Cyclic ureas as novel building blocks for bent-core liquid crystals

Article abstract of DOI:10.1039/b703907g

Cyclic ureas represent a new class of bent-core liquid crystals which, depending on the ring size and other structural parameters, can form a series of polar (ferroelectric and antiferroelectric), as well as non-polar, tilted and non-tilted smectic and un

Full text of DOI:10.1039/b703907g

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Cyclic ureas as novel building blocks for bent-core liquid crystals{
Benjamin Glettner,a Sara Hein,a R. Amaranatha Reddy,{a Ute Baumeisterb and Carsten Tschierske*a
Received (in Cambridge, UK) 15th March 2007, Accepted 26th March 2007
First published as an Advance Article on the web 12th April 2007
DOI: 10.1039/b703907g
so-called B7 phases, for example formed by 2-nitro-substituted
resorcinol derivatives, are characterized by helical superstructures.6
Herein we report that cyclic ureas represent new useful bent
building blocks for the design of bent-core LCs (see the structure in
Table 1). In comparison to other central units, the bending angle
can be easily tailored by changing the ring size. Moreover, the
polar carbonyl group is fixed in a defined direction in the bay
region of the bent core. This is important for the systematic
investigation of the effects of polar groups on phase behaviour.
Synthesis (Scheme 1) was achieved by a double Pd0-catalyzed
C–N cross coupling reaction7 between the cyclic ureas 1/n (n = 0, 1,
2) and ethyl 4-bromobenzoate or 4-bromobenzaldehyde to yield
the diesters 2/n and the dialdehydes 3/n, respectively. Compounds
Cyclic ureas represent a new class of bent-core liquid crystals
which, depending on the ring size and other structural
parameters, can form a series of polar (ferroelectric and
antiferroelectric), as well as non-polar, tilted and non-tilted
smectic and undulated smectic phases.
Currently there is great scientific interest in bent-core liquid
crystals (LCs), as these materials show special properties not
achievable with other liquid crystalline materials.1 Smectic and
columnar phases of these molecules can, for example, show polar
order, which leads to antiferroelectric and ferroelectric switching.
This is of significant interest for numerous applications, such as
fast electrooptical switches, phase modulators and non-linear optic
(NLO) materials.2 The polar order in these mesophases is due to
the restricted rotation of the molecules around their long axes
because of the bend in their rigid aromatic cores. There is also the
potential that biaxial nematic phases can be formed by these
molecules.3 If the molecules adopt a tilted organisation in polar
layers (SmCPA/FE phases), these layers possess an inherent
chirality, as layer-normal, tilt-direction and polar direction describe
either a right- or left-handed system (see Fig. S1 in the ESI{).1,4
The occurrence of chiral superstructures in these mesophases,
formed by achiral molecules, and the possibility of switching this
chirality by electric fields are additional unusual properties of
significant general scientific interest.1
Table 1 Mesophases, transition temperatures (T/uC) and associated
enthalpy values (DH/kJ mol21, lower lines) of the bent-core molecules
6/n, 7/n and 8/na
No. X, Y Z n
T/uC
From the point of view of molecular design, bent-core LCs are
obtained by introducing a bend angle in the range ca. 110 to 140u
between two rod-like moieties (wings) by using 1,3-disubstituted
benzenes, 2,5-disubstituted pyridines, 2,7-disubstituted naphtha-
lenes or 1,3-disubstituted five-membered aromatic heterocycles
(oxadiazoles, oxazoles), or by joining two rod-like molecules by
non-cyclic spacers with an odd number of atoms (carbonyl, oxo,
thia, 1,n-dioxaalkanes, etc.). Non-aromatic cyclic bent units are
rare and to the best of our knowledge there is only one reported
example—a substituted tetrahydropyrane derivative.5§
6/0 OOC, H 0 Cr 189 SmC 314 SmA 329 Iso
b
COO
6/1 OOC, H 1 Cr 198
COO
38.9c
6/2 OOC, H 2 Cr 107 (SmAPA 98) SmA 159 Iso
COO 71.1 0.2 8.0
7/1 COO, H 1 Cr 119 143 SmCPA 174 Iso
9.2 18.3
B50 107 B59PFE 119 B5PFE
6.8d 0.2d
36.8
—
6.7
(M1 186) SmCPA 200 SmA 208 Iso
7.3
10.6
M2
OOC
8/1 COO, F 1 Cr
OOC
52.6
86
54.9
B5PFE 138 USm- 165 USmC 187 Iso
CPFE
e
3.2
—
23.2
For bent-core molecules, it is also of significant interest to have
polar substituents that can contribute to the magnitude of
polarization and significantly modify the phase structures. The
a
Determined in the first DSC heating run (10
K
min21).
Abbreviations: Cr = crystalline solid (Cr–Cr transitions occurring
below the melting temperature are included in the enthalpy value),
SmC = tilted smectic phase with uniform (synclinic) tilt direction,
SmA = non-tilted smectic phase, Iso = isotropic liquid, SmAPA
antiferroelectric (AF) switching non-tilted smectic phase, SmCPA
=
=
aInstitute of Chemistry, Organic Chemistry, Martin-Luther-University
Halle-Wittenberg, Kurt-Mothes-Straße 2, D-06120 Halle, Germany.
E-mail: carsten.tschierske@chemie.uni-halle.de; Fax: +49 345 5527 346;
Tel: +49 345 5525 664
AF switching tilted smectic phase, B50 = non-switching smectic low
temperature phase with in-plane order, B5PFE and B59PFE
ferroelectric (FE) switching smectic phases with in-plane order,
USmC = undulated (wavy deformed) smectic phase, USmCPFE
FE switching undulated smectic phase, M1 and M2 = mesophases
=
=
bInstitute of Chemistry, Physical Chemistry, Martin-Luther-University
Halle-Wittenberg, Mu¨hlpforte 1, D-06108 Halle, Germany
{ Electronic supplementary information (ESI) available: Scheme explain-
ing the origin of chirality, syntheses and analytical data. See DOI: 10.1039/
b703907g
b
c
with unknown structures. Second order phase transition. Not
resolved due to rapid crystallization. Peak splits only in the cooling
d
run; in the heating run there is only one peak at 119 uC for both
transitions. Transition between USmC and USmCPFE is not
associated with an enthalpy change (see Fig. S5).
e
{ Present address: Department of Chemistry & Biochemistry, University
of Colorado at Boulder, Boulder, CO 80309-0215, USA
2596 | Chem. Commun., 2007, 2596–2598
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Fig. 1 Textures, as seen between crossed polarizers: (a) Compound 6/1 at
200 uC; due to a temperature gradient, the SmA phase is seen on top and
the SmCPA phase at the bottom. (b) SmA phase of compound 6/2 at
110 uC. (c) SmAPA phase of 6/2 at 90 uC.
Compound 6/2, incorporating a seven-membered ring as a bent
unit, also shows a SmA phase at high temperature (Fig. 1b) and an
antiferroelectric switching low temperature phase (see Fig. 1c and
Fig. 2a), but the phase transition is completely different from that
observed for 6/1. Though the occurrence of a weakly birefringent
schlieren texture in the homeotropically-aligned regions (see
Fig. S2{) indicates a transition to a biaxial smectic phase at
98 uC, the fan shape texture is retained (see Fig. 1b,c). This means
that there is no layer shrinkage at this phase transition, and
therefore it is assumed that the polar smectic phase of 6/2 is a non-
tilted SmAPA phase.I This type of mesophase is extremely rare.9 It
seems that this seven-membered ring, due to its increased flexibility
in comparison to the related six-membered urea, leads to a
preference for non-tilted (polar and non-polar) over strongly-tilted
smectic LC phases.
Scheme 1 Synthesis of compounds 6/n–8/n. Reagents and conditions: (i)
Pd2(dba)3, XANTphos1, Cs2CO3, dioxane; (ii) 1: KOH, EtOH, H2O,
2: H+, H2O; (iii) m-CPBA, CHCl3; (iv) 1: SOCl2, 2: phenolic component,
DMAP, Et3N, CH2Cl2.
2/n were hydrolyzed to the dicarboxylic acids 4/n, whereas the
diphenols 5/n were obtained by the Bayer–Villiger oxidation8 of
aldehydes 3/n. Esterification reactions of 5/n with benzoic acids or
4/n with 4-alkoxyphenols lead to the bent-core molecules 6/n, 7/n
and 8/n, which were purified by crystallization (see ESI{).
Preliminary investigations of the mesomorphic properties of
bent-core ureas 6/n, 7/1 and 8/1 are based on polarized light
microscopy on a hot stage (Mettler FP 82 HAT), differential
scanning calorimetry (DSC-7, Perkin-Elmer), X-ray diffraction, as
well as on electrooptical experiments.
Inversion of the direction of the ester linkages, in the case of
compound 6/1, gives rise to a significant reduction of the phase
transition temperatures. This leads to broader mesophase ranges at
lower temperatures. For compound 7/1, the SmA phase is
completely removed and an antiferroelectric switching SmCPA
phase (see Fig. S3{) is found, as is typical for bent-core
molecules.** Below 143 uC, the SmCPA phase is replaced by a
non-switchable mesophase characterized by sharp reflections, also
The mesophases and transition temperatures of the synthesized
compounds are collated in Table 1. Though all compounds show
LC phases, the mesophase stability and the mesophase type
strongly depend on the size of the central ring, on the direction of
the COO linking units and on the substitution at the aromatic core
(H, F). Attention is first focused on the effect of the size of the
cyclic urea. Compound 6/0, with a five-membered ring, has
extremely high transition temperatures and exclusively non-polar
smectic A (SmA) and smectic C (SmC) phases. These are LC
phases, as typically observed for rod-like molecules. This indicates
that the five-membered heterocyclic unit provides a nearly linear
molecular shape and therefore this compound behaves like a rod-
like molecule. The transition temperatures can be significantly
reduced by enlarging this ring.
For compound 6/1, with a six-membered ring, there is also a
non-polar SmA phase at high temperature, but a polar-tilted
smectic phase is observed at lower temperature. The transition can
be seen optically by the occurrence of a dense stripe pattern in the
fan texture of the original SmA phase, as shown in Fig. 1a
(bottom). This mesophase shows antiferroelectric switching
behaviour (Ps = 950 nC cm22 at T = 190 uC) and is therefore
assigned as SmCPA (B2 phase)." Hence, this compound behaves
more like a bent-core molecule, and the phase structure can be
changed from a non-polar to a polar smectic phase by a reduction
of temperature.
Fig. 2 Repolarisation current response curves: (a) SmAPA phase of 6/2
at T = 90 uC (Vpp = 100 V, f = 30 Hz, cell thickness = 5 mm, Ps =
630 nC cm22); the appearance of two peaks in each half period of the
applied triangular wave voltage indicates antiferroelectric switching. (b)
B5PFE phase of 8/1 at T = 130 uC (Vpp = 385 V, f = 5 Hz, cell thickness =
5 mm, Ps = 520 nC cm22); the appearance of only one peak is a first
indication of a ferroelectric switching process.
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Chem. Commun., 2007, 2596–2598 | 2597

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silyl units12 and by combining with dendritic or polymeric
structures will lead to new series of bent-core LC materials with
interesting materials properties.
The authors are grateful to the Deutsche Forschungs-
gemeinschaft (GRK 894) and the Fonds der Chemischen
Industrie for financial support. R. A. R. is grateful to the
Alexander von Humboldt Foundation for a Research Fellowship.
Notes and references
Fig. 3 X-Ray diffraction pattern of aligned samples of 8/1: (a) B5FE
phase at T = 120 uC. (b) USmCPFE phase at T = 150 uC. Alignment was
achieved by surface alignment upon slow cooling on a glass substrate,
where the X-ray beam was parallel to the substrate (HI-Star 2D detector
(Siemens)).
§ 3,6-Di-O-[4-(4-octyloxybenzoyloxy)benzoyl]-1,5-anhydro-2-desoxy-D-ara-
binohexitol: This chiral compound, besides a cholesteric phase, also shows
polar switching LC phases, but the spontaneous polarization is relatively
low (Ps = 10–40 nC cm22), comparable with values typically observed for
SmC* phases of rod-like molecules. Hence, the molecules reported herein
represent the first examples of bent-core molecules with a non-aromatic
and cyclic bent unit that form mesophases characterized by high
polarization values in the absence of molecular chirality.
in the wide angle region of the X-ray diffraction pattern (crystalline
mesophase M2).
" Antiferroelectric switching was additionally confirmed by the observation
of a tristable switching by optical investigation.
Fluorination at the periphery of the aromatic core, adjacent to
the terminal alkyl chains, gives rise to a further modification of the
LC phases.10 The melting temperature was further reduced and the
antiferroelectric switching SmCPA phase of the non-fluorinated
compound 7/1 was replaced by a series of different LC phases in
compound 8/1. In the temperature range 86 to 138 uC, the X-ray
diffraction pattern of the aligned samples are characterized by a
series of sharp layer reflections, in combination with relatively
sharp and diffuse wide angle scatterings. The diffraction pattern
between 120 and 138 uC (Fig. 3a) is very typical of a so-called B5
phase. This is a smectic phase with additional in-plane order, but
without correlation between the layers.11 The diffraction pattern
only slightly changes at the transition to a phase assigned as B59 at
119 uC on cooling. However, the wide angle reflections become
sharper, and additional wide angle reflections appear at the next
phase transition at 107 uC. This low temperature mesophase seems
to be a more ordered version of the B59 phase and is assigned as
B50. At temperatures above 138 uC, only layer reflections remain in
the X-ray diffraction pattern, whereas the wide angle scattering
becomes diffuse, indicating a fluid smectic phase (Fig. 3b). The first
order reflection is accompanied by satellite reflections that indicate
an undulation of the layers. As the texture of this high temperature
phase is also similar to that of a columnar phase (see Fig S4{), it is
assumed that this phase is an undulated or modulated smectic
phase. In the temperature range 107 to 167 uC, these mesophases
show ferroelectric switching (B59PFE, B5PFE, USmCPFE) char-
acterized by the occurrence of only one relatively sharp polarisa-
tion current response peak in the half period of an applied
triangular wave field (Fig. 2b).{{ In the USmC phase above 167 uC
and in the B50 phase below 107 uC, no switching could be
observed. It seems that in the non-polar USmC phase at high
temperature, the rotation of the molecules is fast, and polar order
is only achieved at reduced temperature (USmCPFE phase). The
threshold voltage increases and the position of the peak changes at
the transition to the B5PFE and B59PFE phases. Below 107 uC, the
relatively high order seems to inhibit molecular reorientation, and
the ability of polar switching is again lost (B50 phase).
I Due to rapid crystallization, confirmation of the non-tilted organization
was not possible by X-ray scattering. However, the presence of two-brush
and four-brush disclinations in the schlieren texture confirmed a SmAPA
structure. Also, in optical investigations, no rotation of the extinction
crosses could be seen during the switching, as expected for SmAPA phases.
** Though there is only a single peak per half period of the applied
triangular wave voltage, optical investigations clearly indicate a tristable
switching characterized by a relaxation at 0 V, Ps = 200 nC cm22. The ratio
of experimentally determined layer distance d (4.3 nm, X-ray) to molecular
length L (5.8 nm in the most stretched conformation) is in line with a tilted
organization of the molecules.
{{ This single peak cannot be split by using a modified triangular wave
voltage, where a pause is introduced at 0 V, thereby confirming FE
switching. The switching process in all phases seems to take place by a
collective rotation around the long axis, but a detailed understanding of the
phase structures and switching behaviour requires further investigation. B5
phases usually show AF switching, whereas FE switching B5 phases are
very rare.11
1 (a) R. Amaranatha Reddy and C. Tschierske, J. Mater. Chem., 2006,
16, 907; (b) H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., 2006, 45,
597.
2 (a) F. Kentischer, R. Macdonald, P. Warnick and G. Heppke, Liq.
Cryst., 1998, 25, 341; (b) J. Ortega, N. Pereda, C. L. Folcia, J. Etxebarria
and M. B. Ros, Phys. Rev., E, 2000, 63, 011702.
3 (a) L. A. Madsen, T. J. Dingemans, M. Nakata and E. T. Samulski,
Phys. Rev. Lett., 2004, 92, 145505; (b) B. R. Acharya, A. Primak and
Satyendra Kumar, Phys. Rev. Lett., 2004, 92, 145506.
4 D. R. Link, G. Natale, R. Shao, J. E. Maclennan, N. A. Clark,
E. Ko¨rblova and D. M. Walba, Science, 1997, 278, 1924.
5 G. Gesekus, I. Dierking, S. Gerber, M. Wulf and V. Vill, Liq. Cryst.,
2004, 31, 145.
6 G. Pelzl, S. Diele, A. Jakli, C. Lischka, I. Wirth and W. Weissflog, Liq.
Cryst., 1999, 26, 135.
7 J. Yin and S. L. Buchwald, J. Am. Chem. Soc., 2002, 124, 6043.
8 G. W. Yeager and D. N. Schissel, Synthesis, 1991, 63.
9 (a) A. Eremin, S. Diele, G. Pelzl, H. Nadasi, W. Weissflog,
J. Salfetnikova and H. Kresse, Phys. Rev., E, 2001, 64, 051707; (b)
R. Amaranatha Reddy and B. K. Sadashiva, J. Mater. Chem., 2004, 14,
310.
10 R. Amaranatha Reddy and B. K. Sadashiva, J. Mater. Chem., 2002, 12,
2627.
11 H. Nadasi, W. Weissflog, A. Eremin, G. Pelzl, S. Diele, B. Das and
S. Grande, J. Mater. Chem., 2002, 12, 1316.
12 (a) C. Keith, R. Amaranatha Reddy, A. Hauser, U. Baumeister and
C. Tschierske, J. Am. Chem. Soc., 2006, 16, 3051; (b)
R. Amaranatha Reddy, U. Baumeister, C. Keith and C. Tschierske,
J. Mater. Chem., 2007, 17, 62; (c) C. Keith, R. Amaranatha Reddy,
U. Baumeister, H. Kresse, J. Lorenzo Chao, H. Hahn, H. Lang and
C. Tschierske, Chem.–Eur. J., 2007, 11, 2556.
In summary, we have reported a new universal building block
for the design of novel bent-core liquid crystalline materials with
ferroelectric and antiferroelectric LC phases. It can be expected
that further structural modifications, for example, by introducing
2598 | Chem. Commun., 2007, 2596–2598
This journal is ß The Royal Society of Chemistry 2007