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¶ That no switching peak could be observed even in the SmC* phases
could be due to the known weak effect of the (S)-2-methylbutyl group on
polar order, leading to very small values of spontaneous polarization.5,14b
Also optically, no switching can be observed; this confirms that there is no
switching or it takes place by rotating around the molecular long axis.
8 This kind of chiral SmC phase was previously reported for 4-cyano-
resorcinol based BC molecules with azobenzene wings, and a local SmCsPF
structure was assumed to be responsible for chirality.18 However the recent
observation of chiral segregation in isotropic liquids13 supports the
possibility of spontaneous chiral segregation in SmC phases with a dense
packing of the aromatic cores. This chiral segregation provides a helical
distortion of the layers, thus giving rise to layer distortion and leading to
amplification of small chirality effects.
** TGB-like twist states and TGB phases were observed for mixtures of
Fig. 4 Textures of the homeotropic SmC phases (a–c) of rac-1/12 and
(d–f) of (S)-1/12 at T = 75 1C as observed (b,e) between crossed polarizers
and between uncrossed polarizers with the analyzer rotated by 51 (a, d)
clockwise or (c, f) counter-clockwise; reversal of brightness in the series
(a–c) indicates chiral domains with opposite handedness, whereas there is
no effect in the series (d–f) (see also Fig. S6, ESI†).
chiral rod-like molecules with achiral bent-core molecules19 or hockey-
stick compounds20 and for chiral dimesogens with odd spacers.17
A
TGB-like structure has also been discussed as a possible organization in
the dark conglomerate (DC) phases of achiral BC molecules,21a but only
recently a DC-like random grain boundary phase of achiral hockey-stick
LC was reported.21b
1 D. B. Amabilino, Chirality at the Nanoscale, Wiley-VCH, Weinheim,
2009.
2 Handbook of Liquid Crystals, ed. J. W. Goodby, P. J. Collings, T. Kato,
C. Tschierske, H. F. Gleeson and P. Raynes, Wiley-VCH, Weinheim,
2014.
3 C. Tschierske, Angew. Chem., Int. Ed., 2013, 52, 8828–8878.
4 I. Dierking, Symmetry, 2014, 6, 444–472.
5 Chirality in liquid crystals, ed. H.-S. Kietzerow and C. Bahr, Springer,
New York. NY, 2001.
6 (a) J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and
J. S. Patel, Nature, 1989, 337, 449–452; (b) J. W. Goodby, Curr. Opin.
Colloid Interface Sci., 2002, 7, 326–332; (c) M. Brunet, L. Navailles
and N. A. Clark, Eur. Phys. J. E: Soft Matter Biol. Phys., 2002, 7, 5–11.
7 S. R. Renn and T. C. Lubensky, Phys. Rev. A: At., Mol., Opt. Phys.,
1988, 38, 2132–2147.
8 H. T. Nguyen, M. Ismaili, N. Isaert and M. F. Achard, J. Mater. Chem.,
2004, 14, 1560–1566.
9 (a) R. A. Reddy and C. Tschierske, J. Mater. Chem., 2006, 16, 907–961;
(b) H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., 2006, 45,
597–625; (c) A. Eremin and A. Jakli, Soft Matter, 2013, 9, 615.
10 L. E. Hough, M. Spannuth, M. Nakata, D. A. Coleman, C. D. Jones,
coupling between them. The cybotactic nature of the nematic phase
and the absence of clear transition enthalpies at the phase transi-
tions indicate a nearly continuous growth of the coherence length of
the smectic clusters throughout the NcybA–SmA–SmC transitions
(Fig. 3d, Table S1, ESI†). This indicates that already in the nematic
phase the cybotactic clusters are relatively large and continue to
increase in the smectic phases. Thus, in the SmA and SmC phases
there is a remaining layer distortion providing soft layers which can
be easily deformed into a TGB-like superstructure (Fig. 1). Therefore,
the weak helical twisting power of the (S)-2-methylbutyl group can
provide a sufficiently strong chirality effect such that TGB states are
formed at the N*–SmA transition. These can be further stabilized
over broader temperature ranges by planar surface anchoring.
In summary, the first observation of TGB structures in meso-
phases of bent-core mesogens is reported.** These are formed by
molecules with a (S)-2-methylbutoxy stereogenic centre, known to
have only weak helical twisting power. However, strong chirality
of the molecular conformers, an imperfect layer structure and
surface anchoring can stabilize the TGB states.
¨
G. Dantlgraber, C. Tschierske, J. Watanabe, E. Korblova, D. M.
Walba, J. E. Maclennan, M. A. Glaser and N. A. Clark, Science,
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11 H. Takezoe, Top. Curr. Chem., 2012, 318, 303–330.
12 V. P. Panov, R. Balachandran, J. K. Vij, M. G. Tamba, A. Kohlmeier
and G. H. Mehl, Appl. Phys. Lett., 2012, 101, 234106.
13 C. Dressel, T. Reppe, M. Prehm, M. Brautzsch and C. Tschierske,
Nat. Chem., 2014, 6, 971–977.
14 (a) H.-C. Jeong, S. Aya, S. Kang, F. Araoka, K. Ishikawa and
H. Takezoe, Liq. Cryst., 2013, 40, 951–958; (b) H. Ocak, B. Bilgin-
Eran, M. Prehm, S. Schymura, J. P. F. Lagerwall and C. Tschierske,
Soft Matter, 2011, 7, 8266–8280.
The work was supported by DFG (Ts 39/24-1); H. O. is grateful
to the Alexander von Humboldt Foundation for a research fellow-
ship at Martin Luther University, Halle-Wittenberg; B. B.-E. is
grateful to the Alexander von Humboldt Foundation for financial
support toward LC research.
¨
15 (a) L. Kovalenko, M. W. Schroder, R. A. Reddy, S. Diele, G. Pelzl and
W. Weissflog, Liq. Cryst., 2005, 32, 857–865; (b) C. Keith, A. Lehmann,
U. Baumeister, M. Prehm and C. Tschierske, Soft Matter, 2010, 6,
1704–1721.
Notes and references
16 I. Dierking and S. T. Lagerwall, Liq. Cryst., 1999, 26, 83–95.
‡ The helical twisting power is an empirical measure of the effect of 17 C. V. Yelamaggad, A. S. Achalkumar, N. L. Bonde and A. K. Prajapati,
chirality on LC superstructures. It depends on molecular structural para- Chem. Mater., 2006, 18, 1076–1078.
meters, conditions (e.g. T), enantiomeric purity and the type of super- 18 M. Alaasar, M. Prehm, M. Nagaraj, J. K. Vij and C. Tschierske, Adv.
structure. It is inversely proportional to the helical pitch and depends on Mater., 2013, 25, 2186–2191.
the helicity of the molecular conformations, the energy barriers between 19 P. Archer and I. Dierking, Liq. Cryst., 2006, 33, 257–265.
enantiomorphic conformers and the degree of coupling of the stereogenic 20 V. Novotna, M. Glogarova, V. Kozmik, J. Svoboda, V. Hamplova,
unit with the molecular conformational helicity.5
M. Kaspar and D. Pociecha, Soft Matter, 2013, 9, 647–653.
§ The birefringence in the homeotropic SmC* phases is much lower 21 (a) S. K. Lee, L. Shi, M. Tokita and J. Watanabe, J. Phys. Chem. B,
than in the SmC phase of rac-12 (see Fig. 4), indicating the presence of a
helix along the layer normal being larger than the wavelength of light.
2008, 112, 6762–6766; (b) D. Chen, H. Wang, M. Li, M. A. Glaser,
J. E. Maclennana and N. A. Clark, Soft Matter, 2014, 10, 9105–9109.
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Chem. Commun., 2015, 51, 7512--7515 | 7515