Synthesis of aromatic liquid crystals with asymmetric diester based on rod-like multi-ring system by two-step esterification method

Article abstract of DOI:10.1080/15421406.2016.1146862

A novel method of two-step esterifications was developed to synthesize compounds with asymmetric double ester groups. By using this method, six rod-like double ester compounds were prepared with p-hydroxy benzaldehyde, p-hydroxy benzoic acid bicyclohexyl carboxylic acid, cyclohexyl benzoic acid and biphenyl carboxylic acid substituted by n-propyl and n-pentyl as main reactants. The structures and properties of target compounds were confirmed by IR, MS, ¹H NMR, elemental analysis, differential scanning calorimetry (DSC) and hot stage polarizing optical microscope (HS-POM). Typical yields of the target molecules were more than 70%. All the molecules have mesophases with the textures of nematic type, indicating a rod-like molecule with a longer rigid skeleton can keep its mesophases. There was no clearing point observed for any of the derivatives before they decomposed so that the temperature ranges of the mesophases could not be determined. The energy differences between frontier molecular orbitals (HOMO-LUMO) (E_g) of the compounds were calculated by cyclic voltammetry (CV). The terminal ring system has an obvious influence on the energy levels and the energy gaps (E_g).

Full text of DOI:10.1080/15421406.2016.1146862

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis of aromatic liquid crystals with
asymmetric diester based on rod-like multi-ring
system by two-step esterification method
Min-Yan Zheng, Yong-Sheng Wei, Wei Geng & Yuan-Zi Gu
To cite this article: Min-Yan Zheng, Yong-Sheng Wei, Wei Geng & Yuan-Zi Gu (2016) Synthesis
of aromatic liquid crystals with asymmetric diester based on rod-like multi-ring system by
two-step esterification method, Molecular Crystals and Liquid Crystals, 630:1, 19-27, DOI:
10.1080/15421406.2016.1146862
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1146862
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Date: 02 July 2016, At: 03:16

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
Synthesis of aromatic liquid crystals with asymmetric diester
based on rod-like multi-ring system by two-step esterification
method
Min-Yan Zheng, Yong-Sheng Wei, Wei Geng, and Yuan-Zi Gu
School of Chemistry and Chemical Engineering, Xianyang Normal University, Xianyang , P. R. China
ABSTRACT
KEYWORDS
Asymmetric diester;
esterification; liquid crystal;
the frontier molecular
orbitals
A novel method of two-step esterifications was developed to synthe-
size compounds with asymmetric double ester groups. By using this
method, six rod-like double ester compounds were prepared with p-
hydroxy benzaldehyde, p-hydroxy benzoic acid bicyclohexyl carboxylic
acid, cyclohexyl benzoic acid and biphenyl carboxylic acid substituted
by n-propyl and n-pentyl as main reactants. The structures and proper-
ties of target compounds were confirmed by IR, MS, 1H NMR, elemental
analysis, differential scanning calorimetry (DSC) and hot stage polariz-
ing optical microscope (HS-POM). Typical yields of the target molecules
were more than 70%. All the molecules have mesophases with the tex-
tures of nematic type, indicating a rod-like molecule with a longer rigid
skeleton can keep its mesophases. There was no clearing point observed
for any of the derivatives before they decomposed so that the temper-
ature ranges of the mesophases could not be determined. The energy
differences between frontier molecular orbitals (HOMO-LUMO) (Eg) of
the compounds were calculated by cyclic voltammetry (CV). The termi-
nal ring system has an obvious influence on the energy levels and the
energy gaps (Eg).
1. Introduction
Usually, the incorporation of a larger group, such as (2,2,6,6-Tetramethyl-piperidin-1-yl)
oxyl (TEMPO) or nitronyl nitroxide radical (NIT), linked with a rod-like liquid crystal core
tends to inhibit the formation of a mesophase [1,2]. A common strategy for maintaining
mesophase is to increase the linear length of terminal alkyl or alkoxy chains [3–6]. However,
the molecules with alkyl or alkoxy (usually more than C7) would have high viscosity so that
the temperature range of their mesophase is narrow or even has no mesophase resulting from
their terminal longer alkyl or alkoxy chain (flexible longer linker between rigid parts) were
easily tied together [7]. It is found by our previous experience that the long rigid core is able
to produce a stable mesophase and even to covert the nematic phase into the smectic phase by
increasing the number of its core units (aromatic rings) or the length of a linking group. All of
these provide a possibility to prepare molecules with long rigid core and termination by bulky
group (such as nitroxyl) which form a liquid crystalline phase, especially a nematic phase. The
CONTACT Min-Yan Zheng
zmy@.com
sity,Xianyang, Shaanxi, , China.
School of Chemistry and Chemical Engineering, Xianyang Normal Univer-
Supplemental data for this article can be accessed on the publisher’s website.
©  Taylor & Francis Group, LLC

20
M.-Y. ZHENG ET AL.
Scheme . The synthesis process of compounds with double ester from literature [].
nematic phase is of great interest because it may be useful as a dopant in the practically used
liquid crystal mixtures to improve their order parameter and elastic properties [8]. Based
on our previous studies [9], a series of molecules with multi-ring, diester, terminal n-propyl
or n-pentyl and aromatic aldehyde were synthesized by a simple pathway using the ester
group as a linking bridge. The target compounds in the paper were important intermediates
in designing functional molecules of new types [8]. By comparing these derivatives with
similar compounds containing only single ester group, the influences from the length of rigid
skeletons of these molecules on their mesophase were also investigated.
Normally, functional group protection is often used to synthesize compounds with asym-
metric diester groups [10,11]. More steps are needed in this process, such as protection and
deprotection of one or more functional groups, providing a complex process to obtain asym-
metric diester compounds (Scheme 1). A better method of one-step oxidation and two-step
esterifications to synthesize target compounds was designed by our group several years ago
[12] (Scheme 2), which included no group protection and had better yields. To further sim-
plify the synthesis process, a new method involving a two step esterification is described in
this paper with fewer reaction steps and no group protection.
In this article, we describe the synthesis, characterization, thermal stability, mesomorphic
behavior, electrochemical property of target compounds. Their electrochemical properties
were also carried out by cyclic voltammetric method. The energy gaps between their HOMO
and LUMO were calculated to assess the influence of molecular structures on their energy
gaps.
Scheme . The method recorded in literature [] with the yield about %.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
21
Series 1:
a: R = n-propyl, A = trans-cyclohexyl, B = phenyl;
b: R = n-propyl, A = trans-cyclohexyl, B = trans-cyclohexyl;
c: R = n-propyl, A = phenyl, B = phenyl;
Series 2:
d: R = n-pentyl, A = trans-cyclohexyl, B = phenyl;
e: R = n-pentyl, A = trans-cyclohexyl, B = trans-cyclohexyl;
f: R = n-pentyl, A = phenyl, B = phenyl.
2. Experimental
2.1. Materials and characterization
All initial intermediates used in the process were synthesized in our lab. The purities of the
intermediates were determined by LC-10A (Shimadzu) instrument with methanol as eluent
and flowing rate was 1 mL/min and higher than 99%. Their structures were characterized by
IR, GC-MS and 1H NMR methods. Other reagents were from commercial sources and used
as received without further purification.
Elemental analyses were conducted by PE-2400 analyzer (Perkin Elmer). UV spectra were
determined by Agilent 8453 Spectrometer (Agilent Technologies). IR (KBr) spectra were
recorded on a Vertex 70 spectrophotometer (Bruker). Mass spectral data were recorded on a
GCMS-QP2010 (Shimadzu) and IE was 70 ev. 1H NMR spectra were obtained on an Avance
500 (Bruker, 500 MHz, solvent CDCl3). Cyclic voltammetric measurements were performed
on an Epsilon (BAS) which was run by a General Purpose Electrochemical System software.
The conventional three electrodes system was measured against SCE, Platinum wire and
glassy carbon severed as reference, counter and working electrodes, respectively.
2.2. The synthesis of compounds 1
Take the synthesis of 1a as an example. 1a was made with 4-n-propyl-(4-trans-cyclohexyl
benzoyl chloride and p-hydroxybenzaldehyde as main reactants according to the literature
[8]. 1a was obtained as a white crystal (0.61, 87%). 1b, 1c, 1d, 1e, 1f were made with the same
method with the yields of 85%, 83%, 86%, 84%, and 82%, respectively.
1a, C23H26O3, molecular weight 350.20, white needle, 1H NMR (CDCl3, 500 MHz)δ: 0.91
(t, 3H, J = 7.5 Hz, CH3), 1.08 (m, 2 H, CH2), 1.23 (m, 2 H, CH2), 1.35–2.58 (m, 10 H, the
protons of cyclohexane), 7.36 (d, 2 H, J = 8.5 Hz, the protons of aromatic), 7.38 (d, 2 H, J =
9 Hz, the protons of aromatic), 7.97 (d, 2 H, J = 8.5 Hz, the protons of aromatic), 8.11 (d, 2 H,
J = 8.5 Hz, the protons of aromatic), 10.03 (s, 1 H, CHO) [12].
2.3. The synthesis of compounds 4
Take the synthesis of 4a as an example. 2a was made according to the literature [9]. 2a was
obtained as a white crystal (1.6 g, 88%).
2a, C23H26O4, molecular weight 366.45, white needle, 1H NMR(CDCl3, 500 MHz)δ: 0.78
(t, 3 H, J=7.5 Hz, CH3), 0.97 (m, 2H, CH2), 1.11 (m, 2H, CH2), 1.21–2.55 (m, 10H, the H
of cyclohexane), 7.30 (d, 2H, J = 8.5 Hz, the protons of aromatic), 7.35 (d, 2H, J = 9 Hz, the
protons of aromatic), 7.98 (d, 2H, J = 8.5 Hz, the protons of aromatic), 8.01 (d, 2H, J = 8.5 Hz,
the protons of aromatic), 13.00 (s, 1H, COOH).

22
M.-Y. ZHENG ET AL.
Table . Molecular formula, molecular weight, mass fraction, appearance, yield, m.p. and elementary anal-
ysis for target compounds.
Elementary analysis (calcd.)
(%)
Molecular Molecular Mass fraction
Compd.
formula
weight
(LC) (%) Appearance Yield (%)
m.p. (°C)
C
H
4a
4b
4c
4d
4e
4f
CHO
CHO
HO
CHO
CHO
CHO
.
.
.
.
.
.
.
.
.
.
.
.
White crystal
White crystal
White needle
White needle
White flake






– . (.) .(.)
–
–
–
–
–
.(.) .(.)
. (.) .(.)
.(.) .(.)
.(.) .(.)
.(.) .(.)
C
White flake
3a was prepared by heating and stirring a mixture of 1a (3.7 g, 10 mmol), SOCl2 (1.8 g,
15 mmol) and DMF of two to three drops with the method in literature elsewhere [8]. A
green solid was obtained and used to next step without further purification. 3a(3.80 g, 99%),
C23H25ClO3, molecular weight 384.90, green solid, 1H NMR(CDCl3, 500 MHz)δ: 0.91 (t, 3H,
J=7.5 Hz, CH3), 1.25 (m, 2H, CH2), 1.31 (m, 2H, CH2), 1.52–2.72 (m, 10H, the H from cyclo-
hexane), 7.45 (d, 2H, J = 8.5 Hz, the protons of aromatic), 7.58 (d, 2H, J = 9 Hz, the protons
of aromatic), 8.14 (d, 2H, J = 8.5 Hz, the protons of aromatic), 8.22 (d, 2H, J = 8.5 Hz, the
protons of aromatic).
4a was synthesized by dissolving p-hydroxybenzaldehyde in CH2Cl2 (20 mL) and dropping
the mixture into the solution of CH2Cl2 (20 mL) and p-hydroxybenzaldehyde, then triethy-
lamine of 1.2 mL (10 mmol) was added. The reaction was worked up according to method
from the literature [7] to give 4a as a white crystal (1.8 g, 86%). The 4b, 4c, 4d, 4e, and 4f
were also synthesized by the same way. The synthesis of all target compounds is summarized
in Scheme 2 and their structural data are shown in Tables 1 and S1. The spectral data are in
accordance with their assigned structures.
3. Results and discussion
3.1. The discussion of method in synthesizing compounds with double ester group
Group protection is a most common method to synthesize asymmetric aromatic diesters.
Nowadays, the methods used to synthesize asymmetric aromatic diesters are shown in
Scheme 1 and Scheme 2. The total yield of the target products in the process shown in
Scheme 1 were about 30%. The pathway involved the protection and de-protection of the
hydroxide group so that more reaction steps were needed. The method in Scheme 2 used
alkyl substituted carboxylic acids which were esterified with p-hydroxybenzaldehyde. Then
the esterified products were oxidized by KMnO4 in an acetone solution. The target products
with double ester groups were then obtain via esterification a second time. There was no any
functional group needed to protect, so that the total yield of about 60% can be obtained by the
method in Scheme 2. Due to asymmetric diester compounds often used as a moiety of organic
material, it is worth to developing a simple route with a higher yield. During the preparation
of a series of azo materials [9], through a great deal of attempts, we found that esterification
of the hydroxyl in p-hydroxy azo benzoic acid can be done at room temperature, while ester-
ification of the carboxyl group of p-hydroxy azo benzoic acid would be carried out at heated
condition. Once p-hydroxy azo benzoic acid was substituted by p-hydroxy benzoic acid to
participate the reaction, if the reaction still work is worth studying. So, the reaction pathway

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
23
Scheme . A simplified synthetic pathway for two step-esterification of target compounds with yields of
more than %.
was attempted with p-hydroxy benzoic acid as a main reactant, the result showed the reaction
process was still work well here. The modified method in Scheme 3 results in greater than
70% yields and fewer reaction steps that previous methods.
3.2. The mesophase of target molecules
The mesophases of target compounds were determined by polarizing microscope. The six
compounds showed mesophasic textures of nematic type (on heating process). No smectic
and cholesteric phase was observed. No clearing point was found before their decomposition.
The POM textures of the six compounds are shown in Figures 1(A)–(F), respectively.
3.3. The determination of DSC thermograms of compounds 4
DSC thermograms were recorded at the rate of 10°C/min during heating process. The
compounds decompose before clearing to isotropic. The transition temperatures, associated
enthalpy values obtained from DSC studies and the ranges of decomposing from thermo-
gravimetric analysis (TG) for the target compounds are summarized in Table 2. The melting
temperatures of these series are observed in the range between 104°C and 160°C. The tem-
perature ranges of their decomposing points measured by TG determination are from 216°C
to 327°C. Among them, 4f with terminal n-pentyl and two benzene rings is least stable, while
Table . The DSC data of compounds  and compounds 1.
Structural
formula
m.p. ꢀHm d.p. ꢀHd L/W
Structural
formula
m.p. ꢀH L/W
∗
∗
Series Compd.
(°C) (J.g−) (°C) (J.g−
)
Compd.
(°C) (J.g−
)
1
4a
4b
4c
4d
4e
4f
    . 1a
    . 1b
    . 1c
    . 1d
    . 1e
    . 1f
  .
  .
  .
  .
  .
  .
2
∗
L/W represents the ratios of the molecular length to the its width calculated by Chembio D Ultra ..

24
M.-Y. ZHENG ET AL.
Figure . The polarizing optical microscope textures of compounds . (A)–(F): The marbled texture of 4a,
4b, 4c, 4d, 4e, and 4f taken at °C, °C, °C, °C, °C, °C, respectively, ×.
4a with terminal n-propyl cyclohexane and one benzene ring is the most stable. The com-
pounds 4c or 4f with terminal double benzene rings are the least stable ones in their own
series. The transition temperature obtained from DSC analysis is in good agreement with a
result observed from POM.
The influence of the benzoyloxy group (marked in black line in 4a) on thermodynamic
property in compounds 4 can be obtained by comparing the melting points between the two
kinds of compounds with similar structures (4 and 1). Table 2 tells us that, compounds 1 and
compounds 4 have the same trends in m.p. changes. For example, among compounds 1 and 4,
the lowest melting points was produced by the compounds with terminal double cyclohexane
rings. The influence of the extra benzoyloxy on the melting points can be found by comparing
their melting points (between 1a and 4a, 1b and 4b, 1c and 4c, 1d and 4d, 1e and 4e, 1f, and
4f), In comparing 4a–4d to compounds 1a–1d, the melting points are increased in Series 1
by between 49°C and 73°C and the melting points are also increased in Series 2 by 40°C to
47°C.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
25
Figure . The Cyclic voltammogram of 4a in dilute THF solution with tetrabutylammonium perchlorate as
supporting electrolyte. The dotted line shows CV of blank run under identical condition.
The data in Table 2 also indicates the benzoyloxy has the bigger influence on the melting
points of these compounds with terminal n-propyl bi-cyclohexane rings, while it has a smaller
effect on the melting points of those compounds with terminal n-pentyl bi-cyclohexane rings.
Comparing with compounds 1, we found the fact that the melting points of compounds 4 are
even increasing more in series 1 than those in the series 2 of compounds 4 also suggests the
terminal n-pentyl has more influence on melting point than the terminal n-propyl. In other
words, the terminal n-pentyl can somehow shield the influence of the one more benzoyloxy
in molecules.
3.4. Electrochemical analysis
The energy levels and energy gaps of the compounds 4 and 1 were measured out by cyclic
voltammetric method with three electrodes system (SCE, platinum wire and glassy car-
bon serves as reference, counter and working electrodes, respectively, concentration is 4 ×
10−3 mol/L with THF as solvent and tetrabutyl ammonium perchlorate as supporting elec-
trolyte). The EHOMO, ELUMO and the corresponding band gap (Eg) were calculated by following
equations:
ox
EHOMO = −e (Eonset + 4.4) (eV) ;
ꢀ
ꢁ
red
ELUMO = −e Eonset + 4.4 (eV) ;
Eg = −eꢀꢁ, ꢀꢁ = ꢁp–ꢁn [13]. where e is elementary charge, ꢁp is the onset oxidation
potential,ꢁn is the onset reduction potential and Eg is the Egap presumably. The typical voltam-
mogram of 4a is shown in Fig. 2. A obvious oxidation peak can be observed in the CV curve
of 4a (Fig. 2). All these compounds (4 and 1) each has an oxidation peak, but no reduction
peak is observed. In the CV curve, Eox is the potential value of the intersection point between
the electric current datum line and the tangent line of the starting point of the oxidation peak.
The EHOMO, ELUMO, and Eg of every compound were summarized in Table 3.

26
M.-Y. ZHENG ET AL.
Table . Redox potentials, EHOMO, ELUMO and energy gaps of target compounds.
Eox
(ꢁp) λmax(λonset
Series Compd. (eV)
)
EHOMO
(eV)
ELUMO
(eV)
Eg
Eox (ꢁp) λmax(λonset) EHOMO ELUMO Eg
(nm)
(eV) Compd.
(eV)
(nm)
(eV) (eV) (eV)
1
4a
4b
4c
4d
4e
4f
.
.
.
.
.
.
()
()
()
()
()
()
−.
−.
−.
−.
−.
−.
−.
−.
−.
.
.
.
1a
1b
1c
1d
1e
1f
.
.
.
.
.
.
() −. −. .
() −. −. .
() −. −. .
() −. −. .
() −. −. .
() −. −. .
2
−. .
−.
−.
.
.
a
Optical band gaps were calculated from onset absorption wavelengths using equation, Eg = /λonset [].
The data of the EHOMO, ELUMO, and the Eg of each compound were different from each other
and has obvious change rules. These energy levels have a relationship with their terminal ring
structures. Compared with compounds 1, having a terminal benzene ring impacts Eg, but
having an extra benzoyloxy group has little effect on Eg.
In Table 3, we have found that the only structural difference that yielded a change in electro-
chemical properties was the inclusion of additional terminal benzene rings. So 1c, 1f, 4c, and
4f have the narrowest Eg in their own series, respectively. Their Eox are higher, their EHOMO
,
E
LUMO, and Eg are lower. The result is in good agreement that compounds composed with
longer conjugate system (double benzene rings) have lower energy gaps between their fron-
tier molecular orbitals (HOMO and LUMO). The terminal n-propyl and n-pentyl show no
obvious effect on Eox, EHOMO, ELUMO, and Eg.
4. Conclusion
A new method was developed to synthesize compounds of asymmetric double ester group
with yields of more than 70% and without functional group protection. With other methods
for making similar compounds, more reaction steps were required with the total yield about
60% [12] or less than 30% [9].
By comparing compounds 4 and 1, the influences of an additional ester group, terminal
ring system and alkyl chain on the thermodynamic property of those compounds could be
determined.
The result in this paper tell us when the linear length of the molecule were increased by a
rigid skeleton of multi-ring system and multi-ester groups, it would be easier to maintain its
mesophase than those compound only with longer alkyl or alkoxy chain. It is because that the
longer rigid skeleton would increase the sequence of molecules in a longer rod-like molecule
in order to keep their mesophase. The CV illustrate that the terminal ring system have an
obvious influence on the energy levels (HOMO, LUMO, and Eg) of target molecules.
Acknowledgments
We gratefully acknowledge the financial support from the National Natural Science Foundation of
China (No. 21102121), the Natural Science Foundation of Shaanxi Province (No. 2014JM2-2014).
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