Chiral polymorphism in new imine based rod-like liquid crystals

Article abstract of DOI:10.1016/j.molstruc.2020.128755

New imine based rod-like compounds, composed of three-benzene-ring molecular core linked with azomethine as well as ester groups and terminated with (S)-3,7-dimethyloctyloxy chiral unit at one side have been synthesized. The other terminal position has been varied by using n-octyloxy/decyloxy/dodecyloxy groups to reveal chain length effect on mesophase behavior. The liquid crystalline properties of the new compounds have been investigated by polarizing optical microscopy, differential scanning calorimetry and electro-optic studies. The novel chiral calamitics derived from Schiff base or salicylaldimine-core show a chiral polymorphism comprising of blue phase and chiral nematic as well as smectics such as chiral tilted smectic and unidentified smectic mesophase.

Full text of DOI:10.1016/j.molstruc.2020.128755

Journal Pre-proof
Chiral polymorphism in new imine based rod-like liquid crystals
Fatma Pınar Çağlar, Huriye Akdaş-Kılıç, Hale Ocak, Belkız Bilgin Eran
PII:
S0022-2860(20)31080-2
DOI:
https://doi.org/10.1016/j.molstruc.2020.128755
MOLSTR 128755
Reference:
To appear in: Journal of Molecular Structure
Received Date: 1 June 2020
Revised Date: 21 June 2020
Accepted Date: 23 June 2020
Please cite this article as: Fatma.Pı. Çağlar, H. Akdaş-Kılıç, H. Ocak, Belkı.Bilgin. Eran, Chiral
polymorphism in new imine based rod-like liquid crystals, Journal of Molecular Structure (2020), doi:
https://doi.org/10.1016/j.molstruc.2020.128755.
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2020 Published by Elsevier B.V.

CRediT author statement
Fatma Pınar Çağlar: Investigation
Huriye Akdaş-Kılıç: Investigation, Writing - Original Draft
Hale Ocak: Investigation, Resources, Conceptualization, Writing - Review & Editing
Belkız Bilgin Eran: Resources, Conceptualization, Writing - Review & Editing, Supervision,
Project administration

Graphical Abstract
O
O
(
S)
*
O
N
R
OH
R= a: -OC H b: -OC10H21 c: -OC12H25
4
a-c: X= -OH ; 5a,b: X= -H
8
17
2
00
00
0
2
0
1
-
100
200
-
2
-
-
0,001
0,000
0,001
0,002
t / s

1
2
Chiral Polymorphism in New Imine Based Rod-Like Liquid
Crystals
3
4
*
*
5
6
7
Fatma Pınar Çağlar, Huriye Akdaş-Kılıç, Hale Ocak , Belkız Bilgin Eran
Yildiz Technical University, Department of Chemistry, 34220 Esenler, Istanbul, Turkey
8
9
0
1
*Corresponding Authors: hocak@yildiz.edu.tr, bbilgin@yildiz.edu.tr
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3
Abstract
New imine based rod-like compounds, composed of three-benzene-ring molecular core linked
with azomethine as well as ester groups and terminated with (S)-3,7-dimethyloctyloxy chiral
unit at one side have been synthesized. The other terminal position has been varied by using
n-octyloxy/decyloxy/dodecyloxy groups to reveal chain length effect on mesophase behavior.
The liquid crystalline properties of the new compounds have been investigated by polarizing
optical microscopy, differential scanning calorimetry and electro-optic studies. The novel
chiral calamitics derived from Schiff base or salicylaldimine-core show a chiral
polymorphism comprising of blue phase and chiral nematic as well as smectics such as chiral
tilted smectic and unidentified smectic mesophase.
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
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0
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2
3
4
5
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7
8
9
0
Keywords: Imine, rod-like liquid crystals, chiral mesophases, polymorphism.
1

1
2
3
1. Introduction
Liquid crystal state, which is a unique form of matter, have attracted much attention both in
the basic sciences and applications, and gives rise to a wide range of advanced technologies
such as digital displays, sensors, liquid crystal displays (LCDs), high-performance polymers,
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
1
,2,3,4,5
transporting of electron, ion or molecule and drug-delivery systems, hybrid composites.
The wide applicability and commercial success of LCs are based on switching based
6
,7,8,9,10,11,12
technologies.
The synthesis of optically active liquid crystals (LCs) especially represents one of the most
attractive research area due to their remarkable mesomorphic and electro-optic properties as
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
1
3,14,15
compared to achiral analogs.
The introduction of asymmetric carbon atom/s into
calamitic molecule structure generally leads to the occurrence of a macroscopic twist in the
organization of the constituent mesogens which results in the formation of frustrated
mesophases such as chiral nematic (N*) phase, blue phases (BPs) as well as complex ordered
1
6
LC phases such as chiral smectics with unique properties. In recent years, BPs, whose
structure is of highly twisted, have attracted a scientific curiosity and developed to emerge
1
7,18,19,20,21
unique materials for new applications.
In general, BPs temperature range is very
narrow such as between 0.5-1 °C and it emerges between cholesteric (N*) and isotropic
2
2,23
phase.
Most attractive features of BPs are sub-millisecond response times and optical
2
4,25
26
isotropic behavior
which make them perfect candidates for a number of applications.
The main interest of researches on mesogens with BPs broaden their mesomorphic range in
order to make them potential materials for commercial applications such as displays and
2
7
photonic technology.
The investigation of the structure-property relationships in order to synthesize one-component
chiral LCs carrying the desired characteristics for technological applications play a key role
for the optimization of important parameters like anisotropy, linearity, polarizability and
2
8,29
molecular shape.
Some modifications on linking units, terminal chains and central
3
0
aromatic cores of rod-like molecules can induce striking effects on their mesomorphism. An
azomethine linking group is generally incorporated to the rod-like structure to increase the
3
1
length, molecular core polarizability and mesophase stability. These molecules carrying it
defined as Schiff bases or imine compounds widely exhibit conventional calamitic LC
3
2
mesophases such as nematic and/or smectic mesophases. Optically active moieties are
widely used in rod-like liquid crystals for the occurrence chiral calamitic
3
1,33,34,35,36,37,38,39
mesophases.
Especially, the studies on three-ring calamitics carrying an
2

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
Schiff base linker as well as an ester linking group in the same structure show that the
occurrence of chiral mesomophism such as the blue phase-I/II (BPI or BPII) and chiral
4
0,41,42,43
nematic (N*) and chiral smectic C (SmC*) phases.
Additionally, chiral liquid crystals
carrying a salicylaldimine core are more stable due to the presence of intramolecular
3
1,41,43
hydrogen bonding between the hydroxy group and the imine group.
Due to the fact that the introduction of chiral moieties gives rise to the occurence of a rich
variety of the polar liquid crystalline mesophases (S)-3,7-dimethyloctyloxy chiral chain has
been incorporated into terminal positions in a number of previous studies.
,
4
0,44,45,46,47
In this
study, the synthesis, structural and mesomorphic characterization of novel chiral calamitics
derived from Schiff base or salicylaldimine-core are reported. Aromatic cores in target
molecular three-rings architectures are connected with each other by azomethine (-CH=N-) as
well as ester (-COO-) linkers which induce lateral polarity. By the aim of understanding the
effect of the presence of molecular chirality as well as the variation of chain length on the
effect of mesophase behavior, (S)-3,7-dimethyloctyloxy chiral moiety at one of the terminals
and n-octyloxy/decyloxy/dodecyloxy chains at the other were incorporated to the rod-like
structure (see Scheme 1). The liquid crystalline properties of the new mesogens were
investigated by differential scanning calorimetry, optical polarizing microscopy and electro-
optic methods.
1
1
1
1
1
1
1
1
1
1
2
2
2
0
1
2
2. Experimental
2.1. Materials
2
2
2
2
2
2
2
3
3
3
3
3
3
4
5
6
7
8
9
0
1
2
3
4
2,4-dihydroxybenzaldehyde (Aldrich), 4-hydroxybenzaldehyde (Aldrich), glacial acetic acid
(Merck), Ethanol (Merck, 96%), N,N′-dicyclohexylcarbodiimide (Merck) and 4-
(dimethylamino)pyridine (Merck) were purchased commercially. For efficient esterification,
anhydrous solvent methylene chloride (CH Cl ) was obtained by drying over di-phosphorus
2
2
pentoxide (Merck) and distilled under a N atmosphere. Solvents used in the purification step
2
such as crystallization and column chromatography (hexane, ethyl acetate, chloroform,
acetone, methanol and ethanol) were distilled. Analytical thin-layer chromatography (TLC)
was carried out on aluminium plates coated with silica gel 60 F254 (Merck) for aldehyde
intermediates and aluminium plates coated with aluminium oxide 60 F254, neutral (Merck)
for target imine products. Column chromatography was performed using silica gel 60 (Merck,
pore size 60 Å, 230-400 mesh particle size).
3

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
2.2. Instrumentation
1
13
New chiral imine compounds (4a-c and 5a,b) were characterized H-NMR and C-NMR
(Bruker Avance III 500 spectrometer, in CDCl solution, with tetramethylsilane as internal
3
standard).
The mesomorphic properties of compounds were investigated by using a Mettler FP-82 HT
hot stage and control unit in conjunction with a Leica DM2700P polarizing microscope. DSC-
thermograms of chiral imine compounds were recorded on a Perkin-Elmer DSC-6, heating
−
1
and cooling rate: 10 °C min in a nitrogen atmosphere. Electro-optical investigation of
compound 4a were performed in a 10 μm polyimide (PI) non-coated ITO cell (EHC, Japan)
2
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
with a measuring area of 1 cm . Switching experiments were carried out with the triangular-
4
8
wave method using a combination of a function synthesizer (Agilent, model 33220A, load
was set to 10 kΩ), an amplifier (Tabor electronics, model 9400), and the current response
traces were recorded using an oscilloscope (Agilent, model DSO3202A) across a 5 kΩ
resistance.
2.3. Synthesis and characterization
The synthesis of intermediate compounds 2,3 and new imine based rod-like compounds
4a-c and 5a,b
The synthesis of new imine based rod-like compounds 4a-c and 5a,b which are structurally
distinguished by the functionalization of 2-position of the central aromatic ring by one
hydroxyl group is presented in Scheme 1. The imine based compounds 4a-c and 5a,b carrying
ester linking unit were prepared with a procedure which is similar to that used for compounds
4
0
,
44 49
reported earlier. The starting compound (S)-4-(3,7-dimethyloctyloxy)benzoic acid
was
synthesized by the alkylation of ethyl 4-hydroxybenzoate with (S)-3,7-dimethyloctyl-1-
bromide in 2-butanone using K CO and then basic hydrolysis of the obtained product with
2
3
NaOH/H O in ethanol. Spectroscopic data and preparation procedures of (S)-4-(3,7-
2
4
0,49
Dimethyloctyloxy)benzoic acid was reported in previous studies.
4

O
O
H
(
S)
*
O
+
HO
OH
1
X
i
O
O
(
S)
*
O
O
H
+
H N
2
R
X
2
: X= -OH ; 3: X= -H
ii
O
O
(
S)
*
O
N
R
X
4
a-c: X= -OH ; 5a,b: X= -H
a: -OC H
8
17
R= b: -OC H
1
0
21
25
c: -OC H
12
1
2
Scheme 1. Synthesis of the imine based rod-like compounds 4a-c and 5a,b. Reagents and
conditions: (i) DCC, DMAP, dry CH Cl , r.t., overnight; (ii) Glacial acetic acid, dry ethanol,
3
4
5
6
7
8
9
0
1
2
3
4
5
2
2
reflux at 100 °C, 6h.
The esterification of (S)-4-(3,7-dimethyloctyloxy)benzoic acid by treatment with 2,4-
hydroxybenzaldehyde or 4-hydroxybenzaldehyde using N,N′-dicyclohexylcarbodiimide
(DCC) and 4-(dimethylamino)pyridine (DMAP) as catalyst yield the compounds 2,3. The
condensation reaction of compound 2 or 3 and the corresponding aniline compounds p-
octyloxyaniline / p-decyloxyaniline / p-dodecyloxyaniline with glacial acetic acid as catalyst
in ethanol gives the target imine compounds 4a-c and 5a,b. The aniline compounds were
1
1
1
1
1
1
3
1
synthesized as described previously by the reaction of the corresponding alkyl bromide with
5
0,51,52
p-nitrophenol in DMF and then the obtained p-alkyloxynitrobenzene
were reduced by
5
3,54,55
catalytic hydrogenation (H , 10% Pd/C in ethyl acetate) to the corresponding anilines.
2
5

1
2
3
The procedures for the synthesis of intermediate compounds 2,3 and imine based chiral
compounds 4a-c and 5a,b
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
Compounds 2,3:
To a solution of 2.0 mmol of (S)-4-(3,7-dimethyloctyloxy)benzoic acid and 2.0 mmol of 2,4-
dihydroxybenzaldehyde or 4-hydroxybenzaldehyde in 75 ml of dry dichloromethane, 2.0
mmol of N,N’-dicyclohexylcarbodiimide (DCC) and cataytic amount of 4-
(dimethylamino)pyridine (DMAP) were added and the reaction mixture was stirred for 24h at
room temperature under argon atmosphere. The end of reaction was monitored by TLC
(hexane:ethyl acetate/5:1). The reaction mixture was filtered on silica gel with CH Cl and the
1
1
1
1
1
1
1
1
1
2
2
solvent was removed in vacuo. The crude product was purified by column chromatography on
silica gel, eluting with hexane:ethylacetate/10:1.
Spectroscopic
data
and
preparation
procedures
of
(S)-4-(3,7-
0
4
dimethyloctyloxy)benzoyloxy]benzaldehyde (3) was reported in our previous study.
4-Formyl-3-hydroxyphenyl (S)-4-(3,7-dimethyloctyloxy)benzoate (2):
1
Yield: 70 %; colorless crystals. H-NMR (500 MHz, CDCl ): δ (ppm) = 11.31 (s, 1H, OH),
3
9.95 (s, 1H, CHO), 8.18 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 7.67 (d, J ≈ 8.4 Hz, 1H, Ar-CH), 7.04 (d,
J ≈ 8.9 Hz, 2H Ar-CH), 6.98 (dd, J ≈ 8.4 Hz, J ≈ 2.1 Hz, 1H, Ar-CH), 6.94 (d, J ≈ 2.1 Hz,
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
1
2
1H, Ar-CH), 4.23-4.05 (m; 2H, OCH ), 2.00-1.86, (m, 1H, CH), 1.83-1.50 (m, 3H, CH, CH ),
2
2
13
1.50–1.13 (m, 6H, CH ), 1.03 (d, J ≈ 6.5 Hz, 3H, CH ), 0.94 (d, J ≈ 6.6 Hz, 6H, CH ). C-
2
3
3
NMR (125 MHz, CDCl ): δ (ppm) = 195.52 (CHO), 163.95 (COO), 163.94, 163.25, 157.98,
3
120.78, 118.64 (Ar-C), 134.97, 132.52, 114.51, 114.22, 110.94 (Ar-CH), 66.81(OCH ), 39.27,
2
37.30, 36.02, 24.70 (CH ), 29.87, 28.02 (CH), 22.74, 22.65, 19.68 (CH ).
2
3
Compounds 4a-c and 5a,b:
In a 100 mL round-bottomed flask, 2.0 mmol of compound 2 or 3 and 2.4 mmol of the
corresponding aniline compound were dissolved in 50 mL absolute ethanol. To this solution, a
few traces of glacial acetic acid was added as catalyst and the reaction mixture were heated to
reflux for 6 h at 100 °C under argon atmosphere. The end of reaction was monitored by TLC
(hexane:ethyl acetate/2:1). After cooling, the reaction mixture was kept under cool conditions
to allow the precipitation of colorless solid compound for 24 h. The crude product obtained
was collected by filtration and repeatedly washed with hot absolute ethanol. It was purified by
recrystallization with a mixture of methanol/acetone.
6

1
2
3
4
3-Hydroxy-4-{[(4-(Octyloxy)phenylimino]methyl}phenyl
(S)-4-(3,7-
dimethyloctyloxy)benzoate (4a):
1
Yield: 69 %; colorless crystals. H-NMR (500 MHz, CDCl ): δ (ppm) = 13.82 (s, broad 1H,
3
Ar-OH), 8.63 (s, 1H, HC=N), 8.15 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 7.43 (d, J ≈ 8.5 Hz, 1H, Ar-
CH), 7.29 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 6.99 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 6.96 (d, J ≈ 8.9 Hz,
2H, Ar-CH), 6.91 (d, J ≈ 2.2 Hz, 1H, Ar-CH), 6.84 (dd, J ≈ 8.4 Hz, J ≈ 2.2 Hz, 1H, Ar-CH),
5
6
7
1
2
4.14-4.06 (m, 2H, OCH ), 4.00 (t, J ≈ 6.6 Hz, OCH ), 1.92-1.79, 1.74-1.45, 1.41-1.16 (3m,
2
2
1
3
8
9
0
1
2
3
4
5
6
7
8
22H, CH, CH ), 0.98 (d, J ≈ 6.5 Hz, 3H, CH ), 0.91-0.89 (m, 9H, CH ). C-NMR (125 MHz,
2 3 3
CDCl ): δ (ppm) = 164.42 (CO), 163.65, 162.45, 158.50, 154.49, 140.87, 121.28, 117.31 (Ar-
3
1
1
1
1
1
1
1
1
1
C), 159.40 (HC=N), 132.73, 132.37, 122.24, 115.22, 114.36, 112.94, 110.57 (Ar-CH), 68.37,
66.70 (OCH ), 39.22, 37.26, 36.00, 31.90, 29.83, 29.56, 29.40, 29.32, 29.26, 27.97, 26.03,
2
−
1
24.65 (CH, CH ), 22.68, 22.60, 19.64, 14.11 (CH ). FT-IR: γ (cm ) = 1719 (C=O), 1622
2
3
(C=N).
3-Hydroxy-4-{[(4-(Decyloxy)phenylimino]methyl}phenyl
(S)-4-(3,7-
dimethyloctyloxy)benzoate (4b):
1
Yield: 65 %; colorless crystals. H-NMR (500 MHz, CDCl ): δ (ppm) = 13.84 (s, broad 1H,
3
Ar-OH), 8.63 (s, 1H, HC=N), 8.15 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 7.42 (d, J ≈ 8.5 Hz, 1H, Ar-
CH), 7.29 (d, J ≈ 8.8 Hz, 2H, Ar-CH), 6.99 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 6.96 (d, J ≈ 8.9 Hz,
2H, Ar-CH), 6.89 (d, J ≈ 2.2 Hz, 1H, Ar-CH), 6.83 (dd, J ≈ 8.4 Hz, J ≈ 2.2 Hz, 1H, Ar-CH),
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
1
2
4.14-4.06 (m, 2H, OCH ), 4.00 (t, J ≈ 6.6 Hz, 2H, OCH ), 1.92-1.79, 1.73-1.45, 1.41-1.16
2
2
1
3
(3m, 26H, CH, CH ), 0.98 (d, J ≈ 6.5 Hz, 3H, CH ), 0.92-0.89 (m, 9H, CH ). C-NMR (125
2
3
3
MHz, CDCl ): δ (ppm) = 164.39 (CO), 163.66, 162.45, 158.51, 154.49, 140.92, 121.30,
3
117.32 (Ar-C), 159.41, (HC=N), 132.73, 132.36, 122.23, 115.23, 114.37, 112.93, 110.52 (Ar-
CH), 68.38, 66.71 (OCH ), 39.22, 37.26, 36.00, 31.81, 29.84, 29.35, 29.26, 29.23, 27.97,
2
−
1
26.04, 24.65 (CH, CH ), 22.65, 22.59, 19.64, 14.08 (CH ). FT-IR: γ (cm ) = 1720 (C=O),
2
3
1618 (C=N).
3-Hydroxy-4-{[(4-(Dodecyloxy)phenylimino]methyl}phenyl
(S)-4-(3,7-
dimethyloctyloxy)benzoate (4c):
1
Yield: 68 %; colorless crystals. H-NMR (500 MHz, CDCl ): δ (ppm) = 13.76 (s, broad 1H,
3
Ar-OH), 8.62 (s, 1H, HC=N), 8.14 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 7.41 (d, J ≈ 8.5 Hz, 1H, Ar-
CH), 7.27 (d, J ≈ 8.8 Hz, 2H, Ar-CH), 6.98 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 6.95 (d, J ≈ 8.9 Hz,
7

1
2
2H, Ar-CH), 6.90 (d, J ≈ 2.2 Hz, 1H, Ar-CH), 6.83 (dd, J ≈ 8.4 Hz, J ≈ 2.2 Hz, 1H, Ar-CH),
1 2
4.13-4.06 (m, 2H, OCH ), 3.99 (t, J ≈ 6.6 Hz, 2H, OCH ), 1.91-1.78, 1.74-1.44, 1.38-1.15
2
2
1
3
3
4
5
6
7
8
9
0
1
2
3
(3m, 30H, CH, CH ), 0.98 (d, J ≈ 6.5 Hz, 3H, CH ), 0.91-0.88 (m, 9H, CH ). C-NMR (125
2 3 3
MHz, CDCl ): δ (ppm) = 164.36 (CO), 163.66, 162.48, 158.53, 154.60, 140.80, 121.30,
3
117.24 (Ar-C), 159.39, (HC=N), 132.74, 132.34, 122.20, 115.25, 114.37, 112.96, 110.53 (Ar-
CH), 68.39, 66.71 (OCH ), 39.21, 37.24, 36.00, 31.89, 29.84, 29.63, 29.60, 29.56, 29.55,
2
29.36, 29.31, 29.24, 27.95, 26.01, 24.62, (CH, CH ), 22.65, 22.56, 19.61, 14.06 (CH ). FT-IR:
2
3
−
1
γ (cm ) = 1724 (C=O), 1624 (C=N).
1
1
1
1
4-{[(4-(Octyloxy)phenylimino]methyl}phenyl (S)-4-(3,7-dimethyloctyloxy)benzoate (5a):
1
Yield: 59 %; colorless crystals. H-NMR (500 MHz, CDCl ): δ (ppm) = 8.56 (s, 1H, HC=N),
3
8.22 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 8.02 (d, J ≈ 8.6 Hz, 2H, Ar-CH), 7.39 (d, J ≈ 8.6 Hz, 2H, Ar-
CH), 7.30 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 7.05 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 7.00 (d, J ≈ 8.9 Hz,
2H, Ar-CH), 4.20-4.12 (m, 2H, OCH ), 4.05 (t, J ≈ 6.6 Hz, 2H, OCH ), 1.97-1.83, 1.78-1.51,
1
4
2
2
1
3
1
1
1
1
1
2
2
2
2
2
2
5
6
7
8
9
0
1
2
3
4
5
1.46-1.20 (3m, 22H, CH, CH ), 1.04 (d, J ≈ 6.5 Hz; 3H, CH ), 0.98-0.94 (m, 9H, 3 CH ). C-
2 3 3
NMR (125 MHz, CDCl ): δ (ppm) = 164.67 (CO), 163.72, 157.09, 153.28, 144.60, 134.09,
3
121.34 (Ar-C), 158.01 (HC=N), 132.40, 129.79, 122.22, 122.25, 115.07, 114.43 (Ar-CH),
68.38, 66.76 (OCH ), 39.27, 37.31, 36.05, 31.87, 29.89, 29.42, 29.36, 29.29, 28.02, 26.11,
2
−
1
24.70, 22.74 (CH, CH ), 22.70, 22.64, 19.69, 14.14 (CH ). FT-IR: γ (cm ) = 1730 (C=O),
2
3
1625 (C=N).
4-{[(4-(Decyloxy)phenylimino]methyl}phenyl (S)-4-(3,7-dimethyloctyloxy)benzoate 5b:
1
Yield: 71 %; colorless crystals. H-NMR (500MHz, CDCl ): δ (ppm) = 8.56 (s, 1H, HC=N),
3
8.22 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 8.02 (d, J ≈ 8.6 Hz, 2H, Ar-CH), 7.39 (d, J ≈ 8.6 Hz, 2H, Ar-
CH), 7.30 (d, J ≈ 8.8 Hz, 2H, Ar-CH), 7.05 (d, J ≈ 8.9 Hz, 2H, Ar-CH), 7.00 (d, J ≈ 8.9 Hz,
2H, Ar-CH), 4.20-4.12 (m, 2H, OCH ), 4.05 (t, J ≈ 6.6 Hz, OCH ), 1.97-1.84, 1.79-1.51, 1.46-
2
2
2
2
3
3
3
3
6
7
8
9
0
1
2
3
2
2
1
3
1.22 (3m, 26H, CH, CH ), 1.04 (d, J ≈ 6.5 Hz, 3H, CH ), 0.97-0.94 (m, 9H, 3 CH ). C-
2
3
3
NMR (125 MHz, CDCl ): δ (ppm) = 164.66 (CO), 163.72, 157.08, 153.28, 144.59, 134.11,
3
121.34 (Ar-C), 158.02 (HC=N), 132.40, 129.79, 122.25, 122.23, 115.07, 114.43 (Ar-CH),
68.37, 66.76 (OCH ), 39.28, 37.31, 36.06, 31.95, 29.89, 29.63, 29.61, 29.46, 29.37, 28.02,
2
−
1
26.11, 24.70, (CH, CH ), 22.73, 22.65, 19.70, 14.16 (CH ). FT-IR: γ (cm ) = 1726 (C=O),
2
3
1623 (C=N).
8

1
2
3
4
5
3. Results and discussion
3.1. Liquid Crystal Properties of Compounds 4a-c and 5a,b
The liquid crystal properties of the salicylaldimine and imine-based chiral calamitic molecules
4a-c and 5a,b were investigated by using optical polarizing microscope (PM) and differential
scanning calorimeter (DSC). The mesophase type, transition temperatures (T, in degree
6
7
8
9
0
1
2
−1
Celsius) and the corresponding enthalpies (ΔH, in kJ mol ) of 4a-c and 5a,b are presented in
Table 1.
1
1
1
Table 1. Mesophase, phase transition temperatures and the corresponding transition
enthalpies of the compounds 4a-c and 5a,b.
O
(
S)
O
*
O
N
R
X
1
3
a
Compound
X
R
T/°C [ΔH kJ/mol]
H →: Cr 84.2 [26.43] SmC* 157.1 [2.97] N* 174.3 [1.27] Iso
4
a
OH -OC H
8 17
b
b
Cr 28.5 [14.43] SmX 98.0 SmC* 152.1 [2.66] N* 169.0 BP 169.6 [1.33]
Iso : ← C
4
b
OH -OC10
OH -OC12
H
H
21 H →: Cr 74.2 [27.98] SmC* 153.4 [2.52] N* 182.2 [1.45] Iso
b
b
Cr 24.7 [11.87] SmX 102.0 SmC* 144.2 [2.18] N* 174.0 BP 174.2
c
[
1.46] Iso : ← C
4
c
25 H →: Cr 71.3 [28.05] Cr 80.4 [19.70] SmC* 157.0 [3.02] N* 166.7 [1.12]
1 2
Iso
b
b
c
Cr 37.2 [26.56] SmX 96.0 SmC* 152.5 [2.94] N* 162.1 BP 162.7 [1.10]
Iso : ←C
5
a
H
-OC
8
H
17
H →: Cr 73.4 [21.49] Cr 81.2 [1.33] SmC* 143.1 [3.11] N* 170.4 [1.19]
2
Iso
Cr 46.8 [18.09] Cr 58.7 [0.60] SmX 65.7 [0.32] SmC* 139.8 [3.04] N*
1
2
b
1
67.1 BP 167.8 [1.05] Iso : ← C
5
b
H
H
-OC10
-OC12
H
H
21 H →: Cr 70.9 [22.22] SmC* 145.4 [3.69] N* 164.2 [1.07] Iso
Cr 36.3 [13.04] Cr 57.6 [0.46] SmX 67.2 [0.76] SmC* 142.7 [3.62] N*
1
2
b
1
61.3 BP 162.2 [1.47] Iso : ← C
25 H →: Cr 78.8 [29.40] SmC* 145.4 [3.80] N* 158.7 [1.07] Iso
Cr 54.3 [28.14] SmX 67.3 [0.63] SmC* 142.3 [3.72] N* 156.0 BP 156.5
40
5
c
b
c
[
1.39] Iso : ←C
a
1
1
1
1
4
5
6
7
Mesophases and phase transition temperatures as observed on heating (H→) and cooling (←C) and
a
corresponding transition enthalpies of the compounds 4a-c and 5a-c. Perkin-Elmer DSC-6; enthalpy values in
italics in brackets taken from the 2nd heating and cooling scans at a rate of 10 °C min ; Abbreviations: Cr =
crystalline, SmX = unidentified smectic mesophase, SmC* = chiral smectic C phase, N* = chiral nematic phase,
−
1
9

b
c
1
2
3
BP = blue phase, Iso = isotropic liquid phase. These transitions were determined by PM. Enthalpy value
involves two transitions N*–BP–Iso.
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
New imine based calamitic molecules 4a-c and 5a,b with a (S)-3,7-Dimethyloctyloxy chiral
unit at one of terminal positions show a polymorphism composed of chiral mesophases such
as blue phase (BP) and chiral nematic (N*) as well as smectics such as chiral tilted smectic
(SmC*) and unidentified smectic mesophase (SmX). Salicylaldimine based compounds 4a-c
carrying (S)-3,7-Dimethyloctyloxy chiral group and an n-alkyloxy chain with n= 8, 10, 12 at
terminals exhibit enantiotropic chiral nematic (cholesteric) N* mesophase which is identified
by the characteristic oily-streak texture and chiral tilted smectic (SmC*) with a finger-print
texture. Additionally, monotropic mesophases such as blue phase (BP) and unidentified
smectic mesophase (SmX) were appeared on cooling. On heating differential thermogram of
compound 4a, three endotherms which corresponds to a phase transition sequence of crystal
(Cr)-chiral smectic C* (SmC*)-chiral nematic (N*)-isotropic phase (iso) were detected (see
Fig.1a). On cooling from isotropic phase, firstly platelet texture of BP with various colors was
appeared in a very narrow temperature range about 0.5 °C and then a rapid transition from BP
phase to chiral nematic (N*) mesophase with a characteristic oily streak texture was occured
(see Fig. 1b and 1c). The first exotherm on cooling differential thermogram of compound 4a
corresponds to the isotropic phase (Iso) to BP and N* mesophases and enthalpy value
involves these two transitions. When temperature arrived at T= 152.1 °C, the oily streak
texture of N* phase completely disappeared and a finger-print texture which clearly evidences
the presence of a SmC* mesophase starts to appear. The fingerprint texture become more
clear by applying direct current voltage (see Fig. 1e which shows the field-induced texture of
SmC* mesophase). Below about T= 98.0 °C, an additional monotropic mesophase referred to
here as the SmX phase, was observed (see Fig. 2). This transition was not detectable with an
exothermic peak on DSC cooling thermogram but a distinct change from dark to in a
sequence of red→green→bluish color on texture as well as a textural transformation of
finger-print regions point out that a subphase of tilted chiral smectic phase depending on the
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
*
40,56
complexity of the chiral SmC type structure.
DSC thermogram and typical textures of a
series of chiral mesophases observed for compounds 4a are shown in Fig. 1 and 2.
1
0

1
2
3
4
5
6
7
8
9
0
a)
8
4.2 °C
4a
1
57.1
°
C
174.3 °C
1
52.1 °C
169.6 °C
28.5
50
°C
0
100
150
200
250
T /
°C
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
b)
c)
d)
e)
3
3
3
3
3
3
4
4
4
4
4
4
4
5
6
7
8
9
0
1
2
3
4
5
nd
-1
Fig. 1. a) DSC thermogram of compounds 4a on 2 heating and cooling (10 °C min ) and
optical textures of mesophases of compound 4a as observed between crossed polarizers
(indicated by arrows) in a ordinary glassplates (magnification ×100); b) platelet texture of BP
at T= 169.5 °C ; c) oily-streak texture of N* phase at T= 167.5 °C; d) fingerprint texture of
SmC* phase at T= 132.0 °C, e) the fingerprint texture of a SmC* phase obtained under a DC
voltage of +3 V at T= 118.4 °C and the inset shows the fingerprint texture at T= 106.0 °C in a
10 μm PI non-coated ITO cell providing planar alignment on cooling.
1
1

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
a)
b)
c)
d)
1
1
1
1
1
1
1
1
1
1
2
6
7
8
9
0
Fig. 2. Optical textures of smectic mesophases of compound 4a showing a textural
transformation until crystallization as observed between crossed polarizers (indicated by
arrows) in a ordinary glassplates (magnification ×100); (a) SmC* mesophase at T= 98.0 °C;
(b,c,d) SmX mesophase at T= 85.0 °C, 67.0 °C and 46.0 °C, respectively.
2
2
1
2
Salicylaldimine based compounds 4b and 4c with n-decyloxy and n-dodecyloxy chain,
respectively, behave similarly to compound 4a. Compound 4b exhibits a phase transition
sequence of Cr–SmC*–N*–Iso which are in aggrement with three endotherms in DSC heating
curves whereas compound 4c has an additional endotherm at 71.3 °C which corresponds to
Cr →Cr transition (see Fig. 3). On cooling of both compounds from isotropic phase, the
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
1
2
chiral mesophases appeared in a phase transition sequence of blue phase (BP)-chiral nematic
(N*)-chiral smectic C* (SmC*)-unidentified smectic mesophase (SmX)-crystal (Cr) as
observed during cooling of compound 4a. For both compounds, the transition temperature of
BP→N* phase was determined by PM due to undetectable enthalpy changes by DSC.
Additionally, no transition enthalpy could be detected in the DSC cooling scans for SmC*-
SmX transition of compounds 4b and 4c. The clearing point of compound 4c is lower about
15 °C than 4b due on the increase in the length of alkoxy chain at terminal. However, one
point we noted here for that 4c shows a wider SmC* mesophase interval than the
salicylaldimine derivative compounds 4a and 4b which followed each other with the increase
of 2 CH groups in chain. Typical textures of various chiral mesophases observed for
2
compounds 4b and 4c are shown in Fig. 4.
1
2

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
7
4.2
°
C
80.4
°
C
4
b
4c
71.3 °C
157.0
152.5
°
C
166.7 °C
1
53.4 °C
1
82.2 °C
1
°C
62.7 °C
1
44.2 °C
74.2
°C
1
2
4.7
°
C
37.2
50
°C
0
50
100
150
200
250
0
100
150
200
T /
°C
T /
°C
nd
-1
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
Fig. 3. DSC thermograms of compounds 4b and 4c on 2 heating and cooling (10 °C min ).
1
3

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
a)
b)
c)
e)
BP, 162.6 °C
BP, 174.1 °C
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
f)
N*, 173.0 °C
N*, 162.0 °C
g)
SmC*, 135.0 °C
SmC*, 125.0 °C
d)
h)
SmX, 60.0 °C
SmX, 63.0 °C
4
4
4
1
2
3
Fig. 4. Optical textures of the chiral mesophases of a-d) compound 4b and e-h) compound 4c
at the indicated temperatures between crossed polarizers (indicated by arrows) in a ordinary
glassplates as observed on cooling from the isotropic liquid (magnification ×100).
4
4
4
5
The imine compounds 5a and 5b which are differentiate from compounds 4a-c by the absence
of OH group in the aromatic ring of salicylaldehyde exhibit similar mesomorphic properties
as observed in salicylaldimine analogs 4a-c in meaning of the occurrence of chiral
4
4
6
7
1
4

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
mesophases. As seen in Fig. 5, compound 5a exhibits a phase transition sequence of
Cr →Cr →SmC*→N*→Iso which is in agreement with four endotherms on heating curve.
1
2
On cooling from isotropic phase, blue phase (BP) with a platelet texture firstly appeared at
167.8 °C and then focal conic texture of chiral nematic phase (N*) emerged at 167.1 °C. The
oily streaks texture of N* phase was formed after shearing the focal conic pattern of sample
(see Fig. 6b). The enthalpy value which corresponds to the first exotherm involves both two
transitions. On further cooling, the SmC* mesophase at 139.8 °C appeared by the occurrence
of fan-shaped regions with stripe lines which represent the helical order of smectic layers.
When the temperature was further decreased, SmC* phase undergoes transformation to a non-
specific texture of unknown smectic mesophase (SmX) which was also detected by a
calorimetric peak at 65.7 °C in cooling DSC thermogram (see Fig. 5). Below 58.7 °C,
crystallization started and an additional transition corresponds to Cr →Cr was detected at
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
2
1
46.8 °C in cooling DSC thermogram.
Compound 5b with a n-dodecyloxy terminal chain shows polymorphism which is in
agreement with the behaviour observed for analogous imine compound 5a carrying a n-
octyloxy terminal chain (see Fig. 5). As compared with 5a, temperatures which involves
Iso→BP and BP→N* transitions decreased about 5-6 °C whereas the occurrence
temperatures of SmC* as well as SmX mesophases increased about 2-3 °C. This behavior is
not in agreement observed for analogous salicylaldimine compounds 4a and 4b. By increasing
on terminal chain length, the transition temperatures in a phase sequence of Iso→BP →N*
increased about 5 °C whereas transition temperature of SmC* phase decreased about 8 °C.
One point we noted here that the change in terminal alkoxy chain length gives rise to more
pronounced increase or decrease on transition temperatures of mesophases of
salicylaldimines. For example, the comparison of clearing temperatures of salicylaldimine
compound 4b with 4c and the imine analog 5b with 5c depending on the increase of 2 CH2
groups in terminal shows that the decrease about 6 °C appeared for imine compounds whereas
this difference is about 15 °C for salicylaldimine compounds. We would like to mention here
4
0
that the compound 5c were previously reported by our group. It is clear from the
mesomorphic investigations reported here, mesophase type, the mesophase sequence and
changes on transition temperatures depending on chain length are in aggrement with that of
the compound 5c carrying with n-dodecyloxy chain. Typical textures of various mesophases
observed for compound 5a are shown in Fig. 6, as representative case for also imine
compound 5b.
1
5

1
2
3
4
5
6
7
7
0.9 °C
5
b
5
a
7
3.4 °C
1
45.4 °C
1
64.2 °C
1
43.1 °C
81.2
°
C
170.4 °C
6
7.2 °C
6
5.7
°
C
57.6 °C
1
67.8 °C
1
62.2 °C
5
8.7 °C
1
39.8
°C
1
42.7 °C
8
9
0
1
4
6.8
40
°
C
36.3
40
°
C
1
1
20
60
80
100
120
140
160
180
200
220
20
60
80
100
120
140
160
180
200
220
T / °C
T / °C
nd
-1
1
2
Fig. 5. DSC thermograms of compounds 5a and 5b on 2 heating and cooling (10 °C min ).
1
1
1
1
1
1
1
2
2
2
2
3
4
5
6
7
8
9
0
1
2
3
a)
b)
c)
Fig. 6. Optical textures of mesophases of compound 5a as observed between crossed
polarizers (indicated by arrows) in a ordinary glassplates (magnification ×100); a) platelet
texture of BP at T= 167.6 °C ; b) oily-streak texture of N* phase at T= 156.0 °C; c)
fingerprint texture of SmC* phase at T= 130.0 °C.
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
3.2. Electro-optical Investigation of Compound 4a
For a representative case, the salicylaldimine based chiral compound 4a was investigated by
triangular wave field experiments. Electro-optic investigations were carried out in 10 μm
indium tin oxide (ITO) coated cell without an additional polyimide (PI) alignment layer. The
isotropic sample was filled into cell through capillary action. No current response could be
observed for the N* phase of 4a with regardless of the temperature under an applied voltage
-1
of up to 38 Vpp µm . On further cooling, a very small single polarization current peak in
each half period of an applied triangular wave field can be observed below ~152 °C,
indicating a ferroelectric like switching. The result of quantitative analysis, obtained by
-2
integrating the area under the peak, reveals a polarization value (P ) of around 230 nC cm .
S
1
6

1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
The measured polarization value is in the typical range as known for structurally similar
4
1,42,43
calamitic compounds.
The comparison of polarization value (P ) shows that the
S
-2
polarization value of salicylaldimine based compound 4a (P = 230 nC cm ) is slightly higher
S
-2 40
than that of the imine analog 5c (P = 130 nC cm ). Similar results were reported for the
S
4
3
range of SmC* mesophase of three-ring Schiff bases and salicylaldimines. Triangular wave
field experiments show no polar switching in SmX phase of 4a, even at an applied voltage of
380 V . As a representative case, switching current responses obtained under a triangular
pp
wave field at 122.4 °C and 115.0 °C, respectively are shown in Fig. 7.
a)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
200
2
1
00
0
0
-100
-
2
-200
-0,001
0,000
0,001
0,002
t / s
b)
2
200
150
100
50
0
0
-50
-100
-
150
200
-
-2
-0,0010
-0,0005
0,0000
0,0005
0,0010
0,0015
0,0020
t / s
Fig. 7. Switching current response obtained for 4a in a 10 μm noncoated ITO cell under a
triangular wave field; a) at T = 122.4 °C, 380 Vpp, 500 Hz, 5 kΩ and b) at T = 115.0 °C, 380
Vpp, 500 Hz, 5 kΩ.
1
7

1
4. Conclusion
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
In this study, we have reported the synthesis and characterization of novel mesogens which
are composed of three phenyl rings linked with ester and azomethine groups and terminated
with (S)-3,7-dimethyloctyloxy chiral unit and n-octyloxy/decyloxy/dodecyloxy chains. The
new azomethine based chiral rod-like molecules show a chiral polymorphism comprising of a
chiral mesophases such as blue phase (BP) and chiral nematic (N*) as well as smectics such
as chiral tilted smectic (SmC*) and unidentified smectic mesophase (SmX). On cooling, all
investigated compounds exhibited blue phases with a very short range of temperatures.
Comparison of the mesophase behaviors of salicylaldimine derivative compounds and imine
compounds revealed that no change on mesophase type whereas terminal alkoxy chain length
gives rise to more pronounced variation on the mesophase intervals of salicylaldimines
derivatives. The electro-optical investigations show that the presence of a polar switching in
the SmC* mesophase with a relatively small single peak with a low polarization value (PS=
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
-2
230 nC cm ) as reported in structurally similar calamitic compounds. It can be concluded that
this type of imine based rod-like compounds terminated with an (S)-3,7-dimethyloctyloxy
group can generate high chirality which are result in the occurence of a blue phases and
switchable smectics which have a significant interest for the development of fast switching
displays and new technological applications. LC-Nanoparticle (NP) mixtures prepared by the
usage of new materials as a chiral dopant can find a wide applicability in electro-optic,
information transfer, memory storage or sensor applications by the improvement in phase
transition behavior and stability of blue phases.
Acknowledgement
This research has been supported by Yildiz Technical University Scientific Research Projects
Coordination Department. Project Number: FBA-2017-3056. H. Akdaş-Kılıç is grateful to the
TÜBİTAK for a TÜBİTAK-2232 International Fellowship for Outstanding Researcher at
Yildiz Technical University, Istanbul, Turkey.
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Highlights
1. New chiral calamitics based on Schiff base or salicylaldimine-core have been synthesized.
2. The new azomethine based chiral rod-like molecules exhibited a chiral polymorphism.
3. The blue phase and chiral nematic mesophase as well as chiral smectics were observed.

The author(s) declare(s) that there is no conflict of interest for the manuscript titled "Chiral
Polymorphism in New Imine Based Rod-Like Liquid Crystals".
Yours Sincerely,
Prof. Dr. Belkız Bilgin Eran
Prof. Dr. Hale OCAK
Corresponding authors