Nonsymmetrical cholesterol dimers constituting regioisomeric oxadiazole and thiadiazole cores: an investigation of the structure-property correlation

Article abstract of DOI:10.1039/c6nj03141b

Three series of chiral nonsymmetrical dimers were prepared by connecting promesogenic cholesterol to a bent structure derived from a substituted 1,3,4-oxadiazole or 1,2,4-oxadiazole or 1,3,4-thiadiazole moiety. These two mesogenic segments are interconnected through spacers of varying lengths and parity. The structures of the bent achiral unit were systematically varied with different central heterocyclic cores to understand the influence of bent angles on the thermal and gelation behavior. The bent angle of the achiral unit, which is determined by the heterocyclic core, has a major role in the stabilization of frustrated phases. Dimers based on the 1,3,4-oxadiazole unit with a more bent structure stabilized frustrated phases like blue phases and twist grain boundary phases. The bent system with a wider bent angle preferred to stabilize chiral nematic and smectic A phases. It is interesting to note that an increased bent structure reduced the mesophase stability as in the case of dimers based on the 1,3,4-oxadiazole unit, where many compounds exhibited monotropic phases. In the case of dimers with a wider bent angle, enantiotropic mesomorphism was observed. All the compounds showed blue light emission in the solution. Among these chiral dimers, only the compounds based on the 1,3,4-oxadiazole unit showed the gelation ability, which emphasizes how small structural changes like bent angle, dipole moment and the type of heteroatom in the heterocyclic unit affect the macroscopic self-assembly.

Full text of DOI:10.1039/c6nj03141b

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The role of atropisomers on the photo-reactivity and fatigue of
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Nonsymmetrical cholesterol dimers constituting regioisomeric oxadiazole and thiadiazole
cores: an investigation on structure-property correlation
Balaram Pradhan,a# Nirmalangshu Chakraborty,a# Ravindra Kumar Gupta,a G. Shanker,b
Ammathnadu S. Achalkumara*
aDepartment of Chemistry, Indian Institute of Technology Guwahati, PINꢀ781 039, India.
b Department of Studies in Chemistry, Bangalore University, Bangalore, India
#Equally contributed
Eꢀmail: achalkumar@iitg.ernet.in
Abstract
Three series of chiral nonsymmetrical dimers were prepared by connecting promesogenic
cholesterol to a bent structure derived from substituted 1,3,4ꢀoxadiazole or 1,2,4ꢀoxadiazole or
1,3,4ꢀthiadiazole moiety. These two mesogenic segments are interconnected through spacers of
varying length and parity. The structures of the bent achiral unit were systematically varied with
different central heterocyclic cores to understand the influence of bent angles on the thermal and
gelation behavior. The bent angle of the achiral unit, which is determined by the heterocyclic
core has a major role in the stabilization of frustrated phase. Dimers based on 1,3,4ꢀoxadiazole
unit with a more bent structure stabilized frustrated phases like blue phase and twist grain
boundary phases. The bent system with wider bent angle preferred to stabilize chiral nematic and
smectic A phase. It is interesting to note that increased bent structure reduced the mesophase
stability as in the case of dimers based on 1,3,4ꢀoxadiazole unit, where many compounds
exhibited monotropic phases. In the case of dimers with wider bent angle enantiotropic
mesomorphism was observed. All the compounds showed blue light emission in solution.
Among these chiral dimers only the compounds based on 1,3,4ꢀoxadiazole unit showed the
gelation ability, which emphasizes how a small structural changes like bent angle, dipole
moment and the type of hetero atom in the heterocyclic unit affects the macroscopic selfꢀ
assembly.
TOC
Effect of bent angle on the stabilization of liquid crystalline and organogel self-assemblies
1

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Introduction
Liquid crystals (LC) are unique materials, which possess the intermediate order and fluidity
between solids and liquids.1 Chiral LCs are intriguing as they display unique macroscopic
assemblies with extraordinary properties.2ꢀ6 In fact the discovery of liquid crystallinity itself was
with a chiral mesophase which was later named as chiral nematic (N*) or cholesteric phase.7
Application of cholesteric or chiral smectic C (SmC*) phases in different modes of display
applications is well documented. The selective reflection of the light based on the helix pitch
length of the N* phase can be exploited in the development of thermal sensors in diverse areas
like medicine, packing industry and electronics.8aꢀcSimilarly the SmC* phase could be utilized to
realize fast switching ferroelectric display.3,8d When the mesogens are highly chiral they exhibit
frustrated phases. The term frustrated is used because these mesophases are arising out of a
situation where there is a struggle between chiral forces and molecular inclination for parallel
alignment or to form a layered structure. Blue phases (BPs) and twist grain boundary (TGB)
phases are the two classes of frustrated phases encountered with chiral LCs. These mesophases
are the subject of great curiosity owing to their complex super structures and potential
applications.4,6 Blue phases are also helical like N* and SmC* phases but they exist with a
double twist, with the twist axes existing in all directions. There are three types of blue phases,
such as BPI (body centered cubic), BPII (simple cubic) and BPIII (amorphous phases), which
comprise double twist cylinders.5c,29 Usually, BPs prevails over a small thermal range between
the cholesteric or smectic phase and the isotropic (I) liquid state. These three phases exist in the
order of increasing temperature, where BPIII exists at the higher end, while BPI exists at the
lower end. Blue phases are promising in applications related to novel displays (transmissive9a,b
and reflective displays,9c–e) and photonics.9f–i Enhancement of the thermal range of these
frustrated phases is vital to realize their applications.9j,k Similar to BPs, TGB phases also known
to appear over a short thermal range at the phase transition from the isotropic/N* to smectic A
(SmA) or SmC* phases.5, 29 Even though their significance in applied science is not realized until
now, there is an increasing interest in them. This is because, they are considered as LC
equivalents of the Abrikosov flux phases of typeꢀII superconductors. They are classified as
TGBA, TGBC and TGBC*. This classification is based on the characters of the constituent slabs,
whether it is orthogonal smectic A (SmA), tilted smectic C (SmC) and helical SmC*
respectively. They arise from a competition between the need for the molecules to form a helical
2

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organization and a layered structure. Molecules alleviate this frustration by forming the twist
grain boundary phase that consists of blocks of smectic layers, where the consecutive blocks
being rotated by a constant angle to provide a helical structure, with helix axis being orthogonal
to the layer normal direction. Since the smectic layers cannot be continuously twisted, they are
separated from one another through screw dislocations, which lead to the formation of periodic
grain boundaries, leading to the presence of layering as well as helical twist.5 Unlike blue phases,
there are many wide range TGB phases reported in excess of 100 degrees or a super cooled
TGBC* phase.10
Considering the importance of BPs and TGB phases towards the potential applications,
several molecular designs have been tried to obtain a single molecular system that stabilizes
them over a wide thermal range. Chiral systems formed by joining cholesterol to the one end of a
rodꢀlike molecule through a flexible spacer of variable length and parity, exhibit interesting
thermal behavior.11 These systems are credited for different kinds of frustrated phases, reꢀentrant
phases and unique phase sequences due to their unique structural tunability.11 The length and
parity of the spacers have definite roles on the transition temperatures and associated enthalpy
changes of these mesogens. Further, the nature of the nonꢀcholesteryl segment in the dimer also
affects the thermal behavior. Although there is a huge work is accumulated on cholesterolꢀbased
dimers, a complete understanding of the structure–property correlation is still establishing, owing
to the delicate interplay of various factors as stated above. 11
From the literature, it is evident that whenever a chiral entity is connected to a bent
system or when it is physically mixed with a bent system led to the stabilization of frustrated
phases. Nakata et al. reported that the blue phase could be induced by doping a small amount of
bentꢀshaped achiral molecules into N* LCs, and this has become an efficient method to prepare
blue phase liquid crystals with wide temperature ranges.12a Recently, Wang et. al. reported the
blue phase induced by achiral bentꢀshaped 1,3,4ꢀoxadiazoles. 12b They found that the thermal
range of the blue phase was significantly affected by the terminal alkyl chain length. It was
observed that the widest range could be obtained when the alkyl chain was of moderate length.
Further research into the mechanism suggested that the induced blue phase is mainly due to the
bent structure of the 1,3,4ꢀoxadiazoles, and it does not depend on the mesogenicity of the 1,3,4ꢀ
oxadiazole derivatives. Yang's group12c,d systematically explored the effect of different rigid
cores and lateral substituents of 2,5ꢀdisubstituted 1,3,4ꢀoxadiazoles on the thermal range and
3

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electroꢀoptical effects of blue phases. They found that the bentꢀshaped molecules with higher
miscibility and twisted molecular conformation significantly contributed to the stabilization of
blue phases. Yelamaggad et. al. reported several series of cholesterolꢀbased dimers having a bent
chalcone core which augmented the twist in the overall shape of the molecule, led to a phase
sequence IꢀTGBꢀTGBC*.13 Bentꢀcore systems are known to show spontaneous polarization
through chiral induction in selfꢀorganized layered phases and also to form super helical
structures irrespective of the absence of molecular chirality.14 Recently such cores have been
introduced in cholesterolꢀbased dimers to explore the interaction between the molecular chirality
and the bent conformation. One such dimer with a salicylaldimine based bent system exhibited a
phase sequence, viz., I–BPIII–N*–Mx3 (unknown smectic phase), where the BPIII phase existed
o
over a broad thermal range (~20 C).15 Yoshizawa et. al. reported the stabilization of cubic blue
phase and BPIII phase along with N* phase, on doping the Uꢀshaped oligomers formed from
catechol and biphenyl derivatives possessing two 2,3ꢀdifluoroꢀ1,4ꢀdiphenylbenzene units, with a
chiral additive.16a A homologous series of chiral dimeric Uꢀshaped binaphthyl compounds with
an odd spacer parity showed an unusual phase transition between blue phase to smectic A. 16b
Gorecka et.al. reported chiral mesogenic dimers with an odd parity spacer (thus a bent shape),
17
which exhibited the highest temperature N* phase on heating or blue phases on cooling.
Novotna et. al. reported the stabilization of several frustrated phases (BPs, TGBA and TGBC
phases) over a broad thermal and concentration ranges in the binary mixtures of an achiral
hockey stick mesogen and a chiral rodꢀlike mesogen.18 Similarly, doping of an azobenzeneꢀbased
bentꢀshaped molecule into a cholesteric LC induced a blue phase with a thermal range of 25◦C.19
Till then there is no attempt to correlate the effect of bent angle of the achiral unit on the
stabilization of frustrated system in a dimeric system.
Introduction of five membered heterocycle as a central core provides a tunable handle to
alter the bent angle, dipole moment and intermolecular interactions, which in turn control the
molecular selfꢀassembly. The motivation to introduce these bent units in chiral dimers arose
from the banana systems with central heterocyclic unit. Generally, in banana molecules, 1,3ꢀ
phenylene is used as a central unit to provide the bent shape. On the other hand, biaxial smectic
phases were reported in bent systems containing a 1,3,4ꢀoxadiazole central unit.20 2,5ꢀsubstituted
1,3,4ꢀoxadiazole ring has been used either in the center or in terminus of the mesogenic rigid
core by several research groups to induce biaxiality20,21 while certain other groups used the same
4

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unit as a central core of bentꢀshaped lightꢀemitting liquid crystalline materials.22 Similarly, the
regioisomeric heterocycles like 3,5ꢀdisubstitutedꢀ1,2,4ꢀoxadiazoles have also been employed in
the synthesis of bent shaped compounds (Fig. 1).23 The altered bent angle in comparison to the
1,3ꢀdisubstituted benzene derivatives, places these compounds on the midpoint between
calamitic LCs and bentꢀcore LCs.24 Recently Majumdar et.al. reported the occurrence of wide
o
temperature range TGBC* phase over 100 C in cholesterol based dimers containing 1,3,4ꢀ
oxadiazole derivative connected through an odd spacer.25 In comparison to 1,3,4ꢀoxadiazole unit,
bent systems based on 1,3,4ꢀthiadiazole and 1,2,4ꢀoxadiazole based systems which have different
bent angles are not utilized in designing chiral dimers. This motivated us to systematically
explore the effect of bent angle of the achiral unit connected to cholesterol through the spacers of
varied length and parity. Importance of the bent angle in stabilizing the blue phases are stressed
again in a series of hydrogenꢀbonded dimeric complexes, where the blue phase was preferred to
exist when the bent angle is within a range of 132.1ꢀ152.9o. Hꢀbonded complexes with bent
angles, which fall outside this range, did not stabilize any blue phase.31
In the present work, bent systems based on different heterocyclic units like 1,3,4ꢀ
oxadiazole, 1,2,4ꢀoxadiazole and 1,3,4ꢀthiadiazole, are connected to cholesterol unit through
spacers of varying length and parity. The bent angle of exocyclic bonds in the 2ꢀ and 5ꢀpositions
of these different cores varies around 134o, 141o and 160o respectively. Further to understand the
effect of the terminal chain length we have also varied the chain length in one series of
compounds.
Figure 1. Schematic showing the bond angle difference of different core units.
5

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Results and Discussion
Synthesis and Molecular structural characterization
N
N
O
R
O
O
n
O
1-3/8 : n = 3, R = n-OC8H17
1-4/8 : n = 4, R = n-OC8H17
1-5/8 : n = 5, R = n-OC8H17
1-7/8 : n = 7, R = n-OC8H17
1-3/9 : n = 3, R = n-OC9H19
1-4/9 : n = 4, R = n-OC9H19
1-5/9 : n = 5, R = n-OC9H19
1-7/9 : n = 7, R = n-OC9H19
1-3/10 : n = 3, R = n-OC10H21
1-4/10 : n = 4, R = n-OC10H21
1-5/10 : n = 5, R = n-OC10H21
1-7/10 : n = 7, R = n-OC10H21
N
N
2-3/8 : n = 3, R = n-OC8H17
S
2-4/8 : n = 4, R = n-OC8H17
2-5/8 : n = 5, R = n-OC8H17
2-7/8 : n = 7, R = n-OC8H17
R
O
O
n
O
N
O
3-3/8 : n = 3, R = n-OC8H17
3-4/8 : n = 4, R = n-OC8H17
3-5/8 : n = 5, R = n-OC8H17
3-7/8 : n = 7, R = n-OC8H17
N
R
O
O
n
O
Figure 2. Molecular structures of cholesterolꢀbased dimers belonging to 1,3,4ꢀ and 1,2,4ꢀ
oxadiazole and 1,3,4ꢀthiadiazole series.
The synthetic route adopted to prepare three series of target dimers (Fig.2) and their precursors is
outlined in Scheme 1 (See the Electronic Supporting Information (ESI)). Cholesteryl ωꢀ
bromoalkanoates were synthesized as reported in the literature.13 The key intermediates, namely
ethyl 4ꢀnꢀalkoxy benzoates (16a-c) and 4ꢀnꢀoctyloxy benzonitrile (14) were prepared by the Oꢀ
alkylation of ethyl 4ꢀhydroxybenzoate and 4ꢀhydroxybenzonitrile respectively following
Williamson’s protocol. Similarly, ethyl 4ꢀbenzyloxy benzoate (12) was prepared by Oꢀ
benzylation of ethyl 4ꢀhydroxybenzoate. Hydrolysis of the benzyl ester 12 yielded 4ꢀbenzyloxy
benzoic acid (11). Refluxing of 4ꢀnꢀalkoxy benzoates (16a-c) with hydrazine hydrate in ethanol
yielded benzhydrazide intermediates (15a-c), which on further refluxing with 4ꢀbenzyloxy
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benzoic acid chloride in THF in presence of triethylamine base afforded benzoyl
benzohydrazides (8a-c). Then dehydrocyclization of compounds 8a-c using phosphorous
oxychloride yielded 2ꢀ(4ꢀ(benzyloxy) phenyl)ꢀ5ꢀ(4ꢀ(alkoxy)phenyl)ꢀ1,3,4ꢀoxadiazoles (5a-c).26
Similarly the intermediate 8a on refluxing with Lawesson’s reagent in toluene afforded 2ꢀ(4ꢀ
(benzyloxy) phenyl)ꢀ5ꢀ(4ꢀ(octyloxy) phenyl)ꢀ1,3, 4ꢀthiadiazole (7).26a,b Refluxing of the nitrile
(14) with hydroxylamine hydrochloride in ethanol led to benzamidoxime (13), which on
refluxing with 4ꢀbenzyloxy benzoic acid chloride in pyridine afforded 3ꢀ(4ꢀ(benzyloxy)phenyl)ꢀ
5ꢀ(4ꢀ(octyloxy)phenyl)ꢀ1,2,4ꢀoxadiazole (10).26c Debenzylation of 2ꢀ(4ꢀ(benzyloxy)phenyl)ꢀ5ꢀ(4ꢀ
(alkoxy)phenyl)ꢀ1,3,4ꢀoxadiazoles (5a-c), 2ꢀ(4ꢀ(benzyloxy) phenyl)ꢀ5ꢀ(4ꢀ(alkoxy) phenyl)ꢀ1,3,4ꢀ
thiadiazole (7) and 3ꢀ(4ꢀ(benzyloxy) phenyl)ꢀ5ꢀ(4ꢀ(alkoxy) phenyl)ꢀ1, 2, 4ꢀoxadiazole (10) on
debenzylation yielded the corresponding hydroxyl derivatives 4a-c, 6 and 9.24 Oꢀalkylation of
these key intermediates with appropriate cholesterylꢀωꢀbromoalkanoates
under typical
Williamson ether synthesis conditions yielded the target dimers in good yield. The structures of
1
all the intermediates and target molecules were confirmed by spectroscopic technique like H
NMR, 13C NMR, IR spectroscopy, HRMS and MALDIꢀTOF mass spectrometry (See the SI).
Thermal behavior
The mesomorphic behavior of these three new series of cholesterol dimers consisting of
thiadiazole and regioisomeric oxadiazole moieties (Fig.2) was investigated by polarizing optical
microscopy (POM) and differential scanning calorimetric (DSC) studies. The incidence of
thermotropic phases was initially verified based on the observation of strong birefringence and
fluidity under microscope. The characteristic textural pattern seen on POM concluded the precise
LC phase. The peak temperatures due to transitions obtained in the DSC traces were found to be
well accordance with those of the POM studies. Transition temperatures, phase transitions of
each compound and their corresponding enthalpies derived from DSC investigation were
presented in Table 1. The thermal behavior is graphically represented in Fig. S97. Optical
observations were made either with untreated clean glass slides and cover slips or with
ethyltriethoxysilane coated slides (for homeotropic orientation) or with polyimide coated slides
(for planar orientation). Out of the three series containing twenty compounds, only five of them
stabilized frustrated phases and the rest exhibited either smectic or nematic mesophase.
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Table 1.Phase transition temperaturesa (oC) and corresponding enthalpies (kJmolꢀ1) of
cholesterol dimers.
Entry
Phase sequence
2nd Heating
1st Cooling
Cr 128.8 (41.3) Cr 134.8 (6.2) SmA 161.7
(1.0) I
I 156.7 (7.5) SmA 106.6 (109.2) Cr
1-3/8
Cr 152.1 (122.8) I
I 151.1 (11.9) SmA 102.8 (20.6) Cr 97.8 (4.8)
Cr 94.7 (11.1) Cr
1-4/8
Cr 151.1 (184.2) I
I 146.3 (11.0) SmA 90.6 (164.4) Cr
1-5/8
1-7/8
Cr 118.7 (210.1) TGBA 135.9 (1.7) N*
160.4 (18.0) I
Cr 137.9 (94.5) SmA 166b I
Cr 142.0 (154.8) I
I 159.6 (17.2) BP 157.9 (0.1) N* 135.1 (2.7)
TGBA 91.6 (219.1) Cr
I 165.6 (5.2) SmA 109.5 (117.6) Cr
1-3/9
1-4/9
I 141.0 (10.4) SmA 99.8 (19.7) Cr 92.7 (28.4)
Cr 88.3 (18.3) Cr 64.2 (5.8) Cr
Cr 142.8 (183.9) SmA 153.8 (1.5) I
I 153.7 (0.8) TGBA 142.6 (13.7) SmA 109b
1-5/9
TGBA 105b TGBC*100.3 (17.0) Cr
Cr 87.9 (65.8) SmA 115.2 (4.4) I
Cr 135.2 (175.9) SmA 187 (3.2) I
I 120.8 (5.2) SmA 66.3 (61) Cr
1-7/9
1-3/10
1-4/10
I 185.2 (13.1) SmA 109.7 (165.3) Cr
I 149.9 (24.9) SmA 135.8bSmC* 85.1 (143.4)
Cr
Cr 88.6 (6.3) Cr 115.9 (20.2) Cr 130.3
(110.4) SmA 149.9 (21.8) I
Cr 132.9 (179.1) TGBA 158.6 (38.5) I
I 158.2 (35.1) TGBA 91.4 (156.9) Cr
1-5/10
1-7/10
Cr 120.4 (291.2) TGBA 136.4 (3.2) N*
148.5 (16.2) I
I 147.6 (15.3) BP 144.9 (0.2) N* 134.9 (5.4)
TGBA 89.5 (2.4) Cr
Cr 132.1 (89.6) N* 203.7 (3.2) I
Cr 148.7 (113.9) SmA 260.6 (4.7) I
Cr 136.3 (164.6) N* 225.8 (7.1) I
I 187.7 (2.2) N* 69.8 (66.4) Cr
2-3/8
2-4/8
2-5/8
I 243.4 (1.0) SmA 95.6 (91.0) Cr
I 220.9 (17.7) BPIII 217.4b N* 79 (89.1) Cr
Cr 137.7 (175.8) SmA 230.3 (4.7) I
Cr 140.4 (197.2) SmA 221.6 (38.7) I
Cr 165.7 (34.1) SmA 173.1 (26.0) I
Cr 176.2 (243.0) SmA 183.6 (6.5) I
Cr 137.4 (239.7) SmA 185.3 (13.6) I
I 212.9 (3.3) SmA 82.36 (118.9) Cr
I 213.9 (38.2) SmA 57.8 (59.4) Cr
I 167.7 (13.4) SmA 112.4 (119.5) Cr
I 180.8 (35.1) SmA 119.9 (165.2) Cr
I 179.9 (12.7) SmA 84.3 (142.9) Cr
2-7/8
3-3/8
3-4/8
3-5/8
3-7/8
aPeak temperatures in the DSC thermograms obtained during the second heating and first cooling cycles at 5 oC/min.
Cr = Crystal phase; BP = Blue phase; N* = chiral nematic; TGBA = Twist grain boundary phase formed by chiral
SmA blocks; TGBC* = Twist grain boundary phase formed by chiral SmC* blocks; SmA = Smectic A; I = Isotropic
phase. btransition detected in the POM but not observed in DSC.
Compounds 1-3/8, 1-4/8 and 1-5/8 on slow cooling from the isotropic phase exhibited SmA
phase with focal conic fans with dark field of view (Fig.3a) or focal conic fans exclusively (Fig.
o
3bꢀc, S81ꢀ83). Compound 1-7/8 on slow cooling (0.01 C/min) from the isotropic liquid change
to a foggy blue colored mesophase with green platelets resembling the characteristic textures for
BPI or BPII phase (Fig.4b). This texture spanned over a short range of ~1.8 oC (Fig.4a). Further
cooling showed a transition to chiral nematic phase, which exhibited an oilyꢀstreak texture
(Fig.4c) on shearing the small focal conics obtained upon cooling. At around 135.1oC a phase
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transition occurred from chiral nematic phase to a blurred pattern obtained for TGB phase (Fig.
o
4d). This phase was stable up to 92 C, below which it crystallizes as evident in DSC
thermogram (Fig. 4a), which is in well agreement with the POM observations.
T = 135 oC
T = 139 oC
T = 149 oC
(a)
(b)
(c)
Figure 3. POM images obtained for the SmA phases on cooling from isotropic state for
compounds 1-3/8 (a); 1-4/8 (b) and for compound 1-5/8 (c).
BP
A
A
(a)
(b)
(d)
(c)
Figure 4. DSC thermogram in the second heating (red trace) and first cooling (blue trace) at the
scan rate of 5oC/min (a); POM images obtained for compound 1-7/8 on cooling from isotropic
phase, BP at 158.9oC (b); oily streak texture of N*phase (c); and TGBA phase (d).
Chiral dimers with nꢀnonayloxy terminal chain, i.e.1-3/9, 1-4/9 and 1-7/9 exhibited a
SmA phase. In case of dimers 1-3/9 and 1-7/9, the SmA phase was enantiotropic (Fig. S84, S86)
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A
(b)
(a)
(c)
(d)
(e)
Figure 5.DSC thermograms of compound 1-5/9 for the second heating (red trace) and first
o
cooling (blue trace) cycles (a) Filament texture in the high temperature TGBA phase at 150 C
(b); homeotropically aligned smectic A phase (c); reentrant TGBA phase (d); and TGBC* phase
at 101 oC (e).
while in the case of compound 1-4/9, the mesophase was monotropic (Fig. S85). Chiral dimer of
the same series with the spacer length of six carbon atoms (1-5/9) exhibited an interesting
thermal behavior. This compound exhibited an enantiotropic SmA phase (Fig.5a). The sample
sandwiched between a glass slide treated for homeotropic alignment and a coverslip, on cooling
from the isotropic melt exhibited a filament texture originating from the dark field of view,
o
which is the characteristic of the TGBA phase (Fig.5b). On cooling further, at ≈143 C, the
filaments disappear leaving the field of view completely dark (Fig.5c), indicating that the phase
could be a SmA phase. On further lowering of temperature, the filamentary texture reenters as in
o
Fig. 5d at 109 C, suggesting the existence of another TGBA phase i.e. reꢀentrant TGBA phase.
Both the high and low temperature TGBA phases had straight filaments without any undulation.
However SmAꢀTGBA (reentrant) transition was not detected in DSC. Further cooling showed an
undulation in the filaments, which is the characteristic of TGBC* phase, where the molecules are
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tilted in the smectic blocks (Fig. 5e).27,28 As per the proposed model, the constituent blocks are
tilted (SmC–like) and organized to form a helix along the direction of smectic layer normal, in
addition to the typical TGB helix, which is parallel to the smectic planes. Another specific
feature of this model is the existence of the undulating grain boundaries. The TGBC* has been
seen in very few materials27 for example in a trimesogen, which stabilizes TGBC* phase over a
wide thermal range of 90 degrees. From these observations the phase sequence can be written as
Iso–TGBA–SmA–ReꢀTGBA–TGBC*ꢀCr. First two dimers with terminal nꢀdecyloxy chain with
spacers containing four and five carbon atoms (1-3/10 and 1-4/10) exhibited an enantiotropic
SmA phase (Fig. S87, S88). In the case of dimer 1-4/10, a monotropic SmC* was also seen with
a broken focal conic fan texture. Compound 1-5/10 (in slides treated for homeotropic alignment)
on cooling from isotropic liquid state exhibited the formation of filaments (Fig.6b,c) which is
characteristic of TGBA phase. This phase existed over a wide thermal range of 67 degrees, with
a crystallization temperature of ≈ 91oC (Fig.6a). Chiral dimer 1-7/10 on slow cooling
(0.01◦C/min) from the isotropic liquid showed a texture corresponding to BPI or BPII
(Fig.7b).29a This phase existed over very narrow thermal ranges of 2.7 degrees. On cooling, this
pattern was converted to the small focal conics of chiral nematic phase, which on shearing
A
A
Figure 6.DSC thermograms of compound 1-5/10 for the second heating (red trace) and first
cooling (blue trace) cycles (a); filament texture in the high temperature TGBA phase at 156oC
(b); and at 149 oC (c).
furnished an oily streak texture of N* phase (Fig. 7c). Further cooling showed a blurred pattern
as in Fig. 7d, which is corresponding to the TGBA phase. The occurrence of TGBA mesophase
was further supported by the filamentary texture obtained on slides treated for homeotropic
alignment. This mesophase spanned over a thermal range of 45 degrees and was crystallized at
around 90 oC (Fig.7a).
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Figure 7. (a) DSC thermogram in the second heating (red trace) and first cooling (blue trace)
obtained for dimer 1-7/10 (a); POM image of the texture obtained for BPI or BPII (b); oily streak
texture of N* phase on untreated slide (c); blurred planar texture obtained for TGBA on cooling
from N* phase (d).
All the thiadiazole based chiral dimers 2-3/8, 2-4/8, 2-5/8 and 2-7/8 exhibited
enantiotropic mesophases. Compounds 2-3/8 and 2-5/8 exhibited a wide range N* phase (Fig.
S89, S91). Dimer 2-5/8 exhibited BPIII over a range of 3 degrees in addition to N* phase.
Similarly compounds 2-4/8 and 2-7/8 exhibited wide range SmA phase with characteristic
textures (Fig. S90, S92). It should be noted that considering the larger bent angle of 1,3,4ꢀ
thiadiazole, the propensity of the stabilization of the frustrated phases has been reduced. Chiral
dimers based on 1,2,4ꢀoxadiazole exhibited longꢀrange SmA phases irrespective of the spacer
length (Fig.S93ꢀS96). Among these compounds the dimer with a spacer containing four carbon
atoms exhibited a SmA phase of widest thermal range (Fig. S97).
Thus on examining the thermal behavior of these three different series, it is evident that
bent angle of the mesogenic unit connected to cholesterol plays a major role in stabilization of
the frustrated phases. Thus the first series based on 1,3,4ꢀoxadiazole derivatives (1-7/8, 1-5/9, 1-
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5/10 and 1-7/10) stabilized frustrated phases like blue phases and TGBA phases. Further
compound 1-5/10 is unique as it stabilized enantiotropic TGBA phase over a wide temperature
range. This is the first example of dimer stabilizing TGBA phase exclusively. The length of the
terminal tail did not show any regular trend in the mesomorphic behavior, instead the dimers
with six and eight carbon atom spacers showed a propensity to stabilize frustrated phases. We
should also note that many compounds of this series exhibited monotropic mesophases, but the
scenario changed dramatically on moving to their thiadiazole or 1,2,4ꢀoxadiazole analogues
which possess wider bent angle in comparison to 1,3,4ꢀoxadiazole based dimers. All the dimers
based on 1,2,4ꢀoxadiazole stabilized SmA phases, in comparison to dimers based on 1,3,4ꢀ
oxadiazole. It is also noteworthy that even though thiadiazole based dimers do not show any
TGB phase in comparison to 1,3,4ꢀoxadiazole derivatives (which exhibit both blue phase and
TGB phases), all the molecules show enantiotropic mesomorphic behavior, which is due to the
enhanced interaction provided by the sulphur atom in the heterocycle along with an increase in
the molecular linearity.
Photophysical properties
We were interested in the photophysical properties of these dimers considering the
incorporation of highly luminescent oxadiazole and thiadiazole moieties in the molecular
structures. The UVꢀvisible and fluorescence spectra of four dimers in each series were recorded
in micromolar THF solution and the results are tabulated (Table 2). The absorption and emission
spectra of both series of isomeric oxadiazole derivatives were quite different.
Dimers based on 1,3,4ꢀoxadiazoles exhibited a single absorption maximum centered at
303 nm, while dimers based on 1,2,4ꢀoxadiazole derivatives showed a blue shifted absorption
maximum centered at 289 nm. As expected 1,3,4ꢀthiadiazole derivatives exhibited, a red shifted
absorption maximum at around 330 nm (Fig.8). Based on the similarity of the previously
reported systems this band corresponds to the πꢀπ* transition of aromatic system.26 Emission
spectra of dimers based on 1,3,4ꢀoxadiazoles show three different bands at 337, 355, 370 nm,
whereas the dimers based on 1,3,4ꢀthiadiazoles showed a maximum at 398 nm along with a
shoulder at 415 nm. Dimers based on 1,2,4ꢀoxadiazoles exhibited a single emission maximum
centered at 366 nm. All the molecules exhibited a large Stoke’s shift ranging from 5976 – 7280
cmꢀ1.
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Figure 8. Normalized absorption spectra (solid line) and emission spectra (dotted line) in
micromolar THF solution obtained for 1,3,4ꢀoxadiazole derivatives (a); 1,3,4ꢀthiadiazole
derivatives and (b); 1,2,4ꢀoxadiazole derivatives (c).
Table 2.Photophysical a properties of selected dimers.
Entry
Absorption (nm)
Emissionb (nm)
Stokes shift (cmꢀ1)
303
303
303
303
330
330
330
330
289
289
289
289
337, 355, 370
337, 355, 370
337, 355, 370
337, 355, 370
398, 415
398, 415
398, 415
398, 415
366
5976
5976
5976
5976
6207
6207
6207
6207
7280
7280
7280
7280
1-3/8
1-4/8
1-5/8
1-7/8
2-3/8
2-4/8
2-5/8
2-7/8
3-3/8
3-4/8
3-5/8
3-7/8
366
366
366
amicromolar solutions in THF; bexcited at the respective absorption
maxima.
Gelation studies
The gelation properties of the 1,3,4ꢀoxadiazole, 1,3,4ꢀthiadiazole and 1,2,4ꢀoxadiazole
based cholesterol dimers were analyzed in various solvents such as hexane, decane, dodecane,
hexadecane (alkanes), chloroform, dichloromethane, ethanol, dimethylsulfoxide (DMSO),
dimethyl formamide (DMF), tetrahydrofuran (polar aprotic solvents) nꢀbutanol, methanol, (polar
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protic solvents), benzene, toluene and mꢀxylene (aromatic hydrocarbons) at a standard
concentration of 1.5% (w/v). Interestingly, 1,3,4ꢀoxadiazole derivatives (1-3/8, 1-4/8, 1-5/8 and
1-7/8) were able to gelate only in hexadecane but 1,3,4ꢀthiadiazole and 1,2,4ꢀoxadiazole
derivatives did not show any gelation (Table 1ꢀ2 in SI). Immediately after adding the compound
in hexadecane the suspension was heated to get clear solution. The resulting solution was
allowed to cool for around 20ꢀ30 minutes to form the gel at room temperature. The gel formation
was confirmed by “stable to inversion of the glass vial” method, i.e. the gel was unable to flow
on inversion, which confirms the formation of a stable opaque gel (Fig. 9f). The 1,3,4ꢀthiadiazole
and 1,2,4ꢀoxadiazole derivatives precipitated on cooling the clear solution. The critical gelation
concentration (CGC in wt.%) is the minimum concentration necessary for gelation and Tgel (oC)
is the thermal stability of the gels, are determined by ‘dropping ball’ method30 (ESI table 1,2).
The compound 1-7/8 was found to have CGC of 1.12 wt. % and Tgel of 52 oC.
Figure 9. AFM images obtained for compound 1-7/8 at 1mM hexadecane solution (a); 20µM
hexadecane solution (b); distanceꢀheight profile of fibers (c); FESEM images obtained for
compound 1-7/8 at 20µM hexadecane solution (d); expanded region (e); solꢀgel form of
compound in day light and UV light (365 nm)(f).
The surface morphology of the xerogel of 1-7/8 obtained from the organogel in
hexadecane was studied with the help of field emission scanning electron microscopy (FEꢀSEM)
and atomic force microscopy (AFM). All the microscopy studies were carried out after drop
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casting 1 mM and 20 µM hexadecane solution followed by evaporation in vacuum. AFM
microscopic image showed a very thick network of fibers when the sample was prepared from 1
mM concentration as depicted in (Fig. 9a). Similar experiment carried out with a solution at
lower concentration (20 µM, Fig. 9b), helped us to determine the thickness and height of single
fiber. The fibers are found to be of several micrometers in length with heights of 15ꢀ20 nm and
thickness of 250ꢀ350 nm, (Fig. 9c). FESEM studies also revealed the entangled network of fibers
as shown in Fig. 9d and 9e. These fibers are of several micrometers in length, which entangle
further to form a network, that encapsulate a large volume of solvent to stabilize gelation. It is
interesting to note that in spite of the structural similarity, 1,2,4ꢀoxadiazole based dimer did not
undergo gelation. Even the difference of one atom as in the case of 1,3,4ꢀthiadiazole based dimer
had a destabilizing affect on the gelation ability.
Conclusion
In this paper, we reported three new series of chiral nonꢀsymmetric dimers containing
cholesterol moiety covalently linked to regioisomeric oxadiazole/thiadiazole based bent systems
through the spacers of varying length and parity. These compounds are investigated for their
thermal, photophysical and gelation behavior. This investigation was undertaken to understand
the effect of the bent angle in stabilizing the frustrated phases, by varying the central core of the
bent system, which alters the bent angle. The series containing 1,3,4ꢀoxadiazole core is found to
stabilize frustrated phases like BP, N*, TGBA, TGBC* and reꢀentrant TGBA phases along with
SmA and SmC* phases in some cases, whereas series containing regioisomeric 1,2,4ꢀoxadiazole
core exhibited only smectic mesophases. This shows that bent angle of the central core of the
achiral unit is critical in stabilizing the frustrated phases. 1,3,4ꢀthiadiazole based chiral dimers
exhibited N*and/or BP and SmA phases. There is also pronounced effect of the type of
heterocycle on the aggregation behavior in solvents; the dimers with 1,3,4ꢀoxadiazole moiety
were found to gelate in hexadecane while the dimers with 1,2,4ꢀoxadiazole and 1,3,4ꢀthiadiazole
moieties precipitate. The present study clearly reveals how a minute structural modification
affects the macroscopic selfꢀassemblies like liquid crystals and gels.
Acknowledgement
This work was supported by DST, India. The financial assistance from Department of Science
and Technology through DSTꢀSERB Fast track project no. CSꢀ140/2014 is gratefully
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acknowledged. ASA sincerely thanks Science and Engineering Board (SERB), DST, Govt. of
India and BRNSꢀDAE for funding this work through project No. SB/S1/PCꢀ37/2012 and
No.2012/34/31/BRNS/1039 respectively. We thank the Ministry of Human Resource
Development for Centre of Excellence in FAST (F. No. 5ꢀ7/2014ꢀTSꢀVII). ASA acknowledges
CIF, IIT Guwahati for analytical facilities.
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