Highly frustrated liquid crystal phases in optically active dimers: Synthesis and rich phase transitional behavior

Article abstract of DOI:10.1039/c8nj03520b

Herein we report on the synthesis and characterization of four new series of optically active, nonsymmetric dimers in which cholesterol is covalently linked to a Schiff base core through an ω-oxyalkanoyl spacer. While the Schiff base core is substituted with n-butyloxy, n-hexyloxy, n-octyloxy, n-decyloxy and n-dodecyloxy tails, three even-parity spacers, namely, 4-oxybutanoyl, 6-oxyhexanoyl, 8-oxyoctanoyl, and an odd-parity spacer, namely, 5-oxypentanoyl, have been used to join the two cores. The experimental results show that the length and parity of the spacer and the length of the terminal tail play a vital role in deciding the phase sequences of the dimers. In general, the dimers possessing an even-parity spacer display enantiotropic LC phases such as chiral nematic (N?), twist grain boundary (TGB), smectic A (SmA), chiral smectic C (SmC?) and twist grain boundary phase with SmC? slabs (TGBC?). Some of these dimers display TGBC? over a wide temperature range. The dimers with an odd-parity (5-oxypentanoyl) spacer display, unlike their even-membered counterparts, blue phases (BPIII/II/I); besides, they stabilize N? and/or unknown smectic (SmX) phases. The circular dichroism (CD) measurements were carried out as a function of temperature on the planar texture formed by three even-membered dimers and an odd-membered dimer. The occurrence of a strong negative CD band in the N? phase of the even-membered dimers suggests a left-handed screw sense of the macroscopic helical structure, and the scenario is opposite in the case of an odd-membered dimer.

Full text of DOI:10.1039/c8nj03520b

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Highly frustrated liquid crystal phases in optically
active dimers: synthesis and rich phase transitional
behavior†
Cite this:DOI: 10.1039/c8nj03520b
Rashmi Ashwathama Nayak,‡*a Sachin A. Bhat,a G. Shanker,b D. S. Shankar Raoa
a
and C. V. Yelamaggad
*
Herein we report on the synthesis and characterization of four new series of optically active, nonsymmetric
dimers in which cholesterol is covalently linked to a Schiff base core through an o-oxyalkanoyl spacer. While
the Schiff base core is substituted with n-butyloxy, n-hexyloxy, n-octyloxy, n-decyloxy and n-dodecyloxy tails,
three even-parity spacers, namely, 4-oxybutanoyl, 6-oxyhexanoyl, 8-oxyoctanoyl, and an odd-parity spacer,
namely, 5-oxypentanoyl, have been used to join the two cores. The experimental results show that the length
and parity of the spacer and the length of the terminal tail play a vital role in deciding the phase sequences of
the dimers. In general, the dimers possessing an even-parity spacer display enantiotropic LC phases such
as chiral nematic (N*), twist grain boundary (TGB), smectic A (SmA), chiral smectic C (SmC*) and twist grain
boundary phase with SmC* slabs (TGBC*). Some of these dimers display TGBC* over a wide temperature
range. The dimers with an odd-parity (5-oxypentanoyl) spacer display, unlike their even-membered
counterparts, blue phases (BPIII/II/I); besides, they stabilize N* and/or unknown smectic (SmX) phases. The
circular dichroism (CD) measurements were carried out as a function of temperature on the planar texture
formed by three even-membered dimers and an odd-membered dimer. The occurrence of a strong negative
CD band in the N* phase of the even-membered dimers suggests a left-handed screw sense of the
macroscopic helical structure, and the scenario is opposite in the case of an odd-membered dimer.
Received 14th July 2018,
Accepted 3rd December 2018
DOI: 10.1039/c8nj03520b
rsc.li/njc
exist in a variety of LC oligomers.2 Fundamentally, the intrinsic
self-assembly of oligomesogens into LC phases is rather different
I. Introduction
Liquid crystal (LC) oligomers (oligomesogens) formed by from that of the conventional (monomeric) low molar mass
covalently tethering two or more mesogenic cores through LCs.1 For example, the first (simplest) constituent of oligomers,
flexible spacer(s) have been attracting considerable scientific namely LC dimers, wherein two mesogens are bound to each
attention largely due to the fact that they constitute a novel other via a flexible (polymethylene) spacer, displays atypical
class of nonconventional mesomorphic materials providing a mesomorphism which varies from those of the corresponding
plethora of opportunities to modulate the thermal properties monomeric mesogens.1a,d,3,4 In fact, their thermal behavior
and other functional characteristics with great ease.1–4 Inter- depends not only on the chemical nature of two constituent
estingly, in a recent review article by Richard J. Mandle, one of mesogenic segments but also on the length and parity of
the most remarkable phase transitional properties of oligo- the central spacer.3 While the symmetric LC dimers, made by
mesogens, which was hitherto masked, has been revealed: the joining two chemically identical mesogenic units, display a
existence of the twist-bend nematic (Ntb) phase, which signifies rich smectic polymorphism, the non-symmetric LC dimers com-
a structural correlation among the well-known uniaxial nematic prising two chemically dissimilar mesogens show intercalated
(N) and the chiral nematic (N*) phases, has been reported to structures. Indeed, both classes of dimers show an archetypal
spacer parity directed odd–even effect.1a,d,3 That is, the clearing
a Centre for Nano and Soft Matter Sciences, P. B. No. 1329, Prof. U. R. Rao Road,
Jalahalli, Bengaluru 560013, India. E-mail: yelamaggad@cens.res.in,
yelamaggad@gmail.com
(LC phase to isotropic phase) transition temperatures and the
associated entropies show a pronounced alternation when the
spacer parity is varied with even or odd number of methylene
units. This behavior has generally been ascribed to the dependence
of the overall shape of the dimeric molecules on the parity of the
spacer in all-trans conformation. A dimer with an even-parity spacer
attains a rod-like shape where the mesogenic units are antiparallel,
b Department of Chemistry, Jnana Bharathi Campus, Sneha Bhavan,
Bangalore University, Bengaluru, 560056, India
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8nj03520b
‡ Present address: CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road,
Pune-411 008, India.
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whereas in a dimer with an odd-numbered spacer, the mesogens segments display the richest variety of mesophases. Hitherto
are inclined with respect to each other to give an overall bent accumulated experimental results show that the mesomorphism
shape to the molecule. The bent shape of the odd-spaced dimer of such dimers is extremely sensitive to the nature of Schiff base
obviously decreases the molecular shape anisotropy and thus the and its terminal substituents. On the other hand, it is well known
order of the LC phase (due to inefficient packing), so clearing that the thermal behavior of Schiff base LCs can be altered
temperatures and the associated enthalpy/entropy values are effectively if the imine (azomethine) linkage is reversed.21 Hence,
lower in such dimers.
in continuation of our earlier study on such LCs,7c we intended to
In recent years, chirality5 in LC dimers has become one of realize cholesterol-based Schiff base dimers where the orientation
the major research topics not only for fundamental research (direction) of the imine linkage is reversed. Accordingly, we
reasons but also from the technological point of view.1a,d,3,4 have synthesized and investigated the thermal behavior of four
This is mainly because molecular asymmetry imparting form series of cholesterol-based dimers comprising Schiff base
chirality to LC dimers has profoundly influenced their thermal wherein the direction of imine linkage is inverted (see Chart 1).
properties. In fact, several remarkable phase transitional properties The even-parity spacers such as 4-oxybutanoyl(trimethylene),
have been witnessed in a wide variety of both symmetric and 6-oxyhexanoyl(pentamethylene) and 8-oxyoctanoyl(heptamethylene),
nonsymmetric LC dimers.1a,d,3,4 This is especially noteworthy in the and an odd-parity spacer 5-oxypentanoyl(tetramethylene) have been
case of optically active, nonsymmetric dimers in which cholesterol used to join the two mesogenic segments. Given the fact that a slight
is covalently linked to a conventional calamitic (two-ring meso- change in the nature and length of the terminal chain affects both
genic) core via a flexible spacer of varying length and parity. Their transition temperatures and mesophase sequences that will be
unique molecular structural characteristics, mostly stemming from formed, in the present study the Schiff base unit is substituted with
the presence of cholesterol, have not only enabled them to display n-butyloxy, n-hexyloxy, n-octyloxy, n-decyloxy and n-dodecyloxy tails.
exceptional phase sequences, highly frustrated fluid structures, These new dimers are represented by the mnemonic DSB-n,R
reentrant phases, etc., but also provided distinctive strategies and where DSB denotes dimeric Schiff base, n indicates the total
platforms to modulate the properties of LC phases featuring number of methylene carbons solely (excluding the carbonyl
technological values.4 Jin and co-workers were the first to work carbon) in the spacer and R signifies the n-alkoxy tail. A general
on cholesterol-based Schiff base dimers, I-1 to I-4 (Chart 1), molecular structure of the four series of cholesterol-based non-
reporting the occurrence of the richest variety of mesophases in symmetric dimers (DSB-3,R, DSB-4,R, DSB-5,R and DSB-7,R)
one of the dimers (I-1) which included the fluid incommensurate realized in the present study is shown in Chart 1.
smectic (Smic) phase.6 Inspired by these findings, a wide variety of
cholesterol-based dimers numbering over 400 have been realized
II. Synthesis and molecular
where mesogenic cores such as Schiff base,6,7,8c,d o-hydroxy Schiff
base (salicylaldimine),8 isoflavone,9 sydnone,10 azobenzene,11 bent structural characterization
core,12 cinnamate,13 chalcone,14 1,4-diphenyl-1H-1,2,3-triazole,15
biphenyl,16 diphenylbutadiene,17 tolane,18 1,3,4-oxadiazole/
1,2,4-oxadiazole/1,3,4-thiadiazole,19 and pentaalkynylbenzene20
have been interlinked with cholesterol to understand the
structure–property correlations. In essence, the molecular
architecture of cholesterol-based dimers is such that it provides
strategies to design multifunctional soft materials through the
incorporation of suitable molecular fragments such as spacers,
mesogens and terminal segments.
Four series of target Schiff base dimers, DSB-3,R, DSB-4,R, DSB-5,R
and DSB-7,R, were prepared as shown in Scheme 1. Cholesteryl
o-bromoalkanoates (1a–d) were prepared in excellent yields
by the esterification of cholesterol with freshly synthesized
4-bromobutanoyl, 5-bromopentanoyl, 6-bromohexanoyl or
8-bromooctanoyl chlorides as described in the literature.16b,22
These alkanoates 1a–d were reacted with 4-nitrophenol to get
the corresponding nitro derivatives 2a–d which were subs-
equently subjected to catalytic hydrogenation (10% Pd–C;
H2 1 atm., balloon), yielding the key amines 3a–d.
As mentioned earlier, optically active nonsymmetric dimers
derived from the combination of cholesterol and Schiff base6,7,8c,d
4-n-Alkoxybenzaldehydes (4a–e), prepared by O-alkylating
4-hydroxybenzaldehyde with the requisite n-alkyl bromides in the
presence of a mild base, were condensed with amines 3a–d in the
presence of a mild acid to obtain four series of dimers. The
spectroscopic data and the results of elemental analyses were found
to be consistent with the proposed molecular structures of all
the intermediates and final compounds. The detailed synthetic
procedures and molecular structural characterization data of the
target LC dimers and their intermediates are presented in the ESI.†
III. Results and discussion
Chart 1 Molecular structures of novel Schiff base LC dimers reported by
Jin and co-workers (ref. 6 and 7a) and similar dimers with reversed imine
linkage and n-alkoxy terminal tails investigated in the present study.
The thermal behavior of these four series of dimers, DSB-3,R,
DSB-4,R, DSB-5,R and DSB-7,R, and their key intermediates,
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Scheme 1 Reagents and conditions: (i) n-bromoalkanoyl chlorides, pyridine, THF, 0–5 1C to rt (82–88%). (ii) 4-Nitrophenol, anhydrous K2CO3,
butanone, reflux 12 h (76–84%). (iii) H2 (1 atm., balloon), 10% Pd–C, THF, rt, 4 h (87–94%). (iv) n-Bromoalkanes, anhydrous K2CO3, butanone, reflux 12 h
(85–90%). (v) EtOH, AcOH (catalytic), reflux, 1 h (66–85%).
2a–d and 3a–d, was established using standard complementary textural observations under POM. The textures seen for different
techniques. Generally, the samples placed between a clean untreated LC phases of dimers are shown in Fig. 2a–i. It is especially
glass slide and a cover slip were used for the POM study. However, apparent from the results that all the five dimers of the series
for confirmation of the LC phase assignment, two differently surface- display enantiotropic mesomorphism. The first member DSB-3,4
coated slides, one treated for homogeneous orientation and the exhibits SmA and N* phases. When a thin film of this compound
other for homeotropic alignment of the mesogens, were employed. was placed between two glass plates and cooled from the
The intermediates 2a–d and 3a–d show LC phases; in particular, isotropic phase, focal-conic fan texture appeared, which, when
these compounds exhibit N*, TGB and SmA phases that were sheared, showed the characteristic Grandjean texture of the N*
adjudged based on the optical textural observation under POM phase. On cooling the sample further, a transformation to a
(Fig. S1, ESI†). The phase sequences, transition temperatures and focal-conic fan texture of the SmA phase occurs. The next
enthalpies of these intermediates are summarized in Table S1 (ESI†). member DSB-3,6 shows the TGB phase in addition to the SmA
In the following sections, the LC properties of dimers belonging to and N* phases. The TGB phase gave a filament texture on
DSB-3,R, DSB-4,R, DSB-5,R and DSB-7,R series have been discussed. heating from a homeotropic SmA phase, as expected. Indeed,
if the sample is heated at a higher rate (43 1C minÀ1), the N*,
IIIa. Thermal behavior of DSB-3,R series
SmA and TGB phases coexist. Interestingly, the dimer DSB-3,8
The thermal properties of dimers belonging to the DSB-3,R stabilizes the TGBC* phase along with the TGB and N* phases.
series are presented in Table 1, which reveals that all the On cooling from the isotropic state, it displayed a focal-conic
dimers are mesomorphic. Fig. 1a depicts the effect of the length fan texture which on mechanical stress transformed into a
of the terminal tail on the LC behavior of dimers observed Grandjean planar texture (Fig. 2a) of the N* phase.23 The planar
during the cooling cycle. The mesophase identification and texture of the N* phase on further cooling transformed into a
phase sequence confirmation were achieved purely based on TGB phase (which may have either a SmA or a SmC block) at
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Table 1 Phase sequences, transition temperatures (1C)a and enthalpies of transitions (kJ molÀ1) of DSB-3,R series
Phase sequence
Heating
Dimers
Cooling
DSB-3,4
Cr 141 (19.2) SmA 168.6b N* 230.4 (3.7) I
I 229.6 (3.5) N* 167.1b SmA 116 (8.3) Cr
DSB-3,6
DSB-3,8
DSB-3,10
DSB-3,12
Cr 148 (35.3) SmA 156.2b TGB 158.7b N* 220.4 (3.4) I
I 219.5 (2.8) N* 157.1b TGB 154.8b SmA 98.2 (25.2) Cr
Cr 118.5 (38.4) TGBC* 172.1b TGB 195.2b N* 214.3 (3.8) I
I 213.4 (3.7) N* 194.2b TGB 169.9b TGBC* 80.8 (11.2) Cr
Cr 119.9 (31.6) SmC* 173.1b SmA 197.4b TGB 202.4 (0.3) N* 208.8 (4.7) I
I 207.9 (4.6) N* 201.4 (0.3) TGB 195.7b SmA 172.3b SmC* 87 (25.5) Cr
Cr 117.8 (40.6) SmC* 173.2b SmA 202b TGB 203 (0.5) N* 204.3 (3.4) I
I 203.4 (3.5) N* 201.9 (0.4) TGB 200.1b SmA 170.2b SmC* 82.6 (19.9) Cr
a
Transition temperatures determined by both POM and peak values of the DSC traces during the first heating/cooling cycles at 5 1C minÀ1 rate.
The phase transition was observed under the microscope, but it was too weak to be resolved by DSC.
b
Fig. 1 (a) A plot of transition temperature as a function of the number of carbon atoms in the terminal tail for mesogens belonging to the DSB-3,R series.
(b) DSC traces obtained for the first and second heating–cooling cycles of mesogen DSB-3,10 at a rate of 5 1C minÀ1. Notice that the traces of two
heating–cooling cycles closely match, suggesting that these compounds are thermally stable.
about 194 1C as evidenced by the observation of a planar texture dark background at a well defined temperature. Its presence
(Fig. 2b). Upon cooling the sample further, a square grid pattern was clearly recognized by a focal-conic fan texture which
(Fig. 2c) appeared at 169.9 1C over the gray planar texture, transformed into a Grandjean planar texture when the top
indicating the presence of a TGBC* phase.23 Despite several glass substrate was pressed very gently. Upon cooling the
attempts, this dimer could not be aligned homeotropically, and pressed sample further, the TGB phase (having either SmA or
thus, the expected filament and undulated filament textures of SmC block) appeared with a planar texture. On lowering the
the TGB and TGBC* phases, respectively, could not be observed. sample temperature further, the planar texture of the TGB
However, in a wedge type cell treated for planar orientation, phase sharply changed to a pattern comprising both a focal-
Grandjean Cano (GC) dislocation lines were observed in the N* conic fan texture and a pseudo-isotropic texture; these textural
(Fig. 2d), TGB (Fig. 2e) and TGBC* (Fig. 2f) phases, demonstrating features strongly suggest the presence of the SmA phase.23 In
that there is a helical twist normal to the plates in these phases; fact, upon heating the homeotropically aligned (pseudo-
notably, in the TGBC* phase the GC lines superposed on the isotropic texture of) SmA phase, a striking filament texture of
square grid pattern (Fig. 2f). These observations rule out the the TGB phase was observed. When the SmA phase was cooled
possibility that the origin of the square grid is due to some further, an optical texture comprising the banded focal-conic
instability in the helical structure of the SmC* phase. In fact, in fans and cloudy pattern featuring a faint schlieren texture,
the TGBC* structure, besides the presence of a helical super- arising due to planar and homeotropic orientations of the
structure due to the TGB helix, there exists another helical dimers, respectively, were observed, which suggests the presence
structure due to the bulk SmC* phase; the helical axis of the of the SmC* phase. Hence, the occurrence of N*-TGB-SmA-SmC*
latter is perpendicular to that of the TGB helix.
phase sequence was evidenced using slides treated for planar
The dimers DSB-3,10 and DSB-3,12 showed an identical and homeotropic alignments. As expected, the TGB phase
mesomorphic behavior. In fact, the richest polymorphic showed a filament texture when slides treated for homeotropic
sequence was observed for both of these dimers, which we alignment were employed. The presence of the SmA phase
describe as follows. When a thin film of any of the two dimers, displayed the focal-conic texture in slides treated for planar
placed between clean glass slides, was cooled gradually from orientation, as shown in Fig. 2g, and a dark field of view in
the isotropic state, the N* phase was found to emanate from the slides treated for homeotropic alignment. The focal-conic fan
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Fig. 2 Photomicrographs of the optical textures seen for the mesophases of dimers belonging to the DSB-3,R series: (a) the Grandjean planar texture of
the N* seen for DSB-3,8 (206 1C); (b) the planar texture of the TGB phase of DSB-3,8 (180 1C); (c) the square grid pattern of the TGBC* phase emerging
from the planar texture of the TGB phase of DSB-3,8 (169 1C); (d) the Grandjean Cano (GC) dislocation lines observed for the N* phase of DSB-3,8
(200 1C); (e) GC dislocation lines noticed for the TGB phase of DSB-3,8 (190 1C); (f) the square grid pattern superposed on the GC dislocation lines seen
for the TGBC* phase of DSB-3,8 (155 1C); (g) the focal-conic fan texture of the homogeneously aligned SmA phase of DSB-3,10 (190 1C); (h) an
equidistant line pattern superimposed on the focal-conics of SmC* fans of DSB-3,10 (165 1C); (i) the dull yellow greyish schlieren texture of the
homeotropically oriented SmC* phase of DSB-3,12 (151 1C).
texture with dechiralization lines on top of it was observed solely, exhibited enantiotropic mesophases. The first two members,
as shown in Fig. 2h, for the homogeneously aligned SmC* phase. DSB-4,4 and DSB-4,6, showed an identical thermal behavior
However, for the homeotropic orientation it showed a schlieren that was evidenced based on the characteristic optical textures
texture surrounded by a dull yellow greyish uniform pattern of the mesophases. As a representative case, the microphoto-
(Fig. 2i); the schlieren texture mostly contained four-brush graphs of textural patterns observed for the mesophases of
disclinations (strength S = 1). As a representative case, the DSC dimer DSB-4,4 are shown in Fig. 4a–f. When a thin film of the
traces obtained for DSB-3,10 during two consecutive heating and sample, placed between a glass slide and a coverslip, was
cooling cycles are shown in Fig. 2b where the close resemblance cooled slowly (0.1 1C minÀ1) from the isotropic phase, a platelet
among the traces can be seen. Thus, the calorimetric results texture grew on top of the black background and filled the field
corroborate the microscopic observation that these dimers are of view (Fig. 4a), suggesting the presence of BPI or BPII. In
thermally stable. It is important to note that the dimer DSB-3,8 fact the cubic BPI and BPII phases, comprising double twist
shows the TGB and TGBC* phases over a wide thermal width, cylinders, possessed 3D orientational ordering with periods
viz. B24 1C and B85 1C, respectively.
nearly equal to 500 nm that is of the order of the wavelength
of visible light. Thus, they Bragg-scattered light in the visible
range. They comprised differently oriented domains exhibiting
IIIb. Thermal behavior of the DSB-4,R series
The phase sequences, transition temperatures and associated the textural pattern characterized by a mosaic of different
enthalpies of the DSB-4,R series of dimers having an odd-parity colors.23 The textural pattern seen under the POM suggests
spacer are summarized in Table 2. The data shown here were the existence of a cubic blue phase which can be either BPI or
derived from DSC traces of the first heating–cooling cycles BPII.23 With further cooling, the N* phase began to grow
obtained at a rate of 5 1C minÀ1. Fig. 3a shows the DSC traces instantaneously on top of the platelet texture (Fig. 4b) and
obtained for one of the dimers. Fig. 3b shows the dependence filled the field of view quickly displaying an atypical bright
of the transition temperatures of the dimers on the number of texture, which upon gentle shearing showed the Grandjean
carbon atoms in the terminal tail. All five members of the series planar texture of the N* phase (Fig. 4c).
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Table 2 Transition temperatures (1C)a and enthalpies of transitions (kJ molÀ1
of the DSB-4,R series. SmX = an unknown, higher ordered smectic phase
)
odd-membered spacer show BPs;1d,24–26 this implies that the
chirality/pitch of the LC structure is vital27 which shows a
dependence on the parity of the spacer.26 This observation
can be ascribed to the bent shape of dimers (Fig. 5, top portion)
which are less elongated when compared to their even spacer
counterparts (Fig. 5, bottom portion). In particular, this can be
understood in terms of the smaller helical pitch for the odd-
member which stems from the smaller twist elastic constant
that in turn linked to their lower orientational order. In other
words, the higher orientational order found for even-members
precluded stabilization of BPs by them. When the sample
(DSB-4,8) was continued to cool from the Grandjean planar
texture of the N* phase, a mesophase, hereafter referred to as
the SmX phase, set in at B108 1C. It manifests optically in the
Phase sequence
Heating
Compounds
Cooling
DSB-4,4
Cr 102.6 (16.4) N* 169.8 (0.2) I
I 167.8 BPI/II-N* (0.2)b 79.4 (5.9)Cr
DSB-4,6
DSB-4,8
DSB-4,10
DSB-4,12
Cr 109.3 (33.2) N* 166.7 (0.6) I
I 166.0 BPI/II-N* (0.5)b 76.8 (20.9) Cr
Cr 107.7 (25.7) SmX 124.6c N* 160.9 (0.7) I
I 160.3 (0.6) BPIII 151c N*108.3c SmX 59.8 (19.1) Cr
Cr 126.5 (52.7) SmX 144.7c N* 157.1 (0.7) I
I 156.3 (0.6) BPIII 144.2c N* 123.2c SmX 60.8 (34.6) Cr
Cr 124.4 (52) SmX 139.5c N* 155.3 (1.4) I
I 154.8 (1.4) BPIIIc 150 N*133.3c SmX 55 (43) Cr
a
Transition temperatures determined by both POM and peak values of form of oily streak texture having a bluish background as
the DSC traces during the first heating/cooling cycles at 5 1C minÀ1 rate.
shown in Fig. 4f. It can be mentioned here that the optical
b
The I-BP and BP-N* phase transitions were seen under POM, but they
textural patterns for these mesophases were found to be
were too weak to be detected by DSC; thus, the DH value represents the
c
combined enthalpy for the I-BP and BP-N* transitions. The phase identical when the samples were either examined using clean
transition was observed under the microscope, but it was too weak to be
glass slides or treated for planar or homeotropic orientation.
Thus, the exact nature of the SmX phase could not be revealed
resolved by DSC.
by optical study alone.
Mesophase characterization by optical textural observation
In order to probe the structure of the SmX phase, XRD
revealed that the dimers DSB-4,8, DSB-4,10 and DSB-4,12 measurements were carried out on an unaligned (powder)
display nearly the same transitional behavior; the occurrence sample DSB-4,8. While cooling the sample, the XRD patterns
of three distinct phase transitions was evidenced. For example, as a function of temperature were collected in the SmX phase
the dimer DSB-4,8, either placed between clean glass slides or region. The diffractograms obtained for all the temperatures
treated for planar or homeotropic geometry, and cooled slowly were found to be practically identical; as a typical case, the
from the isotropic (I) phase, showed a LC phase having a dark pattern obtained at 90 1C is shown in Fig. 6a. It consists of two
bluish-foggy appearance as shown in Fig. 4d, which is a low angle reflections, the first one very intense (d1) and the
characteristic texture of the BPIII.23 A signature due to I-BPIII other very weak (d2), and a diffuse reflection at wide angles. The
transition with DH = 0.6 kJ molÀ1 (Table 2) seen in the DSC wide angle diffuse peak centered at 4.9 Å corresponds to
trace corroborated the optical observation. On cooling the the intermolecular separation within the smectic layer arising
sample further, at 151 1C the BPIII texture changed to a pattern, due to the liquid like order. The spacings (d1) derived from the
which on gentle shearing showed the Grandjean planar texture low angle Bragg reflections obtained at different temperatures
typical of the N* phase (Fig. 4e). Thus, the BPIII existed over are depicted in Fig. 6b. Indeed, this spacing (d1) corresponds to
9 1C thermal range; it can be noted here that the other two the smectic layer thickness in the SmX phase. The second peak
dimers DSB-4,10 and DSB-4,12 stabilized this phase over with the spacing (d2 = 20.8 Å) nearly half the molecular length
B12 1C and B5 1C, respectively. It must be mentioned here of the dimer must be arising from the combination of different
that the existence of the BPIII phase in these dimers (DSB-4,8, parts of the molecule; in fact, such a feature has been observed
DSB-4,10 and DSB-4,12) and the BPI/II phase in lower members for many cholesterol-based dimers.4,6,7a,b The spacing d1 of the
DSB-4,4 and DSB-4,6 is in good agreement with the general low-angle reflections and the d1/L ratio calculated by considering
observation that chiral, non-symmetric dimers possessing an the molecular length L in an all-trans conformation are presented
Fig. 3 (a) DSC thermograms obtained for the first and second heating–cooling cycles of mesogen DSB-4,6 at a rate of 5 1C minÀ1. (b) Plot of transition
temperature as a function of the number of carbon atoms in the terminal tail for LC dimers of the DSB-4,R series.
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Fig. 4 Microphotographs of the optical textures observed for different LC phases of dimers belonging to the DSB-4,R series: (a) the platelet texture of
BPI/BPII seen for DSB-4,4 (167.8 1C); (b) the textural pattern showing the co-existence of both BPI/BPII and N* phases for dimer DSB-4,4 (167 1C); (c) the
oily streak texture of the N* seen for DSB-4,4 (110 1C); (d) the blue fog texture of the BPIII seen for DSB-4,8 (155 1C); (e) the oily streak texture of the N*
phase observed for DSB-4,8 (120 1C); (f) the uncharacteristic texture observed for the SmX phase of DSB-4,8 (80 1C).
lowering the temperature. Accordingly, the SmX phase can be
considered as a tilted smectic phase.
IIIc. Thermal behavior of the DSB-5,R series
Table 3 shows the phase transition temperatures and the
corresponding enthalpy changes of the DSB-5,R series of
dimers. All the dimers of the series show enantiotropic LC
phases. The dependence of the transition temperatures of the
dimers on the number of carbon atoms in the terminal chain is
shown in Fig. 7a. It is worth mentioning here that these dimers,
like the other three series, were found to be thermally stable
during repeated heating and cooling cycles through the meso-
phase–isotropic transition, as confirmed by both optical and
Fig. 5 The most stable (energy minimized) space-filling models of an odd-
membered dimer DSB-4,4 (top) and an even-membered dimer DSB-3,4
(bottom).
in Table S2 (ESI†). It is apparent from the collected data that at the DSC experiments. For example, as shown in Fig. 7b, the
higher temperatures the layer spacing d1 is slightly more when DSC traces recorded for the two consecutive heating–cooling
compared to the estimated all-trans molecular length (42 Å); thus, cycles were virtually identical. The textures seen for different LC
the d1/L ratio is slightly more than one. However, at lower phases of some of these dimers are shown in Fig. 8a–f.
temperatures the layer spacing d1 is less than the molecular
Compounds DSB-5,4 and DSB-5,6 behaved identically, show-
length and thus the d1/L ratio is less than one, suggesting tilting ing three LC phases. The occurrence of enantiotropic N*, TGB
of the molecules within the smectic layers, which increases on and SmA phases was ascertained based on their characteristic
Fig. 6 (a) X-ray intensity profile obtained for the SmX phase of DSB-4,8 at 90 1C showing two reflections (d1 and d2) at low angles and a diffuse peak at
the wide angle region. (b) The temperature dependence of the layer spacing d1 in the SmX phase.
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textural pattern observed under POM. The focal-conic fan established with the help of POM wherein each mesophase
texture growing just below the isotropic phase and a planar showed characteristic optical textural patterns when viewed
texture having oily streaks seen for the N* phase are shown in through the polarizing microscope. Given the fact that the TGB
Fig. 8a and b, respectively. The planar texture of the TGB phase phase is short-lived, the N* and SmA phases coexisted along with
appearing for a very short thermal range is shown in Fig. 8c it, which was evident from fact that the respective textures of
which immediately transforms into the SmA phase where a these three LC phases existed concurrently.
pattern comprising both focal-conic fan texture and a pseudo-
IIId. Thermal behavior of the DSB-7,R series
isotropic texture could be seen. Fig. 8d shows the filament
texture of the TGB phase obtained upon heating the pseudo- The transitional properties of the five dimers belonging to the
isotropic texture of the SmA phase. The next dimer DSB-5,8 DSB-7,R series are presented in Table 4. Fig. 9a shows the
showed a dimesomorphic sequence involving a transition from dependence of the thermal properties on the number of carbon
the TGBC* phase to the N* phase; a square grid texture pattern atoms in the terminal chain of the DSB-7,R series of dimers.
of TGBC* observed at 120 1C is shown in Fig. 8e. Compound These dimers, despite the fact that they comprise the Schiff
DSB-5,10 showed three enantiotropic LC phases, namely base core, exhibited good thermal stability that was noted
TGBC*, TGB and N* phases, that was adjudged based on during the optical and calorimetric studies. In fact, the peaks
textural observation as described before. The last member of of phase transitions observed in DSC thermograms for any
the series, DSB-5,12, like the dimers DSB-3,10 and DSB-3,12 of number of heating–cooling cycles were found to be highly
the DSB-3,R series, showed a rich variety of LC phases. That is, reproducible; for example, as shown in Fig. 9b, repeated heat-
it exhibited four mesophases, N*, TGB, SmA and SmC* phases. ing–cooling scans of the sample DSB-7,12 showed almost
As a representative case, the microphotograph of the optical texture identical thermograms. From the collected data, the following
of the planarly aligned SmC* phase featuring dechiralisation inference can be made. In general, all the compounds of
lines on top of the focal conic fan texture is shown in Fig. 8f. the series show enantiotropic LC phases. They commonly show
Needless to say, the occurrence of these LC phases was unequivocally the N* phase in spite of variation in their terminal tail length.
Table 3 Transition temperatures (1C)a and enthalpies of transitions (kJ molÀ1) of the DSB-5,R series
Phase sequence
Heating
Compounds
Cooling
DSB-5,4
Cr 130.1 (43.4) SmA 170.9 (0.3) TGB-N* 208.3 (3.3)b
I 207.5 (3.3)b N*-TGB 170.2 (0.2) SmA 55 (33.3) Cr
I
DSB-5,6
DSB-5,8
DSB-5,10
DSB-5,12
Cr 119.8 (23) SmA 165.4c TGB-N* 199.5 (2.7)b
I
I 199.1 (2.6)b N*-TGB 165.1c SmA 99.5 (23.6) Cr
Cr 118.5 (45.8) TGBC* 142.1c N* 194.5 (5.1) I
I 193.7 (5) N* 139.7c TGBC* 79.7 (27.6) Cr
Cr 111.1 (44.4) TGBC* 158.2c TGB 177.5c N* 188.9 (4.8) I
I 188.2 (4.8) N* 175.7c TGB 156.6c TGBC* 68.4 (17.3) Cr
Cr 101.7 (37.7) SmC* 164.3c SmA 181 (0.8) TGB-N* 184.3 (4.6)b
I
I 183.7 (4.5)b N*-TGB 180.1 (0.6) SmA 162.8c SmC* 69.1 (20.7) Cr
a
Transition temperatures determined by both POM and peak values of the DSC traces during the first heating–cooling cycles at 5 1C minÀ1 rate.
b
The TGB phase is a transient phase that was seen under POM only; the TGB-N*/N*-TGB transitions were too weak to be detected by DSC and thus,
c
the DH value represents the combined enthalpy TGB-N*/N*-TGB transitions. The phase transition was observed under the microscope, but it was
too weak to be detected by DSC.
Fig. 7 (a) The influence of the number of carbon atoms in the terminal chain on the transition temperature for the DSB-5,R series; (b) DSC traces
obtained for the first and second heating–cooling cycles of mesogen DSB-5,10 at a rate of 5 1C minÀ1
.
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Fig. 8 Photomicrographs of the optical textures seen for the mesophases exhibited by dimers of the DSB-5,R series: (a) the focal-conic fan texture of
the N* of dimer DSB-5,4 developing on top of the isotropic liquid (207 1C); (b) the oily streak texture of the N* phase formed after shearing the focal-
conic fan texture of the N* phase of DSB-5,4 (185 1C); (c) the planar texture of the TGB phase of dimer DSB-5,6 (165 1C); (d) the filament texture of the
TGB phase obtained when a homeotropically aligned SmA phase of the dimer DSB-5,6 is heated (155 1C); (e) the square grid pattern of TGBC* observed
for DSB-5,8 (120 1C); (f) the dechiralization lines on the top of the broken focal-conic texture of the SmC* phase of DSB-5,12 (100 1C).
This was identified based on the observation of characteristic cooled, a phase transition occurred with a deep gray blurry
Grandjean planar texture (Fig. 10a), formed upon shearing the planar texture which is the characteristic texture of the TGB
natural fan-like texture developed while cooling the samples phase. As the temperature was lowered, a square grid pattern
from the isotropic state.
developed sharply on top of grayish planar texture of the TGB
The first three members DSB-7,4, DSB-7,6 and DSB-7,8 phase at 133 1C, implying the presence of the TGBC* phase. In
displayed SmA and TGB phases additionally. As expected, the order to verify the presence of the N* and frustrated (TGB and
SmA phase showed, when examined using untreated glass TGBC*) phases, the sample was examined in a wedge type cell
microscope slides, the regions of focal-conic fan and homeo- treated for homogeneous alignment. The N* and TGB phases
tropic textures. The TGB phase displayed the filament texture obtained in succession while cooling from the isotropic phase
(Fig. 10b) on heating from a homeotropic SmA phase. The next showed an array of GC lines. On further cooling, the TGBC*
dimer DSB-7,10 showed TGB and TGBC* phases besides the N* phase came into existence with a square grid pattern appearing
phase. When the Grandjean planar texture of the N* phase was on top of the dislocation lines. Thus, the TGB and TGBC*
phases, respectively, exist over thermal range of B14 1C and
B65 1C which is noteworthy given the fact that such phases,
Table
4
Transition temperatures (1C)a and enthalpies of transitions
(kJ molÀ1) of the DSB-7,R series
being frustrated structures, are known to exist over a narrow
temperature range.5,23 Compound DSB-7,12 stabilized the
TGBC* phase besides the N* phase. On cooling the planar
texture of the N* phase, the TGBC* phase occurred with a
characteristic square grid textural pattern (Fig. 10c). To confirm
that the origin of square grid texture is due to the TGBC* phase,
the sample was examined in wedge type cell treated for planar
orientation. The N* phase obtained while cooling from the
isotropic phase showed an array of GC lines; on cooling the N*
phase, the TGBC* phase appeared with a square grid pattern
developing over these dislocation lines.
Phase sequence
Heating
Compounds Cooling
DSB-7,4
DSB-7,6
DSB-7,8
DSB-7,10
DSB-7,12
Cr 130.6 (41.4) SmA 166.3b TGB-N* 190.1 (2.8)c I
I 188.3 (2.7)c N*-TGB 164.9b SmA 89.1 (16.0) Cr
Cr 120.3 (51.6) SmA 166.8 (0.4) TGB-N* 183.7 (3.5)c I
I 182.6 (3.3)c N*-TGB 164.9 (0.3) SmA 75.1 (20.1) Cr
Cr 117.4 (37.4) SmA 150.4b TGB-N* 175.9 (3.6)c I
I 174.5 (3.2)c N*-TGB 149.6b SmA 98.3 (38.9) Cr
Cr 109.6 (34.2) TGBC* 134.4b TGB 148.5 N* 170.4 (3.4) I
I 169.5 (3.2) N* 147.1 TGB 133.0b TGBC* 68.1 (13.6) Cr
Cr 99.5 (35.2) TGBC* 156.4b N* 164.1 (4.1) I
Thus, the foregoing microscopic texture observations sug-
gest that the dimers DSB-7,10 and DSB-7,12 stabilize the TGB
and/or TGBC* phase over a wide thermal span. To substantiate
the results of POM studies, XRD experiments were performed
I 163.7 (4) N* 155.0b TGBC* 60.1 (24.6) Cr
a
Transition temperatures determined by both POM and peak values of
the DSC traces during the first heating–cooling cycles at 5 1C minÀ1
b
rate. The phase transition was observed under POM, but it was too on unaligned (powder) samples DSB-7,10 and DSB-7,12 taken
weak to be detected by DSC. The TGB phase was seen under POM
in Lindemann capillary tube individually. The XRD profiles
c
only; the TGB-N*/N*-TGB transitions were too weak to be detected by
DSC and thus, the DH value represents the combined enthalpy TGB-N*/
were recorded as a function of temperature in the TGBC*
N*-TGB transitions.
and/or TGB mesophases while cooling these dimers, mounted
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Fig. 9 (a) Dependence of the transition temperature for the DSB-7,R series on the number of carbon atoms in the terminal tail. (b) DSC traces obtained
for the first and second heating–cooling cycles of DSB-7,12 at a rate of 5 1C minÀ1. Notice the occurrence of the TGBC* phase over a wide thermal range.
inside a temperature-controlled Mettler hot stage, from their can be recognized as the TGBA phase. The accumulated data in the
isotropic phase. The diffractograms obtained for the frustrated subsequent frustrated (TGBC*) phase clearly show that the layer
phases of both the dimers were found to be roughly the same, spacing d1 is less than the estimated all-trans molecular length and
each showing two small angle (0 o 2y o 51) reflections, the thus the d1/L ratio is less than one. This abrupt drop in the d1 value
first one very intense (d1) and the second one weak (d2), and a at the transition from the TGBA to the TGBC* phase is anticipated
diffuse reflection at wide angles (2y E 201). Table S3 (ESI†) and given the fact that in the TGBC* phase the smectic C*-like blocks,
Fig. 11a–d illustrate the results/various aspects corresponding which remains orthogonal to the helix of the TGB structure,
to XRD studies. As representative cases, the 1D intensity vs. 2y comprise mesogens that are tilted with respect to the layer normal
profile obtained for the TGBC* phase of dimers DSB-7,10 and direction. It can be seen that on cooling the TGBC* phase gradually
DSB-7,12 are shown in Fig. 11a and b, respectively. Table S3 the d1-value decreases in a monotonic manner, suggesting that the
(ESI†) presents the spacing d1 of the small-angle reflections and tilt angle of the mesogens within the smectic layers increases on
the d1/L ratio, where L is the estimated all-trans molecular lowering the temperature.
length derived from a Chem3D molecular model.
Likewise, the L of the dimer DSB-7,12 estimated from its
While the presence of second peak (d2) in the small angle all-trans configuration was found to be higher than the layer
region in each profile is an important one as it may be spacing d1 values obtained in the entire thermal range of the
originating from the combination of different parts of the TGBC* phase (see Fig. 11d and Table S3, ESI†). This implies the
dimer,4,6,7a,b the spacing d1 pertains to the smectic layer thick- tilted organization of the mesogens within the smectic layers
ness in the TGB and TGBC* phases. The large angle diffuse and obviously, the layer thickness will be smaller. Thus, the
band centered at B4.9 Å (Table S3, ESI†) corresponds to the XRD study ensures the tilted organization of the molecules
intermolecular separation within the smectic layer arising due within the smectic layers supporting the occurrence of the
to the liquid like order. Fig. 11c and d depict the layer spacing TGBC* phase in both dimers.
d1 of the lowest angle reflection obtained as a function of
From the foregoing presentation concerning the thermal
temperature in the TGB, TGBC* and TGBC* phases of dimers behavior of four series of cholesterol based dimers, the following
DSB-7,10 and DSB-7,12, respectively. It is apparent from key remarks can be made. It is important to mention here that
Fig. 11c and Table S3 (ESI†) that the layer spacing values these new chiral dimers comprise Schiff base wherein the imine
in the TGB phase are comparable to the length L of the dimer linkage is reversed when compared to the known Schiff
DSB-7,10 in its all-trans conformation; that is, the ratio of base dimers.6,7a Besides, with the intention of establishing
d1/L B 1 is archetypal of the monolayer SmA phase, meaning the structure–property relationships, the length and parity of
that the TGB phase possesses SmA blocks and thus, the TGB the spacer, and the length of terminal tail have been varied.
Fig. 10 Photomicrographs of the optical textures of mesophases obtained for the dimers of the DSB-7,R series: (a) the oily streak texture of the N*
phase formed by DSB-7,6 (180 1C); (b) the co-existence of textures of both SmA and TGB phases seen for DSB-7,6 (165 1C); (c) the square grid pattern
observed for the TGBC* phase of DSB-7,12 (130 1C).
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Fig. 11 1D intensity vs. 2y profile obtained for the TGBC* phase of dimers DSB-7,10 (a) and DSB-7,12 (b). Note that both the profiles possess two
reflections (d1 and d2) at low angles and a diffuse peak at the wide angle region. The temperature dependence of the layer spacing d1 in the TGB and
TGBC* phases of dimer DSB-7,10 (c) and TGBC* phase of dimer DSB-7,12 (d).
As shown in Fig. 12, an odd–even effect is prominently seen when the overall molecular shape of the mesogens governed by the
the parity of the spacer is varied; that is, the number of carbon geometry and flexibility of the spacer. The lower clearing tem-
atoms in the spacer affects the thermal behavior of the com- perature values of the dimers with odd-parity spacer are due to
pounds. For example, the dimers with even-parity spacer show their bent conformation (the reduced shape-anisotropy); on the
high clearing temperature values than dimers comprising odd- other hand, the even-members with their rod-like conformation
parity spacer. Most importantly, the LC property of dimers having show the higher values. This behavior is, in fact, the characteristic
odd-parity spacer differs from those of even-members. Needless to thermal property of LC dimers. It is quite apparent that the
say, the observed odd–even effect can be interpreted in terms of lengths of terminal tail also influence the thermal behavior of
the compounds. For example, in general, within each series the
clearing temperature values decrease with the increase in tail
length. Likewise, within the series, the length of the terminal tail
influences the phase transitional behavior of the compounds.
IV. Chiroptical behavior of the N*
phase
The chiral nematic (N*) phase is an orientationally ordered
system characterized by a helical superstructure wherein the
preferred orientation of the mesogens twists spatially around
an orthogonal (helical) axis.5 It is, being helical, described by its
pitch (P) (the distance corresponding to a full 3601 rotation
of the constituent mesogens along the helix director) and
Fig. 12 The influence of the number of carbon atoms in the o-oxyalkanoyl
spacer on the clearing temperatures for the DSB-3,R, DSB-4,R, DSB-5,R
and DSB-7,R series of dimers. In all the plots dashed lines joining points are
handedness; the latter typically varies when the temperature
is altered. Most importantly, it exhibits special optical proper-
ties such as Bragg-like light reflection, high rotatory power and
circular dichroism (CD). The CD measurement has been
suggestive of the general trend; it can be noted that experimental data do
not cover for n = 7.
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demonstrated to be one of the valid probes to reveal the clearly implying that the chromophores are not influenced by
chiroptical behavior of the N* phase.27,28 The thin-film cell of the molecular chirality, but they are affected by the chirality
planar texture, where the molecular director aligns parallel (handedness) of the phase; in other words, the CD activity
to the glass substrate surfaces and helix axis remaining stems from the macroscopic helical arrangement of dimers.
perpendicular to cell window, displays a strong optical activity Genuinity of the CD signals observed in the N* phase was
where birefringence contribution remains almost negligible or evaluated by recording at four and six different orientations
generally nil; thus, the CD activity resulting from planar texture performed by in-plane rotation of the sample cells by 901 and
of the N* phase is valid and reliable. In fact, the homogeneous 601, respectively. As expected, the CD spectra recorded in these
texture contained in a cell interacts very effectively with the experiments were found to be indistinguishable from those
circularly polarized light (CPL) with a wavelength l = nP (n is the seen for the original position of the sample cell, meaning that
mean refractive index); the direction of light propagation, being there is no LD contribution. The CD equipment used in the
perpendicular to the cell walls, remains parallel to the helix axis current investigation has a built-in provision to measure the LD
and thus maximizes interaction of CPL with the N* phase. The directly and thus, samples were subjected to LD measurement
helix of the N* phase reflects CPL of the same handedness where the activity was found to be nil (see red traces in
while transmitting CPL of opposite handedness; that is, Fig. 13e–h); that is, as shown in the insets of Fig. 13e–f, the LD
circular dichroism of the N* phase is the difference between spectra have no meaningful signatures. The foregoing experimental
the reflection coefficients for right and left hand CPL waves, results evidently suggest that the CD activity of the N* phase is
instead of the absorption coefficients. The helical/twist sense indisputable and certainly not stemming from the LD artifact.
(handedness) (left or right) of the phase shows the direction in
As can be seen in Fig. 13a, c and d, the even-membered
which rotation of the director occurs from layer to layer. Thus, dimers DSB-3,4, DSB-5,4 and DSB-7,4 commonly show an
with the help of CD measurements it is possible to determine intense negative band in the range of 420 to 370 nm and a less
handedness of the N* phase. It should be however noted that intense positive band at B280 nm. While the dimer DSB-4,4
CD experiments are delicate and thus should be carried out with odd-numbered spacer shows an intense positive band in
with extreme care. In particular, the possibility of the linear the region of 420 to 380 nm (Fig. 13b). Hence, both even- and
dichroism (LD) and birefringence contributions should be odd-membered dimers display CD bands at the same position
eliminated so as to obtain valid and reliable CD data.
with an almost matching magnitude but of opposite signs. In
CD spectra as a function of temperature were recorded in general, the intensity of the signals decreases with decreasing
the entire N* phase regime of the four samples, viz. DSB-3,4, temperature, as summarized in Table S4 (ESI†). The occurrence
DSB-4,4, DSB-5,4 and DSB-7,4, chosen as representative cases of CD bands can be attributed to the selective reflection
from each series. The temperature-dependent CD spectra wavelengths of circularly polarized light by the helical structure
obtained in the N* phase are shown in Fig. 13a–d. Table S4 of the phase. The appearance of a less intense peak can be
(ESI†) presents the CD data of the samples in terms of the ascribed to the presence of phenyl rings.29 So, chiroptical
number of CD signals obtained with their positions (lmax, nm), measurement in the N* phase by CD formally indicates its
signs (positive or negative) and intensities (in millidegrees). In optically active (helical) supramolecular macroscopic organization
the following, we describe the method adopted for the sample in which intermolecular interactions between electronic transitions
preparation and CD measurements carried out on the planarly occur. It can be pointed out here that the observation of an intense
oriented N* phase and isotropic state of samples. A minute positive CD signal in the N* phase of odd-membered dimer
amount (B1–2 mg) of dimers sandwiched between two clean DSB-4,4 indicates a right-handed screw sense of the helical
quartz plates was heated to its isotropic phase, using a pro- structure.4,29a Needless to say, the parity of the spacer appears to
grammable hot stage which gives an accuracy of about Æ0.6 1C dictate the handedness of the N* phase formed by LC dimers.
variations, and the CD spectra were recorded. As shown in
Fig. 13e–h (black traces), the CD activity, as expected, remains
silent in this state. The samples were then cooled slowly into
the N* phase and sheared mechanically over and over again not
V. Conclusion
only to ensure uniform spreading but also to realize very thin Four series of nonsymmetric, optically active dimeric liquid
film of the samples. It must be mentioned here that the crystalline compounds, in which cholesterol and Schiff base
preparation of such thin films of the N* phase was indispen- mesogen are interlinked via an o-oxyalkanoyl flexible spacer,
sable to overcome the saturation setback where intense CD have been synthesized and characterized using optical micro-
bands beyond the measurable range of the CD spectrometer scopy and differential scanning calorimetry. With the purpose
existed. Precisely, the standard cells with known thickness of gaining insight into the parameters governing the structure–
could not be used and thus, the CD experiments presented property relations, the lengths of the central o-oxyalkanoyl
herein are purely qualitative. The CD spectra as a function of spacers and those of the terminal alkoxy chains have been
decreasing temperature were recorded in the entire tempera- varied. All the twenty dimers belonging to four series exhibit
ture range of the N* phase. As illustrated in Fig. 13a–d, the CD enantiotropic liquid crystal behavior. The study especially
phenomenon, unlike in the isotropic liquid state (Fig. 13e–h), revealed that the transitional properties of these LC dimers
was observed in the entire thermal width of the mesophase depend not only on the length and parity of the flexible spacer
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Fig. 13 Temperature dependent CD spectra of the N* phase obtained while cooling the dimers DSB-3,4, DSB-4,4, DSB-5,4, and DSB-7,4, from their
isotropic phase (a–d). LD (red trace) and CD (black trace) spectra recorded, respectively, in the N* phase and isotropic phase of the samples (e–h); notice
that, as expected, in these spectra the CD signals are not seen; the insets show that there is no LD activity.
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38, 1563–1589; (i) Handbook of Liquid Crystals; Volume 5:
Non-Conventional Liquid Crystals, ed. J. W. Goodby, P. J.
Collings, T. Kato, C. Tschierske, H. Gleeson and P. Peter
Raynes, Wiley-VCH, Weinheim, 2nd edn, 2014.
but also on the lengths of the terminal tails. The dimers with
odd-parity spacer show considerably lower N*-I (clearing) tran-
sition (isotropization) temperature as compared to their even-
membered counterparts; within the series the isotropization
temperature generally decreases with the increase in terminal
tail length. The even-members commonly show SmA and N*
phases; some of them, depending upon the lengths of the
spacer and terminal chain, display TGB, SmC* and TGBC*
phases additionally. Remarkably, some of these dimers stabilize
highly frustrated LC phases such as TGB and TGBC* over a wide
thermal width. On the other hand, the dimers with an odd-
numbered spacer commonly show the blue phase (BPI/II/III) and
N* phases, while some of the dimers (higher members) also
stabilize an unfamiliar tilted smectic phase; this observation is
in accordance with the general important finding that BPs are
stabilized by optically active, non-symmetric dimers comprising
an odd-membered spacer, and not by their counterpart dimeric
liquid crystals possessing an even spacer. Chiroptical properties
of the chiral nematic phase of the samples were investigated by
CD measurements; this study reveals that the handedness of
cholesteric superstructure depends on the parity of the spacer. In
essence, the study clearly illustrates the complex interplay of the
different molecular sub-units of the dimers in stabilizing meso-
phases such as blue, chiral nematic, twist grain boundary,
smectic A and chiral smectic C phases. Thus, the cholesterol-
based dimers described here are early examples of what could be
a vast family of materials that may secure a vital position in both
basic and applied research.
2 R. J. Mandle, Soft Matter, 2016, 12, 7883–7901.
3 (a) C. T. Imrie, in Structure and Bonding - Liquid crystals, II,
ed. D. M. P. Mingos, Springer-Verlag, 1999, p. 149; (b) C. T.
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Conflicts of interest
There are no conflicts to declare.
8 (a) C. V. Yelamaggad, Uma S. Hiremath and D. S. Shankar
Rao, Liq. Cryst., 2001, 28, 351–355; (b) C. V. Yelamaggad,
U. S. Hiremath, S. Anitha Nagamani, D. S. Shankar Rao and
S. K. Pasad, Liq. Cryst., 2003, 30, 681–690; (c) K. C. Majumdar
and P. K. Shyam, Mol. Cryst. Liq. Cryst., 2010, 528, 3–9;
(d) K. C. Majumdar, P. K. Shyam, D. S. Shankar Rao and
S. K. Prasad, Liq. Cryst., 2012, 39, 1117–1123; (e) D. D. Sarkar,
R. Deb, N. Chakraborty, G. Mohiuddin, R. Kanti Nath and
N. V. S. Rao, Liq. Cryst., 2013, 40, 468–481; ( f ) M. Kumar,
T. Padmini and K. Ponnuvel, J. Saudi Chem. Soc., 2017, 21,
S322–S328.
Acknowledgements
CVY sincerely thanks the Department of Science and Technology
(DST), New Delhi, Govt. of India, for financial support through
SERB project No. EMR/2017/S1/000153.
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