Bent-core mesogens with branched carbosilane termini: Flipping suprastructural chirality without reversing polarity

Article abstract of DOI:10.1002/anie.200800814

(Figure Presented) Back flip: Field-induced transformations between the two homogeneously chiral enantiomers of a bent-core mesogen (see picture) take place by two different mechanisms (A and B). Their combination leads to an unexpected field-induced chirality flipping (C) between the oppositely tilted structures without polarity reversal.

Full text of DOI:10.1002/anie.200800814

Communications
DOI: 10.1002/anie.200800814
Liquid Crystals
Bent-Core Mesogens with Branched Carbosilane Termini: Flipping
Suprastructural Chirality without Reversing Polarity**
Yongqiang Zhang,* Michael J. OꢀCallaghan, Ute Baumeister, and Carsten Tschierske*
Dedicated to Professor Keith H. Pannell on the occasion of his 68th birthday
Materials with macroscopic polar order have widespread
potential applications in molecular electronics and photonics.
The discovery that achiral bent-core (bow- or banana-shaped)
molecules can organize into fluid phases with polar order[1]
and macroscopic chirality[2] opened an intriguing new area in
the field of liquid-crystal (LC) research.[3] The polar order in
these mesophases results from the restricted rotation of these
closely packing bent-core molecules around their long axes
(Figure 1a), while the chirality arises from the tilted organ-
ization of these molecules in polar layers. The relationship
between three vectors—layer normal, tilt direction, and bend
direction—defines a right- or left-handed system (Figures S1
[4]
and S2in the Supporting Information), by which the four
basic structures, SmCsPA (racemic), SmCaPA (chiral), SmCsPF
(chiral), and SmCaPF (racemic), are described (Figure S3 in
the Supporting Information). Most bent-core molecules
possess antiferroelectric (AF = PA) layer structures owing to
the favorable antiparallel organization of bent-core mole-
cules.[3a] The AF states can be switched to the corresponding
ferroelectric states (FE = PF) upon the application of a
sufficiently strong external electric field (Figure 1a). Usually
a tristable (AF) switching with a relaxation of the FE states to
an AF ground state (at 0 V) is observed.[2] This switching
process is characterized by the presence of two polarization
current peaks in each half period of an applied triangular
wave voltage (see for example Figure 2h). In some cases a
bistable switching between the FE states of opposite polarity
also takes place without relaxation to the AF state (“FE
switching”); this is characterized by only one polarization
current peak per half period (see for example Figure 2i).[5]
Two switching mechanisms have been observed for FE
and AF switching: 1) rotation of the molecules around their
long axes,[6,7] (Figure 1b, A) and 2) rotation of the molecules
about a tilt cone (Figure 1b, B).[2]) The first switching
mechanism changes the polarity of the layers while retaining
their chirality, while the second changes both polarity and
chirality (chirality switching).[8]
Herein, we report a new, multistep type of field-induced
chirality switching which takes place between oppositely
tilted structures without reversing polarity (Figure 1b, C).
This was observed with a bent-core molecule that incorpo-
rates a highly branched carbosilane terminus (compound 3,
see Scheme 1). The molecular design is based on the
observation that incorporation of siloxanes into the termini
of tails of bent-core molecules (e.g. compound 1) has a strong
effect on molecular self-assembly.[9,10] The bulkiness of such
groups could lead to a transition from switching about a cone
to switching around the long axis and to a change of the
mesophase structure.[7] Moreover, the capability of siloxanes
to generate microsegregated sublayers leads to a transition
from antiferroelectricity to ferroelectricity.[9] As far as prac-
tical applications are concerned, carbosilanes have significant
advantages over siloxanes owing to their greater chemical
stability. However, previously reported carbosilane-substi-
tuted bent-core molecules have three CH2 units between two
adjacent silicon atoms.[11,12] Hence, the dimethylsilane units in
the carbosilane mesogens are further apart than in their
Figure 1. a) Polar order in smectic phases formed by bent-core mole-
cules and molecular reorganization during an FE and AF switching
process. b) Distinct switching mechanisms: A=switching around the
long axis reverses layer chirality, B=switching about a cone retains
layer chirality, and C=switching tilt direction with retention of polar
direction would reverse chirality (the combination of A and B). The
views are along the polar axis, and the sign of handedness is defined
in Figure S1 in the Supporting Information.
[*] Dr. Y. Zhang, Dr. M. J. O’Callaghan
Displaytech Inc.
2602 Clover Basin Drive, Longmont, CO 80503 (USA)
Fax: (+1)303-772-2193
E-mail: zhang@displaytech.com
Prof. Dr. C. Tschierske
Institute of Chemistry, Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Strasse 2, 06120 Halle (Germany)
Fax: (+49)345-55-27223
E-mail: tschierske@chemie.uni-halle.de
Dr. U. Baumeister
Institute of Chemistry, Physical Chemistry
Martin-Luther-University Halle-Wittenberg (Germany)
[**] This work was supported by NSF under grant nos. OII-0422196 and
IIP-0646460.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200800814.
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Scheme 1. Synthesis of compounds 2 and 3. Compound 1 was
synthesized following a hydrosilylation route. DCC=N,N’-dicyclohex-
ylcarbodiimide, DMAP=4-dimethylaminopyridine.[9a]
Table 1: Mesophases, transition temperatures, enthalpy values, and lattice
parameters for compounds 1–3.[a]
Cmpd.
T 8C [DH kJmolꢀ1 [b]
]
Lattice
parameters
[nm]
(T [8C])[c]
1[d]
2
Cr 63
[8.9]
Cr1 67
[4.3]
Cr 76
[17.9]
SmCPFE 116
[21.1]
74
[7.7]
SmCsPFE 91
[2.3]
Iso
d=4.4
(100)
Iso d=4.39
(100)
Iso d=4.56
(80)
[*]
[*]
Cr2
SmCPFE 107
[21.5]
107
[17.4]
3
ColobPA
a=2.22,
b=4.64,
g=958
Figure 2. Investigation of compound 3: left column: ColobPA phase;
right column: SmCsPFE phase. Textures as seen upon cooling the
isotropic liquid in a 6.0 mm ITO cell: a) ColobPA phase at T=998C,
b) SmCsPFE phase at T=808C. X-ray diffraction patterns
(99)
[a] Abbreviations: Cr=crystalline solid state; SmCsPFE =FE switching polar
smectic phase with the synclinic tilted organization of molecules; ColobPA =
AF switching oblique columnar phase; SmCPFE[*] =FE switching dark
conglomerate phase, [*] indicates the formation of optically active domains,
though the molecules themselves are achiral; Iso=isotropic liquid state.
[b] 10 Kminꢀ1, first heating scan. [c] X-ray diffraction data (T/8C); see
Tables S1 and S2 in the Supporting information for details. [d] Data from
Ref. [9a].
(I(T)ꢀI(1158C, isotropic liquid)) of an aligned sample: c) ColobPA
phase at T=998C (the inset shows the indexation of the small-angle
reflections), d) SmCsPFE phase at T=808C. Models showing the
organization of the molecules: e) ColobPA phase (magenta=2D colum-
nar lattice with relative ratio of X-ray lattice parameters a, b, and g,
blue dotted line=interribbon interfaces, and indigo=carbosilane
layers); f) field-induced ColobPF structure (unstable); g) SmCsPF struc-
ture as proposed for the SmCsPFE phase. Switching current response
curves as obtained under a triangular wave field (ꢁ30 V, 10 Hz) in a
1.2 mm ITO cell with parallel nylon alignment layers: h) AF switching at
T=1008C, i) FE switching at T=758C.
SiCH2Si units. This compound exhibits an FE switching
dark conglomerate type smectic phase (SmCsPFE[*],see Figur-
es S4 and S5 in the Supporting Information), which is similar
to that reported previously for its siloxane analogue 1.[9a]
siloxane analogues, which might reduce the effect of micro-
segregation. We describe here the first examples of bent-core
molecules that incorporate branched carbosilane termini with
only one (2) or two CH2 units (3) between silicon atoms.
Compounds 2 and 3 were synthesized by esterification of 3,4’-
biphenol[9a] with benzoic acids containing carbosilane termini
(Scheme 1, see the Supporting Information). The mesophases
and transition temperatures are summarized in Table 1.
Compound 2 is a direct analogue of the siloxane
derivative 1 in which the SiOSi units are replaced by
For 3, which has a bulkier carbosilane unit, two LC phases,
[13]
ColobPA and SmCsPFE
,
were identified. The high-temper-
ature phase has a texture typical of columnar mesophases
(Figure 2a). The X-ray diffraction (XRD) pattern of a
surface-aligned sample (Figure 2c) confirms a columnar
mesophase with an oblique lattice with lattice parameters
a = 2.22 nm, b = 4.64 nm, and an oblique angle of g = 958 (T=
998C). In the wide-angle region, the diffuse maximum at
0.48 nm, assigned to the mean distance within the aromatic/
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aliphatic parts, is split into two crescent like halos which are
ing Information). Hence, this smectic phase is assigned as
SmCsPFE (Figure 2g) and its behavior is very similar to that of
the SmCsPFE phase of compound 2.
inclined with meridian and equator, indicating a tilted
organization of the molecules. The intensities of the two
inclined maxima are unequal (see also Figure S6 in the
Supporting Information), consistent with a synclinic tilted
organization of the molecules.[14] The columnar mesophase is
a modulated smectic phase (ribbon phase) with a ribbon size
of about three molecules (Figure 2e, see Table S2 in the
Supporting Information). The modulation can be attributed
to the bulky tertiary carbosilane unit that requires additional
space, and the ribbon-type organization of molecules can
overcome this steric frustration. Within the ribbons the
molecules are tilted, and the angle between the director
(the molecular long axis) and the b direction of the oblique
cell is about 428; this is deduced from the direction of the
maxima of the wide-angle diffuse scattering with respect to
the position and distribution of the small-angle reflections of
the oblique lattice (see Figure S6 in the Supporting Informa-
tion). Based on X-ray observations, we propose a ribbon
structure (ColobPA phase) with partial overlapping of the
rodlike wings of the bent-core molecules in adjacent ribbons,
as shown in Figure 2e (see also Figure S7 in the Supporting
Information).
[*]
In the high-temperature phase, the switching current
curve shows two well-resolved current peaks in the half
period of the applied triangular electric field (Figure 2h),
which clearly confirms the AF ground-state structure of the
ColobPA phase. The measured polarization is about
670 nCcmꢀ2 (T= 1008C). The threshold voltage required for
this switching process is much higher than that observed for
the SmCsPFE phases of 1–3.[9,10] It is thought that an AF
switching from the ColobPA ground state to a FE field-induced
ColobPF state requires a high energy owing to the strong steric
interactions at the interribbon interfaces between synpolar
ribbons in the field-induced ColobPF state (Figure 2f, and
Figure S7 in the Supporting Information). As these steric
interactions would be removed in nonmodulated SmCsPF
layers, it appears likely that in the field-induced FE state
the layer modulation could be partly or completely removed
(Figure 2g). The synclinic FE structure induced from the
ColobPA phase is assigned as (+)- or (ꢀ)-SmCsPF-like.[16]
Circular domains with extinction crosses inclined with
polarizer and analyzer were developed for electrooptical
A
phase transition was
observed at 868C (cooling) by
differential scanning calorimetry
(DSC). No change in texture
(Figure 2b) was observed at this
transition, which suggests that the
synclinic tilted organization of the
columnar phase is retained in the
low-temperature phase. The 2D
XRD pattern shows that, at this
temperature, reflections corre-
sponding to the 2D lattice com-
pletely disappear with the excep-
tion of the layer reflections (d =
4.54 nm at 808C). These are
retained as shown in Figure 2d,
indicating the smectic nature of
this low-temperature mesophase.
The position and intensity distri-
bution of the wide-angle diffuse
scattering does not change, con-
firming a synclinic tilted organi-
zation of the molecules with a tilt
angle of about 468.[15] This smectic
mesophase showed only one
single polarization current peak
in the half period of the applied
triangular wave field at both high
(10 Hz, Figure 2i) and low fre-
quency (0.1 Hz), suggesting FE
switching. The measured polar-
ization is about 700 nCcmꢀ2 (T=
808C) and bistable switching by
rotation about a cone was con-
firmed by electrooptical investi-
gations (Figure S8 in the Support-
Figure 3. Investigation of compound 3 under a square-wave electric field (1.2 mm ITO cell with parallel
nylon alignment layer) at 1008C (cooling): a–f) Textures as seen under the polarizing microscope
between crossed polarizers (polarizers are vertical and horizontal); g) the proposed structural models
reflecting the organization of molecules in (a–f); Col1 and Col2 represent the two oppositely tilted
orientations of the ColobPA racemate at 0 V; (ꢀ)-F1 and (ꢀ)-F2 are the two oppositely tilted synclinic
orientations of the field-induced (ꢀ)-SmCsPF-like structures, and (+)-F1 and (+)-F2 are their
enantiomers, that is, (+)- and (ꢀ)-F1 (as well as (+)- and (ꢀ)-F2) have the same tilt direction but
opposite chirality. The fact that only one enantiomer is observed in (b,c) and (f,e) does not mean that
the symmetry is broken, since the other enantiomer was observed in other areas; the molecular polar
direction (indicated by dots and crosses) is from positive to negative charges as used in chemistry
and it is assumed that the molecular polar vector is parallel to the bend direction (see also discussion
in Figure S2 in the Supporting Information); PS=polar switching;[18] Vth =threshold voltage. Carbosi-
lane sublayers, possible interribbon interfaces, and modulation are omitted in F1 and F2 (see detailed
drawings in Figure 2 f) for clarity only.
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investigations (Figure 3a).[17] Under the applied triangular
wave electric field, two polarization peaks were observed,
though extinction crosses did not move; this suggests that AF
switching occurs around the molecular long axis. When a
square-wave voltage (ꢁ 30 V, 1 Hz) was applied, rotation of
extinction crosses from one synclinic orientation state to the
other one with an opposite tilt direction was observed
(Figure 3b–c, or f–e), indicating that polar switching about a
cone takes place.[18] However, switching off the square electric
field did not result in any rotation of extinction crosses
(Figure 3a and d), suggesting that under these conditions the
molecules relax to the ground state ColobPA phase by rotation
around the molecular long axis (Figure 3 g).[19]
By applying a modified low-frequency square-wave field
(0.1 Hz) where additional breaks at 0 V were introduced after
each period (Figure 4), it was possible to realize transforma-
tions among the two SmCsPF-like structures and their two
oppositely titled orientations, abbreviated here as (+)-F1,
(+)-F2, (ꢀ)-F1, and (ꢀ)-F2, and between the two oppositely
tilted AF orientations of the ColobPA structure, abbreviated as
Col1 and Col2(Figure 3g). Starting with the Col obPA ground-
state orientation Col1 (by switching on a positive electric
field), the FE orientation state (ꢀ)-F1 was achieved by
rotation around the long axis (extinction brushes did not
move but birefringence increased slightly). Switching off the
field after the field reversal (i.e. after switching about a cone
from (ꢀ)-F1 to (ꢀ)-F2, extinction brushes rotated) makes
(ꢀ)-F2relax to Col2by rotation around the molecular long
axis (extinction brushes did not move). After a pause at 0 V
the reversed field (positive) was turned on to switch Col2to
(+)-F2via rotation around the molecular long axis (extinction
brushes did not move). The field was switched on until (after
field reversal) the rotation of extinction brushes indicates that
(+)-F2was switched to ( +)-F1 by rotation on a cone.
Switching off the field makes (+)-F1 relax to the starting
structure Col1 (extinction brushes did not move). Thus, the
complete counterclockwise cycle shown in Figure 3 g with
dashed arrows (Col1 to (ꢀ)-F1 to (ꢀ)-F2to Col2to ( +)-F2to
(+)-F1 to Col1) was realized. If the cycle starts instead with
the opposite field direction, the sequence is reversed (shown
with solid arrows in Figure 3g).
irreversible changes between homochiral and racemic struc-
tures in SmCPA
and SmCPFE phases[9b,22] have been
[20,21]
reported (for example, by using either rectangular or square-
wave fields[21]), these processes involve transitions between
diastereomeric superstructures with different free energies
rather than transitions between enantiomeric superstructures
as reported here. The transformations from (+)-F1 to (ꢀ)-F2
or from (+)-F2to ( ꢀ)-F1 (between the oppositely tilted states
with identical free energy and opposite chirality), and even
from Col1 to Col2(between the two oppositely tilted
orientations of a racemic structure) have not previously
been reported.[23]
This new mode of chirality flipping is enabled by
competition between two switching mechanisms. When
driven by a slowly changing voltage (triangle wave), com-
pound 3 has time to relax to its antiferroelectric ground state
(ColobPA) by rotation about the long axis before a transition to
the other field-induced state (e.g. (ꢀ)-F2!Col2!(+)-F2). In
this case chirality is reversed without a change in tilt direction.
We speculate that rotation about the tilt cone is inhibited by
strong interactions generated at ColobPA interribbon inter-
faces (high energy barrier) during switching. When driven by
a square wave (with abrupt voltage changes) compound 3
appears to switch directly from one SmCSPF-like orientation
to another by rotation around the tilt cone (e.g. (+)-F1!(+)-
F2). The tilt direction is reversed and chirality is unchanged.
In this case molecules do not have time to relax to the ColobPA
ground state within the short duration of the switching event.
Interactions at the interribbon interfaces of field-induced
SmCsPF-like states are therefore reduced (Figure S7 in the
Supporting Information), and rotation around the tilt cone
becomes the favored switching pathway. The multistep drive
was designed to combine both fast and slow response modes
within a single sequence, creating a series of transformations
not attainable with the simpler waveforms. In contrast to the
use of the simple square-wave drive, a relaxation period at
0 V was introduced as a new element. The response to this is
the same as that seen for the slow ꢀ30 V! + 30 V transition
of the triangle wave. In principle, the effect of the multistep
drive waveform should also be obtainable with a sawtooth
waveform which intersperses abrupt + 30 V!ꢀ30 V steps
(see Figure 4) with slow linear changes of ꢀ30 V! + 30 V.
In summary, we have reported the first examples of bent-
core molecules with branched carbosilane termini. Com-
pound 3, with the bulkiest carbosilane unit, exhibits a
columnar ColobPA phase (ribbon phase) at high temperature
and an FE smectic phase with synclinic organization at low
temperature. We have demonstrated for the first time the
field-induced transformation between the two homogene-
ously chiral (+)- or (ꢀ)-SmCsPF-like enantiomers with
opposite orientations and between the two oppositely tilted
orientations of the ColobPA racemate. These transformations
involve the combination of two switching mechanisms which
allows the unexpected flipping of superstructural chirality by
changing tilt direction without reversing polar direction (see
Figure 1b, C). Compound 2 having a less bulky carbosilane
unit shows only an FE switching smectic phase.
Previously reported AF switching processes in columnar
phases involved either switching from (+)-F1 to (ꢀ)-F1 via
Col1 or from (+)-F2to ( ꢀ)-F2via Col2(chirality switching by
flipping only the polar direction).[6e] Although reversible or
Figure 4. The application of a modified low-frequency (0.1 Hz) square-
wave field results in the transformation along the cycle indicated by
the dashed arrows in Figure 3 g from Col1 via [(ꢀ)-F1], [(ꢀ)-F2], Col2,
[(+)-F2], and [(+)-F1] to Col1.
These investigations indicate that the combination of
steric frustration and interlayer segregation by careful
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[10] The nanosegregated siloxane sublayers were directly observed
by diffuse scattering in the wide-angle region of X-ray scattering:
C. Keith, R. Amaranatha Reddy, A. Hauser, U. Baumeister, C.
Tschierske, J. Am. Chem. Soc. 2006, 128, 3051.
[11] C. Keith, R. Amaranatha Reddy, H. Hahn, H. Lang, C.
Tschierske, Chem. Commun. 2004, 1898.
[12] a) G. Dantlgraber, U. Baumeister, S. Diele, H. Kresse, B.
Lühmann, H. Lang, C. Tschierske, J. Am. Chem. Soc. 2002,
124, 14852; b) H. Hahn, C. Keith, H. Lang, R. Amaranatha R-
eddy, C. Tschierske, Adv. Mater. 2006, 18, 2629.
molecular design can lead to interesting response character-
istics of stimuli-responsive functional materials. Studies on a
series of bent-core molecules with other carbosilane termini
in are currently in progress.
Received: February 19, 2008
Revised: March 4, 2008
Published online: July 21, 2008
[13] The phases for 1 and 2 were designated as SmCPFE[*] in Table 1 as
cooling the isotropic liquid without an applied field gives a
conglomerate of optically active domains typical for dark
conglomerate phases, and single-peak FE switching is observed
experimentally but the ground-state structure is not exactly
determined (either SmCPA or SmCPF).[10] No chiral domains are
observed for the low-temperature smectic phase of compound 3
and hence, this was designated as SmCsPFE. Note that SmCPFE
might differ from the well-defined SmCPF that describes a
specific state/phase structure.
Keywords: chirality · electrooptics · ferroelectric switching ·
liquid crystals · phase transitions
.
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[15] Calculated using cosq = d/L, q = 438; the molecular geometry
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[16] It is possible that in the field-induced FE state the layer
modulation could be not completely removed. Therefore, the
synclinic FE structure induced from the ColobPA phase is
assigned as (+)- or (ꢀ)-SmCsPF-like so that it can be distin-
guished from (+)- or (ꢀ)-SmCsPF structures, induced in the
nonmodulated smectic phases (SmCsPFE phases of compounds 1
and 2), where such layer modulations can be excluded.
[17] This is achieved by cooling the isotropic liquid under a square
electric field (ꢁ 30 V, 10 Hz) and then switching to a triangular
electric field (ꢁ 30 Vpp, 10 Hz).
[18] The application of a square-wave field always results in single-
peak switching since there is no time (at 0 V) to allow polar
states to relax to the ground-state AF structure. Hence, under a
square-wave field ferroelectric and antiferroelectric switching
cannot be distinguished, and therefore the term polar switching
(PS) is used here.
[19] It should be noted that the initial application of a square-wave
electric field leads to AF switching from ColobPA to SmCsPF-like
structures by rotation around the molecular long axis, and polar
switching by rotation about a cone occurs subsequently as the
sign of the electric field is changed.
[20] a) G. Heppke, A Jakli, S. Rauch, H. Sawade, Phys. Rev. E 1999,
60, 5575; b) S. Rauch, C. Selbmann, P. Bault, H. Sawade, G.
Heppke, O. Morales-Saavedra, M. Y. M. Huang, A Jakli, Phys.
Rev. E 2004, 69, 021707.
[21] A. Jµkli, C. Lischka, W. Weissflog, G. Pelzl, S. Rauch, G. Heppke,
Ferroelectrics 2000, 243, 239.
[22] Y. Zhang, M. Wand, M. J. OꢀCallaghan, C. M. Walker, W.
Thurme, Ferroelectrics 2006, 344, 11.
[23] Inversion of macroscopic chirality with time in an USmCPF
phase was recently reported, which probably results from the
reorganization of molecules in anticlinic interfaces upon the
application of a dynamic ac field: R. Amaranatha Reddy, M. W.
Schröder, M. Bodyagin, H. Kresse, S. Diele, G. Pelzl, W.
Weissflog, Angew. Chem. 2005, 117, 784; Angew. Chem. Int.
Ed. 2005, 44, 774.
6896 www.angewandte.org
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Angew. Chem. Int. Ed. 2008, 47, 6892 –6896