 8d) and re-entrant SmA (Figure 8 f) phases ex-
hibit uniaxial interference patterns. Indeed, the conoscopic pat-
terns were also observed in the absence of an external field
where the splitting of isogyres in the SmAb phase was found
to be very weak. In view of all of the foregoing observations it
can be concluded that the dimer I(S)-7 shows the BP-N*-TGB-
SmA-SmAb-SmA sequence with the uniaxial smectic A phase
being re-entrant.
As mentioned earlier, the thermal behavior and overall con-
formation of LC dimers decisively depends upon the length
and parity of the spacer. Figure 9 presents, in the form of a hor-
izontal bar chart, the dependence of the LC behavior of the
cooling cycle of (R)-enantiomers, except the dimer I(R)-3 as it
is non-mesomorphic, on the number of carbon atoms in the
central spacer. It especially shows the odd-even effect, the in-
fluence of length and parity of the spacer on the phase se-
quences, transition temperatures, and clearing temperatures.
In general, the thermal behavior of the present series of com-
pounds was found to be identical to that of LC dimers report-
ed hitherto.[28–31,52] For example, the clearing temperatures of
the even-members with short-spacer lengths (n=4, 6, and 8)
vary considerably showing higher values. However, upon in-
creasing the spacer length (n=10) this effect seems to attenu-
ate; by contrast, the compounds with an odd-numbered meth-
ylene spacer show lower clearing temperatures. As discussed
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(sanidic) or banana-shaped, respectively. In other words, the
apolar SmAb phase formed by sanidic molecules possesses D2h
symmetry and nematic-type biaxiality, while banana-like meso-
gens stabilize the SmAb phase of C2V symmetry where at least
one direction is polar. However, it is interesting to note that
the former type of SmAb phase occurs less commonly[54,58,73–75]
as compared to the latter structure.[76–83] In fact, the present
study is the extension of our previous work reported on
a chiral dimer I(R)-7 that showed the SmAb phase in an unique
and unprecedented phase sequence, that is, SmA-SmAb-SmA
(re-entrant). As expected, such a phase sequence could be
only observed in the (S)-enantiomer, I(S)-7, of the above dimer,
indicating that designing the mesogens capable of stabilizing
the SmAb phase or the aforesaid phase sequence is not a trivial
task necessitating the development of either single-compo-
nent LC systems or mixtures; however, a literature survey sug-
gests that the latter strategy is proven to be fruitful and less
cumbersome.[74,77,83]
Figure 9. A horizontal bar graph depicting the dependence of mesomorphic
behavior during cooling cycle (phase sequence, phase transition tempera-
tures and clearing temperatures) on the number of carbon atoms of the
central spacer of (R)-enantiomers. Note that the TGB phase is not included
in the profile as it exists over a very short thermal range.
Binary mixtures derived from mixing of two pure mesogens
closely resembling in their molecular structure and geometry
(conformation) are known to stabilize thermodynamically
stable LC phases of the constituent compounds.[86] For exam-
ple, it has been observed that a binary mixture made from two
nematogens with nearly identical chemical structures and geo-
metries show an enantiotropic nematic phase.[86–88] These ob-
servations prompted us to select two mesogens exhibiting the
SmAb phase. Scheme 1 shows the molecular structures of the
two LCs, along with their LC behavior, used for the preparation
of binary mixtures. Precisely, the achiral dimer I-5, 4’-((5-(4-(((4-
(decyloxy)phenyl)imino)methyl)-3-hydroxyphenoxy)pentyl)oxy)-
[1,1’-biphenyl]-4-carbonitrile,[58] was mixed with the chiral
dimer I(R)-7.[54] The weight ratio percentages in the nine binary
mixtures of I-5 and I(R)-7, respectively, are 10:90 (M1-9), 20:80
(M2-8), 30:70 (M3-7), 40:60 (M4-6), 50:50 (M5-5), 60:40 (M6-4),
70:30 (M7-3), 80:20 (M8-2) and 90:10 (M9-1); thus, weight per-
centages will be used throughout this text. Thorough mixing
of the dimers I-5 and I(R)-7 was achieved by blending in their
isotropic state. The dimers were weighed in their crystalline
form using a microbalance with an accuracy of 1 mg; subse-
quently, the mixtures were heated to about 1458C on a hot
plate (equipped with a thermometer) and stirred continuously
for about 5 min to obtain well-homogenized bi-component
systems.
previously, such behavior can be interpreted in terms of the
overall conformation (linear- or bent-shape) of the molecules
governed by the geometry and flexibility of the central spacer,
which basically affects the mutual orientation of the mesogen-
ic entities. Another interesting aspect to be noted is that all
the odd-members (with n=5, 7, and 9), except dimers I(R)-3/
I(S)-3, show BPs; this behavior is in complete agreement with
the general observation that chiral dimers with an odd-parity
spacer show BPs, which is not the case with even-membered
dimers, suggesting that the chirality of the liquid crystal struc-
ture shows a dependence on the parity of the spacer.[28] Per-
haps, odd-members, being bent-shaped, reduce the pitch of
LC phases due to a smaller twist elastic constant that is associ-
ated to their lower orientational order.[28]
Binary mixture study
A binary mixture study was undertaken mainly to explore the
possibility of stabilizing the SmAb phase and/or SmA-SmAb-
SmA (re-entrant) phase sequence. Among the various types of
smectic LC (layered) phases, which essentially differ in their
molecular packing, symmetry of 1D and 2D lattices, and tilt
angle with respect to the layer normal, the optically uniaxial,
monolayered SmA phase having D symmetry is the simplest
The thermal behavior and phase transition temperatures of
the mixtures determined using POM in conjunction with a hot
stage are listed in Table 3. The phase diagram, which illustrates
LC phases (sequences) exhibited by different mixtures of two
mesogens over a range of temperatures, is also shown in
Figure 10. As can be seen, the compositions move from pure
dimer I-5 on the left, through nine mixtures, to pristine I(R)-7
on the right of the diagram. It must be mentioned here that I-
5 mixes well with the chiral dimer I(R)-7 at all the concentra-
tions, as evidenced by sharp phase transitions (width
~500 mK) observed under POM. This implies a good structural
and geometrical compatibility of both compounds. In all mix-
tures, the SmAb and/or re-entrant SmA phases exist as meta-
stable (monotropic) LC phases. The first three mixtures M1-9,
1h
and perhaps one of the most exhaustively studied smectic
phases.[69,70] In this phase, the mesogens within the layer are
oriented along the layer normal (director n) and freely rotate
about their long molecular axis. On the contrary, one of the
most sought after phases in the SmA LCs is the SmAb
phase[54,58,73–76] as it is supposed to be a very promising and
viable medium for future advanced technologies.[84] In this
phase, the constituent molecules are oriented along the layer
normal (director n), but have an additional director (m) in the
plane of the layers due to the restricted rotation of mesogens
around their long molecular axis. The symmetry of this phase,
however, is proposed[85] to be dependent on the type of con-
stituent mesogens: D2h or C2V if the molecules are board-like
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Table 3. LC Phase sequences and transition temperatures (oC)[a] of nine binary mixtures formed by mixing achiral dimer I-5 (10 to 90 wt%) with optically
active dimer I(R)-7 (90 to 10 wt%). Cr=Crystal; SmA=uniaxial smectic A phase; SmAb =biaxial smectic A phase; TGB=twist grain boundary phase; N*=
chiral nematic phase; BP=blue phase; I=isotropic liquid state.
Mixture
Phase sequence
Wt% ratio
I-5:I(R)-7
Heating
Cooling
M1–9
(10:90)
M2–8
(20:80)
M3–7
(30:70)
M4–6
(40:60)
M5–5
(50:50)
M6–4
(60:40)
M7–3
(70:30)
M8–2
(80:20)
M9–1
(90:10)
Cr 67.9 SmA 73 TGB[b]-N* 103.1 I
I 101.5 BP 92.8 N*-TGB[b] 70.2 SmA 52.1 SmAb 51.2 SmA (re-entrant) 35 Cr
Cr 69.5 SmA 79 TGB[b]-N* 106.1 I
Cr 69.0 SmA 79.8 TGB[b]-N* 108 I
Cr 83.8 SmA 86.5 TGB[b]-N* 113 I
Cr 100 SmA 105.7 TGB[b]-N* 128 I
Cr 97 SmA 102.1 TGB[b]-N* 122.6 I
Cr 100.9 SmA 108.4 TGB[b]-N* 127 I
Cr 106.2 SmA 122.4 TGB[b]-N* 136.9 I
Cr 107 SmA 124.9 TGB[b]-N* 138 I
I 104.3 BP 100 N*-TGB[b] 75.2 SmA 56.2 SmAb 48.2 SmA (re-entrant) 30 Cr
I 104.1 BP 100 N*-TGB[b] 75.9 SmA 60.1 SmAb 43.1 SmA (re-entrant) 30 Cr
I 112 BP 109 N*-TGB[b] 83.3 SmA 61.8 SmAb 42 Cr
I 126 N*-TGB[b] 104.2 SmA 77.7 SmAb 65.3 Cr
I 122 N*-TGB[b] 101.8 SmA 73.1 SmAb 60.9 Cr
I 126.3 N*-TGB[b] 107.2 SmA 76.2 SmAb 63.5 Cr
I 136 N*-TGB[b] 122.3 SmA 82.1 SmAb 76.1 Cr
I 137.7 N*-TGB[b] 124.3 SmA 92.1 SmAb 74.9 Cr
[a] The phase transitions and the corresponding temperatures were deduced from polarizing microscope study. [b] A transient phase.
is, BP-N*-TGB-SmA-SmAb-SmA (re-entrant) that was adjudged
mainly based on the textural observation under POM.
On slow cooling the isotropic liquid of these mixtures held
between clean glass slides and observed under a POM, a char-
acteristic blue fog texture of the amorphous BPIII phase ap-
pears and following this a colorful platelet texture featuring
crinkled lines among the plates of BPI grows instantaneously.
On cooling further, the N* phase comes into existence, which
upon shearing showed oily streaks. Upon lowering the temper-
ature, a transition from N* phase to SmA phase occurs with
textural pattern comprising both focal-conic fan and homeo-
tropic textures; Figure 11a represents such an pattern ob-
served for the mixture M3-7 [I-5:I(R)-7; 30:70]. If the sample is
heated slowly from the homeotropically aligned SmA phase,
the filaments of the TGB phase grow which eventually coalesce
to an unfamiliar texture. On cooling the sample further, the
SmA phase transforms into the SmAb phase with homeotropic
regions exhibiting a schlieren pattern with a strong director
fluctuation (Figure 11b). Indeed, the schlieren texture consisted
of both two- and four-brush strength defects. Besides, in some
Figure 10. The phase diagram of the mixtures of dimers I-5 and I(R)-7.
M2-8 and M3-7, behave exactly analogous to that of chiral
dimer I(R)-7 exhibiting a rich poymesomorphic sequence, that
Figure 11. Photomicrographs of the optical textures seen for the LC phases of binary mixture M3-7 [I-5:I(R)-7; 30:70]: (a) texture of SmA phase comprising
both focal-conic fan texture and homeotropic regions (658C); (b) texture of the SmAb phase consisting of both stripped focal-conic fan and schlieren patterns
(558C); (c) the pattern observed for the homeotropically oriented SmAb phase (568C) where alternate bright and dark bands persist over the schlieren tex-
ture.
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of regions of the sample, a striped texture with alternate
bright and dark bands could also be observed (Figure 11c). In
the planar regions, very fine striations appear across the elon-
gated fans of the focal-conic texture (Figure 11). As the sample
in the homeotropic case was cooled further, the SmA phase re-
appears where the striped and the schlieren textures vanish
completely leaving a dark field of view. Conoscopic studies
were carried out to confirm the existence of biaxial and uniax-
ial SmA phases. The homeotropic cells required for the experi-
ments were fabricated first, as described before, and then the
samples were inserted in them. The observations between
crossed polarizers were made in the SmA, SmAb and SmA (re-
entrant) phases. As a representative case, the conoscopic pat-
terns seen for mixture M2-8 [I-5:I(R)-7; 20:80] are shown in Fig-
ure 12a–c. As can be seen, the high- and low-temperature (re-
entrant) SmA phases show uniaxial interference patterns (Fig-
ure 12a,c), while for the middle SmAb phase the isogyres split
to give a biaxial pattern (Figure 12b). It can be seen that the
thermal range of SmAb phase in M1-9 is about 18C, which is
less than that present in the case of dimer I(R)-7 (ꢁ2.78C). In-
terestingly, the SmAb phase, at the expense of re-entrant SmA
phase, becomes thermally more stable in the mixtures M2-8
and M3-7; in the latter case, the phase exists over a tempera-
ture range of 188C.
Conclusion
In this article, the results of our extensive studies encompass-
ing the synthesis and evaluation of phase transitional behavior
of sixteen non-symmetric polar dimers, in the form of eight
enantiomeric pairs are presented; the study also includes prep-
aration and characterization of binary mixtures. The dimers
were made by covalently linking an electron-deficient cynobi-
phenyl core and an electron-rich salicylaldimine unit through
a conformationally flexible alkylene spacer. Each pair of enan-
tiomers has been derived from (R)-2-octyloxy and (S)-2-octyloxy
tails. The number of methylene units in the flexible spacer has
been varied from 3 to 10 yielding eight pairs. Thus, the effect
of varying the length and parity of the spacer on thermal prop-
erties has been examined with the aid of polarized optical mi-
croscopy, differential scanning calorimetry and powder X-ray
diffraction. The length and parity of the spacer greatly influen-
ces the thermal behavior of dimers. The members with odd-
parity (5, 7 and 9) spacers, except a pair of non-mesomorphic
enantiomers with oxypropoxy (3) spacer, show commonly blue
phases (BPs) and chiral nematic (N*) phase, while a pair of
enantiomers with oxypentoxy (5) spacer shows additionally
a twist grain boundary (TGB) phase and smectic A (SmA)
phases. Notably, the (S)-enantiomer with oxyheptoxy (7) spacer
shows, analogous to its counterpart (R)-enantiomer reported
earlier by us, TGB, SmA, biaxial SmA (SmAb), and re-entrant
SmA (SmAre) phases as well. The orthoscopic and conoscopic
patterns confirm occurrence of the SmA-SmAb-SmAre sequence.
All the dimers with even-membered (4, 6, 8 and 10) spacers
stabilize the N* phase, while the members bearing 4 and 10
methylene units in the spacer show additionally TGB and/or
SmA phases. X-ray measurements reveal the interdigitated or
intercalated arrangement of mesogens in the SmA phase,
which is not only associated with the interplay among the rela-
tive lengths of the spacer and terminal tail but also linked to
electrostatic quadrupolar interactions between dissimilar meso-
genic entities. CD experiments were carried out to reveal the
handedness of the N* phases stabilized by two enantiomers;
mirror-imaging CD bands of virtually equal magnitude but
with opposite sign were obtained, confirming the enantiomer-
ic purity of the isomers. To develop materials capable of show-
ing the SmAb phase or a rare phase sequence (i.e., SmA-SmAb-
SmAre sequence), binary mixtures of two known, achiral and
chiral, dimers were prepared and characterized. The occurrence
of interesting phase sequences such as BP-N*-TGB-SmA-SmAb-
SmAre, BP-N*-TGB-SmA-SmAb and N*-TGB-SmA-SmAb were es-
tablished; notably, the latter two sequences appear to be unre-
ported hitherto. Thus, the dimers and mixtures studied herein
serve as model systems for the design and development of
smectic phase having D2h symmetry and nematic-type biaxiali-
ty.
Figure 12. Photomicrographs of the conoscopic patterns observed for ho-
meotropically aligned smectic phases of mixture M2-8 [I-5:I(R)-7; 20:80]:
(a) The SmA phase (708C); (b) the SmAb phase (528C); (c) re-entrant SmA
phase (408C). Note that isogyres are moderately separated in the conoscopic
pattern (b) of the SmAb phase.
The mixture M4-6 shows the sequence BP-N*-TGB-SmA-
SmAb while cooling from the isotropic phase. In this case,
unlike in the abovementioned mixtures, the re-entrant SmA
vanishes completely whereas the SmAb phase stabilizes over
a wider thermal range of ~208C. The next five mixtures
namely, M5-5, M6-4, M7-3, M8-2 and M9-1 display a phase se-
quence viz., N*-TGB-SmA-SmAb upon cooling wherein the BP
gets extinguished. In mixtures M5-5, M6-4, M7-3 and M8-2 the
SmAb phase occurs in the thermal range of 6-128C which is
slightly less than (148C) that is present in the pure dimer I-5.
However, in the case of M9-1 it increases to ~178C. Thus, all
the mixture prepared herein exhibit SmAb phase and in some
cases it appears in a sequence of SmA-SmAb-SmA (re-entrant).
This implies that the phase transitional behavior of binary mix-
tures of the non-symmetric dimers is possibly determined by
molecular structural similarity and conformation. Such structur-
al parameters facilitate thorough core–core interactions of
mesogens leading to their excellent miscibility.
Experimental Section
Materials, methods and instrumentation
Unless otherwise stated, all reagents procured from commercial
chemical companies were used as received. 4-Nitrophenol, anhy-
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drous potassium carbonate, potassium iodide, acetic acid, etc.
were purchased from local sources, whereas 2,4-dihydroxy benzal-
dehyde, (R)-2-octanol, (S)-2-octanol, triphenylphosphine, diisoprop-
yl-azodicarboxylate, 10% Pd-C, and a,w-dibromoalkanes were ob-
tained from overseas companies. Absolute ethanol was distilled
twice with magnesium ethoxide (magnesium turnings and iodine);
butanone was stored over anhydrous K2CO3 for 24 h and then sub-
jected to fractional distillation. THF was dried over sodium and
benzophenone under inert atmosphere. Ethyl acetate and hexanes
obtained from local sources were distilled twice before use. Ultra-
pure nitrogen (99.99%) was used for maintaining the inert atmos-
phere. For column chromatography, silica gel (60–120 and 100–
200 mesh) was used as the stationary phase. Thin layer chromatog-
raphy (TLC) was performed on aluminum-backed sheets pre-
coated with silica gel (0.2 mm, Merck, Kieselgel 60, F254); this tech-
nique was employed to assess the purity of the compounds, espe-
cially to monitor the progress of the reactions. The TLC plates were
either stained with iodine vapor or exposed to ultraviolet light for
the detection of organic spots.
Acknowledgements
This work is fully funded by SERB, DST, Govt. of India, under Proj-
ect No. SR/S1/OC-04/2012. The corresponding author expresses
his gratitude to SERB for this support.
Keywords: chiral oligomers · enantiomers · liquid crystals ·
phase transitions · structure–property relationship
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Manuscript received: July 2, 2016
Accepted Article published: && &&, 0000
Final Article published: && &&, 0000
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FULL PAPER
Soft Matter
Eight new pairs of enantiomeric
dimers, synthesized by joining cyanobi-
phenyl and salicylaldimine cores via
a flexible spacer of varying length and
parity, including binary mixtures derived
from such dimeric motifs, are evidenced
to show helical, frustrated, optically
biaxial and re-entrant liquid crystal
phases by orthoscopic, conoscopic, dif-
ferential scanning calorimetric and X-ray
powder diffraction techniques.
Vediappen Padmini,
Palakurthy Nani Babu, Geetha G. Nair,
D. S. Shankar Rao,
Channabasaveshwar V. Yelamaggad*
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Optically Biaxial, Re-entrant and
Frustrated Mesophases in Chiral, Non-
symmetric Liquid Crystal Dimers and
Binary Mixtures
Chem. Asian J. 2016, 00, 0 – 0
www.chemasianj.org
15
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ