The synthesis, mesomorphic and dielectric investigation of new unsymmetrical bent-core mesogens derived from 3-hydroxybenzoic acid

Article abstract of DOI:10.1007/s11696-020-01203-4

Abstract: The synthesis, mesomorphic and dielectric investigation of new unsymmetrical bent-core mesogens derived from 3-hydroxybenzoic acid central core with linked two rodlike units carrying (S)-3,7-dimethyloctyloxy group and an n-alkyloxy chain at both terminals are presented. The liquid crystalline properties of the new unsymmetrical bent-core materials have been investigated by polarizing optical microscopy and differential scanning calorimetry. New compounds with the reversed ester linking unit exhibit a monotropic or enantiotropic columnar mesophase at lower temperatures as compared to analogs with a straight chain depending on the presence of the branched (S)-3,7-dimethyloctyloxy terminal chain. Dielectric measurements for one of the bent-core mesogens, 3-[[4-[[4-(octyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid 4-[[4-((S)-3,7-dimethyloctyloxy)phenoxy]carbonyl]phenyl ester (OBDPE), have been carried out on the angular frequency range from 25.12 to 50265k?rad/s at different temperatures. Thanks to dielectric measurements, real and imaginary dielectric constant, conductivity mechanism and dielectric relaxation mechanism of OBDPE were obtained. Graphic abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s11696-020-01203-4

Chemical Papers
https://doi.org/10.1007/s11696-020-01203-4
ORIGINAL PAPER
The synthesis, mesomorphic and dielectric investigation of new
unsymmetrical bent‑core mesogens derived from 3‑hydroxybenzoic
acid
Hale Ocak1 · Esma Ahlatcıoğlu Özerol2 · Fulya Çağla Çelikel1 · Mustafa Okutan3 · Belkız Bilgin Eran1
Received: 22 February 2020 / Accepted: 16 May 2020
© Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
The synthesis, mesomorphic and dielectric investigation of new unsymmetrical bent-core mesogens derived from 3-hydroxy-
benzoic acid central core with linked two rodlike units carrying (S)-3,7-dimethyloctyloxy group and an n-alkyloxy chain
at both terminals are presented. The liquid crystalline properties of the new unsymmetrical bent-core materials have been
investigated by polarizing optical microscopy and diferential scanning calorimetry. New compounds with the reversed
ester linking unit exhibit a monotropic or enantiotropic columnar mesophase at lower temperatures as compared to ana-
logs with a straight chain depending on the presence of the branched (S)-3,7-dimethyloctyloxy terminal chain. Dielectric
measurements for one of the bent-core mesogens, 3-[[4-[[4-(octyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid 4-[[4-((S)-
3,7-dimethyloctyloxy)phenoxy]carbonyl]phenyl ester (OBDPE), have been carried out on the angular frequency range from
25.12 to 50265k rad/s at diferent temperatures. Thanks to dielectric measurements, real and imaginary dielectric constant,
conductivity mechanism and dielectric relaxation mechanism of OBDPE were obtained.
Graphic abstract
Keywords Liquid crystals · Bent-core compounds · 3-Hydroxybenzoic acid derivatives · Columnar mesophase · Dielectric
measurement
Introduction
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-020-01203-4) contains
supplementary material, which is available to authorized users.
Bent-core liquid crystals have attracted great interest since
the discovery of their polar switching behavior (Niori et al.
1996) in their mesophases. The polar ordering of bent-
shaped materials generates structural layer chirality, which
* Hale Ocak
hocak@yildiz.edu.tr
Extended author information available on the last page of the article
Vol.:(0123456789)
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Chemical Papers
results in ferroelectric or antiferroelectric properties (Kishi-
kawa et al. 1999).
chiral terminal group derived from lactic acid exhibit colum-
nar B1REV type of phase (Novotná et al. 2012). The attaching
polar groups such as F, Cl and CF3 at one of the terminals
of these materials result in the occurrence of polar tilted
smectic phase (Shreeninvasa Murthy and Sadashiva 2004b).
The presence of alkoxy groups at both terminals gives rise to
columnar phase (Colob) as well as the switchable lamellar
(SmCP) phase (Shreeninvasa Murthy and Sadashiva 2005).
In this paper, we report the synthesis, structural charac-
terization, mesomorphic and dielectric investigation of new
unsymmetrical bent-core mesogens derived from 3-hydroxy-
benzoic acid central core with linked two rod-like units car-
rying (S)-3,7-dimethyloctyloxy group and n-alkyloxy chain
at both terminals.
A great number of symmetrical and unsymmetrical bent-
core liquid crystals have been reported till date. The meso-
phases exhibited by these molecules have been assigned as
B-phases, but also a large number of bent-shaped materi-
als which show nematic (Schröder et al. 2003; Bailey et al.
2009; Kohout et al. 2009) and columnar phases (Reddy and
Tschierske 2006; Gorecka et al. 2008) have been reported.
In recent years, bent-core compounds exhibiting various
tilted and non-tilted smectic phases have attracted signif-
cant interest (Dunemann et al. 2005; Pratibha et al. 2000;
Keith et al. 2010; Gupta et al. 2011, Pociecha et al. 2006;
Shreeninvasa Murthy and Sadashiva 2004a).
The molecular structure of bent-core liquid crystals is
composed of two rodlike units, which are covalently linked
to 1,3-position of a bent central unit such as resorcinol,
3-hydroxybenzoic acid (Shreeninvasa Murthy and Sadashiva
2004a; Sadashiva et al. 2001, 2002; Reddy and Sadashiva
2004), 4′-hydroxybiphenyl-3-carboxylic acid (Kovarova
et al. 2014; Guo et al. 2010; Radhika et al. 2010) and
3′-hydroxybiphenyl-4-carboxylic acid (Kovarova et al. 2014;
Reddy et al. 2011). The structural variations on the bent-core
compounds are clearly more efective on the formation of
mesophase as compared to calamitic compounds. The meso-
phase type is strongly dependent on the type and orientation
of linking units (Weissfog et al. 2005), the number of the
aromatic rings (Ocak and Bilgin-Eran 2015), the type and
the position of lateral substituents on the central unit (Ocak
et al. 2011, Weissfog et al. 2001, 2004, Pelzl et al. 1999) and
type and length of terminal chains (Ocak et al. 2012, 2013).
The orientation of the linking units connecting the aro-
matic rings of the bent-core plays a decisive role on the
type of the formed mesophase (Weissfog et al. 2005, 2006;
Krishnan et al. 2006; Reddy et al. 2010) and strongly afects
the transition temperatures as it gives rise to the changes
on the polarity, distribution of electron density and bending
angle of the molecule.
Experimental
Materials and instrumentation
New unsymmetrical bent-core mesogens derived from
3-hydroxybenzoic acid central core were characterized by
1H-NMR and 13C-NMR (Bruker Avance III 500 spectrom-
eter, in CDCl3 solution, with tetramethylsilane as internal
standard). Microanalysis was performed using a Thermo
Fischer Scientifc FlashEA 1112 Series elemental analyzer.
The mesomorphic properties of compounds were evalu-
ated by a Mettler FP-82 HT hot stage and control unit in
conjunction with a Leica DM2700P polarizing microscope
(PM). Transition temperatures were confrmed by using
PerkinElmer DSC-6, heating and cooling rate: 10 °C min−1
in a nitrogen atmosphere.
Hioki LCR Meter IM3536 within the angular frequency
interval of 25.12–50265482.46 rad/s was used for investi-
gation of dielectric and impedance properties at diferent
temperatures with a high accuracy. At the stage of pre-meas-
urement preparation, the liquid crystal compound was flled
into a planar aligned indium tin oxide (ITO)-coated glass
plates (5 μm) cells purchased from Instec Colorado Inc.
3-Hydroxybenzaldehyde (Sigma-Aldrich), NaClO2
(Fluka), NaH2PO4.H2O (Merck), resorcinol (Merck),
t-BuOH (Merck), N,N′-dicyclohexylcarbodiimide (Merck)
and 4-dimethylaminopyridine (Merck) were commer-
cially available. Anhydrous solvent CH2Cl2 was dried over
diphosphorus pentoxide (Merck) and distilled under a N2
atmosphere. Solvents used in the purifcation step such as
crystallization and column chromatography (ethyl acetate,
chloroform and dichloromethane) were distilled. Thin-layer
chromatography (TLC) was carried out on aluminum plates
coated with silica gel 60 F254 (Merck). Column chroma-
tography was performed using silica gel 60 (Merck, pore
size 60 Å).
The bent-shaped materials generally utilize two diferent
functional groups connecting two rod-like units to the cen-
tral core to generate a non-symmetrical molecular structure.
The hydroxylic and carboxylic groups as in 3-hydroxyben-
zoic acid are used more frequently to form a non-symmetri-
cal bent-core mesogen (Tůma et al. 2016a, b; Kohout et al.
2017). As a result of this, the reversed orientation of ester
groups is placed at both sides of the bent-core molecular
structure and the infuence of the ester unit orientation on
mesomorphic properties is expected due to changed fex-
ibility and polarity of the molecule (Kohout et al. 2009). The
mesophase type exhibited by 3-hydroxybenzoic acid-based
bent-core materials mostly depends on the terminal substi-
tuted groups. For example, bent-core materials which were
built up from 3-hydroxybenzoic acid as a central unit with a
1 3

Chemical Papers
(Güzeller et al. 2015). The esterifcation of 4-(4-n-alky-
loxybenzoyloxy)benzoic acids (1a–e) and 3-hydroxybenza-
ldehyde using N,N′-dicyclohexylcarbodiimide (DCC) and
4-(dimethylamino)pyridine (DMAP) as catalyst (Tschierske
and Zaschke 1989) in dry CH2Cl2 yielded 3-[[4-[[4-(n-alky-
loxy)benzoyl]oxy]benzoyl]oxy]benzaldehydes (Srinivasan
et al. 2013) (2a–e). Then, the reaction mixture composed
of compounds 2a–e and resorcinol in t-BuOH reacted
with aqueous solution of NaClO2 and NaH2PO4 to obtain
3-[[4-[[4-(n-alkyloxy)benzoyl]oxy]benzoyl]oxy]benzoic
acids (Nagaveni et al. 2012) (3a–e) by oxidation (Lee et al.
2000). Finally, the esterifcation of compounds 3a-e with
(S)-4-(3,7-dimethyloctyloxy)phenyl 4-hydroxybenzoate (4)
(Güzeller et al. 2015) using DCC and DMAP as catalyst
Synthesis and characterization of bent‑core
mesogens derived from 3‑hydroxybenzoic acid
The new bent-core compounds 3-[[4-[[4-(n-alkyloxy)ben-
zoyl]oxy]benzoyl]oxy]benzoic acid 4-[[4-((S)-3,7-dimeth-
yloctyloxy)phenoxy]carbonyl]phenyl esters with a bent unit
representing ester of 3-hydroxybenzoic acid have been syn-
thesized starting from commercially available 3-hydroxy-
benzaldehyde and following the procedure as reported in
Prasad et al. (2004) (see Scheme 1). The 4-(4-n-alkyloxy-
benzoyloxy)benzoic acids (1a–e) (Achten et al. 2004) were
obtained by esterifcation of 4-(n-alkyloxy)benzoic acids
(Jongen et al. 2003) with 4-hydroxybenzaldehyde, followed
by NaClO2 oxidation (Lee et al. 2000) as reported previously
Scheme 1 The synthesis of
O
O
new unsymmetrical bent-core
compounds 5a–e derived from
3-hydroxybenzoic acid central
unit
RO
O
OH
1a-e
DCC, DMAP
dry CH2Cl2
H
HO
O
O
H
O
O
O
2a-e
O
RO
NaClO2, NaH2PO4.H2O
Resorcinol, t-BuOH
O
OH
O
O
O
3a-e
O
RO
(S)
*
O
O
O
DCC, DMAP
dry CH2Cl2
OH
4
O
O
O
O
O
O
O
5a-e
O
O
*
RO
(S)
R= CnH2n+1; n= 6,8,10,12,14
1 3

Chemical Papers
at room temperature gave rise to the target bent-core com-
CH2), 1.43–1.38 (m; 2H, CH2), 1.33–1.09 (m; 4H, 2 CH2),
pounds 5a–e.
0.83 (t, J≈6.9 Hz; 3H, CH3).
3‑[[4‑[[4‑(Decyloxy)benzoyl]oxy]benzoyl]oxy]benzaldehyde
Procedure for the synthesis of 3‑[[4‑[[4‑(n‑alkyloxy)
benzoyl]oxy]benzoyl]oxy]benzoic acid
4‑[[4‑((S)‑3,7‑dimethyloctyloxy)phenoxy]carbonyl]
phenyl esters (5a–e)
(2c) C31H34O6; 502.60 g/mol: Yield: 72%, colorless crys-
1
tals. H-NMR (500 MHz, CDCl3): δ (ppm) = 9.98 (s; –
CHO), 8.22 (d, J≈8.8 Hz; 2 Ar–H), 8.09 (d, J≈8.9 Hz; 2
Ar–H), 7.75 (d, J≈8.8 Hz, 1H, Ar–H), 7.70 (s, 1H, Ar–H),
7.56 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.45 (d, J ≈ 7.6 Hz, 1H,
Ar–H), 7.32 (d, J≈8.8 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 1.85–1.81 (m; 2H,
OCH2CH2), 1.79–1.73 (m; 2H, CH2), 1.68–1.63 (m; 2H,
CH2), 1.44–1.38 (m; 2H, CH2), 1.29–1.15 (m; 8H, 4 CH2),
0.82 (t, J≈6.5 Hz; 3H, CH3).
The corresponding 3-[[4-[[4-(n-alkyloxy)benzoyl]
oxy]benzoyl]oxy]benzoic acid (3a–e) (1.0 mmol) and
(S)-4-(3,7-dimethyloctyloxy)phenyl 4-hydroxybenzo-
ate (4) (1.1 mmol) were dissolved in dry CH2Cl2, and
N,N′-dicyclohexylcarbodiimide (DCC) (1.60 mmol) and
4-(dimethylamino)pyridine (DMAP) (0.1 mmol) were added
to this solution. The reaction mixture was stirred under argon
atmosphere for 3 h at room temperature. The reaction was
followed by TLC (CHCl3). The mixture was fltered through
a silica gel layer and washed with CHCl3. After removing
the solvent in vacuo, the product was purifed by column
chromatography on silica gel, eluting with chloroform, and
then recrystallized from a mixture of dichloromethane and
ethyl acetate.
3‑[[4‑[[4‑(Dodecyloxy)benzoyl]oxy]benzoyl]oxy]benzalde‑
hyde (2d) C33H38O6; 530.65 g/mol: Yield: 75%, colorless
1
crystals. H-NMR (500 MHz, CDCl3): δ (ppm)=9.98 (s;
–CHO), 8.22 (d, J≈8.8 Hz; 2 Ar–H), 8.09 (d, J≈8.9 Hz; 2
Ar–H), 7.75 (d, J=7.6 Hz, 1H, Ar–H), 7.70 (s, 1H, Ar–H),
7.56 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.45 (d, J ≈ 7.6 Hz, 1H,
Ar–H), 7.32 (d, J≈8.8 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 1.86–1.81 (m; 2H,
OCH2CH2), 1.79–1.73 (m; 2H, CH2), 1.68–1.66 (m; 2H,
CH2), 1.44–1.38 (m; 2H, CH2), 1.29–1.11 (m; 12H, 6 CH2),
0.82 (t, J≈6.7 Hz; 3H, CH3).
All intermediates and 5a–e bent-core compounds were
1
characterized by H-NMR and 13C-NMR. The proposed
structures are in full agreement with the obtained spectro-
scopic data (see SI).
Spectroscopic data and preparation procedures of
4-(4-n-alkyloxybenzoyloxy)benzoic acid (1a–e) are given in
Ref. (Muhammad et al. 2011). (S)-4-(3,7-dimethyloctyloxy)
phenyl 4-hydroxybenzoate (4) was frst reported by Güzeller
et al. (2015).
3‑[[4‑[[4‑(Tetradecyloxy)benzoyl]oxy]benzoyl]oxy]benzalde‑
hyde (2e) C35H42O6; 558.70 g/mol: Yield: 95%, colorless
1
crystals. H-NMR (500 MHz, CDCl3): δ (ppm)=9.98 (s;
–CHO), 8.22 (d, J≈8.8 Hz; 2 Ar–H), 8.09 (d, J≈8.9 Hz; 2
Ar–H), 7.75 (d, J≈7.6 Hz, 1H, Ar–H), 7.70 (s, 1H, Ar–H),
7.56 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.45 (d, J ≈ 7.6 Hz, 1H,
Ar–H), 7.32 (d, J≈8.8 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.6 Hz; 2H, OCH2), 1.86–1.81 (m; 2H,
OCH2CH2), 1.79–1.73 (m; 2H, CH2), 1.68–1.66 (m; 2H,
CH2), 1.43–1.38 (m; 2H, CH2), 1.29–1.11 (m; 16H, 8 CH2),
0.81 (t, J≈6.8 Hz; 3H, CH3).
3‑[[4‑[[4‑(Hexyloxy)benzoyl]oxy]benzoyl]oxy]benzaldehyde
(2a) C27H26O6; 446.50 g/mol: Yield: 70%, colorless crys-
1
tals. H-NMR (500 MHz, CDCl3): δ (ppm) = 9.98 (s; –
CHO), 8.22 (d, J≈8.8 Hz; 2 Ar–H), 8.09 (d, J≈8.8 Hz; 2
Ar–H), 7.75 (d, J≈7.6 Hz, 1H, Ar–H), 7.70 (s, 1H, Ar–H),
7.56 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.45 (d, J ≈ 7.6 Hz, 1H,
Ar–H), 7.32 (d, J≈8.8 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.6 Hz; 2H, OCH2), 1.79–1.73 (m; 2H,
CH2), 1.45–1.39 (m; 2H, CH2), 1.31–1.14 (m; 4H, 2 CH2),
0.85 (t, J≈7.0 Hz; 3H, CH3).
3‑[[4‑[[4‑(Hexyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid
(3a) C27H26O7; 462.49 g/mol: Yield: 95%, colorless crys-
1
tals. M.p.: 162–164 °C. H-NMR (500 MHz, CDCl3): δ
(ppm)=8.22 (d, J≈8.7 Hz; 2 Ar–H), 8.09 (d, J≈8.8 Hz; 2
Ar–H), 7.96 (d, J≈7.7 Hz, 1H, Ar–H), 7.89 (s, 1H, Ar–H),
7.49 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.42 (d, J ≈ 7.5 Hz, 1H,
Ar–H), 7.32 (d, J≈8.7 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 1.79–1.73 (m; 2H,
CH2), 1.45–1.39 (m; 2H, CH2), 1.31–1.14 (m; 4H, 2 CH2),
0.85 (t, J≈6.9 Hz; 3H, CH3).
3‑[[4‑[[4‑(Octyloxy)benzoyl]oxy]benzoyl]oxy]benzaldehyde
(2b) C29H30O6; 474.54 g/mol: Yield: 79%, colorless crys-
1
tals. H-NMR (500 MHz, CDCl3): δ (ppm) = 9.98 (s; –
CHO), 8.22 (d, J≈8.8 Hz; 2 Ar–H), 8.09 (d, J≈8.9 Hz; 2
Ar–H), 7.75 (d, J≈7.6 Hz, 1H, Ar–H), 7.70 (s, 1H, Ar–H),
7.56 (t, J ≈ 7.8 Hz, 1H, Ar–H), 7.45 (d, J ≈ 7.6 Hz, 1H,
Ar–H), 7.32 (d, J≈8.7 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.6 Hz; 2H, OCH2), 1.86–1.81 (m; 2H,
OCH2CH2), 1.79–1.73 (m; 2H, CH2), 1.68–1.66 (m; 2H,
3‑[[4‑[[4‑(Octyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid
(3b) C29H30O7; 490.54 g/mol: Yield: 83%, colorless crys-
1
tals. M.p.: 161–163 °C. H-NMR (500 MHz, CDCl3): δ
1 3

Chemical Papers
(ppm)=8.22 (d, J≈8.6 Hz; 2 Ar–H), 8.09 (d, J≈8.8 Hz; 2
Ar–H), 7.95 (d, J≈7.7 Hz, 1H, Ar–H), 7.88 (s, 1H, Ar–H),
7.49 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.42 (d, J ≈ 7.5 Hz, 1H,
Ar–H), 7.32 (d, J≈8.6 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 1.79–1.14 (m; 12H,
6 CH2), 0.83 (t, J≈6.9 Hz; 3H, CH3).
CH3), 0.85–0.80 (m; 9H, 3 CH3). 13C-NMR (125 MHz,
CDCl3): δ (ppm) = 164.78, 164.32, 164.31, 163.89 (4 s;
COO), 163.74, 156.97, 155.67, 154.89, 151.08, 144.17,
130.68, 129.91, 127.86, 127.52, 127.46, 126.34, 123.67,
120.88 (10 s; 10 Ar–C ve 4d; 4 Ar–CH), 132.45, 131.92,
131.87, 122.36, 122.26, 121.94, 115.15, 114.46 (8d; 8 Ar–
CH), 68.42, 66.77 (2t; 2 OCH2), 29.86, 27.99 (2d; 2 CH),
39.26, 37.30, 36.22, 31.55, 29.06, 25.67, 24.67, 22.60 (8t;
8 CH2), 22.72, 22.62, 19.66, 14.04 (4q; 4 CH3). C50H54O10
(814.96); Anal. Calc.: C, 73.69; H, 6.68. Found: C, 73.54;
H, 6.93.
3‑[[4‑[[4‑(Decyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid
(3c) C31H34O7; 518.60 g/mol: Yield: 96%, colorless crys-
1
tals. M.p.: 153–155 °C. H-NMR (500 MHz, CDCl3): δ
(ppm)=8.22 (d, J≈8.8 Hz; 2 Ar–H), 8.09 (d, J≈8.9 Hz; 2
Ar–H), 7.95 (d, J≈7.7 Hz, 1H, Ar–H), 7.87 (s, 1H, Ar–H),
7.46 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.39 (d, J ≈ 7.5 Hz, 1H,
Ar–H), 7.31 (d, J≈8.9 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 1.88–1.82 (m; 2H,
OCH2CH2), 1.79–1.73 (m; 2H, CH2), 1.66–1.62 (m; 2H,
CH2), 1.44–1.38 (m; 2H, CH2), 1.29–1.16 (m; 8H, 4 CH2),
0.82 (t, J≈6.6 Hz; 3H, CH3).
3‑[[4‑[[4‑(Octyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid
4‑[[4‑((S)‑3,7‑dimethyloctyloxy)phenoxy]carbonyl]phenyl
ester (OBDPE) (5b) C52H58O10; 843.01 g/mol: Yield: 0.56 g
1
(66%), colorless crystals. H-NMR (500 MHz, CDCl3): δ
(ppm) = 8.24–8.21 (m; 4 Ar–H), 8.10–8.08 (m; 3 Ar–H),
8.02 (s, 1H, Ar–H), 7.56 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.48
(d, J≈8.2 Hz, 1H, Ar–H), 7.34–7.31 (m; 4 Ar–H), 7.05 (d,
J≈9.0 Hz; 2 Ar–H), 6.92 (d, J≈8.8 Hz; 2 Ar–H), 6.87 (d,
J≈9.0 Hz; 2 Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 3.95–
3.91 (m; 2H, OCH2), 1.88–1.08 (m; 20H, 10 CH2), 0.88 (d,
J≈6.6 Hz; 3H, CH3), 0.84–0.80 (m; 9H, 3 CH3). 13C-NMR
(125 MHz, CDCl3): δ (ppm) = 164.79, 164.33, 164.31,
163.89 (4 s; COO), 163.74, 156.97, 155.66, 154.88, 151.08,
144.16, 130.68, 129.92, 127.86, 127.53, 127.46, 126.34,
123.67, 120.87 (10 s; 10 Ar–C ve 4d; 4 Ar–CH), 132.45,
131.92, 131.87, 122.36, 122.26, 121.94, 115.15, 114.45 (8d;
8 Ar–CH), 68.42, 66.76 (2t; 2 OCH2), 29.85, 27.99 (2d; 2
CH), 39.26, 37.29, 36.22, 31.81, 29.33, 29.23, 29.09, 25.99,
24.67, 22.62 (10t; 10 CH2), 22.72, 22.66, 19.66, 14.11 (4q;
4 CH3). C52H58O10 (843.01); Anal. Calc.: C, 74.09; H, 6.93.
Found: C, 73.97; H, 7.25.
3‑[[4‑[[4‑(Dodecyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid
(3d) C33H38O7; 546.65 g/mol: Yield: 88%, colorless crys-
1
tals. M.p.: 151–153 °C. H-NMR (500 MHz, CDCl3): δ
(ppm)=8.22 (d, J≈8.4 Hz; 2 Ar–H), 8.09 (d, J≈8.5 Hz; 2
Ar–H), 7.96 (d, J≈7.6 Hz, 1H, Ar–H), 7.89 (s, 1H, Ar–H),
7.49 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.42 (d, J ≈ 7.4 Hz, 1H,
Ar–H), 7.32 (d, J≈8.4 Hz; 2 Ar–H), 6.92 (d, J≈8.6 Hz; 2
Ar–H), 3.99 (t, J≈6.4 Hz; 2H, OCH2), 1.89–1.82 (m; 2H,
OCH2CH2), 1.79–1.73 (m; 2H, CH2), 1.66–1.61 (m; 2H,
CH2), 1.45–1.38 (m; 2H, CH2), 1.29–1.18 (m; 12H, 6 CH2),
0.81 (t, J≈6.8 Hz; 3H, CH3).
3‑[[4‑[[4‑(Tetradecyloxy)benzoyl]oxy]benzoyl]oxy]benzoic
acid (3e) C35H42O7; 574.70 g/mol: Yield: 98%, colorless
crystals. M.p.: 149–151 °C. 1H-NMR (500 MHz, CDCl3):
δ (ppm)=8.22 (d, J≈8.6 Hz; 2 Ar–H), 8.09 (d, J≈8.8 Hz;
2 Ar–H), 7.97 (d, J≈7.7 Hz, 1H, Ar–H), 7.89 (s, 1H, Ar–H),
7.49 (t, J ≈ 7.9 Hz, 1H, Ar–H), 7.43 (d, J ≈ 7.4 Hz, 1H,
Ar–H), 7.32 (d, J≈8.6 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2
Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 1.99–1.91 (m; 2H,
OCH2CH2), 1.79–1.73 (m; 2H, CH2), 1.68–1.58 (m; 2H,
CH2), 1.45–1.38 (m; 2H, CH2), 1.29–1.15 (m; 16H, 8 CH2),
0.81 (t, J≈6.4 Hz; 3H, CH3).
3‑[[4‑[[4‑(Decyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid
4‑[[4‑((S)‑3,7‑dimethyloctyloxy)phenoxy]carbonyl]phenyl ester
(5c) C54H62O10; 871.06 g/mol: Yield: 0.63 g (72%), color-
less crystals. 1H-NMR (500 MHz, CDCl3): δ (ppm)=8.32–
8.28 (m; 4 Ar–H), 8.17–8.15 (m; 3 Ar–H), 8.09 (s, 1H,
Ar–H), 7.63 (t, J≈7.9 Hz, 1H, Ar–H), 7.55 (d, J≈7.9 Hz,
1H, Ar–H), 7.41–7.38 (m; 4 Ar–H), 7.12 (d, J ≈8.8 Hz; 2
Ar–H), 6.99 (d, J≈8.6 Hz; 2 Ar–H), 6.94 (d, J≈8.8 Hz; 2
Ar–H), 4.06 (t, J≈6.5 Hz; 2H, OCH2), 4.02–3.98 (m; 2H,
OCH2), 1.86–1.15 (m; 24H, 12 CH2), 0.95 (d, J≈ 6.5 Hz;
3H, CH3), 0.90–0.86 (m; 9H, 3 CH3). 13C-NMR (125 MHz,
CDCl3): δ (ppm) = 164.78, 164.33, 164.31, 163.89 (4 s;
COO), 163.74, 156.97, 155.67, 154.89, 151.07, 144.17,
130.68, 129.92, 127.86, 127.52, 127.46, 126.34, 123.67,
120.87 (10 s; 10 Ar–C ve 4d; 4 Ar–CH), 132.45, 131.92,
131.87, 122.36, 122.26, 121.94, 115.15, 114.45 (8d; 8 Ar–
CH), 68.42, 66.77 (2t; 2 OCH2), 29.86, 27.99 (2d; 2 CH),
39.26, 37.30, 36.22, 31.94, 31.91, 29.56, 29.37, 29.33,
29.09, 25.99, 24.67, 22.73 (12t; 12 CH2), 22.69, 22.62,
3‑[[4‑[[4‑(Hexyloxy)benzoyl]oxy]benzoyl]oxy]benzoic acid
4‑[[4‑((S)‑3,7‑dimethyloctyloxy)phenoxy]carbonyl]phenyl ester
(5a) C50H54O10; 814.96 g/mol: Yield: 0.44 g (54%), color-
less crystals. 1H-NMR (500 MHz, CDCl3): δ (ppm)=8.24–
8.21 (m; 4 Ar–H), 8.10–8.08 (m; 3 Ar–H), 8.02 (s, 1H,
Ar–H), 7.56 (t, J≈8.0 Hz, 1H, Ar–H), 7.48 (d, J≈8.2 Hz,
1H, Ar–H), 7.34–7.31 (m; 4 Ar–H), 7.05 (d, J ≈8.9 Hz; 2
Ar–H), 6.92 (d, J≈8.9 Hz; 2 Ar–H), 6.87 (d, J≈9.0 Hz; 2
Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 3.96–3.90 (m; 2H,
OCH2), 1.81–1.07 (m; 16H, 8CH2), 0.88 (d, J≈6.6 Hz; 3H,
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19.66, 14.13 (4q; 4 CH3). C54H62O10 (871.06); Anal. Calc.:
phenyl ester (5e) C58H70O10; 927.17 g/mol: Yield: 0.72 g
1
C, 74.46; H, 7.17. Found: C, 74.07; H, 7.30.
(78%), colorless crystals. H-NMR (500 MHz, CDCl3): δ
(ppm) = 8.25–8.21 (m; 4 Ar–H), 8.10–8.08 (m; 3 Ar–H),
8.02 (s, 1H, Ar–H), 7.56 (t, J ≈ 8.0 Hz, 1H, Ar–H), 7.48
(d, J≈8.0 Hz, 1H, Ar–H), 7.34–7.30 (m; 4 Ar–H), 7.05 (d,
J≈9.0 Hz; 2 Ar–H), 6.92 (d, J≈8.9 Hz; 2 Ar–H), 6.87 (d,
J≈9.0 Hz; 2 Ar–H), 3.99 (t, J≈6.5 Hz; 2H, OCH2), 3.95–
3.91 (m; 2H, OCH2), 1.80–1.07 (m; 32H, 16 CH2), 0.88 (d,
J≈6.6 Hz; 3H, CH3), 0.83–0.80 (m; 9H, 3 CH3). 13C-NMR
(125 MHz, CDCl3): δ (ppm) = 164.79, 164.33, 164.32,
163.89 (4 s; COO), 163.74, 156.97, 155.66, 154.89, 151.08,
144.16, 130.68, 129.92, 127.85, 127.53, 127.46, 126.34,
123.68, 120.87 (10 s; 10 Ar–C ve 4d; 4 Ar–CH), 132.46,
131.93, 131.88, 122.36, 122.26, 121.94, 115.15, 114.46 (8d;
8 Ar–CH), 68.42, 66.77 (2t; 2 OCH2), 29.86, 27.99 (2d; 2
CH), 39.26, 37.30, 36.23, 31.94, 29.71, 29.69, 29.67, 29.66,
29.60, 29.57, 29.38, 29.37, 29.09, 25.99, 24.68, 22.73 (16t;
16 CH2), 22.71, 22.63, 19.67, 14.14 (4q; 4 CH3). C58H70O10
(927.17); Anal. Calc.: C, 75.13; H, 7.61. Found: C, 75.15;
H, 7.86.
3‑[[4‑[[4‑(Dodecyloxy)benzoyl]oxy]benzoyl]oxy]benzoic
acid 4‑[[4‑((S)‑3,7‑dimethyloctyloxy)phenoxy]carbonyl]
phenyl ester (5d) C56H66O10; 899.11 g/mol: Yield: 0.68 g
1
(76%), colorless crystals. H-NMR (500 MHz, CDCl3): δ
(ppm) = 8.32–8.28 (m; 4 Ar–H), 8.17–8.15 (m; 3 Ar–H),
8.09 (s, 1H, Ar–H), 7.63 (t, J ≈ 8.0 Hz, 1H, Ar–H), 7.55
(d, J ≈ 8.0 Hz, 1H, Ar–H), 7.41–7.38 (m; 4 Ar–H), 7.12
(d, J≈8.9 Hz; 2 Ar–H), 6.99 (d, J≈8.8 Hz; 2 Ar–H), 6.94
(d, J ≈ 9.0 Hz; 2 Ar–H), 4.06 (t, J ≈ 6.5 Hz; 2H, OCH2),
4.02–3.96 (m; 2H, OCH2), 1.86–1.14 (m; 28H, 14 CH2),
0.95 (d, J≈6.6 Hz; 3H, CH3), 0.90–0.87 (m; 9H, 3 CH3).
13C-NMR (125 MHz, CDCl3): δ (ppm)= 164.78, 164.32,
164.31, 163.89 (4 s; COO), 163.73, 156.97, 155.67, 154.89,
151.08, 144.17, 130.68, 129.91, 127.86, 127.53, 127.46,
126.34, 123.67, 120.87 (10 s; 10 Ar–C ve 4d; 4 Ar–CH),
132.45, 131.92, 131.87, 122.36, 122.26, 121.94, 115.15,
114.45 (8d; 8 Ar–CH), 68.42, 66.77 (2t; 2 OCH2), 29.86,
27.99 (2d; 2 CH), 39.26, 37.30, 36.23, 31.93, 29.67, 29.65,
29.60, 29.57, 29.36, 29.09, 25.99, 24.71, 24.67, 22.73 (14t;
14 CH2), 22.70, 22.62, 19.67, 14.14 (4q; 4 CH3). C56H66O10
(899.11); Anal. Calc.: C, 74.81; H, 7.40. Found: C, 74.86;
H, 8.04.
Results and discussion
Liquid crystalline properties
3‑[[4‑[[4‑(Tetradecyloxy)benzoyl]oxy]benzoyl]oxy]benzoic
acid 4‑[[4‑((S)‑3,7‑dimethyloctyloxy)phenoxy]carbonyl]
The liquid crystalline properties of new unsymmetrical bent-
core compounds 5a–e are given in Table 1.
Table 1 Mesophase, phase transition temperatures and the corresponding transition enthalpies of the compounds 5a–e
O
O
O
O
O
O
O
O
*
O
RO
(S )
R = C nH 2n+ 1
Compound
CnH2n+1
C6H13
T/°C [ΔH kJ/mol]a
5a
H→: Cr 120.51 [50.32] Iso
Cr 92.88 [32.30] Col 102.83 [8.18] Iso:←C
5b (OBDPE)
C8H17
H→: Cr 110.22 [33.68] Iso
Cr 74.42 [19.92] Col 99.50 [5.40] Iso:←C
5c
5d
5e
C10H21
C12H25
C14H29
H→: Cr 101.72 [21.79] Col 119.51 [13.09] Iso
Cr 70.05 [21.25] Col 115.27 [13.34] Iso:←C
H→: Cr 101.95 [21.65] Col 113.69 [3.52] Iso
Cr 77.27 [22.67] Col 105.97 [9.69] Iso:←C
H→: Cr 103.88 [25.67] Col 122.72 [12.45] Iso
Cr 79.71 [22.95] Col 119.92 [13.33] Iso:←C
Cr crystalline, Col columnar mesophase, Iso isotropic liquid phase
aPerkinElmer DSC-6; enthalpy values in italics in brackets taken from the second heating and cooling scans at a rate of 10 °C min−1
1 3

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The DSC thermograms of both compounds 5a and 5b
exhibit one endotherm on heating and two exotherms on
cooling which correspond to the presence of a monotropic
mesophase (Fig. 1). Compounds 5a and 5b exhibit a meso-
phase with a mosaic texture (see Fig. 2) of a columnar phase
between ordinary glass plates under polarizing microscope
as observed for the achiral analog with n-octyloxy chains at
both terminals reported in Weissfog et al. (2005). The enan-
tiotropically occurred mesophase of the achiral analog was
assigned as a Colob mesophase as a result of X-ray analysis
(Weissfog et al. 2005).
peaks corresponding to Cr–Col–Iso transitions. On cooling
from isotropic liquid under PM, mesophase was detected
by the observation of a mosaic texture or the occurrence of
circular domains under very slow cooling rate. No further
investigations such as X-ray analysis could be performed
for the determination of a subclass of columnar mesophase
of 5a–e due to the fact that the crystallization immediately
started. The textures of Col mesophases for 5a, 5b and 5c,
5d, 5e are shown in Figs. 2 and 3, respectively.
The comparison of compounds 5a–e shows that the
increase in the number of carbon atoms in alkoxy chain
length at terminal gives rise to an enantiotropic mesomor-
phism as well as a deeply increase in Col mesophase inter-
vals preserving mesophase type (Table 1).
Comparison of 5b (OBDPE) with the achiral analog
(Weissfog et al. 2005) involving the same chain length
shows that the melting and clearing point of 5b dramati-
cally decreased by the efect of chain branching in (S)-
3,7-dimethyloxy group at one of the terminals. Additionally,
the Col mesophase of 5b appears monotropically which was
detected by two calorimetric peaks at 99.50 °C and 74.42 °C
in the DSC second cooling scans (Fig. 1).
Dielectric properties of OBDPE (5b)
The determination of dielectric parameters is very impor-
tant because these parameters give information about the
electromagnetic energy of a material at a wide broad of fre-
quency. For this purpose, compound 5b as representative
case has been investigated on the angular frequency range
from 25.12 rad/s to 50265k rad/s at diferent temperatures.
The complex dielectric constant (ꢀ∗(ꢁ)) consists of two
Similarly, compounds 5c, 5d and 5e possessing longer
terminal alkyl chains showed only an enantiotropic columnar
mesophase which is in agreement with the behavior observed
for achiral analogs (Weissfog et al. 2005) (Fig. 3). On both
heating and cooling, compounds 5c, 5d and 5e showed two
Fig. 1 DSC thermograms of compounds 5a and 5b on second heating and cooling (10 °C min−1
)
Fig. 2 The mosaic texture of
the Col phase of compound 5a
at 93.0 °C during crystalliza-
tion; compound 5b at 91.0 °C
as observed between crossed
polarizers (indicated by arrows)
in ordinary glass plates on cool-
ing (magnifcation×100)
5a
5b
Cr
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Chemical Papers
Fig. 3 DSC thermograms on second heating and cooling (10 °C
min−1) and the Col phase of compounds 5c, 5d and 5e; the mosaic
texture at 108.3 °C, circular-like domains at 94 °C on very slow cool-
ing and the mosaic texture at 119.0 °C, respectively (magnifcation
×200 for 5c and 5e)
The complex dielectric constant is ꢀ∗(ꢁ) that can be
indicated by Eq (1) (Fernandes et al. 2017; Korkmaz et al.
2017, 2018)
parameters which are real ꢀ�(ꢁ) and imaginary components
ε″(ω) of dielectric constant. ω is the angular frequency and
is equal to 2πƒ.
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Chemical Papers
Figure 5 shows the angular frequency dependence of plot
of imaginary part of dielectric constant. The maximum value
is critic angular frequency, and for low angular frequency
the critic angular frequency (ωc1) is equal to 24.14 rad/s
and maximum imaginary part of dielectric constant value is
38.41. At high frequency, the critic angular frequency (ωc2)
is equal to 46366 k rad/s and maximum imaginary part of
the dielectric constant value is 3.024 for high frequency in
the columnar mesophase of the liquid crystal. At low angular
frequency, the imaginary part of dielectric constant increases
up to critical angular frequency and, after critical angular
frequency, decreases up to 1×107 rad/s. The imaginary part
of dielectric constant increases with an increase in angular
ꢀ∗(ꢁ) = ꢀ�(ꢁ) − iꢀ��(ꢁ)
(1)
Figure 4 shows that variation of real part of dielectric con-
stant depending on angular frequency for OBDPE (5b). The
real part of dielectric constant decreases with an increase in
frequency up to 1k rad/s, and after this angular frequency
value, the real part of dielectric constant is almost unchanged
up to 468k rad/s and it decreases again after this value. As
shown in Fig. 4, there are two regions with a dipole efect.
Molecular orientation causes the dipole effect, and the
dipole efect is much stronger at low frequency than at high
angular frequency. At high angular frequency, the molecular
orientation is stable in the presence of electric feld.
Fig. 4 Angular frequency
dependence of plot of real
part of dielectric constant for
OBDPE (5b)
Fig. 5 Angular frequency
dependence of plot of imaginary
part of dielectric constant for
OBDPE (5b)
1 3

Chemical Papers
frequency due to the decrease in the oriented electrical
molecular dipole efect.
the relaxation is called as Nearly Debye model. For Debye-
type relaxation model (α=0), the dipoles are not interacting
with each other. The relaxation types for frst and second
regions are Nearly Debye and non-Debye type, respectively.
Absorption coefcient (α) and relaxation times (τ0) parame-
ters values are calculated, and temperature-dependent graphs
are shown in Fig. 6a. The dielectric strength (Δεs) values
are measured liquid crystal phases; crystalline, columnar
mesophase and isotropic temperature-dependent graphs are
shown in Fig. 6b.
Cole–Cole equation is given in Eq. (2)
(
)
1−ꢃ
1 + ꢁꢂ0
sin 1 ꢃꢄ
2
ꢀ�(ꢁ) = ꢀ∞ + (ꢀs − ꢀ∞)
1 + 2(ꢁꢂ0)1−ꢃ sin 1 ꢃꢄ + (ꢁꢂ0)2(1−ꢃ)
2
(2)
α is the absorption coefcient and τ0 is the relaxation time.
The dielectric strength (Δεs) is calculated as given in
Eq. (3)
Cole–Cole plot is shown in Fig. 7 for information of die-
lectric relaxation of mechanism for OBDPE (5b). As shown
in Fig. 7, there is an open (inset graph: equivalent parallel
resistance–capacitance circuit) and a closed semicircle in
two Cole–Cole plots two R–C series for OBDPE (5b). In
this system, there accommodates two serial connected paral-
lel RC devices.
Δꢀs = ꢀs − ꢀ∞.
(3)
ꢀs is the limiting low-frequency and ꢀ∞ is the limiting high-
frequency dielectric constant (Ahlatcıoğlu et al. 2014).
Absorption coefficient (α) and relaxation times (τ0)
parameters have also been ftted by Origin Pro 2015 graphic
program in terms of Cole–Cole Eq. (2) on the angular fre-
quency dependency of the relative dielectric constant
(ꢀ� − ln ꢁ) curves. Absorption coefficient value varies
0<α<1. If α value ranges between 0 and 1, the relaxation
type is non-Debye. Absorption coefcient is nearly zero, and
According to Jonscher’s universal law (σ=σdc +Aωs). σdc
is a dc component, Aωs is an ac component, and s parameter
is frequency exponent (Cole and Cole 1941). As shown in
Fig. S22 (see SI), the frequency exponent value can be cal-
culated. There are three regions on the plot of ln(σ) versus
Fig. 6 The dielectric param-
eters; (a) absorption coefcient
and relaxation times, (b) dielec-
tric strength of OBDPE (5b)
(a)
(b)
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Chemical Papers
Fig. 7 Real dielectric constant
part dependence of plot of
imaginary dielectric constant
part for OBDPE (5b)
ln(ω) graph. The s parameters are 0.698 (sI) and 0.936 (sIII)
for the frst and third region, respectively. According to Jon-
scher’s universal law, if s parameter value is between 0<s<1
region, the conductivity mechanism is correlated barrier
hopping (CBH). The second region (sII =0.003) obeys nearly
DC mechanism because s parameter is very close to 0. The
values of the calculated s parameter at regions are given in
Table S1 (see SI).
dipole efect decreases in OBDPE (5b) at high angular fre-
quency. The relaxation types of OBDPE (5b) are Nearly
Debye and non-Debye type, for the frst and second regions,
respectively. According to Cole–Cole plot, there is an open
(inset graph: equivalent parallel resistance–capacitance cir-
cuit) and a closed semicircle in two Cole–Cole plots two
R–C series for OBDPE (5b). In this system, there accom-
modates two serial connected parallel RC devices. The con-
ductivity mechanisms of OBDPE (5b) are correlated barrier
hopping (CBH) for frst and third regions and Nearly DC for
second region.
Conclusions
Acknowledgements This research has been supported by Yildiz Tech-
nical University Scientifc Research Projects Coordination Department
with the Projects Numbers: 2013-01-02-YL05.
In this work, the synthesis, structural and mesomorphic
characterization of a new unsymmetrical bent-core meso-
gens 5a–e derived from 3-hydroxybenzoic acid central core
with linked two rodlike units carrying (S)-3,7-dimethyloc-
tyloxy group and n-alkoxy chain at both terminals has been
reported. The compounds 5a–e show a monotropic or enan-
tiotropic columnar phase with a mosaic texture at a narrow
mesomorphic range depending on alkoxy chain length at
terminal. Additionally, the comparison with analogs with a
straight chain shows that a decrease in the mesophase transi-
tions by the efect of a chain branching at one of terminals.
As a representative case, dielectric parameters of the mem-
ber with a n-octyloxy chain of bent-core mesogens series
have been evaluated. According to real part of dielectric con-
stants, there are two regions with a dipole efect. Molecular
orientation causes the dipole efect, and the dipole efect is
much stronger at low frequency than at high angular fre-
quency. At high angular frequency, the molecular orientation
is stable in the presence of electric feld. The imaginary part
of dielectric constant increases with an increase in angular
frequency due to the fact that oriented electrical molecular
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Afliations
Hale Ocak1 · Esma Ahlatcıoğlu Özerol2 · Fulya Çağla Çelikel1 · Mustafa Okutan3 · Belkız Bilgin Eran1
1
3
Department of Chemistry, Yildiz Technical University,
34220 Esenler, Istanbul, Turkey
Department of Physics, Yildiz Technical University,
34220 Esenler, Istanbul, Turkey
2
Department of Bioengineering, Yildiz Technical University,
34220 Istanbul, Turkey
1 3