Thioether-linked benzylideneaniline-based twist-bend nematic liquid crystal dimers: Insights into spacer lengths, mesogenic arm structures, and linkage types

Article abstract of DOI:10.1016/j.tet.2021.132351

The first examples of thioether-linked benzylideneaniline-based liquid crystal dimer homologues, namely symmetric N-(4-cyanobenzylidene)aniline-based homologues [i.e., (CN)BASnSBA(CN)] and unsymmetric N-(4-cyanobenzylidene)aniline- and 4-cyanobiphenyl-bas

Full text of DOI:10.1016/j.tet.2021.132351

Tetrahedron 95 (2021) 132351
Contents lists available at ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Thioether-linked benzylideneaniline-based twist-bend nematic liquid
crystal dimers: Insights into spacer lengths, mesogenic arm structures,
and linkage types
Yuki Arakawa*, Yuko Ishida, Kenta Komatsu, Yuto Arai, Hideto Tsuji
Department of Applied Chemistry and Life Science, Graduate School of Engineering, Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho,
Toyohashi, Aichi, 441-8580, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
The first examples of thioether-linked benzylideneaniline-based liquid crystal dimer homologues,
namely symmetric N-(4-cyanobenzylidene)aniline-based homologues [i.e., (CN)BASnSBA(CN)] and
unsymmetric N-(4-cyanobenzylidene)aniline- and 4-cyanobiphenyl-based homologues [i.e., (CN)
BASnSCB] possessing different alkylene spacers (CnH2n, n ¼ 3, 5, 6, 7, 9, and 11) were synthesized and
characterized. The odd-n (CN)BASnSBA(CN) and (CN)BASnSCB homologues formed twist-bend nematic
(NTB) phases below the temperatures of conventional nematic (N) phases upon cooling. Remarkably, the
NTB phases of the unsymmetric (CN)BASnSCB (n ¼ 3, 5, 7, and 9) homologues were supercooled below
room temperature and vitrified as the NTB glass phases. The impacts of the N-(4-cyanobenzylidene)an-
iline structure on the LC phase-transition behavior were revealed in terms of the spacer lengths,
mesogenic arm structures (or symmetry), and linkage types.
Received 23 April 2021
Received in revised form
29 June 2021
Accepted 13 July 2021
Available online 16 July 2021
Keywords:
Twist-bend nematic phase
Liquid crystal dimer
Sulfur
© 2021 Elsevier Ltd. All rights reserved.
Thioether
Schiff base
Benzylideneaniline
1. Introduction
bent core molecules [34,35] that exhibit the NTB phase. In addition
to the fact that the vast majority of the twist-bend nematogens are
Bent molecules often form unique liquid crystal (LC) phases,
which are associated with spontaneous polarization and symmetry
breaking [1e4]. The twist-bend nematic (NTB) phase based on a
heliconical aggregation of the molecular directors is the newest
among such LC phases, and has attracted much attention in soft
matter and LC fields. The possibility of this kind of phase of matter
was previously independently predicted by Meyer [5] and Dozov
[6], was simulated by Memmer [7], and was initially identified for
an unknown nematic (Nx) phase at a temperature below that of the
conventional nematic (N) phase for an achiral cyanobiphenyl-based
LC dimer with a heptamethylene spacer, viz. CB7CB [8]. To date, the
heliconical pitches have been found to be remarkably short, typi-
cally being around the length of several molecules, or ~10 nm, as
confirmed by microscopy [9,10] and resonant X-ray scattering
[11,12]. The identification and characterization of the NTB phase has
encouraged the development of a large number of bent dimers
[13e26] and less-developed oligomers [27e32], polymers [33], and
based on achiral molecules, their chiral molecules have also been
reported [36e39]. Although the typical thermal phase sequence is
in the order of Isotropic (I)eNeNTB phases upon decreasing tem-
perature, formation of the NTB phase directly from the I phase
(IeNTB phase sequence) was observed in a few rare cases [40e42].
Most importantly, all twist-bend nematogens reported to date are
related to a bent molecular geometry. Both early and more recent
theoretical works also confirm that the whole bent shapes dictate
the occurrence of the phase of matter and its detailed properties
[5e7, 43e46].
Among the various rigid mesogenic structures reported to date,
an imine- or Schiff-base-based mesogen, viz. benzylideneaniline,
has often been exploited as a component of the mesogenic arm
structures in LC dimers [47e57], some of which exhibited the NTB
phases irrespective of the direction of CH]N in the mesogenic
arms [58e63]. In particular, the 4-cyanosubstituted analog, viz. N-
(4-cyanobenzylidene)aniline, could be considered a good candidate
as a mesogenic arm for not only LC dimers but also twist-bend
nematogenic dimers. This is due to the fact that the 4-cyano sub-
stitution of a mesogenic arm of the LC dimers is likely to cause self-
assembly into anti-parallel packing via intermolecular
* Corresponding author.
E-mail address: arakawa@tut.jp (Y. Arakawa).
https://doi.org/10.1016/j.tet.2021.132351
0040-4020/© 2021 Elsevier Ltd. All rights reserved.

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
dipoleedipole interactions, such as that observed for
cyanobiphenyl-based LCs [64]. This is likely to lead to not only LC
appearance but also additional NTB phase formation for dimeric
molecules. Indeed, we previously reported that although non-
substituted and mono-4-cyano-substituted biphenyl-based LC di-
mers were non-mesogenic, their di-4-cyano-substituted analog
(the so-called cyanobiphenyl-based dimer analog) was mesogenic
and exhibited the NTB phase [62]. To the best of our knowledge, at
this time, only a few examples of N-(4-cyanobenzylidene)aniline-
based LC dimers that exhibit the NTB phase have been reported.
These are based on methylene- [58,59,63], ether-, methylene-/
ether- [63], and thioether-linked LC dimers [62], of which our group
reported a symmetric thioether-linked N-(4-cyanobenzylidene)
aniline-based LC dimer, viz. (CN)BASnSBA(CN), wherein (CN), BA, n,
and S denote a 4-substituted cyano group, N-benzylideneaniline,
the carbon atom number of the alkylene spacer, and thioether
moieties, respectively, with a heptamethylene spacer (n ¼ 7) (Fig.1)
[62]. This species forms a monotropic NTB phase with a crystalli-
zation tendency upon cooling at a typical scanning rate. However,
other homologues have yet to be developed, and hence, it is un-
known how the spacer length and the mesogenic arm symmetry
affect the phase-transition behavior and mesomorphism. For an
unsymmetric mesogenic counter arm structure, the 4-
cyanobiphenyl arm can be considered to be a potential counter
mesogenic arm structure. Such unsymmetric mesogenic arm
structures linked with thioether bonds often provide the crystal-
lized NTB LC dimers with a wide-temperature-range NTB phase and
drastic vitrifiable ability as an NTB glass (NTBG) phase [65,66].
Thus, we herein report the preparation of two homologous se-
ries of thioether-linked N-(4-cyanobenzylidene)aniline-based LC
dimers, i.e., symmetric (CN)BASnSBA(CN), and unsymmetric N-(4-
cyanobenzylidene)aniline- and 4-cyanobiphenyl-based homo-
logues, (CN)BASnSCB, which contain flexible alkylene spacers
(CnH2n, n ¼ 3, 5, 6, 7, 9, and 11). Subsequently, the phase-transition
behavior of these dimers is investigated in detail by polarized op-
tical microscopy (POM) and differential scanning calorimetry
(DSC). To understand the influence of the N-(4-cyanobenzylidene)
aniline arm structure on the resulting LC dimers, the phase-
transition properties of (CN)BASnSBA(CN) and (CN)BASnSCB are
compared with those of previously reported structurally similar
analogs, viz. cyanobiphenyl-based CBSnSCB dimers [23,25] and
azobenzene-based (CN)AzoSnSAzo(CN) and (CN)AzoSnSCB dimers
(Fig. 1) [66], and different linkage counterparts [59,63].
2. Experimental
Unless otherwise noted, all chemical reagents and solvents were
commercially
Aminobenzenethiol, 4-bromobenzenethiol, 4-cyanobenzaldehyde,
all -dibromoalkanes, and tetrakis(triphenylphosphine)palla-
available
and
used
as
received.
4-
a,u
dium(0) [Pd(PPh3)4] were purchased from Tokyo Chemical Industry
Co., Ltd. (TCI). 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)
benzonitrile was purchased from BLD Pharmatech Ltd. Cesium
carbonate (Cs2CO3) was purchased from FUJIFILM Wako Pure
Chemical Corporation. Ethanol (EtOH), dichloromethane (CH2Cl2),
and chloroform were purchased from Nacalai Tesque, Inc. Hexane,
acetone, acetonitrile, and tetrahydrofuran (THF) were purchased
from Kanto Chemical Co., Inc. The synthetic schemes of the two
homologous series are shown in Schemes 1 and 2. The symmetric
(CN)BASnSBA(CN) homologues were synthesized in reference to
the previously reported procedures for (CN)BAS7SBA(CN) [62]. The
unsymmetric (CN)BASnSCB homologues were synthesized in the
same procedures for (CN)BAS7SCB, which are representatively
described in the supporting information. The molecular structures
were characterized using 1H and 13C nuclear magnetic resonance
(NMR) spectroscopy on a JNM-ECS400 (400 MHz for 1H and
100 MHz for 13C) or a JNM-ECX500 (500 MHz for 1H NMR and
126 MHz for 13C NMR) instrument (JEOL Ltd., Tokyo, Japan) and
high-performance liquid chromatography/high resolution mass
spectrometry (HPLC/HRMS) with Agilent 1200 HPLC-Chip and 6520
Accurate-Mass Q-TOF. The thermal phase transitions and phase
identification were performed using POM with an Olympus polar-
ized optical microscope (BX50, Tokyo, Japan) and a Linkam (Surrey,
UK) temperature controller LK-600PM. The phase-transition tem-
peratures and the associated entropy changes were determined by
using DSC with a Shimadzu DSC 60 (Kyoto, Japan) at a rate of 10 ꢀC
min⁻1 under a nitrogen gas flow (50 mL min⁻1). X-ray diffraction
(XRD) measurements were conducted using a Rigaku Rint 2500
Fig. 1. Thioether-linked liquid crystal dimers that exhibited the NTB phase in (a) the previous study and (b) the present study.
2

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
Scheme 1. Synthesis of the symmetric (CN)BASnSBA(CN).
Scheme 2. Synthesis of the unsymmetric (CN)BASnSCB.
instrument with a Cu K
a
X-ray source for pristine solution-
BASnSBA(CN) and unsymmetric (CN)BASnSCB, respectively. In the
following paragraphs, we describe the phase-transition behavior of
the symmetric (CN)BASnSBA(CN), and then discuss the results ob-
tained for the unsymmetric (CN)BASnSCB.
crystallized and mesophase samples on a silicon substrate at
25 ꢀC. UVevisible absorption spectra in THF solution were recorded
on a JASCO V-630 UVevis spectrophotometer.
Upon first heating of each solution-crystallized pristine sample,
although the symmetric (CN)BASnSBA(CN) (odd n ¼ 7, 9, and 11)
formed N phases, (CN)BASnSBA(CN) (n ¼ 3, 5, and 6) did not exhibit
any mesophases. Upon first cooling from each I phase, all odd-n
(CN)BASnSBA(CN) (n ¼ 3, 5, 7, 9, and 11) dimers exhibited N and NTB
phases upon, whereas the even-n homologues (n ¼ 6) did not form
3. Results and discussion
The determined phase-transition temperatures and the scaled
entropy changes (
DS/R) upon first heating and cooling at a rate of
10 ꢀC min⁻1 are listed in Tables 1 and 2 for the symmetric (CN)
3

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
Table 1
Phase-transition temperatures (T, ꢀC) and associated entropy changes scaled by the gas constant R (
10 ꢀC minꢁ1 for (CN)BASnSBA(CN).
DS/R) upon first heating (upper line) and cooling (bottom line) at a rate of
Code
n
Phase
T (ꢀC)
DS/R
Phase
T (ꢀC)
Phase
T (ꢀC)
DS/R
Phase
(CN)BASnSBA(CN)
3a
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
Cr
142.9
81.9
148.1
81.3
189.0
176.1
123.0
57.6
124.5
66.6
13.2
4.6
e
e
I
I
I
I
I
I
I
I
I
I
I
I
NTB
e
56.0b
86.3
N
102.4
131.7
0.12
0.28
5
15.6
12.8
18.8
17.9
12.5c
9.0
NTB
e
N
e
e
N
N
N
N
N
N
6
e
c
7
e
136.8
134.4
136.3
134.2
132.5
130.2
-
NTB
e
91.4
93.4
95.0b
0.55
c
9
14.9c
12.2
16.6c
14.1
-
NTB
e
0.77
c
11
125.2
82.5
-
NTB
0.97
a
n ¼ 3 exhibited Tg ¼ 9.4 ꢀC.
b
c
Determined by POM.
Total DS/R value at melting and isotropic transitions due to insufficient peak separation.
Table 2
Phase-transition temperatures (T, ꢀC) and associated entropy changes scaled by the gas constant R (
10 ꢀC minꢁ1 of (CN)BASnSCB.
DS/R) upon first heating (upper line) and cooling (bottom line) at a rate of
Code
n
Phase
T (ꢀC)
D
S/R
Phase
T (ꢀC)
Phase
T (ꢀC)
D
S/R
Phase
(CN)BASnSCB
3
Cr
G
Cr
G
Cr
Cr
Cr
G
Cr
G
115.5
10.9
124.7
12.2
171.4
158.7
111.8
13.8
88.2
11.4
104.7
66.1
11.8
10.5
e
e
N
e
N
N
N
N
N
N
N
N
N
I
I
I
I
I
I
I
I
I
I
I
I
NTB
e
46.3
80.1
91.7
0.11
5
NTB
e
117.5
177.4
174.2
129.1
127.2
127.3
124.5
124.9
122.3
0.29
e
6
17.9a
11.8
13.2
e
2.25
0.00
0.46
0.67
0.66
1.04
1.08
7
e
NTB
e
91.4
90.2
89.7
9
11.9
NTB
e
11
Cr
Cr
17.0
12.7
NTB
a
Total DS/R value at melting and isotropic transitions due to insufficient peak separation.
discernible mesophases. Representative POM images of the NTB
phases observed for (CN)BAS9SBA(CN) are shown in Fig. 2. All odd-
n (CN)BASnSBA(CN) dimers exhibited blocky, striped, or polygonal
textures at temperatures below those of the N phases, which are
typical of the NTB phase [8,67e69]. In addition, all NTB phases of the
symmetric odd-n (CN)BASnSBA(CN) dimers crystallized during
cooling to room temperature at a rate of 10 ꢀC min⁻1. Interestingly,
during POM, the majority of the N phase domain of (CN)BAS3S-
BA(CN) underwent no crystallization, and transitioned to the NTB
phase at 56.0 ꢀC, which is the lowest such transition among the
symmetric (CN)BASnSBA(CN) homologues. This observation is good
agreement with the result that the DSC for (CN)BAS3SBA(CN)
revealed a glass transition temperature (Tg) of ~9 ꢀC. Together with
the lowest the DSIN/R of (CN)BAS3SBA(CN), this also implies a more
bent geometry for the shortest central spacer.
Subsequently, the phase-transition behavior of the unsym-
metric (CN)BASnSCB was examined. In this case, upon first heating
their solution-crystallized pristine samples, the unsymmetric (CN)
BASnSCB (n ¼ 6, 7, 9, and 11) formed N phases, whereas the (CN)
BASnSCB (n ¼ 3 and 5) did not exhibit discernible mesophases.
Fig. 2. POM images of the symmetric (CN)BAS9SBA(CN) homologues; (a) the N phase at 100 ꢀC and (b) the NTB phase at 80 ꢀC.
4

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
Specifically, (CN)BAS6SCB exhibited the appearance of an N phase
between ~171 and ~177 ꢀC only upon first heating; however, this N
phase was not observed upon second heating, with only one
melting temperature (Tm) at ~177 ꢀC to the I phase. The different
crystalline species between the solution-crystallized and thermally
crystallized (CN)BAS6SCB samples were confirmed by XRD mea-
surements, as shown in Fig. S15 (Supporting Information). In
addition , (CN)BAS5SCB exhibited an N-phase transition from the
partial cold-crystallization region at 103.7 ꢀC during the second
heating process carried out after cooling to ꢁ10 ꢀC, as confirmed by
the DSC curves and POM images [Fig. 4(b) and S6, respectively].
Upon cooling, all odd-numbered (CN)BASnSCB species formed NTB
phases at temperatures below those of the conventional N phases,
and (CN)BAS6SCB exhibited the N phase alone. The representative
POM images of the N and NTB phases observed for (CN)BASnSCB
(n ¼ 7 and 11) are shown in Fig. 3, and those of the other homo-
logues are given in the supporting information. The shortest (CN)
BAS3SCB recorded the lowest NeNTB transition temperature
(TNNTB) of 46.3 ꢀC. To our surprise, the NTB phases of (CN)BASnSCB
(n ¼ 3, 5, 7, and 9) could be supercooled to room temperature, and
were finally vitrified as the NTBG phases, which is in marked
contrast to the crystallized symmetric (CN)BASnSBA(CN) homo-
logues. The DSC curves recorded for (CN)BASnSCB (odd-n ¼ 3, 5, 7,
and 9) revealed their Tg at temperatures below those of the NTB
phases. The DSC curves for (CN)BAS11SCB showed that the NTB
phase was crystallized, as shown in Fig. S14 (supporting
information).
value of 4.2 Å represent an intermolecular liquid-like correlation.
This result is in good agreement with usual NTB phase behavior
with a pseudo layered and liquid-like correlation or an N-type
nature, thereby confirming that the NTB phase of (CN)BAS7SCB
underwent supercooling to room temperature without
crystallization.
Overall, all odd-n members for both the symmetric (CN)
BASnSBA(CN) and unsymmetric (CN)BASnSCB homologues were
found to exhibit the NTB phase, while the even-n members did not.
This was ascribed to the relatively linear or bent molecular geom-
etries for the LC dimers imposed by the alkylene spacers with even
or odd numbers of carbon atoms, respectively. It is worth noting
that the NTB phases of the asymmetric (CN)BASnSCB (n ¼ 3, 5, 7, and
9) homologues were supercooled to room temperature, and vitri-
fied as the NTBG phases. The remarkable NTB stability and vitrifi-
cation ability exhibited by the asymmetric (CN)BASnSCB
homologues were attributed to the combination of thioether link-
ages with a small bond angle for a more bent geometry, and
unsymmetric mesogenic-arm structures (especially with a cyano-
biphenyl counter arm), which promote NTB phase formation and
prevent crystallization, respectively, or mutually, as reported by our
previous papers [65,66]. Since the flexible thioether (CeSeC) group
has a smaller bond angle (~100ꢀ) than those of the methylene
(CeCH2eC, ~110ꢀ) and ether (CeOeC, ~118ꢀ) groups, the smaller
bond angle of the thioether linkage effectively contributes to build
more bent geometries for the odd-n homologues to form the NTB
phase [23]. However, in the case of even alkylene spacers, the thi-
oether linkage results in the two mesogenic arms of the LC dimers
being relatively deviated from the linear molecular axis, and hence
give rise to a greater rotational volume or steric hindrance, ulti-
mately disturbing LC formation [23,70e72]. Consequently, for the
In addition, we conducted XRD measurements for (CN)BAS7SCB
(Fig. 5). The pristine crystal sample showed many diffraction peaks.
For the NTB phase at 25 ꢀC, only a broadened peak centered close to
2
q
¼ 21.4ꢀ was observed. This broadened peak and its d-spacing
Fig. 3. POM images of the unsymmetric (CN)BASnSCB homologues; (a) the N phase at 100 ꢀC and (b) the NTB phase at 60 ꢀC for n ¼ 7; (c) the N phase at 120 ꢀC and (d) the NTB phase
at 50 ꢀC for n ¼ 11.
5

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
Fig. 4. DSC curves of (CN)BASnSCB upon first and second heating (black solid and dashed blue curves, respectively) and first cooling (red solid curve) at a rate of 10 ꢀC minꢁ1; (a)
n ¼ 3, (b) n ¼ 5, (c) n ¼ 7, and (d) n ¼ 9.
AzoSnSAzo(CN) and (CN)AzoSnSCB were quoted from our previous
papers [66]. As shown in Fig. 6, it is apparent that the value of TIN
decreased in the order of (CN)AzoSnSAzo(CN)
>
(CN)
AzoSnSCB > (CN)BASnSBA(CN) > (CN)BASnSCB > CBSnSCB. More
specifically, the TIN values of the Schiff base-based (CN)BASnS-
BA(CN) and (CN)BASnSCB were found to be between those of the
present dimers for the same value of n, i.e., lower than those of the
azobenzene-based (CN)AzoSnSAzo(CN) and (CN)AzoSnSCB dimers,
and higher than those of CBSnSCB. The TIN values of both the
symmetric azobenzene- and the Schiff base-based dimers were
clearly higher than those of the unsymmetric dimers containing a
counter cyanobiphenyl arm. The results obtained for TNNTB are also
mostly similar to the trend observed for TIN, wherein the TNNTB
values of the azobenzene-based dimers are clearly higher than
those of the Schiff base-based and cyanobiphenyl-based dimers,
whereas those of the Schiff base-based dimers are marginally
higher than or comparable to those of the cyanobiphenyl-based
dimers. Furthermore, the azobenzene-based dimers, especially
the symmetric (CN)AzoSnSAzo(CN) [66], clearly showed a tendency
to exhibit a narrow-temperature NTB phase due to the higher
crystallization tendency compared to the two Schiff base-based
dimers and the biphenyl-based CBSnSCB. In terms of the sym-
metric dimers, cyanobiphenyl-based CBS7SCB clearly exhibited an
enhanced vitrifiable tendency compared with the crystallized
azobenzene-based (CN)AzoS7SAzo(CN) and the Schiff base-based
(CN)BASnSBA(CN) [23,66]. These results could correspond to the
Fig. 5. One dimensional-XRD profiles of (CN)BAS7SCB for pristine Cr phase at 25 ꢀC
(top) and the NTB phase at 25 ꢀC (bottom).
thioether-linked LC dimers, the odd-n homologues exhibited well-
defined LC phases compared with those observed for the even-n
homologues [23]. Indeed, in contrast to the present non-mesogenic
(CN)BAS6SBA(CN) and the monotropic (CN)BAS6SCB, it is known
that methylene- and ether-linked analogs with an even spacer do
exhibit enantiotropic LC phases [58,59,63]. Furthermore, the long
alkylene spacer endows the LC dimers with enantiotropic LC pha-
ses, which can be attributed to the increasing molecular anisotropy
as well as the improved flexibility. As can be seen in Table 1, the
DS/
order of the
p-conjugation length and the dipole moment of the
R value at the IeN transition ( SIN/R) increases with increasing
D
mesogenic arms, i.e., azobenzene > benzylideneaniline > biphenyl,
as discussed below.
spacer length or n; it was therefore assumed that the molecular
anisotropy increased with increasing n.
The UVevisible spectra of the dimers in dilute THF solutions are
shown in Fig. 7. It was assumed that the absorption wavelengths
To discuss the influence of the N-(4-cyanobenzylidene)aniline
structure on the phase-transition behavior of the LC dimers, the
IeN transition temperature (TIN), the TNNTB, the Tg, and the crys-
tallization temperature (TCr) upon cooling of the (CN)BASnSBA(CN),
(CN)BASnSCB, (CN)AzoSnSAzo(CN), (CN)AzoSnSCB, and CBSnSCB
dimers were plotted in Fig. 6 as a function of n [23,25,66]. In the
present study, the phase-transition data of CBSnSCB (n ¼ 3, 5, 7, 9,
and 11) were recorded upon first heating and cooling at a rate of
10 ꢀC min⁻1 (Table S1). The data for azobenzene-based (CN)
which correspond to the
p-conjugation length for each ground
state of the mesogenic arm structures decreased in the following
order: 4-cyanoazobenzene > N-(4-cyanobenzylidene)aniline > 4-
cyanobiphenyl. The absorption peaks of the present dimers
appeared to be red-shifted compared to those of the conventional
non-substituted core structures due to the presence of the sulfur-
containing weak thioether donor and the cyano group acceptor.
6

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
Fig. 6. (a) TIN and (b) TNNTB values of (CN)BASnSBA(CN) (or BAeBA), (CN)BASnSCB (or BAeCB), (CN)AzoSnSAzo(CN) (or AzoeAzo), (CN)AzoSnSCB (or AzoeCB), and CBSnSCB (or
CBeCB), as a function of odd n. The values for (CN)AzoSnSAzo(CN) and (CN)AzoSnSCB were obtained from the literature [23,66]. In the figures, the dimers are represented by each
abbreviation which is explained in the above parentheses.
mesogenic arm fragments, which are spread over the long molec-
ular axes, also correspond to the order of 4-cyano-4ʹ-methyl-
thioazobenzene (6.70) > N-(4ʹ-cyanobenzylidene-4-methylthio)
aniline (5.69) > 4-cyano-4ʹ-methylthiobiphenyl (4.31), for which
the data was collected from our previous study [62]. In addition to
the UVevis spectra, the determined dipole moment orders imply
that the magnitude of the intermolecular attractive interaction
caused by the dipoleedipole interaction should follow the order
azobenzene dimers > benzylideneaniline dimers > biphenyl di-
mers, thereby supporting observations made for their phase tran-
sition behavior.
The Tg values of the Schiff base-based (CN)BASnSCB homologues
were found to be marginally lower than those of the azobenzene-
based (CN)AzoSnSCB and biphenyl-based CBSnSCB species. The Tg
values of LC dimers are usually influenced to a lesser extent than
the TIN values by the
marginally lower Tg value for (CN)BASnSCB could be attributed to a
reduced -conjugation (or fewer dipole moments) and may be
counterbalanced by the unsymmetric mesogenic arms in compar-
ison with the unsymmetric azobenzene-based (CN)AzoSnSCB and
the symmetric biphenyl-based CBSnSCB respectively. On the other
p-conjugation and molecular anisotropy. The
Fig. 7. Non-normalized UVeVisible spectra of in THF solution.
p
Hence, the UVevisible absorption bands derived from the benzy-
lideneaniline and azobenzene arms in the present dimers were
assigned in reference to that of the cyanobiphenyl dimer CBS7SCB.
As shown in Fig. 7, CBS7SCB exhibits a main absorption peak at
hand, the
DSIN/R values of the five homologous series were rela-
312 nm. This absorption was ascribed to the pep* transition of the
tively similar to those reported in the literature for the same value
of n, and increased upon increasing n. These trends imply that the
molecular biaxiality in the vicinity of the NeI phase transition is
relatively consistent for the various analogous homologues at the
same value of n, but increases with decreasing n. These trends are
consistent with the fact that the molecular biaxiality of an LC dimer
is predominantly influenced by the linkage type and spacer length
rather than the mesogenic arm structure [16,65,66]. In the present
study, since the compared five dimer homologues possess the same
thioether linkages, the differences in the TIN values observed for the
LC dimer homologues could be predominantly attributed to the
mesogenic arm structures and their number.
Finally, we compared the phase-transition behavior of sym-
metric 4-cyanobenzylideneaniline-based LC dimers with different
linkages (X, see Table 3), methylene (CH2, abbreviated as C), ether
(O) and thioether (S) with the same total spacer length 7, as shown
in Table 3 and Fig. 8. The Tm, NeI phase transition temperature (TNI),
cyanobiphenyl moiety, considering the peak observed at 300 nm
for n-octyl-4ʹ-cyanobiphenyl (8CB) in solution, which contained no
apparent vibronic structures [73]. For the two Schiff base-based
dimers, a main band close to 365 nm were observed, which were
attributed to the
[74,75]. Another peak at 265 nm of (CN)BASnSBA(CN) was assigned
to the * transition of benzylideneaniline moiety [74,75]. This
pep* transition of the benzylideneaniline moiety
sep
peak was ambiguous for the asymmetric (CN)BASnSCB, due to the
lack of one benzylidene arm. For the two azobenzene-based di-
mers, a main peak close to 390 nm were observed, which were
attributed to the
More specifically, the
p
e
p
pep
* transition band of the trans-isomer [76].
* transition bands of the trans-isomers of
the azobenzene-based (CN)AzoSnSAzo(CN) and (CN)AzoSnSCB di-
mers were shifted to higher wavelengths than those of non-
substituted azobenzene due to the presence of a weak electron-
donating thioether group and an electron-withdrawing cyano
group, and were found to overlap partially with the nep* transi-
and their associated
DS/R values upon heating for the compounds
tion. The above observations for the 4-cyanobenzylideneaniline-
and 4-cyanoazobenzen-based dimers are similar to those obtained
for the aminoazobenzenes and similar compounds based on a weak
push-pull system [76]. Hence, in terms of the absorption spectra,
the thioether- and 4-cyano-based benzylideneanilines and the 4-
cyanoazobenzens could be classified into an aminoazobenzene-
type spectroscopic class. In addition, the dipole moments of the
bearing heptamethylene (C7) and oxypentyloxy (OC5O) spacers (i.e.,
CN-7-CN and CNeO5OeCN, respectively, in the literature) were
extracted from the literature [63]. Since X ¼ S [i.e., (CN)BAS5S-
BA(CN)] in the present study exhibited the monotropic LC behavior
as described earlier, information relating to the transition between
the N and I phases was taken from the cooling data. Hence, it is
noted that although the TIN of S (132 ꢀC) is seemingly much lower
7

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
Table 3
with carbon atom numbers of n ¼ 3, 5, 6, 7, 9, and 11. It was revealed
that all bent symmetric (CN)BASnSBA(CN) and unsymmetric (CN)
BASnSCB homologues with odd n ¼ 3, 5, 7, 9, and 11 formed the NTB
phases below the conventional N phases upon cooling. In contrast,
even-n symmetric (CN)BAS6SBA(CN) exhibited no discernible
mesophases, and unsymmetric (CN)BAS6SCB formed an N phase. It
is noteworthy that (CN)BASnSCB (n ¼ 3, 5, 7, and 9) formed NTB
phases over a wide temperature range, which were vitrified upon
cooling below room temperature to give NTBG phases. This prom-
inent stability of the NTB phase was attributed to a supercooling
effect assisted by the combination of the unsymmetric mesogenic
arm structures and the molecular bend resulting from the thioether
linkages. The phase-transition temperatures were higher for the
symmetric (CN)BASnSBA(CN) dimers than for the unsymmetric
(CN)BASnSCB dimers. Overall, the present Schiff base-based LC di-
mers exhibited intermediate phase-transition temperatures
compared with those of the structurally similar 4-
cyanoazobenzene-based and 4-cyanobiphenyl-based analogs,
which can be assumed to correspond to the differences in their
intermolecular interactions. Furthermore, their phase transitions
were evaluated in terms of the different linkage types, such as
methylene and ether bonds. The present study therefore adds new
molecular structural insights with respect to N-(4-
cyanobenzylidene)aniline-based LC dimer chemistry from the
viewpoint of the spacer length, the mesogenic arm structure (or
symmetry), and the linkage type.
The phase-transition properties of the symmetric 4-cyanobenzylideneaniline-based
LC dimers containing different linkages X ¼ C (CH2), O, and S upon heating. Those for
X ¼ C and O has been taken from the literature [63].
X
Tm (ꢀC)
TNNTB (ꢀC)a
TNI (ꢀC)
D
SCr/R
DSNI/
R
C (CH2)
O
S
127
176
148.1
124
e
154
10.1
14.7
15.6
0.22
0.71
222
86.3
131.7a
0.28a
a
Data upon cooling.
Declaration of competing interest
The authors declare no conflict of interest.
Acknowledgements
This work was financially supported by the Japan Society for the
Promotion of Science KAKENHI (grant numbers 17K14493 and
20K15351), the Naito Research Grant, and research grants from the
Toukai Foundation for Technology and Toyohashi University of
Technology. We would like to thank Ms. Tsugumi Shiokawa and Dr.
Hiroko Tada in the Division of Instrumental Analysis (Okayama
University) for carrying out the mass measurements. We would
also like to acknowledge the support of the Cooperative Research
Facility Centre at Toyohashi University of Technology for the XRD
measurements.
Fig. 8. The molecular structure and phase-transition temperatures of the symmetric 4-
cyanobenzylideneaniline-based LC dimers containing different linkages X ¼ C (CH2), O,
and S, as a function of the linkages type.
than TNI of C (154 ꢀC), the former would exhibit a supercooling
effect upon cooling. Overall, it is clear that the phase-transition
temperatures and
DSNI/R are significantly higher for O than for C
and S. Indeed, the values for C and S depend on the phases, and
these results are consistent with usual trends observed for calam-
itic LCs, where the presence of an ether bond with a larger bond
angle causes calamitic LCs to be more linear or less sterically hin-
dered compared with the methylene and thioether counterparts,
which possess smaller bond angles and greater flexibility [70e72].
In addition, although the ether-based dimer did not appear to form
the NTB phase [63], the methylene- and thioether-linked dimers did
form this phase [59,62]. Such a difference can also be ascribed to
the different geometries caused by the linkage types, i.e., the rela-
tively more linear geometry for the ether dimer, and the bent ge-
ometries for the methylene- and thioether-linked dimers. It was
found that the phase-transition behavior of benzylideneaniline-
based LC dimers would supports the expected molecular geome-
tries in terms of different linkage bond types: C, O, and S.
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.tet.2021.132351.
References
[1] T. Niori, T. Sekine, J. Watanabe, T. Furukawa, H. Takezoe, Distinct ferroelectric
smectic liquid crystals consisting of banana shaped achiral molecules, J. Mater.
Chem. 6 (7) (1996) 1231e1233.
[2] D.R. Link, G. Natale, R.F. Shao, J.E. Maclennan, N.A. Clark, E. Korblova,
D.M. Walba, Spontaneous formation of macroscopic chiral domains in a fluid
smectic phase of achiral molecules, Science 278 (5345) (1997) 1924e1927.
[3] T. Sekine, T. Niori, J. Watanabe, T. Furukawa, S.W. Choi, H. Takezoe, Sponta-
neous helix formation in smectic liquid crystals comprising achiral molecules,
J. Mater. Chem. 7 (8) (1997) 1307e1309.
4. Conclusions
[4] S.P. Sreenilayam, Y.P. Panarin, J.K. Vij, V.P. Panov, A. Lehmann, M. Poppe,
M. Prehm, C. Tschierske, Spontaneous helix formation in non-chiral bent-core
liquid crystals with fast linear electro-optic effect, Nat. Commun. 7 (2016)
11369.
[5] R.B. Meyer, in: R. Balian, G. Weil (Eds.), Structural Problems in Liquid Crystal
Physics, Molecular Fluids: summer School in Theoretical Physics, les Houches
lectures 1973, Gordon and Breach, New York, 1976, pp. 271e343.
[6] I. Dozov, On the spontaneous symmetry breaking in the mesophases of achiral
banana-shaped molecules, Europhys. Lett. 56 (2) (2011) 247e253.
We established the first examples of two homologous series of
thioether-linked Schiff base, N-(4-cyanobenzylidene)aniline-based
LC dimers, namely symmetric N-(4-cyanobenzylidene)aniline-
based homologues, (CN)BASnSBA(CN), and unsymmetric N-(4-
cyanobenzylidene)aniline- and 4-cyanobiphenyl-based homo-
logues, (CN)BASnSCB, that possess flexible alkylene spacers (CnH2n
)
8

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
[7] R. Memmer, Liquid crystal phases of achiral banana-shaped molecules: a
computer simulation study, Liq. Cryst. 29 (4) (2002) 483e496.
crystal dimers and trimers: alternate induction of smectic and twist-bend
nematic phases depending on spacer parity for liquid crystal trimers, J. Mol.
Liq. 326 (2021) 115319.
[8] M. Cestari, S. Diez-Berart, D.A. Dunmur, A. Ferrarini, M.R. de la Fuente,
D.J.B. Jackson, D.O. Lopez, G.R. Luckhurst, M.A. Perez-Jubindo, R.M. Richardson,
J. Salud, B.A. Timimi, H. Zimmermann, Phase behavior and properties of the
liquid-crystal dimer 100,700-bis (4-cyanobiphenyl-40-yl) heptane: a twist-bend
nematic liquid crystal, Phys. Rev. E. 84 (3) (2011) 31704.
[9] D. Chen, J.H. Porada, J.B. Hooper, A. Klittnick, Y. Shen, M.R. Tuchband,
E. Korblova, D. Bedrov, D.M. Walba, M.A. Glaser, J.E. Maclennan, N.A. Clark,
Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of
[33] W.D. Stevenson, J. An, X.B. Zeng, M. Xue, H.X. Zou, Y.S. Liu, G. Ungar, Twist-
bend nematic phase in biphenylethane-based copolyethers, Soft Matter 14
(2018) 3003.
[34] D. Chen, M. Nakata, R. Shao, M.R. Tuchband, M. Shuai, U. Baumeister,
W. Weissflog, D.M. Walba, M.A. Glaser, J.E. Maclennan, N.A. Clark, Twist-bend
heliconical chiral nematic liquid crystal phase of an achiral rigid bent-core
mesogen, Phys. Rev. E 89 (2014) 22506.
achiral molecular dimers, Proc. Natl. Acad. Sci. U. S.
A
110 (2013)
[35] P. Sreenilayam, V.P. Panov, J.K. Vij, G. Shanker, The NTB phase in an achiral
asymmetrical bent-core liquid crystal terminated with symmetric alkyl
chains, Liq. Cryst. 44 (2017) 244e253.
[36] A. Zep, S. Aya, K. Aihara, K. Ema, D. Pociecha, K. Madrak, P. Bernatowicz,
H. Takezoe, E. Gorecka, Multiple nematic phases observed in chiral mesogenic
dimers, J. Mater. Chem. C 1 (2013) 46e49.
[37] E. Gorecka, N. Vaupotic, A. Zep, D. Pociecha, J. Yoshioka, J. Yamamoto,
H. Takezoe, A twist-bend nematic (NTB) phase of chiral materials, Angew.
Chem. Int. Ed. 54 (2015) 10155e10159.
[38] R.J. Mandle, J.W. Goodby, Optically active bimesogens incorporating branched
central spacers, RSC Adv. 8 (2018) 18542e18548.
[39] R. Walker, D. Pociecha, J.M.D. Storey, E. Gorecka, C.T. Imrie, The chiral twist-
bend nematic phase (N*TB), Chem. Eur J. 25 (2019) 13329e13335.
[40] A.A. Dawood, M.C. Grossel, G.R. Luckhurst, R.M. Richardson, B.A. Timimi,
N.J. Wells, Y.Z. Yousif, On the twist-bend nematic phase formed directly from
the isotropic phase, Liq. Cryst. 43 (1) (2016) 2e12.
[41] C.T. Archbold, E.J. Davis, R.J. Mandle, S.J. Cowling, J.W. Goodby, Chiral dopants
and the twist-bend nematic phaseeinduction of novel mesomorphic behav-
iour in an apolar bimesogen, Soft Matter 11 (38) (2015) 7547e7557.
[42] Y. Arakawa, K. Komatsu, T. Shiba, H. Tsuji, Methylene- and thioether-linked
cyanobiphenyl-based liquid crystal dimers CBnSCB exhibiting room temper-
15931e15936.
ꢀ
[10] V. Borshch, Y.-K. Kim, J. Xiang, M. Gao, A. Jakli, V.P. Panov, J.K. Vij, C.T. Imrie,
M.G. Tamba, G.H. Mehl, O.D. Lavrentovich, Nematic twist-bend phase with
nanoscale modulation of molecular orientation, Nat. Commun. 4 (2013) 2635.
[11] C. Zhu, M.R. Tuchband, A. Young, M. Shuai, A. Scarbrough, D.M. Walba,
J.E. Maclennan, C. Wang, A. Hexemer, N.A. Clark, Resonant carbon K-edge soft
X-ray scattering from lattice-free heliconical molecular ordering: soft dilative
elasticity of the twist-bend liquid crystal phase, Phys. Rev. Lett. 116 (2016)
147803.
ꢀ
ꢀ
[12] M. Salamonczyk, R.J. Mandle, A. Makal, A. Liebman-Pelaez, J. Feng,
J.W. Goodby, C. Zhu, Double helical structure of the twist-bend nematic phase
investigated by resonant X-ray scattering at the carbon and sulfur K-edges,
Soft Matter 14 (48) (2018) 9760e9763.
[13] M. Sepelj, U. Baumeister, T. Ivsic, A. Lesac, Effects of geometry and electronic
structure on the molecular self-assembly of naphthyl-based dimers, J. Phys.
Chem. B 117 (29) (2013) 8918e8929.
ꢀ
ꢀ
ꢀ
[14] N. Sebastian, D.O. Lopez, B. Robles-Hernandez, M.R. de la Fuente, J. Salud,
ꢀ
M.A. Perez-Jubindo, D.A. Dunmur, G.R. Luckhurst, D.J.B. Jackson, Dielectric,
calorimetric and mesophase properties of 100-(20,4-difluorobiphenyl-40-
yloxy)-900-(4-cyanobiphenyl-40-yloxy) nonane: an odd liquid crystal dimer
with a monotropic mesophase having the characteristics of a twist-bend
nematic phase, Phys. Chem. Chem. Phys. 16 (2014) 21391e21406.
[15] R.J. Mandle, E.J. Davis, C.T. Archbold, C.C.A. Voll, J.L. Andrews, S.J. Cowling,
J.W. Goodby, Apolar bimesogens and the incidence of the twist-bend nematic
phase, Chem. Eur. J. 21 (2015) 8158e8167;
ature twist-bend nematic phases and glasses, Mater. Adv.
2 (2021)
1760e1773.
[43] C. Greco, G.R. Luckhurst, A. Ferrarini, Molecular geometry, twist-bend nematic
phase and unconventional elasticity: a generalised maier-saupe theory, Soft
Matter 10 (2014) 9318e9323.
[14a] Z. Ahmed, C. Welch, G.H. Mehl, The design and investigation of the self-
[44] N. Vaupotic, M. Cepic, M.A. Osipov, E. Gorecka, Flexoelectricity in chiral
nematic liquid crystals as a driving mechanism for the twist-bend and splay-
bend modulated phases, Phys. Rev. E 89 (2014) 30501.
[45] N. Vaupotic, S. Curk, M.A. Osipov, M. Cepic, H. Takezoe, E. Gorecka, Short-
range smectic fluctuations and the flexoelectric model of modulated nematic
liquid crystals, Phys. Rev. E 93 (2016) 22704.
assembly of dimers with two nematic phases, RSC Adv.
5 (2015)
93513e93521.
ꢁ
ꢁ
ꢁ
[16] R.J. Mandle, C.C.A. Voll, D.J. Lewis, J.W. Goodby, Etheric bimesogens and the
twist-bend nematic phase, Liq. Cryst. 4 (2016) 13e21.
ꢁ ꢀ
ꢀ
ꢁ
ꢀ
[17] T. Ivsic, M. Vinkovic, U. Baumeister, A. Mikleusevic, A. Lesac, Towards un-
derstanding the NTB phase: a combined experimental, computational and
spectroscopic study, RSC Adv. 6 (6) (2016) 5000e5007.
[46] L. Longa, W. Tomczyk, Twistebend nematic phase from the landauede Gen-
nes perspective, J. Phys. Chem. C 124 (41) (2020) 22761e22775.
ꢁ ꢀ
ꢁ
ꢀ
[18] T. Ivsic, U. Baumeister, I. Dokli, A. Mikleusevic, A. Lesac, Sensitivity of the NTB
phase formation to the molecular structure of imino-linked dimers, Liq. Cryst.
44 (1) (2017) 93e105.
[47] J.-I. Jin, J.-H. Park, Thermotropic compounds with two terminal mesogenic
units and a central spacer. 51. Homologous series of polymethylene-
a, u-bis
(p-Oxybenzylidene aniline), Mol. Cryst. Liq. Cryst. 110 (1e4) (1984) 293e308.
[19] R.J. Mandle, C.T. Archbold, J.P. Sarju, J.L. Andrews, J.W. Goodby, The de-
pendency of nematic and twist-bend mesophase formation on bend angle, Sci.
Rep. 6 (2016) 36682.
[20] D.A. Paterson, J.P. Abberley, W.T.A. Harrison, J.M.D. Storey, C.T. Imrie, Cyano-
biphenyl-based liquid crystal dimers and the twist-bend nematic phase, Liq.
Cryst. 44 (2017) 127e146.
[48] J.L. Hogan, C.T. Imrie, G.R. Luckhurst, Asymmetric dimeric liquid crystals the
preparation and properties of the
a-(4-cyanobiphenyl-4'-oxy)-u-(4-n-alky-
lanilinebenzylidene-4'-oxy)hexanes, Liq. Cryst. 3 (5) (1988) 645e650.
[49] G.S. Attard, S. Garnett, C.G. Hickman, C.T. Imrie, L. Taylor, Asymmetric dimeric
liquid crystals with charge transfer groups, Liq. Cryst. 7 (4) (1990) 495e508.
[50] V. Prasad, D.S.S. Rao, S.K. Prasad, Liquid crystalline dimeric compounds with
an alkylene spacer, Liq. Cryst. 28 (5) (2001) 761e767.
ꢁ
ꢀ
ꢁ
[21] A. Knezevic, M. Sapunar, A. Buljan, I. Dokli, Z. Hamersak, D. Kontrec, A. Lesac,
Fine-tuning the effect of interactions on the stability of the NTB phase,
p
ep
[51] C.V. Yelamaggad, S. Anitha Nagamani, Uma S. Hiremath, D.S. Shankar Rao,
S. Krishna Prasad, Salicylaldimine-based symmetric dimers: synthesis and
thermal behaviour, Liq. Cryst. 29 (2002) 1401e1408.
Soft Matter 14 (42) (2018) 8466e8474.
[22] K. Watanabe, T. Tamura, S. Kang, M. Tokita, Twist bend nematic liquid crystals
prepared by one-step condensation of 4-(4-Pentylcyclohexyl) benzoic acid
and alkyl diol, Liq. Cryst. 45 (2018) 924e930.
[23] Y. Arakawa, K. Komatsu, H. Tsuji, Twist-bend nematic liquid crystals based on
thioether linkage, New J. Chem. 43 (2019) 6786.
€
[52] R. Achten, A. Koudijs, Z. Karczmarzyk, A.T.M. Marcelis, E.J.R. Sudholter, Liquid
crystalline properties of salicylaldimine-based dimers: influence of terminal
alkyl chain length and central part, Liq. Cryst. 31 (2) (2004) 215e227.
[53] G.Y. Yeap, T.C. Hng, S.Y. Yeap, E. Gorecka, M. Ito, K. Ueno, M. Okamoto,
W.A.K. Mahmood, C.T. Imrie, Why do non-symmetric dimers intercalate? The
[24] Y. Arakawa, H. Tsuji, Selenium-linked liquid crystal dimers for twist-bend
nematogens, J. Mol. Liq. 289 (2019) 111097.
synthesis and characterisation of the
oxy)-
-(2-methylbutyl-40-(400-phenyl) benzoateoxy) alkanes, Liq. Cryst. 36
(12) (2009) 1431e1441.
a
-(4-benzylidene-substituted-aniline-40-
ꢀ
[25] E. Cruickshank, M. Salamonczyk, D. Pociecha, G.J. Strachan, J.M.D. Storey,
u
C. Wang, J. Feng, C. Zhu, E. Gorecka, C.T. Imrie, Sulfur-linked cyanobiphenyl-
based liquid crystal dimers and the twist-bend nematic phase, Liq. Cryst. 46
(2019) 1595.
[54] L. Marin, A. Zabulica, M. Sava, New symmetric azomethinic dimer: the influ-
ence of structural heterogeneity on the liquid crystalline behaviour, Liq. Cryst.
38 (2011) 433e440.
[55] L. Marin, A. Zabulica, M. Sava, Symmetric liquid crystal dimers containing a
luminescent mesogen: synthesis, mesomorphic behavior, and optical prop-
erties, Soft Mater. 11 (2013) 32e39.
[26] R.J. Mandle, J.W. Goodby, Molecular flexibility and bend in semi-rigid liquid
crystals: implications for the heliconical nematic ground state, Chem. Eur J. 25
(2019) 14454.
[27] R.J. Mandle, J.W. Goodby, A liquid crystalline oligomer exhibiting nematic and
twist-bend nematic mesophases, ChemPhysChem 17 (2016) 967e970.
[28] F.P. Simpson, R.J. Mandle, J.N. Moore, J.W. Goodby, Investigating the Cusp
between the nano-and macro-sciences in supermolecular liquid-crystalline
twist-bend nematogens, J. Mater. Chem. C 5 (2017) 5102e5110.
[29] S.M. Jansze, A. Martínez-Felipe, J.M. Storey, A.T. Marcelis, C.T. Imrie, A twist-
bend nematic phase driven by hydrogen bonding, Angew. Chem. 127 (2)
(2015) 653e656.
[56] G.-Y. Yeap, H.-C. Lee, W.A.K. Mahmood, C.T. Imrie, D. Takeuchi, K. Osakada,
Synthesis, thermal and optical behaviour of non-symmetric liquid crystal
dimers
a-(4-benzylidene-substituted-aniline-49-oxy)-x-[pentyl-4-(49-
phenyl)benzoateoxy]hexane, Phase Transitions 84 (2011) 29e37.
[57] E. Perju, L. Marin, Mesomorphic behavior of symmetric azomethine dimers
containing different chromophore groups, Molecules 26 (8) (2021) 2183.
[58] P.A. Henderson, O. Niemeyer, C.T. Imrie, Methylene-linked liquid crystal di-
mers, Liq. Cryst. 28 (2001) 463e472.
[59] P.A. Henderson, C.T. Imrie, Methylene-linked liquid crystal dimers and the
twist-bend nematic phase, Liq. Cryst. 38 (11e12) (2011) 1407e1414.
[60] A.N. Scarbrough, M.R. Tuchband, E.D. Korblova, R. Shao, Y. Shen,
J.E. Maclennan, M.A. Glaser, N.A. Clark, D.M. Walba, The heliconical nematic
twist-bend phase from “classic” bent-core benzylideneanilines with oligo-
methylene cores, Mol. Cryst. Liq. Cryst. 647 (1) (2017) 430e438.
[30] Y. Wang, G. Singh, D.M. Agra-Kooijman, M. Gao, H. Krishna Bisoyi, C. Xue,
M.R. Fisch, S. Kumar, Q. Li, Room temperature heliconical twist-bend nematic
liquid crystal, CrystEngComm 17 (2015) 2778.
[31] R. Saha, G. Babakhanova, Z. Parsouzi, M. Rajabi, P. Gyawali, C. Welch,
ꢀ
G.H. Mehl, J. Gleeson, O.D. Lavrentovich, S. Sprunt, A. Jakli, Oligomeric
oddeeven effect in liquid crystals, Mater. Horiz. 6 (2019) 1905e1912.
[32] Y. Arakawa, K. Komatsu, T. Shiba, H. Tsuji, Phase behaviors of classic liquid
9

Y. Arakawa, Y. Ishida, K. Komatsu et al.
Tetrahedron 95 (2021) 132351
[61] D.A. Paterson, C.A. Crawford, D. Pociecha, R. Walker, J.M. Storey, E. Gorecka,
C.T. Imrie, The role of a terminal chain in promoting the twist-bend nematic
phase: the synthesis and characterisation of the 1-(4-cyanobiphenyl-40-yl)-6-
(4-alkyloxyanilinebenzylidene-40-oxy) hexanes, Liq. Cryst. 45 (13e15) (2018)
2341e2351.
studies of the nematic NTB phase of 1,11-di-(100-cyanobiphenyl-4-yl) unde-
cane, J. Mater. Chem. C 2 (2014) 556e566.
[69] J. Zhou, W. Tang, Y. Arakawa, H. Tsuji, S. Aya, Viscoelastic properties of a
thioether-based heliconical twistebend nematogen, Phys. Chem. Chem. Phys.
22 (17) (2020) 9593e9599.
[62] Y. Arakawa, K. Komatsu, S. Inui, H. Tsuji, Thioether-linked liquid crystal di-
mers and trimers: the twist-bend nematic phase, J. Mol. Struct. 1199 (2020)
126913.
[63] E. Forsyth, D.A. Paterson, E. Cruickshank, G.J. Strachan, E. Gorecka, R. Walker,
J.M.D. Storey, C.T. Imrie, Liquid crystal dimers and the twist-bend nematic
phase: on the role of spacers and terminal alkyl chains, J. Mol. Liq. 320 (2020)
114391.
[64] A. Leadbetter, R. Richardson, C. Colling, The structure of a number of nema-
togens, J. Phys. Colloq. 36 (C1) (1975) 37e43.
[65] Y. Arakawa, Y. Ishida, H. Tsuji, Ether- and thioether-linked naphthalene-based
liquid-crystal dimers: influence of chalcogen linkage and mesogenic-arm
symmetry on the incidence and stability of the twist-bend nematic phase,
Chem. Eur J. 26 (2020) 3767e3775.
[66] Y. Arakawa, K. Komatsu, Y. Ishida, H. Tsuji, Thioether-linked azobenzene-
based liquid crystal dimers exhibiting the twist-bend nematic phase over a
wide temperature range, Liq. Cryst. (2020), https://doi.org/10.1080/
02678292.2020.1800848.
[67] V.P. Panov, M. Nagaraj, J.K. Vij, Y.P. Panarin, A. Kohlmeier, M.G. Tamba,
R.A. Lewis, G.H. Mehl, Spontaneous periodic deformations in nonchiral planar-
aligned bimesogens with a nematic-nematic transition and a negative elastic
constant, Phys. Rev. Lett. 105 (16) (2010) 167801.
[70] Y. Arakawa, S. Kang, H. Tsuji, J. Watanabe, G. Konishi, Development of novel
bistolane-based liquid crystalline molecules with an alkylsulfanyl group for
highly birefringent materials, RSC Adv. 6 (2016) 16568e16574.
[71] Y. Arakawa, H. Tsuji, Phase transitions and birefringence of bistolane-based
nematic molecules with an alkyl, alkoxy and alkylthio group, Mol. Cryst.
Liq. Cryst. 647 (1) (2017) 422e429.
[72] Y. Arakawa, Y. Sasaki, H. Tsuji, Supramolecular hydrogen-bonded liquid
crystals based on 4-n-alkylthiobenzoic acids and 4,40-bipyridine: their
mesomorphic behavior with comparative study including alkyl and alkoxy
counterparts, J. Mol. Liq. 280 (2019) 153e159.
[73] A. Shah, M.S. Sannaikar, S.R. Inamdar, B. Duponchel, R. Douali, D.P. Singh,
Photoluminescence modulation in the graphene oxide dispersed 4-n-octyl-4'-
cyanobiphenyl molecular system, J. Lumin. 226 (2020) 117509.
ꢀ
[74] H.H. Jaffe, S.-J. Yeh, R.W. Gardner, The electronic spectra of azobenzene de-
rivatives and their conjugate acids, J. Mol. Spectrosc. 2 (1958) 120e136.
€
[75] L.R. Knopke, A. Spannenberg, A. Brückner, U. Bentrup, The influence of sub-
stituent effects on spectroscopic properties examined on benzylidene aniline-
type imines, Spectrochim. Acta Mol. Biomol. Spectrosc. 95 (2012) 18e24.
[76] Z. Mahimwalla, K.G. Yager, J. Mamiya, A. Shishido, A. Priimagi, C.J. Barrett,
Azobenzene photomechanics: prospects and potential applications, Polym.
Bull. 69 (2012) 967e1006.
[68] R.J. Mandle, E.J. Davis, C.T. Archbold, S.J. Cowling, J.W. Goodby, Microscopy
10