Preparation, structure and optical properties of thermochromic liquid crystal compounds containing (?)-menthyl with selective reflection

Article abstract of DOI:10.1016/j.molliq.2018.11.088

A series of new chiral liquid crystal compounds named MOPnB containing (?)-menthyl group has been prepared and characterized by FT-IR and ¹H NMR spectra. The thermal properties and optical textures were investigated by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM). Increasing the length of the terminal alkoxy group tended to decrease the clearing points and narrow the temperature range of the N* phase as well as favored the development of SmC* phase. Furthermore, Bragg selective reflection spectra of the compounds were measured by ultraviolet/visible spectrometer (UV/Vis). All compounds MOPnB (n = 2, 4, 6, 8, 10) revealed the thermochromic property or selective reflection of visible light in the N* phase. Additionally, only MOP10B and MOP12B in the SmC* phase had the thermochromic phenomena and gave red-saffron colour.

Full text of DOI:10.1016/j.molliq.2018.11.088

Accepted Manuscript
Preparation, structure and optical properties of thermochromic
liquid crystal compounds containing (−)-menthyl with selective
reflection
Cong-Cong Luo, Xin-Jiao Wang, Lu-Juan Han, Ying-Gang Jia,
Shao-Ming Ying, Ji-Wei Wang
PII:
S0167-7322(18)35193-6
DOI:
Reference:
https://doi.org/10.1016/j.molliq.2018.11.088
MOLLIQ 9998
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
8 October 2018
14 November 2018
19 November 2018
Please cite this article as: Cong-Cong Luo, Xin-Jiao Wang, Lu-Juan Han, Ying-Gang
Jia, Shao-Ming Ying, Ji-Wei Wang , Preparation, structure and optical properties of
thermochromic liquid crystal compounds containing (−)-menthyl with selective reflection.
Molliq (2018), https://doi.org/10.1016/j.molliq.2018.11.088
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Preparation, Structure and Optical Properties of Thermochromic Liquid Crystal
Compounds containing (-)-Menthyl with Selective Reflection
Cong-Cong Luo a, Xin-Jiao Wang a, Lu-Juan Han a, Ying-Gang Jia a, *, Shao-Ming Ying b and
Ji-Wei Wang b, *
aCollege of Science, Northeastern University, Shenyang, 110004, PR China. E-mail: jiayinggang@mail.neu.edu.cn.
bFujian Province University Key Laboratory of Green Energy and Environment Catalysis, Ningde Normal University,
Ningde, Fujian, 352100, PR China, E-mail: jlwjw@sina.com
Abstract
A series of new chiral liquid crystal compounds named MOPnB containing (-)-menthyl
group has been prepared and characterized by FT-IR and 1H NMR spectra. The thermal
properties and optical textures were investigated by differential scanning calorimetry
(DSC) and polarizing optical microscopy (POM). Increasing the length of the terminal
alkoxy group tended to decrease the clearing points and narrow the temperature range
of the N* phase as well as favored the development of SmC* phase. Furthermore, Bragg
selective reflection spectra of the compounds were measured by ultraviolet/visible
spectrometer (UV/Vis). All compounds MOPnB (n = 2, 4, 6, 8, 10) revealed the
thermochromic property or selective reflection of visible light in the N* phase.
Additionally, only MOP10B and MOP12B in the SmC* phase had the thermochromic
phenomena and gave red-saffron color.
Keywords: (-)-menthol; chiral liquid crystals; selective reflection; blue phase; chiral
smectic C; thermochromic

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1. Introduction
Chiral liquid crystal compounds (CLCs) have been receiving growing interest as the
reduced symmetry of chirality in the CLCs leads to a helical supramolecular structure,
which yields some interesting properties and elegant phenomena, such as the selective
reflection of light, thermochromism and ferroelectricity, rendering these periodic
dielectric fluid media for a number of potential applications, particularly as
thermography, tunable color filters and polarizers, reflective displays, mirrorless lasing,
etc. [1-10] Due to their unique properties, many efforts have been made to fabricate
novel materials with periodic structures, not only focus on the desired reflection [11],
but also tuning the reflection through light [1], electricity [12, 13], stress [14],
temperature [15], pH [16], solvent [17], and humidity [18]. In addition, the stimuli-
responsive behavior of CLCs and their promising applications have been drawn much
more attentions of scientists and engineers from different backgrounds [19-28].
In the CLC phases, there are two basic helical arrangements of the molecules. In one
case, when the orientational ordering of the molecules occurs in one preferential
direction, a single-twist helical structure is formed. The molecules with this structure
usually show chiral nematic phase (N*) and chiral smectic C phase (SmC*). In the other
case, a double-twist helical structure is created while the helix forms in more than one
preferential direction, and blue phase (BP) is the typical LC phase of molecules with
this structure [19]. Therefore, periodic helical structures in N*, SmC* or BP phase of
CLCs induce these systems to show unique optical properties, such as Bragg selective
reflection. The most striking is the so-called selective reflection, where circularly

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polarized light of a specific handedness and wavelength is reflected. This phenomenon
can readily be observed by the naked eye as an iridescent color play when the pitch of
the N* and SmC* phase structure is in the range of visible spectrum. Also in nature the
selective reflection of the cholesteric-like structures is well known from the bright colors
exhibited by some beetles [20]. For the cubic BPs, however, there may be several
selective reflection wavelengths corresponding to various crystal planes, none of which
coincide with the helical selective reflection wavelength. The selective reflection from
BPs also provides iridescent light.
As the selective reflection of CLCs in visible light could generate iridescent colour,
and the colour can be tuned by external stimuli, extensive studies have been conducted
to design novel CLCs for practical applications. Until now, selecting or preparing novel
optical active chiral entity for new CLC construction attracts many attentions of
researchers. Cholesterol ([a] D20 = -36˚) is one of the most widely used chiral reagents
for building thermochromic CLCs. The N* and SmC* phases with the wavelength of
selective reflection in the visible range of the spectrum often have a twist with a pitch P
well below P < 1 μm [21]. And also, to generate BPs, molecules require high chirality
[22-24]. BPs are generally observed between the isotropic phase and the cholesteric or
smectic phases of highly chiral mesogens, where the N* phase at lower temperature
often exhibits selective reflection in the visible range. That is, short pitch is essential for
them.

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Optically active (-)-menthol has three chiral centers and shows a large specific
20
rotation of -50˚ ([a]D = -50˚), which can cause the plane of polarization to rotate
greatly, so we expect that CLCs containing (-)-menthyl possess a short pitch in
mesophase for generating BPs and N*/SmC* with thermochromic properties. Recently,
we have successfully synthesized CLCs based on (-)-menthol by introducing
oxyacetyloxy spacer (-OCH2COO-) [25-29] or succinyloxy group (-OOCCH2CH2COO-)
[30-32] between the mesogenic core and the bulky terminal menthyl group. And these
CLCs reveal BPs and thermochromic properties in N* phase. In this study, we
synthesised a series of new CLCs MOPnB (n = 2 – 18) with various terminal chain
lengths and investigated the effect of chain lengths on the thermal properties and pitches
of CLCs. In this series, the alkoxy terminal group is substituted with n-ethoxy, n-
butyloxy to n-octadecaneloxy tails in even numbers (n). We found that different lengths
of terminal alkoxy groups have a significant influence on the selective reflection and
thermal properties. Furthermore, we also discussed the influence of the structure of
alkoxy terminal groups on the mesomorphism and thermochromism of the CLCs in
detail.
2. Experimental
2.1 Materials
(-)-Menthol was purchased from Shanghai Kabo Chemical Co., Ltd. Succinic anhydride,
1-bromobutane, 1-bromohexane, 1-bromooctane, 1-bromodecane, 1-bromododecane,
1-bromotetradecane, 1-bramohexadecane, 1-bromooctadecane, 4-hydroxybenzoic acid,

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ethyl 4-hydroxybenzoate, 4-ethoxylbenzoic acid and other reagents and solvents were
all purchased from Sinopharm Chemical Reagent Co., Ltd. All reagents were used
directly without further purification. Solvents were purified by standard methods.
2.2 Characterization
The chemical structure of intermediates and CLCs were analyzed by proton magnetic
resonance spectroscopy using a Bruker ARX400 spectrometer with deuterated
chloroform as a solvent and tetramethylene silane (TMS) as internal standard. Fourier
transform infrared (FT-IR) spectra of the synthesized intermediates and CLCs in solid
state were obtained by the KBr method performed on PerkinElmer spectrum One
spectrometer (PerkinElmer, Foster City, CA). Transition temperatures and enthalpies
were measured by a DSC 204 (Netzsch, Hanau, Germany) equipped with a cooling
system. The optical textures of mesophases were identified by polarizing optical
microscopy (Leica, Germany) with a Linkam cooling and heating stage. XRD
measurements were performed with a nickel-filtered Cu-Ka radiation (Rigaku, Japan).
The selective reflection wavelength was measured using a PerkinElmer 950
ultraviolet/visible (UV/Vis) spectrometer with hot stage.
2.3 Synthesis of the intermediates and CLCs
The synthetic procedures of all the intermediates and CLCs are shown in scheme 1.
Intermediate compound, 4-hydroxyphenyl 4-(4-menthyloxy-4-oxobutanoyloxy)
benzoate (3) was prepared according to our previous reports [31]. Intermediates 4-
alkoxybenzoic acids 5-n (n = 4, 6, 8, 10, 12, 14, 16, 18) were synthesised via previously

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published methods (5-2 was purchased from Sinopharm Chemical Reagent Co. ,Ltd.
directly) [33].
All the CLCs 4-(4-(4-menthyloxy-4-oxobutanoyloxy)benzoyloxy)phenyl 4-
alkoxylbenzoate (MOPnB) were synthesised by the Steglich esterification reaction. 4-
Alkoxy benzoic acids with intermediate 3 were catalyzed by N,N´-dicyclohexyl
carbodiimide (DCC) and N,N´-dimethylaminopyridine (DMAP) to prepare CLCs
MOPnB (n = 2 – 18) series. The detailed synthetic procedures and characterizations of
CLCs were described in the Supporting Information.
Scheme 1. Synthesis of the compounds MOPnB (n=2, 4, 6, 8, 10, 12, 14, 16, 18)

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3. Results and discussion
3.1 Elucidation of molecular structures
The series of CLCs based on menthol were designed as target compounds, in which the
terminal alkoxy groups increased in even carbon numbers from 2 to 18. The structures
of new CLCs MOPnB (n = 2 – 18) were characterised by FT-IR and 1H NMR. FT-IR
measurements confirmed the formation of the compounds. In the IR spectra, we can see
that the hydroxyl stretching absorption band of intermediate 3 disappear; meanwhile,
ester stretching bands move to around 1731, 1764 cm-1 for all CLCs (Fig. 1). The
representative 1H NMR spectrum of compound MOP10B is given in Fig. 2. In the 1H
NMR spectra, all signals of protons were assigned well for the proposed structures.
Especially, the proton of aryl skeleton exhibits three multiplets and one symmetrical
doublet, indicating the mesogenic core section of MOP10B. Additionally, the signal δ
4.78 – 4.72 ppm corresponds to the proton of menthyl group linking to ester group, and
δ 4.07 – 4.00 ppm corresponds to methylene protons in alkoxy section. And 2.94 – 2.91,
2.78 – 2.74 ppm corresponds to four protons of methylene in the succinyloxy group. All
the characteristic data are in accordance with the proposed structures of the compound
MOP10B.

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Figure 1. IR spectra of intermediate 3 and compound MOP10B.
Figure 2. The 1H NMR spectrum of compound MOP10B.
3.2 Thermal analysis
The thermal properties of the new CLCs with different alkoxy tails were detected by
DSC. The phase transition temperatures were measured at a rate of 10 °C min-1 on the
first heating and cooling cycles and Figure 3 shows the DSC curves for MOP2B and
MOP10B as examples. Their phase sequences, transition temperatures and enthalpies
of the MOPnB (n = 2 – 18) are presented in Table 1.

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Figure 3. DSC thermograms of compounds MOP2B and MOP10B.
Table 1. Transition Temperatures (°C) and Enthalpies (J/g) of MOPnB series
Phase transitionsa °C (corresponding enthalpy changesb; J g-1)
n
heating
cooling
2
4
6
8
Cr129.9(71.27)N*170.2(0.62)Iso
Cr111.6(52.66)N*148.3(0.75)Iso
Cr107.0(53.80)N*142.7(0.81)Iso
Cr120.7(46.84)N*138.0(1.21)Iso
Iso168.1(0.59)BP164.5cN*80.1(48.66)Cr
Iso147.0(0.81)BP144.7cN*68.7(22.37)Cr
Iso139.4(0.87)BP138.8cN*66.3(47.57)Cr
Iso135.0(1.00)BP134.2cN*76.0(46.02)Cr
Iso130.7(1.24)BP129.6cN*84.6(4.31)TGBA*83.6
cSmA*83.4cSmC*58.4(38.22)Cr
10 Cr86.8(51.23)N*132.6(1.29)Iso
12 Cr105.5(72.8)N*124.3(0.89)Iso
14 Cr109.0(71.78)N*120.6(0.65)Iso
16 Cr96.8(47.74)N*110.7(0.37)Iso
18 Cr104.6(57.08)N*111.0(0.10)Iso
Iso122.2(0.92)BP121.9cN*86.0(3.45)TGBA*85.8
cSmA*77.2cSmC*61.1(22.67)Cr
Iso118.4(0.96)BP117.5cN*90.7(3.48)SmC*70.0(3
6.74)Cr
Iso108.0(0.33)BP107.4cN*84.9(0.58)SmC*69.5(3
9.61)Cr
Iso110.0(0.99)BP109.7cN*94.0(3.30)SmC*78.7(5
8.82)Cr
aPhase transition temperatures determined by both POM and peak values of the DSC traces. bValues of
enthalpy are given in brackets. n indicates the number of carbon atoms in the terminal alkoxy chains. cThe
transition seen under POM was too weak to be recognized by DSC.

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For MOPnB (n = 2, 4, 6, 8), DSC heating thermograms show not only a melting
transition, but also a chiral nematic phase (N*) to isotropic phase (Iso) transition on
heating cycles. On the cooling cycles, an isotropic to N* phase transition and a
crystallization phase transition (Cr) were observed. MOPnB (n = 10, 12, 14, 16, 18)
have the same phase transition to MOPnB (n = 2, 4, 6, 8) on heating and cooling cycles,
but as an unusual, the monotropic chiral smectic C phase (SmC*) phase after N* phase
in compounds MOPnB (n = 10, 12, 14, 16, 18) appear while cooling the samples from
their isotropic phase. In addition, the enthalpies of all compounds MOPnB in Table 1
gave lower values irrespective of the length of the alkoxy chains. This may be accounted
in terms of the reduction of length-to-breadth ratio and the increased biaxiality arising
from the orientation of the menthyl group with respect to the long axes of the mesogenic
unit, which could reduce the transition temperatures and enthalpies [34-37].
Figure 4. A plot of transition temperature as a function of number of carbon atoms (n) in the terminal alkoxy
groups for MOPnB series: (a) heating mode and (b) cooling mode.
The relationship of phase transition temperatures (heating and cooling mode) with
different lengths of the terminal alkoxys (n) is presented in Figure 4. The number n in

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alkoxys has a modest influence on the mesomorphic properties. It shows that increasing
the lengths in alkoxy groups changs the transition temperature and the mesomorphic
temperature ranges. The clearing points (Ti) and the phase transition temperature of Iso
– N* transition decrease with increasing n. The effect of increasing the length of
terminal alkoxys is two-fold; first, the anisotropic properties of the molecule are
increased resulting in an increased temperature of Ti and Iso – N* transition; second, the
dilution of the cores is increased resulting in a decrease in Ti and Iso – N* transition.
Thus, the overall effect of increasing the length of a terminal alkoxys depends
essentially on the interaction strength parameter of the core [37, 38]. In our case, the
core of a three ring system of MOPnB (n = 2 – 18) is large, thus the dilution effect
dominates and the transition temperatures simply fall more or less with increasing chain
length. Figure 4 also revealed that MOP2B – MOP8B are enantiotropic N* liquid
crystal compounds. But MOP10B – MOP18B show not only enantiotropic N* phase
but also monotropic SmC* phase. And the N* – SmC* transition temperatures are found
to rise gently. The temperature range of the N* phase narrows, whereas the thermal
stability of the SmC* phase gradually increases with n increasing. This identifies that
the increase of alkoxy groups favours the formation of SmC* phase over N* phase,
presumably due to the enhanced interlayer interactions derived from the increase of the
terminal alkoxy groups [39].
3.3 Optical analysis
The optical textures for studied compounds MOPnB (n = 2 – 18) were characterized
by POM for identifying their mesophase. Their thermochromic property or selective

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reflection in visible light was analysed by UV-vis spectrometer. In MOPnB (n = 2 – 18)
series, all compounds show a phase transition sequence of Cr – N* – Iso on heating
cycles. During slowly cooling, compounds MOPnB (n = 2, 4, 6, 8) show Iso – BP – N*
– Cr; MOP10B and MOP12B exhibit two new phase transitions of twist grain boundary
A (TGBA*) and chiral smectic A phase (SmA*) and present a phase transition sequence
of Iso – BP – N* – TGBA* – SmA* – SmC* – Cr; and MOPnB (n = 14, 16, 18) reveal
a phase transition sequence of Iso – BP – N* – SmC* – Cr. The differences of
measurement results between DSC and POM were the BP, TGBA* and SmA* phases
which were only observed by POM. Because the Iso – BP and N* – TGBA* – SmA*
transitions are too weak to record in the DSC measurements.
3.3.1 CLCs MOP2B, MOP4B, MOP6B and MOP8B
Figure 5. Optical textures of MOP2B

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Figure 6. Sets of photographs of thermochromic process produced by MOP2B (a), MOP4B (b), MOP6B (c) and
MOP8B (d) on heating process in the N* phase.
Compounds MOPnB (n = 2, 4, 6, 8), while slowly heating from their melting points,
display the planar oily streak texture (Fig. 5a). The planar texture of the N* phase
exhibits an important optical phenomena, the selective reflection. In general, the
selective reflection wavelength depends on the molecular chirality and ordering of the
molecules within the phase. Due to the unique helical structure of N* phase, their
orientation of the molecules varies in a helical fashion and thus selectively reflects light
according to Bragg’s law [40]. The variation of temperature resulted in the change of P,
and led to the thermochromic character (selective reflection) at different temperatures
and different observation angles. This is the reason for CLCs having thermochromic
phenomena, which can readily be observed by the naked eye as an iridescent colour. As
a representative, sets of photographs of thermochromic process for compounds MOPnB
(n = 2 – 8) on heating process in the N* phase are shown in Figure 6. The color of the

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reflected light of compounds MOP2B and MOP8B are varied in the red region. Then
compounds MOP4B and MOP6B are in the yellow-green or red-green region.
Figure 7. UV/Vis spectra of MOP2B (a), MOP4B (b), MOP6B (c) and MOP8B (d).
Selective reflection requires the light with the same handedness as the helix and the
wavelength satisfying  = nP, where n is an average refractive index,  is the maximum
wavelength. The representative reflection spectra of MOP2B in the N* phase on heating
cycles are shown in Figure 7a. Based on the observations, we can estimate the selective
reflection wavelength changing from 617 nm to 677 nm upon increasing temperature
with the temperature interval from 135 °C to 160 °C for MOP2B. With the temperature

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increasing, the reflective colours of the N* phase show an obvious red shift, which is
caused by the Bragg selective reflection from the plane of oily streak textures of the N*
phase. This red shift phenomenon of N* phase with the increase of temperature has been
reported before [41]. The reflection colours of MOP2B mainly shift to a long-
wavelength region with the increasing of temperature due to the P of N* phase altered
with temperature and it unwound with increasing temperature. However, the selective
reflection wavelengths of MOP4B and MOP6B revealed a slight red-shift firstly with
increasing temperature, until closing to the clearing point, the wavelengths show blue
shift (Fig. 7b-7c). Further, the obvious blue shift was observed by compound MOP8B.
The wavelengths of MOP8B shift 49 nm with increasing temperature (Fig. 7d). The
reason for blue shift with increasing temperature was that the helical structure started to
twist. So, the wavelengths may have red shift or blue shift with increasing the
temperature in the N* phase of CLCs MOPnB (n = 2 – 8).
On the cooling cycles for MOPnB (n = 2 – 8), the BP appeared firstly as an iridescent
platelet texture with a small temperature range about ~4 °C between isotropic and N*
phases as shown in Figure 4b. Their allure and beauty mesophases originate mainly
from their self-organized three-dimensional photonic crystal structure. With the lattice
spacing on the order of several hundred nanometers, Bragg selective reflection could be
any colour positioned in the visible region [43]. The high chirality of (-)-menthyl in
CLCs forces the mesogenic molecules to twist with respect to each other, resulting in
the typical double twisted cylinder of BPs. Defect theory well explains why BPs are
usually found to appear in a narrow temperature range (Seen in the SI). On further

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cooling, the focal conic texture of N* phase with a wider temperature range could be
observed until the sample crystallized.
3.3.2 CLCs MOP10B and MOP12B
Compounds MOPnB (n = 10, 12) also display the oily streak textures on the heating
cycles (Fig. 8a). Based on the observation, the texture contains coloured grains, with the
colour corresponding to the pitch length of the N* helix. The N* phase of MOP10B
exhibits reflections in the red region. The representative thermochromic image of
MOP10B is shown in Figure 8b. The mesomorphic properties and the thermochromic
phenomena disappear at 132.6 °C (Fig. 8c) and UV-Vis spectra of N* phase for
MOP10B is shown in Figure 8d. We can estimate the value of the wavelength changing
from 752 nm to 652 nm upon increasing temperature within the temperature interval
from 114 °C to 130 °C. The helical structure started to wind with increasing temperature.
The plot of wavelength against temperature showed a linear relationship, and dλ/dT < 0
in the N* phase for MOP10B. As the length of the alkoxy increases, the helical pitch
length of the N* phase increases gradually, and when n = 12, the pitch length of the N*
phase of this series CLCs is out of the order of the wavelength of white light, so no
selective reflection in N* phase of MOP12B appears.
When slowly cooling from isotropic phase for MOP10B and MOP12B, the BP
appears as a fuzzy blue texture with a small temperature range about 0.5 °C between
isotropic and N* phases. Since the range of BP phase is too narrow, the fully structure
of BP phase cannot formed. On further cooling, the focal conic texture of N* phase also
grew (Fig. 9a). As the temperature continued to cool down from the N* phase, the

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filament texture appeared (Fig. 9b), indicating the characteristic of TGBA* phase. And
then the SmA* phase was confirmed by the microscopic observation of a dark field of
view for homeotropic orientation (Fig. 9c). Similar to the short pitch cholesteric phase,
the short pitch SmC* phase exhibits the phenomenon of selective reflection as well. On
further cooling, the SmC* phase appeared with selective reflection as the temperature
declined and the equivalent POM, thermochromic images and UV-Vis spectra in SmC*
phase for MOP10B are shown in Figure 9d-9f. First, the red petal texture (Fig. 9d (1))
started growing and with further cooling other colour petal textures (Fig. 9e (1) - f (1))
formed. For SmC* phase, where the pitch length of the helix is of a similar magnitude
to the wavelength of light, light will be selectively reflected. Through the reflected
image we found that the thermochromic process in cooling stage passes through red
(Fig. 9d (2)) to saffron (Fig. 9f (2)) for MOP10B. The corresponding value of reflection
wavelength changes from 703 nm (Fig. 9d (3)) to 581 nm (Fig. 9f (3)) with the
temperature interval from 80 °C to 70 °C. The reflection wavelengths of MOP10B in
SmC* phase are mainly blue shift with decreasing the temperature. The colours of petal
textures, reflected colour and reflection wavelength changed with temperature caused
by the variation of helical pitches with changing temperature in the visible region of the
spectra [44]. The reason for this outcome is that the molecules are tilted at an angle with
respect to the layer planes, and the tilt angle () in SmC* phase usually increases as the
temperature decreases. Generally, after the SmA* – SmC* phase transition, the tilt 
varies with respect to temperature in the following:
0.5
( )
휃 푇 = 휃0(푇푁 − 푇)

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Where TN is the phase transition temperature, T is the temperature, 0 is a constant.
Thus, the tilt angle  increases as the temperature is reduced, and as a consequence the
pitch in a helical SmC* is shortened. CLC MOP12B also displays similar texture and
thermochromic phenomena in SmC* phase with MOP10B, however, the temperature
ranges of iridescent colours for MOP12B are a little narrower than that of MOP10B.
On further cooling, the CLCs MOP10B and MOP12B began to crystallize.
Both N* phase and SmC* phase with short pitch exhibit the phenomenon of selective
reflection. MOP10B in both N* and SmC* phases can reveal thermochromic
phenomena through its spectral range, but MOP12B is only observed thermochromic
process in SmC* phase.
Figure 8. Optical textures and reflection spectra of MOP10B in N* phase on heating.

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Figure 9. Optical textures of MOP10B in N* phase (a), TGBA* phase (b) and SmA* phase (c). And the equivalent
POM, reflected images and UV/vis spectra in SmC* phase on cooling to 80 °C (d), 76 °C (e) and 72 °C (f).
3.3.3 CLCs MOP14B，MOP16B and MOP18B
Compounds MOPnB (n = 14, 16, 18) also displayed oily streak texture on heating
cycles. On slowly cooling from their isotropic phases, the BP appeared as a fuzzy blue
texture with increasing alkoxy chain length. This suggested that extending the terminal
alkoxy chain length could increase the intermolecular interaction such that it limited the
formation of the BP in this series. Typically, the compounds were found to exhibit N*
phase which possessed focal conic textures on cooling cycles. Subsequently cooling of
the N* phase produced a transition to the SmC* phase which exhibited fingerprint
texture for MOP14B and MOP16B (Fig. 10a) and bonnet texture for MOP18B (Fig.
10b).

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Figure 10. Optical textures of MOP14B and MOP18B.
The short pitch N* phase or SmC* phase exhibits the phenomenon of selective
reflection, and N* phases with a pitch smaller than approximately P < 1 µm are
generally referred to as short pitch materials. As the length of the alkoxy increases, the
helical pitch increases gradually and long pitch N* phase or SmC* phase forms. Due to
the pitch length of these phases is out of the order of the wavelength of white light, so
no selective reflection of MOP14B, MOP16B and MOP18B appears.
3.4 XRD studies
To ascertain the mesophase assignments of the N* and SmC* phases, XRD
measurements were carried out for the sample MOP10B. The sample was filled into a
Lindemann capillary (1 mm diameter) tube in the isotropic phase and both ends of the
tube were flame sealed. The diffraction patterns in the mesophase obtained on cooling
from their isotropic phases were collected on an image plates. The intensity versus 2θ
plots derived from the diffraction patterns of MOP10B are shown in Figure 11. At
110 °C, a broad diffraction peak was observed in the large-angle region (2θ = 19.3°),
corresponding to the intermolecular distance of 4.59 Å, which is assigned to the N*
phase. When it cooled from 110 °C to 82 °C, the diffraction pattern showed a sharp
diffraction peak 2θ = 2.6° (d-spacing 33.9 Å) and a diffused peak at 2θ = 19.2° (d-
spacing 4.62 Å). The sharp diffraction peak in the small-angle region is characteristic

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of smectic structures, while the diffused peak indicates the lateral packing. The length
of the molecule (l) including alkoxy groups in all trans conformation of MOP10B
measured using a Chem3D molecular model was estimated to be 38 Å. The tested d-
spacing at 82 °C is less than the molecular length, suggesting a titled smectic C
molecular arrangement. When it cooled from 82 °C to 75 °C, the interlayer distance

decreased from 33.9 Å to 32.68 Å. The tilt angle ( ) is calculated to be 29.8° at 82 °C
and 34.1° at 75 °C from the equation   cos1(d /l). The d-spacing at 70 °C does not
change, but the intensity of the peak was significantly enhanced, which demonstrated
that the order of molecule arrangement enhanced in intermolecular layers at 70 °C [45].
Figure 11. Intensity versus 2θ graph derived from the XRD diffraction of compound MOP10B at 110 °C, 82 °C,
75 °C and 70 °C, respectively.
4. Conclusions
In this work, we have synthesized a series of new CLC compounds MOPnB (n = 2 –
18) derived from (-)-menthol and investigated their mesomorphism and optical
properties. In general, the compounds MOPnB (n = 2 – 18) all exhibit oily streak

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textures on heating cycles. Upon cooling, the compounds MOPnB (n = 2 – 8) exhibit a
phase transition sequence as Iso – BP – N* – Cr; compounds MOPnB (n = 10, 12)
reveal an Iso – BP – N* – TGBA* – SmA* – SmC* – Cr phase transition process;
MOPnB (n = 14, 16, 18) reveal a phase transition sequence of Iso – BP – N* – SmC*
– Cr. And with increasing the length of the terminal alkoxy tail, the corresponding Ti
and the phase transition temperature of Iso – N* decrease. The temperature range of the
N* phase narrows and the thermal stability of the SmC* phase gradually increases with
the increase of n. CLCs MOPnB (n = 2 – 10) in N* phase reveal thermochromic
phenomena due to the selective reflection of visible light, however, when n ≥ 12,
thermochromic phenomena in N* phase disappear. Additionally, iridescent petal
textures with thermochromic process in SmC* phase were only observed for MOP10B
and MOP12B, and the wavelengths are blue shift with decreasing temperature. CLC
MOP10B reflects selectively visible light or reveals thermochromic property not only
in N* phase but also in SmC* phase. High chirality and high twist power are approved
for (-)-menthol and it is a good candidate for preparation of CLCs with BPs or short
pitch.
Acknowledgements
This work was supported by National Nature Science Foundation of China (31560268)
and Program for New Century Excellent Talents in Fujian Province University
(JA14325).

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Graphical Abstract

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Highlights
Novel CLCs MOPnB based on (-)-menthol with high chirality were synthesized.
All chiral liquid crystals MOPnB displayed blue phase.
MOPnB (n = 2 - 10) revealed the thermochromic property in N* phase on heating.
MOPnB (n = 10, 12) revealed the thermochromic property in SmC* phase on cooling.