Liquid-crystalline quaternary block molecules incorporating bent-core units

Article abstract of DOI:10.1002/anie.200503402

(Figure Presented) A flick of a switch: The combination of fluorinated segments and oligo(dimethyl-siloxane) units with bent mesogenic cores leads to new liquid-crystalline materials (see picture) that display a temperature-dependent change from ferroelec

Full text of DOI:10.1002/anie.200503402

Communications
Liquid Crystals
DOI: 10.1002/anie.200503402
Liquid-Crystalline Quaternary Block Molecules
Incorporating Bent-Core Units**
R. Amaranatha Reddy, Gert Dantlgraber,
Ute Baumeister, and Carsten Tschierske*
Dedicated to Professor Siegfried Hünig
on the occasion on his 85th birthday
The design of functional supramolecular systems by the self-
organization of single molecules to give complex super-
structures is a contemporary challenge. Whereas classical
supramolecular chemistry focuses on assemblies formed by a
limited number of molecules (host–guest chemistry),[1] the
design of self-organized bulk materials with special properties
is also an important task. The design of materials with polar
order, for example, is of importance for piezoelectric, pyro-
electric, and nonlinear optical applications. Polar ordered
liquid-crystalline (LC) soft matter, in which the configuration
of polar order can be switched by means of electric fields, can
be used in fast-switching electrooptical microdisplays and
light modulators.
Chiral rodlike molecules organized in a tilted configura-
tion in layers (SmC* phases) were initially used as ferro-
electric (FE) and antiferroelectric (AF) switchable liquid-
crystalline materials.[2] In the last decade, achiral bent-core
molecules were found to provide a novel class of AF switching
LC materials.[3–5] Here, the polar order is due to the shape of
the molecules which restricts the rotation around the long axis
and leads to a spontaneous polarization (P) parallel to the
layer planes (see Figure 1a). In most of these mesophases, the
bent direction in adjacent layers is antiparallel so that the
layer polarization alternates from layer to layer, which leads
to a macroscopic apolar AF structure (SmCPA phases).[4]
More recently, FE switching mesophases (SmCPF) also have
been reported for such materials.[6–13] In such cases, the bent
direction in adjacent layers is parallel and switching takes
place between two energetically equivalent polar structures
(see Figure 1a).
[*] Dr. R. A. Reddy, Dr. G. Dantlgraber, Prof. Dr. C. Tschierske
Institute of Organic Chemistry
Martin-Luther-University Halle-Wittenberg
Kurt-Mothes-Strasse 2, 06120 Halle (Germany)
Fax: (+49)345-5525-346
E-mail: carsten.tschierske@chemie.uni-halle.de
Dr. U. Baumeister
Institute of Physical Chemistry
Martin-Luther-University Halle-Wittenberg (Germany)
[**] This work was supported by the DFG and the Fonds der Chemischen
Industrie. R.A.R. is grateful to the Alexander von Humboldt
Foundation for a research fellowship.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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therefore ferroelectric switching materials are rare. One
group of compounds that have a pronounced tendency to
form such FE switching smectic phases incorporate oligo(di-
methylsiloxane)[6,7] or carbosilane units[10] (see, for example,
compounds Hn in Figure 2). This observation indicates that a
polyphilic molecular structure and the segregation of oligo-
(dimethylsiloxane) units into separate sublayers is a success-
ful way to generate new bulk properties.[5b,6,7,10] Whereas the
majority of bent-core molecules are built up of only two
distinct parts, namely, the rigid bent core and the terminal
alkyl chains, these oligo(dimethylsiloxane)-substituted mole-
cules are composed of three distinct and incompatible units
and therefore represent ternary block molecules (Figure 2).
Herein, we report the first examples of bent-core mole-
cules composed of four distinct units. As shown in Figure 2,
these quaternary block molecules Fn combine two different
end chains—an oligo(dimethylsiloxane) and a perfluoroalkyl
chain—as well as alkyl spacers and aromatic bent-core units.
As a result of their special structure, these molecules show
interesting properties, such as a temperature-dependent
transition from ferroelectric to antiferroelectric switching
and a temperature-dependent transition between diastereo-
meric superstructures. In addition, it is shown for the first time
that the switching behavior of diastereomeric mesophase
structures can be different, with one being ferroelectric and
the other being antiferroelectric.
Figure 1. Modes of organization of bent-core molecules in polar
smectic LC phases. a) Side views: gray arrows indicate the polar
directions; dotted arrows indicate out-of-plane interlayer fluctuations,
which are easier in the SmCPA structure than in the SmCPF structure.[4]
b) Views parallel to the bend direction: subscript s denotes synclinic
organization (the tilt direction is the same in adjacent layers);
subscript a denotes anticlinic (the tilt direction changes from layer to
layer); subscript F denotes ferroelectric (synpolar); subscript A denotes
antiferroelectric (antipolar). The chirality of the layers is indicated by
the gray or white color of the molecules (see Supporting Information
for an explanation of layer chirality). SmCaPF and SmCsPA represent
racemic structures (chirality sense changes in adjacent layers); SmCsPF
and SmCaPA are homogeneous chiral structures (chirality sense is
identical in the layers). Dotted arrows indicate out-of-plane interlayer
fluctuations, which are easier in the SmCsP structures than in the
SmCaP structures. c) Switching on a cone changes both the polar and
tilt directions, that is, layer chirality is retained.[4]
Compounds F1, F2, and Fi2 were synthesized in an
analogous manner as reported for the non-fluorinated
oligo(dimethylsiloxane) derivatives Hn,[6] by hydrosilylation
of the appropriate fluorinated olefinic precursor F (see
Supporting Information). The properties of these new com-
pounds were determined by polarizing microscopy, X-ray
scattering, and electrooptical techniques. The observed mes-
ophase types and transition temperatures are collated in
Table 1and compared with the olefinic precursor F. All
compounds Fn (n = 1, 2) show optically isotropic mesophases
Note that the molecules are tilted relative
to the layer normal in most of these smectic
phases which gives rise to chirality in their LC
superstructures (the layer normal, tilt direction,
and polar axis define a chiral system; see
Supporting Information).[4] As with organic
molecules where the covalent connection of
stereogenic centers leads to diastereomers, the
combination of chiral layers in liquid-crystal-
line phases leads to diastereomeric superstruc-
tures. As shown in Figure 1b, the chirality sense
can be either identical in adjacent layers,
leading to homogeneously chiral structures
(SmCsPF and SmCaPA), or opposite, leading to
macroscopic racemic superstructures (SmCaPF
and SmCsPA).[4] These structures can be distin-
guished by means of their distinct optical
properties using a polarizing microscope.
Figure 2. Structures of a classical bent-core molecule (top),[20] ternary bent-core molecules Hn,[6]
and quaternary bent-core molecules Fn.
As mentioned above, in soft matter systems
macroscopic polar structures are unstable and
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Table 1: Mesophases, transition temperatures, and transition enthalpy values (in brackets) of the
fluorinated compounds Fn in comparison with compound F, which lacks an oligo(dimethylsiloxane)
group.[a]
Switching experiments with compound
F1 (10-mm thick noncoated ITO cell, Fig-
ure 3b) indicate only one peak in the
switching current curve down to a frequency
of 0.1Hz and point to a FE switching
process. However, optical investigations
indicated
a
more complex behavior.
Although the shape of the polarization
current curve does not change over the
whole temperature range, a clear temper-
ature-dependent change of the switching
process is seen. Domains with an SmCaPF
structure[16] were grown under a triangular
ac electric field at 100 Hz (see Figure 5b).
These domains are characterized by extinc-
tion crosses that coincide with the directions
of the polarizer and analyzer, indicating an
anticlinic organization of the molecules.
Under the applied ac field, this SmCaPF
structure is stable at all temperatures.
Upon switching off the field at T> 1108C,
there is no change in the orientation of the
extinction crosses (see Figure 5a) which
means that the structure does not change
and remains SmCaPF also at zero voltage;
Compound
n
T [8C] (DH [kJmolÀ1])[b]
F
–
1
2
2
Cr 111 (29.3) SmCPA 123 (20.0) Iso
[*]
F1
F2
Fi2[c]
Cr 94 (21.0) SmCPA+F ꢀ110 (À) SmCPF[*] 130 (21.1) Iso
Cr 104 (22.6) SmCPF[*] 127 (19.2) Iso
Cr 105 (22.8) SmCPF[*] 127 (20.1) Iso
[a] Abbreviations: Cr=crystalline solid state; SmCPA =AF switching polar smectic phase, with tilted
arrangement of the molecules; SmCPF[*] =FE switching polar smectic phase, with tilted arrangement of
the molecules; superscript [*] indicates the formation of a chiral superstructure, as seen by domains of
opposite chirality (see Supporting Information) although the molecules themselves are configuration-
ally achiral; Iso=isotropic liquid state. [b] 10 KminÀ1, first heating scan. [c] In this compound the
branched (Me3SiO)2Si(Me) group is used instead of the linear Me3Si(OSiMe2)2 unit as used in
compound F2.
composed of domains of opposite chirality sense (see
Supporting Information) as also reported for the non-
fluorinated analogues Hn.[6,7]
X-ray scattering of the mesophase of compound F1 (see
Figure 3a) indicates a fluid layer structure without in-plane
order, as characterized by a layer reflection together with the
fourth order.[14] The layer thickness is 4.2 nm, which is much
smaller than the calculated molecular length (l = 5.95 nm in
the most-extended conformation with all-trans alkyl chains,
according to CPK models) and is in line with a strongly tilted
arrangement of the molecules in a smectic monolayer
structure (Figure 4a). This observation also indicates that
there is no segregation of the fluorinated and the oligo(di-
methylsiloxane) end chains into distinct sublayers which
would otherwise lead to a bilayer structure with alternating
oligo(siloxane) and perfluoroalkane sublayers (see Fig-
ure 4b).[15] Nevertheless, the diffuse scattering in the wide-
angle region has a very unsymmetrical profile in which two
maxima can be distinguished (see inset in Figure 3a). The
maximum at 0.46 nm corresponds to the mean distance
between the hydrocarbon parts of the molecules, while the
shoulder corresponds to a mean distance of 0.62 nm and is
assigned to the mean distance between the nanosegregated
pentamethyldisiloxane units. Interestingly, this shoulder is
absent in the diffraction pattern of the corresponding non-
fluorinated pentamethyldisiloxane derivative H1[6] which
indicates that although the fluorinated segments do not
form their own sublayers they can facilitate the segregation of
Figure 3. Investigation of compound F1: a) X-ray diffraction pattern at
T=1108C; the inset shows the wide-angle region with two maxima,
and the arrow within the inset shows the fourth-order smectic
reflection. b) Switching current response at T=1158C in a 10-mm thick
noncoated indium tin oxide (ITO) cell (EHC, Japan) on applying an
alternating normal (1 Hz) and modified triangular-wave voltage.[11]
the oligo(dimethylsiloxane) parts. The mixed sublayers con-
taining the fluorinated chains and aliphatic spacer segments
(see Figure 4a) are probably more incompatible with the
pentamethyldisiloxane units than the alkyl chains themselves
and favor a segregation of these units into separate sublayers.
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Hence, there is a transition from a FE switching process at
high temperature to AF switching at lower temperatures.
Such a change of the switching type has only rarely been
observed for other bent-core molecules,[13] mostly in mix-
tures[12] or with a reverse sequence.[9]
Besides this temperature dependence, there is a remark-
able frequency dependence of the switching process. At lower
frequency (< 0.5 Hz) or under a dc field, the extinction
crosses are only slightly inclined (ca. 158) with the direction of
the polarizer and analyzer (see Figure 6a). This can be
explained with a structure in which an anticlinic organization
(SmCaPF, extinction crosses would be parallel to the polar-
izers) is mixed with a synclinic organization (SmCsPF,
extinction crosses would be inclined by 458) on a mesoscopic
scale (see models shown in Figure 6a).[17] In other words, two
diastereomeric organizations, chiral (SmCsPF) and racemic
(SmCaPF), coexist.
Figure 4. Models of the organization of compounds Fn in the smectic
phases: a) Monolayer structure in which the fluorinated chains and the
hydrocarbon chains are organized in common sublayers. b) Bilayer
structure in which the fluorinated chains and hydrocarbon chains are
organized in different sublayers (not observed for Fn).
Again, a significant change of the switching process can be
seen at around 1108C. At T> 1108C the extinction crosses do
not change their position after switching off the field
(compare Figure 6a and b), but upon application of an
electric field with the opposite sign the crosses rotate by about
608. In this way, the extinction crosses again become inclined
with the direction of the polarizer and analyzer by 158 but in
the opposite direction (compare Figure 6b and c). This
indicates a bistable switching process, that is, a direct switch-
ing between two different FE states. Reorganization of the
molecules takes place in each layer by a rotation on a cone
which reverses the polar direction and the tilt direction in the
synclinic as well as in the anticlinic domains (follow the black
line in Figure 6 from a to c).
At T< 1108C a different behavior is observed (see
Figure 6a,c,d). At this temperature the extinction crosses
Figure 5. Textures and mesophase structures of compound F1 in a
10-mm noncoated ITO cell obtained a) at T>1108C after switching off
the ac field, b) under the applied ac field (Vpp =peak-to-peak
voltage), and c) at T<1108C after switching off the field.
Yellow arrows show the positions of the polarizer (P) and
analyzer (A).
that is, the polar FE state seems to be stable (or long-
living metastable) under the experimental condi-
tions. These results indicate a bistable switching
process, that is, a direct switching between two
different polar FE states without a nonpolar AF
state at zero voltage (FE switching).
Switching off the field at T< 1108C leads to a
different behavior. In this case, the extinction
crosses slowly relax at zero voltage to a position
that is inclined 458 with respect to polarizer and
analyzer and the texture becomes more colored (i.e.
the birefringence is increased, see Figure 5c). These
observations indicate that the molecules now adopt
a synclinic organization, with the molecules tilted by
approximately 458 in the layers, achieved by a
rotation of the molecules on a cone into the AF
SmCsPA state. There are three stable states (the two
polar FE states and the nonpolar AF state) which
indicates an antiferroelectric type of switching.
Figure 6. Temperature dependence of the switching process of compound F1 (10-mm
noncoated ITO cell) at low frequency (<0.5 Hz) or under a dc field. Textures seen between
crossed polarizers a) under an applied field (35 Vdc) at 1008C or at 1208C, b) after switching
off the field at 1208C, c) after reversal of the applied field at 1208C or 1008C, and d) after
switching off the field at 1008C.
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relax at zero voltage to a position that is inclined by 458 with
respect to the polarizer and analyzer, and the texture becomes
more colored (i.e. the birefringence is increased, see Fig-
ure 6d). This behavior indicates that at zero voltage all
molecules adopt a synclinic organization with the molecules
tilted by about 458 in the layers. Hence, it seems that at T<
1108C only the anticlinic microdomains relax to the synclinic
AF state (follow the pink line in Figure 6 from a to d),
whereas the synclinic microdomains do not relax (follow the
dotted blue line in Figure 6 from a to d). Upon field reversal,
the extinction crosses continue rotation to the state shown in
Figure 6c, inclined with the polarizers by approximately 158,
indicating that again synclinic and anticlinic microdomains
coexist. This is achieved if in the homogeneously chiral
microdomains all molecules switch around a cone from one
synclinic FE structure to the other synclinic FE structure with
an opposite tilt direction and polar direction (follow the
dotted blue line in Figure 6 from d to c), whereas in the
racemic domains the molecules in every second layer switch
around a cone leading to a transition from the synclinic AF
structure to the anticlinic FE structure (follow the pink line in
Figure 6 from d to c). This means that the chiral synclinic FE
microdomains (SmCsPF) are more stable and retain their
polar organization at zero voltage. Only after field reversal do
these synclinic FE domains switch to the opposite FE state.
The racemic anticlinic FE microdomains (SmCaPF) are less
stable and readily relax to the AF state at zero voltage. Hence
at T< 1108C there is a coexistence of FE and AF switching
microdomains and there is a temperature-dependent change
of the switching mechanism from FE to AF only for the
anticlinic (racemic) organization, whereas for the synclinic
(chiral) organization the FE structure is stable also at zero
voltage over the whole temperature range and FE switching is
exclusively observed. This is the first time that the observation
of such a distinct switching behavior is reported for diaste-
reomeric configurations of a mesophase. However, the AF
switching is slow and therefore at all the temperatures only
one polarization reversal current peak is found under a
triangular wave voltage.
Figure 7. Changes in the molecular organization of compound F2 as
seen upon cooling under a triangular-wave electric ac field (150 Vpp
;
200 Hz; 5-mm noncoated ITO cell): a) Exclusively SmCsPF domains;
b) Transition from SmCsPF toSmC aPF; c) Nearly complete transition to
SmCsPF domains (see also Supporting Information).
In summary, the first examples of quaternary four-block
bent-core molecules are reported. Compounds F2 and Fi2
with large heptamethyltrisiloxane units show exclusively
ferroelectric switching, whereas a clear antiferroelectric
switching is observed for compound F, which lacks a siloxane
unit (Table 1). It is thought that the segregation of the
siloxane units favors the synpolar organization that leads to
FE switching. For compounds F2 and Fi2, the siloxane
sublayers are robust and FE switching is observed at all
temperatures. However, for compound F1 with a short
pentamethyldisiloxane unit there seems to be a competing
influence of the fluorocarbon chains. At lower temperature,
the fluorinated segments become more rigid and therefore
the out-of-plane interlayer fluctuations between the layers
become more important. These fluctuations are entropically
favored and can take place more easily across synclinic and
antipolar interfaces than across anticlinic and synpolar
interfaces (see dotted arrows in Figure 1a,b).[6a,18] For this
reason the SmCaPF domains, which have both types of
unfavorable interfaces, are more strongly destabilized by a
decrease in the temperature than the SmCsPF domains, which
have only one type. This could explain why these domains
relax to the SmCsPA state at zero voltage (AF switching),
whereas the SmCsPF domains remain stable (FE switching).
Hence, variations in the balance of fluorinated and siloxane
end groups lead to interesting bulk properties and allow a
fine-tuning of the mesophases of bent-core mesogens.
In the case of compounds F2 and Fi2, which have larger
nonbranched or branched heptamethyltrisiloxane segments,
optical investigations under a dc field indicate a bistable (FE)
switching over the whole temperature range (see Supporting
Information).[6]
Another remarkable feature of these compounds is seen
during the growing process of the domains under an applied
ac field (see Figure 7). Whereas for compound F1 the racemic
SmCaPF organization was exclusively obtained under these
conditions (see Figure 5b), in the case of compound F2 there
is a temperature-dependent reorganization of the molecules.
For F2 a homogeneously chiral synclinic organization (extinc-
tion brushes inclined with the polarizers, SmCsPF) is obtained
upon cooling the isotropic liquid state, but on further cooling
a slow transition to the (racemic) anticlinic organization
(extinction brushes parallel to the polarizers, SmCaPF) is
observed. This means that there is a temperature-dependent
change of the diastereomeric superstructures from synclinic
FE at high temperature to anticlinic FE at lower temper-
atures.[19]
Received: September 26, 2005
Published online: February 21, 2006
Keywords: chirality · electrooptics · liquid crystals ·
.
phase transitions
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