Angewandte
Chemie
Liquid Crystals
A Twist-Bend Nematic (NTB) Phase of Chiral Materials**
ˇ
ˇ
Ewa Gorecka,* Natasa Vaupotic, Anna Zep, Damian Pociecha, Jun Yoshioka, Jun Yamamoto,
and Hideo Takezoe
Abstract: New chiral dimers consisting of a rod-like and
cholesterol mesogenic units are reported to form a chiral twist-
bend nematic phase (NTB*) with heliconical structure. The
compressibility of the NTB phase made of bent dimers was
found to be as large as in smectic phases, which is consistent
with the nanoperiodic structure of the NTB phase. The atomic
force microscopy observations in chiral bent dimers revealed
a periodicity of about 50 nm, which is significantly larger than
the one reported previously for non-chiral compounds (ca.
10 nm).
structure, with the director inclined from the helix axis by an
angle lower than 90 deg (oblique helix) is believed to be
formed because of a negative bend elastic constant[6] or
a large flexoelectric effect.[7,8] Such conditions are realized for
the bent molecules, for example dimers with rigid cores
connected by sufficiently flexible linkers with an odd number
of carbon atoms[9] or hydrogen bonds,[10] and bent-core
molecules.[11] Since the heliconical structure of the NTB
phase is not related to molecular chirality, the obvious
question arises: what effect does the molecular chirality
would have on the heliconical structure? So far only one
report was published, in which the authors showed that for
chiral twin molecules the strong competition between the
effects induced by chirality and molecular bend might lead to
a sequence of modulated nematic phases with distinct phase
transitions in a narrow temperature range between the
regular cholesteric phase (N*) and new nematic phase,
preliminarily identified as a “splay-bend” nematic struc-
ture,[12] in agreement with theoretical predictions.[13] Here we
extend our studies to several chiral dimeric materials showing
the N-N phase transition; by miscibility studies the lower-
temperature nematic phase was identified as the NTB phase
and the upper temperature phase as a cholesteric one.
Asymmetric bi-mesogens are studied, in which rod-like
units, azo (Azo series) or imine (SB series), are connected
with a cholesteric unit by an alkyl spacer with an odd number
of carbon atoms (Figure 1, for synthesis details see the
Supporting Information). Such molecular geometry promotes
bent conformers, in which two mesogenic units make an
average angle of 120 deg; it also ensures large flexibility
because of a low energy barrier for rotations around chemical
bonds within the spacer. Two basic molecular modifications
were applied for both series of compounds; the length (n) of
a terminal alkyl or alkoxy chain at the rod-like azo/imine unit
and the length of the flexible spacer (m) were changed. For
T
here is a growing number of reports on achiral compounds
that display a new type of nematic phase, having a chiral
structure.[1,2] The existence of such a phase, named the “twist-
bend nematic (NTB)” is an intriguing and unique example of
spontaneous mirror symmetry breaking that occurs in a fluidic
state. The question about mechanism that might lead to
formation of chiral structures in achiral liquid is of funda-
mental importance. It is also important to understand how
weak chiral interactions manifest themselves in such struc-
tures. Transmission electron microscopy (TEM)[1–3] and
atomic force microscopy (AFM)[4] measurements showed
that the wavelength of spatial helical modulations in the NTB
phase is extremely short, just about 3 molecular lengths. Such
a short pseudo-layer structure is responsible for characteristic
focal conic textures of the NTB phase.[4,5] The short heliconical
[*] Prof. E. Gorecka, A. Zep, Dr. D. Pociecha, Prof. H. Takezoe
Department of Chemistry, Warsaw University
ul. Zwirki i Wigury 101, 02-089 Warsaw (Poland)
E-mail: gorecka@chem.uw.edu.pl
ˇ
Prof. N. Vaupotic
Faculty of Natural Sciences and Mathematics
ˇ
University of Maribor, Koroska 160, Maribor (Slovenia)
and
Jozef Stefan Institute
Jamova 39, 1000 Ljubljana (Slovenia)
a fixed terminal chain (n = 1), compounds show the N*-NTB
*
phase transition, except for the azo compound with the
longest m = 15 tail, for which the N*-SmA phase transition
was found (related DSC scans are presented in the Supporting
Information). Apparently changing the length of the inner
spacer does not influence much the phase sequence. On the
contrary, extending the terminal alkyl chain (for a chosen
spacer length, m = 5) exerts much more profound influence
on the phase sequence; the NTB* phase was stable only for
short homologues, n = 1–3 for imine and only for n = 1 for azo
derivatives. The homologues with alkyl tails longer than n = 4
for imine and n = 2 for azo derivatives show complex
sequences of modulated smectic phases. The phase sequence
for studied compounds is presented in Figure 1.
Dr. J. Yoshioka
Department of Physics and Applied Physics
Waseda University
3-4-1 Okubo, Shinjuku-ku, Tokyo169-8555 (Japan)
Prof. J. Yamamoto
Department of Physics, Kyoto University
Kitashirakawaoiwake-cho, Sakyo-ku, Kyoto
606-8502 (Japan)
Prof. H. Takezoe
Toyota Physical and Chemical Research Institute
41-1, Yokomichi, Nagakute, Aichi 480-1192 (Japan)
[**] This work was financed by Foundation for Polish Science under
program MASTER 3/2013 and the research program P1-0055
financed by the Slovenian Research Agency.
In the lower-temperature nematic phase the viscosity was
comparable to the viscosity of the smectic phase, but the X-
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2015, 54, 10155 –10159
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
10155