A bow-phase mesogen showing strong, robust analog electro-optics

Article abstract of DOI:10.1039/b102732h

The design and synthesis of the first of a new class of bow-phase (also known as banana-phase) mesogens possessing resorcylidene aniline groups in the core is described. The new achiral material, NORABOW, shows two chiral electro-optically active smectic phases, both existing over a wide temperature range. The high temperature smectic, which exists as the thermodynamically stable phase between 120°C and 150°C, exhibits electro-optic behavior not previously reported for a bow-phase. In particular, high susceptibility analog electro-optics ("V-shaped switching") are observed in a texture which is stable to large applied electric fields. The electro-optic behavior of NORABOW is of potential utility in photonic switching applications.

Full text of DOI:10.1039/b102732h

View Article Online / Journal Homepage / Table of Contents for this issue
A bow-phase mesogen showing strong, robust analog electro-optics1{
David M. Walba,*a Eva Ko¨rblova,a Renfan Shaob and Noel A. Clarkb
aDepartment of Chemistry and Biochemistry, 215 UCB, the Ferroelectric Liquid Crystal
Materials Research Center, University of Colorado, Boulder, CO 80309, USA.
E-mail: walba@colorado.edu
bDepartment of Physics, 390 UCB, the Ferroelectric Liquid Crystal Materials Research
Center, University of Colorado, Boulder, CO 80309, USA
Received 26th March 2001, Accepted 6th June 2001
First published as an Advance Article on the web 26th September 2001
The design and synthesis of the first of a new class of bow-phase (also known as banana-phase) mesogens
possessing resorcylidene aniline groups in the core is described. The new achiral material, NORABOW, shows
two chiral electro-optically active smectic phases, both existing over a wide temperature range. The high
temperature smectic, which exists as the thermodynamically stable phase between 120 uC and 150 uC, exhibits
electro-optic behavior not previously reported for a bow-phase. In particular, high susceptibility analog electro-
optics (‘‘V-shaped switching’’) are observed in a texture which is stable to large applied electric fields. The
electro-optic behavior of NORABOW is of potential utility in photonic switching applications.
provide a straightforward synthesis, the target bis-p-NonylOxy-
1. Introduction
Resorcylidine Aniline 3 was prepared as shown in Fig. 1. In
keeping with the trivial nomenclature first suggested by the
Chalmers group,7 we name this core unit ‘‘NORA’’, and dub
the new bent-core liquid crystal NORABOW. In this structure
the Schiff base units in the core are ‘‘flipped’’ relative to their
orientation in the classic bis-nonyloxybenzylideneamine Schiff
base material NOBOW (structure and phase transitions are
given in Fig. 1 for comparison). As we have previously
reported, NOBOW exhibits the antiferroelectric SmCSPA
macroscopic racemate as the thermodynamic phase, and the
SmCAPA macroscopic conglomerate as a metastable phase
easily obtained upon melting of the B4 ‘‘blue crystal’’ phase.4b
Upon cooling of NORABOW from the isotropic liquid
In a structural evolution paralleling that of the calamitic SmC*
LCs, most bent-core (banana-shaped, or bow-shaped) meso-
gens2 exhibiting the remarkable SmCP antiferroelectric3 and
ferroelectric4 phases are Schiff bases. These bow-phase
materials have recently been the subject of intense physical
investigation.2 But, as for the early SmC* materials, physical
studies can be complicated by the hydrolytic instability of the
molecules. While SmCP mesogens without Schiff base units
have been reported,5 variations on the theme of the bis-Schiff
base bent-core system have proven remarkably useful for
providing interesting polar smectics.
The resorcylidene aniline core, present in such classic
calamitic FLCs as the MBRA6 and MORA7 series, actually
seems superior to the benzylidene aniline core with respect to
mesogenicity, and is more stable towards hydrolysis. This core
also played a key role in the first discovery of antiferroelectrics
from achiral molecules, being the core present in the pioneering
Soto-Bustamante/Blinov anticlinic bilayer antiferroelectrics
(SmAPA).8 Since anticlinic layer interfaces are important for
obtaining ferroelectric bow-phases, the Soto-Bustamante/
Blinov result reinforces the idea that this structural feature
may prove interesting when incorporated into bow-phase
mesogens.
between the plates of a transparent capacitor test cell,9
a
smectic focal conic liquid crystal texture develops at 145 uC, as
seen in the polarized light microscope (Fig. 2). The texture is
reminiscent of the B2 bow-phase texture, with the apparent
formation of two distinct types of focal conic domains
(isotropic regions are still present in this image). Domains of
type A have a green birefringence color and are ‘‘SmA-like’’,
i.e. the optic axis is along the layer normal. Domains of type B
have a pink birefringence color, and exhibit a tilt of the optic
axis from the layer normal of about ¡20u. No changes in the
texture occur upon further cooling.
Given the enhanced stability and excellent mesogenicity of
the resorcylidene aniline core, we proceeded to explore this
modification to the classic bow-phase mesogen structure. As
described below, this effort has led to a new achiral electro-
optically active bow-phase material exhibiting potentially
useful high strength analog electro-optics in standard LC
cells (V-shaped switching).
3. Electro-optics of NORABOW
Application of small electric fields (v2 V mm21
) to a
NORABOW cell evidences additional complexity and struc-
tural information not seen in the initial textures. First, a
distinct analog rotation of the extinction brushes in the tilted
domains of type B can be seen with application of the field. The
response appears to be quite unsymmetrical (i.e. there is more
rotation for one sign of the field than for the other), but
domains of opposite tilt direction can be easily seen. This
behavior proves that the tilted domains of type B are chiral,
and that the sample is a conglomerate, containing domains of
opposite handedness.
2. Synthesis and liquid crystalline texture of the new
bow-phase mesogen NORABOW
To keep the structure close to that of the well-characterized
Schiff base mesogens exhibiting SmCP phases, but also to
Interestingly, domains of type A, exhibiting the green
birefringence color, occur in two distinctly different variations,
{Basis of a presentation given at Materials Discussion No. 4, 11–14
September 2001, Grasmere, UK.
DOI: 10.1039/b102732h
J. Mater. Chem., 2001, 11, 2743–2747
This journal is # The Royal Society of Chemistry 2001
2743

View Article Online
Fig. 1 The structure, synthesis, and transition temperatures of compound 3 (NORABOW) (temperatures are in degrees centigrade). The structure,
phases and transition temperatures of the classic bow-phase mesogen NOBOW are also shown for comparison.
as indicated in Fig. 3. In one variety (labeled A2 in the figure),
there is no brush rotation nor other visible electro-optic
behavior at all upon application of a small field. In a second
variety (A1) of these green ‘‘SmA-like’’ domains a complex
change in texture occurs, where thin stripes appear parallel to
the layers in the focal conics, as can be seen in Fig. 3. This
response is quite easily seen, but there is no visible macroscopic
brush rotation in the domains, and the response seems achiral.
At temperatures between 145 uC and 120 uC, upon applica-
tion of a large field (6 V mm21), the texture changes drama-
tically. Removal of the field then gives a smooth SmA-like
focal conic texture (domains of type C) with a second order
pink birefringence color, as shown in Fig. 4 (E~0). All of the
initially formed domains of type A and type B change to this
new texture upon application of the field then removal of the
field. This texture is also obtained if the sample is cooled from
the isotropic liquid phase in the presence of a field.
Fig. 2 Liquid crystal texture observed by polarized light microscopy
(polarizer vertical, analyzer horizontal) upon cooling of NORABOW
from the isotropic melt. This sample has not been exposed to an electric
field.
Fig. 3 Focal conic domains of type A with application of a small
(v5 V mm21) electric field.
2744
J. Mater. Chem., 2001, 11, 2743–2747

View Article Online
in optic axis orientation is analog, with no hint of a threshold as
would be expected for an antiferroelectric.
Upon cooling the sample below 74 uC a dramatic change in
the electro-optic susceptibility of the system occurs, but with no
change in the zero field texture. This new phase, SmCP2, shows
a higher birefringence (Dn*d~1600 nm; green birefringence
color) than that of SmCP1 of type C at zero field, and the
birefringence change and brush rotation in response to applied
fields is smaller, but still easily observable. The SmCP2 phase is
restored when the sample is heated to 120 uC.
4. Discussion
While much work remains to be done to develop an under-
standing of the structure and electro-optics of the NORABOW
SmCP phases, several features of the system deserve comment.
First, it is interesting that none of the textures formed on
cooling from the isotropic phase (focal conics of type A1, A2
and B) is similar to that obtained upon application of a field
(either during formation of the texture or afterwards); focal
conics of type C. Since domains of type C have never been seen
to ‘‘relax’’ to another texture, we feel it is likely that this texture
represents an alignment of the thermodynamic phase of the
material. Textures of type A1, A2, and B are then either
metastable phases (this often occurs with bow-phase materials),
or different alignment modes of the thermodynamic phase, or
some of both.
Second, the magnitude of the analog electro-optic response
(y 7 degrees/(V mm21)) is similar to the largest electroclinic
effect observed to date for a SmA* material,10 though not as
large as the coefficients observed for ‘‘V-shaped switching’’
SmC* materials.11 Even so, the electro-optic behavior of
NORABOW is interesting in the context of potential applica-
tions of the bow-phases. While analog electro-optics have been
reported for the SmCSPF bow-phase of the racemic
MHOBOW in the low birefringence ‘‘gold focal conic’’ B7
texture,4a,12 in that case the analog susceptibility is small (y1
degree/(V mm21), and the texture changes irreversibly to a high
birefringence ‘‘bistable blue’’ SSFLC-like alignment at high
field. The bistable texture is also produced when this B7
material is cooled from the isotropic phase in the presence of a
field.
Fig. 4 Focal conic domains of type C obtained from NORABOW after
application of a field above 6 V mm21 (fields indicated are V mm21).
The new texture changes dramatically upon application of a
field. As can be seen in Fig. 4, thick stripes appear in the focal
conic domains parallel to the layers, and the birefringence color
changes quite dramatically. Upon progressing from zero field
to an applied field of 5 V mm21, the birefringence color
progresses in a continuous and reversible way through purple,
blue, green, salmon, and finally a light yellow color in Newton’s
third order.
When the sample is driven with a triangular driving
waveform centered about zero volts, it can be easily seen
that the stripes appearing in the focal conic domains represent
counter-rotating extinction brushes from chiral domains of
opposite handedness. That is, the seemingly uniform focal
conic domains are actually conglomerates with walls between
heterochiral domains. These walls are invisible at zero applied
field, but become visible as the brushes in the domains counter-
rotate as the field increases.
Fig. 5 shows an area of the cell where cylindrical focal conic
domains of opposite handedness can be seen. The counter-
rotating nature of the brushes in these domains is very obvious
when observing the cell’s response to a triangular driving
waveform. The rotation of the optic axis in these domains is
quite large; y¡35u, saturating at about 5 V mm21. The change
We have proposed a mechanism for the analog electro-optics
exhibited by MHOBOW and the change to bistable switch-
ing,4a as illustrated in Fig. 6. In this scenario, the low-
birefringence gold focal conic domains derive from ‘‘steric’’
alignment where the bow-plane is parallel to the substrate
surfaces, forcing the layers and polarization to tilt from the
surface normal (Fig. 6A). This alignment is identical to that of
the ‘‘Sony mode’’ analog switch developed for calamitic SmC*
materials.13 Application of electric fields along the substrate
Fig. 6 Illustration of the proposed alignment modes for V-shaped
analog switching (A) and bistable switching (B) in the SmCSPF bow-
phase.
Fig. 5 Switching of heterochiral domains of type C showing the large
saturation rotation of the optic axis at fields of 5 V mm21
.
J. Mater. Chem., 2001, 11, 2743–2747
2745

View Article Online
normal causes an analog rotation of the director on the (tilted)
tilt cone.
(30%):16 1H NMR (300 MHz, CDCl3) d: 11.48 (1H, s, CHLO),
9.70 (1H, s, OH), 7.33 (1H, d, J~8.55 Hz, Ar), 6.52 (1H, d,
J~10.99 Hz, Ar), 6.40 (1H, s, Ar), 4.00 (2H, t, J~6.59 Hz,
OCH2), 1.83–1.23 (14H, m, 7 CH2), 0.89 (3H, t, CH3).
In this alignment mode, however, the polarization cannot
orient parallel to the applied field. If the field passes a threshold
value, then the layers ‘‘straighten’’ in a manner similar to the
formation of quasi-bookshelf alignment obtained when a large
field is applied to a chevron SSFLC cell, such that the
polarization can orient parallel to the field (6B). Bistable
switching is then observed for this ‘‘bookshelf’’ alignment.
This mechanism is inconsistent with our observations
concerning NORABOW electro-optics, however. In the case
of NORABOW, the analog alignment is obtained when the
sample is cooled from the isotropic phase in the presence of a
field. This certainly implies that the analog switching is
occurring in an alignment where the polarization (either
induced or spontaneous) is parallel to the field at high fields.
At this stage the behavior of NORABOW is, not surprisingly,
mysterious. Some possible interpretations are: 1) the phase is
antiferroelectric with a very low threshold and almost no
hysteresis; 2) the phase is a SmA showing electroclinic
switching into an induced SmCSPF; 3) the phase is a
ferroelectric SmCSPF with high polarization and small screen-
ing from ions, providing electrostatically driven V-shaped
switching analogous to that observed in SmC* materials;14 or
4) the layers (and tilt cone) are tilted when the polarization is
parallel to the field, that is, the phase is a SmCG,15 and there is
strong biaxial anchoring preferring the bow plane parallel to
the substrates.
4-(2-Hydroxy-4-nonyloxybenzylideneamino)benzoic acid (2)
To a solution of aldehyde 1 (3.28 g, 0.012 mol) in toluene
(190 mL) was added 4-aminobenzoic acid (1.70 g, 0.012 mol),
and the resulting mixture was refluxed for 24 h using a Dean–
Stark trap for azeotropic removal of water. After cooling, the
product (2.85 g) crystallized from the solution and was isolated
by filtration. The resulting solid was washed with hexane and
recrystallized from absolute ethanol to provide 2.58 g (56%) of
acid 2: 1H NMR (500 MHz, d7-DMF) d: 8.98 (1H, s, CH), 8.09
(2H, d, J~8.52, Ar), 7.60 (1H, d, J~8.72 Hz, Ar), 7.52 (2H, d,
J~8.32, Ar), 6.61 (1H, d, J~10.90 Hz, Ar), 6.53 (1H, s, Ar),
4.09 (2H, t, J~6.54, OCH2), 3.50 (1H, br s, OH), 1.78–1.26
(14H, m, 7 CH2), 0.86 (3H, t, CH3); 13C NMR (500 MHz, d7-
DMF) d: 167.68, 165.09, 164.79, 164.48, 153.05, 135.35, 131.59,
129.49, 122.02, 113.89, 108.19, 101.99, 68.88, 32.36, 26.46,
23.11, 14.27. MS (m/z): theor.: 383.2089; EI 383.3. EA for
C23H29NO4 (383.49) calculated: C 72.04%, H 7.62%, N 3.65%;
found: C 72.13%, H 7.91%, N 3.61%.
1,3-Phenylene bis[4-(2-hydroxy-4-
nonyloxybenzylideneamino)benzoate] (3)
To a solution of resorcinol (0.880 g, 0.008 mol) and acid 2
(1.15 g, 0.003 mol) in dichloromethane (110 mL) were added
dicyclohexylcarbodiimide (0.742 g, 0.0036 mol) and 4-dimethyl-
aminopyridine (20 mg). The resulting reaction mixture was
stirred at room temperature for 48 h. The urea which formed
during the reaction was removed by filtration and the
dichloromethane was evaporated in vacuo. The solid residue
was purified by flash chromatography on silica gel using
dichloromethane–2% methanol as eluent and then crystallized
from ethyl acetate, from absolute ethanol, and again from ethyl
acetate, providing 170 mg (13%) of the target liquid crystal 3 as
a yellow solid. 1H NMR (500 MHz, CDCl3) d: 8.87 (2H, s,
2CH), 8.24 (4H, d, J~8.54, Ar), 7.50 (1H, t, J~8.24, Ar), 7.36–
7.18 (9H, m, Ar), 6.50 (4H, s, Ar), 4.01 (4H, t, J~6.55 Hz, 2
OCH2), 1.83–1.28 (28H, m, 14 CH2), 0.89 (6H, t, 2 CH3), 0.37
(2H, br s, 2 OH); 13C NMR (500 MHz, CDCl3) d: 164.37,
164.29, 164.09, 163.12, 153.36, 151.46, 134.02, 131.72, 129.87,
126.73, 121.26, 119.24, 115.85, 112.74, 108.11, 101.52, 68.36,
31.87, 29.51, 29.35. 29.25, 29.04, 25.97, 22.67, 14.11; MS (m/z):
theor.: 840.4350, electrospray positive 841, exact mass found:
840.4381; EA for C52H60N2O8 (841.06) calculated: C 74.26%,
H 7.19%, N 3.33%; found: C 73.84%, H 7.13%, N 3.28%.
5. Conclusions
In conclusion,
a new bow-phase mesogen possessing a
bis-resorcylidene aniline core, and showing a novel chiral,
electro-optically active bow-phase, has been synthesized. The
electro-optic behavior is especially interesting, showing a high
susceptibility analog response to applied fields, with a large
change in birefringence on tilting of the optic axis. These
properties are of potential utility in a variety of photonic
switching applications.
6. Experimental
NMR spectra were obtained with a Varian VXR-300S
spectrometer (300 MHz) or with a Varian VXR-500 spectro-
meter (500 MHz). Spectra were referenced to CHCl3 (7.24 ppm
for 1H, 77.0 ppm for 13C). Liquid crystal phases and phase
transitions were determined using polarized light microscopy
with a Nikon Optiphot2 POL microscope equipped with an
INSTEC temperature controlled hot stage.
Dichloromethane was dried by distillation from CaH.
Dry dimethylformamide (DMF), 2,4-dihydroxybenzaldehye,
4-aminobenzoic acid, resorcinol, dicyclohexylcarbodiimide,
4-dimethylaminopyridine, 95% sodium hydride (NaH), and
1-bromononane were used as supplied from Aldrich Chemical
Company.
Acknowledgements
This work was supported by the Ferroelectric Liquid Crystal
Materials Research Center (National Science Foundation
MRSEC award No. DMR-9809555).
2-Hydroxy-4-nonyloxybenzaldehyde (1)
References
To a solution of 2,4-dihydroxybenzaldehyde (15 g, 0.109 mol)
in DMF (90 mL), cooled to 0 uC, was added 95% NaH (3.28 g,
0.130 mol). The reaction mixture was allowed to stir at room
temperature for 1 h, then 1-bromononane (27 g, 25 mL,
0.130 mol) was added dropwise. The resulting mixture was
allowed to stir overnight, then poured into 210 mL of water,
and acidified to pH 3 with 5% aqueous HCl. This mixture was
shaken with ether, and the organic layer was separated, washed
with water and brine, then dried over MgSO4. The crude
product obtained after evaporation of solvent was purified by
flash chromatography on silica gel using hexane–25% ethyl
acetate as eluent. Known aldehyde 1 was obtained 8.5 g
1
This is paper Part 30 in the series Design and Synthesis of New
Ferroelectric Liquid Crystals. Previous papers in the series:
(a) D. M. Walba, E. Ko¨rblova, R. Shao, J. E. Maclennan,
D. R. Link, M. A. Glaser and N. A. Clark, J. Phys. Org. Chem.,
2000, 12, 830; (b) D. M. Walba, E. Ko¨rblova, R. Shao,
J. E. Maclennan, D. R. Link, M. A. Glaser and N. A. Clark, in
Anisotropic Organic Materials - Approaches to Polar Order, eds.
R. Glaser and P. Kaszynski, American Chemical Society,
Washington, DC, in press; (c) D. M. Walba, E. Ko¨rblova,
R. Shao, J. E. Maclennan, D. R. Link, M. A. Glaser and
N. A. Clark, Science, 2000, 288, 2181; (d) D. M. Walba, L. Xiao,
P. Keller, R. Shao, D. Link and N. A. Clark, Pure Appl. Chem.,
1999, 51, 2117; (e) D. M. Walba, D. J. Dyer, X. H. Chen,
2746
J. Mater. Chem., 2001, 11, 2743–2747

View Article Online
U. Mu¨ller, P. Cobben, R. Shao and N. A. Clark, Mol. Cryst. Liq.
Cryst., 1996, 288, 83 and references therein.
G. Pelzl, S. Diele and W. Weissflog, Adv. Mater., 1999, 11, 707.
(a) T. Akutagawa, Y. Matsunaga and K. Yashuhara, Liq. Cryst.,
1994, 17, 659; (b) T. Niori, T. Sekine, J. Watanabe, T. Furukawa
and H. Takezoe, J. Mater. Chem., 1996, 6, 1231.
(a) D. M. Walba, E. Ko¨rblova, R. Shao, J. E. Maclennan,
D. R. Link, M. A. Glaser and N. A. Clark, Science, 2000, 288,
2181; (b) E. Gorecka, D. Pociecha, F. Araoka, D. R. Link,
M. Nakata, J. Thisayukta, Y. Takanishi, K. Ishikawa, J. Watanabe
and H. Takezoe, Phys. Rev. E Stat. Phys., Plasmas, Fluids, Relat.
Interdiscip., 2000, 62, R4524.
from our laboratories. These cells, which are commercially
available from Displaytech (www.displaytech.com), have glass
substrates with a 3.5–4.5 mm cell gap, coated with low-pretilt
polyimide alignment layers. The rubbing directions on the two
substrates are parallel.
2
3
10 An electroclinic coefficient of 6.2 degrees/(V mm21) was recently
reported for a SmA* material: M. S. Spector, J. W. Naciri,
R. Shashidhar and P. A. Heiney, Proc. SPIE-Int. Soc. Opt. Eng.,
2000, 4107, 99.
4
11 In the V-shaped switching alignment, analog tilts of 30 degrees/
(V mm21) have been reported: P. Rudquist, J. P. F. Lagerwall,
M. Buivydas, S. T. Lagerwall, N. A. Clark, J. E. Maclennan,
R. Shao, D. A. Coleman, S. Bardon, T. Bellini, D. R. Link,
G. Natale, M. A. Glaser, D. M. Walba, M. D. Wand and
X.-H. Chen, J. Mater. Chem., 2000, 10, 801.
12 D. M. Walba, E. Ko¨rblova, R. Shao, J. E. Maclennan, D. R. Link
and N. A. Clark, Proc. SPIE-Int. Soc. Opt. Eng., 1999, 3800, 21.
13 K. Nito, H. Takanashi and A. Yasuda, Liq. Cryst., 1995, 19, 653.
14 N. A. Clark, D. Coleman and J. E. Maclennan, Liq. Cryst., 2000,
27, 985.
5
6
7
S. D. Diele, I. Wirt and C. Tscheirske, Chem. Commun., 1998, 23,
2573.
B. I. Ostrovskii, A. Z. Rabinovich, A. S. Sonin, E. L. Sorkin,
B. A. Strukov and S. A. Taraskin, Ferroelectrics, 1980, 24, 309.
(a) A. Hallsby, M. Nilsson and B. Otterholm, Mol. Cryst. Liq.
Cryst., 1982, 82, 61; (b) B. Otterholm, M. Nilsson, S. T. Lagerwall
and K. Skarp, Liq. Cryst., 1987, 2, 757.
E. A. Soto Bustamante, S. V. Yablonskii, B. I. Ostrovskii,
L. A. Beresnev, L. M. Blinov and W. Haase, Liq. Cryst., 1996, 21,
829.
8
9
15 P. G. de Gennes, The Physics of Liquid Crystals, Oxford
University Press, London, 1974.
The LC cells used in the present work are of the same type as those
used for previously reported work on the bow-phase materials
16 R. U. Safina, B. M. Bolotin, N. B. Etingen and N. T. Kuznetsova,
Zh. Vses. Khim. O-va. im. D. I. Mendeleeva, 1983, 28, 596.
J. Mater. Chem., 2001, 11, 2743–2747
2747