Axially polar columnar phase made of polycatenar bent-shaped molecules

Article abstract of DOI:10.1021/ja044597k

The polycatenar bent-shaped molecules are able to form columnar phases with column stratum built of few molecules, arranged in coplanar or conelike geometry. In the latter case, the phase becomes axially polar, with electric spontaneous polarization reori

Full text of DOI:10.1021/ja044597k

Published on Web 11/13/2004
Axially Polar Columnar Phase Made of Polycatenar Bent-Shaped Molecules
Ewa Gorecka,* Damian Pociecha, Jozef Mieczkowski, Joanna Matraszek, Daniel Guillon,† and
Bertrand Donnio†
Chemistry Department, Warsaw UniVersity, Al. Z˙ wirki i Wigury 101, 02-089 Warsaw, Poland, and Groupe des
Mate´riaux Organiques, IPCMS, 23 rue du Loess, BP 43, 67037 Strasbourg Cedex, France
Received September 7, 2004; E-mail: gorecka@chem.uw.edu.pl
Although, in general, the mesoscopic order is a result of interplay
between many competing and/or cooperative molecular interactions,
it can be argued that to a great extent the type of mesophase, lam-
ellar or columnar, is simply determined by the shape of molecules.
The columnar phases are predominantly formed by molecules
having disklike1 or bowlike2 shapes. The other mechanism, observed
for polycatenars,3 i.e., molecules having a rodlike mesogenic core
and multiple terminal alkyl chains, involves the microsegregation
Figure 1. Chemical formula and phase sequences for the studied materials.
induced by the incompatible space required to accommodate the
aromatic and alkyl parts of molecules. The polar order in columnar
phases is rather rare. It was claimed to exist for bowlike molecules;2
however, recent studies do not confirm ferroelectric properties for
these materials.4 So far, polar properties of columnar phases were
unambiguously proved only for chiral disklike molecules.5 In 1996,
it was shown that bent-shaped molecules offered a new opportunity
to design polar, lamellar, and broken-layer columnar mesophases
made of achiral materials.6-8 In this contribution, we combined
the ability of polycatenar molecules to form columnar phases with
induction of polar order by bent-shaped core molecules to obtain
an axially polar columnar phase.
The DSC, optical, and X-ray studies revealed three columnar
mesophases for the studied compounds (Figure 1). The highest
temperature mesophase was identified as a columnar one with a
two-dimensional hexagonal lattice (Colh). The optical texture shown
in Figure 2a is characteristic of a Colh phase (smooth fans with
extinction brushes aligned along the polarizers were observed). In
the X-ray studies (Table 1), a few low-angle Bragg reflections
corresponding to a two-dimensional hexagonal lattice of columns
and the diffused high-angle reflection, corresponding to the
liquidlike order of molecules within the columns with a mean
intermolecular distance of 4.5 Å, were detected. The calculated unit
cell dimension correlates to the molecular length (distance between
the terminal chain ends), which is 70 and 73 Å for Pol12 and Pol14,
respectively. As usual, the intercolumnar distance is smaller than
the molecular diameter because the alkyl chains are in disordered
conformations and interpenetrate between neighboring columns.
Calculated from density (∼1 g cm-3) and lattice volume, the number
of molecules that are necessary to fill a 4.5 Å stratum of each
column was found to be ∼4 molecules.
The transition temperatures are given in °C along with the transition enthalpy
changes (in parentheses) in J g-1
.
Figure 2. Textures of compound Pol14 in the Colh phase at T ) 170 °C
(a) and ColhPA phase at T ) 110 °C (b). Appearance of black, pseudoiso-
tropic regions in the texture confirms the hexagonal structure of the phase.
Table 1. Low-Angle Bragg Reflections (in Å) and, in Parentheses,
Their Miller Indicesa
Pol12 T ) 150 °C, Colh
55.0 (10), 31.7 (11), 27.5 (20) a ) 63.5 Å
T ) 110 °C, ColhPA 51.7 (10), 29.7 (11), 25.7 (20), a ) 59.7 Å
34.7, 19.5 (21)
T ) 70 °C, Colr
50.4 (11), 31.6 (20), 29.0 (21), a ) 63.2 Å,
25.3 (22), 20.2 (31) b ) 83.5 Å
55.8 (10) a ) 64.4 Å
Pol14 T ) 170 °C, Colh
T ) 110 °C, ColhPA 53.1 (10), 32.1, 27.4 (20)
a ) 61.3 Å
a ) 65.6 Å,
b ) 92.4 Å
T ) 70 °C, Colr
53.5 (11), 32.8 (20),
30.8 (21), 26.7 (22)
a Indices are assigned assuming a hexagonal lattice for Colh and ColhPA
phases and a rectangular lattice for the Colr phase. Unit cell parameters are
given in the last column. The additional signal that could not be indexed
into the hexagonal structure in ColhPA corresponds to the size of the blocks
made of broken columns (see the text and Figure 4).
relaxation frequency critically decreases (Figure 4a). This behavior
is similar to the soft mode observed in the paraelectric SmA phase
in the vicinity of lower temperature polar SmC* or SmC*A phases.10
The phase transition from Colh to lower temperature phase is
marked by a distinct lowering of the optical birefringence (from
0.07 to 0.03) (Figure 2). The texture of the low-temperature phase
can be rebuilt to give zero birefringence by applying an electric
field. This suggests that, under the electric field, the columns
reorient to have their axes perpendicular to the glass plate. A clear
current peak is observed upon reversal of the applied voltage (Figure
4b), which proves the polar nature of the phase. The switching
occurs without optical changes. Thus, it can be assumed that
polarization exists along the columns. The low dielectric response
Thus, it can be assumed that a few molecules self-assemble to
form an overall disk-shaped object (Figure 3a) in a similar way as
in dendrimeric materials.9 Such a structure would have compensated
dipole moments, which is in agreement with results of electrooptic
studies. The Colh phase is not sensitive to electric field: no current
peak or any optical changes upon applying electric field have been
detected. In the Colh phase, the clear dielectric mode is observed,
which on approaching the lower temperature phase departs from
Arrhenius-type fr ∼ exp(-Ea/kT) temperature behavior. Close to
the phase transition, the mode intensity critically increases and its
† IPCMS.
9
15946
J. AM. CHEM. SOC. 2004, 126, 15946-15947
10.1021/ja044597k CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
an additional, very weak signal was recorded that does not match
with a hexagonal structure (Table 1). The estimated height of
column fragments is 30-35 Å in both compounds.
Upon further decrease in the temperature, another disordered
columnar phase is observed. The X-ray studies confirm the
rectangular columnar structure (Colr) with p2gg symmetry. The
distortion from the hexagonal lattice is not significant, as the ratio
b/a is 1.3 and 1.4, for Pol12 for Pol14, respectively, while b/a )
x
3 for an ideal hexagonal structure. The transition from the
ColhPA phase to the Colr phase occurs with a decrease of
birefringence to almost zero, without any noticeable change of the
domain’s shape. The Colr phase is not switchable under the electric
field. The structure of this phase is not clear at present.
Figure 3. (a) Schematic drawing of the column cross-section made of four
bent-shaped molecules. (b) In the ColhPA phase, the molecules form a
conelike structure with noncompensated dipole moment depicted by an
arrow. (c) Broken-column-type structure of ColhPA phase.
In summary, it was shown that polycatenar bent-shaped mol-
ecules are able to form columnar phases with column stratum built
of few molecules, arranged in coplanar or conelike geometry. In
the latter case, the phase becomes axially polar, with electric
spontaneous polarization reorientable in an electric field. It should
be noticed that previous attempts to obtain polycatenar bent-shaped
molecules resulted only in nonpolar Colh or smectic phases, most
probably due to the much smaller bend in thienyl derivatives.12
Although we cannot exclude that dipole interactions play some role
in this system, it is more plausible that the cone formation is rather
due to steric interactions. The filling of a column section with flat
bent-shaped molecules might lead to some voids between molecule
arms, and the tilting of the molecules improves packing by
decreasing the distances between the aromatic parts of the
molecules. To avoid an internal electric field, the antiferroelectric
order is imposed by breaking the columns and forming a three-
dimensional structure.
Figure 4. (a) Inverse temperature dependence of the relaxation frequency
fr and the mode strength ∆ꢀ in the Colh phase of Pol12 compound. Far
from the Colh-ColhPA phase transition frequency follows Arrhenius law
(solid line) with Ea ) 195 kJ mol-1. (b) Current peak registered for Pol12
compound in the ColhPA phase at 130 °C upon applying the triangular
waveform voltage. The calculated spontaneous electric polarization is
∼70nC cm-2
.
Acknowledgment. This work was supported by KBN Grant
4T09A 00425 and organic synthesis by UW Grant BW-1637/7/04.
is observed (ꢀ ∼ 3.5), which together with the switchable nature
of the phase reveals its antiferroelectric properties. Surprisingly,
the X-ray studies showed that the hexagonal structure is preserved
in the switchable phase (Table 1). Thus, the phase will be referred
to further as ColhPA, where PA stands for polar, antiferroelectric
properties. The column diameter in the ColhPA phase is only slightly
smaller than in the Colh phase, so it is reasonable to assume that
the number of molecules in the column stratum remains the same
in both phases; however, in the low-temperature phase, the
molecules depart from coplanar geometry, forming a cone with a
noncompensated dipole moment along the cone axis (Figure 3b).
Simple geometrical considerations show that the cone angle is about
150-140°. This assumption is also consistent with measurements
of birefringence. It is expected that for light propagation perpen-
dicular to the column axis, ∆n should decrease upon transition from
a flat disk to a conelike structure of the phase. However, the
question arises how to compromise the two-dimensional hexagonal
structure and antiferroelectric order of columns, as it is well-known
that antiparallel dipoles cannot be accommodated without frustration
in a lattice of triangular symmetry.11 One possibility is that the
lattice is quasihexagonal, i.e., dipoles are randomized in the two-
dimensional triangular structure. The other possibility is the
existence of an additional three-dimensional superstructure (Figure
3c). In the xy plane perpendicular to the column axis the ferroelectric
hexagonal order exists but the columns are broken along the z
direction and polarization direction alternates between the blocks.
It should be noticed that for the phase made of conelike objects,
the best space filling is obtained if the blocks are shifted in the xy
plane by half of the cell lattice distance and the ABAB stacking of
blocks with alternating polarization is enforced. This three-
dimensional superstructure is supported by X-ray studies in which
Supporting Information Available: Chemical synthesis, spectral
characterization of materials, and experimental details (PDF). This
material is available free of charge via the Internet at http://pubs.acs.org.
References
(1) (a) Levelut, A. M. J. Chim. Phys. 1983, 80, 149. (b) Guillon, D. Struct.
Bonding 1999, 95, 41.
(2) (a) Serrette, A. G.; Swager, T. M. J. Am. Chem. Soc. 1993, 115, 8879.
(b) Bing Xu; Swager, T. M. J. Am. Chem. Soc. 1993, 115, 1159.
(3) Nguyen, H. T.; Destrade, C.; Maltheˆte, J. AdV. Mater. 1997, 9, 375.
(4) Hasse, W.; Athanassopoulou, M. A.; Kilian, D.; Swager, T. M.; Wrobel
S. In Relaxation Phenomena; Hasse, W., Wrobel, S., Eds.; Springer: New
York, 2003.
(5) (a) Bock, H.; Helfrich, W. Liq. Cryst. 1992, 12, 697. (b) Scherowsky, G.;
Chen, X. H.; Levelut, A. M. Liq. Cyst. 1998, 24, 157.
(6) (a) Niori, T.; Sekine, T.; Watanabe, J.; Furukawa, T.; Takezoe, H. J. Mater.
Chem. 1996, 6, 1231. (b) Link, D. R.; Natale, G.; Shao, R.; Maclennan,
J.; Clark, N. A.; Korblova, E.; Walba, D. M. Science 1997, 278, 1924.
(7) Pelzl, G.; Diele, S.; Weissflog, W. AdV. Mater. 1999, 9, 707.
(8) (a) Szydlowska, J.; Mieczkowski, J.; Matraszek, J.; Bruce, D. W.; Gorecka,
E.; Pociecha, D.; Guillon, D. Phys. ReV. E 2003, 67, 031702. (b) Pelz,
K.; Weissflog, W.; Baumeister U.; Diele, S. Liq. Cryst. 2003, 30, 1151.
(c) Schroeder, M. W.; Diele, S.; Pelzl, G.; Weissflog, W. ChemPhysChem
2004, 5, 99.
(9) (a) Balagurusamy, V. S. K.; Ungar, G.; Percec, V.; Johansson, G. J. Am.
Chem. Soc. 1997, 119, 1539. (b) Donnio, B.; Barbera´, J.; Gime´nez, R.;
Guillon, D.; Marcos, M.; Serrano, J. L. Macromolecules 2002, 35, 370.
(c) Gehringer, L.; Bourgogne, C.; Guillon, D.; Donnio, B. J. Am. Chem.
Soc. 2004, 126, 3856.
(10) Lagerwall, S. T. In Ferroelectric and Antiferroelectric Liquid Crystals;
Wiley-VCH: Weinheim, 1999.
(11) See, for example: Harris, M. Nature 1999, 399, 311.
(12) (a) Paraskos, A. J.; Swager, T. M. Chem. Mater. 2002, 14, 4543. (b)
Eichhorn, S. H.; Paraskos, A. J.; Kishikawa, K.; Swager, T. M J. Am.
Chem. Soc. 2002, 124, 12742
JA044597K
9
J. AM. CHEM. SOC. VOL. 126, NO. 49, 2004 15947