Synthesis and Characterization of a Series of Cholesterol-Based Liquid Crystalline Dimers with a Chiral (-)- Menthyl Terminal Group

Article abstract of DOI:10.1080/15421406.2014.953759

A novel series of cholesterol-based liquid crystalline (LC) dimers with a menthyl terminal group in the biphenyl base side have been synthesized. The chemical structures and LC properties of this series of compounds are characterized by FT-IR, ¹H-NMR, elemental analysis, hot-stage coupled polarizing microscopy and differential scanning calorimetry. All the dimers exhibit enantiotropic mesophases. These dimesogenic compounds with a short spacer are chiral nematic LCs, while with long spacers, they are SmA and chiral nematic LCs. Some of these LC dimers exhibit iridescent colors in their chial nematic (N) states.

Full text of DOI:10.1080/15421406.2014.953759

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Synthesis and Characterization of a
Series of Cholesterol-Based Liquid
Crystalline Dimers with a Chiral (-)-
Menthyl Terminal Group
a
Changcheng Wu
a
Tianjin Municipal Key Laboratory of Fiber Modification &
Functional Fiber, School of Material Science & Chemical Engineering,
Tianjin Polytechnic University, Tianjin, P.R. China
Published online: 11 Apr 2015.
Click for updates
To cite this article: Changcheng Wu (2015) Synthesis and Characterization of a Series of Cholesterol-
Based Liquid Crystalline Dimers with a Chiral (-)- Menthyl Terminal Group, Molecular Crystals and
Liquid Crystals, 609:1, 31-39, DOI: 10.1080/15421406.2014.953759
To link to this article: http://dx.doi.org/10.1080/15421406.2014.953759
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Mol. Cryst. Liq. Cryst., Vol. 609: pp. 31–39, 2015
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2014.953759
Synthesis and Characterization of a Series
of Cholesterol-Based Liquid Crystalline Dimers
with a Chiral (-)- Menthyl Terminal Group
∗
CHANGCHENG WU
Tianjin Municipal Key Laboratory of Fiber Modification & Functional Fiber,
School of Material Science & Chemical Engineering, Tianjin Polytechnic
University, Tianjin, P.R. China
A novelseries of cholesterol-basedliquid crystalline (LC)dimers with a menthyl terminal
group in the biphenyl base side have been synthesized. The chemical structures and LC
1
properties of this series of compounds are characterized by FT-IR, H-NMR, elemental
analysis, hot-stage coupled polarizing microscopy and differential scanning calorimetry.
All the dimers exhibit enantiotropic mesophases. These dimesogenic compounds with a
short spacer are chiral nematic LCs, while with long spacers, they are SmA and chiral
nematic LCs. Some of these LC dimers exhibit iridescent colors in their chial nematic
∗
(
N ) states.
Keywords Chiral nematic phase; cholesterol; dimesogenic; liquid crystalline dimer;
menthyl
1
. Introduction
LC dimers, also called dimesognic compounds, are molecules containing two identical
symmetric) or nonidentical mesogenic moieties (unsymmetric) connected by a flexible
(
spacer. LC dimers have been attracted intensive attention not only because they are re-
garded as model compounds for LC polymers but also due to their inherently interesting
LC properties [1–6]. Due to the great structural variety of unsymmetric compounds, the
quantity of unsymmetric LC dimers is much more than that of symmetric LC dimers.
In particular, unsymmetric cholesterol-based LC dimers which containing one cholesteryl
moiety and another substituted aromatic mesogenic unit such as benzoate ester, Schiff’s
base, azobenzene, or biphenyl are especially interesting [7–25]. This is because the chi-
ral cholesteryl moiety entitles many unique optical properties such as selective reflection,
circular dichroism, and thermo-optical effects besides the properties of conventional rigid
aromatic LC dimers. In this aspect, they could be applied in optical storage, optical switch-
ing, nonlinear optics, and liquid crystal display device [14–18].
(-)- Menthol is another natural chiral compound; it has three chiral carbon atoms
in its molecule. Introduce the menthyl group to the LC compounds or polymers also can
∗
Address correspondence to Changcheng Wu, Tianjin Municipal Key Laboratory of Fiber
Modification & Functional Fiber, School of Material Science & Chemical Engineering, Tianjin
Polytechnic University, Tianjin 300160, P.R. China. E-mail: ccwu@tjpu.edu.cn
Color versions of one or more of the figures in the article can be found online at
www.tandfonline.com/gmcl.
31

32
C. Wu
∗
produce N , blue or chiral smectic phases [26,20]. To reveal the properties of the LC dimers
containing cholesteryl moiety and a chiral (-)- menthyl terminal group, a novel series of
cholesterol-based LC dimers with menthyl terminal group in the biphenyl base side have
been synthesized, also their LC properties are also studied.
2. Experimental
2.1. Materials
(-)- Menthol (98%) was obtained from Aladdin chemical. Cholesterol was obtained
from Tianjin Yingbo Biochemical Reagent Company (Tianjin, P.R. China). N, N-
dicylcohexylcarbodiimide (DCC) and 4-dimethylaminopyridine (DMAP) were used as
received (Arcos). Dichloromethane (DCM) was dried with CaH2. Tetrahydrofuran (THF)
was distilled from sodium-benzophenone. Other solvents and reagents were AR or CP
grade and used without further purification.
2.2. Synthesis
The synthetic route used to prepare this series of LC dimers is shown in the Scheme 1. Di-
carboxylic acid monocholesteryl esters were synthesized according to a reported procedure
[20, 21].
Scheme 1. Synthetic route of the cholesterol-based LC dimers containing (-) menthyl terminal
groups. Regents and conditions: (a) THF, DMAP, refluxing 12 hr; (b) DCC/DMAP/DCM, 24 hr,
room temperature; and (c) DCC/DMAP/DCM, 24 hr, room temperature.
2.2.1. Synthesis of (-)-monomenthyl succinate (1). (-)- Monomenthyl succinate was syn-
thesized using the common method esterification of (-)-methanol and succinic anhydride
◦
in THF with the catalyst DMAP. Yield: 78.6%. M.p. 61–63 C.
−1
FR-IR (KBr, cm ): 2917–3440, 1727, 1710, 1388, 1285, 1224, 1177, 1037, 1011.

Synthesis and Characterization of a Series of Cholesterol-Based LCs
33
1HNMR (CDCl3, ppm): .4.67–4.74 (m, 1H), 2.67–2.72 9 (t, 2H), 2.58–2.63 (t, 2H),
.72–2.01 (broad, 18H).
0
ꢀ
2
.2.2. Synthesis of (4 -hydroxybiphenyl) 4-menthyloxycarboxyl propionate (2). Ten mil-
limole compound (1) and 32 mmol 4, 4-biphenyl were dissolved in 40 mL dry THF,
0 mmol DCC, and 2 mmol DMAP were added with stirring at room temperature for 24
1
hr. The dicyclohexylurea formed as precipitation was removed by filtration and the filtrate
was concentrated under reduced pressure. The crude product was purified by column chro-
◦
matography on silica gel (200–300 mesh) using a mixture of petroleum ether (60–90 C)
and ethyl acetate (4/1, v/v) as eluent, white solid powder was obtained. Yield: 51.4%. mp.
◦
1
35.5–137 C.
−
1
FR-IR (KBr, cm ): 3475, 2956, 1751, 1731, 1611, 1530, 1408, 1368, 1267, 1207,
153, 969, 891, 824. HNMR (CDCl3, ppm): 7.74–7.49 (d, 2H), 7.37–7.40 (d, 2H),
.09–7.12 (d, 2H), 6.82–6.95 (d, 2H), 5.29 (s, 1H), 4.70–4.79 (m, 1H), 2.89–2.93 (t,
H), 2.73–2.77 (t, 4H), 0.74–2.02 (broad, 18H).
1
1
7
4
2.2.3. Synthetic of the cholesterol-based LC dimers containing (-) menthyl terminal groups
(3) (chmen-1, chmen-2, chmen-3, and chmen-4). Four millimole compound (2) and 4 mmol
corresponding dicarboxylic acid monocholesteryl ester were dissolved in 40 mL dry DCM.
Four millimole DCC and 0.6 mmol DMAP were added with stirring at room temperature
for 24 hr. The dicyclohexylurea formed as precipitation was removed by filtration and
the filtrate was concentrated under reduced pressure. The crude product was purified by
column chromatography on silica gel (200–300 mesh) using a mixture of petroleum ether
◦
◦
(
(
60–90 C) and ethyl acetate (4/1, v/v) as eluent, then recrystallized from petroleum ether
60–90 C), white solid powder was obtained.
−1
Chmen-1, Yield: 53.8%. FT-IR (KBr,cm ): 2955, 1760, 1731, 1622, 1493, 1313,
198, 1166, 1136. H-NMR (CDCl3, ppm): 7.52–7.55 (d, 4H), 7.14–7.17 (dd, 4H), 5.38
1
1
(s,1H), 4.70–4.78 (m, 2H), 2.89–2.91 (t, 4H), 2.71–2.75 (t, 4H), 0.73–2.35 (broad, 61H).
Element analysis for C57H80O8; calculated C 76.64%, H 9.03%; found C 76.58%, H 9.27%.
−
1
Chmen-2, Yield: 50.1%. FT-IR (KBr,cm ): 2936, 1760, 1731, 1628, 1495, 1413,
1
1
5
2
7
254, 1202, 1167, 1136. H-NMR (CDCl3, ppm): 7.53–7.55 (d, 4H), 7.13–7.7 (dd, 4H),
.37 (s,1H), 4.59–4.78 (m, 2H), 2.88–2.92 (t, 2H), 2.72–2.76 (t, 2H), 2.61–2.63 (t, 2H),
.31–2.38 (t, 4H), 0.67–2.02 (broad, 63H). Element analysis for C59H84O8; calculated C
6.92%, H 9.19%; found C 77.00%, H 9.49%.
−1
Chmen-3, Yield: 52.5%. FT-IR(KBr,cm ) : 2936, 1760, 1732, 1601, 1495, 1414,
1
1
5
2
7
253, 1202, 1168, 1136. H-NMR (CDCl3, ppm): 7.53–7.56 (d, 4H), 7.12–7.17 (dd, 4H),
.38 (s,1H), 4.60–4.78 (m, 2H), 2.88–2.92 (t, 2H), 2.72–2.76 (t, 2H), 2.55–2.60 (t, 2H),
.27–2.32 (t, 4H), 0.67–2.02 (broad, 67H). Element analysis for C61H88O8; calculated C
7.17%, H 9.34%; found C 77.18%, H 9.30%.
−1
Chmen-4, Yield: 63.7%. FT-IR (KBr,cm ): 2933, 1760, 1733, 1628, 1495, 1367,
1
1
5
2
7
254, 1202, 1168, 1136. H-NMR (CDCl3, ppm): 7.53–7.56 (d, 4H), 7.12–7.7 (dd, 4H),
.38 (s,1H), 4.60–4.78 (m, 2H), 2.88–2.92 (t, 2H), 2.72–2.76 (t, 2H), 2.54–2.59 (t, 2H),
.26–2.32 (dd, 4H), 0.67–2.02 (broad, 71H). Element analysis for C63H92O8; calculated C
7.42%, H 9.49%; found C 77.29%, H 9.46%.
2.3. Characterization
Chemical structures of the intermediates and LC dimers were characterized with FT-IR by
1
using Bruker Tensor 37 spectrometer and H-NMR spectrum by using Bruker 300 MHz

34
C. Wu
spectrometer with tetramethylsilane as an internal standard. Elemental analyses were car-
ried out on a Germany Elementary Vario EL III instrument. The phase transitions of the
LC dimers were determined by means of Nikon Eclipse E200 polarized optical microscope
(POM) equipped with an Instec STC200 hot stage. A differential scanning calorimeter
(
DSC) (Netzsch DSC204) was used to measure the transformation temperature and trans-
◦
formation enthalpy at the heating and cooling rate of 5 C/min.
3. Results and Discussions
In this work, a series of Cholesterol-based LC dimers have been synthesized, the two
nonidentical mesogenic moieties cholesteryl and biphenyl are linked through dicarboxylic
ester bonds, the spacer lengths are 2, 4, 6, 8 methylenes, on the other side of biphenyl
containing another chiral (-)-menthyl terminal group, they are depicted as chmen-1, chmen-
2, chmen-3, and chmen-4. In the previous studies of unsymmetric cholesterol-based LC
dimers, mostly, they had only one chiral moiety in their molecular structure, another
mesogenic group such as benzoate ester, Schiff’s base, azobenzene, or biphenyl did not
catch a chiral tail [7–25]. Only a few researches reported the tolane mesogenic moiety was
attached chiral tails such as 2S-chloro-3S-methylpentanoyloxy or 1, 3-dioxalane on the
unsymmetric cholesterol-based LC dimers [26].
Menthyl is a well-known chial group which can introduce chiral nematic phase when
it is incorporated in the LC molecules or polymers [27–31]. To the best to our knowledge,
there is no report about the LC dimers containing chiral cholesteryl and (-)-menthyl groups
together. So the aim of this study to reveal the structure-property relationships of novel
Cholesterol-based dimeogenic compounds containing (-)-menthyl group.
Figures 1 and 2 show the DSC thermograms of LC dimers (Chmen-1, Chmen-2,
Chmen-3, and Chmen-4) in the heating and cooling process, respectively. In the heating
◦
process of Chmen-1, there are two peaks appeared at 114.4 and 162.3 C. Very similar
to Chmen-2, Chmen-3, and Chmen-4, there are three peaks appeared in heating, but the
transition temperatures are decreased in turn. In the cooling processes, one peak appeared
◦
◦
at 159.9 C for Chmen-1, two peaks appeared at 146.5 and 141.2 C for Chmen-2, but three
peaks appeared in the Chmen-3 and Chmen-4 DSC curves.
◦
In the POM observation coupled with hot stage, when Chmen-1 was heated to 115 C,
the broken fan-shaped texture appeared (Fig. 3(a)), the oily-streak texture formed when the
sample was slightly sheared (Fig. 3(b)). The LC phase transited to isotropic phase until
◦
◦
hated to 163 C. When Chmen-1 was cooled from isotropic phase at 160 C, fan-shaped
texture (also can changed to oily-streak texture after sheared) appeared again, but it could
not crystallized even cooled down to room temperature. Therefore, the LC phase formed
∗
of Chmen-1 during and cooling is N phase. In the POM observation of Chmen-2, when it
was hated to 123 C, a typical focal-conic fan SmA texture was found (Fig. 4(a)), after it
◦
◦
∗
is heated to 143 C, the texture changed to oily-streak texture of N phase until isotropic at
◦
∗
1
48 C (Fig. 4(b)). In the cooling process of Chmen-2, the isotropic to N phase transition
◦
∗
◦
taken place at 146 C, and the N phase transmitted to SmA phase at 141 C, then the
SmA phase retained at room temperature. The LC behavior of Chmen-3 and Chmen-4
are very similar to that of Chmen-2, but Chmen-3 and Chmen-4 could crystallized at low
◦
◦
temperature 34 Cand 40 C, respectively.
Therefore, from combination of the results of DSC studies and POM observation, we
can summarize the LC properties of LC dimers in Table 1.
From the above results of their LC properties, we could conclude, with the alkylene
spacer lengths increased in the LC dimers, Chmen-1 appeared only one chiral nematic

Synthesis and Characterization of a Series of Cholesterol-Based LCs
35
Figure 1. DSC traces for the LC dimers in the heating process.
Figure 2. DSC traces for the LC dimers in the cooling process.
phase, but Chmen-2, Chmen-3, and Chmen-4 exhibited SmA and chiral nematic phases.
∗
∗
∗
The temperatures of N to isotropic phase transition, isotropic phase to N phase and N to
SmA phase transitions decreased as alkylene spacer lengths increased, this indicated that

36
C. Wu
Figure 3. Optical polarizing micrographs of Chmen-1 in heating process: (a) fan-shaped texture at
◦
1
30 C and (b) oily-streak texture formed after sheared.
the longer of alkylene spacer length, the more flexible the dimesogenic compound was, so
that the LC phase appeared at lower temperature as the alkylene spacer lengths increased.
Except Chmen-1, both in the heating and cooling processes of the LC dimers Chmen-2,
∗
Chmen-3, and Chmn-4, the iridescent colors appeared in their N LC states. This property
∗
of N phase is due to the presence of helical superstructure. The variation of temperature
∗
results in the change of the pitch of helical structure of the N phase, this effect leads
to the color changes at different temperature on the different observation angles [32].
∗
unfortunately, the N phases temperature ranges of these LC dimers are rather low, about
◦ ◦
4
.5 C–8.1 C, we roughly describe the phenomena as blow: Chmen-2 shows red to green
color change during in the heating 143 C–148 C, the inverse color change of N state takes
◦
◦
∗

Synthesis and Characterization of a Series of Cholesterol-Based LCs
37
Table 1. Thermal transition temperatures of cholesterol-based LC dimers
Compound
Heating
Cooling
∗
∗
I159.9 N
Chmen-1
Chmen-2
Chmen-3
Chmen-4
Cr114.4 N 162.3I
∗
∗
Cr 122.7SmA143.3 N 147.8I
Cr 99.8SmA120.2 N 126.1I
Cr88.7SmA103.7 N 111.8I
I146.5 N 141.2SmA
∗
∗
∗
∗
I124.3 N 118.2SmA34.0Cr
I109.9 N 101.9SmA39.5Cr
∗
N = chiral nematic and I = isotropic, the temperature was taken at DSC curves peaks value.
Figure 4. Optical polarizing micrographs of Chmen-2 in heating process: (a) fan-shaped texture at
◦ ◦
30 C and (b) oily-streak texture formed at 145 C.
1

38
C. Wu
place in the cooling process. Chmen-3 and Chmen-4 mainly appear red and yellow color
∗
in the N phase.
4. Conclusion
In this work, we have synthesized a series of Cholesterol-based LC dimers combining of
biphenyl mesogen with (-)-menthyl terminal group, the two mesogenic moieties are linked
through dicarboxylic ester bonds; the alkylenen spacers lengths are 2,4,6,8 mehylenes. The
∗
LC dimer with a short flexible spacer is N phase while longer flexible spacer is SmA and
∗
N phase. The phase transition temperatures are lower as the flexible spacers are longer in
general. Except Chmen-1, other dimeogenic compounds exhibit iridescent colors in the N
∗
states both in heating and cooling process.
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