Silicon-containing polyphilic bent-core molecules: The importance of nanosegregation for the development of chirality and polar order in liquid crystalline phases formed by Achiral molecules

Article abstract of DOI:10.1021/ja057685t

Polyphilic molecules composed of a bent aromatic core, oligo(siloxane) units, and alkyl segments were synthesized, and the self-organization of these molecules was investigated. Most materials organize into polar smectic liquid crystalline phases. The swi

Full text of DOI:10.1021/ja057685t

Published on Web 02/09/2006
Silicon-Containing Polyphilic Bent-Core Molecules: The
Importance of Nanosegregation for the Development of
Chirality and Polar Order in Liquid Crystalline Phases Formed
by Achiral Molecules
Christina Keith,† R. Amaranatha Reddy,† Anton Hauser,‡ Ute Baumeister,‡ and
Carsten Tschierske*,†
Contribution from the Institute of Organic Chemistry, Martin-Luther UniVersity
Halle-Wittenberg, Kurt-Mothes Strasse 2, D-06120 Halle, Germany, and Institute of Physical
Chemistry, Martin-Luther UniVersity Halle-Wittenberg, Mu¨hlpforte 1, D-06108 Halle, Germany
Received November 11, 2005; Revised Manuscript Received January 9, 2006; E-mail: carsten.tschierske@chemie.uni-halle.de
Abstract: Polyphilic molecules composed of a bent aromatic core, oligo(siloxane) units, and alkyl segments
were synthesized, and the self-organization of these molecules was investigated. Most materials organize
into polar smectic liquid crystalline phases. The switching process of these mesophases changes from
antiferroelectric for the nonsilylated compounds via superparaelectric to surface-stabilized ferroelectric with
increasing segregation of the silylated segments. It is proposed that the siloxane sublayers stabilize a
polar synclinic ferroelectric (SmCsPF) structure, and the escape from a macroscopic polar order as well as
steric effects leads to a deformation of the layers with formation of disordered microdomains, giving rise to
optical isotropy. Another striking feature is the spontaneous formation of chiral domains with opposite
handedness. For two compounds, a temperature-dependent inversion of the optical rotation of these domains
was found, and this is associated with an increase of the tilt angle of the molecules from <45° to >50°.
This observation confirms the recently proposed concept of layer optical chirality (Hough, L. E.; Clark, N.
A. Phys. Rev. Lett. 2005, 95, 107802), which is a new source of optical activity in supramolecular systems.
With increasing length of the alkyl chains, segregation is lost and a transition from smectic to a columnar
phase is found. In the columnar phase, the switching process is antiferroelectric and takes place by rotation
of the molecules around the long axes, which reverses the layer chirality; that is, the racemic ground-state
structure is switched into a homogeneous chiral structure upon application of an electric field.
materials for electronic circuits and photovoltaics,4 switchable
NLO materials, and sophisticated materials for optical switches
and phase modulators.5 Hence, the fundamental understanding
of the self-organization in such systems is of importance for
future developments in science and technology. This requires
the detailed understanding of the effects of molecular-structural
parameters upon nanoscale morphologies and macroscopic
properties of these systems. The introduction of molecular
chirality6 and polyphilicity7 into LC materials has been used to
provide novel, more complex self-organized LC structures,8
among them, cubic and other 3D phases,9 cylinder phases,8f,i,j,l,m,o,p,r
Introduction
Supramolecular chemistry aims at developing complex sys-
tems from programmed molecules in a thermodynamically
controlled self-assembly process.1 Liquid crystals (LC) represent
a special type of self-organized systems, which is fluid and has
a well defined periodic nanoscale structure, giving rise to
anisotropic physical properties. In addition, these materials can
easily change their configuration under the influence of different
external stimuli. This unique combination of long range order
and mobility is also a basic requirement for the development
of living matter and provides the foundation of numerous future
technological applications. Early work on liquid crystalline (LC)
materials gave quite simple (nematic, smectic, and columnar)
phase structures,2 which already find broad use, for example,
as materials in displays of mobile communication and data
processing devices. Future LC materials might, for example,
provide new photonic materials,3 semiconducting organic
(3) (a) Coles, H. J.; Pivnenko, M. N. Nature 2005, 436, 997-1000. (b) Hwang,
J.; Song, M. H.; Park, B.; Nishimura, S.; Toyooka, T.; Wu, J. W.; Takanishi,
Y.; Ishikawa, K.; Takezoe, H. Nat. Mater. 2005, 4, 382-387.
(4) (a) Simpson, C. D.; Wu, J.; Watson, M. D.; Mu¨llen, K. J. Mater. Chem.
2004, 14, 494-504. (b) Schmidt-Mende, L.; Fechtenko¨tter, A.; Mu¨llen,
K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science 2001, 293, 1119-
1122.
(5) O’Callaghan, M. J.; Wand, M. D.; Walker, C. M.; Nakata, M. Appl. Phys.
Lett. 2004, 85, 6344-6346.
† Institute of Organic Chemistry.
(6) (a) Pansu, B. Mod. Phys. Lett. B 1999, 13, 769-782. (b) Kitzerow, H.-S.;
Bahr, C. Chirality in Liquid Crystals; Springer: New York, 2001.
(7) (a) Tschierske, C. J. Mater. Chem. 1998, 8, 1485-1508. (b) Tschierske,
C. J. Mater. Chem. 2001, 11, 2647-2671. (c) Tournilhac, F.; Blinov, L.
M.; Simon, J.; Yablonsky, S. V. Nature 1992, 359, 621-623. (d) Ostrovskii,
B. I. In Structure and Bonding 94, Liquid Crystals I; Mingos, D. M. P.,
Ed.; Springer; Berlin, 1999; pp 200-240.
‡ Institute of Physical Chemistry.
(1) (a) Lehn, J.-M. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4763-4768. (b)
Lehn, J.-M. Science 2002, 295, 2400-2403.
(2) Handbook of Liquid Crystals; Demus, D., Goodby, J. W., Gray, G. W.,
Spiess, H.-W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany, 1998;
Vols. 1-3.
9
10.1021/ja057685t CCC: $33.50 © 2006 American Chemical Society
J. AM. CHEM. SOC. 2006, 128, 3051-3066
3051

A R T I C L E S
Keith et al.
quasi liquid crystalline phases,10 and LC phases showing
ferroelectric (FE) and antiferroelectric (AF) properties.6b
Reduction of the symmetry of rigid segments is an additional
way to increase the complexity of LC superstructures. This was
shown for molecules with a bent aromatic core structure
(banana-shaped LC), which can give rise to macroscopic polar
order in fluid systems.11 Polar order results from the packing
of the bent-core molecules in layers where the bent structure
restricts their rotation around the long axis, leading to a
spontaneous polarization (P) parallel to the layer planes.12 In
adjacent layers, both tilt direction and polar direction can be
either identical or opposite, leading to, in total, four different
structures, as shown in Figure 1.13 Two of them are macroscopi-
cally polar (SmCsPF and SmCaPF); the others are macroscopi-
cally nonpolar (SmCaPA and SmCsPA). Usually, the AF states
(SmCaPA and SmCsPA) are stable and can be switched into the
corresponding FE states (SmCsPF and SmCaPF, respectively)
under the influence of an external electric field. Upon switching
off the field, the polar FE states usually relax back to the
nonpolar AF states. This switching process has three stable states
and therefore represents an AF switching process.
We have recently enhanced the complexity of bent-core
molecules by introduction of an additional incompatible unit,
leading to a new class of liquid crystals composed of three
incompatible units: a bent rigid aromatic core, two flexible alkyl
chains, and a highly flexible and bulky oligosiloxane end group
at one terminus.14 These polyphilic bent-core molecules combine
two fundamental concepts of LC design, the concept of
polyphilicity7 and the concept of bent-core molecules. The
molecules are abbreviated as Six-mBn, where x gives the number
Figure 1. (a) The switching of bent-core molecules (side views) and (b)
the four possible arrangements in tilted polar smectic phases. The subscripts
‘s’ and ‘a’ indicate the correlation of the tilt direction in adjacent layers:
s ) synclinic means an identical tilt direction; a ) anticlinic indicates an
opposite tilt direction. Subscripts ‘A’ and ‘F’ indicate the correlation of
the polar direction in adjacent layers. PA is indicative of an antipolar structure
(polar direction alternates); this structure is macroscopically nonpolar and
usually assigned as antiferroelectric. PF stands for a synpolar structure (polar
direction is identical); this structure is macroscopically polar and usually
assigned as ferroelectric (and therefore abbreviated as PF). The color
indicates the chirality of the layers (superstructural chirality; for details,
see Figure S1 in Supporting Information). The SmCaPA and SmCsPF
structures are homogeneous chiral (identical chirality sense in the layers),
and the SmCsPA and SmCaPF structures are macroscopic racemic (alternating
chirality sense in the layers). Switching usually takes place on a cone (shown
in the middle). It reverses polar direction and tilt direction and retains the
layer chirality.
(8) (a) Tschierske, C. Ann. Rep. Prog. Chem., Ser. C 2001, 97, 191-267. (b)
Goodby, J. W.; Mehl, G. H.; Saez, I. M.; Tuffin, R. P.; Mackenzie, G.;
Auze´ly-Velty, R.; Benvegnu, T.; Plusquellec, D. Chem. Commun. 1998,
2057-2070. (c) Lee, M.; Cho, B.-K.; Zin, W.-C. Chem. ReV. 2001, 101,
3869-3892. (d) Kato, T. Science 2002, 295, 2414-2418. (e) Barbera´, J.;
Donnio, B.; Gime´nez, R.; Guillon, D.; Marcos, M.; Omenat, A.; Serrano,
J. L. J. Mater. Chem. 2001, 11, 2808-2813. (f) Ko¨lbel, M.; Beyersdorff,
T.; Cheng, X. H.; Kain, J.; Diele, S. J. Am. Chem. Soc. 2001, 123, 6809-
6818. (g) Tschierske, C. Curr. Opin. Colloid Interface Sci. 2002, 7, 69-
80. (h) Gehringer, L.; Bourgogne, C.; Guillon, D.; Donnio, B. J. Am. Chem.
Soc. 2004, 126, 3856-3867. (i) Cheng, X. H.; Prehm, M.; Das, M. K.;
Kain, J.; Baumeister, U.; Diele, S.; Leine, D.; Blume, A.; Tschierske, C. J.
Am. Chem. Soc. 2003, 125, 10977-10996. (j) Chen, B.; Zeng, X.;
Baumeister, U.; Diele, S.; Ungar, G.; Tschierske, C. Angew. Chem., Int.
Ed. 2004, 43, 4621-4625. (k) Oh, N.-K.; Zin, W.-C.; Im, J.-H.; Ryu, J.-
H.; Lee, M. Chem. Commun. 2004, 1092-1093. (l) Cheng, X.; Das, M.
K.; Baumeister, U.; Diele, S.; Tschierske, C. J. Am. Chem. Soc. 2004, 126,
12930-12940. (m) Chen, B.; Baumeister, U.; Diele, S.; Das, M. K.; Zeng,
X.; Ungar, G.; Tschierske, C. J. Am. Chem. Soc. 2004, 126, 8608-8609.
(n) Saez, I. M.; Goodby, J. W. J. Mater. Chem. 2005, 15, 26-40. (o) Cook,
A. G.; Baumeister, U.; Tschierske, C. J. Mater. Chem. 2005, 15, 1708-
1721. (p) Chen, B.; Zeng, X. B.; Baumeister, U.; Ungar, G.; Tschierske,
C. Science 2005, 307, 96-99. (q) Schaz, A.; Lattermann, G. Liq. Cryst.
2005, 32, 407-415. (r) Chen, B.; Baumeister, U.; Pelzl, G.; Das, M. K.;
Zeng, B.; Ungar, G.; Tschierske, C. J. Am. Chem. Soc. 2005, 127, 16578-
16591. (s) Ungar, G.; Zeng, X. Soft Matter 2005, 1, 95-106.
Scheme 1. Structure and Assignment of the Compounds under
Investigation
(9) (a) Diele, S. Curr. Opin. Colloid Interface Sci. 2002, 7, 333-342. (b)
Borisch, K.; Diele, S.; Go¨ring, P.; Kresse, H.; Tschierske, C. J. Mater.
Chem. 1998, 8, 529-543. (c) Ungar, G.; Liu, Y.; Zeng, X.; Percec, V.;
Cho, W.-D. Science 2003, 299, 1208-1211. (d) Percec, V.; Mitchell, C.
M.; Cho, W.-D.; Uchida, S.; Glodde, M.; Ungar, G.; Zeng, X.; Liu, Y.;
Balagurusamy, V. S. K.; Heiney, P. A. J. Am. Chem. Soc. 2004, 126, 6078-
6094.
(10) Zeng, X.; Ungar, G.; Liu, Y.; Percec, V.; Dulcey, A. E.; Hobbs, J. K. Nature
2004, 428, 157-160.
of Si atoms in the siloxane units, m the length of the spacer
unit between siloxanes and bent cores; B stands for the bent-
core unit, and n is the number of C atoms in the terminal alkyl
chain (see Scheme 1). Three of such compounds (Six-11B12
with x ) 2, 3, i3) have been reported in a previous communica-
tion.14 It turned out that the siloxane units have a large impact
on the properties of the mesophases of these molecules. It leads
to a change of the switching behavior from AF to an apparently
(11) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater.
Chem. 1996, 6, 1231-1233.
(12) Reviews: (a) Pelzl, G.; Diele, S.; Weissflog, W. AdV. Mater. 1999, 11,
707-724. (b) Tschierske, C.; Dantlgraber, G. Pramana 2003, 61, 455-
481. (c) Amaranatha Reddy, R.; Tschierske, C. J. Mater. Chem. 2005, DOI
10.1039/b504400f.
(13) (a) Link, D. R.; Natale, G.; Shao, R.; Maclennan, J. E.; Clark, N. A.;
Ko¨rblova, E.; Walba, D. M. Science 1997, 278, 1924-1927. (b) Walba,
D. M. Topics in Stereochemistry (Materials-Chirality); Green, M. M., Nolte,
R. J. M., Meijer, E. W., Eds.; Wiley: New York, 2003; Vol. 42, p 475.
9
3052 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Si-Containing Polyphilic Bent-Core Molecules
A R T I C L E S
Scheme 2. Synthesis of Compounds Six-mBna
ferroelectric type, and also the optical appearance of the
mesophase is completely changed. The highly birefringent
textures usually observed for smectic phases are replaced by
optically isotropic phases composed of chiral domains with
opposite handedness. In the meanwhile, these “dark conglomer-
ate phases” were observed for several other bent-core molecules
with FE or AF switching smectic phases.15-18 Recently, a related
phenomenon was also observed in a cubic mesophase.19
However, though the subject of intensive discussions, the origin
of optical isotropy, chirality, and ferroelectricity remained
unclear.
To clarify the situation, we performed a systematic study,
concerning the influence of the molecular structure of these
oligosiloxane-derived polyphilic bent-core LCs upon their self-
organization in LC phases, which is reported herein. The most
important result is that two of these compounds show a
temperature-dependent inversion of the optical rotation. This
is the first report on a temperature-induced inversion of chirality
in a supramolecular system formed by achiral molecules. This
observation, together with the recently proposed concept of layer
optical chirality,20 allows now for the first time one to assign
the phase structure of the dark conglomerate phases of the
investigated compounds as synclinic tilted and ferroelectric
(SmCsPF). This leads to a fundamental understanding of the self-
organization in these systems and the switching behavior of these
mesophases. The SmCsPF organization is stabilized as the
segregation of oligosiloxane segments is increased, and this is
a function of the size of the oligosiloxane units, but also alkyl
chain length and spacer length are important. A transition from
a Reagents and conditions: (a) cat. [Pd(PPh3)4], NaHCO3, H2O, glyme,
reflux, 8 h;23 (b) 4-(4-n-alkoxybenzoyloxy)benzoic acid,26 DCC, DMAP,
CH2Cl2, 20 °C, 24 h;24 (c) H2, Pd/C, THF, 40 °C, 16 h; (d) 4-(ω-
alkenyloxy)benzoic acid,25 DCC, DMAP, CH2Cl2, 20 °C, 24 h;24 (e)
EtMe2SiH (x ) 1) or Me3Si(OMe2Si)x-1H (x ) 2,3), Karstedt’s catalyst,
toluene, 20 °C, 24 h.14,22
AF via superparaelectric to surface-stabilized FE switching is
found with increasing segregation. In addition, with increasing
length of the alkyl chains, a transition from smectic phases via
wavy deformed layers to a columnar phase is found, which is
mainly due to steric reasons. In contrast to the smectic phases,
in the columnar phase, the switching process is antiferroelectric
and takes place by rotation of the molecules around the long
axes, which reverses the superstructural chirality; that is, the
racemic nonpolar ground-state structure is switched into a
homogeneous chiral polar structure upon application of an
electric field. These results provide a lesson in principles of
self-assembly in polar ordered soft matter and offer fundamental
clues for future design of new bent-core mesogens with specific
properties, namely, superstructural chirality and polar order, in
a predictable way.
(14) Preliminary communication: Dantlgraber, G.; Eremin, A.; Diele, S.; Hauser,
A.; Kresse, H.; Pelzl, G.; Tschierske, C. Angew. Chem., Int. Ed. 2002, 41,
2408-2412.
(15) (a) Thisayukta, J.; Nakayama, Y.; Kawauchi, S.; Takezoe, H.; Watanabe,
J. J. Am. Chem. Soc. 2000, 122, 7441-7448. (b) Thisayukta, J.; Kamee,
H.; Kawauchi, S.; Watanabe, J. Mol. Cryst. Liq. Cryst. 2000, 346, 63-75.
(c) Shreenivasa Murthy, H. N.; Sadashiva, B. K. Liq. Cryst. 2002, 29,
1223-1234. (d) Ortega, J.; Folcia, C. L.; Etxebarria, J.; Gimeno, N.; Ros,
M. B. Phys. ReV. E 2003, 68, 011707. (e) Weissflog, W.; Schro¨der, M.
W.; Diele, S.; Pelzl, G. AdV. Mater. 2003, 15, 630-633. (f) Jakli, A.; Huang,
Y.-M.; Fodor-Csorba, K.; Vajda, A.; Galli, G.; Diele, S.; Pelzl, G. AdV.
Mater. 2003, 15, 1606-1610. (g) Schro¨der, M. W.; Diele, S.; Pelzl, G.;
Dunemann, U.; Kresse, H.; Weissflog, W. J. Mater. Chem. 2003, 13, 1877-
1882. (h) Schro¨der, M. W.; Pelzl, G.; Dunemann, U.; Weissflog, W. Liq.
Cryst. 2004, 31, 633-637. (i) Weissflog, W.; Sokolowski, S.; Dehne, H.;
Das, B.; Grande, S.; Schro¨der, M. W.; Eremin, A.; Diele, S.; Pelzl, G.;
Kresse, H. Liq. Cryst. 2004, 31, 923-933.
(16) Field-induced SmCP[*] phases: (a) Heppke, G.; Parghi, D. D.; Sawade,
H. Liq. Cryst. 2000, 27, 313-320. (b) Etxebarria, J.; Folcia, C. L.; Ortega,
J.; Ros, M. B. Phys. ReV. E 2003, 67, 042702.
(17) Bent-core dimers and oligomers and polymers based on oligosiloxane and
carbosilane units: (a) Dantlgraber, G.; Diele, S.; Tschierske, C. Chem.
Commun. 2002, 2768-2769. (b) Dantlgraber, G.; Baumeister, U.; Diele,
S.; Kresse, H.; Lu¨hmann, B.; Lang, H.; Tschierske, C. J. Am. Chem. Soc.
2002, 124, 14852-14853. (c) Keith, C.; Amaranatha Reddy, R.; Tschierske,
C. Chem. Commun. 2005, 871-873. (d) Keith, C.; Amaranatha Reddy,
R.; Hahn, H.; Lang, H.; Tschierske, C. Chem. Commun. 2004, 1898-1899.
(18) FE switching bent-core materials: (a) Walba, D. M.; Ko¨rblova, E.; Shao,
R.; Maclennan, J. E.; Link, D. R.; Glaser, M. A.; Clark, N. A. Science
2000, 288, 2181-2184. (b) Nakata, M.; Link, D. R.; Araoka, F.; Thisayukta,
J.; Takanishi, Y.; Ishikawa, K.; Watanabe, J.; Takezoe, H. Liq. Cryst. 2001,
28, 1301-1308. (c) Bedel, J. P.; Rouillon, J. C.; Marcerou, J. P.; Laguerre,
M.; Nguyen, H. T.; Achard, M. F. J. Mater. Chem. 2002, 12, 2214-2220.
(d) Amaranatha Reddy, R.; Sadashiva, B. K. J. Mater. Chem. 2002, 12,
2627-2642. (e) Amaranatha Reddy, R.; Sadashiva, B. K. Liq. Cryst. 2003,
30, 1031-1050. (f) Rauch, S.; Bault, P.; Sawade, H.; Heppke, G.; Nair,
G. G.; Jakli, A. Phys. ReV. E 2002, 66, 021706. (g) Kumazawa, K.; Nakata,
M.; Araoka, F.; Takanishi, Y.; Ishikawa, K.; Watanabe, J.; Takezoe, H. J.
Mater. Chem. 2004, 14, 157-164. (h) Amaranatha Reddy, R.; Raghunathan,
V. A.; Sadashiva, B. K. Chem. Mater. 2005, 17, 274-283. (i) Nadasi, H.;
Weissflog, W.; Eremin, A.; Pelzl, G.; Diele, S.; Das, S.; Grande, S. J. Mater.
Chem. 2002, 12, 1316-1324. (j) Bedel, J. P.; Rouillon, J. C.; Marcerou, J.
P.; Nguyen, H. T.; Achard, M. F. Phys. ReV. E 2004, 69, 061702. (k)
Takezoe, H.; et al. J. Am. Chem. Soc. 2005, 127, 11085-11091.
(19) Kajitani, T.; Kohmoto, S.; Yamamoto, M.; Kishikawa, K. Chem. Mater.
2005, 17, 3812-3819.
Results and Discussion
1. Synthesis. The synthesis of the compounds is shortly
outlined in Scheme 2. Accordingly, compounds En-(m-2)Bn
with a terminal double bond were synthesized first,21 and in
the final step, the silicon-containing units were attached by
hydrosilylation reaction, yielding the silylated derivatives Six-
mBn.14,22 Isolation and purification was achieved by repeated
chromatography and crystallization. The experimental proce-
dures and analytical data are described in the Supporting
Information. The characterization of the mesophases was done
(21) Transition temperatures and transition enthalpies of compounds En-(m-
2)Bn are collated in Tables S2 and S3 of the Supporting Information. In
the series of the olefins En-9Bn with increasing chain length, nonswitching
rectangular columnar phases (n ) 4-8; B1 phases, see Figure S4) are
replaced by AF switching polar smectic C phases (n ) 10-22), character-
ized by a single-layer structure without in-plane order and birefringent
textures (SmCPA phases ) B2-type phases). This B1-B2 sequence is usually
observed in the homologous series of bent-core molecules upon elongation
of alkyl chains; see ref 26.
(20) Hough, L. E.; Clark, N. A. Phys. ReV. Lett. 2005, 95, 107802.
(22) Mehl, G. H.; Goodby, J. W. Chem. Ber. 1996, 129, 521-525.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3053

A R T I C L E S
Keith et al.
Table 1. Mesophases, Phase Transition Temperatures, and d
Values of the Smectic Phases of the Bent-Core Molecules 12B12
and En-9B1214 and the Silylated Derivatives Six-11B12
Table 2. Mesophases and Phase Transitions of Compounds
Si3-11Bna
(x ) 1, 2,14 314
)
1b
n
T/
°C
∆
H/kJ mol-
d, a, b/nm,
γ
/
°
L/nm
1
Cr 130 Iso
38.7
4
6
8
Cr 93 SmCsPF *]
113 Iso
17.1
118 Iso
20.9
117 Iso
18.3
117 Iso
22.3
113 Iso
22.3
115 Iso
24.3
4.4
5.0
5.2
5.4
5.6
5.7
5.8
6.0
6.2
6.4
6.6
[
10.9
Cr 100 SmCsPF
[
]
*
4.4
31.6
Cr 94 SmCsPF *]
4.5
[
20.3
10 Cr 61 SmCsPF *]
4.4
4.3
[
12.5
11 Cr 63 SmCsPF *]
[
30.2
[
12 Cr 70 SmCsPF *]
4.4
29.3
[
14 Cr 61 (SmX[*] 60) SmCsPF *]
113 Iso 4.5
22.6
11.4
111 Iso
22.9
96 USmCsPF 109 Iso 4.4
1.8
20.4
3.7c
4.5
[
16 Cr 66 SmCsPF *]
23.9
compound
T/
°
Ca
d/nm (T/°C)
L/nmb
18 Cr 87 USmCsPF′
23.9
20 Cr 83 USmCsPF
22.5
22 Cr 88 ColobPA
28.8
16.3
En-9B12c
12B12
Si1-11B12
Si2-11B12c
Si3-11B12c
Sii3-11B12c,d
Cr 108 (SmCPA 98) Iso
Cr 88 SmCPA 111 Iso
n.d.
96 ColobPA
100 Iso 4.4d
3.7 (90)
3.9 (105)
4.2 (95)
4.4 (95)
n.d.
5.2
5.4
5.6
5.8
17.3
4.5, 2.2,103°
4.9, 2.2, 100° 6.8
Cr 100 SmCsPF */°]114 Iso
[
98 Iso
13.1
Cr 77 SmCsPF */°]118 Iso
[
Cr 70 SmCsPF *]115 Iso
[
Cr 63 SmCsPF *]116 Iso
[
a Abbreviations: USmCsPF, USmCsPF′ ) undulated (wavy deformed)
polar smectic phases with synclinic and synpolar correlation between
adjacent layers (see Figure 9b); ColobPA ) AF switching oblique columnar
phase (see Figure 9c); SmX[*] ) nonswitchable smectic phase with
additional in-plane order; values in parentheses indicate monotropic
(metastable) mesophases; for other abbreviations, see Table 1. b Determined
a Abbreviations: Cr ) crystalline solid state; Iso ) isotropic liquid state;
[
SmCPA ) tilted polar smectic phase with antipolar structure; SmCsPF *] )
distorted synclinic tilted polar smectic phase with synpolar order and chiral
[
domains of opposite handedness in the ground state; */°] indicates that
by DSC, first heating scan, 10 K min-1
.
c Refers to the SmX[*] phase.
macroscopic chiral domains coexist with a birefringent texture; g ) glass
transition; n.d. ) not determined. b L ) molecular length assuming an
overall V-shaped conformation with a bending angle of 120°, all-trans
conformation of the alkyl chains and most stretched conformation of the
Si-containing units. c See ref 14. d Branched oligosiloxane unit (2-substituted
1,1,1,2,3,3,3-heptamethyltrisiloxane).
d Refers to the USmCsPF phase
alkyl chains and therefore were expected to reduce the stability
of LC phases.
However, the mesophases of the silylated compounds are
quite distinct from those formed by related nonsilylated
molecules.26 These mesophases appear as fractal nuclei which
coalesce to a grainy unspecific texture which is completely dark
(optically isotropic) between crossed polarizers, showing only
very small irregularly distributed bright spots in some cases. A
remarkable feature of the mesophases of compounds Six-11B12
and Si3-11Bn (n ) 4-16) is that, in these mesophases, domains
of opposite handedness can be distinguished, as shown for the
mesophase of Si3-11B14, as an example, in Figure 2a,b. Upon
rotating the analyzer by a small angle (5-10°), dark and bright
domains become visible. If the analyzer is rotated in the opposite
direction, the brightness of the domains is reversed. The light
transmission does not change if the sample is rotated. This
indicates a conglomerate of macroscopic chiral domains with
opposite chirality sense. These optically isotropic mesophases
composed of a conglomerate of chiral domains are designated
by polarizing microscopy, differential scanning calorimetry
(DSC), X-ray scattering, and investigation of the switching
behavior by electrooptical methods.
2. Mesomorphic Properties. 2.1. Size of the Silicon-
Containing Unit: Transition from Conventional SmCPA
Phases to Dark Conglomerate Phases. The phase transition
temperatures of the synthesized compounds Six-mBn are
collated in Tables 1-3. In Table 1, the influence of the size of
the Si-containing unit upon the liquid crystalline properties
is shown for the dodecyl-substituted compounds Six-11B12
(x ) 1-3), and these compounds are compared with the olefinic
precursor En-9B1214 and the alkyl-substituted compound 12B12.
The nonsilylated and terminally unsaturated compound En-9B12
has a monotropic (metastable) birefringent AF switching SmCPA
phase (B2-type phase) as typical for numerous other bent-
core molecules and also found for the dialkyl-substituted
compound 12B12 without CdC double bond (see Figures S2
and S3). Hydrosilylation22 of En-9B1214 leads to the silyl-
substituted materials Six-11B12 (x ) 1-3), which show
enantiotropic (thermodynamically stable) mesophases with
enhanced mesophase stability compared to 12B12 and En-9B12,
despite that the Si-containing units are more bulky than linear
(23) (a) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483. (b) Hird,
M.; Gray, G. W.; Toyne, K. J. Mol. Cryst. Liq. Cryst. 1991, 206, 187-
204.
(24) Tschierske, C.; Zaschke, H. J. Prakt. Chem. 1989, 331, 365-366.
(25) Adams, N. W.; Bradshaw, J. S.; Bayona, J. M.; Markides, K. E.; Lee, M.
L. Mol. Cryst. Liq. Cryst. 1987, 147, 43-60.
(26) Shen, D.; Pegenau, A.; Diele, S.; Wirth, I.; Tschierske, C. J. Am. Chem.
Soc. 2000, 122, 1593-1601.
9
3054 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Si-Containing Polyphilic Bent-Core Molecules
A R T I C L E S
Table 3. Mesophases and Phase Transitions of Compounds Si3-mB12a
1
compound
m
T/
°
C
∆
H/kJ mol-
d/nm
L/nm
[
Si3-5B12
5
Cr 105 (SmCsPF */°] 83) Iso
53.2
[
Si3-6B12
Si3-7B12
Si3-8B12
Si3-9B12
Si3-10B12
Si3-11B12
6
7
Cr 86 SmCsPF */°]
93 Iso
14.8
104 Iso
20.6
4.8 (85)
3.8 (90)
4.0 (90)
5.3
5.4
5.5
5.6
5.7
5.8
18.1
[
Cr 75 SmCsPF */°]
21.3
[
8
Cr 71 SmCsPF */°]
102 Iso
16.3
21.8
[
9
Cr1 58 Cr2
28.8
68 (SmX[*] 61) SmCsPF *]
108 Iso
21.1
4.0 (100)
3.0 (40)
4.4 (97)
11.9
115 Iso
22.3
[
10
11
Cr 63 SmCsPF *]
39.2
[
Cr 70 SmCsPF *] 115 Iso
4.4 (95)
a Abbreviations and conditions as in Tables 1 and 2.
as “dark conglomerate phases”, and this type of supramolecular
chirality, which is not based on a molecular configurational
chirality, is assigned with the symbol “[*]”. For all compounds
with a heptamethyltrisiloxane unit (Si3-11Bn with n ) 4-16
and Sii3-11B12), this dark conglomerate texture is found
exclusively. However, for compounds Si1-11B12 and Si2-11B12
with shorter Si-containing units, sometimes also regions with a
birefringent schlieren texture can coexist with the dark con-
[
glomerate texture (assigned with */°]).
X-ray investigations indicate simple layer structures without
in-plane order for the mesophases of all compounds Six-11B12
(x ) 1-3). The layer thickness is always significantly smaller
than the molecular length, which is in line with a strongly tilted
arrangement of the molecules within a monolayer structure with
estimated tilt angles of about 35-45° with respect to the layer
normal (SmCP phases).
In addition, there is an important change in the profile of the
diffuse wide-angle scattering. For compound Si3-11B12 with a
large oligosiloxane unit, this scattering has a very asymmetric
profile in which two maxima can be separated, one maximum
at 0.47 nm corresponding to the mean distance between the fluid
alkyl chains and between the aromatic cores and the second
one with a maximum at about 0.7 nm corresponding to the mean
distance between the disordered siloxane units (see for example
curve “a” in Figure 4, which actually shows the wide-angle
scattering of the higher homologue Si3-11B14). This is an
indication of the fluid nanosegregated organization in the smectic
mesophase of this compound; that is, the siloxane units build
up their own sublayers, leading to the triple layer structure, as
shown in Figure 3a. The presence of different mean distances
between hydrocarbon units (cross sectional area ) 0.21 nm2)
and between siloxane units (cross sectional area ) 0.38 nm2)
can be realized by adopting an antiparallel packing of adjacent
molecules, leading to a monolayer structure instead of the
expected bilayer structure.
Figure 2. Chiral domains observed for compound Si3-11B14 (a), (b) in
the SmCsPF *] phase at 65 °C between slightly uncrossed polarizers (arrows
[
indicate the positions of the polarizers); (c) same region between crossed
polarizers, (d, e) SmX[*] phase at 55 °C between slightly uncrossed
polarizers; from (a) to (b) and from (d) to (e), the direction of the polarizers
is changed, from (a) via (c) to (d), the sample is cooled, from (e) via (c) to
(b), it is heated. Texture (c) is actually observed in the SmCsPF[*] phase at
65 °C under crossed polarizers, but the same appearance can be found at
the transition from (a) to (d) and (e) to (b) depending on the temperature.
This correlates with the observed differences in the textural
features between these compounds.
2.2. Effect of the Length of the Terminal Alkyl Chains:
Transition from Smectic to Columnar Phases. As shown in
Table 2, with increasing length of the terminal chain of
compounds Si3-11Bn, a series of different mesophases is found,
which is discussed in the following sections.
2.2.1. The Fluid Dark Conglomerate Smectic Phases of
Compounds Si3-11B4 to Si3-11B16. All compounds Si3-11Bn
with n ) 4-16 show the same textural features as Si3-11B12
The diffuse wide-angle reflection at D ) 0.7 nm is missing
for compounds Si1-11B12 and Si2-11B12 with smaller silicon-
containing segments, which means that for these compounds
the alkyl chains and the Si-containing units are not segregated
(there are only two instead of three sublayers; see Figure 3b).
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3055

A R T I C L E S
Keith et al.
Figure 3. Models of the organization of the siloxane-substituted bent-core molecules (a) in the triply segregated smectic phases, and (b) in the simple
smectic phases where aliphatic chains and oligo(dimethylsiloxane) units are not segregated; (c) shows the a model of a sponge phase (from ref 28a), where
a random web of the aromatic layers divides the fluid regions (segregated oligosiloxane sublayers are not shown).
Figure 5. Dependence of the d values of the smectic phases of compounds
Si3-mBn on the number of C atoms m in the spacer unit (compounds Si3-
mB12, n ) 12, filled squares and solid line) and on the number of C atoms
n in the terminal alkyl chains (compounds Si3-11Bn, m ) 11, open squares
and dotted line).
Figure 4. Wide-angle region of selected X-ray diffraction pattern: (a) two
[
diffuse maxima in the SmCsPF *] phase of compound Si3-11B14 at 100 °C
indicate a nanosegregated structure with discrete oligosiloxane sublayers;
(b) one diffuse maximum in the USmCsP′F phase of compound Si3-11B18
at 92 °C indicates the absence of separate oligosiloxane sublayers; (c) several
reflections in the SmX[*] phase of compound Si3-11B14 at 50 °C indicate
an additional in-plane order.
mesophases of compounds Si3-11Bn with n ) 18-22 (see
Figure 4, curve b, which shows the wide-angle profile of the
wavy deformed smectic phase (USmCsP′F) of compound Si3-
11B18).
(dark conglomerates). X-ray investigations confirm smectic
phases without in-plane order (see Table S1 in the Supporting
Information). Remarkably, there is no change of the layer
distance on increasing the length of the alkyl chain. Compounds
Si3-11B4 (d ) 4.4 nm) and Si3-11B16 (d ) 4.5 nm) have
practically the same d value, though the difference of the
molecular length is 12 CH2 groups (see Table 2 and Figure 5)!
This unique behavior is in line with the suggested triple layer
model shown in Figure 3a. This means that the layer thickness
is exclusively determined by the combined lengths of the spacer
unit, bent-core, and siloxane unit. The alkyl end chains have to
be accommodated within the sublayers provided by the spacer
units. For molecules with extremely long chains, however
(compounds Si3-11Bn with n ) 18-22), this is not possible
and segregation of alkyl chains and siloxane units is lost. This
is seen in the profile of the diffuse wide-angle scattering in the
X-ray diffraction pattern. For the smectic phases of all com-
pounds Si3-11Bn with n ) 4-14, there is a shoulder at D )
0.7 nm, corresponding to the mean distance between the siloxane
units (see Figure 4, curve a). This shoulder is very weak for
compound Si3-11B16, and it is completely absent for the
The optical isotropy and the spontaneous formation of
domains with opposite chirality sense are quite unusual features
of the ground-state structures of the smectic mesophases of
compounds Si3-11Bn with n ) 4-16. As suggested by Jakli et
al.,15f an optical isotropic tilted smectic phase can be explained
if the organization would be SmCaPA and if the bending angle
R and tilt angle θ are related to each other according to R ) 2
tan-1(1/cos θ). However, this uniform ratio is unlikely to occur
for a whole series of compounds. In addition, as will be shown
in section 3, the actual mesophase structure is synclinic and
synpolar in most cases. Another remarkable feature of these
mesophases is the disordered structure of the layers which can
even be observed under the polarizing microscope in some cases
(grainy appearance). Also, the diffraction peaks are not resolu-
tion limited, indicating a limited size of the smectic domains.
AFM (see Figure 6) and electron microscopy with freeze fracture
samples27 confirm a strongly distorted structure of the layers
(the layers are strongly folded), similar to lyotropic sponge
(27) Clark, N. A.; et al. Poster Presented at the 9th International Liquid Crystal
Conference, Ljubljana, July 4-9, 2004, Book of Abstracts, SYN-P026.
9
3056 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Si-Containing Polyphilic Bent-Core Molecules
A R T I C L E S
[
Figure 6. AFM image of the surface of a glassy solidified sample of the SmCsPF *] phase of compound Sii3-11B12 at 20 °C, showing a very rough surface
structure. Beside the long-scale modulation, there is an additional fine grainy structure with a length scale of about 100 nm (see the enlarged part).
phases.28 This means that optical isotropy is due to a random
microdomain structure, where the uniformly aligned smectic
domains have a size which is smaller than the wavelength of
light. This distorted structure makes it difficult to assign the
ground-state structure to one of the four possible structures
shown in Figure 1 because no aligned samples for X-ray
scattering could be obtained. However, compound Si3-11B14
shows a temperature-dependent inversion of chirality, and this
provides the possibility to make a conclusion on the structure
of these dark conglomerate phases (see section 2.2.2.).
tilt angle of θ ) 43° was calculated according to dX-ray/dmodel
) cos θ for the SmCsPF *] phase, which is the same value as
[
that determined by electrooptical investigations (see section 3).
In the SmX[*] phase, the calculated tilt angle is about 53°. No
additional reflections occur in the small-angle region, which
indicates that this mesophase also has a layer structure. The
maximum of the broad diffuse peak in the wide-angle region,
however, is split into several reflections (see Figure 4, curve
c), confirming an additional in-plane ordering in the low-
temperature mesophase.30 As no birefringence is introduced at
the phase transition, the distorted nature of the smectic phase
should be maintained at the phase transition. The proven tilted
arrangement of the molecules distinguishes this phase from the
2.2.2. The Soft Crystalline Dark Conglomerate Phase of
Si3-11B14. Compound Si3-11B14 shows a transition from the
SmCsPF *] phase to a higher ordered low-temperature dark
[
[
conglomerate mesophase at 56 °C on cooling (phase transition
at 60 °C upon heating), which is accompanied by a reversible
inversion of the chirality sense of the chiral domains. Bright
domains in the high-temperature phase become dark in the lower
temperature phase and vice versa (Figure 2a-e). Though
temperature-dependent inversion of chirality has been observed
for helical superstructure in chiral nematic (N*) and chiral
smectic C phases (SmC*) of chiral molecules,29 this is the first
example of a temperature-dependent reversal of the optical
rotation in a mesophase formed by achiral molecules. X-ray
studies indicate that the layer thickness decreases from 4.4 nm
soft crystalline B4 *] phase,31 which is a nontilted low-
temperature phase with a TGB-like helical superstructure.32
2.2.3. Possible Structures of the Dark Conglomerate
Phases. 2.2.3.A. Origin of Layer Distortion. Layer distortion
in the investigated systems might arise due to a steric frustration
of the areas required by the aromatic and the nonaromatic
molecular parts at the interfaces, which could lead to a Gaussian
curvature of these interfaces.33 Optical isotropic bicontinuous
cubic phases8a,c,g,9a,b,34 are known to occur as intermediate states
(30) This is in line with the fact that no electrooptical switching is observed in
this soft crystalline mesophase.
[
(31) (a) Niwano, H.; Nakata, M.; Thisayukta, J.; Link, D. R.; Takezoe, H.;
Watanabe, J. J. Phys. Chem. B 2004, 108, 14889-14896. (b) Araoka, F.;
Ha, N. Y.; Kinoshita, Y.; Park, B.; Wu, J. W.; Takezoe, H. Phys. ReV.
Lett. 2005, 94, 1-4. (c) Walba, D. M.; Eshdat, L.; Ko¨rblova, E.; Shoemaker,
R. K. Cryst. Growth Des. 2005, 5, 2091-2099.
in the fluid smectic phase (SmCsPF *]) to 3.7 nm in the low-
temperature SmX[*] phase. Using the model shown in Figure
3a and assuming a bending angle of 120° (dmodel ) 6.2 nm), a
(32) In principle, it is possible that also in the SmX[*] phase the smectic slabs
[
(28) (a) Strey, R.; Winkler, J.; Magid, L. J. Phys. Chem. 1991, 95, 7502-7507.
(b) Roux, D.; Coulon, C.; Cates, M. E. J. Phys. Chem. 1992, 96, 4174-
4187. (c) Porte, G. Curr. Opin. Colloid Interface Sci. 1996, 1, 345-349.
(d) Hyde, S. T. Colloids Surf. A 1997, 129-130, 207-225.
(29) (a) Vill, V.; von Minden, H. M.; Bruce, D. W. J. Mater. Chem. 1997, 7,
893-899. (b) Langner, M.; Praefcke, K.; Kru¨erke, D.; Heppke, G. J. Mater.
Chem. 1995, 5, 693-699. (c) Kaspar, M.; Gorecka, E.; Sverenyak, H.;
Hamplova, V.; Glogarova, M.; Pakhomov, S. A. Liq. Cryst. 1995, 19, 589-
594. (d) Loubser, C.; Wessels, P. L.; Styring, P.; Goodby, J. W. J. Mater.
Chem. 1994, 4, 71-79.
might adopt a TGB-like helical organization as in the B4 *] phases, but
there is no indication of such a helical structure.
(33) (a) Seddon, J. M.; Templer, R. H. Handbook of Biological Physics;
Lipowsky, R., Sackmann, E., Eds.; Elsevier: Amsterdam, 1995; Vol. 1,
pp 97-160. (b) Hyde, S. T. Handbook of Applied Surface and Colloid
Science; Holmberg, K., Ed.; Wiley: Chichester, 2001; Vol. 2, pp 299-
332.
(34) (a) Gharbia, M.; Gharbi, A.; Nguyen, H. T.; Malthete, J. Curr. Opin. Colloid
Interface Sci. 2002, 7, 312. (b) Bruce, D. W. Acc. Chem. Res. 2000, 33,
831-840.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3057

A R T I C L E S
Keith et al.
at the transition from flat smectic layers to an organization in
columns. The fact that the mesophases under discussion occur
upon introduction of a bulky siloxane unit into the molecule
12B12, itself forming a nondistorted smectic phase, and that
elongation of the alkyl chains leads to the formation of columnar
phases (see section 2.2.5.) would be in line with this explanation.
However, a cubic lattice can be excluded for the mesophases
under investigation (rather high fluidity, no X-ray evidence),
and fluid isotropic intermediate phases are extremely rare in
thermotropic systems and usually occur only for few homo-
logues in limited temperature ranges.35,36 In contrast, the dark
conglomerate phases were found over the whole temperature
range for a wide variety of different compounds.15-18
In addition, chirality can also be a source of layer distortion.
Chirality in self-organized superstructures, formed by achiral
bent-core molecules, arises from the tilted organization of these
molecules in polar layers.13 The SmCP layers have a C2 phase
symmetry which lacks a mirror plane, and therefore, these layers
are inherently chiral. Layer normal, tilt direction, and polariza-
tion vector describe either a right-handed or a left-handed system
(see Figure S1 in the Supporting Information). Changing either
the direction of layer polarization or the tilt direction changes
the chirality sense of the layer.13 The SmCsPA and SmCaPF
phases have opposite chirality sense in adjacent layers (see
Figure 1, indicated by different color), leading to macroscopic
racemic superstructures,39 whereas in the SmCaPA and SmCsPF
phases, the layers have identical chirality sense and hence these
structures are homogeneously chiral (Figure 1). As bent-core
molecules preferably adopt helical molecular conformations,40,41
diastereomeric relations of these chiral conformers with the layer
chirality could lead to a (partial) deracemization of molecular
conformations; that is, one helix sense is dominating in the (+)-
SmCsPF domains, whereas the opposite helix sense is dominating
in the (-)-SmCsPF domains.42 As chiral molecules cannot align
completely parallel in soft matter systems, this conformational
chirality could contribute to layer distortion.12c
Sponge phases (L3 phases) are optically isotropic mesophases
which are quite common in lyotropic systems. These mesophases
occur beside lamellar phases over a quite large range of dilution,
if the surfactant bilayers are swollen by solvents. In these sponge
phases, the layers are curved and form a continuous nonperiodic
web of bilayers (Figure 3c).27,28 Interestingly, also the dark
conglomerate phases can be found for molecules with relatively
long alkyl chains or chains which are functionalized with bulky
end groups (siloxanes, carbosilanes). In addition, if n-dodecane
is added to the nonsilylated compound 12B12, the typical
birefringent schlieren texture of the SmCPA (B2-type) phase is
completely replaced by a well developed dark conglomerate
texture, similar to that shown in Figure 2a,b, and the layers are
expanded from d ) 3.7 nm for the pure compounds to d ) 3.9
nm for the solvent-saturated sample.37 Hence, a sponge-like
random web of the aromatic sublayers that divides the fluid
regions into homogeneous (siloxanes and alkyl chains are mixed)
or inhomogeneous (siloxanes are separated from the alkyl
chains) labyrinths is a likely model for the observed dark
conglomerate phases (see Figure 3c).
2.2.3.B. Origin of Optical Activity. Recently, Hough and
Clark have predicted that a significant optical activity arises
from the layer chirality of the polar tilted smectic phase
structure.20 This means that a significant optical rotation is an
intrinsic property of the homogeneous chiral layer structures
(FE, SmCsPF; or AF, SmCaPA), which could only be observed
if the birefringence of the mesophase is low. The unique
temperature-dependent inversion of the chirality sense at the
phase transition from the fluid smectic SmCsPF *] phase to the
[
higher ordered SmX[*] phase observed for compounds Si3-
11B14 (section 2.2.2.) and Si3-9B12 (section 2.3.) can be
satisfactorily explained by the idea of layer optical chirality.
For this optical activity, the sign of the optical rotation is
dependent on the tilt angle of the long axis of the bent aromatic
core with respect to the layer plane,20 and such a change of the
tilt angle was found at this phase transition. This change in tilt
angle should lead to an inversion of optical rotation as proposed
by this model.
As mentioned above, layer distortion in these sponge-like
structures might result from a slight misbalance of the areas
required by the incompatible segments at the interfaces. Layer
distortion might also arise as a result of the escape from a
macroscopic polar order if the ground-state structure is ferro-
electric (SmCsPF or SmCaPF). Splay of the polar directions in
SmCsPF layers can, for example, give rise to a regular wavy
deformation of the layers which leads to the birefringent B7-
type phases.38 A nonregular deformation of polar layers in all
three dimensions could give rise to a optically isotropic sponge-
like structure.
2.2.3.C. Phase Structure and Polar Order. This means that
the ground-state structure of the dark conglomerate phases of
the silylated compounds should be homogeneous chiral, either
SmCsPF or SmCaPA, and the chiral domains of opposite handed-
ness represent regions with a distinct sense of the layer chirality.
Hence, if the correlation of the tilt direction could be determined,
it would be possible to decide if the ground-state structure is
either ferroelectric (synclinic tilt, SmCsPF) or antiferroelectric
(anticlinic tilt, SmCaPA). However, due to the distorted structure
(35) Optically isotropic mesophase in achiral LC: (a) Warmerdam, T. W.; Nolte,
R. J. M.; Drenth, W.; van Miltenburg, J. C.; Frenkel, D.; Zijlstra, R. J. J.
Liq. Cryst. 1988, 3, 1087-1104. (b) Weissflog, W.; Letko, I.; Pelzl, G.;
Diele, S. Liq. Cryst. 1995, 18, 867-870. (c) Pietrasik, U.; Szydlowska, J.;
Krowczynski, A.; Pociecha, D.; Gorecka, E.; Guillon, D. J. Am. Chem.
Soc. 2002, 124, 8884-8890. (d) Bilgin-Eran, B.; Tschierske, C.; Diele, S.
Baumeister, U. J. Mater. Chem. 2005, DOI 10.1039/b511832h.
(36) Optically isotropic mesophase in chiral LC: (a) Yamamoto, J.; Nishiyama,
I.; Inoue, M.; Yokoyama, H. Nature 2005, 437, 525-528. (b) Nishiyama,
I.; Yamamoto, J.; Goodby, J. W.; Yokoyama, H. Chem. Mater. 2004, 16,
3212-3214. (c) Nguyen, H. T.; Ismaili, M.; Isaert, N.; Achard, M. F. J. J.
Mater. Chem. 2004, 14, 1560-1566. (d) Takanishi, Y.; Ogasawara, T.;
Yoshizawa, A.; Umezawa, J.; Kusumoto, T.; Hiyama, T.; Ishikawa, K.;
Takezoe, H. J. Mater. Chem. 2002, 12, 1325-1330. (e) Goodby, J. W.;
Dunmur, D. A.; Collings, P. J. Liq. Cryst. 1995, 19, 703-709.
(39) Concerning a discussion of the actual structure of the SmCaPF and SmCsPA
phases, see: (a) Folcia, C. L.; Ortega, J.; Etxebaria, J. Liq. Cryst. 2003,
30, 1189-1191. (b) Pyc, P.; Mieczkowski, J.; Pociecha, D.; Gorecka, E.;
Donnio, B.; Guillon, D. J. Mater. Chem. 2004, 14, 2374-2379.
(40) (a) Osipov, M. A.; Kuball, H.-G. Eur. Phys. J. E 2001, 5, 589-598. (b)
Nakata, M.; Takanishi, Y.; Watanabe, J.; Takezoe, H. Phys. ReV. E 2003,
68, 041710. (c) Gorecka, E.; Cepic, M.; Mieczkowsky, J.; Nakata, M.;
Takezoe, H.; Zenks, B. Phys. ReV. E 2003, 67, 061704.
(37) Induction of an optically isotropic mesophase by addition of organic solvents
was also reported by Jakli et al., but no chiral domain structure was observed
for these mesophases and the switching was reported to be ferroelectric,
which is in contrast to our observations: (a) Jakli, A.; Cao, W.; Huang,
Y.; Lee, C. K.; Chien, L.-C. Liq. Cryst. 2001, 28, 1279-1283. (b) Huang,
M. Y. M.; Pedreira, A. M.; Martins, O. G.; Figueiredo Neto, A. M.; Jakli,
A. Phys. ReV. E 2002, 66, 031708.
(41) Earl, D. J.; Osipov, M. A.; Takezoe, H.; Takanishi, Y.; Wilson, M. R.
Phys. ReV. E 2005, 71, 021706.
(42) It should be mentioned here that chirality might also arise from a
spontaneous deracemization of molecular conformations as proposed for
the B4 *] phases.31
[
(38) Clark, N. A.; et al. Science 2003, 301, 1204-1211.
9
3058 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Si-Containing Polyphilic Bent-Core Molecules
A R T I C L E S
of these smectic phases, it is impossible to get uniformly aligned
samples, and therefore, the layer correlation cannot be derived
from X-ray data. As it will be shown in section 3.3., the layer
distortion can be squeezed out under an electric field, leading
to birefringent textures with circular domains. Viewed between
crossed polarizers, these domains are characterized by dark
extinction brushes (see, for example, Figure 13b,c). The tilt
direction can be deduced from the position of the extinction
brushes with respect to the positions of polarizer and analyzer.
If these brushes are inclined with the direction of the polarizers,
the tilt is synclinic, and if the brushes coincide with the
directions of the polarizers, it is anticlinic. Under a DC field,
only the synclinic structure is found for the smectic phases of
all compounds Si3-11Bn with n ) 4-16 (see section 3.3.). If it
is assumed that a DC field does not change the tilt correlation,
then the ground-state structure should also be synclinic. The
only homogeneously chiral structure with synclinic tilt is
SmCsPF, and this means that the correlation of the polar
directions should be ferroelectric in these smectic phases.43
Hence, the escape from a macroscopic polarization in the
SmCsPF structure might contribute to the distortion of a flat
layer structure. However, it should be mentioned that in the
mixed system 12B12 + dodecane, where a dark conglomerate
phase is induced by the solvent, the switching remains clearly
AF (two polarization current peaks in a half period of an applied
triangular wave field), as in the SmCPA (B2) phase of the solvent
free material. This confirms that formation of dark conglomerate
textures is independent of polar order, and layer distortion is
also found in nonpolar SmCPA phases.15c-h Therefore, it can
be concluded that escape from polar order can reinforce layer
distortion, but steric effects and possibly also chirality effects
are also required.
Figure 7. Textures of compound Si3-11B18 as seen between crossed
polarizers; (a) USmCsPF phase at 106 °C, as obtained by cooling from the
isotropic liquid; (b) USmCsP′F phase at 90 °C as obtained by cooling from
the USmCsPF phase.
2.2.4. The Undulated Smectic Phases of Si3-11B18. The
mesophases of compound Si3-11B18 are distinct from the lower
homologues. On cooling, at 104 °C, spherulitic and lancet-like
textures as are typical for a mesophase with 2D lattice (Figure
7a) were observed.18h,j,44,45 On further cooling, at 96 °C, a phase
transition takes place, the texture becomes broken, and a fluid
fine schlieren texture is formed (Figure 7b). However, X-ray
investigation indicates a layer structure with d ) 4.4 nm at all
temperatures (Figure 8a). The absence of the diffuse scattering
around D ) 0.7 nm of the oligosiloxane sublayers (see Figure
4, curve b) proves a nonsegregated structure where oligosiloxane
units and alkyl chains are mixed up, and the unsymmetrical
profile of the wide-angle scattering in the aligned sample46
(higher intensity at the right, see Figure 8a) indicates a synclinic
organization with an extremely large tilt angle (θ ≈ 49°).
Figure 8. X-ray diffraction pattern of aligned samples. (a) USmCsP′F phase
of compound Si3-11B18 at 85 °C (the 03 reflection is very weak); the
diffraction pattern does not change at the transition to the USmCsPF phase;
(b) ColobPA phase of Si3-11B22 at 97 °C; in both cases, the stronger intensity
of the wide-angle scattering on the right is indicative of a synclinic tilt of
the molecules; in (b), synclinic domains with opposite tilt direction coexist,
and therefore, a wide-angle scattering is also found on the left; the distinct
intensity of these scatterings indicates a synclinic organization, but if both
would have the same intensity, a synclinic or anticlinic tilted organization
could not be distinguished.
Though a phase transition can be detected at 96 °C by a
significant change of the texture and by a peak in the DSC (∆H
) 1.8 kJ mol-1), no change was found at this temperature in
the X-ray diffraction pattern. This means the organization is
synclinic in both mesophases. In addition, the layer reflections
are extended to lines parallel to the equator in both phases (see
Figure 8a), which points to mesophases with a wavy deformed
(undulated) layer structure, as shown in Figure 9b.38,45 The
difference between these two phases might arise from a different
undulation wavelength and a different correlation length of the
undulations in adjacent layers. In the smectic-like low-temper-
ature phase (USmCsP′F), there might be only a short-range
correlation of the undulations, whereas in the high-temperature
phase (USmCsPF), the correlation might become medium or long
range, leading to a change of the optical appearance of the
mesophase from a smectic-like texture to a texture more
reminiscent of a mesophase with 2D lattice.
(43) In addition, a synclinic organization of the molecules was proven for the
undulated variants of the smectic phase (compound Si3-11B18) by X-ray
scattering of aligned samples (see Figure 8a).
(44) Examples of columnar phases formed by bent-core molecules: (a)
Watanabe, J.; Niori, T.; Sekine, T.; Takezoe, H. Jpn. J. Appl. Phys. 1998,
37, L139-L142. (b) Mieczkowski, J.; Gomola, K.; Koseska, J.; Pociecha,
D.; Szydlowska, J.; Gorecka, E. J. Mater. Chem. 2003, 13, 2132-2137.
(c) Amaranatha Reddy, R.; Sadashiva, B. K.; Raghunathan, V. A. Chem.
Mater. 2004, 16, 4050-4062. (d) Pelz, K.; Weissflog, W.; Baumeister,
U.; Diele, S. Liq. Cryst. 2003, 30, 1151-1158. (e) Kardas, D.; Prehm, M.;
Baumeister, U.; Pociecha, D.; Amaranatha Reddy, R.; Mehl, G. H.;
Tschierske, C. J. Mater. Chem. 2005, 15, 1722-1733. (f) See ref 21b. (g)
Nguyen, H. T.; Bedel, J. P.; Rouillon, J. C.; Marcerou, J. P.; Achard, M.
F. Pramana J. Phys. 2003, 61, 395-404.
(45) Keith, C.; Amaranatha Reddy, R.; Baumeister, U.; Tschierske, C. J. Am.
Chem. Soc. 2004, 126, 14312-14313.
(46) These mesophases are distinct from the dark conglomerate phases of the
lower homologues, and therefore, aligned samples can be obtained.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3059

A R T I C L E S
Keith et al.
to lines), indicating a USmCsPF phase. As the organization of
the molecules is synclinic in the undulated layer structures, the
undulations should not be sinusoidal (symmetric), but asym-
metric (see Figure 9b), as in the asymmetric ripple phases of
phospholipids.48
Hence, as shown in Figure 9a-c, elongation of the alkyl
chains leads to a stepwise transition from dark conglomerate
phases (SmCsPF *], Si3-11Bn with n ) 4-16) via undulated
[
layers (USmCsPF and USmCsP′F mesophases of Si3-11B18 and
low-temperature phase of Si3-11B20) to a ribbon phase built
up by fragmented layers (ColobPA, high-temperature phase of
Si3-11B20 and compound Si3-11B22).
2.3. Effect of the Length of the Spacer Unit. The influence
of the length of the aliphatic spacer unit between the bent-core
and the oligosiloxane unit was investigated for the series of
dodecyloxy-substituted compounds Si3-mB12 (m ) 5-11; see
Table 3). Only smectic phases were found in this series. It can
be clearly seen that reduction of the spacer length gives rise to
a reduction of the mesophase stability, but also the textures are
slightly different. Optical isotropic mesophases composed of
chiral domains were observed for all homologues with relatively
long spacer units with m g 9, whereas the homologues with
shorter spacers (m ) 6-8) are characterized by the coexistence
of this dark conglomerate texture and a birefringent schlieren
texture (see Figure S8 in the Supporting Information). Depend-
ing on the conditions (cooling rate, type of glass surfaces, etc.),
the birefringent regions can be dominating or minor. In the X-ray
diffraction pattern, a diffuse scattering with a shoulder at D )
ca. 0.7 nm (siloxane sublayers) is observed for compounds Si3-
mB12 with n ) 7-11, whereas this shoulder is absent for Si3-
6B12. In addition, there is a remarkable and unique dependence
of the layer spacing on the length of the spacer unit. Upon
reduction of the spacer length, the layer spacing decreases first
from d ) 4.4 nm (m ) 11) to d ) 3.8 nm (m ) 7) as expected,
but for Si3-6B12, it strongly rises again, so that this compound
has a larger layer distance (d ) 4.8 nm) than that of Si3-11B12
with seven additional CH2 groups (d ) 4.4 nm; see Table 3
and Figure 5). It seems that for compound Si3-6B12 the thin
sublayers of the alkyl spacers cannot accommodate the relatively
long terminal chains. The segregation of the siloxane units is
lost, and these units become randomly distributed within the
aliphatic sublayers of the alkyl chains, as indicated in the X-ray
diffraction pattern by the complete absence of the shoulder at
0.7 nm corresponding to the oligosiloxane sublayers (see Figure
S9 in the Supporting Information). As the limitation for the
expansion of the aliphatic sublayers is removed, also the tilt of
the aromatic cores is reduced and both effects contribute to the
huge layer expansion from Si3-7B12 to Si3-6B12.
Figure 9. Possible way for the transition from (a) a smectic phase via (b)
a nonsinusoidal undulated smectic phase (wavy deformed layers, the
undulation period can be much larger than actually shown) to (c) an oblique
columnar phase (broken layers, the resulting ribbons are organized in an
oblique lattice) as observed with increasing steric frustration. The transition
from wavy deformed layers to broken layers is associated with a change of
the polar order from ferroelectric to antiferroelectric. The possibilities
concerning the polar directions in the ColobPA phase are discussed in Figure
S6, Supporting Information.
2.2.5. The Undulated Smectic and Columnar Phases of
Si3-11B20 and Si3-11B22. The textural features of the meso-
phases of compounds Si3-11B20 and Si3-11B22 (see, for
example, Figure S5, Supporting Information) are identical with
those seen for the high-temperature phase of Si3-11B18.47 The
diffraction pattern of an aligned sample of compound Si3-11B22
is shown in Figure 8b as an example. The small-angle reflections
can be indexed to an oblique columnar phase with the lattice
parameters a ) 4.9 nm, b ) 2.2 nm, and an oblique angle γ )
100°. Within this mesophase, the organization is synclinic
(different intensity of the wide-angle reflections) and the tilt
angle is 50°. A model of this mesophase is shown in Figure 9c.
Accordingly, the layers are broken into ribbons (modulated
layers), and these ribbons are organized into an oblique 2D
lattice. This phase is assigned as columnar oblique (Colob). From
switching experiments (see section 3.4.), it can be concluded
that the organization of the molecules in adjacent ribbons is
antipolar. There are at least three possible structures depending
on the polar direction in adjacent ribbons, which are discussed
in more detail in ref 12c and in Figure S6 of the Supporting
Information.
The diffraction pattern of Si3-11B22 is the same in the whole
temperature region, but for compound Si3-11B20, there is a
change of the diffraction pattern at 96 °C. At higher temperature,
the small-angle diffraction pattern is indicative of a ColobPA
phase (a ) 4.5 nm, b ) 2.2 nm, and γ ) 103°, Figure S7a) as
in the case of Si3-11B22. Around 96 °C, all out-of-meridian
reflections completely disappear, indicating a smectic structure
until crystallization (see Figure S7b, Supporting Information).
No change can be observed in the wide-angle region, indicating
a synclinic tilt of the molecules in both mesophases. The
diffraction pattern of this low-temperature phase is identical with
the mesophase(s) of Si3-11B18 (layer reflections are extended
It is remarkable that also in the homologous series of
compounds Si3-mB12 there is one compound (Si3-9B12) which
shows a transition to a higher ordered low-temperature phase
(see Figure S10 in the Supporting Information) with an inversion
of chirality at the phase transition, very similar to that of
compound Si3-11B14. The neighboring homologues Si3-8B12
and Si3-10B12 do not show this type of low-temperature phase.
Remarkably, the ratio of end chain length to spacer length is
(47) Though a synclinic organization in the ColobPA and USmCsPF phases is
clearly proven by X-ray scattering, in the textures, the extinction crosses
coincide with the positions of polarizer and analyzer (see Figure S5). This
indicates that these 2D-modulated smectic phases preferably adopt an
alignment of the ribbons with the polarization vector parallel to the glass
surfaces, which cannot be distinguished from an anticlinic organization by
optical methods. See: Gorecka, E.; Vaupotic, N.; Pociecha, D.; Cepic, M.;
Mieczkowski, J. ChemPhysChem 2005, 6, 1087-1093.
(48) (a) MacKintosh, F. C. Curr. Opin. Colloid Interface Sci. 1997, 2, 382-
387. (b) Katsaras, J.; Tristram-Nagle, S.; Liu, Y.; Headrick, R. L.; Fontes,
E.; Mason, P. C.; Nagle, J. F. Phys. ReV. E 2000, 61, 5668-5677. (c)
Sengupta, K.; Raghunathan, V. A.; Katsaras, J. Phys. ReV. E 2003, 68,
031710.
9
3060 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Si-Containing Polyphilic Bent-Core Molecules
A R T I C L E S
of compounds Six-mBn are optically isotropic and composed
of domains of opposite chirality sense. However, above a certain
threshold voltage, the texture becomes strongly birefringent.
Also cooling from the isotropic liquid state under an applied
electric field with a certain threshold strength leads to strongly
birefringent textures with circular domains (see, for example,
Figure 11a,e). This means that under an electric field the strongly
distorted layer structures of the dark conglomerate phases are
transformed into nearly nondistorted layers. A probable explana-
tion could be that the alignment force provided by the electric
field above a certain threshold is stronger than the layer
distorting forces. More generally speaking, the structure of the
mesophase investigated in the electric field experiments is
distinct from the mesophase formed without applied field.
Figure 10. Switching current response of compound Si3-11B4 in a 6 µm
polyimide-coated ITO cell ((105 V, 1 Hz, 5 kΩ) on applying an alternating
simple (regime of the downward current peak) and modified triangular wave
voltage (regime of the upward current peak/s) at T ) 85 °C.
3.1. Surface-Stabilized (Metastable) Ferroelectric Switch-
ing. For the optical investigation of the switching process,
circular domains were grown by slow cooling under a DC field.
Within the obtained domains, the layers are aligned perpen-
dicular to the cell surfaces (bookshelf geometry), and these
layers are circularly arranged around the centers of these
domains. Hence, the molecules are organized parallel to the cell
surfaces. The characteristic features of these domains are
extinction brushes, where the direction of the brushes are parallel
and perpendicular to the direction of the optical axis of the
smectic layers. The change of the position of these brushes,
depending on the applied voltage, gives information about the
change of the direction of the optical axes and hence about the
reorganization of the molecules under the influence of the
electric field.13a Different types of switching behavior were
observed, depending on the structure of the molecules, though
only one current-response peak was observed in all cases.
the same in both compounds (ratio ) 1.3). Also, for compound
Si3-9B12, the layer distance decreases from d ) 4.0 nm in the
SmCsPF *] phase to d ) 3.0 nm in the SmX[*] phase, indicating
[
an increase of the tilt angle from ca. 44 to ca. 58°. It seems
that, for this special ratio of spacer length to end chain length,
the terminal chains can be accommodated between the aliphatic
spacer units in such a way that optimal space filling is possible
if the aromatic cores adopt a strongly tilted organization in
layers. This dense packing might allow the crystallization of
the aromatic and possibly also of the aliphatic parts, whereas
the oligosiloxane units seem to remain in a more disordered
state, which retains some softness of this smectic mesophase.
3. Electrooptical Investigations. Switching experiments in
electric fields can give information concerning the organization
of the molecules with respect to their polar direction.13 All fluid
smectic phases of compounds Six-mBn show only one very
sharp single current peak in the half period of a triangular wave
voltage, also at low frequencies (down to 1 to 0.1 Hz,).49 The
calculated polarization values are between 700 and 800 nC cm-2
in all cases. This single peak is a first indication of a FE
switching process, but this is not an unambiguous proof of
ferroelectricity. For example, if the relaxation to the AF ground
state would be slow, then the two polarization current peaks of
an AF switching can merge to only one. Therefore, it was
proposed to use a modified triangular wave field, where a delay
is introduced at zero voltage.18c Also under these conditions,
using noncoated ITO cells,50 independent of the temperature
and cell thickness (5 or 10 µm), only one peak is observed in
the modified triangular wave field (see Figure 10). This indicates
that a bistable switching takes place, and that switching always
occurs after zero-voltage crossing of the applied field.
For compounds Si3-mBn with dark conglomerate phases (m
) 7-11 and n ) 12 or m ) 11 and n ) 4-16) under an applied
DC field, circular domains were obtained, in which the
extinction crosses are inclined with polarizer and analyzer,
indicating a synclinic FE organization (see Figure 11a). The
extinction crosses of these homogeneously chiral circular
domains do not relax at zero voltage (see Figure 11b); they
change their position only after application of the opposite
voltage (Figure 11c). This indicates a bistable switching process
(only two distinct states at + voltage and at - voltage can be
observed). As also shown in Figure 11a-c, the texture of
compound Si3-11B4 at zero voltage has nearly the same
birefringence as the textures under the field. However, the
birefringence slowly decreases after terminating the field, and
after 30 min, the texture has become completely dark in most
parts of the sample, as shown in Figure 11d. This texture is
reminiscent of the virgin dark conglomerate texture, obtained
by cooling from the isotropic phase without applied field, with
the difference that no chiral domains could be identified
(probably these domains are too small to be visible). This
indicates that the switching process itself is ferroelectric, but
the field-induced FE states (SmCsPF) are metastable at zero
voltage and, on a longer time scale, relax to the macroscopically
nonpolar (disordered) ground-state structure. For compound Si3-
11B8, the relaxation takes about 1 h, and for the long chain
compounds Si3-11Bn (n ) 12-16), it cannot be observed, even
after prolonged storage. Hence, the FE states, once formed in
an electric field, are stabilized by surface interactions (surface
stabilized FE switching).
Additional information was gained from the optical investiga-
tion of the switching process. Increasing the voltage of an
applied electric field gives rise to a significant change of the
appearance of the smectic mesophases of the silylated com-
pounds. In the virgin ground states, most smectic mesophases
(49) In some cases, under special conditions, the single peak can be split into
two very close fused peaks, though optical investigations exclude an AF
switching (see, for example, Figure S11). Probably, this is due to the
coexistence of diastereomeric (energetically different) SmCsPF and SmCaPF
states in these cases. It is also possible that this splitting results from an
energetic difference for the molecules in the center of the sample and at
the surfaces.
(50) In polyimide-coated cells, the switching behavior can be more complex in
some cases, due to the effect of an induced electric double layer, which
will be discussed in detail in a separate paper.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3061

A R T I C L E S
Keith et al.
Figure 11. (a-c) Bistable (FE) switching of homogeneous chiral domains of the SmCsPF phase of compound Si3-11B4 (crossed polarizers, indicated by
arrows, T ) 100 °C, 6 µm, polyimide-coated ITO cell, 1 Hz); (d) texture obtained after terminating the applied field for about 30 min; (e-g) switching
process of the polar smectic phase of compound Si3-11B16 (T ) 105 °C, 6 µm polyimide-coated ITO cell).
of birefringence during switching can be observed for all
apparently bistable switching smectic phases of the series Si3-
11Bn. It is only marginal for Si3-11B4 (see Figure 11a-c), and
it becomes stronger with increasing chain length and is the
strongest for Si3-11B16. It seems that immediately after switch-
ing off the electric field in the center of the sample the flat
smectic layers become distorted (optically isotropic), as in the
ground-state structure. Only at the surfaces do the nondistorted
(birefringent) layers remain. In these surface layers, the position
of the extinction crosses is retained. This indicates that the
SmCsPF structure is stabilized due to surface alignment, and
these surface layers switch into the other polar state after field
reversal. In the field-on state, the disordered layer fragments in
the center reorganize into the configuration defined by the
surface layers, and the texture becomes highly birefringent again.
Figure 12. Organization of bent-core molecules in the ITO cell at zero
voltage after removal of the field: (a) the position of the extinction brushes
and the birefringence of the texture remain after switching off the electric
field (compound Si3-11B4) due to the robust layer structure; (b) the texture
becomes weakly birefringent due to the disordered structure in the center,
and the position of the extinction crosses is not changed, due to surface
coupling; (c) optically isotropic regions, where throughout the whole sample
the smectic layers are distorted in such a way that the ferroelectric clusters
are disordered (weak coupling to the surface).
It seems that the degree of coupling between the molecules with
respect to the strength of the surface anchoring is responsible
for the chain length dependence of this effect (see Figure 12).
If the layers are robust (short alkyl chains), the surface-stabilized
polar structure is stable in the whole sample for a certain period
of time. However, the coupling to the surfaces is not strong
enough to maintain the polar order after switching off the electric
field, and hence, in the whole sample, the polar layers slowly
relax to the macroscopically apolar dark conglomerate structure
(see Figure 12c). If the coupling between the molecules is weak
(long alkyl chains), then a significant amount of the material
immediately relaxes and only thin surface layers remain (see
Figure 12b). Because in this case the coupling between the
molecules within the smectic phase is weaker than anchoring
to the surfaces, the texture within the surface layers is stable
3.2. Combined Surface-Stabilized FE Switching and Su-
perparaelectric Switching. With increasing chain length, the
optical appearance of the zero-voltage state changes. This is
most clearly seen for compound Si3-11B16 (see Figure 11e-
g). Here the birefringence significantly drops after switching
off the field, though the direction of the extinction crosses does
not change; that is, the organization should remain synclinic
and therefore should remain highly birefringent. This reduction
9
3062 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Si-Containing Polyphilic Bent-Core Molecules
A R T I C L E S
and is not affected by the relaxation of the majority of the
material into the optical isotropic disordered structure.
Hence, for compounds Si3-11Bn with n ) 6-16, the
switching process is not classical ferroelectric, where one
macroscopically polar state switches directly into another one
with opposite polarity. This process can only be seen at the
surfaces where the ferroelectric domains are stabilized by surface
alignment (surface-stabilized FE switching). In the center (where
the influence of the surfaces is weak), the polar layers are
unstable and relax into smaller ferroelectric domains which
adopt a random orientation in space (as in the dark conglomerate
ground-state structure). This model is similar to that suggested
for a superparaelectric switching process.51 These investigations
confirm that the ground-state structure is ferroelectric (SmCsPF)
and the disordered (optically isotropic) structure allows an
escape from the macroscopic polar order. In other words, though
the local microstructure is polar, the mesophase itself is
macroscopically apolar.
3.3. The Field-Induced Smectic Phases of Si3-11B18: At
the Borderline to AF Switching. Also for compound Si3-11B18
with an octadecyl terminal chain, only one peak was observed
for the switching under a triangular wave field. This type of
current response was found in the whole mesomorphic temper-
ature range, that is, in the temperature range of both undulated
smectic mesophases (USmCsP′F and USmCsPF). However, it
must be considered that under the influence of the electric field
the layer undulation is squeezed out, as the texture is different
from the ground-state structure and identical to those observed
for flat layer structures.52,53 In addition, the phase transition at
96 °C cannot be observed in the field-aligned samples. This
means that the actually investigated mesophase is not one of
the undulated USmCsPF phases, but a field-induced nonundu-
lated smectic phase. Nevertheless, the switching process ob-
served under the polarizing microscope indicates some signifi-
cant differences to those observed for the smectic phases of the
other compounds Si3-11Bn with shorter chains (n ) 4-16).
As shown in Figure 13b, on cooling under a triangular wave
AC field, only domains with extinction brushes parallel to
analyzer and polarizer could be observed, as usually seen for
anticlinic tilted smectic phases.54 However, the textural changes
seen during switching of these domains are distinct from those
usually observed for typical anticlinic domains of classical bent-
core molecules.13a To explain this observation, it is assumed
that the fundamental structure is still SmCsPF, but there are
additional synpolar (PS) and anticlinic interfaces. Due to these
SmCaPS interfaces, the tilt direction between adjacent synclinic
ferroelectric (SmCsPF) domains (stacks of layers) alternates. The
size of the domains is smaller than the wavelength of light, and
therefore, the extinction brushes coincide with the direction of
polarizer and analyzer (see Figure 13b). At high temperature
(Figure 13a), on terminating the applied field, no change in the
position of the extinction brushes could be seen, except for a
significant decrease in the birefringence. As mentioned earlier,
this reduction of the birefringence can be explained by the
Figure 13. Temperature dependence of the switching process of compound
Si3-11B18, (b) under the applied AC field (6 µm polyimide-coated ITO
cell, 25 Vpp/µm, f ) 1 Hz); (a) after switching off the field at T > 95 °C,
the position of the dark extinction brushes does not change (FE state is
stable at the surfaces and in the center of the cell relaxes to a dark
conglomerate phase); and (c) after switching off the field at T < 95 °C, in
the bright regions the dark brushes become inclined with respect to polarizer
and analyzer (the FE state relaxes to the AF ground state by rotation on a
cone); in the dark regions, the texture is the same as in (a); the models
below show sections of the proposed microdomain structures composed of
SmCsPF domains with synpolar and anticlinic interfaces in (a) and (b) and
with antipolar and synclinic interfaces in (c); to avoid confusion the terms
synpolar (subscript S) and antipolar (subscript A) are used to characterize
the polar correlation between domains, whereas the polar correlation between
the individual layers is described as ferroelectric () synpolar) and
antiferroelectric () antipolar).
formation of a dark conglomerate structure in the center of the
sample (SmCsPF domains become disordered and escape from
the macroscopic polar order), whereas the polar structure at the
cell surfaces remains. Hence, the switching is surface-stabilized
FE plus superparaelectric.
At the temperature where a phase transition is optically
observed in the ground-state structure, a change of the switching
behavior is also observed in the field-induced smectic phase.
Upon termination of the field at low temperature (Figure 13c),
the extinction brushes slowly relax with rotation to positions
inclined with the directions of polarizer and analyzer, and the
birefringence strongly increases in the zero-voltage state. This
clearly indicates a relaxation to a ground-state structure, in which
the synclinic domains adopt a uniform tilt direction. The
relaxation takes place on a cone and leads to a synclinic and
antipolar (PA) organization of the SmCsPF domains with discrete
SmCsPA boundaries between them, yielding a total nonpolar
ground-state structure, as shown in the lower part of Figure 13c.
The antipolar organization of the domains removes a main
(51) Achard, M. F.; Bedel, J. Ph.; Marcerou, J. P.; Nguyen, H. T.; Rouillon, J.
C. Eur. Phys. J. E 2003, 10, 129-134.
(52) Examples of field-induced Col to Sm transitions: (a) Ortega, J.; de la Fuente,
M. R.; Etxebarria, J.; Folcia, C. L.; Diez, S.; Gallastegui, J. A.; Gimeno,
N.; Ros, M. B. Phys. ReV. E 2004, 69, 011703. (b) Etxebarria, J.; Folcia,
C. L.; Ortega, J.; Ros, M. B. Phys. ReV. E 2003, 67, 042702.
(53) Such a change of the mesophase structure was also observed for B7-type
phases; see ref 38.
(54) Provided that the layers are organized in a bookshelf geometry; see ref 47.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3063

A R T I C L E S
Keith et al.
driving force for layer distortion, giving rise to a nondistorted
and therefore highly birefringent texture. It should be mentioned
that not the whole sample adopts this synclinic antipolar SmCsPF
domain structure. The dark areas, seen in Figure 13c, represent
regions with the same low birefringent texture as shown in
Figure 13a (with an anticlinic synpolar organization of the
SmCsPF domains in the surface layers). Hence, the switching
process seems to be metastable ferroelectric, with two distinct
relaxation mechanisms. One of them leads to a distorted layer
structure (superparaelectric type of switching combined with
surface-stabilized FE switching), the other one to nondistorted
layers with synclinic antipolar SmCsPA interfaces between the
SmCsPF domains. The latter one can be regarded as a nonclassic
type of AF switching (reorientation of FE domains, which is
distinct from the AF switching of the B2-type SmCPA phases,
where every second layer is switched).55
Figure 14. Switching current response of the ColobPA phase of com-
pound Si3-11B22 in a 5 µm noncoated ITO cell ((150 V, 20 Hz) at T )
90 °C.
The same switching behavior as observed for Si3-11B18 was
also found for compounds Six-11B12 with short silicon-con-
taining units (x ) 1 or 2; see Figure S12 in the Supporting
Information) and compounds Si3-mB12 with short spacer units
(m ) 5 and 6), that is, it is found for all compounds forming
smectic phases without discrete siloxane sublayers, where alkyl
chains and siloxane units are mixed in common sublayers (and
the diffuse wide angle scattering at D ) 0.7 nm is missing).
The stepwise change of the organization of the molecules
and their switching behavior can be understood by a continuous
transition from a discrete triple layer organization to a simple
layer structure composed of only two types of sublayers.
Compounds with discrete siloxane sublayers have a strong
preference for the SmCsPF structure under all conditions. For
silylated compounds without these siloxane sublayers, where
the siloxane groups are organized in common layers with the
alkyl chains, the stability of the ferroelectric structure is reduced,
so that also antipolar interfaces can emerge, but the SmCsPF
structure is still dominating. This reduction of segregation also
leads to a change of the textures from the purely dark
conglomerate type (SmCsPF *]) to a coexistence of birefringent
[
[
and dark conglomerate textures (SmCsPF */°]). For compounds
En-9Bn and 12B12 without siloxane units, the SmCsPF stabiliz-
ing effect is completely missing in the ground state, and for
these compounds, antiferroelectric ground-state structures are
strongly favored, leading to birefringent textures and classical
AF switching under all conditions.
3.4. The AF Switching ColobPA Phase of Compound Si3-
11B22: Switching of Superstructural Chirality. For com-
pound Si3-11B22, which shows exclusively the ColobPA phase,
the switching process is clearly AF as indicated by the presence
of two well separated repolarization current peaks (see Figure
14). The value of the spontaneous polarization (350-400 nC
cm-2) is significantly smaller than that in the smectic phases.56
The extinction crosses are always inclined with the directions
of polarizer and analyzer, and this indicates that the synclinic
organization of the ground-state structure (see section 2.2.5.)
Figure 15. Switching process of the polar ColobPA phase of compound
Si3-11B22 (5 µm noncoated ITO cell), as seen between crossed polarizers
(T ) 90 °C); the position of the extinction cross does not change, only the
birefringence slightly decreases in the zero-field state, which indicates a
switching around the long axis.
is not significantly changed by the field (Figure 15). The layer
modulation remains under the electric field, as can be seen by
the textures of the field-aligned samples (Figure 15), which
are very different from those typically observed for the smectic
phases. Because of the modulation of the layers, the switching
on a cone is restricted by the inter-ribbon boundaries, and
therefore, the switching takes place around the long axis as is
typical for ColobPA phases.45,57 Indeed, in the circular domains,
(55) The observation of only a single peak in switching experiments under a
triangular wave field might be due to the fact that the relaxation to the
synclinic antipolar organization (nonclassic AF switching) is slow. Hence,
under a triangular AC field, the synclinic antipolar boundaries, seen in DC
field experiments, cannot be formed to a considerable extend, possibly
because the anticlinic defects in the surface layers inhibit the fast
reorganization to a purely synclinic structure.
(56) In addition, the threshold voltage (∼50 Vpp/µm) is much higher than that
for the smectic phases (Si3-11B4 ∼ 16 Vpp/µm to Si3-11B16 ∼ 7 Vpp/µm,
USmCsPF phase of Si3-11B16 ∼ 30 Vpp).
(57) Examples for switching of suprastructural chirality by collective rotation
around the long axis: (a) Schro¨der, M. W.; Diele, S.; Pelzl, G.; Weissflog,
W. ChemPhysChem 2004, 5, 99-103. (b) Szydlowska, J.; Mieczkowski,
J.; Matraszek, J.; Bruce, D. W.; Gorecka, E.; Pociecha, D.; Guillon, D.
Phys. ReV. E 2003, 67, 031702. (c) Amaranatha Reddy, R.; Schro¨der, M.
W.; Bodyagin, M.; Kresse, H.; Diele, S.; Pelzl, G.; Weissflog, W. Angew.
Chem., Int. Ed. 2005, 44, 774-778. (d) Weissflog, W.; Dunemann, U.;
Schro¨der, M. W.; Diele, S.; Pelzl, G.; Kresse, H.; Grande, S. J. Mater.
Chem. 2005, 15, 939-946.
9
3064 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006

Si-Containing Polyphilic Bent-Core Molecules
A R T I C L E S
where the extinction brushes are inclined with polarizer and
analyzer (synclinic domains), the position of the extinction
crosses does not change, neither on terminating the field nor
on reversing the field. This confirms a switching by rotation
around the long axis, which changes the polar direction with-
out changing the tilt direction of the molecules.45 It should
be noted that this switching process gives rise to a change
of the superstructural chirality of the ribbons from racemic
at zero voltage to homogeneously chiral under the applied
field, and the change of the sign of the applied field
switches between the enantiomeric chiral states (see Figure
15).45,57
With increasing segregation of the oligosiloxane sublayers,
the switching process changes from a classical AF-type for the
nonsilylated compounds (e.g., 12B12) via a nonclassical AF
and superparaelectric switching to long-living surface-stabilized
FE for the triple layer phases. The effect of the additional
oligosiloxane sublayers seems to be largely entropic. Usually,
the AF organization in the liquid crystalline phases of bent-
core molecules allows an easy fluctuation of the molecules from
layer to layer because the wings of the bent cores are organized
in a synclinic fashion at the interlayer interfaces (see Figure 1).
These fluctuations are more difficult in the FE structures, where
the wings of the bent cores are organized in an anticlinic fashion
at these interfaces and hence disturb these fluctuations. This
leads to an entropic penalty for the FE structure. Because the
isotropic siloxane sublayers can suppress these fluctuations,
these sublayers remove the entropic penalty for the FE structure
and make the FE structure more favorable.12c If the oligosiloxane
sublayers are disturbed (short spacers, long alkyl chains, or small
siloxane units), the entropic effect becomes more important,
which is favorable for the AF organization. However, in addition
to this entropic effect, there should also be an energetic
stabilization of a synpolar FE structure by these sublayers. The
reasons for this energetic effect are not completely clear, and
further investigations are required.
The high-temperature ColobPA phase of compound Si3-11B20
shows the same behavior as the ColobPA phase of Si3-11B22,
but for this compound, the switching process changes at the
phase transition (96 °C) to the low-temperature USmCsPF phase,
where one of the two peaks disappears. In this low-temperature
mesophase, the switching behavior is essentially the same as
in the field-induced smectic phases of Si3-11B18. Hence, in this
compound, there is a temperature-dependent change from
surface stabilized FE switching (on a cone) at lower temperature
to AF switching (around the long axis) at higher temperature.58
Summary and Conclusion
In addition, with increasing length of the alkyl chains, steric
effects also become important, and this leads to a transition from
smectic phases via undulated smectic phases to an oblique
columnar phase formed by ribbon-like layer fragments. Simul-
taneously, the switching process changes from a rotation around
a cone in the smectic phases, which retains the layer chirality,
to a switching around the long axis in the columnar phases,
which reverses the layer chirality.
The self-organization of bent-core molecules was modified
in a systematic way by introduction of polyphilicity, provided
by silicon-containing units, which are incompatible with the
aromatic cores and the alkyl chains. Segregation of the siloxane
and alkyl segments takes place if the oligosiloxane units have
a sufficient length. This leads to a triple layer structure, where
spacer length, length of the rigid bent core, and the size of the
oligosiloxane units determine the layer spacing. If spacer length
and length of the terminal alkyl chain are very different, then
the end chains cannot be accommodated completely in the
aliphatic regions, which disturbs segregation and gives rise to
a simple layer structure where only the aromatic and nonaro-
matic segments are segregated. The incompatibility of the
oligosiloxane units with the aromatic cores and the aliphatic
chains is thought to be due to the high flexibility and the more
spherical average shape of these units. Hence, related effects
were observed with molecules, where carbosilane units replace
the siloxane units.17d
In summary, this work has contributed to a significantly
improved understanding of the relations between molecu-
lar structure, emergence of polar order, and supramolecu-
lar chirality in bent-core systems. It highlights the importance
of the engineering of interfaces for directed LC materials
design.59
Acknowledgment. The work was supported by the DFG
(GRK 894/1) and the Fonds der Chemischen Industrie; R.A.R.
is grateful to the Alexander von Humboldt Foundation for the
research fellowship. We thank Noel Clark (University of
Colorado at Boulder) for helpful discussions.
Segregation of alkyl chains and siloxane units significantly
changes the mesophase structure and the switching behavior.
In the ground-state structure, the synclinic ferroelectric orga-
nization of the molecules is favored (SmCsPF). The escape from
macroscopic polar order in such mesophases contributes to a
collapse of flat layers with formation of an optically isotropic
Note Added after ASAP Publication: In the version
published on the Internet February 9, 2006, the central biphenyl
benzoate units were drawn in the wrong direction in the formula
of compounds En-9B12 and Six-mBn in Scheme 1 and in Tables
1-3. The formula are correct in the version published February
22, 2006, and in the print version.
[
microdomain structure (SmCsPF *]). In this optically isotropic
mesophase, the superstructural layer chirality of the homoge-
neously chiral SmCsPF structure can be observed by an optical
activity of the distinct domains (dark conglomerate phase). The
observation of a change of the sense of optical rotation by
changing the orientation of the aromatic cores with respect to
(58) Related transitions between AF and FE switching were observed for B5
phases, where the FE switching phase appears at low temperature: (a)
Nadasi, H.; Weissflog, W.; Eremin, A.; Pelzl, G.; Diele, S.; Das, B.; Grande,
S. J. Mater. Chem. 2002, 12, 1316-1324. It was also observed in mixtures
of FE and AF switching materials with B2-type phases, but in this case,
the phase sequence is reversed, here the FE switching phase is the high-
temperature phase: (b) Kumazawa, K.; Nakata, M.; Araoka, F.; Takanishi,
Y.; Ishikawa, K.; Watanabe, J.; Takezoe, H. J. Mater. Chem. 2004, 14,
157-164.
(59) In a similar way, bulky hydrocarbon units were recently used to in-
fluence the interlayer correlation in SmC phases of rod-like molecules:
Cowling, S. J.; Hall, A. W.; Goodby, J. W. Liq. Cryst. 2005, 32, 1483-
1498.
the layer plane at the phase transition of SmCsPF *] to SmX[*]
[
from ca. 43-44 to ca. 52-58° can be explained on the basis of
the recently suggested concept of layer optical chirality20 in
these polar smectic phases. This is a new source of optical
activity in supramolecular systems, distinct from helical super-
structures.
9
J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006 3065

A R T I C L E S
Keith et al.
Supporting Information Available: Figures explaining the
occurrence of suprastructural chirality (S1) and possible orga-
nizations in the distinct ColobPA phases (S6); switching current
response curves for compounds 12B12 (S3) and Si3-11B16
(S11); textures of the SmCPA phase (S2), the Colr phase (S4),
Si3-9B12 (S7, S9, and S10), X-ray data (Table S1); transi-
tion temperatures of the olefinic precursors En-(m-2)Bn (Tables
S2, S3), experimental procedures and analytical data (NMR,
MS, elemental analysis) of the reported compounds. This
material is available free of charge via the Internet at
http://pubs.acs.org.
the SmCsPF */°] phase of Si3-6B12 (S8), the ColobPA phase of
[
Si3-11B20 (S5); optical changes during the switching of Si1-
11B12 (S12); diffraction pattern of Si3-11B20, Si3-6B12, and
JA057685T
9
3066 J. AM. CHEM. SOC. VOL. 128, NO. 9, 2006