Twist-bend nematic liquid crystals based on thioether linkage

Article abstract of DOI:10.1039/c8nj06456c

We developed twist-bend nematogens based on cyanobiphenyl dimers with thioether or sulfur linkage, as opposed to the commonly used methylene and ether linkages. Based on the smaller bond angle of thioether C-S-C than methylene C-CH₂-C and ether

Full text of DOI:10.1039/c8nj06456c

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Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
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Twist-bend nematic liquid crystals with thioether linkages
Yuki Arakawa,* Kenta Komatsu, and Hideto Tsuji
Received 00th January 20xx,
Accepted 00th January 20xx
We developed twist-bend nematogens based on cyanobiphenyl dimers with thioether or sulfur linkages, as opposed to the
commonly used methylene and ether linkages. Based on the smaller bond angle of thioether C-S-C than methylene C-CH2-C
and ether C-O-C, we designed two dimer homologous series that are more bent, symmetric CBSnSCB with two thioether
linkages and asymmetric CBSnOCB with a thioether and ether linkage, containing odd carbon numbers n = 3, 5, 7, 9, and 11,
and also synthesized even-n members n = 6 for each system. Both CBSnSCB with n = 3, 5 and 7 and CBSnOCB with n = 5 and
7 formed a broad range of twist-bend nematic phases that were stable down to room temperature and showed remarkably
enhanced glass-forming ability. DFT calculations revealed that twist-bend nematogens CBS7SCB and CBS7OCB have smaller
bend angles made of the para axes of the two cyanobiphenyl moieties and larger dipole moments than those of conventional
nematogen ether-linked CBO7OCB.
DOI: 10.1039/x0xx00000x
www.rsc.org/
significant focus of research on liquid crystals and condensed
matter. In addition to low-molecular-weight molecules, twist-
bend nematogens have been found for oligomers [9] as well as
Introduction
Molecular geometry or shape plays an important role in liquid
crystals located between three-dimensional solid crystals and
isotropic (I) liquids. In 2001, Dozov predicted the possibility of a
unique helical-nanostructured nematic (N) phase with conical
director distribution along its helical axis for bent-shaped
mesogens, referred to as twist-bend nematic (NTB) phase [1]. In
experimental research, the N to another undefined N (Nx)
transition was initially observed for bent dimers with different
odd-numbered alkylene spacers [2]. Then, Cestari et al.
revealed that, based on 2H-NMR measurement using a
deuterated achiral dimer with Nx transition, a chiral structure or
symmetry breaking was generated in the Nx phase, and then,
the Nx phase was identified as the NTB phase [3]. The periodic
formation has been observed by freeze-fracture transmission
electron microscopy [4] and scanning electron microscopy
based on photo-polymerized materials [5]. Recently, resonant
carbon K-edge soft X-ray scattering measurement [6] has
revealed that the local director is tilted against its helix axis and
the helix pitch is close to 10 nm in the NTB phase [6a,b)], and
that it also supports the double helix formation in the NTB phase
[6b,c)] proposed previously [7]. From the view point of
molecular design, furthermore, there is the fact that a duplexed
hexamer forms a wider NTB phase than the one parent trimer,
sustaining such a double helical structure formed in NTB phase
[8].
polymers [10]. Twist-bend nematogens are expected to have
new applications owing to their fast electrooptical response
[11] and helicoidal cholesteric structure [12]. In addition, the
twist-bend smectic phase has recently been discovered [13].
Some methylene-linked cyanobiphenyl dimers (CBnCB -
wherein CB refers to cyanobiphenyl units and n denotes the odd
carbon number in the alkylene spacer), form the enantiotropic
NTB phase [3-6,14], in which the first report for CBnCB
postulated the Nx or NTB phase as a smectic A phase [15].
Although the ether-linked counterparts CBOmOCB are
recognized as conventional nematogens [16], a recent in-depth
analysis by Paterson et al. indicated that CBO5OCB forms a
narrow monotropic NTB phase range [17]. Sebastián et al.
reported that some dual ether-linked fluorinated CB dimers are
capable of forming kinetically-dependent NTB phase with a
broad range [18]. On the other hand, Mandle et al. have
explored the structure-property relationship extensively for the
development of twist-bend nematogens [19]. They reported
that not only the structures of mesogenic parts but also the
bend angles made by the para axes of the two mesogenic
moieties are essential determinants of the formation of the NTB
phase. Moreover, they found that ether linkages are less likely
to induce the NTB phase alongside methylene linkages due to
the larger bond angle for C-O-C than C-CH2-C, making the
molecular geometry of ether-linked dimers close to a straight
rod shape. Accordingly, it is interesting to note that the
asymmetrically methylene- and ether-linked cyanobiphenyl
dimers CB6OCB [20] and CB8OCB [18b)] form monotropic NTB
phases. Dawood et al. reported that a Schiff base dimer with a
propylene spacer forming NTB phase transited directly from the
I phase without an intermediate N phase, in which the
molecular design is based
Inspired by the recent discovery of the NTB phase, the
development of new twist-bend nematogens has been a
Department of Environmental and Life Sciences, Graduate School of Engineering,
Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi,
Aichi 441-8580, Japan. E-mail: ya017@edu.tut.ac.jp.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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S
S
S
m
SH
K2CO3
S
S
m
DOI: 10.1039/C8NJ06456C
+
Br
Br (CH2)m
Br
Br
Br
Acetonitrile
NC
NC
CN
O
B
O
m
O
S
S
m
NC
Pd(PPh3)4 / Cs2CO3
NC
CN
Br
CBSmSCB
m = 3, 5, 6, 7, 9, 11
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CN
THF
18-Crown 6-ether
Fig. 1. Molecular structures of the thioether-linked dimers CBSnSCB and CBSnOCB.
K2CO3
OH
+
O
Br
Br (CH2)m
m
Acetonitrile
Br
on the theory that postulates more bent structures for shorter
spacer dimers [21]. Furthermore, Jansze et al. and Walker et al.
found that hydrogen-bonded oligomers based on non-twist-
bend nematogenic or non-mesogenic molecules also work as
twist-bend nematogens [22]. Thus, a large variety of twist bend
nematogenic dimers or oligomers has recently been developed.
Nevertheless, the inserted linking atoms between a mesogenic
part and a spacer for twist-bend nematogens are typically based
exclusively on carbon, followed by oxygen.
This study presents, for the first time, twist-bend
nematogens based on thioether (C-S-C)-linked dimers, that is,
sulfur is included in the linking atom. So far, we have been
addressing the development of thioether-containing rod-like
mesogens [23], wherein we noted the bond-angle features for
the smaller bond angle of ca. 100° for C-S-C in thioether than ca.
118° for C-O-C in ether and ca. 110° for C-CH2-C in alkylene. In
the light of such a small bond angle of thioether, then, we
perceived that a thioether linkage could be preferable to induce
a more-bent molecular curvature geometry. Herein, we
designed two dimer families that are more bent; symmetric
CBSnSCB with two thioether linkages and asymmetric CBSnOCB
with a thioether and ether linkage, with carbon numbers n = 3,
5, 7, 9, and 11 (Fig. 1). In the acronyms CBSnSCB and CBSnOCB,
S and O respectively indicate the thioether and ether linkages.
In addition, even members CBS6SCB and CBS6OCB, which are
considered close to rod-like (or strictly Z-shaped) molecular
geometry, were also prepared.
Br
SH
Br
K2CO3
S
O
m
Br
Br
Acetonitrile
O
B
O
S
O
m
NC
Pd(PPh3)4 / Cs2CO3
NC
CN
CBSmOCB
THF
m = 3, 5, 6, 7, 9, 11
Fig. 2. Synthetic routes of CBSnSCB and CBSnOCB.
60, Shimadzu) with heating and cooling scans performed at a
rate of 3 °C min-1 under nitrogen gas. CBS3SCB, CBS7SCB and
CBS7OCB were subject to further DSC measurements on a DSC
8500 (Perkin Elmer) equipped with an Intracooler II at different
rates under nitrogen gas. X-ray diffraction (XRD) measurements
were conducted using a Bruker D8 DISCOVER equipped with a
Vantec-500 detector using Cu-Kα radiation. The specimens, kept
in capillary glass tubes of 1.5 mm diameter (purchased from
WJM-Glass Müller GmbH), were aligned under a magnetic field
(B = 1 T). In order to estimate the molecular parameters
including molecular conformation, dipole moment, and charge
distribution for the optimized molecular geometries, density
functional theory (DFT) calculations were used. Molecular
structure optimization was carried out using Gaussian 16 [25]
with the spacer in the all-trans conformation at the B3LYP/6-
31G(d) level of theory [26]. For visualization and analysis of the
calculated results, Gauss View 6 [27] was used.
Experimental
All chemicals were purchased from commercial suppliers as
depicted in ESI and used without further purification. The
synthetic routes for CBSnSCB and CBSnOCB are shown in Fig. 2.
Both homologous series were synthesized through a Williamson
ether reaction between 4-bromobenzenethiol or 4-
bromophenol and corresponding α,ω-dibromoalkanes, and a
Suzuki-Miyaura coupling reaction [24]. Their respective
characterization data are described in ESI.
Results and Discussion
The mesomorphic behaviors of CBSnSCB and CBSnOCB were
evaluated by a combination of POM and DSC. Upon heating,
almost all the homologs, with the exception of CBS3SCB,
CBS5SCB, and CBS6SCB, showed only conventional N phases;
their phase transition behavior is summarized in Table S1. As a
representative result, the N phase had a wider range of ca. 25
°C for CBS7OCB compared with ca. 4 °C for CBS7SCB.
The phase sequences, transition temperatures and enthalpy
changes upon cooling are listed in Table 1. POM observation for
CBS7SCB was performed in uniaxially planar-aligned polyimide-
Molecular structures were characterized by 1H NMR and 13
C
NMR measurements on a JEOL LNM-EX 400 and a Bruker
Avance III 400, respectively. The phase transitions were
investigated by polarizing optical microscopy (POM) (BX50,
Olympus) with a temperature controller (LK-600PM, Linkam
Scientific Instruments) and differential scanning calorimetry
(DSC) (DSC-
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phases. On the other hand, although CBS3OCB showed just one
melting peak at 142.9 °C without any mesophases upon the first
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DOI: 10.1039/C8NJ06456C
heating, it formed N phase after the melt at 120.2 °C upon the
second heating; it has a polymorph depending on the
crystallization conditions. It is interesting to note that CBS3SCB
also formed NTB phase, but CBS3OCB did not, at a rate of 3 °C
min-1.
5
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9
In marked contrast with mesogenic odd CBSnSCB members,
an even CBS6SCB member, that is thought of as a straight rod
shape, does not show any mesophases. This is thought to be
due to the steric hindrance derived from the less bond angle
[23] and rotational energy barrier [28] for thioether, disturbing
the mesophase formation for rod-shaped molecules [23]. This
suggests that dimeric curvature on the basis of thioether has a
beneficial effect in the mesogenic induction. CBS6OCB forms
enantiotropic N phase. Both even members show drastically
higher transition temperatures and associated enthalpy
changes compared with odd ones, reflecting the difference of
each molecular geometry.
We plotted the transition temperatures at N-I [T(NI)] and
NTB-N [T(NTBN)] against n for both odd-n members in Fig. 4. One
can see that the T(NI) values are wholly lower for sole thioether-
containing CBSnSCB than for ether-containing CBSnOCB. This is
similar to the trend observed for the calamitic mesogens with
each alkylthio and alkoxy group, which is ascribed to the steric
hindrance due to the smaller bond angle and lower rotation
barrier for the former than the latter. In the present case for
bent dimers, the T(NI) depression for sole thioether-containing
CBSnSCB are attributed to the more bent molecular geometry
compared to CBSnOCB as discussed later in DFT calculations.
The T(NI) values slightly ascend with an increasing carbon
number and then reaches the maxima at the middle length. The
T(NTBN) values also increase with an increasing carbon number
within the twist bend nematogens, resulting in the wider range
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Fig. 3. Photomicrographs of CBS7SCB for the N phase at 100 °C (a), for the NTB phase at
85 °C (b) and 50 °C (c) and (d). Crossed arrows and lower arrows represent polarizers
and rubbing directions, respectively.
Table 1. Phase transition temperature (°C) and enthalpy changes (kJ mol-1) at a rate of 3
°C min-1 upon cooling.
Sample
Phase sequence
CBS3SCB
CBS5SCB
CBS6SCB
CBS7SCB
CBS9SCB
NTB 44 (-)a Cr 63.9 (11.9)b N 83.1 (0.2) I
Cr 76.6 (19.9)b NTB 78 (-)a N 107.7 (0.7) I
Cr 186.5 (61.6) I
G 15.9 NTB 88.3 (0.3) N 115.2 (1.4) I
Cr 100.8 (33.4) N 116.7 (2.4) I
CBS11SCB Cr 101.4 (42.0) N 114.4 (3.0) I
CBS3OCB Cr 101.1 (25.4) N 137.4 (0.66) I
CBS5OCB Cr 76.6 (17.0)b NTB 90.1 (0.04) N 143.5 (1.4) I
CBS6OCB Cr 176.7 (49.2) N 191.3 (7.8) I
CBS7OCB Cr 55.0 (16.7)b NTB 95.9 (0.06) N 146.7 (2.0) I
CBS9OCB Cr 98.0 (36.3) N 143.0 (2.1) I
CBS11OCB Cr 102.5 (37.4) N 134.1 (2.9) I
a Observed by POM, b Imperfect crystallization.
surface cells with a cell thickness of 3 μm; representative of NTB phases. Here, it is interesting to note that although
photomicrographs are shown in Fig. 3. Figure 3(a) shows the N CBSnSCB form narrower N phases than CBSnOCB, the former
phase at 95 °C with a single green birefringent texture securing has wider NTB ranges than the latter, in a case where kinetically
its uniaxial alignment. It should be noted that at 86 °C, the line precedented crystallization is not considered. Wholly, the
along the rubbing direction gradually emerges, resulting in a observed trends are consistent with molecular field theory for
stripe texture, as shown in Figs. 3(b)–(d). This textural behavior V-shaped nematogens; As the molecular bend angle decreases
strongly suggests the emergence of the NTB phase. The optical from 140° or more bent geometry, its uniaxial N (Nu) phase
texture was not altered by the transition to crystal phases by region decreases by the T(NI) decrement and incidence of
cooling to ambient temperature. We found that CBSnSCB with biaxial N (NB) phase or the NTB phase, giving rise to a widened
n = 3, 5, 7 and CBSnOCB with n = 5, 7 show monotropic NTB region for the latter phases [29], wherein a certain bend angle
phases after the conventional N phase (Figs. S5–S8). This is the further close to 90° contributes to stabilize Nu again or
first report of thioether-linked twist- bend nematogens that destabilize NB or NTB phases.
include linking atoms other than carbon and oxygen. Combined
In addition, we estimated the entropy changes (ΔS) scaled
with the fact that ether-linked CBOnOCB dimers are mostly a by gas constant (R) at NTB-N transition [ΔS(NTBN)/R] and at N-I
conventional nematogen [16], the results for asymmetric transition [ΔS(NI)/R], with which the latter are listed in Table S2
CBS5OCB and CBS7OCB highlight the benefits of a thioether and plotted in Fig. 5. ΔS(NTBN)/R of 0.30 for CBS7SCB is larger
linkage for inducing the NTB phase. Since the mesogenic than 0.04 for CBS5OCB and 0.06 for CBS7OCB and similar to 0.31
behavior for the shortest spacer CBS3SCB strongly relies on for CB7CB [3]. Whilst, ΔS(NI)/R values are determined as 0.42
kinetics and its N-NTB phase transition is preceded by for CBS7SCB and 0.57 for CBS7OCB which is not so much
crystallization at ca. 64 °C, we captured its NTB phase by POM different from 0.34 for CB7CB [3,14] and 0.66 for CBO7OCB
observation as shown in Fig. S8. Although a Schiff base dimer [15a)]. As can be seen in Fig. 5, the ΔS(NI)/R values linearly
with a propylene spacer formed NTB phase transited directly correlate with n in odd members for both systems. Based on the
from I phase [21], it exhibited conventional sequences; NTB-N-I slope of the plots, the increase in ΔS(NI)/R per ethylene (-CH2-
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Fig. 4. The transition temperatures at NTB-N (circle) and N-I (square) as a function of odd
spacer number n for CBSnSCB (a) and CBSnOCB.
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Fig. 6. DSC curves for CBS7SCB upon cooling after heating. G: glass, Cr: crystal, N: nematic
phase, and I: isotropic phase.
asymmetrically methylene- and ether-linked monotropic twist
bend nematogens CB6OCB [20] and CB8OCB [19b)] crystallized
at the given rates. The present results suggest that a thioether
linkage prevents crystallization and enhances the glass-forming
ability for bent-dimeric mesogens, allowing us to easily keep the
desirable molecular aggregation states. Intriguingly, this runs
counter to the fact that highly polarizable thioether-containing
rod-like mesogens have an intrinsically stronger crystallization
ability due to stronger intermolecular attractive interactions
than those of their alkyl and alkoxy counterparts [23].
Next, CBS7SCB and CBS7OCB were used to obtain
temperature-dependent XRD measurements in a magnetic field
(B = 1 T). The 2D- and 1D-XRD patterns for CBS7SCB are shown
in Figs. 7(a)–(c), wherein the long molecular axis corresponds to
the magnetic field direction represented by an arrow. In the
small-angle region, subtle diffraction reminiscent of molecular
clusters was observed. The d-spacings were ca. 13 Å (Table S3
and Fig. S14), which were close to half of the length of each
molecule. Traditionally, it is known that nematogenic dimers
often show X-ray diffraction associated with close to or less than
a molecular half length. Although it has been ascribed to their
intercalated structures in a molecular cluster, such a model yet
requires further careful discussion. Here, it is curious that the
small-angle diffraction intensities become rather low after the
transition to the NTB phase from N phase and a similar behavior
is observed for CBS7OCB (Figs. S12 and S13). Salamończyk et al.
reported that the presence and absence of the resonant X-ray
scattering corresponding to a full and half pitch of a helix in an
NTB phase, respectively, suggests the two interlocked and
shifted double helices [6b)]. Based on the above knowledges,
the present results may reflect such a change of the local
molecular clusters formed in the N and NTB phases, so as to form
shifted double-helical structures in the latter.
Fig. 5. The plots of ΔS(NI)/R values for CBSnSCB (circle) and CBSnOCB (square) as a
function of carbon number in the odd spacer.
CH2-) is estimated as 0.11 for odd-n CBSnSCB and 0.08 for odd-
n CBSnOCB, respectively, leading to the estimation of its
increase of ca. 0.1 per an ethylene spacer for odd-sparer dimers.
It seems that ΔS(NI)/R values are smaller for CBSnSCB than for
CBSnOCB in short spacer dimers, but they inversely become
similar or slightly larger for the former than the latter in long
spacer dimers above n = 9. This may be attributed to the
speculation; short-spacer dimers reflect the bent molecular
geometry but long-spacer ones give rise to deviation from bent
geometry due to more conformers and the dilution effect of
mesogenic parts, in each mesophase [21]. Accordingly, CBSnSCB
would show the lower and similar or slightly higher values than
CBSnOCB in small and large n, respectively.
CBS7SCB was subjected to further DSC measurements at
different heating and cooling rates of 1, 3, and 10 °C min-1, as
depicted in Fig. 6. At any given cooling rate, the transition from
the N phase to NTB phase was discernible. Subsequently, the NTB
phase was supercooled to room temperature, and the glass
transitions were observed at ca. 16 °C without crystallization at
a conventional rate of 10 °C min-1. Even at slower rates of 1 and
3 °C min-1, a partially crystallized glassy state was observed,
which is supported by the existence of the glass transition and
double cold-crystallization upon subsequent heating, as shown
in Fig. S9. CBS3SCB, CBS5SCB, CBS5OCB, and CBS7OCB are also
capable of being vitrified with partial crystallization at a rate of
3 °C min-1.
We estimated their orientational order parameters in each
N phase based on the azimuthal intensity distribution centered
at 2θ = ca. 4.5 Å located on the equatorial line for CBS7SCB and
CBS7OCB and compiled them in Table S3 and plotted in Fig. 7
(d). As is the case with conventional uniaxial N phases, the
estimated S values for both molecules also show a gradual
increase on declining temperature within the range between
NTB phases that are stable down to room temperature are
highly rare [30]. The representative methylene-linked CB7CB
[14a)] and CB9CB [14b)] show low glass transition temperatures
under 10 °C and require a high cooling rate of over 30 °C min-1
to be vitrified with partial crystallization. In addition, the
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phase needs careful explanation, since the faVcitewtAhrtaictle Othnleinye
increase with decreasing temperature iDsOcIo: 1m0.m10o39n/lCy8aNcJc0e6p45te6Cd
based on other methods; 2H-NMR measurements using
deuterated methylene linkages and para-axis of cyanobiphenyl
parts in deuterated CBnCB [20,31] and Polarized Raman
Spectroscopy [32]. Here, we will carefully consider about the
origin of the broadened azimuthal intensity distribution of
wide-angle X-ray diffraction in NTB phase. In the conventional N
phase, molecules averagely uniaxially align along the magnetic
field. In the NTB phase, whereas, molecules could form each
helix with a tilt from its helical axis. Consequently, the azimuthal
intensity distribution of a wide-angle diffraction could be
expected to be wider than in the upper conventional N phase.
Actually, Agra-kooijman et al. reported the plateau-liked
azimuthal intensity distribution of a wide-angle diffraction
which can be individually fitted by gaussian equation, and is
reminiscent of such a molecular tilt [33]. Then, they estimated
that the tilt angle increases with decreasing temperature based
on the wide-angle diffractions. In the present study, in addition,
the obtained diffraction became further relaxed in the lower NTB
region, making estimation of their order parameters rather
difficult. In general, Helfrich-Hurault undulation (buckling)
instability with undulated pseudo-layer structures as observed
in smectic phases, resulting in the characteristic stripe and focal
conic textures formed in NTB phases, is accepted for NTB phase
[34]. The Helfrich-Hurault undulation instability model has a
trend where the pitch declines with decreasing temperature.
Such a broadened diffraction distribution or orientational
relaxation in NTB phase may be attributed to not only molecular
tilt but also helical tilt in undulations. Indeed, high-magnetic
field like 25 T irradiation can achieve effective molecular
alignment even in NTB phase [35] and the above described NMR
measurements also usually used around 10 T. It seems that our
condition of 1 T would not achieve the effective molecular
orientation in NTB phase, which is similar to the previous
reported XRD results using below 1.0 T [32, 36]. For reference,
the order parameters estimated in each NTB phase with perhaps
imperfect alignments are also given in Table S2 showing
decreased values than those in each N phase. In addition,
birefringence (Δn) is also one of the major factors associated
with the order parameter. Whereas, Δn-measurement in NTB
phase is very hard due to the difficulty of obtaining sufficiently
large and uniform NTB domains. Mayer et al. reported the
precise measurement of the birefringence of
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Fig. 7. 2D-XRD patterns of CBS7SCB for the N phase at 100 °C (a) and for the NTB phase at
85 °C (b), in which the long molecular axes are directed along each magnetic field
direction, as indicated by arrows, 1D-XRD patterns for CBS7SCB (c), and temperature
dependence of order parameters in each N phase for CBS7SCB and CBS7OCB based on
the azimuthal intensity distribution of wide-angle diffraction centered at 2θ = ca. 4.5Å(d).
0.3 and 0.6, which is a natural consequence of the thermal
fluctuations with respect to the director. Here, it is interesting
to note that the orientational order parameters in each N phase
were lower for CBS7SCB than for CBS7OCB, differing
substantially from the propensity observed for thioether-
containing rod-like mesogens, which tend to show higher order
parameters than those of alkoxy counterparts due to enhanced
intermolecular attractive interaction [23]. The present
observation is an embodiment of the more bent geometry for
the former than the latter in each N phase, as discussed later in
DFT calculations. The lateral molecular distances evaluated
from the wide-angle diffractions were smaller for CBS7SCB than
for CBS7OCB at a similar reduced temperature (Table S2 and Fig.
S10), implying the existence of effective intermolecular
attractive interaction enhanced by two high-polarizable sulfur
atoms.
CB7CB from its alternating double-generated single domains
with opposite handedness in the NTB phase [37]. Wherein, they
showed that the Δn once exhibits a positive jump and then
suddenly decreased below the transition from N to NTB phase,
which is an indicative of the macroscopic non-uniformity in NTB
phase. They revealed that the tilt angle increases from 9° at NTB-
N transition to saturated 37° with decreasing temperature
based on Δn measurement. With respect to tilt angle, the NMR
[31], XRD [33], PRS [34] and birefringence [37] measurements
do not run counter to one another.
As can be seen in Fig. 7(a) and (b), the orientation in the
magnetic field became distinctly more disordered in the NTB
phase than in the N phase. We fully understand that the
discussion on the orientational order parameter in the NTB
Finally, molecular geometries and electronic parameters for
CBSnSCB and CBSnOCB were determined based on DFT
calculations using Gaussian 16 and the basis function of
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of 0.145, but ether-oxygen has a negative chargeVoiewf -A0rt.ic5le2O2n.liInne
addition, thioether has less electron donor properties than
ether in the ground state [23,38], suggesting that they could
DOI: 10.1039/C8NJ06456C
have
a considerable difference in their intermolecular
6
interactions [23]. Accordingly, it can be concluded that the
nature of the NTB phase with an enhanced glass-forming ability
for the present dimers can mainly be attributed to the
synergistic effects of the largely-bent molecular geometry and
high polarizability based on thioether.
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Conclusions
Fig. 8. Optimized molecular geometries and dipole moments (arrows) of conventional
nematogen CBO7OCB, twist-bend nematogens CBS7SCB and CBS7OCB estimated based
on DFT calculations at b3lyp/6-31g(d).
In summary, the results of this study offer a new molecular
design and structural insight for the development of twist-bend
nematogens based on thioether-linked dimers. In previous
studies, twist-bend nematogenic dimers exclusively used
carbon and oxygen in the linking atoms. As a result of the more
bent dimeric curvature and high polarizability of thioether, the
present dimers show a wide range of NTB phases supercooled to
room temperature and enhanced glass-forming ability. The
present molecular designs lead to the facile synthetic method
according to Williamson ether reaction to develop linkages.
Furthermore, because of the intrinsically contained resonant
sulfur atoms in the molecular structures, the present molecular
design is expected to make it possible to apply the twist-bend
nematogens to traditional resonant X-ray scattering at the
sulfur k-edge without doping molecules with target resonant
atoms [6c),39]. We will report the detailed phase-transition
behavior, structural and physical properties of the present
dimers, and design other thioether-linked twist-bend
nematogens in future studies.
b3lyp/6-31g(d) [26]. As a reference, the DFT calculation was
applied to conventional nematogen ether-linked CBO7OCB,
too, which is consistent with the reported molecular bend
values of 140° [19b)]. The optimized molecular structures for
CBS7SCB, CBS7OCB and CBO7OCB are shown in Fig. 8. The
estimated molecular bend angles, made of the para axes of the
two cyanobiphenyl moieties in each system, were 109° for
CBS7SCB and 126° for CBS7OCB. The bend angle for CBS7SCB
was markedly smaller than, whereas that for CBS7OCB was
highly similar to, the favorable value of 125° for twist-bend
nematogens [19(b)]. Such more bent geometries for both
systems are strongly ascribed to the bond angle of thioether
linkages C-S-C estimated as ca. 103° since those for ether
linkages of C-O-C were ca. 117°, by the present DFT calculations.
It is clear that the dimers with a shorter spacer or a smaller bend
angle have a tendency to form NTB phase. This is ascribed to the
fact that short-spacer dimers reflect the bent molecular
geometry compared to long-spacer ones with more conformers
and the dilution effect of mesogenic parts, resulting in a
deviation from bent geometry. The inner benzene rings and the
directly connected thioether linkages are coplanar with each
other which is similar to the relationship for ether linkages but
different from methylene linages.
It is possible that such a bent geometry led by two thioether
linkages for the former is not only more effective for the
prevention of crystallization, as demonstrated by DSC, but also
causes the mesogenic parts to deviate from the director,
leading to the lower order parameter than that of the latter in
each N phase, as determined by XRD. It should also be
emphasized that
thioether or alkylthio groups can present markedly contrasting
phase transitions and mesogenic behaviors depending on the
inherent molecular geometry between curves and rods [23]. In
addition, their dipole moments are directed to molecular
transverse axes, as shown in Fig. 8. Notably, the dipole moment
value of 4.25 for CBS7SCB is larger than 3.26 for CBS7OCB,
where both values are larger than the 1.96 for nematogen
CBO7OCB (Fig. 8). This is achieved by the highly polarizable
sulfur atom, and such laterally large dipole moments could also
induce stable NTB phases with high glass-transition
temperatures for the present thioether-linked dimers. On the
other hand, the sulfur atom in thioether has a positive charge
Conflicts of interest
There are no conflicts to declare.
Acknowledgments
This work was supported by KAKENHI 15H06285 and
17K14493, Toyota Physical and Chemical Research Institute
Scholars, and the Naito Research Grant. We are grateful to Prof.
Masatoshi Tokita for permission to use the instruments for DSC
and temperature-variable XRD measurements.
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