Design and phase transition behavior of siloxane-based monomeric and dimeric liquid crystals bearing cholesteryl mesogenic groups

Article abstract of DOI:10.1016/j.jorganchem.2019.02.016

A series of siloxane-based monomeric and dimeric liquid crystals (LCs) bearing cholesteryl mesogenic groups were newly synthesized and their phase transition behavior was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). All the siloxane-based LCs obtained in this study showed a smectic A phase in a wide temperature range. Among the siloxane-based LCs obtained, only the dimeric LC with a shortest siloxane unit at a central part of a spacer was found to exhibit a chiral nematic phase and a blue phase in addition to the smectic A phase. The difference in the thermal properties of the siloxane-based dimeric LCs was investigated in terms of their conformations. Furthermore, the results obtained for the siloxane-based LCs were discussed with comparing with those of homologues having the normal alkyl chains and spacers.

Full text of DOI:10.1016/j.jorganchem.2019.02.016

Journal of Organometallic Chemistry 886 (2019) 34e39
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Design and phase transition behavior of siloxane-based monomeric
and dimeric liquid crystals bearing cholesteryl mesogenic groups
, b,
Kaito Katsuki a, Kosuke Kaneko a *, Kimiyoshi Kaneko a, Riki Kato b,
Nobuyoshi Miyamoto b, Tomonori Hanasaki a
a Department of Applied Chemistry, College of Life Sciences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan
b Department of Life, Environment, and Applied Chemistry, Faculty of Engineering, Fukuoka Institute of Technology, 3-30-1 Wajiro-higashi, Higashi-ku,
Fukuoka, 811-0295, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of siloxane-based monomeric and dimeric liquid crystals (LCs) bearing cholesteryl mesogenic
groups were newly synthesized and their phase transition behavior was investigated by polarized optical
microscopy (POM), differential scanning calorimetry (DSC), and X-ray diffraction (XRD). All the siloxane-
based LCs obtained in this study showed a smectic A phase in a wide temperature range. Among the
siloxane-based LCs obtained, only the dimeric LC with a shortest siloxane unit at a central part of a spacer
was found to exhibit a chiral nematic phase and a blue phase in addition to the smectic A phase. The
difference in the thermal properties of the siloxane-based dimeric LCs was investigated in terms of their
conformations. Furthermore, the results obtained for the siloxane-based LCs were discussed with
comparing with those of homologues having the normal alkyl chains and spacers.
Received 26 December 2018
Received in revised form
14 February 2019
Accepted 18 February 2019
Available online 20 February 2019
Keywords:
Liquid crystal
Siloxane
Cholesteryl group
Blue phase
© 2019 Elsevier B.V. All rights reserved.
1. Introduction
and inorganic parts leads to nanosegregation into highly ordered
nanostructures [3].
Self-assembly in molecular science is one of the outstanding
tools for fabricating functional nanomaterials. Highly ordered ar-
chitectures in the nanostructured materials are fundamental fac-
tors dictating unique features in molecular biology, chemistry,
polymer science, materials science, and engineering [1]. Such mo-
lecular self-assembly systems accelerate a significant advance in
modern scientific technology, and are attracting a great deal of
attention. Over the past few decades, the self-assembly of liquid
crystals (LCs) responsive to external environments, has proven to
be a very useful tool creating the well-ordered materials [2].
Depending on the chemical structure and molecular shape, a va-
riety of LC phases can be formed, and furthermore their structures
are changeable by external stimuli for example electric/magnetic
field, light, temperature, and mechanical forces. For the construc-
tion of the nanostructured materials using the LCs, hybrid organic/
inorganic molecules such as those based on siloxane moieties are
attractive and feasible. The chemical immiscibility of the organic
There has been an increasing interest in LC dimers consisting of
two mesogenic groups connected by a flexible spacer, due to their
interesting phase behavior which significantly differs from that of
the corresponding monomeric LCs. Since LC dimers were firstly
€
reported by Vorlander in 1927 [4], symmetrical dimers composed
of two structurally identical mesogenic groups linked through a
flexible central spacer such as a polymethylene group have been
widely investigated, especially during the past two decades [5]. The
transition properties of the LC dimers are strongly dependent on
the nature and length of the flexible spacer [6]. For example, an
odd-even effect for the clearing and melting points has been
observed by changing the length of the spacer between the two
mesogenic groups. In addition, the design of twist bend nematic
(Ntb) phases arising from the LC dimers is a very hot topic in the
molecular science of the LCs recently [7]. From the scientific
background mentioned above, the study of such unique phase
properties in the LC dimers is still an important subject. Presently, a
standard approach for understanding of the properties is to
investigate stabilities of two identical mesogenic groups (sym-
metrical dimers) [8], unsymmetrical instabilities (unsymmetrical
dimers) [9], innate mesogenic properties depending on its molec-
ular structure [8], length and flexibilities of spacers [8], connection
* Corresponding author. Department of Applied Chemistry, College of Life Sci-
ences, Ritsumeikan University, 1-1-1 Nojihigashi, Kusatsu, Shiga, 525-8577, Japan.
E-mail address: kaneko@fc.ritsumei.ac.jp (K. Kaneko).
https://doi.org/10.1016/j.jorganchem.2019.02.016
0022-328X/© 2019 Elsevier B.V. All rights reserved.

K. Katsuki et al. / Journal of Organometallic Chemistry 886 (2019) 34e39
35
mode between mesogenic groups and spacers (terminally, verti-
cally) [10], and conformations of overall molecule (linear, tilt, bent)
[11].
purchased from Nacalai Tesque, Inc. Toluene was dried over sodium
and then distilled, and other solvents were used without further
purification. Synthetic routes of the objective siloxane-based LCs
are shown in Fig. S1.
The present study is focused on the siloxane-based monomeric
and dimeric LCs bearing cholesteryl mesogenic groups, which could
be categorized as hybrid organic/inorganic LC dimers, and their
phase transition behavior was characterized by polarising optical
microscopy (POM), differential scanning calorimetry (DSC) and X-
ray diffraction (XRD). The incorporation of the siloxane unit into the
spacers for the dimeric LCs is an essential scientific topic for
studying the molecular design of the LC materials with the unique
properties deriving from its flexibility and nanosegregation. We
investigated also how the siloxane unit in the spacer and tail in-
fluences on the thermal properties of the homologues having the
normal alkyl chains and spacers. The synthetic scheme of the
siloxane-based LCs is presented in Fig. 1.
2.2. Characterization
Thermal properties were measured using a Diamond DSC (Per-
kin-Elmer, Inc. MA, USA) at heating and cooling rates of 5 ꢀC minꢁ1
.
The optical textures of the liquid crystalline phases were observed
on a Nikon ECLIPSE LV100 polarising optical microscope (POM)
equipped with Mettler-Toledo FP-82HT hot-stage and FP-90 central
processor. 1H NMR spectra were recorded on an ECS 400 NMR
spectrometer (JEOL Ltd., Tokyo, Japan) at 400 MHz, using CDCl3 as
the solvent. Chemical shifts (
tetramethylsilane (TMS,
¼ 0.0 ppm) as internal standard for A, B
and C, or from the residual chloroform signal (CHCl3,
d) are expressed in ppm relative to
d
d
¼ 7.24 ppm)
for D-n, (n ¼ 2, 3) and E-n (n ¼ 2, 3, 4). The alignment of the LCs was
checked on the POM observation by using the non-treated glass
cells (E.H.C. Co. Ltd, Japan). Structural analyses by SAXS were per-
2. Experimental
2.1. Materials
formed with the Rigaku NANOPIX (Cu K
a radiation at 30 V and
40 mA) with a camera distance of 220 mm. For SAXS measure-
ments, the samples were introduced into a thin glass capillary with
a thickness of 1.5 mm.
The reagents, allyl bromide, methyl 4-hydroxybenzoate,
cholesterol, 1,1,1,3,3-pentamethyldisiloxane, 1,1,1,3,3,5,5-hepta
methyltrisiloxane,
1,1,3,3-tetramethyldisiloxane,
1,1,3,3,5,5-
hexamethyltrisiloxane, 1,1,3,3,5,5,7,7-octamethyltetrasiloxane, and
the organic solvents, 2-butanone, methanol, dichloromethane,
toluene were purchased from Tokyo Chemical Industry Co., Ltd. 1-
Ethyl-3-(3-dimethylaminopropyl)-carbodiimide (EDC) and N,N-
dimethyl-4-aminopyridine (DMAP) were purchased from FUJIFILM
Wako Pure Chemical Corporation. Potassium carbonate was
3. Results and discussion
The phase transition behavior of the siloxane-based LCs ob-
tained in this study is discussed in the following two sections: (1)
the thermal behavior of the monomeric LCs (D-n, n ¼ 2, 3) and
Fig. 1. Synthetic scheme of the siloxane-based LCs bearing the cholesteryl mesogenic groups.

36
K. Katsuki et al. / Journal of Organometallic Chemistry 886 (2019) 34e39
dimeric ones (E-n, n ¼ 2, 3, 4), and (2) the influence of the siloxane
units on the thermal behavior of the corresponding LC materials
having normal alkyl chains. The thermal properties of all the
siloxane-based LCs summarized in Table S1 were studied by dif-
ferential scanning calorimetry (DSC), polarising optical microscopy
(POM) and X-ray diffraction (XRD) analysis.
3.1. Monomeric (D-n) and dimeric (E-n) siloxane-based LCs
First of all, the compound C which is a precursor of all the
siloxane-based LCs was found to show a chiral nematic (N*) phase
in a temperature range from 124 ꢀC to 226 ꢀC [12]. The POM images,
DSC chart and XRD profile of D-2 are shown in Fig. 2. On the POM
observations, D-2 exhibited a focal conic texture typical of a smectic
A (SmA) phase. In the DSC diagram on heating, the endothermic
peak was detected at 110.0 ꢀC. This temperature is in good agree-
ment with the temperature at which the focal conic texture
appeared in the POM studies. On the further heating, the endo-
thermic peak at 161.2 ꢀC corresponds to a clearing point with the
disappearance of the texture in the POM image. The small-angle
XRD pattern for D-2 at 150.0 ꢀC shows a sharp peak in the small
Fig. 3. Illustration of the possible molecular packing of D-2 in the SmA phase.
determining the mesophase stability. D-3 with the longer siloxane
unit possesses the wider LC temperature range, providing not only
the lowered clearing point but also the significant decrease in its
melting point. It is reasonable to conclude that the flexibility of the
siloxane unit suppressed its crystallization. The XRD profile of D-3
demonstrated a typical characteristic of the SmA phase, and its d-
spacing (39.4 Å) is longer than that of D-2. The difference in the d-
spacing is due to the addition of the siloxane unit (the bond dis-
tance of SieO is 1.64 Å, and the SieOeSi angle is 142.5ꢀ [13]).
The drastic change of the phase behavior for D-n (n ¼ 2, 3) was
demonstrated by the simple modification of the chemical struc-
tures in comparison of that of the compound C. The addition of the
siloxane unit resulted in the remarkable decrease in the clearing
point and also the suppression of the crystallization. Furthermore,
the SmA phases were induced by the addition of the siloxane unit.
The phase transition behavior for the dimeric series E-n (n ¼ 2,
3, 4) is described here. With regard to the dimeric siloxane-based
LCs, the interesting phase behavior was unexpectedly observed
depending on the number of the siloxane unit connected in a
central part of the spacer. The POM images, DSC chart and XRD
profile for E-2 are shown in Fig. 4. The dimeric LC (E-2) with a
shortest siloxane central unit showed the enantiotropic smectic A
phase, N* phase, and blue phase (BP), which were characterized on
the basis of the POM textures. The DSC chart displayed the three
endothermic peaks on heating. The melting point of E-2 is 140.0 ꢀC,
whereas its clearing point is 226.1 ꢀC. These results revealed that
the respective transition temperatures are higher than these of D-n
(n ¼ 2, 3). The platelet texture of the BP was observed only upon
scanning with the rate of 2.0 ꢀC minꢁ1 on the POM observation. In
the XRD profile for E-2 at 180.0 ꢀC shown in Fig. 4, a sharp peak in
angle region (2
angle region (2
q
¼ 2.53ꢀ, d ¼ 34.9 Å) and a broad peak in the wide
q
¼ 16.0ꢀ, d ¼ 5.5 Å) corresponding to the lateral
spacing between the rod-like molecules. The focal conic structure
observed in the POM image and the characteristic XRD pattern with
a very sharp reflection and a diffuse reflection of the alkyl tails were
indicative of the formation of the SmA phase.
According to the molecular modelling calculation in the
extended conformation using ChemBio3D-Ultra, employing MM2
minimal energy, the estimated molecular length (L) of the most
extended conformation is about 34.5 Å. As a result, a calculated d/L
ratio is about 1.0 (L z d), suggesting that the LC molecules are
packed in a single smectic layer as demonstrated in Fig. 3. The
hybridization of immiscible organic and inorganic components in
the molecular construction at the nanoscale generally leads to
nanosegregation, and the each site tends to gather separately.
Therefore, such molecular design in D-2 provides the nano-
segregated LC organization formed by the siloxane domains and
the ones consisting of the alkyl chains and the cholesteryl meso-
genic groups, resulting in the stable SmA phase.
The trend in the phase behavior for D-3 was similar to that for D-
2 (Figs. S2eS5). The length of the siloxane unit is a key factor
the small angle region (2
the wide angle region (2
length of the full stretched molecular structure of E-2 is calculated
to be about 31.2 Å, consequently suggesting a U-shaped confor-
mation of the dimer packed in the smectic layers [14], as illustrated
in Fig. 5.
q
¼ 2.80ꢀ, d ¼ 31.5 Å) and a broad peak in
q
¼ 16.0ꢀ, d ¼ 5.5 Å) were detected. The half
In contrast to E-2, only smectic A phase was observed for E-3
and E-4 (Figs. S2eS4). They showed the slight decrease in their
clearing points but the gradually lowered melting points as
increasing the number of the siloxane units. As a result, the wider
LC temperature range was obtained, where D
T (LC range) was 86 ꢀC
for E-2, 128 ꢀC for E-3, and 160 ꢀC for E-4, respectively as shown in
Fig. 6. The change of the melting points for E-n (n ¼ 2, 3, 4) is
resulted from the additional flexibility of the siloxane units and its
associated suppression of the crystallization, which was also
Fig. 2. (a) POM image at 150 ꢀC on heating, (b) DSC chart and (c) XRD profiles recorded
at 50 ꢀC (Cryst.), 150 ꢀC (SmA), and 190 ꢀC (Iso.) for D-2.

K. Katsuki et al. / Journal of Organometallic Chemistry 886 (2019) 34e39
37
observed for D-n (n ¼ 2, 3).
The XRD profiles in a small angle region (2
q
¼ 0e10ꢀ) obtained
for D-n (n ¼ 2, 3) and E-n (n ¼ 2, 3, 4) are displayed in Fig. 7. The
respective data were recorded in their smectic A phases. As
mentioned earlier, the d-spacings of D-2 and D-3 were 34.9 Å and
39.4 Å, respectively, and the difference of the d-spacing is attrib-
uted to the one siloxane unit. On the other hand, from the profiles
of E-n (n ¼ 2, 3, 4), it is apparent that the shift of the d-spacing
corresponding the smectic layers is smaller in comparison with D-n
(n ¼ 2, 3). This different trend can be explained by considering the
conformations of D-n (n ¼ 2, 3) and E-n (n ¼ 2, 3, 4). The rod-
shaped molecules of D-n (n ¼ 2, 3) are perpendicularly oriented
in the smectic layers. The siloxane unit was connected at the ter-
minal part of the molecule, thereby the addition of the siloxane unit
directly affected the difference in the d-spacing. In contrast, the
siloxane unit was introduced at the central part of the spacer for E-
n (n ¼ 2, 3, 4). As illustrated in Figs. 5, S6, and S7, we proposed the
U-shaped conformations of E-n (n ¼ 2, 3, 4) in the smectic layers on
the basis of the d-spacing obtained. Because of this conformation,
the increase of the siloxane unit had a small impact on the d-
spacing for E-n (n ¼ 2, 3, 4). Simultaneously, the proposed U-sha-
ped conformations were also supported by the small shift of the d-
spacing obtained by the XRD analysis. Furthermore, the longer
siloxane unit provides the dimeric molecules wider space enough
to form the U-shaped conformations. This could be linked to the
appearance of the stable smectic A phase for E-3 and E-4. By
contrast, the N* phase and the BP were observed for E-2 by the
insufficient length and space at the central part, which could be
related to the incomplete ordering (like V-shaped) between the
mesogenic groups within the molecule.
Fig. 4. (aec) POM images taken on heating, (d) DSC chart and (e) XRD profile for E-2.
The scanning rate on the POM investigation was (a, b) 5 ꢀC minꢁ1 and (c) 2 ꢀC minꢁ1
.
3.2. Influence of the siloxane units on the thermal properties
Finally, we discuss the influence of the siloxane units on the
thermal properties of the homologues having the normal alkyl
chains reported in the literature. The N* phase was observed for all
the monomeric homologues having the normal alkyl chain
(n ¼ 1e12) [15], and an odd-even effect was apparent in its clearing
temperatures. The monomeric homologues having the long alkyl
chain (n ꢂ 7) showed the smectic phases with the wider
Fig. 5. Illustration of the possible molecular packing of E-2 in the SmA phase.
Fig. 6. Phase transition behavior of the siloxane-based monomeric (D-n; n ¼ 2, 3) and dimeric LCs (E-n; n ¼ 2, 3, 4).

38
K. Katsuki et al. / Journal of Organometallic Chemistry 886 (2019) 34e39
discussed from a view point of the conformation [18]. At the higher
temperature range above the SmA phase, E-2 might form the bent
conformation because of the unstable U-shaped conformation.
4. Conclusion
Identical types of the siloxane-based monomeric and dimeric
LCs bearing the cholesteryl mesogenic groups have been synthe-
sized and characterized. The mesomorphic behavior was found to
be greatly governed by the modification using the siloxane units in
comparison of those of the corresponding LCs having the normal
alkyl chains and spacers. Furthermore, the nature of the LC phases
was associated with the nanosegregation of the constituent parts of
the molecules consisting of the cholesteryl mesogenic groups, the
alkyl chains and the siloxane unit. For all the siloxane-based LCs
obtained in this study, the SmA phases were induced by the
contribution of the siloxane units in compared with those of the
homologues having the normal alkyl spacers and tails. Especially,
the dimeric LC (E-2) with a shortest siloxane central unit showed
interestingly the BP in addition to the SmA and the N* phase. This
precise study provided some useful insights into the molecular
design of hybrid organic/inorganic LCs towards variety of nature of
LC phases.
¼ 0e10ꢀ) in the SmA phases obtained
Acknowledgment
Fig. 7. XRD profiles in a small angle region (2
for D-n (n ¼ 2, 3) and E-n (n ¼ 2, 3, 4).
q
This work was partially supported by MEXT-supported Program
for the Strategic Research Foundation at Private Universities
(2012e2016). We thank Mr. Yuta Kitajima (Ritsumeikan University)
for technical assistance with the experiments and fruitful
discussions.
temperature range as a function of the number of the carbon atoms.
The homologues having the alkyl chain (n ¼ 7, 9) showed the phase
sequences; Cryst. 129 Sm 138 N* 210 Iso. (n ¼ 7) and Cryst. 139 Sm
166 N* 194 Iso. (n ¼ 9), of which the tail length is comparable as D-n
(n ¼ 2, 3), respectively. As mentioned above, D-n (n ¼ 2, 3) showed
only the SmA phase, and the notable difference in the phase
behavior is that the N* phase disappeared for D-n (n ¼ 2, 3) and the
considerably decrease in the melting points and clearing points was
observed. This was due to a stabilization of the SmA phase probably
by a nanosegregation of the incompatible siloxane unit [16].
On the other hand, the dimeric LCs having the normal alkyl
spacers (n ¼ 4e12) had been found to show the N* phase in a
temperature range over 200 ꢀC and the odd-even effect in both the
clearing points and melting points [17]. It is common that LC dimers
exhibit a remarkable odd-even effect of the transition tempera-
tures, and the behavior has been generally explained on the basis of
the molecular shapes of the all trans-conformation of the dimers
depending on the odd- and even-membered spacers. The dimeric
LCs having the normal alkyl spacers (n ¼ 9, 11) showed the phase
sequences; Cryst. 210 N* 265 Iso. (n ¼ 9) and Cryst. 193 N* 260 Iso.
(n ¼ 11) [17], of which the spacer length is comparable as E-n (n ¼ 2,
3), respectively. (There have been no reports about the LC dimer
with the alkyl chain length comparable to that of E-4). The intro-
duction of the siloxane unit in the spacer of the dimeric LCs resulted
in the pronounced enlargement of the LC temperature range and
induced the SmA phase for E-n (n ¼ 2, 3, 4) and a narrow range of
the BP only for E-2. The apparent difference in the phase transition
behavior is the emergence of the SmA phases by the introduction of
the siloxane unit. Such higher SmA stability seen for E-n (n ¼ 3, 4) is
presumably attributed to a close interaction between the meso-
genic groups within the molecule, that would be enhanced by the
U-shaped conformation accompanying with the longer siloxane
unit in the spacer. Furthermore, the emergence of the BP was
interestingly obtained only for E-2. This is attributed to that the
length of the central siloxane moiety in the spacer of E-2 is insuf-
ficient enough to form the stable U-shaped conformations in the
SmA phase. The origin of the BPs for LC dimers has been widely
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jorganchem.2019.02.016.
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