Synthesis, characterization and liquid crystalline properties of a series of hydroxybiphenyl benzoate and biphenyl bis(benzoate)

Article abstract of DOI:10.1016/j.molliq.2016.10.002

Two series of hydroxybiphenyl benzoate 2a–2c and biphenyl bis(benzoate) 3a–3c have been synthesized by the esterification reaction of 4-alkoxybenzoic acid 1a–c with 1,1′-biphenyl-4,4′-diol in presence of DCC and DMAP in dichloromethane at room temperature under nitrogen atmosphere. Products ratio (2 vs 3) were not significantly influenced with the variation of alkoxy substituents. The newly synthesized ester products were characterized by IR and NMR spectroscopy as well as elemental analysis. The transition temperatures and mesophases have been investigated by differential scanning calorimetry and polarized optical microscopy. Although the synthesized ester derivatives, 3a–c are achiral, molecular chirality is exhibited in their mesophases.

Full text of DOI:10.1016/j.molliq.2016.10.002

Synthesis, characterization and liquid crystalline properties of a series of
hydroxybiphenyl benzoate and biphenyl bis(benzoate)
Salhed Ahmed, Md. Mostafizur Rahman, Ashikur Rahman Rabbi, Md.
Kausar Ahmed, S.M. Saiful Islam, Muhammad Younus, Bimal Bhusan
Chakraborty, Sudip Choudhury, Shamim Ahmed, Nasim Sultana
PII:
DOI:
Reference:
S0167-7322(16)31959-6
doi:10.1016/j.molliq.2016.10.002
MOLLIQ 6406
To appear in:
Journal of Molecular Liquids
Received date:
Accepted date:
19 July 2016
1 October 2016
Please cite this article as: Salhed Ahmed, Md. Mostafizur Rahman, Ashikur Rah-
man Rabbi, Md. Kausar Ahmed, S.M. Saiful Islam, Muhammad Younus, Bi-
mal Bhusan Chakraborty, Sudip Choudhury, Shamim Ahmed, Nasim Sultana, Syn-
thesis, characterization and liquid crystalline properties of a series of hydroxy-
biphenyl benzoate and biphenyl bis(benzoate), Journal of Molecular Liquids (2016),
doi:10.1016/j.molliq.2016.10.002
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ACCEPTED MANUSCRIPT
Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
Synthesis, characterization and liquid crystalline properties of a series of
hydroxybiphenyl benzoate and biphenyl bis(benzoate)
Salhed Ahmeda, Md. Mostafizur Rahmana, Ashikur Rahman Rabbia, Md. Kausar Ahmeda, S.M. Saiful Islama,
Muhammad Younusa*, Bimal Bhusan Chakrabortyb, Sudip Choudhuryb, Shamim Ahmedc, Nasim Sultanac
a
b
Department of Chemistry, Shahjalal University of Science and Technology, Sylhet-3114, Bangladesh; Department of Chemistry, Assam
University, Silchar, India; c Bangladesh Council of Scientific and Industrial Research, Dhaka 1000, Bangladesh.
* Corresponding Author. E-mail: myouns-che@sust.edu
Abstract
Two series of hydroxybiphenyl benzoate 2a-2c and biphenyl bis(benzoate) 3a-3c have been synthesized by the esterification reaction of 4-
alkoxybenzoic acid 1a-1c with 1,1′-biphenyl-4,4′-diol in presence of DCC and DMAP in dichloromethane at room temperature under nitrogen
atmosphere. Products ratio (2 vs 3) were not significantly influenced with the variation of alkoxy substituents. The newly synthesized ester
products were characterized by IR and NMR spectroscopy as well as elemental analysis. The transition temperatures and mesophases have been
investigated by differential scanning calorimetry and polarized optical microscopy. Although the synthesized ester derivatives, 3a-c are achiral,
molecular chirality is exhibited in their mesophases.
Key words: Biphenyl benzoate; liquid crystals; achiral; chiral
liquid crystals with chiral phases are available in the
literature [20c-d, 21], rod-like liquid crystals using
1. Introduction
biphenyl and phenylpyrimidine linked via flexible
methylene spacer are also reported [20a-b]. In this study,
we report the synthesis and characterization of a new series
of hydroxybiphenyl benzoate 2a-2c and biphenyl
bis(benzoate) 3a-3c and investigation of their liquid
crystalline properties. The use of long, linear aromatic
biphenyl ring contributes to the thermal stability of the
mesophase. Though the newly formed ester derivatives 3a-
c are achiral, the molecular chirality is exhibited in their
mesophases without introducing chiral dopants.
Thermochromism is a well-known and useful property
frequently observed in different types of materials [1]. The
change of color with temperature is certainly
a
phenomenon which is of interest in the field of liquid
crystals [2], and many applications are based on this
property: thermometers (fever indicator and gadgets),
colored electro optic films and light-emitting diodes [3].
Phototunability of the cholesteric liquid crystals pitch and
the circular polarized reflection wavelength has formed the
basis of many applications including tunable color
reflectors and filters, sensors, tunable lasers and molecular
switches [4-12]. Chirality is a very intriguing issue in liquid
crystal (LC) science [13]. There are many types of chiral
LCs, such as chiral nematic (N*) [14], chiral smectic C
(SC*) [15], blue phase [16-17] and twisted grain boundary
(TGB) phase [18]. Liquid crystal molecules can be made
chiral either by including chirality within the molecules or
by adding chiral dopant into liquid crystal phases [13, 19].
In recent years, there have been many reports on the design
of liquid crystals exhibiting chiral phases, without
possessing any chiral center [20-21]. Among them, bent-
core molecules with one or two flexible tails exhibit a wide
variety of novel structural phenomena involving the
interplay of chiral, polar, and liquid crystalline order [20c-
d]. During controlled heating or cooling, these materials
form isotropic fluid, though they possess short range
2. Results and discussion
2.1 Syntheses
The ester products of hydroxybiphenyl benzoate 2a, and
biphenyl bis(benzoate) 3a were synthesized by the
positional and orientational order. As
a result, the
macroscopic symmetry is broken to produce chirality,
despite the fact that these molecules are achiral. This type
of symmetry breaking is observed in nematic and smectic
phases, and polarized optical microscopy gives regions of
different optical properties. Rotation of one polarizer from
its crossed position clockwise by a certain angle (˂ 20°)
gives two different optical domains, and rotating the
polarizer in the opposite direction by the same angle
reverses the position of the domains. Although a vast
number of achiral bent-core (1-3-substituted phenyl core)
Scheme 1 Synthesis of hydroxybiphenyl alkoxybenzoate
and
biphenyl
bis(alkoxybenzoate)
derivatives
1

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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
Table 1 Synthesis of hydroxybiphenyl alkoxybenzoate and biphenyl bis(alkoxybenzoate) derivativesa
Entry
4-Alkoxybenzoic acid, 1
1,1′-Biphenyl-4,4′-diol
Hydroxybiphenyl alkoxybenzoate
Biphenyl bis(alkoxybenzoate)
(2.32 mmol)
(mmol)
2,i (%)
3,i (%)
1
2
3
1a
1b
1c
2.32
2.32
2.32
21.5 (2a)
25.5 (2b)
20.7 (2c)
18.3 (3a)
24.6 (3b)
14.2 (3c)
a The esterification reactions were performed in DCC (2.32 mmol) and DMAP (2.32 mmol) in dry dichloromethane at room temperature.
i Isolated yields.
esterification reaction [22] of 4-butoxybenzoic acid 1a
with 1,1′-biphenyl-4,4′-diol in presence of DCC and
DMAP in dichloromethane at room temperature under
nitrogen atmosphere. The newly synthesized compounds 2a
and 3a were isolated as white solids in 21.5% and 18.3%
respectively (Table 1, entry 1, and Scheme 1). Using excess
1,1′-biphenyl-4,4′-diol (6 equivalent) in the reaction
increases the yield of the mono-benzoate 2a by 9%
(isolated yield 30.5%), but the formation of bis-benzoate 3a
remains the same. Utilizing excess (6 equivalents) 4-
butoxybenzoic acid 1a in the reaction increases the yield of
the bis-benzoate 3a significantly (54.1 %). The synthesized
ester derivatives were purified and isolated by column
chromatography eluting with hexane and dichloromethane
(2:1). Similarly, other ester products 2b-c, and 3b-c with
longer carbon chains (C8 and C12) were synthesized. The
synthetic route for biphenyl benzoate derivatives is shown
in Scheme 1. Products ratio (2 vs 3) were not significantly
influenced with the variation of alkoxy substituent. The
results were summarized in Table 1. All the synthesized
ester products provided satisfactory elemental analyses.
The structure of ester products 2a-2c and 3a-3c were
confirmed by IR, 1H and 13C NMR spectroscopic
techniques (Table 2). Although the preparation of
compound 2a and 2b were reported previously, 2a was
isolated as an impure product, could not be purified by
recrystallization and column chromatography [23], and
characterization of 2b was limited to IR and 1H NMR
spectroscopy [24].
2.2. Characterization
In the IR spectra, the (C=O) stretching frequency is
diagnostic of the characterization of the ester group (-
COOR), and the absence of the terminal acidic hydrogen, -
COOH confirms the completion of the reaction. The ester
product 2a displays a sharp single absorption band at 1707
cm-1, which is assigned to the stretching vibration of the
ester group (RCOOR). The compound shows no broad
band for the -COOH stretching vibration in the range of
2400-3400 cm-1 confirming that the terminal carboxylic
acidic group (-COOH) undergo the esterification with the
hydroxyl group of 1,1′-biphenyl-4,4′-diol. The spectrum
displays another single absorption band at 3458 cm-1,
characteristic of -OH stretching vibration, demonstrating
that the esterification reaction occurs in one hydroxyl group
and another hydroxyl group is retained in the compound
[25]. Similarly, compound 3a displays the presence of a
sharp absorption at 1728 cm-1 and absence of a broad band
in the range of 2400-3400 cm-1, thus confirming the
formation of the desired ester product (RCOOR).
Compound 2a displays a single IR absorption band at 3458
cm-1, characteristic of -OH stretching vibration, whereas
compound 3a does not display any absorption band 3400-
3600 cm-1, suggesting that both OH groups of the diol takes
part in the esterification reaction. Similarly, other
compounds 2b, 2c, 3b and 3c display characteristic IR
peaks in the expected regions (Table 2).
Table 2 Selected spectroscopic data (IR and 1H NMR) for compounds 2a-c and 3a-c
Compounds
IR (cm-1)
1H NMR (ppm)a
2a
8.15 (d, 2H, biphenyl), 7.56 (d, 2H, biphenyl), 6.98 (d, 2H, biphenyl), 6.90 (d, 2H, biphenyl),
7.46 (d, 2H, aryl), 7.24 (d, 2H, aryl), 4.05 (t, 2H, OCH2), 1.78 (q, 2H, CH2), 1.5 (sx, 2H, CH2-CH3),
and 0.98 (t, 3H, CH3)
1707 ( C=O, ester), 3458
( OH)
2b
2c
8.16 (d, 2H, biphenyl), 7.56 (d, 2H, biphenyl), 6.98 (d, 2H, biphenyl), 6.89 (d, 2H, biphenyl),
7.45 (d, 2H, aryl), 7.24 (d, 2H, aryl), 4.04 (t, 2H, OCH2), 1.79 (q, 2H, CH2), 1.4 (sx, 2H, CH2-CH3),
and 0.85 (t, 3H, CH3)
1708 ( C=O, ester), 3464
( OH)
8.16 (d, 2H, biphenyl), 7.56 (d, 2H, biphenyl), 6.98 (d, 2H, biphenyl), 6.89 (d, 2H, biphenyl),
7.46 (d, 2H, aryl), 7.24 (d, 2H, aryl), 4.04 (t, 2H, OCH2), 1.79 (q, 2H, CH2), 1.57 (sx, 2H, CH2-CH3),
and 0.87 (t, 3H, CH3)
1730 ( C=O, ester), 3331
( OH)
3a
3b
3c
8.17 (d, 2×2H, biphenyl), 6.99 (d, 2×2H, biphenyl), 7.63 (d, 2×2H, aryl), 7.29 (d, 2×2H, aryl),
4.06 (t, 2×2H, OCH2), 1.81 (q, 2×2H, CH2), 1.53 (sx, 2×18H, (CH2)9-CH3), and 0.99 (t, 2×3H, CH3)
1728 ( C=O, ester)
1730 ( C=O, ester)
1728 ( C=O, ester)
8.17 (d, 2×2H, biphenyl), 6.96 (d, 2×2H, biphenyl), 7.63 (d, 2×2H, aryl), 7.29 (d, 2×2H, aryl),
4.04 (t, 2×2H, OCH2), 1.82 (q, 2×2H, CH2), 1.34 (sx, 2×18H, (CH2)9-CH3), and 0.89 (t, 2×3H, CH3)
8.17 (d, 2×2H, biphenyl), 6.99 (d, 2×2H, biphenyl), 7.63 (d, 2×2H, aryl), 7.29 (d, 2×2H, aryl),
4.04 (t, 2×2H, OCH2), 1.82 (q, 2×2H, CH2), 1.34 (sx, 2×18H, (CH2)9-CH3), and 0.88 (t, 2×3H, CH3)
a 1H NMR spectra were referenced to 7.25 ppm (residual proton of CHCl3) of CDCl3.
2

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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
OCH2 carbon and other carbons of the octyloxy group
exhibit signals at 14.11, 22.67, 26.00, 29.12, 29.23, 29.34
and 31.82 ppm. The ester carbon displays signal at 165.23
ppm, and the phenyl and biphenyl ring carbons display
signal at 114.35, 115.70, 121.51, 122.03, 127.72, 1228.37,
132.34, 133.15, 138.50, 150.05 and 155.27 ppm. Compared
to the mono-benzoate 2b, the bis-benzoate 3b gives only
eight signals in the aromatic region at 114.34, 121.51,
122.15, 128.18, 132.33, 138.11, 150.59 and 163.62 ppm,
verifying the symmetrical nature of the compound as
evident in the 1H NMR spectrum.
2.3. Thermal stability
Thermal studies of all compounds were performed by
thermo gravimetric analysis (TGA) under nitrogen
atmosphere. Analysis of the TG trace (heating rate 10
oC/min) shows that compound 2a-2c and 2a-c exhibit good
thermal stability.
Fig 1 1H NMR spectrum of compound 2b
1
The H NMR spectra of compound 2a-2c and 3a-3c could
be fully assigned to the phenyl and alkyl proton resonances.
The spectrum of 2b, for example, displays two doublet
resonances corresponding to the octyloxyphenyl ring,
which are observed at 7.45 and 7.24 ppm, and four sets of
doublet resonances corresponding to the biphenyl ring,
Table 3 Thermogravimetic data for 2a-c and 3a-c
Compounds Decomposition Decomposition
%
of
onset (oC) on endset (oC) on weight
TGA
320
332
328
375
373
360
TGA
389
402
426
428
433
419
loss
80
75
77
65
71
62
2a
2b
2c
3a
3b
3c
Due to the presence of 4,4′-biphenyl aromatic ring based
central core [26], the compounds exhibit high thermal
o
stability, and decompositions are observed at ca. 320 C
for 2a, 332 oC for 2b, 328 oC for 2c, 375 oC for 3a, 373 oC
for 3b, and 360 oC for 3c in the TG traces.
Fig 2 1H NMR spectrum of compound 3b
which are observed at 8.16, 7.56, 6.98, 6.89 ppm.
Additionally, the octyloxy alkyl protons are evident as
triplet, quintet, septet and triplet resonances at 4.04, 1.79,
1.57 and 0.87 ppm, respectively (Table 2). Similar to 2b,
bis-benzoate 3b displays two doublet resonances at 7.63
and 7.29 for the octyloxy phenyl protons. However, in
contrast to 2b, it exhibits two doublets at 8.17 and 6.99
ppm for the biphenyl ring confirming the symmetrical
nature of biphenyl bis(benzoate) 3b. Similarly, compounds
2a, 2c and 3a, 3c display peaks in the expected regions of
the H NMR spectra (Table 2). Representative H NMR
spectra of compound 2b and 3b are shown in Fig. 1 and 2,
respectively. The aliphatic carbon skeletons, aromatic ring
carbons and –C=O and OCH2 functional groups of the
mono- and bis-benzoate 2a-2c and 3a-3c could fully be
characterized by the 13C NMR spectroscopy. The spectrum
of 2b displays a strong signal at 68.04 ppm due to the -
Fig. 3 TGA thermograms of compound 2a
Representative TGA curve of compound 2a is shown in
Fig. 3. In mono-benzoate 2a-2c, thermal stability slightly
increases from 2a to 2b due to the increase of chain length
of terminal moiety from OC4H9 to OC8H17, but decreases in
2c for the further increase. The thermal stability of the bis-
ester 3a-c is higher than that of the monoester 2a-c (Table
3), and in the series, thermal stability decreases with
increase of the chain length of the terminal moiety (Table
1
1
3

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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
3). Among the ester compounds, the highest thermal
stability is observed for 3a (decomposition onset at 375
oC). The weight loss (77-80%) in mono-benzoate is much
higher that of bis-benzoate (62-65%).
2.4. Phase clarifications of compounds 2 and 3
The phase behavior of compounds 2a-c and 3a-c were
investigated by a combination of DSC (Fig 4) and POM
(Fig. 6-11). DSC traces of compounds 2 and 3 predict the
existence of liquid crystal phases. The phase transition
temperatures observed with DSC are consistent with those
detected by POM. The mesophase type and phase transition
temperatures are summarized in Table 4. The DSC
thermogram of 3c is shown in Fig. 4. In POM, all the
compounds exhibit strong birefringences characteristic of
liquid crystalline behavior during the heating and cooling.
Fig. 4 DSC thermogram (10 oC/min heating/cooling) of 3c
Compound 2b, with longer alkoxy chain (OC8H17) than 2a
2.4.1 Mono-benzoate 2a-2c
Compound 2a, on second heating, exhibits crystal to
o
o
mesophase transition at 204.9 C with enthalpy change of
(OC4H9), on heating, shows a glass transition at 86.3 C, a
crystal to crystal transition at 111.5 oC and a nematic
transition at 177.2 oC. On slow cooling it displays an
73.7 J/g, and mesophase to isotropic liquid transition at
o
239.9 C with enthalpy change of 2.1 J/g. POM of the
o
compound displays thread-like textures of an enantiotropic
nematic mesophase. DSC shows that it has a glass
additional smectic A (Sm A) transition at 143.3 C. The
nematic droplets of the nematic phase and batonnet texture
of the Sm A phase are observed on POM (Fig. 6).
Compound 2c, with the longest alkoxy chain (OC12H25),
exhibits only smectic A mesophase. POM demonstrates
batonnet textures, both on heating and cooling, which is
characteristic to Sm A phase (Fig. 7). In the hydroxyl
mono-benzoate series 2a-2c, the length of the alkoxy chain
is varied keeping the core aromatic structure same, and it is
observed in the DSC that the melting point as well as
nematic phase stability decreases from 2a to 2b with
increase of chain length. The nematic ranges for 2a and 2b
o
transition at 121.8 C during the first heating, which is lost
in the second heating.
Table 4 Phase transition temperatures and enthalpies
change of compounds 2a-c and 3a-c
Compounds Heating cyclea
Cooling cyclea
Cr- N 204.9 (73.7), IL-N 262.1 (0.5), N-Cr
N-IL 239.9 (2.1) 131.5 (39.7)
2a
2b
o
o
are 21.5 C and 35 C, respectively; it also decreases with
increase of chain length (Fig. 5). The longest alkoxy chain
(OC12H25) in 2c, compared to 2a and 2b, dictates the
mesophase to be smectic [27].
Cr1-Cr2 111.5 (4.9), IL-N 219.9 (1.6), N-
Cr-N 177.2 (77.1), SmA 143.3 (35.1),
N-IL 198.7 (5.1)
SmA-Cr 120.6 (10.3)
Cr-SmA 97.7 (19.4), IL-SmA 157.5 (33.3),
SmA -IL 155.4 (28) SmA -Cr 93.9 (22.9)
2c
3a
Cr-SmA
151.7, IL-N 362.1 (6.6), N-
(26.5), SmA-N 178.0 SmA 169.4 (40.6),
(44.2), N-IL 360.4 SmA-Cr 116.4 (4.6)
(6.3)
Cr1-Cr2 109.0 (7.1), IL-N 292.4 (2.9), N-
3b
3c
Cr2-SmA
140.0 SmA 213.8 (3.7), SmA-
(39.5), SmA-N 212.9 SmC 138.5 (18.7),
(4.0), N-IL 290.3 SmC-Cr 125.8 (11.0)
(3.5)
Cr1-Cr2 71.6 (11.1), IL-N 249.5 (1.5), N-
Cr-
SmC
115.5 SmA 229.3 (3.9), SmA-
127.6 (16.5),
Fig. 5 Phase temperature ranges of 2a-2c and 3a-3c during
(30.3),
SmC-SmA SmC
heating
125.2 (16.7), SmA-N SmC-Cr 96.4 (36.4),
220.1 (4.9), N-IL Cr2-Cr1 63.9 (3.0)
245.8 (1.3)
2.4.2 Bis-benzoate 3a-3c
Compound 3a (OC4H9 terminal moiety) exhibits three
endothermic and three exothermic transitions as evident by
DSC (Table 4). POM shows enantiotropic nematic and
smectic A mesophases during heating and cooling. The
phases are reflected by schlieren texture (nematic) and
a
Enthalpy change (∆H/Jg-1) given in bracket. Cr = Crystalline
phase, N = Nematic phase, IL = Isotropic Liquid, Sm A = Smectic
A phase, Sm C = Smectic C phase.
4

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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
feather like texture (Sm A) (Fig. 8). Similarly, 3b, on
(a)
heating, displays Sm A and nematic mesophases (Fig. 9).
However, it exhibits an additional Sm C phase during
cooling. Compared to 3a and 3b, 3c exhibits enantiotropic
Sm C, Sm A (Fig. 10) and nematic transitions. The Sm C,
Sm A and nematic phases are reflected by broken focal
conic, focal conic and schlieren textures, respectively. The
new bis-benzoates 3a-3c are symmetrical molecules where
the central biphenyl core, in its 4,4′ position, is linked to an
additional phenyl ring through ester linkage, thus increases
the length to breadth ratio of the molecules. With shorter
o
alkoxy chain, 3a shows the nematic stability 362.1 C with
largest nematic range (182.4 oC) of the series (Fig. 5). With
short alkoxy chain (OC4H9), it also shows smectic phase
with narrow range probably due to the lateral attraction for
the resultant dipole of ester bridged aromatic rings [26-27].
As the terminal chain length increases from C4 to C8, the
melting point and nematic phase stability of 3b decreases,
but the smectic A phase stability increases, as expected
(Fig. 5). In 3c, with the OC12H25 terminal moiety, an
additional smectic C phase (Fig. 5) is observed because of
lamellar packing which is stabilized by the longest (C12)
terminal chain [26-28]. While n-alkyl and n-alkoxy
cyanobiphenyl are commercially important mesogens [29],
and their thermally stable analogues are used in designing
polymer composites [30], the hydroxy functionalized
analogues are used as pendant cores in side chain LC
polymers [31].
(b)
Fig. 7 Optical texture of 2c. Batonnet textures of smectic A
phase at 168.7 ºC during heating (a), and at 165.3 ºC (b)
during cooling.
(a)
(a)
(b)
(b)
Fig. 6 Optical texture of 2b. Batonnet texture (a) of smectic
A phase at 125.3 ºC, and nematic droplets (b) of nematic
phase at 219.0 ºC
Fig. 8 Optical texture of 3a. Feather like texture (a) of
smectic A phase at 145.4 ºC, and (b) schlieren texture of
nematic phase at 305.4 ºC
5

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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
uncrossed polarizer, the texture is split into darker and
brighter domains, and by uncrossing the polarizer in
opposite directions by the same angle, the darker and
brighter domains are exchanged (Fig. 11), indicating that
the domains are twisted in opposite directions [20a,d]. The
chirality of a twist-bend nematic phase, formed by
methylene link, can be indentified NMR spectroscopy [32].
Achiral nematic liquid crystal 5CB can be used to create
template blue phases that can act as mirrorless laser and
switchable opto-electronic devices [33].
(a)
Fig. 9 Optical texture of 3b. Thread like texture of nematic
phase at 210.3 ºC
(a)
(b)
(b)
(c)
Fig. 10 Optical texture of 3c. Broken focal conic texture (a)
of smectic C phase at 122.6 ºC, and focal conic texture (b)
of smectic A phase at 125.19 ºC
3.3 Chiral domains of 3a-3c
POM demonstrates that though compound 3a-3c has no
chiral center of its own, it shows chirality under the
microscope with respect to the alignment of polarizer and
analyzer due to the tilting of molecules or the helical
arrangement of one group of molecules with respect to
another. When the samples are observed under slightly
Fig. 11 Optical texture of 3b under uncrossed and crossed
polarizer at 295 ºC. Domains of opposite handed optical
activity upon decrossing the analyzer relative to the
polarizer, to the left (a), to the right (c). DC phase appears
as dark between crossed polarizer (b).
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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
2.5. Absorption and emission properties of the compounds
The theoretical electronic transitions for the compounds are
presented in Table 5. The results indicate the major and
minor contributors involved in the transition. Interestingly,
for the compound 3a, the major contributor is not HOMO-
>LUMO; rather the orbitals H-2 to L+2 are involved in the
transition. The density of states (DOS) profile (Fig. 13B)
indicates the presence of two sets of closely packed energy
levels for the compound 3a. Such densely spaced orbitals
may be responsible for the involvement of orbitals higher
(and lower) than the frontier orbitals in the transition of
electron. Another important feature is the nature of frontier
orbitals. For 3a, both the HOMO and LUMO are centered
at the terminal aromatic unit, but for the compound 2a, the
HOMO is centered far away at the central aromatic unit,
which in turn is probably responsible for the difference in
DOS.
The investigation of UV/Vis absorption spectroscopy of all
the compounds was carried out at room temperature. The
lowest energy absorption bands in the UV/Vis spectra, in
chloroform solution, at room temperature, for compounds
2a-c, (1.25×10-5 mol/L for 2a and 2c, and 1.11×10-5 mol/L
for 2b) are observed at 269 nm, and that for compounds 3a-
c (1.25×10-5 mol/L for 3a, 1.11×10-5 mol/L for 3b and
1.0×10-5 mol/L 3c) are also observed at the same peak
position (Fig. 12). The variation of alkoxy substituent
(butoxy, octyloxy, and dodecyloxy) in compounds 2a-c and
3a-c has no effect in absorption maxima. By comparing
with the related systems, the lowest energy band, in each
case, is attributed to the absorption at the 1,4-biphenyl core
[34]. The photoluminescence spectrum of 2 and 3 displays
emission bands in the ultraviolet to violet region (Fig. 12).
The room temperature spectral measurement of compounds
2a-c and 3a-c, recorded in chloroform under excitation at
the wavelength of the absorption maximum (max= 269
nm), shows emission maxima at 336 nm for 2a-c, and at
380 nm, 362 nm and 353 nm, for compounds 3a, 3b and 3c,
respectively. The optical (absorption and emission)
properties of some related systems [34] have been reported
recently.
Table
5
Contributors responsible for the electronic
transitions
Absorption
maxima (nm)
Compounds
2a
Contributors
Expl
Theo
HOMO->LUMO (19%)
HOMO->L+1 (68%)
H-5->L+7 (2%)
269
232
H-2->L+4 (3%)
H-1->L+2 (6%)
2a
2b
H-2->LUMO (22%)
H-1->L+1
(35%)
HOMO->L+2 (21%)
H-7->L+7 (4%)
H-6->L+5 (7%)
H-5->L+4 (5%)
3a
269
218
3a
2c
3. Experimental details
3.1. Materials and techniques
Solvents were dried, distilled from appropriate drying
agents and degassed before use [36]. The compounds
H9C4O-C6H4-COOH [37], H17C8O-C6H4-COOH [37] and
H25C12O-C6H4-COOH [37] were prepared by literature
methods. NMR spectra were recorded on Bruker 400 MHz
3c
1
NMR spectrometer in CDCl3 solvent. H and 13C NMR
3b
spectra were referenced to solvents resonances. Infrared
spectra were recorded on Shimadzu Prestige 21 FTIR
spectrometer by using KBr pellets or solution in
dichloromethane. The thermal stability was assessed with a
Shimadzu TGA-50 thermogravimetric analyzer under
nitrogen atmosphere. Samples were heated in the range
Fig. 12 Absorption and emission spectra of 2a-c and 3a-c
o
o
o
2.6. Theoretical Studies
from 25 C to 500 C, at heating rate of 10 C/min, in an
aluminium crucible. The phase transition temperatures and
associated enthalpies were recorded using differential
scanning calorimeter (DSC) with a Shimadzu TA-60A
The origin of the electronic transitions in the target
compounds were analyzed computationally taking 2a and
3a as the representative compounds. The geometries of the
compounds were optimized at B3LYP [35] density
functional level with 6-31++g(d,p) basis set. All
calculations were performed with GAUSSIAN 03 package
on BRAF supercomputing environment.
o
instrument, at a heating/cooling rate of 10 C/min, under
nitrogen atmosphere. Samples were heated in the range
from 25 oC to 350 oC, in an aluminium crucible.
7

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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
Fig. 13 Density of States (DOS) profile showing HOMO and LUMO for the compounds 2a (A) and 3a (B)
MHz, CDCl3):  13.83, 19.21, 31.12, 68.04, 114.33,
The mesophases exhibited by the hydroxybiphenyl
alkoxybenzoate 2a-c and biphenyl bis(alkoxybenzoate) 3a-
c were observed and characterized by using a Olympus
TH4-200 microscope equipped with a Linkam hot stage
and temperature controller (H95-HS).
115.70, 121.51, 122.03, 127.72, 128.39, 132.32, 133.27,
138.37, 150.05, 155.28, 163.59, and 165.23; Anal. Calc. for
C23H22O4: C, 76.22; H, 6.12%. Found: C, 76.30; H, 6.15%.
1
3a: IR (solid state, KBr):  1728 (-CO-, ester) cm-1; H
NMR (400 MHz, CDCl3):  8.17 (d, 2×2H, biphenyl), 7.63
(d, 2×2H, aryl), 7.29 (d, 2×2H, aryl), 6.99 (d, 2×2H,
biphenyl), 4.06 (t, 2×2H, OCH2), 1.81 (q, 2×2H, CH2), 1.53
(sx, 2×2H, CH2-CH3), and 0.99 (t, 2×3H, CH3); 13C NMR
(100 MHz, CDCl3):  13.83, 19.22, 31.16, 68.05, 114.34,
121.52, 122.15, 128.18, 132.34, 138.11, 150.59, 163.62,
and 165.01; Anal. Calc. for C34H34O6: C, 75.82; H, 6.36%.
Found: C, 75.71; H, 5.87%.
3.2. Syntheses
3.2.1. 4′-Hydroxy-[1,1′-biphenyl]-4-yl 4-(butoxy)benzoate
(2a) and [1,1′-biphenyl]-4,4′-diyl bis(4-(butoxy)benzoate)
(3a)
A mixture of 4-butoxybenzoic acid (1a) (0.45 g, 2.32
mmol) and 1,1′-biphenyl-4,4′-diol (0.432 g, 2.32 mmol) in
presence of dicyclohexacarbamide (DCC) (0.478 g, 2.32
mmol) and dimethylaminopyridine (DMAP) (0.283 g, 2.32
mmol) in dichloromethane (20 mL) was degassed under
nitrogen atmosphere and the resulting mixture was stirred
for 24 hours at room temperature. After completion the
reaction, the precipitate was removed by filtration. The
solvent was removed in under reduced pressure, and the
solid residue was redissolved in dichloromethane and the
undissolved solid was removed by filtration. Then, the
resulting solution was washed with 10% cold acetic acid
followed by brine solution, and dried over anhydrous
magnesium sulfate. After that, the solvent was removed
under reduced pressure, and the solid residue was purified
by column chromatography on silica gel eluting with
hexane and dichloromethane (2:1). The first band was
separated and identified as 3a, and the second band as 2a,
as white solids, in 18.34% (0.228 g) and 21.50% (0.18 g)
yield, respectively. 2a: IR (solid state, KBr):  3458 (-OH),
3.2.2.
4′-Hydroxy-[1,1′-biphenyl]-4-yl
4-
(octyloxy)benzoate (2b) and [1,1'-biphenyl]-4,4'-diyl bis(4-
(octyloxy)benzoate) (3b)
The same procedure as for compound 2a and 3a was
followed for the synthesis of 2b and 3b, but 4-
octyloxybenzoic acid 1b, was used instead of 4-
butoxybenzoic acid, 1a. Yield: 3b 25.5%; 2b 24.6%.
2b: IR (solid state, KBr):  3464 (-OH), 1708 (-CO-, ester)
cm-1; 1H NMR (400 MHz, CDCl3):  8.16 (d, 2H,
biphenyl), 7.56 (d, 2H, biphenyl), 7.45 (d, 2H, aryl), 7.24
(d, 2H, aryl), 6.98 (d, 2H, biphenyl), 6.89 (d, 2H, biphenyl),
4.9 (s, 1H, -OH), 4.04 (t, 2H, OCH2), 1.79 (q, 2H, CH2),
1.4 (m, 10H, (CH2)5-CH3), and 0.85 (t, 3H, CH3); 13C NMR
(100 MHz, CDCl3):  14.11, 22.67, 26.00, 29.12, 29.23,
29.34, 31.82, 68.38, 114.35, 115.70, 121.51, 122.03,
127.72, 128.37, 132.34, 133.15, 138.50, 150.05, 155.27,
163.62, and 165.23; Anal. Calc. for C27H30O4: C, 77.48; H,
7.22%. Found: C, 77.12; H, 6.69%. 3b: IR (solid state,
KBr):  1730 (-CO-, ester) cm-1; 1H NMR (400 MHz,
CDCl3):  8.17 (d, 2×2H, biphenyl), 7.63 (d, 2×2H, aryl),
7.29 (d, 2×2H, aryl), 6.96 (d, 2×2H, biphenyl), 4.04 (t,
2×2H, OCH2), 1.82 (q, 2×2H, CH2), 1.34 (m, 2×10H,
(CH2)5-CH3), and 0.89 (t, 2×3H, CH3); 13C NMR (100
1
1707 (-CO-, ester) cm-1; H NMR (400 MHz, CDCl3): 
8.15 (d, 2H, biphenyl), 7.56 (d, 2H, biphenyl), 7.46 (d, 2H,
aryl), 7.24 (d, 2H, aryl), 6.98 (d, 2H, biphenyl), 6.90 (d,
2H, biphenyl), 4.05 (t, 2H, OCH2), 1.78 (q, 2H, CH2), 1.5
(sx, 2H, CH2-CH3), and 0.98 (t, 3H, CH3); 13C NMR (100
8

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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
compounds 2a and 3a were performed with GAUSSIAN 03
package as representative compounds.
MHz, CDCl3):  14.11, 22.67, 26.01, 29.12, 29.24, 29.34,
31.82, 68.37, 114.34, 121.51, 122.15, 128.18, 132.33,
138.11, 150.59, 163.62, and 165.01; Anal. Calc. for
C42H50O6: C, 77.51; H, 7.74%. Found: C, 77.55; H, 7.32%.
3.2.3. 4′-Hydroxy-[1,1′-biphenyl]-4-yl 4-(dodecyloxy)
benzoate (2c) and [1,1'-biphenyl]-4,4'-diyl bis{4-
(dodecyloxy)benzoate} (3c)
Acknowledgements
We acknowledge Higher Education Quality Enhancement
Project (HEQEP) (CP 2524) of University Grants
Commission of Bangladesh for funding.
The same procedure as for compound 2a and 3a was
applied for the synthesis of 2c and 3c, but 4-
dodecyloxybenzoic acid 1c, was used instead of 4-
butoxybenzoic acid, 1a. Yield: 3c 14.2%; 2c 20.7%.
References
2c: IR (solid state, KBr):  3331 (-OH), 1730 (-CO-, ester)
cm-1; 1H NMR (400 MHz, CDCl3):  8.16 (d, 2H,
biphenyl), 7.56 (d, 2H, biphenyl), 7.46 (d, 2H, aryl), 7.24
(d, 2H, aryl), 6.98 (d, 2H, biphenyl), 6.89 (d, 2H, biphenyl),
4.04 (t, 2H, OCH2), 1.79 (q, 2H, CH2), 1.57 (m, 18H,
(CH2)9-CH3), and 0.87 (t, 3H, CH3-); 13C NMR (100 MHz,
CDCl3):  14.13, 22.70, 25.99, 29.12, 29.36, 29.37, 29.57,
29.60, 29.64, 29.67, 31.93, 68.37, 114.33, 115.70, 121.51,
122.02, 127.71, 128.37, 132.22, 133.19, 138.37, 150.05,
155.28, 163.62, and 165.23; Anal. Calc. for C31H38O4: C,
78.45; H, 8.07%. Found: C, 78.17; H, 7.18%. 3c: IR (solid
state, KBr):  1728 (-CO-, ester) cm-1; 1H NMR (400 MHz,
CDCl3):  8.17 (d, 2×2H, biphenyl), 7.63 (d, 2×2H, aryl),
7.29 (d, 2×2H, aryl), 6.99 (d, 2×2H, biphenyl), 4.04 (t,
2×2H, OCH2), 1.82 (q, 2×2H, CH2), 1.34 (m, 2×18H,
(CH2)9-CH3), and 0.88 (t, 2×3H, CH3-); 13C NMR (100
MHz, CDCl3):  14.11, 22.70, 26.00, 29.12, 29.36, 29.38,
29.57, 29.60, 29.65, 29.67, 31.93, 68.37, 114.34, 121.51,
122.15, 128.18, 132.33, 138.11, 150.59, 163.62, and
165.01; Anal. Calc. for C50H66O6: C, 78.70; H, 8.72%.
Found: C, 78.58; H, 8.36%.
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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
Graphical Abstract
Chirality is observed in achiral biphenyl bis-
benzoates as evident by
optical textures under uncrossed (˂ 20o) optical
polarizer
11

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Hydroxybiphenyl Benzoates and Biphenyl Bis(benzoate)s: Syntheses and LC Phases
Highlights



Synthesis and spectroscopic characterization of a
series of biphenyl mono- and bis-benzoates
Biphenyl benzoates exhibit excellent thermal
stability
Chiral domains are observed in achiral biphenyl
bis-benzoates
12