The effect of the position of chiral (?)-menthyl on the formation of blue phase and mesophase behavior in biphenyl-benzoate liquid crystals

Article abstract of DOI:10.1016/j.molliq.2019.112003

Eight new chiral liquid crystal compounds 4-(4-menthyloxy-n-oxoalkanoyloxy) biphenyl-4′-yl 4-butoxybenzoates MnB4OB (n = 1–8), were prepared by modifying the position of chiral (?)-menthyl in the menthol based liquid crystal compounds through gradually increasing the alkyl chain length of the dicarboxylic spacer. All compounds were characterized by FT-IR and NMR spectroscopy in order to prove their chemical structures. Differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray diffraction were carried out to systematically investigate their phase transition behaviors. The position of chiral (?)-menthyl in relation to the core has a great effect on the formation of BPs and mesomorphic behaviors. Only CLCs M1B4OB and M2B4OB with short spacer chains presented blue phases. Furthermore, the length and parity of the flexible spacers have a profound influence on phase structures and phase transition behaviors. An odd-even effect is observed for these chiral liquid crystal compounds.

Full text of DOI:10.1016/j.molliq.2019.112003

Journal Pre-proof
The effect of the position of chiral (−)-menthyl on the formation of blue phase and
mesophase behavior in biphenyl-benzoate liquid crystals
Zhang-Pei Chen, Xin-Jiao Wang, Ling-Xin Meng, Ji-Wei Wang, Ying-Gang Jia
PII:
S0167-7322(19)32090-2
DOI:
https://doi.org/10.1016/j.molliq.2019.112003
MOLLIQ 112003
Reference:
To appear in: Journal of Molecular Liquids
Received Date: 12 April 2019
Revised Date: 23 October 2019
Accepted Date: 23 October 2019
Please cite this article as: Z.-P. Chen, X.-J. Wang, L.-X. Meng, J.-W. Wang, Y.-G. Jia, The effect
of the position of chiral (−)-menthyl on the formation of blue phase and mesophase behavior in
biphenyl-benzoate liquid crystals, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/
j.molliq.2019.112003.
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The effect of position of chiral (-)-menthyl on the formation
of blue phase and mesophase behavior in biphenyl-benzoate
liquid crystals

The effect of the position of chiral (-)-menthyl on the formation of
blue phase and mesophase behavior in biphenyl-benzoate liquid
crystals
Zhang-Pei Chen1, Xin-Jiao Wang1, Ling-Xin Meng1, Ji-Wei Wang2, 3*, Ying-Gang Jia1*
1Department of Chemistry, College of Science, Northeastern University, Shenyang, 110819, P. R.China
2 Fujian Provincial Key Laboratory of Featured Materials in Biochemical Industry, College of Chemistry
and Materials, Ningde Normal University, Ningde, 352100, P. R. China
3Fujian Province University Engineering Research Center of Mindong She Medicine
，College of Chemistry
and Materials, Ningde Normal University, Ningde, 352100, P. R. China
Abstract
Eight new chiral liquid crystal compounds 4-(4-menthyloxy-n-oxoalkanoyloxy)
biphenyl-4’-yl 4-butoxybenzoates MnB4OB (n = 1-8), were prepared by modifying the
position of chiral (-)-menthyl in the menthol based liquid crystal compounds through
gradually increasing the alkyl chain length of the dicarboxylic spacer. All compounds were
characterized by FT-IR and NMR spectroscopy in order to prove their chemical structures.
Differential scanning calorimetry (DSC), polarized optical microscopy (POM) and X-ray
diffraction were carried out to systematically investigate their phase transition behaviors. The
position of chiral (-)-menthyl in relation to the core has a great effect on the formation of BPs
and mesomorphic behaviors. Only CLCs M1B4OB and M2B4OB with short spacer chains
*Corresponding author: jiayinggang@mail.neu.edu.cn (Y.G. Jia). jlwjw@sina.com (J. W.Wang)

presented blue phases. Furthermore, the length and parity of the flexible spacers have a
profound influence on phase structures and phase transition behaviors. An odd-even effect is
observed for these chiral liquid crystal compounds.
Keywords: Chiral liquid crystal, Spacer length, Blue phases, Odd-even effect, (-)-Menthol
1. Introduction
Liquid crystals (LCs) formed by self-assembly of the anisotropic molecules represent a
distinct and interesting state of matter exhibiting both liquid-like fluidity and crystal-like order.
This unique feature leads to self-healing, adaptive, and stimuli-responsive behaviors of
supramolecular systems, which enable LCs to be used as the typical self-organizing molecular
materials in the modern material science [1-4]. Chiral molecules have no plane of symmetry
and are optically active. Molecular asymmetry imposes a reduction in the space symmetry,
which leads to multitudinous chiral liquid crystalline phase, such as cholesteric (N*), chiral
smectic Sm*, twist grain boundary, and cubic blue phases (BPs), each with unprecedented and
unique properties [1, 2]. In these chiral mesophases, BPs possess a cubic lattice structure,
which can be regarded as a self-organized three- dimensional (3D) photonic crystal. Indeed,
BPLCs have attracted considerable attention from a wide range of applications across
numerous fields [5-7]. BPs have been known to consist of doublet-twist cylinders (DTCs) and
can be classified as BPI, BPII and BPIII in light of their emergence order [8]. Both BPI and
BPII have a fluid three-dimensional periodic structure with body-centered cubic and simple
cubic symmetry, respectively. In contrast, BPIII is amorphous or isotropic symmetry with an
arbitrary orientation [9-11]. In recent years, BPs have been widely investigated due to their
potential applications such as display devices, tunable photonic crystals, photonic band-gap
2

materials etc. [12-15]. Therefore, numerous efforts have been made towards the development
of BPLC materials through rational molecular design and efficient synthetic methods [16-18].
Previous studies have demonstrated that the liquid-crystal morphologies or mesogenic
behaviors of LCs strongly depend on their molecular structures, and even could be oriented to
a large extent by altering the numbers, size, and polarity of rigid cores as well as the length
and parity of the spacers [19]. In BPs, the peculiar structures depend on how the LC
molecules self-assemble into DTC structures and pack them to fill the space. Until now, a lot
of BPLCs have been prepared and their physical properties have been investigated. Chirality
is regarded as a very important factor responsible for the appearance of BPs with DTC
structure. Generally, there are two popular methods to prepare LCs with BPs. The first one is
to develop single-component mesogen with short-pitch by introducing chiral reagents into
mesogenic cores. The second strategy is to utilize the host-guest approach by adding chiral
guest into an LC host. When chirality in the host-guest system is high enough, which has a
high twisting power [20, 21]; the chiral molecule can drive LC molecules into DTC structure.
Even though doping is a simple and often used method for the preparation of BPLCs, a
single-component LC with wide temperature interval of BPs and the studies on the
structure-property correlations still attract many researchers.
BPs are frustrated defect phases, generally observed between the isotropic phase and the
cholesteric phase of highly chiral mesogen with short pitch [22]. Optically active (-)-menthol
shows a large specific rotation, and normally is utilized as a guest to induce cholesteric phases
[23, 24]. Due to the bulky menthyl group being a terminal group directly linked to the phenyl
rings, no BPs or mesophase were observed for them. Recently, we have successfully
synthesized chiral liquid crystal compounds based on (-)-menthol by introducing the
3

oxyacetyloxy spacer (-OCH2COO-) or butanedioyloxy spacer (-OOCCH2CH2COO-) between
the mesogenic core and the bulky terminal menthyl moiety and they all reveal BP phases
[25-31]. The spacer made menthyl unbound to mesogenic core and imparts molecular
chirality to mesophase leading to the formation of BPs. In those studies, the effects of
terminal chain length and structure of mesogenic core on BPs were further discussed in details
[29-31]. We found that in those (-)-menthol-derived CLCs, the chirality of (-)-menthol can
lead to the formation of BPs, and terminal chain length and structure of mesogen did not
affect BPs formation except the temperature range of BPs. However, when hexanedioyloxy
group acted as a linking unit between mesogenic core and chiral (-)-menthyl, no BPs were
formed [32]. Thus, we changed the location of chiral (-)-menthyl with respect to the rigid core
by varying the linking spacers from butanedioyloxy to decanedioyloxy and found that the
length of the flexible spacers has a great effect on the formation of BPs and mesomorphic
behaviors [33]. In connection, how the effect of propanedioyloxy as a new spacer together
with those spacers mentioned above on the formation of BP, mesomorphic behavior, and
molecules arrangement is what we consider in this paper.
Firstly, by modifying the position of chiral (-)-menthyl in relation to the mesogenic core
via spacer length, a new homologous CLCs with chiral (-)-menthol as chiral origin, butoxy as
one terminal group, biphenyl-benzoate as core, was designed. Through gradually increasing
the alkyl chain length of the alkanedioyloxy spacer (from propanedioyloxy to decanedioyloxy)
between chiral (-)-menthyl and mesogenic core, eight new CLC compounds:
4-(4-menthyloxy-n-oxoalkanoyloxy)biphenyl-4’-yl 4-butoxybenzoates MnB4OB (n = 1-8)
were prepared, where n indicates the number of carbon atoms in the flexible spacers. The
mesomorphism and thermal behavior of these compounds were investigated by differential
4

scanning calorimeter (DSC), POM and XRD. The selective reflection of light was studied
with UV. The effect of flexible spacers of varying length and parity on the formation of BPs,
and mesomorphic properties has been discussed systemically.
2. Experimental section
2.1. Materials
All chemicals were obtained from the indicated sources. (-)-Menthol was purchased from
Shanghai Kabo Chemical Co. (China). 2,2-Dimethyl-1,3-dioxane-4,6-dione was purchased
from Tianjin Bodi Chemical Co., Ltd. (Tianjin, China). Succinic anhydride, glutaric anhydride,
adipic acid, pimelic acid, suberic acid, azelaic acid, and sebacic acid were purchased from
Sinopharm Chemical Reagent Co., Ltd (China). All other reagents and solvents were
purchased from Tianjin Bodi Chemical co., Ltd. (Tianjin, China). The reagents were used
directly without further purification. The solvents were purified by standard methods. The
purities of all intermediates and LC compounds were checked with column chromatography
using TLC sheets coated with silica gel. All the CLC compounds were recrystallized from
methanol.
2.2. Characterization
FT-IR spectra were measured with the samples pressed into KBr pellets on a PerkinElmer
spectrum One (B) spectrometer. NMR spectra were performed on Bruker ARX 600 MHz or
300 MHz (Bruker, Karlsruhe, Germany) NMR spectrometer, and tetramethylsilane (TMS)
was used as an internal standard to report chemical shifts in ppm. Element analysis (EA) was
performed by use of an Elementar Vario ELIII (Elementar, Germany). The thermal behavior
5

was determined with a Netzsch 204 DSC (Netzsch, Hanau, Germany) with a heating and
cooling rate of 10 °C/ min in a nitrogen atmosphere. The LC optical textures were observed
and recorded with a Leica DMRX POM (Leica, Wetzlar, Germany) equipped with a Linkam
THMSE-600 (Linkam, London, UK) cool and hot stage. X-ray diffraction (XRD)
measurements were performed using a nickel-filtered Cu-Kα (λ = 1.542 Å) radiation
monochromatised with a Rigaku DMAX-3A powder diffractometer (Rigaku, Japan). The
samples were filled individually into two different Lindemann capillary (1 mm diameter)
tubes and both the ends of the tubes were flame-sealed. The selective reflection wavelength
was measured using a PerkinElmer 950 ultraviolet/visible spectrometer with a hot stage.
2.3. Synthesis of the chiral compounds
O
O
n = 1:
, Toluene, Reflux
O
O
O
O
O
O
OH
n = 2, 3:
n = 4~8:
or
, Heating
+ SOCl2
O
OH
O
O
O
n
O
L-Menthol
n = 1, 2, 3, 4, 5, 6, 7, 8
O
O
HO
OH
n
O
O
4,4'-Biphenol
THF, Pyridine
O
O
SOCl2
O
Cl
OH
O
O
n
n
n = 1, 2, 3, 4, 5, 6, 7, 8
MnB-OH
n = 1, 2, 3, 4, 5, 6, 7, 8
HO2C
O(CH2)3CH3
O
O
O
DCM, DCC, DMAP
O
O
O
C
O(CH2)3CH3
n
MnB4OB
n = 1, 2, 3, 4, 5, 6, 7, 8
Scheme 1. Synthetic route of CLC compounds MnB4OB.
Scheme 1 shows the chemical structures and synthetic routes of the CLCs with different
6

lengths of the flexible spacers. All the intermediates and compounds gave single spots on thin
layer chromatography and NMR spectra, which are in agreement with their structures. The
detailed synthesis steps of MnB4OB (n = 1-8) are described below.
2.3.1. Synthesis of the intermediates (MnB-OH)
2.3.1.1 Synthesis of the intermediates (M1B-OH)
(-)-Menthol (1.2g, 7.67 mmol) and 2,2-dimethyl-1,3-dioxane-4,6-dione (1.1 g, 7.63 mmol)
were placed into a 50-mL round-bottom flask, then 8 mL of toluene was added. The mixture
was stirred at reflux for 20 h. After completion of the reaction (checked by TLC with iodine),
the reaction mixture was cooled to room temperature and concentrated. The obtained residue
was applied onto a silica gel column with ethyl acetate/petroleum ether (40:60/v:v) to achieve
1
3-(menthyloxy)-3-oxopropanoic acid as a colorless oil (1.0 g, 54%). H NMR (300 MHz,
Chloroform-d) δ 4.72 (m, 1H), 3.41 (s, 2H), 2.07-0.93 (m, 9H), 0.90 (dd, J = 7.4, 5.6 Hz, 6H),
0.75 (d, J = 6.9 Hz, 3H).
The 3-(menthyloxy)-3-oxopropanoic acid (2.144 g, 8.8 mmol) and thionyl chloride (10.0 mL)
were added into a round flask. The mixture was stirred at 64 °C for 3 h. After removal of the
excess thionyl chloride via distillation under reduced pressure, the intermediate compound
(-)-menthyl malonyl chloride was obtained. Then, this intermediate was dissolved in 20 mL of
anhydrous tetrahydrofuran (THF) and was added dropwise to a solution of
4,4’-dihydroxybiphenyl (1.966 g, 10.5 mmol) in 10 mL THF and 0.85 mL pyridine. The result
reaction mixture was stirred at room temperature for 3 h. After evaporation of the solvent, the
crude product was purified by column chromatography on silica gel (200-300 mesh), with
petroleum ether-ethyl acetate (80:20/v:v) as the eluent, to afford pure white solid M1B-OH
7

(2.015 g, 56%). IR (KBr, cm-1): 3515 (-OH), 2952, 2932, 2864 (-CH3, -CH2-), 1747, 1724
(-COO-), 1497 (Ar-), 1207, 1171 (C-O-C), 831 (Ar-H). 1H NMR (300 MHz, Chloroform-d) δ
7.59-7.48 (m, 2H), 7.46-7.37 (m, 2H), 7.20-7.06 (m, 2H), 6.95-6.80 (m, 2H), 4.91 (s, 1H),
4.81 (td, J = 10.9, 4.4 Hz, 1H), 2.17-1.82 (m, 2H), 1.70 (d, J = 12.1 Hz, 2H), 1.52-0.37 (m,
16H).
2.3.1.2 Synthesis of the intermediates M2B-OH and M3B-OH
4-(Menthyloxy)-4-oxobutanoic acid and intermediates M2B-OH were synthesized according
to the previous reported procedure [29]. Similarly, 5-(menthyloxy)-5-oxopentanoic acid and
intermediates M3B-OH were prepared according to the synthesis method of
4-(menthyloxy)-4-oxobutanoic acid and compound M2B-OH, respectively. The detailed data
of M2B-OH and M3B-OH are given in ESI.
2.3.1.3 Synthesis of the intermediates MnB-OH (n = 4, 5, 6, 7, 8)
Intermediates MnB-OH (n = 4, 5, 6, 7, 8) were synthesized from (-)-menthol and the
corresponding aliphatic dicarboxylic acid with similar methods to our previous paper [32].
The detailed data of MnB-OH (n = 4, 5, 6, 7, 8) were showed in ESI.
2.3.2. Synthesis of liquid crystal compounds (MnB4OB)
4-Butoxybenzoic acid (1.8 mmol), intermediate M1B-OH (1.8 mmol), DCC (1.8 mmol) and
DMAP (0.010 mmol) were dissolved in 20 mL methylene chloride. The reaction mixture was
stirred at room temperature for 24 h. After the precipitation dicyclohexylurea (DCU) was
removed by filtration; the filtrate was washed with dilute HCl aqueous solution, water, dried
over anhydrous MgSO4, and then concentrated under reduced pressure. The crude product
was purified by column chromatography on silica gel (200-300 mesh), with hexane-ethyl
8

acetate (4:1/v:v) as the eluent, to yield a pure white powder M1B4OB (yield 43%). IR (KBr,
cm-1): 2956-2869 (-CH3, -CH2-), 1762, 1740, 1732 (-COO-), 1606, 1510 (Ar-), 1209, 1167
1
(C-O-C), 847 (Ar-H). H NMR (600 MHz, Chloroform-d) δ 8.17-8.15 (d, J = 8.9 Hz, 2H),
7.62-7.57 (m, 4H), 7.28 (d, J = 8.6 Hz, 2H), 7.19 (d, J = 8.6 Hz, 2H), 6.98 (d, J = 8.9 Hz, 2H),
4.82 (td, J = 10.9, 4.4 Hz, 1H), 4.06 (t, J = 6.5 Hz, 2H), 3.62 (s, 2H), 2.10-2.07 (m, 1H), 1.96
(td, J = 7.0, 2.7 Hz, 1H), 1.84-1.79 (m, 1H), 1.70 (dt, J = 12.1, 2.9 Hz, 2H), 1.56 (d, J = 2.7
Hz, 1H), 1.52 (qd, J = 9.2, 8.4, 6.5 Hz, 3H), 1.44 (ddt, J = 14.4, 10.8, 3.1 Hz, 1H), 1.12-1.03
(m, 2H), 1.00 (t, J = 7.4 Hz, 3H), 0.91 (dd, J = 13.0, 6.8 Hz, 7H), 0.79 (d, J = 6.9 Hz, 3H). 13C
NMR (100 MHz, Chloroform-d) δ 165.7 (C=O), 165.3 (C=O), 165.0 (C=O), 163.6 (C), 150.6
(C), 149.9 (C), 138.6 (C), 137.9 (C), 132.3 (CH), 128.3 (CH), 128.2 (CH), 122.2 (CH), 121.7
(CH), 121.4 (C), 114.3 (CH), 76.1 (CH), 68.0 (CH2), 47.0 (CH), 42.1 (CH2), 40.7 (CH2), 34.1
(CH3), 31.4 (CH), 31.1 (CH2), 26.1 (CH), 23.3 (CH3), 22.0 (CH2), 20.8 (CH2), 19.2 (CH2),
16.2 (CH3), 13.9 (CH3). Anal. calcd. for C36H42O7: C, 73.70; H, 7.22%. Found: C, 73.67; H,
7.24%.
CLCs MnB4OB (n = 2, 3, 4, 5, 6, 7, 8) were prepared from MnB-OH (n = 2, 3, 4, 5, 6, 7,
8) and 4-butoxybenzoic acid as described for the synthesis of M1B4OB. The detailed data of
other CLCs MnB4OB were listed in ESI.
3. Results and discussion
3.1. Elucidation of molecular structures
The synthetic routes of CLCs MnB4OB (n = 1-8) are illustrated in Scheme 1 and the last
synthetic step of these compounds is esterification reaction. Except for varying length and
parity of the connecting alkanedioyloxy group, they possess similar structures with biphenyl
9

benzoate as mesogenic core, which was surrounded by a 4-butoxybenzoyl on one side and a
chiral menthyl on the other side. The chemical structures of these intermediates and
1
corresponding target compounds were confirmed by FT-IR and H NMR spectroscopic
techniques.
The FT-IR spectrum of intermediate M1B-OH and CLC M1B4OB are illustrated in
Figure 1. In the spectrum, we can see that the hydroxyl stretching absorption band at 3515
cm−1 of intermediate M1B-OH disappear, while ester stretching bands at 1747, 1724 cm−1 of
M1B-OH move to 1740, and 1732 cm−1 for M1B4OB. Two medium intensity bands around
2956 and 2869 cm−1 are due to aliphatic v (C-H) asymmetric and symmetric stretching
vibrations of methylene (-CH2-) group, and the stretching vibration at 1606 and 1510 cm−1 are
attributed to v (C = C) of benzene ring.
Figure 1. IR spectra of intermediate M1B-OH and CLC M1B4OB.
1
The structures of M1B-OH and CLC M1B4OB were also confirmed by H NMR
spectra (Figure 2). In the spectra, aromatic ring proton peaks of intermediate M1B-OH are
observed as multiplet around 7.59-7.48, 7.46-7.37, 7.20-7.06, 6.95-6.80 ppm. The signal
10

around 4.91 ppm in a singlet form was corresponds to the proton of phenolic hydroxyl group.
The signal around 4.81 ppm in a double triplet form observed corresponds to one proton of
menthyl connecting to ester group. In contrast, hydrogenous chemical shift of aromatic ring of
liquid crystal compound M1B4OB appear around 8.17-8.15, 7.62-7.57, 7.28, 7.19, and 6.98
ppm. Meanwhile, the signal of proton in menthyl connecting to ester group shifted to 4.82
ppm in a double triplet form. The signal around 4.06 ppm in a triplet form was observed due
to the methylene of 4-butoxybenzoyl group. In addition, the signal around 3.62 ppm in a
singlet form was observed, which ascribes to the methylene in malonyl group.
Figure 2. 1H NMR spectra of intermediate M1B-OH and CLC M1B4OB.
Other CLCs MnB4OB (n = 1-8) are structurally identical except different spacer lengths,
and their spectral results are almost similar. Through the FT-IR and NMR analysis, we can
confirm that structures of the CLCs are in good agreement with the designs.
3.2. Thermal properties
In order to explore the influence of the linking spacer group on the mesomorphic behaviors of
11

the liquid crystalline compounds, these CLCs MnB4OB (n = 1 to 8) were characterized by
differential scanning calorimetry (DSC) and polarising optical microscope (POM). The
transition temperatures and the corresponding enthalpies derived from DSC traces of the
second heating-cooling cycles at a rate of 10 °C min−1 are given. Phase transition temperature
and associated enthalpy changes of the synthesized compounds are given in Table 1. Typical
DSC curves of MnB4OB (n = 2 and 7) are shown as examples in Figure 3.
Table 1. Phase transitions and thermodynamic data for MnB4OB.
Phase transitionsa (°C) (corresponding enthalpy changes; kJ mol−1)b
n
Heating
Cr142.9(71.98)Iso
Cooling
1
2
3
4
Iso146.2(2.69)BPI/II142.2cN*117.5(60.55)Cr
Iso127.1(0.66)BPI/II122.8cN*60.7(0.57)SA*5
3.4(32.18)Cr
Cr79.4(37.52)N*128.9(0.68)Iso
Cr112.1(55.19)N*141.8(0.53)Iso
Cr102.2(62.23)N*134.7(0.41)Iso
Iso139.9(0.60)N*95.7(4.67)SC*76.9(35.81)Cr
Iso132.7(0.39)N*96.8(3.04)SC*69.9(1.37)SX*
61.1(16.65)Cr
Cr75.9(20.50)SC*100.5(4.46)N*134.0(0.9 Iso132.3(1.21)N*97.8(4.44)SC*72.9(2.44)SX*
9)Iso 58.8(11.47)Cr
Cr62.6(9.52)SX*76.9(2.03)SC*107.5(3.34) Iso131.8(0.62)N*104.7(3.18)SC*74.2(1.67)SX
N*133.7(0.79)Iso *51.8(9.71)Cr
Cr62.0(8.73)SX*79.0(2.02)SC*110.4(2.86) Iso133.1(0.64)N*107.2(2.94)SC*75.7(1.85)SX
N*135.5(0.71)Iso *51.8(8.40)Cr
Cr73.9(40.93)SC*102.1(4.23)N*125.0(0.1 Iso124.0(0.10)N*99.1(2.94)SC*73.9(2.27)SX*
0)Iso 42.6(5.91)Cr
aCr: Crystal; N*: chiral nematic phase; SA*: chiral smectic A phase; SC*: chiral smectic C phase;
5
6
7
8
SX* = chiral smectic X phase; Iso: isotropic phase.
bValues of enthalpy are given in brackets. n indicates the number of carbon atoms in the spacer
chains.
cThe transition temperatures were observed by POM at cooling rate of 0.1 oC min-1.
12

Figure 3.DSC curves obtained for the CLCs M2B4OB and M7B4OB.
As can be seen from Table 1, CLC M1B4OB only showed a melting transition and did
not exhibit any mesophase on heating cycle due to the high melting point (Tm). During its
cooling process, CLC M1B4OB showed chiral nematic phase (N*), accordingly, CLC
M1B4OB is monotropic. With spacer increasing the long spacer chains add flexibility to the
rigid core structure, which tends to reduce melting points and allows liquid crystalline phases
appear, so the higher members (n = 2-8) show enantiotropic LC behavior. Upon heating,
CLCs M2B4OB, M3B4OB and M4B4OB show N* phases, and CLCs M5B4OB, M8B4OB
reveal two mesophases, chiral smectic C (SC*) and N* phase. However, three mesophases of
chiral smectic X phase (SX*), SC* and N* phases were observed for M6B4OB and M7B4OB.
On cooling process, compounds M2B4OB and M3B4OB show an additional chiral smectic A
(SA*) or SC* phase respectively besides N* phase. The other CLCs MnB4OB (n = 4-8)
exhibit three mesophase of N*, SC*, SX* (an unknown chiral smectic X) from high to low
temperature. The N*, SA*, SC* and SX* phases were further confirmed by POM and XRD
measurements. In addition, Table 1 shows that the clearing enthalpies of CLCs MnB4OB (n =
13

2-8) give lower values irrespective to the length of the flexible spacers, which can attribute to
the bulky nature of the menthyl group that enhances molecular biaxiality and reduces the
change in order at the transition.
The relationship of phase transition temperatures (heating and cooling mode) with the
number of carbon atoms (n) for CLCs MnB4OB (n = 1-8) is exhibited in Figure 4. It can be
seen that as the spacing length with parity count n switches from odd to even the phase
transition temperature reveals a regular changes. Melting temperatures (Tm) and
crystallization temperatures (TCr) exhibited a modest odd-even effect as the parity of the
spacer is varied and the odd members gave higher values than the corresponding even-parity
ones. This indicates that the compound with odd spacer is more compact, which leads to a
higher melting point than the even one as parity. Clearing temperature (Tc) and Iso-N*
transition temperature (TIso-N*) also show the same variation pattern with melting point, the
CLC with an odd spacer shows higher values than that with an even spacer, probably because
even spacer CLCs are more twisted than the corresponding CLCs with odd spacer. These CLC
compounds exhibit odd-even effect of the clearing temperature as the parity of the spacer is
varied, which is similar to the odd-even effect of CLCs in terminal chain length [29] and LC
dimers with various connecting spacer length [34,35]. However, there is no regular change in
N*-Sc* transition temperature (TN*-Sc*) and Sc*-Sx* transition temperature (TSx*-Sc*) with the
increase of n. In addition, the SC* phase appears as n greater than 4. This demonstrates that
increasing the flexible spacers can favor the formation of SC* phases, which is presumably
due to the enhanced interlayer interactions derived from an increase of the spacers.
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Figure 4.Phase diagram of compounds MnB4OB on heating and cooling cycles.
3.3. Optical textures
The mesomorphic transition temperatures observed by POM were consistent with the phase
transition temperatures measured by DSC. Optical texture is one of the most effective means
for identifying mesophase type. The optical textures of LC state were observed by means of
POM with a hot stage, and some representative optical textures are given in Figure 5. On the
heating course, the CLC M1B4OB directly melted to the isotropic phase without any
mesophase and compounds MnB4OB (n = 2-8) showed oily streaks texture (Figure 5a, b) in
their N* phase. The actual oily streaks can be seen as a network of defect lines dispersed in
uniformly helical regions. The oily streaks texture disappeared until the sample entered into
isotropic phase. Due to the enantiotropic behavior the SC* and SX* phases on heating course
were further identified by their optical textures upon cooling. On cooling cycles, CLCs
M1B4OB and M2B4OB revealed a platelet texture of BP (Figure 5c, d) and a broken focal
conic texture (Figure 5e) of N* phase, and an additional pseudo-isotropic texture of SA* phase
for M2B4OB. Due to the enthalpy values of BP transition are too small to be detected by
DSC. BPs of M1B4OB and M2B4OB exist over a very narrow temperature interval, about ~
4 °C because of the thermodynamic disruption. In addition, the other CLCs present an N*
15

phase with a focal conic texture (Figure 5f) and SC* phase with a cloudy texture (Figure 5g).
Here, the dull-greyish/cloudy patterns of SC* phase stemmed from the homeotropic
orientation of the mesogens, which obviously suggested a layered structure of the phase. It is
well known that if the helix axis of the SC* phase propagates along the direction of light
(optic axis), then a greyish/cloudy textural pattern can be seen. When compounds MnB4OB
(n = 4, 5, 6, 7, 8) cooled further from the SC* phase, they exhibited another more ordered
untypical texture (Figure 5h) of SX* phase. The more ordered SX* phase was proved by X-ray
diffraction. Cooled the sample continuously, crystalline phase took place and mesophase
disappeared.
Figure 5. Photomicrographs of optical textures seen under POM for the liquid crystal phases of MnB4OB
series: (a) oily streaks texture of N* phase on heating to 107.1 °C for M2B4OB; (b) oily streaks texture of
N* phase on heating to 105.0 °C for M5B4OB; (c) a platelet texture of BPI/II on cooling to 146.1 °C for
M1B4OB; (d) a platelet texture of BPI/II on cooling to 126.8 °C for M2B4OB; (e) broken focal conic
textures with BPI/II texture of N* phase on cooling to 122.5 °C for M2B4OB; (f) focal conic textures of
N* phase on cooling to 131.9 °C for M7B4OB; (g) cloudy texture of SC* phase on cooling to 102.0 °C for
M7B4OB; (h) untypical texture of SX* phase on cooling to 61.5 °C for M7B4OB.
Notably, only CLCs M1B4OB and M2B4OB reveal a platelet texture composes of red,
green and blue plates on slow cooling from the isotropic phase, which suggests the existence
of a cubic BPI/II. The alluring and beautiful mesophase originates mainly from their
16

self-organized three-dimensional photonic crystal structure. With typical lattice spacing on the
order of several hundred nanometers, selective Bragg reflection could be any color positioned
in the visible region, and the corresponding wavelength is related to the lattice plane. The
theoretical and experimental work has demonstrated that CLCs of short pitch could form up to
three distinct BPs at the transition from isotropic phase to the N* phase. Moreover, the double
twist tube of BPs has been shown theoretically to be more stable than the single twist
structure of the N* phase at a higher temperature. In such a configuration, the energy can be
further lowered by allowing twist in all directions perpendicular to the local director. In
addition, we have also found that the chiral menthyl location with respect to the rigid core is
important in determining the formation of BPs. When n is greater than 2, no BPs are formed
for CLCs MnB4OB (n = 3-8).
3.4. The selective reflection of light
The planar oily streak texture of the N* phase in heating exhibits one of the important optical
phenomena, namely selective reflection. If the pitch length is of the order incident white light
wavelength, then colors are selectively reflected. In general, the selective reflection
wavelength depends on the molecular chirality and ordering of the molecules within the phase.
The maximum wavelength (λm) of the selective reflection is defined by Bragg Equation, λm =
nP, where P is the pitch length of the helical structure and n is the average refraction index of
the LCs. Importantly, helical pitch is temperature dependent; thus, the color of the reflected
light varies with temperature.
In this work, the property of selective reflection in the N* phase was examined with the
help of UV/Vis spectra equipped with a hot stage. Only CLC M2B4OB selectively reflected
visual light on heating, since P length of the N* phase is of the order of the wavelength of
17

incident white light. Figure 6 shows the UV/Vis spectra of N* phase for CLC M2B4OB
obtained during heating cycles. Overall, it can be seen that blue shift of the maximum
reflection wavelength (λm) occurs as the temperature increases. Furthermore, their inherent
reflection peak intensities are gradually weakened. According to the Bragg Equation, the rise
of temperature leads the helical structure to twist and reduces the P value. So, with increasing
temperature a blue shift of λm is observed and the reflection color varies from red to green
(inset in Figure 6). In addition, the incident light beam will be split into its two circularly
polarized components by the helical structure. One of the circularly polarized components
will pass through the specimen, whereas the other component will be reflected. The
component that is reflected has the same rotation sense as that of the screw direction of the
helix.
Figure 6. UV/Vis spectra of M2B4OB at N* phase temperature range
As the length of the spacer increases, when n ≥ 3, the pitch length of the N* phase of this
series CLCs is out of the order of the wavelength of white light, so no selectively reflection
appears. Selective reflection in the visible range also requires a small pitch P for CLCs, so
18

BPs and selective reflection in the visible range are often observed for highly chiral mesogen.
BPLCs normally show iridescent color in their N* phase on heating process.
3.5. XRD studies
XRD analysis presented a complementary evaluation of the phases observed by DSC and
POM, giving additional information about their structural parameters. In the cooling cycle,
CLCs MnB4OB (n = 4-8) reveal a similar phase transition sequence of Iso-N*-SC*-SX*-Cr.
Thus M7B4OB was performed as an example for XRD measurement at the same cooling
temperature operated by POM observation in order to get further insight into the mesophase
structures. The diffraction patterns in the mesophase obtained on cooling from their isotropic
phases. The intensity versus 2θ plots derived from the diffraction patterns of M7B4OB are
shown in Figure 7. In the cooling scan, only a diffused peak at 2θ = 19.8° (d-spacing 4.48 Å)
at 131.0 °C was observed. For this phase, focal conic textures were observed by POM.
Therefore, combining XRD with POM, we can confirm that this phase is assigned as the N*
phase.
Figure 7. X-ray diffraction profiles for M7B4OB.
When cooled to 102 °C, the small-angle XRD scattering diffraction pattern showed one
19

strong reflection at 2θ = 2.9° (d-spacing 30.43 Å), together with a diffused peak at 2θ = 19.8°
(d-spacing 4.48 Å). The sharp reflection in the small angle region indicates smectic layer
arrangement of LC molecules. The theoretical molecular length (l), calculated by the method
of Chem 3D is about 35.85 Å. The d/l ratio of M7B4OB at 102 °C is 0.85, and which noted
that the LC molecules tilt arrangement of SC* mesophase and the tilt angle (α), was calculated
to be 31.79° from the equation α = cos−1 (d/l). This identified that the cloudy texture in the
mesophase observed by POM is SC* phase. The tilt angle in smectic C* increases with the
decrease of temperature. When cooled to 93 °C, the small-angle XRD scattering diffraction
pattern showed one strong reflection at 2θ = 3.0° (d-spacing 29.42 Å) and the tilt angle (α)
was 34.85°. On further cooling to 65 °C, in addition to increase strength a reflection in the
small angle is also located at 2θ = 3.0°, and it is the same to the reflection at 93°C. The
increase in reflection strength implies a more orderly arrangement of LC molecules. And
combining DSC with POM results, a more ordered SX* mesophase was considered.
3.6 Molecules self-assemble in mesophase
(-)-Menthol is optically active and has three chiral centers. (-)-Menthol molecular asymmetry
can impart chirality form to liquid-crystalline phases, which is manifested in the formation of
helical ordering of the constituent molecules of the phase. The mesophase types of CLC
MnB4OB series include Iso, BP, SA*, SC*, Sx* and Cr phase and schematic representation of
the molecular self-assembles is shown in Figure 8.
20

Figure 8. Schematic representation of the molecular arrangements in mesophase of CLC MnB4OB
The molecular arrangements are chaotic in the Iso state (Figure 8a). The N* phase is a
chiral phase, which spontaneously forms a macroscopic helical structure. As the temperature
decreases, BP and the N* phase appear. The chirality of (-)-menthyl in these compounds
forces the mesogenic molecules to twist with respect to each other (inset in Figure 8b),
resulting in the typical double twisted cylinder of BP (Figure 8b). The N* phase (Figure 8c)
also possesses a helix axis and consists of local nematic layers twisted continuously along the
axis. Molecules in SA* phase (Figure 8d) orient along the smectic layer normal, and the
d-spacing is also almost equal to the theoretical molecular length. Helical smectic phases are
also known to exist in the SC* phase (Figure 8e) and in SC* phase the chiral molecules tilt
with respect to the layers normal. It is the precession of the tilt that gives rise to the
macroscopic helical structure. Molecules arrangement of MnB4OB (n = 4-8) in mesophase
SX* phase (Figure 8f) is similar to SC* phase, except that it is more ordered than SC* phase.
The full phase transition sequence in cooling is an Iso-N*-SC*-SX*-Cr process for compounds
MnB4OB (n = 4-8), an Iso-N*-SC*-Cr transition process for M3B4OB, an
Iso-BP-N*-SA*-Cr transition process for M2B4OB and an Iso-BP-N*-Cr transition process
for M1B4OB.
Chiral menthyl location with respect to the rigid core has a great effect on the formation of
BPs and mesomorphic behavior. As chiral menthyl location is closer to the rigid core (n < 3),
chiral menthyl lead molecules into DTC arrangement in BP interval for M1B4OB and
M2B4OB. However, as the distance between the chiral center and core further increases (n ≥
3) from M3B4OB to M8B4OB, menthyl is not powerful enough to make the molecular
arrangement in DTC structure and only N*, SC* and SX* phases form. This is because that
21

when the chiral center is closer to the core, the P of the helix becomes shorter, therefore, the
chirality increases and BPs is formed [36]. In other words, when P exceeds a critical value, Pc,
then BPs no longer exists.
4. Conclusions
A series of CLC compounds was synthesized and characterized by modifying the position of
chiral (-)-menthyl in relation to the mesogenic core. The CLC compound M1B4OB was
monotropic LC, however the other members show enantiotropic LC behaviors. Melting
temperatures (Tm), clearing temperature (Tc), Iso-N* transition temperature (TIso-N*) and
crystallization temperatures (TCr) exhibit an odd-even effect as the parity of the spacer varied
and the odd members show higher values than the corresponding even-parity CLCs. In
addition, the position of chiral (-)-menthyl in relation to core has a great effect on the
formation of BPs and mesomorphic behaviors. CLCs M1B4OB and M2B4OB revealed BPs
with DTC structure. Increasing the length of the flexible spacers tended to favor the
development of smectic phase. On cooling cycles, CLCs MnB4OB (n =4, 5, 6, 7, 8) present a
phase transition sequence from high to low temperature as: Iso-N*-SC*-SX*-Cr.
Disclosure statement
No potential conflict of interest was reported by the authors.
Funding
This work was supported by the National Natural Science Foundation of China (31560268,
21702026), Fundamental Research Funds for the Central Universities (N180705004,
22

N170504018), the Science and Technology Foundation of Liaoning Province (20170520185),
Fujian Natural Science Foundation (2019J0106), scientific research fund project of Ningde
Normal University (2018Y06, 2018T02, 2018Z02) and IRTSTFJ.
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Highlights
Eight novel CLCs MnB4OB were synthesized by varying the distance of
chiral (-)-menthyl in relation to the core.
The position of the chiral (-)-menthyl relative to the core determined
whether the BPs can be formed.
Only CLCs M1B4OB and M2B4OB with short spacer displayed blue
phase.
As the spacer length and parity increasing an odd-even effect was
observed.