Thioether-linked liquid crystal dimers and trimers: The twist-bend nematic phase

Article abstract of DOI:10.1016/j.molstruc.2019.126913

Systematic synthesis of thioether-linked dimers and trimers was carried out to reveal molecular designs for inducing mesophases and twist-bend nematic (N_TB) phases. A five-fold approach based on molecular structural parameters including the ter

Full text of DOI:10.1016/j.molstruc.2019.126913

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Thioether-linked liquid crystal dimers and trimers: The twist-bend nematic phase
Yuki Arakawa, Kenta Komatsu, Satoyoshi Inui, Hideto Tsuji
PII:
S0022-2860(19)31004-X
DOI:
https://doi.org/10.1016/j.molstruc.2019.126913
MOLSTR 126913
Reference:
To appear in: Journal of Molecular Structure
Received Date: 24 June 2019
Revised Date: 5 August 2019
Accepted Date: 8 August 2019
Please cite this article as: Y. Arakawa, K. Komatsu, S. Inui, H. Tsuji, Thioether-linked liquid crystal
dimers and trimers: The twist-bend nematic phase, Journal of Molecular Structure (2019), doi: https://
doi.org/10.1016/j.molstruc.2019.126913.
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Dimers
Timers
Oligomeric effect
S
S
7
CN
NC
S
S
7
CBS7SBS7SCB
O
S
7
CN
CN
NC
NC
S
O
7
CBO7SBS7OCB
S
O
7
O
S
7
CBS7OBO7SCB
Molecular design of thioether-linked dimers and trimers for twist-bend nematic liquid crystals

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Thioether-linked liquid crystal dimers and trimers: the
twist-bend nematic phase1
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Yuki Arakawa,* Kenta Komatsu, Satoyoshi Inui, and Hideto Tsuji
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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. E-mail: arakawa@tut.jp
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1
Abbreviations: LCs, liquid crystals; CB, cyanobiphenyl; CBnCB, molecules containing cyanobiphenyl
fragments with a spacer containing n atoms; CBZnZCB, Z represents the nature of the linkage bond;
POM, polarizing optical microscope; DSC, and differential scanning calorimetry; DFT, density functional
theory.

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Abstract
Systematic synthesis of thioether-linked dimers and trimers was carried out to reveal
molecular designs for inducing mesophases and twist-bend nematic (NTB) phases. A
five-fold approach based on molecular structural parameters including the terminal
substituent, the position of the thioether bond, the nature of the bridge bond linking
benzene rings in the mesogenic fragments, the nature of bonds at the linkage on the side
oppose to the thioether linkage, and oligomeric effects were evaluated. Dimers with
cyano groups at the 4,4’-positions in each mesogenic fragment of the thioether-based
dimeric system were found to afford a large long-axis directional dipole moment in the
fragments and exhibited potential for application as mesogens, with some molecules
indicating the ability to form NTB phase. The dimers containing the thioether bond as a
part of the spacer and not at the linkage position to the mesogenic fragments deteriorate
as twist-bend nematogens but behave as traditional nematogens. Also, Imine-, azo- and
triple-bond bridged mesogenic fragments lead the dimers to be twist-bend nematogenic.
Notably, all asymmetric thioether-linked cyanobiphenyl dimers with different functional
bonds at the linkage on the side oppose to the thioether linkage, such as methylene,
ketone, and two-typed esters (C=OO and OC=O) can form NTB phases, with some of
them retaining the phase upon cooling to room temperature, which highlights the utility
of thioether linkage in the molecular design of twist-bend nematogens. In the trimer
system (or oligomeric effect), a cyanobiphenyl-based trimer composed of interior ether
and exterior thioether linkers formed an NTB phase. This paper, for the first time, not
only reveals the systematic molecular design of thioether-linked dimers but also offers
the prospect of the thioether-linked oligomers, for twist-bend nematic liquid crystals.
Keywords: liquid crystal dimer; liquid crystal trimer; twist-bend nematic phase; sulfur;
thioether.

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1. Introduction
Molecular shape and geometry are key structural characteristics in liquid crystals (LCs)
and are determinants for the emergence of LCs between the solid crystal and isotropic
liquid phases. They are known to dominate the physical properties and mesomorphism
in LCs. LCs have been contributing to furnish LC displays (LCDs) [1], and are expected
to be used in various applications beyond traditional LCD technologies [2]. While
rod-shaped (or calamitic) and disc-shaped (or discotic) LCs are the major structural
classification among LCs, bent-shaped LCs which are a specific sub-type of the former
have attracted a great deal of attention due to their exotic mesomorphism. The discovery
of ferroelectricity by Niori et al. [3] and spontaneous symmetry breaking [4] in achiral
bent-shaped dimers, also known as banana LCs, has served as an impetus for rapid
growth in this area.
LC dimers built using two rigid mesogens linked by an alkylene spacer containing
odd-numbered carbon atoms also behave as bent-shaped molecules, and their discovery
and development has been a fascinating subject in condensed matter science [5].
Particularly, the emergent mesophase, or twist-bend nematic (NTB) phase, previously
predicted by Meyer [6] and Dozov [7], and its recent experimental corroboration by
Cestari et al. [8] has further stimulated interest in LC dimer science. The NTB phase has
pseudo layering structures and a director periodicity based on twist and bend
deformations, forming a heliconical helix with the local director tilted against the helical
axis, which is characterized by right- and left-handed helical nanostructured domains, or
spontaneous breaking of symmetry, despite an achiral system. The helical pitch is
considered ~10 nm based on flexoelectric measurement [9], freeze fracture transmission
electron microscopy (FF-TEM) [10], and soft resonant X-ray measurement [11], and the
theory based on the extended Frank elastic energy density supports such nanometric
pitch in the NTB phase [12]. The development of various applications based on NTB
liquid crystals has recently been addressed [13]. Furthermore, twist-bend smectic
(SmTB) phase [14] and twist-grain-boundary-twist-bend phase (TGBTB) that adjacent
blocks of pseudo-layers of the nanoscale pitch rotate across grain boundaries [15] were
also reported for bent-shaped dimers.
Although molecular designs that induce the NTB phase during the course of
integration are yet to be established, it is apparent that the degree of molecular bending
described by interarm angle (α) between the para axes of the two mesogenic fragments
[Figure 1 (a)] has a crucial bearing on the NTB phase induction. Mandle et al. have been
extensively exploring the structure-property relationship that induces the NTB phase

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from the viewpoint of molecular bend [16]. A linkage bond between each mesogenic
fragment and the flexible spacer is the predominant determinant of an α value. For
instance, a methylene linkage (C–CH2–C) with an α value of ~110° renders the dimer
more bent than an ether linkage (C–O–C) with an α value of ~117°. The most
representative twist-bend nematogen includes dual methylene-linked cyanobiphenyl
(CB) dimers linked by alkylene spacers (CnH2n). These molecules are denoted by the
abbreviation CBnCB, where n represents the carbon number of the spacer used. In this
paper, related structures are described by a similar acronym, CBZnZCB, where Z
denotes the composition of the linkage used. For example, while such dual
methylene-linked dimers CBnCB with an α value of 111°, and asymmetric methylene-
and ether-linked cyanobiphenyl dimers CBnOCB with an α of ~128° display the NTB
phase [16,17], the symmetric dual ether-linked dimers CBOnOCB with an α of ~144°
tend not to result in NTB phase formation [18] due to the large α value, or close to
straightened molecular geometry, with the exception of the considerably narrow NTB
phase displayed by CBO5OCB, [17(a)]. Whereas, a few dual ether-linked dimers are
also known to have the ability to form the NTB phase [19]. Z. Ahmed et al. reported on
methylene-linked twist-bend nematogens with various mesogenic fragments [20].
Watanabe et al. reported on a comparative study on the NTB phase incidence of
two-typed opposite ester [–C=OO–; (COO) and –OC=O–; (OCO)]-linked dimers [21].
OCO-linked cyanobiphenyl dimers (or mesogenic fragment to OC=O to spacer) with a
relatively large α of 139° [21] solely exhibited the uniaxial nematic (NU) phase [22],
while COO-linked pentylcyclohexylphenyl dimers (or mesogenic fragment to COO to
spacer) with α of 97° exhibit the NTB phase (but COO-linked cyanobiphenyl dimers,
CBCOOnOOCCB, are not mesogenic) [21]. Mandle et al. found that some
asymmetrically OCO-ester- and ether-linked dimers exhibited the NTB phase [23], and
Gorecka, Zep and coworkers reported asymmetrically COO- and OCO-linked
cholesterol-based dimers with the NTB phase [24]. Based on the theory that a shorter
spacer is reflected in increased molecular curvature, Dawood et al. reported that a dimer
with imine bonds in the spacer and the shortest propylene spacer displayed an NTB
phase transitioned directly from the isotropic (Iso) phase, as opposed to the common
NTB-NU-Iso phase sequence [25]. Pocock et al. reported that a narrow range of
conformation or bend angle distribution contributes to stabilize the NTB phase and leads
directly to the NTB-Iso phase transitions [26]. Knezěvic et al. reported on several
naphthalene-based twist-bend nematogenic dimers with various linkage groups, and
found that ketone-linked one formed a relatively narrow range of the NTB phase among
them, due to its broad conformation distribution [27]. The above research suggests that

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the nature of linkage bonds in bent-shaped LC dimers and the resulting molecular bent
degree are one of major factors responsible for the emergence of the NTB phase.
Considering that twist-bend nematogens reported so far are based on an archetypal
dimer system composed of rigid two mesogenic fragments and a flexible spacer, it is of
interest to note that classical rigid bent-core LCs [28], a hybrid bent-core LC trimer [29],
hydrogen-bonded oligomers [30], a duplexed hexamer [31], trimers and tetramers [32],
and polymers [33] all exhibit NTB phases.
Although most twist-bend nematogens reported so far are based solely on carbon
and oxygen linkages, during the course of studying alkylthio- (or thioether-) containing
mesogens [34], we reported that thioether-linked cyanobiphenyl-based dimers
CBSnSCB and CBSnOCB display the NTB phase (Figure 1) [35]. Since the thioether
linkage (C–S–C) has a smaller bond angle of ~100° compared to methylene and ether
linkages, the CBS7SCB and CBS7OCB have smaller α values, of 109° and 126°
respectively, compared to analogous molecules containing a methylene-linked CB7CB
and an ether-linked CBO7OCB [Figure 1(a)]. Very recently, we reported that a
homologue of selenium- (or selenoether-) linked cyanobiphenyl dimers (CBSenSeCB)
with smaller α values, of 90°, also has the potential for forming the NTB phase [36].
In this study, as a part of our studies to prepare chalcogen-linked LC oligomers,
especially those that induce the NTB phase, we systematically synthesized
thioether-linked dimers and trimers and evaluated their phase transition behavior. We
report an effective design of molecules that could induce mesophases and the NTB phase,
using a five-fold approach to evaluate the role of the terminal substituent, thioether
position, the bridge bond linking benzene rings in the mesogenic fragments, the linkage
on the side opposite to the thioether linkage, and the oligomeric effects.

(a)
O
O
n
n
CBOnOCB
CBnCB
NC
CN
NC
CN
= 111o
= 144o
twist-bend nematogen
conventional nematogen
O
n
NC
CN
CBnOCB
= 128o
twist-bend nematogen
(b)
S
S
n
S
O
n
CBSnSCB
= 109o
NC
CN NC
CN
CBSnOCB
= 126o
twist-bend nematogen
twist-bend nematogen
Terminal substituents (X)
(c)
S
S
7
X
X
X: CN, H, Me, OMe, CHO, F, Cl, CF3, NH2, NO2
Bridge bonds (Y) in the mesogenic fragments
Linkages (Z) oppose to the thioether linkage
S
S
7
Z
S
n
Y
Y
NC
CN
NC
CN
Y: C=N, N=N,
Z: CH2, CO, OC=O, C=OO, S, O
C
C,C=C,OC=O,C=OO,
Thioether in the spacer
S
S
S
CB1S1CB
CN
n
NC
CN
NC
CN
CB1SnS1CB
Trimers (oligomeric effect)
S
S
7
NC
S
S
7
TCBSS7 (TCBS7)
O
S
S
CN
CN
7
NC
NC
S
O
7
TCBSO7)
O
7
O
S
7
TCBOS7
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Figure 1. Methylene- and/or ether-linked LC dimers (a), thioether-linked dimers
CBSnSCB and CBSnOCB with NTB phases (b), and prepared dimers and trimers(c).

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2. Experimental
All chemicals used in the study were purchased from commercial sources as follows
and were used as received. 4-Bromobenzenethiol, pinacol esters, 1,7-dibromoheptane,
cyclohexanone, bromine, 4-aminobenzenthiol, 4-aminobenzonitrile, 4-bromobenzoic
acid,
4-bromobenzyl
bromide,
4-carboxybenzonitrile,
acid,
4-formylbenzonitrile,
4-hydroxybenzonitrile,
4-mercaptobenzoic
4-hydroxybenzenethiol,
4-cyano-4'-hydroxybiphenyl, 5-bromovaleric acid, 4-Bromo-1-butanol (ca. 80%),
thiophosgene, 2,2’-bipyridine, tetrakis(triphenylphosphine)palladium(0) [Pd(PPh3)4],
triphenylphosphine (PPh3), N,N-dimethylaminopyridine (DMAP), trifluoroacetic acid
(TFA), triethylsilane (TES) and 1,5-cyclooctadiene (COD) were purchased from Tokyo
Chemical Industry Co., Ltd. Dicyclohexylcarbodiimide (DCC), n‐ butyllithium (n‐
BuLi) in n‐ hexane (1.6 M), Copper(I) iodide (CuI), tetrahydrofuran (THF),
N,N-dimethylformamide (DMF), toluene and 1,4-dioxane for reaction solvents were
purchased from Kanto Chemical Co., Inc. Potassium carbonate (K2CO3), cesium
carbonate (Cs2CO3) and calcium carbonate (CaCO3) were purchased from Nacalai
Tesque Inc. Bis(1,5-cyclooctadiene)nickel(0) [Ni(COD)2], triethylamine (TEA) and
Sodium sulfide nonahydrate (Na2S 9H2O) were purchased from FUJIFILM Wako Pure
Chemical Co. 4-Ethynylbenzonitrile, 2,4’-dibromoacetophenone and OXONE®
(monopersulfate
compound) were purchased
from
Sigma-Aldrich Co.
4,4’-Dihydroxybiphenyl was purchased from BLD Pharmatech Ltd. 4-Bromobenzyl
mercaptane was purchased from Fluorochem Ltd. 1,7-Bis(4-bromophenylthio)heptane
was synthesized as described in Ref. 35.
The synthetic procedures and characterization data of the prepared compounds are
described in the supporting information. The molecular structures were evaluated by1H
and 13C nuclear magnetic resonance (NMR) spectra and were recorded on a
1
13
JNM-ECS400 (400 MHz for H and 100 MHz for C) or a JNM-ECX500 (500 MHz
for 1H, 126 MHz for C) instrument (JEOL Ltd., Tokyo, Japan). Chemical shift values
13
(δ) are reported in ppm (tetramethylsilane δ 0.00 ppm for 1H; and residual chloroform δ
77.0 ppm for 13C). Phase transition behaviors were studied through an Olympus (Tokyo,
Japan) polarizing optical microscope (POM; BX50) using a Linkam (Surrey, UK)
temperature controller LK-600PM) and a Shimadzu (Kyoto, Japan) differential scanning
calorimetry (DSC, DSC 60) at a rate of 3 or 10 ꢀmin-1 under a flow of nitrogen gas (50
mL min-1). The POM photographs and DSC results are included in the supporting
information, excluding those described in the manuscript. The molecular structures were
optimized by density functional theory (DFT) using Gaussian 16 [37] at the B3LYP 26
level [38] with 6-31G(d) basis set using the spacer in the all-trans conformation.

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3. Results and Discussion
3.1 Effect of terminal substituent
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We began with the synthesis of a series of thioether-linked biphenyl dimers with varied
terminal substituents [X: H (i.e., no substituent), Me, OMe, F, Cl, Me, CHO, CF3, NH2,
and NCS] at both 4,4’ positions, employing the synthesis routes shown in Figure 2. This
homologous series is based on a heptylene spacer, since CBS7SCB exhibited the best
stability of the NTB phase among the spacers evaluated with the CBSnSCB structure
[39]. All phase-transition temperatures and enthalpy changes (ꢀH) evaluated by DSC
measurements are compiled in Table 1. The dimer with CN groups at the termini
(CBS7SCB) is known to display a NU phase between 88 and 115 ºC, and an NTB phase
below 88 ºC, and is eventually vitrified at ~16 ºC or displays an NTB glassy (NTBG) state
[35]. However, we expected mesophase formation in the prepared dimers and were
surprised to find that no other dimer exhibited a mesophase. Further, the asymmetric
dimer with X = H and CN was not mesogenic at the melting point of 125.7 ꢀ, and the
results from the study indicated that cyanobiphenyls for both mesogenic fragments were
suitable for inducing a mesophase and NTB phase in the dual thioether-linked dimer
system.
In order to understand the mesogenic ability of this homologous series from the
viewpoint of the molecular structures, we estimated the dipole moment of monomeric
models with a methylthio group (SMe) in each mesogenic unit (Table 1), based on the
density function theory at the B3LYP level [37] with the basis set 6-31G(d) using
Gaussian 16 [38]. Because cyanobiphenyl mesogens are predisposed to pack
antiparallelly in the mesophase or to form the mesophase [39], such an approach can
effectively elucidate the structure–property relationship among LC dimers [40]. Among
these, models with X = CN and NO2 exhibit particularly large long-axis directional
dipole moments beyond 5 D (Table 1). Rod-like mesogens with a NO2 group favor
packing in a parallel and head-to-head (or tail-to-tail) manner, due to the strong
intermolecular interactions between NO2 groups [41]. However, such tendency may not
be good for the formation of mesophases and the helical NTB phase in a more bent
thioether-linked dimer system, which agrees with the fact that methylene-linked dimer
containing NO2 groups is not mesogenic [42]. Although some methylene-linked dimers
with NO2 groups are known to form monotropic NTB phases, they also exhibit narrow
NTB ranges or instability compared with the CN counterparts [20].

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Figure 2. Molecular structures and synthetic routes of thioether-linked dimers with
various terminal substituents (X) at both 4,4’-positions.
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Table 1. Transition temperatures (ºC) and enthalpy changes (ꢀH; kJ mol-1) of the dimers
with different substituents (X) upon second heating (upper line) and first cooling
(bottom line) at a rate of 3ºC min-1 on DSC and dipole moment (Debye) taken from
each mesogenic fragment by DFT.
Dipole moment
X
Phase sequence
CN
(CBS7SCB)
H
Cr 112.1 (37.1) NU 115.9 (0.9) Iso
G 15.9 NTB 88.3 (-) NU 115.2 (1.4) Iso
Cr 136.0 (42.1) Iso
5.7020
1.3961
1.9653
2.6032
1.4640
2.6513
4.7305
3.5006
6.0386
2.3251
4.1285
-
Cr 118.4 (42.2) Iso
F
Cr 140.7 (46.9) Iso
Cr 120.7 (44.9) Iso
Cl
Cr 120.8 (18.5) Cr 127.4 (18.0) Iso
Cr 116.0 (26.5) Iso
Me
Cr 137.6 (57.7) Iso
Cr 123.5 (51.6) Iso
OMe
CHO
CF3
NO2
NH2
NCS
Cr 171.7 (67.9) Iso
Cr 160.6 (65.5) Iso
Cr 129.9 (36.6) Iso
Cr 102.5 (32.2) Iso
Cr 122.6 (25.8) Iso
Cr 113.0 (26.4) Iso
Cr 96.9 (35) Iso
Cr 72.5 (31.3) Iso
Cr 129.4 (40.8) Iso
Cr 111.9 (40.8) Iso
Cr 119.7 (50.2) Iso
Cr 111.2 (49.2) Iso
CN and H
(asymmetric)
Cr 125.7 (44.7) Iso
Cr 103.2 (41.3) Iso
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a Observed by POM.
3.2 Effect of thioether position
To probe the role of the thioether position in inducing the NTB phase, we synthesized

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three cyanobiphenyl-based dimers with methylene linkages inserted between the
thioether functionality and cyanobiphenyl arms. The structures are abbreviated as
CB1SnS1CB and CB1S1CB, in which 1 refers to the carbon number of the methylene
linkages (Figure 3). The addition of the methylene linker between the benzene ring and
the sulfur atom places the thioether part in the spacer and not at the linkage to the
mesogenic fragments. The prepared molecules CB1S3S1CB and CB1S5S1CB are
analogous to the twist-bend nematogens CBS5SCB and CBS7SCB, respectively, which
the total number of atoms in the spacers and linkages along their linear chain is the
same. Although CB1S3S1CB formed the monotropic NU phase, which is stable below
ambient temperature, CB1S5S1CB also exhibited the monotropic NU phase with a
considerably narrow range discernible by the POM observation, shown in the
supporting information. However, neither of these molecules formed NTB phases, unlike
CBS5SCB and CBS7SCB. Moreover, CB1S1CB, containing the shortest spacer, was
not mesogenic. These results indicated that the presence of the thioether bond linked
directly to the cyanobiphenyl moiety is preferred to its presence in the spacer for
exhibiting twist-bend nematogen behavior. The NU-Iso transition temperatures of 101.6
and 108.6 ºC for CB1S3S1CB and CB1S5S1CB, respectively, are relatively lower than
108.5 and 115.2 ºC observed for their analogs CBS5SCB and CBS7SCB, respectively.
In addition, the entropy changes scaled by gas constant (R) at NU-Iso transition
[ꢀS(NUI)/R] of 0.2 for CB1S3S1CB are similar to or smaller than those of 0.2 and 0.4
for the analogous CBS5SCB and CBS7SCB structures, respectively. The interarm angle
(α) values of CB1S3S1CB and CB1S5S1CB for each structure were optimized by DFT
calculations at the B3LYP/6-31G(d) level of theory and were calculated to be 129°,
which is larger than 109° for twist bend nematogen CBS7SCB with the thioether bond
at the linkage position. This increase in the α values for CB1S3S1CB and CB1S5S1CB
implies that their mean molecular geometries are close to a straightened rod when
compared with those of more bent CBS5SCB and CBS7SCB with smaller α values.
Nevertheless, CB1S3S1CB and CB1S5S1CB exhibited lower NU-Iso transition
temperatures and ꢀS(NUI)/R values than those of CBS5SCB and CBS7SCB, indicating
that the NU phase order decreases more for the former than for the latter. These results
indicate that a flexible thioether bond with low rotational energy barrier and smaller
dihedral angle of ~100° in the spacer may cause molecular disordering for CB1S3S1CB
and CB1S5S1CB. Additionally, these features of the thioether bond could deteriorate
twist-bend nematogens to form a helical structure. The role of a disulfide S–S bond (not
thioether) in spacers of cyanobiphenyl-based dimers [42] and cholesterol-based dimers
[43] has been reported. Moreover, the smaller ꢀS(NUI)/R values for LC dimers were

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reportedly ascribed to the flexibility of the S–S bond [42] and the biaxiality with its
small dihedral angle of 90° [43]. In addition, Zong et al. reported that cholesterol- and
phenylbenzoate-based asymmetric dimers with terminal thioether groups display lower
transition temperatures and narrower mesophase ranges than those of the alkoxy counter
parts [44]. Thus, considering the reported observations and the results of this study, it
can be concluded that the position of the sulfur atom in LC dimers has a profound effect
on the mesomorphism and phase-transition behavior.
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Figure 3. Synthetic routes for CB1S3S1CB, CB1S5S1CB, and CB1S1CB.
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Table 2. Transition temperatures (ºC) and enthalpy changes (ꢀH; kJ mol-1) of CB1S1CB,
CB1S3S1CB and CB1S5S1CB upon second heating (upper line) and first cooling
(bottom lone) at a rate of 10ºC/min.
Compound
CB1S1CB
Phase sequence
Cr 199.5 (47.4) Iso
Cr 92.8 (35.3) Iso
G -6.9 NU 58.0 (14.7) Cr 89.0 (-)a Cr 101.6 (24.0)b Iso
G -5.3 NU 57.7 (-) c Iso
CB1S3S1CB
CB1S5S1CB
Cr 101.5 (-) Cr’ 108.6 (61.4)b Iso
Cr 63.7 (51.0)d Iso
307
a Cold crystallization.

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b Total ꢀH value in Cr to other Cr (Cr’) and Cr to Iso transitions.
c
Accurate enthalpy change could not be recorded due to the overlap of small
crystallization and the NU-Iso transition.
d Monotropic NU phase were seen at ca. 56 ºC by POM observation as shown in the
supporting information.
3.3 Effect of bridge bond in mesogenic part
After evaluating the effect of terminal substituents, we synthesized two
cyano-substituted dimers with various bridge bonds inserted between two benzene rings,
including N=C (imine), N=N (azo), C≡C (triple bond, or tolane), C=C (double bond, or
stilbene), OCO and COO (opposite directional esters) (Figure 4), and studied their
phase transitions (Table 3). Notably, the imine-, azo-, and tolane-based dimers formed
the monotropic NTB phase as well as the NU phase. The imine dimer formed the NTB
phase between 65 and 93 ºC upon cooling, and the corresponding DSC curves and
photomicrographs are shown in Figures 5 and 6, respectively. The photomicrographs
indicate a typical NU texture at 100 ºC [Figure 6(a)] and a typical NTB phase with a
coexisting of focal conic and rope-like textures at 70 ºC [Figure 6(b)]. Further, the imine
dimer displayed polymorphism which was determined to be dependent on kinetics
(Figure 5) and indicated a melting temperature at 126 ºC upon initial heating from the
solution-crystalized phase of the pristine sample. Upon further heating after the first
cooling from the Iso phase, the transition from the crystal (Cr) to other crystal (Cr’)
occurred at ~86 ºC, which was not observed upon the initial heating. Consequently,
melting at another temperature (115 ºC) emerged and the original melting point of 126
ºC was also detected. For azo and tolane dimers, the NTB–NU transition was not
observed on the acquired DSC thermogram because its peak overlapped with the
crystallization peak due to their considerably narrower NTB range. The NTB phases of
the dimers were observed using POM between 118 and 121 ºC for the azo dimer,
[Figures 6(c) and (d)] and between 87 and 94 ºC for the tolane dimer [Figures 6(e) and
(f)]. However, while the stilbene, OCO- and COO-ester dimers formed the NU phase,
they did not exhibit the NTB phase. The stilbene dimer had a wide NU range between
108 and 176.4 ºC, while the ester-based dimers indicated a NU range of 107.6 to 115.6
ºC for the OCO dimer, and 105.1 to 145.6 ºC for the COO dimer. The compounds
prepared in this homologous series exhibited similar ꢀS(NUI)/R, with values ranging
from 0.4-0.5, in which the exceptionally low value of 0.2 for the stilbene dimer could be
ascribed to its high transition temperature.

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Figure 4. Molecular structures and synthetic routes for thioether-linked dimers with
various bridge bonds (Y).

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Table 3. Transition temperatures (ºC) and enthalpy changes (ꢀH; kJ mol-1) of dimers
with various bridge bonds (Y) in mesogenic fragments upon second heating (upper line)
and first cooling (bottom line) at a rate of 10 ºC/min on DSC. Brackets describe the NTB
phase transition by POM observation alone.
Y
Phase sequence
ꢀS(NUI)/R
(representation)
C=N
Cr 122.9 (37.8) NU 135.7 (1.4) Iso
Cr 58.5 (24.5) NTB 89.8 (-)NU 133.8 (1.6) Iso
Cr 160.8 (60.8) NU 170.0 (1.4) Iso
[Cr 118 NTB 121]a Cr 125.4 (45.4) NU 169.7 (1.6) Iso
Cr 144.9 (45.2) Iso
-
0.5
0.4
0.4
-
(imine dimer)
N=N
(azo dimer)
C≡C
[Cr 87 NTB 94]a Cr 99.2 (33.3) NTB 94 (-)a NU 138.1 (1.3) Iso
(tolane dimer)
C=C
0.4
0.2
0.2
-
Cr 168.8 (22.7) Iso
(stilbene dimer)
OC=O
Cr 108.0 (15.4) NU 176.4 (0.8) Iso
Cr 122.7 (40.3) Iso
(OCO-ester dimer)
C=OO
0.4
-
Cr 107.6 (36.0) NU 115.6 (1.3) Iso
Cr 172.1 (70.8) Iso
(COO-ester dimer)
0.5
Cr 105.1 (53.2) NU 145.6 (1.8) Iso
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a Observed by POM.
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Figure 5. DSC curves for imine dimer upon first cooling (a), first heating (b) and
second heating (c) at a rate of 10 ºC min-1.

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Figure 6. Photomicrographs of NU phase at 100 ºC (a) and NTB phase at 70 ºC (b) for
the imine dimer, of NTB-NU transition at 121 ºC (c) and NTB phase at 120 ºC (d) for the
azo dimer, and of NU phase at 95 ºC (e) and NTB phase at 87 ºC (f) for the tolane dimer.
3.4 Linkage effect
To examine the effect of the functional bonds at the linkage position on the side
opposite to the thioether linkage, we synthesized asymmetric cyanobiphenyl dimers
with a fixed thioether linkage with varied linkages at the other cyanobiphenyl fragment
(Figure 7). These varied linkages included methylene, and ketone groups as well as both
structural variations of the ester bonds [-OCO- and -COO-], which were abbreviated as
CB6SCB, CBCO5SCB, CBOCO4SCB, and CBCOO4SCB, respectively. The total
number of atoms in the spacer backbone and linkage along each linear chain in these
molecules is seven, as shown in Figure 7. Their phase-transition temperatures, ꢀH,
ꢀS(NUI)/R, and α values along with the reported phase-transition temperatures and
ꢀS(NUI)/R values for CB7CB [8], CB6OCB [17], CBS7SCB [35], and CBS7OCB [35]
are listed in Table 4. It must be emphasized that all the prepared thioether-linked

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cyanobiphenyl dimers with different bonds at the linkage on the side oppose to the
thioether, formed NTB phases, thereby strongly highlighting the utility of the thioether
linkage for the molecular design of twist-bend nematic liquid crystals. The NTB phases
are evidenced by the photomicrographs and the DSC thermogram, as shown in Figures
8 and 9, and the supporting information. Further, the prepared dimers exhibited α values
lower than 144° for the conventional nematogen CBOnOCB. CB6SCB displayed
NTB-NU transition temperature at 81.5 °C followed by the NTB phase during cooling to
ambient temperature, and eventually vitrified at 3.9 °C. During cooling, a negligible
broadened peak ascribed to minor crystallization was observed at 55.1 °C, at which
small crystallization did not typically occur during several measurements. Upon heating
from the NTB phase at 70 °C above the crystallization upon cooling, on the other hand, a
NTB-NU phase transition appeared at ~90°C, and the NU phase transitioned to the Iso
phase at ~112 °C, at which the optical textures in the NTB phase are shown in Figure 8
(a) and (b). Next, the ketone-linked CBCO5SCB melted at 145.0 °C and the resultant
NU phase was transitioned to the Iso phase at 151.4 °C upon first heating from the
pristine sample. Upon cooling from the Iso phase, CBCO5SCB exhibited NU-Iso
transition at 149.6 °C and NTB-NU transition at 109.1 °C, followed by crystallization at
84.5 °C with a ꢀH value of 26.7 kJ mol-1. Upon subsequent heating, CBCO5SCB
exhibited the NU phase between 120.3 and 152.2 °C, which the melting at 120.3 is
markedly contrast to that at 145.0 °C upon the first heating. Upon first heating on
re-measurement using the same DSC sample after a month, it displayed the NU phase.
This behavior was similar to that observed upon second heating on the prior
measurement or different to that observed upon the first heating from the
solution-crystallized sample. Hence, the crystalline species from the
solution-crystallization and after melting for CBCO5SCB are different, and the latter
could be thermodynamically stable. Among the different linkage dimers, the
ketone-linked CBCO5SCB displayed a relatively narrow range of the NTB phase
because of the crystallization. Such NTB instability could be similar to other
ketone-linked twist-bend nematogens [16,27]. The dimer CBOCO4SCB respectively
exhibited NTB-NU and NU-Iso transition temperatures at 82.3 and at 150 °C, while the
NTB phase was vitrified at 17.5 °C with prior partial crystallization at 54.5 °C with a
small value of ꢀH (6.6 kJ mol-1). It is known that the asymmetrically OCO-ester- and
ether-linked CBOCO6OCB [16] and the dual OCO-ester-linked dimers
CBOCOnOCOCB [22] are exclusively conventional nematogens. These data indicate
the beneficial use of employing the thioether linkage in cyanobiphenyl dimers to induce
the NTB phase in the presence of an OCO-linkage at the other cyanobiphenyl linkage

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position in the dimer. On the other hand, CBCOO4SCB exhibited relatively distinctive
optical textures and phase-transition behavior. Upon cooling from the Iso phase,
CBCOO4SCB formed the NU phase, followed by transformation to other textures, as
shown in Figure 8 (c). It was slightly difficult to achieve its characteristic domains for
the NTB phase since the sample tends to form small domains in a non-treated glass cell.
Here, the NTB phase showed a clear rope-like (or stripe) texture in relatively thinner
uniaxially planar-aligned specimens, which is an embodiment of the Helfrich–Hurault
undulation instability based on pseudo-layer structures, similar to those observed for Sm
phases [10(a)]. Each line width of stripe textures associates with cell thickness. Then,
we conducted POM observation of CBCOO4SCB and CB6SCB (for comparison) in
uniaxially rubbed polyimide-surface-treated (for planar alignment) cells with 5 ꢁm
thickness. As can be seen in Figure 8, the NTB textures of both molecules formed stripe
textures different from those in untreated cells. However, both the optical textures were
distinct; although CB6SCB displays clear stripe texture which is usual behavior in NTB
phase, CBCOO4SCB forms equivocal stripe-like and small focal conic textures based
on Dupim cyclide. The line width for CBCOO4SCB (~2.9 ꢁm) was apparently smaller
than that for CB6SCB (~4.9 ꢁm similar to the cell thickness). In addition, the transition
of the NU phase to a lower one upon cooling for CBCOO4SCB was detected as a clear
first order transition with the ꢀH value of 0.47, in marked contrast to the second
order-like tendency for those of the other molecules (Figure 9). The results of POM
observation and DSC measurements implied distinct molecular aggregates or the
existence of a specific intermolecular interaction in CBCOO4SCB. Hence, although we
carried out further WAXD measurements for CBCOO4SCB, clear diffractions
reminiscent of Sm layer structures were not seen, in contrast to the SmTB phases [14]. In
this study, it was postulated that the lower phase was the NTB phase. However, further
investigation is needed. CBCOO4SCB displayed remarkably low NTB-NU and NTB-Iso
transition temperatures at 52.3 and 64.5 ºC, respectively. The NTB phase cooled to room
temperature in the absence of a crystallization peak, and the glass transition peak was
observed at 20.7 ºC [Figure 9(d)], which may be ascribed to its bent geometry resulting
from the smallest α value of 105° among the thioether-linked dimers with various other
linkers. Moreover, upon heating, CBCOO4SCB displayed unique phase-transition
behavior as it transitioned from the NU phase to the other Iso (Iso’) phase at ~69 ºC,
when subjected to a broad range of cold crystallization centered at ~86 ºC, and
transitioned from the Cr to Iso phase again. POM photomicrographs indicating this
behavior are shown in the supporting information.
Additionally, CB2SCB was synthesized in order to verify the chain length effect on

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the thioether- and methylene-linked dimer homologs CBnSCB. Its synthetic procedures
and phase-transitions are described in the supporting information and Table 4,
respectively. CB2SCB did not show any liquid crystal phases but displayed two-typed
crystalline species dependent on kinetics (described in the supporting information). The
results showed that a relatively long spacer is required to induce the mesophase and NTB
phase for CBnSCB homologues series.
The NTB-NU and NU-Iso transition temperatures upon cooling for CBZnSCB were
plotted against linkage Z, in which Z indicates methylene (C), ether (O), thioether (S),
ketone (CO), and two ester configurations (OCO and COO) [Figure 10 (a)], as a
function of the α values [Figure 10(b)-(d)]. In the comparison among C, O, and S in
Figure 10(a), O shows apparently higher NTB-NU and NU-Iso transition temperatures and
S displays a lower NTB-NU transition temperature than that of C. These results are
ascribed to the molecular geometries discussed below and agree with previously
reported results [35]. Figures 10(b) and (c) indicate that the NTB-NU and NU-Iso
transition temperatures of the prepared dimers exhibit dependency on their α values,
with the former displaying a plateaued temperature profile centered at around 120°,
which further ascends with increasing the α values; this trend was consistent with the
literature [16]. The variations in NTB-NU and NU-Iso transition temperatures with
differing linkage types are shown in Figure 10(c). In addition, the ꢀS(NUI)/R values are
also plotted as a function of the α values in Figure 10(d). Notably, the ꢀS(NUI)/R values
decreased with decreasing α values, which might be reminiscent of the lower order in
the NU phase for the dimer with a higher degree of bend and smaller α values.

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Figure 7. Molecular structures and synthesis routes of thioether-linked dimers

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CBZnSCB with a different linkage (Z) on the side oppose to the thioether linkage.
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Figure 8. Photomicrographs illustrating NTB phases in a non-treated cell at 80 °C (a)
and in a uniaxially rubbed polyimide-surface treated planar cell with 5 ꢁm thickness at
70 °C (b) for CB6SCB, and in a non-treated cell at 68 °C (c) and in a uniaxially rubbed
polyimide-surface treated planar cell with 5 ꢁm thickness at 26 °C (d) for
CBCOO4SCB.
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Figure 9. DSC curves at a rate of 10 °C min-1 for CB6SCB (a), CBCO5SCB (b),
CBOCO4SCB (c) and CBCOO4SCB (d).
Table 4. Transition temperatures (ºC) and enthalpy changes (ꢀH; kJ mol-1) of the dimers
with oppositely various linkages upon second heating (upper line) and first cooling
(bottom line) at a rate of 10 ºC/min on DSC.
Z
Phase sequence
ꢀS(NUI)/R
α
(abbreviation)
a
d
CH2
NTBG 5.0 Cr 59.3 (14.1)b Cr 96.8 (-)c NU 106.0 (-)d Iso
G 3.9 Cr 55.1 (5.81)e NTB 81.5 (0.77) NU 101.8 (0.71) Iso
Cr’ 112.8 (-) Cr 120.3 (32.7)g NU 152.2 (1.82) Iso
Cr 84.5 (26.7) NTB 109.1 (-)h NU 149.6 (2.36) Iso
G 17.0 Cr 54.8 (6.9)b Cr 83.1 (7.5)b Cr 125.1 (37.4) NU 153.0 (1.33) Iso
G 17.5 Cr 54.5 (6.6) NTB 82.3 (-)h NU 150.0 (1.6) Iso
G 21.0 NTB 53.7 (0.29) NU 69 (-) Iso’i 86.1 (16.4) Cr 129 (29.0)b Iso
G 20 NTB 52.3 (0.47) NU 64.5 (0.08) Iso
-
109
118
127
105
(CB6SCB)
COf
0.23
0.51
0.67
0.38
0.46
-
(CBCO5SCB)
OCO
(CBOCO4SCB)
COO
(CBCOO4SCB)
CB7CBj
0.03
0.34
-
NTB 103 NU 116 Iso
111
128g
109
126
-
CB6OCBk
CBS5SCBl
CBO5SCBm
Cr 100 NTB 109 NU 153
Cr 76.6 NTB 78 NU 107.7 Iso
0.23
0.42
Cr 76.6 NTB 90.1 NU 143.5 Iso
CB2SCB
[Cr 73.6 (16.2) Iso]n G 20.7 Cr 41.7 (4.2) Iso
-
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500
a Polymorphism behavior is described in the main text.
b Cold crystallization.
c The ꢀH is given as a total ꢀH of Cr-NU and NU-Iso transitions, due to their overlap.
d Not determined due to the peak overlap.
e Imperfect crystallization.
f Ketone dimer has a polymorph with a melting temperature of 145.0 ºC with ꢀH = 37.0
kJ mol-1 upon first heating.
g The ꢀH is given as total ꢀH of the peaks at 112.8 and 120.3 ºC.
h Not determined.
i Observed by POM.
j In Ref. 8.
k In Ref. 17.
l The value for all-trans spacer model.
m In Ref. 35.
n
CB2SCB shows two crystalline species dependent on kinetics described in the
supporting information. The phase transition in brackets is upon cooling at a rate of 3

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ºC min-1.
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Figure 10. The NTB-NU (blue squares) and NU-Iso (red circles) transition temperatures
upon cooling against linkage (Z) for CBZnSCB (a), the NTB-NU (b) and NU-Iso (c)
transition temperatures as a function of the α value, and ꢀS(NUI)/R value as a function
of the α value (d).
3.5 Oligomeric effects
In order to probe oligomeric effects, we prepared three thioether-based trimers; solely
thioether-linked trimer (CBS7SBS7SCB), interiorly thioether- and exteriorly
ether-linked trimer (CBO7SBS7OCB), and interiorly ether- and exteriorly
thioether-linked trimer (CBS7OBO7SCB). CBS7SBS7SCB, which is only
thioether-linked, is a structural analog of an alkoxy trimer homologous of TCBOn (n =
7) as previously reported [45]. As shown in the photomicrographs and DSC thermogram
(see the supporting information), CBS7SBS7SCB did not form a mesophase upon
heating with the melting point at 168.8 ºC and exhibited the monotropic NU phase range
between 99.7 and 118.0 ºC, at which the phase range was drastically lower and
relatively narrower compared to that of TCBO7 with the NU phase range between 157
and 206 ºC upon heating and monotropic SmA phase below 157 ºC [45(c)]. Whereas,
the NTB phase incidence was not observed for CBS7SBS7SCB. The instability of the NU

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phase and exclusive nematogenic tendency for CBS7SBS7SCB were also ascribed to
the small bond angle and rotational energy barrier of thioether, which agrees with the
trend observed for thioether-containing mesogens [34,35]. In addition, CBO7SBS7OCB
displayed the enantiotropic NU phase but not the NTB phase. It is noteworthy that only
the interiorly ether-linked and exteriorly thioether-linked trimer CBS7OBO7SCB
formed the metastable NTB phase between 119 and 126 ºC upon cooling, which are
evidenced by POM shown in Figure 12 but not detected on DSC due to prior
crystallization (see the supporting information). This is the first example of a
thioether-linked trimer with NTB phase. Moreover, in the POM, CBS7OBO7SCB was
mainly crystallized at ~130 ºC and the small part of the supercooled NU region
transitioned to the NTB phase accompanied by other Cr’ and Cr transitions (Figure 12).
These results revealed that the combination of interior ether and lateral thioether
linkages has a beneficial effect in inducing the NTB phase for thioether-linked trimers.
Because a trimer with exclusive odd spacers expresses a bent geometry, interior ether
linkers with a larger bond angle than that for thioether could relax the molecular bend,
causing CBS7OBO7SCB to become a twist-bend nematogenic trimer. The development
of twist-bend nematogenic oligomers [29-32] and polymers [33] is known to be highly
challenging because they have not been widely studied. Hence, the present results are
expected to offer a promising prospect for the synthesis of novel twist-bend nematogens
based on thioether-linked oligomers.

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Figure 11. Synthesis routes for the thioether-linked trimers.
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Table 5. Phase-transition temperatures (ºC) and enthalpy changes (ꢀH; kJ mol-1)
for the trimers.
Compd.
Phase sequence
Cr 99.8 (7.9) Cr’ 146.4 (45.7) Iso
Cr 99.7 (25.1) NU 118.0 (1.89) Iso
Cr 181.9 (60.8) Iso
CBS7SBS7SCB
CBO7SBS7OCB
Cr 121.0 (41.2) NU 160.1 (3.8) Iso
Cr 149.4 (72.6) NU 162.4 (2.8) Iso
[Cr’ 119 NTB 126 NU]a Cr 130.0 (68.9) NU 160.2 (2.6) Iso
CBS7OBO7SCB
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a Observed by POM.

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Figure 12. Photomicrographs of NU phase at 130 ºC, NTB phase at 125 ºC, coexistence
of Cr’ and Cr phases at 108 ºC, and Cr phase at 102 ºC for CBS7OBO7SCB.
4. Conclusions
In summary, we systematically synthesized various thioether-linked dimers with varied
terminal groups, bridge bonds linking two benzene rings in the rigid mesogenic parts,
individual cyanobiphenyl fragment linkages, and trimers for twist-bend nematic liquid
crystals. The results indicated that dimers with terminal cyano-group substitutions,
having a large dipole moment along the long mesogenic fragment axis, have the
preferred structural attributes for the preparation of mesogens and twist-bend
nematogens by using the thioether-linked dimer system. Dimers with the thioether bond
as a part of the spacer (not at the linkage position) did not display the NTB phase but
showed the conventional NU phase. Dimers based on imine-, azo-, and
triple-bond-bridged mesogenic fragments formed the NTB phase. Thioether-linked
dimers with different bonds at the linkages on the side oppose to the thioether ones,
exhibited superior potential as twist-bend nematogens with NTB phases cooled to room
temperature. This fact highlighted the importance of the thioether linker in the
molecular design of twist-bend nematic liquid crystals. Among the thioether-linked
trimers, an interiorly ether- and exteriorly thioether-linked cyanobiphenyl-based trimer
behaved as a twist-bend nematogen. The present study offers the systematic molecular
design of thioether-linked dimers and oligomers for the development of twist-bend
nematic liquid crystals.
Acknowledgement
This work was supported by Japan Society for the Promotion of Science KAKENHI
Grant nos. 15H06285 and 17K14493 and research grants from the Naito Foundation and
the Toukai Foundation for Technology.
References and Notes
1 (a) M. Schadt and W. Helfrich, Voltage‐ dependent optical activity of a twisted
nematic liquid crystal, Appl. Phys. Lett., 1971, 18, 127; (b) D. W. Berreman, Optics

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617
in smoothly varying anisotropic planar structures: application to liquid-crystal twist
cells,
J. Opt. Soc. Am., 1973, 63, 1374; (c) P. Kirsch, M. Bremer, Nematic liquid crystals
for active matrix displays: molecular design and synthesis, Angew. Chem. Int. Ed.,
2000, 39, 4216.
2 (a) M. Mitov, Cholesteric liquid crystals with a broad light reflection band, Adv.
Mater., 2012, 24, 6260; (b) R. Dąbrowski, P. Kula and J. Herman, High
birefringence liquid crystals, Crystals, 2013, 3, 443; (c) T. Kato, M. Yoshio, T.
Ichikawa, B. Soberats, H. Ohno and M. Funahashi, Transport of ions and electrons
in nanostructured liquid crystals, Nat. Rev. Mater., 2017, 2, 17001; (d) T. Seki,
Light-directed alignment, surface morphing and related processes: recent trends, J.
Mater. Chem. C, 2016, 4, 7895; (e) M. Funahashi, Integration of electro-active
π-conjugated units in nanosegregated liquid-crystalline phases, Polym. J., 2017, 49,
75; (f) T. Kato, J. Uchida, T. Ichikawa and T. Sakamoto, Functional liquid crystals
towards the next generation of materials, Angew. Chem. Int. Ed., 2018, 57, 4355; (g)
T. Ube and T. Ikeda, Photomobile Polymer Materials with Complex 3D
Deformation, Continuous Motions, Self‐ Regulation, and Enhanced Processability,
Adv. Opt. Mater., 2019, 1900380, DOI: 10.1002/adom.201900380; (h) K. Pal, U. N.
Maiti, T. P. Majumder, P. Dash, N. C. Mishra, N. Bennis, J. M. Otón, Ultraviolet
visible spectroscopy of CdS nano-wires doped ferroelectric liquid crystal, J. Mol.
Liq., 2011, 164, 233; (i) K. Pal, M. L. N. M. Mohan, B. Zhan and G. Wang, Design,
synthesis and application of hydrogen bonded smectic liquid crystal matrix
encapsulated ZnO nanospikes, J. Mater. Chem. C, 2015, 3, 11907; (j) K. Pal, S.
Sajjadifar, M. A. Elkodous, Y. A. Alli, F. Gomes, J. Jeevanandam, S. Thomas, A.
Sigov, Soft, self-assembly liquid crystalline nanocomposite for superior switching,
Electron. Mater. Lett., 2019, 15, 84.
3 T. Niori, T. Sekine, J. Watanabe, T. Furukawa and H. Takezoe, Distinct ferroelectric
smectic liquid crystals consisting of banana shaped achiral molecules, J. Mater.
Chem., 1996, 6, 1231.
4 (a) T. Sekine, T. Niori, J. Watanabe, T. Furukawa, S. W. Choi and H. Takezoe,
Spontaneous helix formation in smectic liquid crystals comprising achiral molecules,
J. Mater. Chem., 1997, 7,1307; (b) D. R. Link, G. Natale, R. F. Shao, J. E.
Maclennan, N. A. Clark, E. Korblova and D. M. Walba, Spontaneous formation of
macroscopic chiral domains in a fluid smectic phase of achiral molecules, Science,
1997, 278, 1924.
5
(a) C. T. Imrie and P. A. Henderson, Liquid crystal dimers and oligomers, Curr. Opin.

618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
Colloid Interface Sci., 2002, 7, 298; (b) C. T. Imrie and P. A. Henderson, Liquid crystal
dimers and higher oligomers: between monomers and polymers, Chem. Soc. Rev., 2007,
36, 2096; (c) C. T. Imrie, P. A. Henderson and G.-Y. Yeap, Liquid crystal oligomers:
going beyond dimers, Liq. Cryst., 2009, 36, 755.
6
7
R. B. Meyer, Structural problems in liquid crystal physics, Les Houches Summer
School in Theoretical Physics. In: R. Balian, G. Weil, editors. 1973 Molecular Fluids.
New York (NY): Gordon and Breach; 1976.
I. Dozov, On the spontaneous symmetry breaking in the mesophases of achiral
banana-shaped molecules, Europhys. Lett., 2001, 56, 247.
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, and H. Zimmermann, Phase behavior and properties of the
liquid-crystal dimer 1′′, 7′′-bis(4-cyanobiphenyl-4′-yl)heptane: a twist-bend nematic
liquid crystal, Phys. Rev. E, 2011, 84, 031704.
9 C. Meyer, G. R. Luckhurst and I. Dozov, Flexoelectrically driven electroclinic effect
in the twist-bend nematic phase of achiral molecules with bent shapes, Phys. Rev.
Lett., 2013, 111, 067801.
10 (a) 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 and O. D. Lavrentovich, Nematic twist-bend phase
with nanoscale modulation of molecular orientation, Nat. Commun., 2013, 4, 2635;
(b) 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 and N. A. Clark,
Chiral heliconical ground state of nanoscale pitch in a nematic liquid crystal of
achiral molecular dimers, Proc. Natl. Acad. Sci., 2013, 110, 15931.
11 (a) C. Zhu, M. R. Tuchband, A. Young, M. Shuai, A. Scarbrough, D. M. Walba, J. E.
Maclennan, C. Wang, A. Hexemer and 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., 2016, 116, 147803;
(b) M. Salamonćzyk, N. Vaupotič, D. Pociecha, C. Wang, C. Zhu and E. Gorecka,
Structure of nanoscale-pitch helical phases: blue phase and twist-bend nematic
phase resolved by resonant soft X-ray scattering, Soft Matter, 2017, 13, 6694.
12 M. P. Rosseto, R. R. Ribeiro de Almeida, R. S. Zola, G. Barbero, I. Lelidis, L. R.
Evangelista, Nanometric pitch in modulated structures of twist-bend nematic liquid
crystals, J. Mol. Liq., 2018, 267, 266–270.
13 (a) J. Xiang, Y. Li, Q. Li, D. A. Paterson, J. M. D. Storey, C. T. Imrie and O. D.
Lavrentovich, Electrically tunable selective reflection of light from ultraviolet to