The heliconical nematic twist-bend phase from “classic” bent-core benzylideneanilines with oligomethylene cores

Article abstract of DOI:10.1080/15421406.2017.1290396

We report the synthesis and properties of several new heliconical twist-bend nematic (N_TB) bimesogens possessing “classic” bent-core benzylideneaniline wings combined with oligomethylene spacers of odd parity. While short alkoxy tails combined with a resorcinol diester core in the classic system appear to give SmA phases (not B2 phases), in the present system, methoxy or ethoxy tails exhibit the N_TB phase as evidenced by textural analysis. Several of these compounds also exhibit microscopic helical defect structures in the N_TB phase, which have been reported in other lamellar and pseudo-lamellar liquid crystal phases, but not in the N_TB phase.

Full text of DOI:10.1080/15421406.2017.1290396

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
The heliconical nematic twist-bend phase from
“classic” bent-core benzylideneanilines with
oligomethylene cores
Alyssa N. Scarbrough, Michael R. Tuchband, Eva D. Korblova, Renfan Shao,
Yongqiang Shen, Joseph E. Maclennan, Matt A. Glaser, Noel A. Clark & David
M. Walba
To cite this article: Alyssa N. Scarbrough, Michael R. Tuchband, Eva D. Korblova, Renfan
Shao, Yongqiang Shen, Joseph E. Maclennan, Matt A. Glaser, Noel A. Clark & David M. Walba
(2017) The heliconical nematic twist-bend phase from “classic” bent-core benzylideneanilines
with oligomethylene cores, Molecular Crystals and Liquid Crystals, 647:1, 430-438, DOI:
10.1080/15421406.2017.1290396
To link to this article: http://dx.doi.org/10.1080/15421406.2017.1290396
Published online: 23 May 2017.
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Date: 23 May 2017, At: 20:05

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –
http://dx.doi.org/./..
The heliconical nematic twist-bend phase from “classic”
bent-core benzylideneanilines with oligomethylene cores
Alyssa N. Scarbrougha, Michael R. Tuchbandb, Eva D. Korblovaa, Renfan Shaob,
Yongqiang Shenb, Joseph E. Maclennanb, Matt A. Glaserb, Noel A. Clarkb,
and David M. Walbaa
aDepartment of Chemistry and Biochemistry and Soft Materials Research Center, University of Colorado, Boulder,
CO, USA; bDepartment of Physics and Soft Materials Research Center, University of Colorado, Boulder, Colorado,
USA
ABSTRACT
KEYWORDS
Nematic twist-bend phase;
double-helical defects;
benzylideneaniline
bimesogens
We report the synthesis and properties of several new heliconical
twist-bend nematic (NTB) bimesogens possessing “classic” bent-core
benzylideneaniline wings combined with oligomethylene spacers of
odd parity. While short alkoxy tails combined with a resorcinol diester
core in the classic system appear to give SmA phases (not B2 phases),
in the present system, methoxy or ethoxy tails exhibit the NTB phase as
evidenced by textural analysis. Several of these compounds also exhibit
microscopic helical defect structures in the NTB phase, which have been
reported in other lamellar and pseudo-lamellar liquid crystal phases, but
not in the NTB phase.
1. Introduction
One of the most exciting developments in liquid crystal (LC) science of the past 20 years,
actually representing a milestone in the history of stereochemistry, was the identification
and supramolecular structure elucidation of the first known fluid conglomerates: the B2
subphases, SmCSPF and SmCAPA, formed by P-9-OPIMB a.k.a. NOBOW (1), Figure 1 [1].
The homologous series including NOBOW, comprised of resorcinol diesters possessing ben-
zylideneaniline wings, can be considered to represent the “classic” bent-core mesogens,
first reported by Matsunaga, [2] then demonstrated to possess polar order by Takezoe and
Watanabe [3]. Within a relatively short time, the latter authors reported a related “bent-core”
possessing an odd-carbon oligomethylene core replacing the resorcinol[4] 8OAM5AMO8 (2),
Figure 1, later shown to exhibit the B7 phase [5]. The B2 and B7 phases are smectics, and as
such, possess long range positional order in one dimension.
A further advance in fundamental LC supramolecular stereochemistry has occurred over
the past several years with the discovery of at least two examples of fluid conglomerates pos-
sessing no long range positional order. One of these, first reported by Tschierske,[6] seems to
exhibit a cybotactic cubic phase, providing an optically isotropic conglomerate with only dif-
fuse scattering in the X-Ray profile. The second recent example, the NTB phase, with its own
rich history in the LC field, as described below.
CONTACT David M. Walba
walba@colorado.edu
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
©  Taylor & Francis Group, LLC

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
[699]/431
Figure . “Classic” bent-core benzylideneanilines and an exemplary member of the new NTB series of
bimesogens.
Interestingly, there seem to be no publications exploring the classic Benzylideneaniline
Bimesogen (BB) bent-core system with simple oligomethylene cores (e.g. (EtO)2-9-BB (3),
Figure 1), though Henderson and Imrie have reported that the constitutionally isomeric
=
system with the C N bonds “flipped” and a five carbon methylene core possesses NTB
phases [7]. Note the similarity in structure of the classic NOBOW and 8OAM5AMO8 sys-
tems and the new “BB” NTB system, but with the rather dramatic difference that the alkyl
chains prefer a 90° dihedral angle at the connection to the aromatic rings, while the alky-
loxy and aryloxydiesters prefer a zero degree dihedral angle, as indicated in the structures in
Figure 1.
Herein, we report on a series of homologues in the BB class exhibiting rich NTB mesomor-
phism as indicated in Table 2, filling a hole in the NTB structure space, and also exhibiting
beautiful microscopic helical defects, which has not been previously reported for the NTB
phase.
Finally, synthesis of the new bent-cores, involving a double-B-alkylborane Suzuki coupling,
is highly convergent and efficient, and to our knowledge has not previously been applied to
the synthesis of bent-core bimesogens with simple oligomethylene cores.
2. The heliconical twist-bend nematic phase
In 2001 Dozov proposed a mean field elasticity model for the structure of a nematic phase
of bent molecules, showing that the molecular bend can produce a new heliconical nematic
ground state with a helical director field [8]. Soon after, Memmer found the NTB phase in
molecular simulations of bend particles, [9] adding weight to the possibility of finding a mod-
ulated nematic phase as laid out by Dozov.
In 1993, Luckhurst presented the synthesis and characterization of the CBnCB series
reporting a phase sequence that they interpreted as Iso – N – SmA [10]. In 2010–11 this
phase sequence was further explored by Panov and Cestari, the latter arguing that the lowest
temperature phase was the heliconical twist-bend nematic, predicted by Dozov and Memmer,
rather than the SmA [11, 12]. Since that time, a variety of experimental approaches, includ-
ing the direct observation of the helical modulation via freeze-fracture transmission electron
microscopy (FFTEM) [13, 14] and Resonant Soft X-Ray Scattering (RSoXS) at the carbon K
edge, [15] have supported the identification of this phase as a heliconical nematic twist-bend
motif.

432/[700]
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Table . The BB series synthesized and examined in this paper. R groups in blue exhibited N and NTB phases.
The general design theme to obtain the NTB phase are molecules possessing two lin-
ear “wings” comprised of rigid mesogenic units, spaced by alkyl chains with an odd num-
ber of carbons introducing, on average, a bent-core. A variety of structural designs that
exhibit the NTB phase have been reported in literature, including: the initial alkyl - spaced
bimesogens with varying cores and end groups; [16] bimesogens with alkyl- heteroatomic
conjugated spacers; [17, 18] bimesogens with hydrogen-bonding end groups; [19] a semi-
flexible structure possessing a 1-benzoyl-4-phenylpiperazine core; [20] bent-core ”hybrid”
[21] mesogens; and an oligomeric mesogen [22]. Other developments include a reversible N
– NTB switchable unsymmetrical azo-benzene bimesogen, [23] benzyloxybenzylidene bime-
sogens with an Iso – NTB transition, [24] and the first observation of an NTB – smectic
transition [25].
Here we focus on the benzylideneaniline rigid “wings,” combined with simple odd-carbon
alkyl cores –with compound (EtO)2-9-BB (3), Figure 1, serving as a prototype. A complete
list of R-n-BB prepared in the current study is given in Table 1.
Among these compounds only the ethoxy homologs exhibit enantiotropic N and NTB
phases. The broadest NTB ranges result from C9- and C11- cores. These materials were stud-
ied via PLM, FFTEM, DSC, and X-Ray diffraction. FFTEM experiments exhibit a distribution
of observed periodicities with a peak at ∼8 – 9 nm in the NTB phase. We also identify micron-
scale helical defects of large Burgers vector in the NTB phase of several members of the R-n-BB
series, which have been reported for other lamellar and pseudo-lamellar liquid crystal phases,
but not, to our knowledge, for the NTB phase.
Table . Phase diagram for R-n-BB compounds exhibiting N and NTB phases. All temperatures reported in
degrees Celsius and determined with PLM. Parenthesis indicate monotropic phase behavior.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
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Scheme . Synthetic scheme for the R-n-BB targets.
3. Synthesis of the R-n-BB series
The general synthetic route for the R-n-BB series is illustrated in Scheme 1. B-alkyl Suzuki
coupling of α,ω-dienes with dimethyl acetal protected aryl halides afforded intermediates, 2 –
5. Subsequent acid-promoted deprotection of the dimethyl acetals in aqueous acetone resulted
in the intermediate dialdehydes, 6 – 9. These dialdehydes underwent Schiff base formation
with aniline and para alkoxy-, para alkyl-, and para hydroxy anilines to afford target R-n-BB
compounds, 10 – 15, which will be discussed in latter sections. Refer to methods section in
supplementary information below for detailed experimental procedures.
4. Results and discussion
As determined by methods described in detail in the supplementary material, it was found that
all R-n-BB compounds 10-15 exhibit the Iso – N – NTB phase sequence on cooling. Compound
10 ((MeO)2-7-BB) exhibits a monotropic N – NTB transition, while compounds 11, 12 and 14
exhibit enantiotropic N phase and a monotropic N - NTB transition on cooling. Bimesogens
13 and 15 exhibit enantiotropic N and NTB phases.
Compounds 16 and 17 ((MeO)2-5-BB, and (EtO)2-5-BB respectively) did not possess any
nematic phases (structures can be found in Supplementary Information). They did, however,
contain a monotropic X2 – X1 transition on heating. The lower temperature uncharacterized
phase, X2, is birefringent, whereas the second uncharacterized, X1, is weakly birefringent.
Neither X2 nor X1 respond to shear stress when between glass slides. This result is coun-
terintuitive because initial predictions by Ferarrini et al. suggest that bimesogens with shorter
alkyl spacers prefer a bent molecular conformation in the isotropic phase, which would favor a
nematic-nematic transition in the material [26, 27]. This lesser amount of flexible alkyl spacer
in these two homologs may result in a more rigid bent-core bimesogen, which we can expect
to promote higher ordered phases, such as those observed for these compounds.
Bimesogens that contained odd-parity alkyl spacers >C5, and which contained alkoxy end
groups longer than OCH2CH3 were either non-mesogenic or contained a higher-ordered liq-
uid crystal phase. This result is not surprising because we expect bent-core structures with
long flexible tails to promote the formation of lamellar phases. What is surprising is that the
mesophases formed from the BB did not follow previously reported trends where the SmCP
phase predominated—instead of the NTB phase—as alkyl tails were lengthened [20, 28].
The bimesogen with terminal hydroxyl groups had no LC phases, which is interesting since
Imrie recently showed that the NTB phase can be driven by hydrogen bonding of terminal
carboxylic groups (more specifically: intermolecular dimerization of carboxylic groups) of an
unsymmetrical bimesogen [19].

434/[702]
A. N. SCARBROUGH ET AL.
Textural analysis of R-n-BB LCs
The BB series were studied with PLM in both untreated glass slides and (antiparallel) planar
aligned polyimide cells. Samples contained between untreated glass slides exhibited random
planar alignment of the N phase which exhibit visible director fluctuations. As the N phase
is cooled, fluctuations decrease and finally cease on transition to the lower temperature NTB
phase. Below this transition, a paramorphic broken focal conic texture of the NTB forms. On
further cooling, a randomly aligned striped texture which we attribute to layer undulations—a
phenomenon originally described by Clark [29, 30]—gradually fills the cell and remains until
a subsequent phase transition occurs.
In the 4 µm planar aligned cell the N phase exhibits uniform alignment and nematic direc-
tor fluctuations. The N - NTB transition subsequently occurs with no significant change in
birefringence and a transition from the smooth N texture to a broken focal conic texture
(Figure 2 (a)). Upon further cooling into the NTB phase a periodic striped deformation—
initially investigated by Panov [11]—grows in, with the stripes parallel to the cell rubbing
direction (Figure 2 (b)). Small domains of elliptical polygonal texture remain throughout the
entirety of the cell with the striped texture.
The 3.4 µm cell preparations exhibited the same uniform alignment and fluctuations of the
N phase and the same broken focal conic texture just below the N – NTB transition (Figure 2
(c)) as the 4 µm sample. However, upon cooling in the NTB phase, the periodic striped pattern
was not consistent across the cell. Rather, the striped pattern grew into larger conglomerate
Figure . PLM of (EtO)--BB 13. a) The texture of the NTB phase just below the N - NTB transition in a  µm
(antiparallel) planar cell at .°C (cooling rate: °C/min). b) Periodic striped texture of the NTB phase in
 µm (antiparallel) planar cell at .°C. The stripes align parallel to the rubbing direction (cooling rate:
°C/min). c) The texture of the NTB phase just below the N - NTB transition at .°C in a . µm (antiparallel)
planar aligned cell (cooling rate: °C/min). d) Striped texture of NTB phase at .°C in a . µm (antiparallel)
planar aligned cell. Periodic stripes grow parallel to the rubbing direction and form into larger conglomerate
domains. (cooling rate: °C/min).

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domains (Figure 2 (d)), which suggests some dependence of these periodic deformations on
cell thickness.
Double helix defects
Double helix defects (DHDs) were observed on fast heating for both enantiotropic BBs, 13
and 15 (Figure 3 (a)). The DHDs are kinetically trapped by first bringing the hot stage of
the PLM to 120°C, for 13, and 118°C for 15. Sample cells were then individually placed on
the hot stage where the NTB fan texture and DHDs quickly grew in. Careful examination of
15 showed both left- and right-handed DHDs (Figure 3 (b) and (c)), but with a surprising
preponderance (>95%) of left-handed DHDs. We believe the DHDs can be modeled by a
pair of two +1/2 line defects formed from the NTB “pseudo-layers”, defined by the position
of constant azimuthal angle in the NTB helix, which coil around each other into a double
helix. This behavior has been observed in other layered and pseudo-layered liquid crystal
media, including cholesterics and smectics, [31–34] but this is the first observation of this
phenomenon in the NTB phase. It can be described as a paired screw dislocation with a large
Burger’s vector of several layers. In the NTB phase, the role of the layers is played by positions of
constant azimuthal angle in the NTB helix which are separated by a single pitch length, making
the Burger’s vector some hundreds of NTB “pseudo-layers” in our samples. This appears to
indicate that these compounds are more strongly “layering” than other NTB-forming materials
which do not readily form these defects.
Differential scanning calorimetry
Thermal behavior of the BB series is reported below. Latent heat peaks were analyzed at atmo-
spheric pressure to support phase transition data originally assigned with PLM. Some latent
heat peaks for the N - NTB transition were unobservable, which is in accordance with other
NTB systems.
All measurements began at 25°C, were heated above the isotropic temperature, then cooled
back down to 25°C. This cycle was repeated twice: once at 10°C/min and then at 5°C/min.
Some N - NTB latent heats were unobservable—even with increased sample size and slower
cycle rates. In these cases, the phase transition temperatures were assigned via PLM.
Figure. Helicaldefectsin13and15,observedinµm(antiparallel)planaralignedpolyimidecells.a)Helical
defects of neat 13 are observed at °C on fast heating. The sample is mostly well aligned, as the sample
is extinguished well everywhere except where the bright helical defects are formed. b) A left-handed helix
and c) right-handed helices at °C in neat 15 on fast heating.

436/[704]
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X-ray diffraction
We carried out small and large angle X-Ray diffraction studies on several of our compounds to
determine the nature of the phase below the temperature of the N phase. Several studies that
used wide- and small- angle X-Ray diffraction at 10 keV have shown that diffraction from
the NTB phase is nearly identical to that of the nematic phase, with a notable lack of sharp
scattering peaks—that would indicate the presence of a smectic phase [12, 35]. At tempera-
tures which exhibit the NTB phase in our compounds, we find that the diffraction pattern is
nematic-like, with no evidence for a lamellar structure (see Figures S7 – S10 in Supplemen-
tary Information). This indicates that the phase is locally nematic, and is consistent with our
designation of the phase as the twist-bend nematic.
Freeze fracture transmission electron microscopy
To visualize and investigate the nanoscale structure of compound 13, we prepared it for
FFTEM imaging. We observed nanometer-scale modulations, which are characteristic of the
NTB phase and are reported in several FFTEM studies [13, 14, 20, 36]. Figure 4 (a) shows an
FFTEM image of twist-bend nematic with domains of pitch around 8.4 nm, the most typi-
cal pitch length for 13 at this quenching temperature. These compounds generally form well-
aligned domains on the order of several hundred nanometers (Figure 4 (b)), somewhat smaller
than the micron-scale domains observed in twist-bend nematic-forming material CB7CB. In
addition, these compounds have a somewhat stronger propensity for fracturing at ‘pseudo-
layer’ steps (Figure 4 (b)), in analogy with layer fracture steps which are a signature of smectic
phases. This may imply an enhanced pseudo-layer stiffness, which has not been observed in
other NTB systems.
Figure . FFTEM images of BB compounds. Green scale bars indicate  nm regions. a) Red line indicates
orientation of heliconical director of the NTB phase in 13 at °C whereas blue arrows highlight examples of
domain boundaries. b) Yellow arrows illuminate an area that contains psuedo-layer steps in the NTB phase
of 13 at °C. c) Nematic phase of 13 at °C. No indication of heliconical fluctuations nor pseudo-layer
steps. d) Normalized pitch distribution of 13 in the NTB phase quenched at °C. The histogram shows the
probability of finding a region in the sample with a given pitch in a bin size of . nm.

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To get the most complete picture of the behavior of the pitch as observed in FFTEM, we
utilize a statistical approach. For each FFTEM image, we measure the pitch length in each NTB
domain and measure the corresponding domain area. The area acts as a weighting factor for
the pitch length to give us the ‘frequency’ that a given pitch length is observed by FFTEM at the
specified quenching temperature. In the FFTEM method, we measure not only the expected
behavior of the pitch in a sample, but we get a broadening of the pitch distribution both by
having fracture planes that are not normal to the helix axis and having the fracture planes
not normal to the electron beam direction [36]. The pitch distribution for 13 is shown in
Figure 4 (d). A resonant X-Ray study of CB7CB [15] shows that a generic trait of the NTB
phase at temperatures just below the N-NTB transition is the presence of a mosaic of domains
of different pitch lengths, leading to an unexpectedly broad distribution. Because we quenched
several degrees below the N-NTB transition, we believe we are in this regime.
5. Conclusions
We have prepared and characterized a series of liquid crystalline bimesogens (R-n-BB) pos-
sessing the “bent-core classic” benzylideneaniline “wings,” combined with simple alkyl cores
of odd carbon number. These are shown to exhibit monotropic or enantiotropic NTB phases.
The optimal spacer length for obtaining the NTB phase is between 9 and 11 carbons, and the
minimum length required to form NTB phases for these homologs is 7 methylenes.
To our knowledge, we report here for the first time the formation of microscopic double
helix dislocations in an NTB phase. It is currently unclear whether these helices are an indi-
cation of phase chirality, or if they are paramorphic features that are kinetically trapped on
heating from the crystal.
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