Laterally azo-bridged H-shaped ferroelectric dimesogens for second-order nonlinear optics: Ferroelectricity and second harmonic generation

Article abstract of DOI:10.1021/ja9069166

Two classes of laterally azo-bridged H-shaped ferroelectric liquid crystals (FLCs), incorporating azobenzene and disperse red 1 (DR-1) chromophores along the FLC polar axes, were synthesized and characterized by polarized light microscopy, differential sc

Full text of DOI:10.1021/ja9069166

Published on Web 11/23/2009
Laterally Azo-Bridged H-Shaped Ferroelectric Dimesogens for
Second-Order Nonlinear Optics: Ferroelectricity and Second
Harmonic Generation
Yongqiang Zhang,*,† Josu Martinez-Perdiguero,‡ Ute Baumeister,⊥
Christopher Walker,† Jesus Etxebarria,*,‡ Marko Prehm,⊥ Josu Ortega,§
Carsten Tschierske,| Michael J. O’Callaghan,† Adam Harant,† and
Mark Handschy†,#
Displaytech Inc., 2602 CloVer Basin DriVe, Longmont, Colorado 80503, UniVersity of the
Basque Country, 48080 Bilbao, Spain, and Institute of Chemistry, MLU Halle-Wittenberg,
Kurt-Mothes-Strasse 2, D-06120 Halle, Germany
Received August 14, 2009; E-mail: yqzhang@micron.com; j.etxeba@ehu.es
Abstract: Two classes of laterally azo-bridged H-shaped ferroelectric liquid crystals (FLCs), incorporating
azobenzene and disperse red 1 (DR-1) chromophores along the FLC polar axes, were synthesized and
characterized by polarized light microscopy, differential scanning calorimetry, 2D X-ray diffraction analysis,
and electro-optical investigations. They represent the first H-shaped FLC materials exhibiting the ground-
state, thermodynamically stable enantiotropic SmC* phase, i.e., ground-state ferroelectricity. Second
harmonic generation measurements of one compound incorporating a DR-1 chromophore at the incident
wavelength of 1064 nm give a nonlinear coefficient of d22 ) 17 pm/V, the largest nonlinear optics coefficient
reported to date for calamitic FLCs. This value enables viable applications of FLCs in nonlinear optics.
Introduction
it is difficult to realize both large macroscopic nonlinear effects
and stable dipole alignment in the same system.1h
Dramatic advances in the study of organic nonlinear optical
(NLO) materials, potential successors to inorganic crystals such
as LiNO3, have occurred in recent years.1 Organic materials
offer potential advantages over inorganic crystals because of
their attractive features such as low dielectric constants, large
and ultrafast responses, facile fabrication and processability, and
wide range of operating frequencies. In poled-polymer materials,
macroscopic nonlinearities are generally achieved by using an
extremely strong electric field. Nevertheless, the poled state is
not thermodynamically stable, causing electro-optic performance
to decay over time. Although it is possible to maintain a stable
dipole alignment by utilizing either side-chain polymers with
high glass transition temperature (Tg) or cross-linking polymers,
Ferroelectric liquid crystals (FLCs), as new types of organic
NLO materials, possess inherent thermodynamically stable polar
order.2,3 If a large second-order susceptibility (ꢀ(2)) can be
incorporated, NLO FLCs will become a compelling alternative
to inorganic and poled-polymer materials. An important advan-
tage is that their polar direction can be controlled via ferro-
electric (FE) switching under an external electric field, enabling
fabrication of more complex devices. Previous attempts to make
rod-shaped NLO FLCs have proved the principle,4 but they have
been deficient in the size of their NLO coefficients because
incorporating chromophores possessing large hyperpolarizabili-
ties (ꢁ) along their polar axis while remaining an FLC is
extremely difficult.5 A new approach to making complex FLCs
with potentially large NLO coefficients was pioneered by the
Walba group,6 based on a novel “H-shaped” molecular structure
including disperse red 1 (DR-1), in which a pair of rod-shaped
molecules are connected by an azo bridge. Note that the
structural array has two important advantages: (i) a wider range
of chromophores can be accommodated, and (ii) it more fully
aligns the chromophore along the FLC’s polar axis, thereby
producing larger electro-optical coefficients. Although few such
† Displaytech Inc. (Displaytech Inc. is now part of Micron Technology
Inc.).
‡ Department of Condensed Matter Physics, University of the Basque
Country.
§ Applied Physics II, University of the Basque Country.
⊥ Physical Chemistry, MLU Halle-Wittenberg.
| Organic Chemistry, MLU Halle-Wittenberg.
# Present address: Office of the Under Secretary, Department of Energy,
1000 Independence SW, Washington, DC 20585.
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J. D.; McGee, J. Science 2002, 298, 1401. (b) Shi, Y.; Zhang, C.;
Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H.
Science 2000, 288, 119. (c) Nonlinear optics of organic molecules
and polymers; Nalwa, H. S., Miyata, S., Eds.; CRC Press, Inc: Boca
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(3) Although polymeric nematic liquid crystals have been shown to possess
permanent polar order and strong SHG activity, they cannot respond
to the applied electric field as well as FLCs owing to the high viscosity
of polymers: (a) Watanabe, T.; Miyata, S.; Furakawa, T.; Takezoe,
H.; Nishi, T.; Migita, A.; Sone, M.; Watanabe, J. Jpn. J. Appl. Phys.
1996, 35, L505. (b) Koike, M.; Yen, C.-C.; Liu, Y.; Tsuchiya, H.;
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9
18386 J. AM. CHEM. SOC. 2009, 131, 18386–18392
10.1021/ja9069166  2009 American Chemical Society

Ferroelectric Dimesogens for Second-Order NLO
A R T I C L E S
Chart 1. H-Shaped FLC Compounds (Chromophores in Blue)
Scheme 1. Synthesis of H-Shaped FLC Compounds 1a-e
Incorporating an Azobenzene Chromophorea
a Reagents and conditions: (a) (-)-R-2-octanol, PPh3, DEAD, THF. (b)
Pd(OH)2/C, H2. (c) 3, HCl, NaNO2; then 4, K2CO3, H2O/EtOH.8 (d) BuNH2,
benzene.9 (e) 4′-Alkoxy-4-biphenylcarboxylic acid chlorides, DMAP, Et3N,
THF. Bn ) benzyl, Bz ) benzoyl.
Scheme 2. Synthesis of H-Shaped FLC Compounds 2a-c
Incorporating a DR-1 Chromophorea
compounds have been reported,6 none, with the exception of a
main-chain polymer6b derived from one of these compounds,
exhibits the desired thermodynamically stable enantiotropic
SmC*, i.e., ferroelectric, phase which is indispensable to
applications of FLCs in nonlinear optics.
Herein, we first report two classes of laterally azo-bridged
H-shaped FLCs, 1a-e and 2a-c (Chart 1), that exhibit a
ground-state, thermodynamically stable enantiotropic SmC*
phase, i.e., ground-state ferroelectricity. Compounds 1a-e
include a conjugated azo linkage, while 2a-c are incorporated
into a DR-1 chromophore. Second harmonic generation (SHG)
measurements show that the extrapolated d22 coefficient of the
second-order susceptibility tensor for 2c is 17 pm/V, the largest
NLO coefficient reported to date for calamitic NLO FLCs. It
should be noted that chromophores in these materials can be
a Reagents and conditions: (a) CH3CHO, Na(AcO)3BH, ClCH2CH2Cl.10
(b) LiOH, EtOH, H2O. (c) 7, HCl, NaNO2, EtOH/H2O; then 6, pyridine,
CH2Cl2, EtOH. (d) 4′-Alkoxy-4-biphenylcarboxylic acid chlorides, DMAP,
Et3N, THF.
Tg), which requires NLO chromophores to possess excellent
thermal and chemical stability.1c,7
readily oriented by a weak electric field (0.1-0.5 V µm-1
)
parallel to the cell surface of a homeotropically aligned cell. In
contrast, the poling process for poled polymers generally
requires a much stronger electric field (10-50 V µm-1) at high
temperature (above the polymer glass-transition temperature,
Results and Discussion
Synthesis. The syntheses of two types of compounds, 1a-e
and 2a-c, are outlined in Schemes 1 and 2, respectively (see
detailed synthetic procedures and analytic data in the Supporting
Information). To prepare compounds 1, two precursors for diazo
coupling reactions, 3 and 4, were synthesized from 4-benzy-
loxyphenol by a literature6a approach (for 3) and a two-step
approach (for 4) involving Mitsunobu reaction and benzyl
deprotection via hydrogenolysis. Subsequent diazo coupling,8
followed by aminolysis of the resulted ester under mild
conditions,9 afforded diphenol 5 in excellent yield. Esterification
of 5 with a variety of 4′-alkoxy-4-biphenylcarboxylic acid
chlorides generated the final mesogens 1a-e in moderate to
good yields (Scheme 1). As shown in Scheme 2, the final
mesogens 2a-c were synthesized from aniline 3 in a multistep
approach similar similar to that used for 1a-e. Two precursors
for diazo coupling reactions, 6 and 7, were prepared by a two-
step approach involving reductive amination10 and hydrolysis
(4) (a) Walba, D. M.; Ros, M. B.; Clark, N. A.; Shao, R.; Robinson, M. G.;
Liu, J. Y.; Johnson, K. M.; Doroski, D. J. J. Am. Chem. Soc. 1991,
113, 5472. (b) Trollsås, M.; Orrenius, C.; Sahle´n, F.; Gedde, U. W.;
Norin, T.; Hult, A.; Hermann, D.; Rudquist, P.; Komitov, L.;
Lagerwall, S. T.; Lindstro¨m, J. J. Am. Chem. Soc. 1996, 118, 8542.
(c) Dubois, J.-C.; Barny, P. L.; Mauzac, M.; Noel, C. In Handbook of
Liquid Crystals; Demus, D., Goodby, J. W., Gray, G. W., Spiess,
H. W., Vill, V., Eds.; Wiley-VCH: Weinheim, Germany, 1998; Vol.
3, p 207. (d) Artal, C.; Ros, M. B.; Serrano, J. L.; Pereda, N.;
Etxebarria, J.; Folcia, C. L.; Ortega, J. Macromolecules 2001, 34, 4244.
(e) Keller, P.; Shao, R.; Walba, D. M.; Brunet, M. Liq. Cryst. 1995,
18, 915. (f) Walba, D. M.; Keller, P.; Shao, R.; Clark, N. A.; Hillmyer,
M.; Grubbs, R. H. J. Am. Chem. Soc. 1996, 118, 2740. (g) Fazio,
V. S. U.; Lagerwall, S. T.; Zauls, V.; Schrader, S.; Busson, P.; Hult,
A.; Motschmann, H. Eur. Phys. J. E 2000, 3, 245. (h) Kapitza, H.;
Zentel, R.; Twieg, R. J.; Nguyen, C.; Vallerien, S. U.; Kremer, L. F.;
Wilson, C. G. AdV. Mater. 1990, 2, 539. (i) Espinet, P.; Etxebarria,
J.; Folcia, C. L.; Ortega, J.; Ros, M. B.; Serrano, J. L. AdV. Mater.
1996, 8, 745.
(5) (a) Ikeda, T.; Sasaki, T.; Ichimura, K. Nature 1993, 361, 428. (b)
Sasaki, T.; Ikeda, T.; Ichimura, K. J. Am. Chem. Soc. 1994, 116, 625.
(6) (a) Walba, D. M.; Dyer, D. J.; Sierra, T.; Cobben, P. L.; Shao, R.;
Clark, N. A. J. Am. Chem. Soc. 1996, 118, 1211. (b) Walba, D. M.;
Xiao, L.; Keller, P.; Shao, R.; Link, D.; Clark, N. A. Pure Appl. Chem.
1999, 71, 2117. (c) Walba, D. M.; Xiao, L.; Korblova, E.; Keller, P.;
Shoemaker, R.; Nakata, M.; Shao, R.; Link, D. R.; Coleman, D. A.;
Clark, N. A. Ferroelectrics 2004, 309, 77.
(7) Wu, X.; Wu, J.; Liu, Y.; Jen, A. K.-Y. J. Am. Chem. Soc. 1999, 121,
472.
(8) Haghbeen, K.; Tan, E. W. J. Org. Chem. 1998, 63, 4503. Note that
14 g of K2CO3 should be used instead of 1.4 g mentioned in the
experimental procedure of this reference.
(9) Bell, K. H. Tetrahedron Lett. 1986, 2263.
(10) Abdel-Magid, A. F.; Carson, K. G.; Harris, B. D.; Maryanoff, C. A.;
Shah, R. D. J. Org. Chem. 1996, 61, 3849.
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A R T I C L E S
Zhang et al.
Table 1. Mesophases, Phase Transition Temperatures, Maximum
Absorption Wavelengths (λmax), and the Corresponding Molar
Extinction Coefficients (ε) of Compounds 1a-e and 2a-ca
compd
T/°C
λmax/nm
ε/M-1 cm-1
1a
Cr 65 SmC* 180 SmA* 218 Iso
Iso 214 SmA* 179 SmC* 40 gSmC*
Cr 56 SmC* 136 SmA* 170 Iso
Iso 160 SmA* 127 SmC* 33 gSmC*
Cr 74 SmC* 157 SmA* 174 Iso
Iso 170 SmA* 155 SmC*8 gSmC*
Cr1 60 Cr2 98 SmC* 180 SmA* 199 Iso
Iso 195 SmA* 178 SmC* 14 gSmC*
Cr 74 SmC* 168 SmA* 214 Iso
Iso 204 SmA* 159 SmC* 43 gSmC*
Cr 73 Iso
385
10757
1b
1c
1d
1e
2a
2b
2c
385
385
385
385
531
531
531
9916
9409
10812
10295
30081
29098
33912
Iso 68 SmC* 10 gSmC*
Cr 69 Iso
Iso 67 SmC* 1 gSmC*
Cr 73 SmC* 78 Iso
Iso 73 SmC* 14 gSmC*
a Phase transition temperatures determined by DSC at 10 K min-1
during heating (top) and cooling (bottom) scans from -10 (or 200) to
200 (or -10) °C, except for 1a and 1e from -10 (or 220) to 220 (or
-10) °C. Abbreviations: Iso, the isotropic liquid; gSmC*, the glassy
state of the SmC* phase; Cr, the crystalline state. The isotropic melt
was stored in vials at room temperature for some time (1 week for 1a-e
and 3 months for 2a-c) prior to DSC measurements. UV-vis spectra
were taken in dilute CH2Cl2 solution.
Figure 1. Textures (a, b, c, d, e, f, h) in a 6 µm ITO-coated cell, switching
current response curve (g), and XRD patterns (i, j) [insets: I ) I(T) - I(177
°C), where the scattering of the isotropic liquid is subtracted to enhance
the effect of the outer diffuse scattering] of 1c: (a, i) SmA* phase at 160
°C; (b, c, d, e, f, h, j) SmC* phase; (b) 150 °C; (c) 150 °C with field off;
(d) 150 °C upon applying a square wave (SW) voltage ((30 V, 100 Hz);
(e) 60 °C upon applying a SW voltage ((30 V, 20 Hz); (f) 60 °C with
field-off; (g) switching current recorded in a 6 µm ITO-coated cell at 100
°C; (h) circular domains (upper, 100 °C) surrounded by void and associated
director orientation (lower); (j) 140 °C.
under basic conditions (for 6) and a literature6a approach (for
7). Diazo coupling between them furnished diphenol 8, which
was further esterified with various 4′-alkoxy-4-biphenylcar-
boxylic acid chlorides to afford 2a-c in good yields. It is notable
that the phenol group ortho to the nitro group showed much
higher reactivity than the one ortho to the diazo unit, making it
possible to efficiently synthesize compounds (e.g., 2c11) having
different alkoxy tails attached to their biphenyl cores. All
compounds were purified by flash columnar chromatography,
and for some compounds, further purification was performed
by crystallization from a mixture of hexanes and ethyl acetate.
Analytic data are in good agreement with the proposed
structures. The liquid crystalline phases were characterized by
polarized-light optical microscopy (POM), differential scanning
calorimetry (DSC), X-ray diffraction (XRD) analysis, and
electro-optical techniques.
LC Phases of Compounds 1a-e Incorporating an Azoben-
zene Chromophore. Phase transition temperatures and UV-vis
spectroscopic data for 1a-e and 2a-c are summarized in Table
1. Compounds 1a-e exhibit a phase sequence of Cr-SmC*-
SmA*-Iso. They show moderate enantiotropic SmA* phase
ranges from 17 to 46 K and extremely broad enantiotropic SmC*
phase ranges from 80 to 115 K during heating. Compound 1a,
with saturated alkoxy tails, shows the highest clearing point of
218 °C, and incorporation of multiple double bonds (e.g., linolyl
group in 1b)12 and large carbosilane units (e.g., dicarbosilane
unit in 1c) into alkoxy tails significantly lowers the clearing
points by 44-48 °C.
a large endothermic peak (two peaks for 1d),13 attributed to a
transition from the crystalline state to the SmC* phase, was
observed, while this peak was not observed during the second
heating, supporting a slow crystallization process. We also
observed the occurrence of crystallization from the supercooled
glassy state of the SmC* phase in cells during storage at room
temperature and found that the speed of crystallization depends
on the structure of the tails. Compounds 1c and 1d, with silane
termini, tend to crystallize more slowly than other analogues.
Cooling the isotropic liquid of 1c with field off yielded
coexisting planar (fan-shaped texture) and homeotropic (pseudo-
isotropic texture) regions (Figure 1a), suggesting a SmA* phase.
Upon cooling further, the homeotropic area evolved into a
Schlieren texture, and in the planar area an equidistant line
pattern (Figure 1b) due to the helical superstructure appeared
on the fan-shaped texture.14 They are typical features for SmC*
phases. We found that the optical retardation of cells filled with
1a-e is strongly dependent on electric fields and temperature.
For example, if no electric field was applied, 1c exhibited
various yellowish textures (Figure 1c,f) depending on temper-
ature. Upon the application of an external electric field, green-
blue fan-shaped textures (Figure 1e) were observed over the
broad temperature range of the SmC* phase. As the temperature
The SmC* phases of 1a-e are supercooled to the glassy state,
which is supported by the DSC measurements of samples after
prolonged storage at room temperature. For instance, during the
first heating of DSC measurements with two consecutive scans,
(13) The first peak in 1d was attributed to a transition from the first
crystalline state to the second crystalline state, and the second peak
was assigned as a transition from the second crystalline state to the
SmC* phase. See DSC data in Table 1.
(14) Dierking, I. Textures of Liquid Crystals; Wiley-VCH: Weinheim,
Germany, 2003.
(11) The synthesis of 2c involved sequential addition of 4′-decyloxy-4-
biphenylcarboxylic acid chloride and 4′-(11-undecenyloxy)-4-biphe-
nylcarboxylic acid chloride, respectively by utilizing the regioselec-
tivity due to the nitro group.
(12) Dyer, D. J.; Walba, D. M. Chem. Mater. 1994, 6, 1096.
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Ferroelectric Dimesogens for Second-Order NLO
A R T I C L E S
approached the SmC*-SmA* phase transition during heating,
green-blue fan textures suddenly changed to violet fan-shaped
textures (Figure 1d) and eventually to yellowish fan-shaped
textures (Figure 1a) in the SmA* phase. This is presumably
due to the changes of birefringence and molecular tilt upon
approaching the C-A transition.
Switching experiments are important for the identification of
ferroelectric (FE) and antiferroelectric (AF) switching in polar
mesophases. We have investigated the switching behavior of
1a, 1c, and 1d by applying a triangular wave (TW) voltage.
All of them show a sharp single-current peak during each half
period of the TW voltage (Figure 1g), indicating FE switching.
Spontaneous polarizations (Ps) were measured to be 112 (1a),
86 (1c), and 103 nC cm-2 (1d), respectively. The polarizations
do not vary significantly except as the temperature approaches
the SmC*-SmA* phase transition, indicating that the tilt angles
and order parameters do not change significantly over the broad
SmC* phase ranges. Cooling the isotropic liquid of 1c under a
TW voltage yielded circular domains with extinction crosses
inclined with respect to crossed polarizers (Figure 1h). Reversing
a DC electric field resulted in the rotation of extinction crosses,
and they remained unchanged upon switching off the field. This
bistable switching via rotation of molecules on a smectic cone
clearly indicates a FE ground state, which confirms the result
of the switching current response under the applied TW voltage.
2D XRD patterns of 1c (Figure 1i,j) confirm the aforemen-
tioned phase sequence with layer spacings of dA ) 3.91 nm
and dC ) 3.84 nm.15 The outer diffuse scattering (ODS) in the
wide-angle region shows one maximum at 0.51 nm for the
SmA* phase and at 0.50 nm for the SmC* phase, which is
related to the average width of the aromatic cores and the fluid
alkyl chains. The tilt angle was calculated as 26° from the
ꢀ-distribution of the ODS (Figures S1 and S2), close to the
optical tilt angle of 22°.16
LC Phase of Compounds 2a-c Incorporating a DR-1 Chro-
mophore. Compound 2c exhibits an enantiotropic SmC* phase
with a wide range of 59 K during cooling, whereas 2a,b exhibit
a monotropic SmC* phase. Like for analogues 1a-e, their
SmC* phase is supercooled to the glassy state. In the DSC
cooling scans only one exothermic peak, attributed to a transition
from the isotropic liquid to the SmC* phase, was observed
beside a broad glass transition step between 1 and 14 °C. Since
the supercooled glassy state is metastable, crystallization can
occur slowly during storage at room temperature (days to
weeks).17 As expected, crystallization of 2c with a combination
of different tails from the supercooled glassy state of the SmC*
phase is slowest. Compound 2c possesses the highest clearing
point, suggesting that mixed alkoxy tails in this type of
mesogenic materials stabilize the SmC* phase.
Figure 2. (a) Texture at 70 °C in a 6 µm ITO-coated cell, (b) switching
current response curve at 70 °C in a 1.8 µm ITO-coated cell with parallel
nylon alignment layer, (c) optical response at 65 °C in a 1.2 µm ITO-
coated cell with parallel nylon alignment layer, and (d) XRD pattern at 60
°C [inset: I ) I(60 °C) - I(80 °C)] of 2c.
Cooling 2c from the isotropic liquid yielded fan-shaped focal
conic textures (Figure 2a), supporting the SmC* phase. The
switching current curve shows only one single current peak
(Figure 2b) during each half-period of an applied TW voltage,
suggesting FE switching. Spontaneous polarizations were
measured to be 65 (2a), 62 (2b), and 70 nC cm-2 (2c),
respectively, which are consistent with the extrapolated polar-
ization (65 nC cm-2) reported for a dimethylamino analogue.6a
Upon the application of a square wave (SW) field (25 V/µm),
the switching time constants of 2c were measured to be 3 ms
at 70 °C and 0.5 s at 30 °C, thousands of times slower than
those for conventional rod-shaped FLCs,18 indicating high
viscosity for this type of material. The FE switching was stopped
by freezing a cell with dry ice but started again upon warming
it up to room temperature, strongly supporting the DSC result
that the SmC* phase is supercooled to the glassy state. The
electro-optical response (Figure 2c) of 2c in an aligned cell upon
the application of a low-frequency (1 Hz) TW voltage shows a
typical shape that does not change except for the change of the
distance between two peaks as the frequency is increased (10
Hz) or decreased (0.2 Hz). These results strongly support a
bistable FE switching rather than an analogous V-shaped
switching.
The XRD pattern (Figure 2d) confirms the SmC* phase with
a layer spacing of dC ) 2.91 nm and a maximum ODS at 0.45
nm. The tilt angle was calculated as 26° (Figures S1 and S3),
close to the optical tilt angle of 21°19 but much smaller than
that (44°) calculated from cos θ ) dC/L,20 suggesting strong
intercalation between adjacent layers.
Textures of Racemic 1c and 2c under Homeotropic Con-
ditions. To confirm whether the ground-state structure is
synclinic FE (SmC*) or anticlinic AF (SmCA*) in these two
(15) The calculated molecular length for 1c from the MOPAC molecular
modeling is 4.4 nm on the basis of the rod with the azo group meta
to the chiral center.
(16) The optical tilt angle was measured at 140 °C in an aligned 1.2 µm
cell with nylon alignment layers upon the application of a SW voltage
((10 V, 10 Hz) and kept almost a constant value of 22° when the
temperature was decreased from 140 to 100 °C.
(17) Crystallization was observed when cells were stored at room temper-
ature for several days (e.g., 2a) or several weeks (e.g., 2c). In addition,
a large endothermic peak due to a transition from the crystalline state
to the SmC* phase was observed during the first heating cycle of a
DSC scan when the samples were stored at room temperature for
months, but the same peak could not be observed if the samples were
stored at room temperature for a short time. These observations
strongly indicate that crystallization from the glassy state takes place
slowly during storage of samples.
(18) Conventional rod-shaped FLCs’ switching speed is 10-150 µs.
(19) The optical tilt angle was measured at 60 °C in an aligned 1.2 µm
cell with nylon alignment layers upon the application of a SW voltage
((20 V, 10 Hz).
(20) L is the sum (3.99 nm) of the molecular length (3.75 nm) from the
MOPAC molecular modeling and van der Waals diameter (0.24 nm)
of an H atom.
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A R T I C L E S
Zhang et al.
Table 2. Mesophases and Phase Transition Temperatures of
Mixturesa
mixture
T/°C
M1
M2
Cr1 55 Cr2 71 SmC* 156 SmA* 173 Iso
Iso 169 SmA* 154 SmC* 5 gSmC*
Cr 69 SmC* 147 SmA*168 Iso
Iso 161 SmA* 145 SmC* 5 gSmC*
a Phase transition temperatures determined by DSC at 10 K min-1
during heating (top row) and cooling (bottom row) scans from -10 (or
200) to 200 (or -10) °C. Abbreviations: Iso, the isotropic liquid;
gSmC*, the glassy state of SmC* phases; Cr, the crystalline state. The
isotropic melt was stored in vials at room temperature for seven months
prior to DSC measurements because only the glassy transition was
observed in the first heating cycle of DSC scans if the samples were
stored for a short time.
Figure 3. Schlieren textures of rac-1c (a) (normal defect strengths, S )
(1) and rac-2c (b) (high defect strengths, S ) ( 1, -2, and -3) at room
temperature in a 1 µm cell under homeotropic anchoring conditions. Note
that the textures at the high-temperature SmC phases are completely
identical, which is additional evidence to show that the SmC* phases are
supercooled to the glassy state.
Figure 5. Plots of X-ray layer spacings (d/Å) (a) and X-ray tilt angles
(θ/°) (b) of mixture M1 versus temperature from 30 to 170 °C.
Figure 4. UV-vis spectra of 1c (a) and 2c (b) in dilute CH2Cl2 solution,
and M2 are also supercooled to the glassy state. Crystallization
slowly took place when samples were stored at room temper-
ature for months. The bistable FE switching was confirmed by
electro-optical investigations (Figure S5) similarly to those
performed for the host 1c.
showing two absorption bands.
types of H-shaped FLC materials,21 we formulated two race-
mates, rac-1c and rac-2c, by mixing equimolar amounts of the
corresponding enantiomers. DSCs (Figure S4) show that both
exhibit almost the same phase transitions as their single
enantiomers. Schlieren textures under homeotropic conditions22
show only 4-brush (rac-1c, Figure 3a) or high-strength 4n-brush
(rac-2c, Figure 3b) singularities. It was shown that SmCA phases
exhibit both 2- and 4-brush singularities while SmC phases show
only 4-brush singularites.23 This strongly supports that both 1c
and 2c possess ground-state FE (SmC*) rather than AF (SmCA*)
phases.
UV-Vis Spectra. Both 1a-e and 2a-c exhibit two absorp-
tion bands in the ultraviolet and visible regions (see examples
in Figure 4). The same absorption band at 304 nm is attributed
to the π-π* transition of biphenyl units since the biphenol
precursors 5 and 8 do not exhibit this band while 4′-decyloxy-
4-biphenylcarboxylic acid chloride shows a similar band at 328
nm. The different absorption bands at 382 (1a-e) and 531 nm
(2a-c) arise from the π-π* transitions of the chromophore
units. Compared with 1a-e, 2a-c show a red-shift of 149 nm
and larger molar extinction coefficients (see Table 1) for the
second absorption bands, which is due to the strong charge-
transfer in the DR-1 chromophore.
2D XRD patterns of mixture M1 from 167 to 30 °C (Figure
S6) confirm the DSC phase sequence of Iso-SmA*-SmC*-g
during cooling. Layer spacings initially decrease from an average
of dA ) 3.87 nm to dC ) 3.80 nm at 130 °C and then gradually
increase to dC ) 4.03 nm at 30 °C (Figure 5a), whereas the
X-ray tilt angles calculated from the ꢀ-distribution of the ODS
keep an almost constant value of 24° (Figure 5b) during the
SmC* phase when the temperature is away from the A-C
transition. This result indicates that there is strong intercalation
in the SmA* phase, but the intercalation gradually weakens as
the temperature is lowered during the SmC* phase (Figure 6).
The increase of layer spacings without changing tilt could also
be explained by a gradual C-chain stretching with decreasing
temperature25 or the combination of two effects. ODS in the
wide angle region slowly increases from 0.46 nm at 30 °C to
0.50 nm at 167 °C, indicating that thermal fluctuation gradually
increases the average lateral distances between the molecules.
SHG Measurements. In order to measure dij coefficients via
SHG, we fabricated cells (see details in Figure S7) with
LC Phases of Mixtures M1 and M2. Since 2a-c have a
strong, broad absorption band at 531 nm (Figure 4b),24 we have
chosen to perform SHG measurements using mixtures instead
of pure compounds. Compound 1c appears to be a good SmC*
host material because it is transparent at 532 nm, has a broad
SmC* phase range, and forms films similar to those formed by
2c. We formulated two mixtures, M1 (10 mol % of 2c in 1c)
and M2 (20 mol % of 2c in 1c), and their mesophases and phase
transition temperatures are listed in Table 2. As expected,
mixtures M1 and M2 exhibit the same phase sequence as the
host 1c, but the phase transition temperatures are slightly lower
than those in 1c. As in the host 1c, the SmC* phases in M1
(21) Sometimes a ground-state SmCA* material can also exhibit one current
peak per half cycle owing to the merging of two current peaks. See
the merge with increasing frequencies in a SmCA* phase in ref 14, p
124.
(22) Cell surfaces were treated with n-octadecyltriethoxysilane to form self-
assembled monolayers to achieve homeotropic alignment.
(23) (a) Takanishi, Y.; Takezoe, H.; Fukuda, A.; Komuara, H.; Watanabe,
J. J. Mater. Chem. 1992, 2, 71. (b) Nishiyama, I.; Yamamoto, J.;
Goodby, J. W.; Yokoyama, H. J. Mater. Chem. 2003, 13, 2429.
(24) The SHG signal at 532 nm generated from the incident beam (λ )
1064 nm) is almost entirely absorbed by 2a-c. Absorption can be
significantly reduced by using a small percentage of 2a-c.
(25) The C-chains might be strongly folded at high temperature due to the
special H-shape of the molecules and gradually stretch with decreasing
temperature.
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18390 J. AM. CHEM. SOC. VOL. 131, NO. 51, 2009

Ferroelectric Dimesogens for Second-Order NLO
A R T I C L E S
Figure 8. Scheme of the geometry of FLC molecules of mixture M1 in
the aligned zone and polarization of the input and output beams used in
SHG experiments: p-p and s-p. Note that the definition of dipole herein
is from positive to negative charges, as used in chemistry. The angles
between the chromophore and the polar axis are about 26-30°.
Figure 6. Models showing the organization of molecules of M1 (a) in the
SmA* phase with strong intercalation of alkoxy tails (dA), (b) in the SmC*
phase with gradually reduced intercalation of alkoxy tails (dC1), and (c) in
the SmC* phase without intercalation of alkoxy tails (dC2). The molecules
in (a-c) are viewed along the direction with the chromophore plane
perpendicular to the paper. It can be conceived that the host 1c might have
the same molecular organization.
Figure 9. Temperature dependence of the SHG signal (a) in a 3.75 µm
thick cell with a gap of 0.65 mm between in-plane electrodes under a DC
voltage of 400 V, and SHG signal intensity normalized to the maximum
signal of a quartz sample versus cell thickness (µm) (b) for mixture M1.
All data in (b) are corrected for the Fresnel factors.
Figure 7. Textures of one of the aligned samples of mixture M1 used for
SHG measurements (1 µm thickness, 120 °C, DC voltage of 300 V across
the gap of 1.7 mm). (a) Polarizers along the indicatrix axes and (b) the cell
rotated by ∼45°, showing a uniform single domain in the gap. Owing to
the large width of the gap, only one of the electrode edges is observed
in the pictures. The electric field direction (E) is sketched in both pictures.
The width of the pictures is 1 mm.
switching off the electric field at 30 °C, the signal dropped to
half after 10 min and to the same level as that at 150 °C after
2 h. The slow dropping of the SHG signal may suggest that the
supercooled SmC* phase slowly relaxes to the ground-state
helical superstructure, as observed in Figure 1b.
To determine dij coefficients, we measured the SHG signal
intensities of M1 in three well-aligned homeotropic cells with
thicknesses of 1.0, 2.0, and 3.75 µm. These data were compared
with the SHG at the maximum of the first Maker fringe of a
quartz sample (d11 ) 0.4 pm/V) under the same conditions of
illumination. Similarly, the SHG of mixture M2 was also
measured in two cells with thicknesses of 1.0 and 3.75 µm.
The data process to extract the d coefficients followed the same
approach proposed by Herman and Hayden for absorbing
materials.27
homeotropic alignment layers in the gap between in-plane ITO
electrodes. Mixtures were introduced into the cells by capillary
action at a temperature well above their clearing points. Samples
were well oriented in these homeotropic cells (see an example
in Figure 7) by applying an external strong DC electric field.
The SHG experimental setup has been described elsewhere.26
The fundamental light source is a Q-switched Nd:YAG (yttrium
aluminum garnet) laser (λ ) 1064 nm). A y-cut quartz crystal
(d11 ) 0.4 pm/V) was used for calibration.
The measurements were carried out at normal incidence. As
shown in Figure 8, the d tensor is expressed in a reference frame
in which z is perpendicular to the smectic layers and y is parallel
to the polar axis. Input-output light polarization configurations,
i.e., input and output parallel or perpendicular to the applied
electric field, p or s, were used to measure SHG.
The temperature dependence of the SHG signal for mixture
M1 (Figure 9a) was investigated to find the optimum temper-
ature for SHG efficiency. Upon cooling the isotropic liquid under
a DC voltage of 400 V, a weak SHG signal was detected at
170 °C. The signal gradually increased with decreasing tem-
perature and finally reached a maximum at 50 °C. Further
cooling to 30 °C hardly affected the signal intensity. Upon
SHG Data Process. Assuming that absorption occurs only at
2ω and the normal incidence is adopted, the SHG power P2ω
from an absorbing material can be simplified to
R2L
2π∆nL
sin2
+ sinh2
(
)
(
)
λ
4
2
2
P2ω ∝ Pω2 deffL2 e-(R L/2)
2
R2L
2
2π∆nL
+
(
)
(
)
λ
4
where deff ) d22 for p-p polarizations, R2 is the absorption
coefficient at 2ω, and its value (0.614 µm-1) for mixture M1 is
extrapolated from the UV-vis spectrum of 2c in dilute CH2Cl2
solution, ∆n ) no(2ω) - no(ω) is the corresponding ordinary
(26) Pereda, N.; Folcia, C. L.; Etxebarria, J.; Ortega, J.; Ros, M. B. Liq.
Cryst. 1998, 24, 451.
(27) Herman, W. N.; Hayden, L. M. J. Opt. Soc. Am. B 1995, 12, 416.
9
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A R T I C L E S
Zhang et al.
refractive index mismatch between the second harmonic and
fundamental waves, λ is the wavelength of the fundamental light,
L is the sample thickness, and Pω is the power of the
fundamental light. A fit of SHG intensities (normalized to
quartz) of the three samples of M1 to a theoretical curve (Figure
9b) gives the d22 coefficient (1.5 pm/V) and the dispersion
parameter ∆n ) 0.01. The same treatment for M2 produces
the d22 coefficient (3.9 pm/V) and the dispersion parameter ∆n
) 0.02. This is the first use of SHG to characterize absorbing
NLO FLC materials. The remaining dij coefficients can be
deduced from the other polarization configuration and were
much smaller than d22. The SHG measurement of the host 1c is
negligible in comparison with M1 and M2. Hence, using a linear
extrapolation formula, the d22 coefficient of 2c was found to be
17 pm/V,28,29 more than 3 times as large as that for the best
calamitic NLO FLC previously reported, Roche-1.30,31 This d
value is also larger than those reported for most bent-core
compounds.32 The electro-optical (EO) coefficient r22 is esti-
mated to be larger than 5 pm/V,33 which is comparable to the
r coefficients of many commercial inorganic EO crystals.
Conclusion
We report two new classes of laterally azo-bridged FLCs that
incorporate azobenzene and DR-1 chromophores along the FLC
polar axes. Compounds 1a-e, containing an azobenzene chro-
mophore, exhibit thermodynamically stable enantiotropic SmA*
and SmC* phases, while 2c, incorporating a DR-1 chromophore,
only has an enantiotropic SmC* phase. They are the first
H-shaped dimesogens that have ground-state enantiotropic FE
phases. Both mixtures M1 and M2 also exhibit ground-state
FE phases. It is remarkable that they can readily achieve good
planar alignment in cells with buffed nylon alignment layers
and good homeotropic alignment under a relatively weak in-
plane electric field.
SHG measurements of mixtures give the extrapolated d22
coefficient of 17 pm/V, the largest NLO coefficient reported to
date for calamitic FLCs. This value enables viable applications
of FLCs in nonlinear optics. To the best of our knowledge, this
work represents the first example that combines intrinsic polar
order with large macroscopic nonlinearities. It should be noted
that, although some bent-core NLO mesogens show large
macroscopic nonlinearities, almost all of them have ground-
state antiferroelectricity. These investigations indicate that
elaborate molecular design and tailoring can lead to interesting
stimuli-responsive functional FLC materials with potentially
useful NLO and EO properties.
(28) Dalton et al. have shown that dipole-dipole interaction significantly
increases with increased chromophore loading for strong chro-
mophores, but the electro-optical coefficients for polymeric materials,
including DR-1 chromophores, are almost linear with chromophore
loading if the chromophore density is less than 25 wt %: Dalton, L. R.;
Steier, W. H.; Robinson, B. H.; Zhang, C.; Ren, A.; Garner, S.; Chen,
A.; Londergan, T.; Irwin, L.; Carlson, B.; Fifield, L.; Phelan, G.;
Kincaid, C.; Amend, J.; Jen, A. J. Mater. Chem. 1999, 9, 1905–1920.
The DR-1 chromophore density in 2c is 23 wt %. Moreover, we have
found that 1c and 2c have similar degrees of polar order (∼20%) (see
details in Table S1). Hence, extrapolating the d22 coefficient of 2c
from mixtures M1 and M2 is valid.
(29) Preliminary SHG measurements of pure films of 2c at the incident
wavelength of 1600 nm (much further from resonance) give an
expected smaller d22 value of 4.5 pm/V, which is still remarkable for
calamitic FLCs. This result, together with SHG measurements of
mixtures M1 and M2 at the same wavelength, will be published
separately.
Acknowledgment. This work was supported by NSF (USA)
grants (OII-0539835 and IIP-0646460) to Displaytech Inc., and by
the CIDYT-FEDER of Spain-EU (Project MAT2006-13571-C02-
02). J.M.-P. thanks the Basque Government for a grant.
(30) Schmitt, K.; Herr, R. P.; Schadt, M.; Fu¨nfschilling, J.; Buchecker, R.;
Chen, X. H.; Benecke, C. Liq. Cryst. 1993, 14, 1735.
(31) It is evident that the d value in Roche-1 including a p-nitroaniline
chromophore is less resonantly enhanced than that in 2c containing a
DR-1 chromophore. However, the ꢁ value of DR-1 is more than 5
times that of p-nitroaniline at the incident wavelength of 1.91 µm (49
vs 9.2 × 10-30 esu): Cheng, L. T.; Tam, W.; Stevenson, S. H.;
Meredith, G. R.; Rikken, G.; Marder, S. R. J. Phys. Chem. 1991, 95,
10631–10643. Their dispersion-free values ꢁ0 (35.2 vs 7.6 × 10-30
esu) are extrapolated using the two-level model: Spackman, M. A. J.
Phys. Chem. 1989, 93, 7594. Furthermore, the degree of polar order
for Roche-1 (∼30%, see ref 6a) is only 50% larger than that (∼20%,
see Table S1) for 2c. Taking these factors into account, a large d22
value (17 pm/V) for 2c is reasonable.
Supporting Information Available: Synthetic procedures for
1a-e and 2a-c; cell fabrication; Figures S1-S7; and Table
S1. The material is available free of charge via the Internet at
http://pubs.acs.org.
JA9069166
(33) The analogue with an opposite DR-1 chromophore dipole orientation
was reported to give an extrapolated coefficient of r ≈ 5 pm/V: Walba,
D. M.; Garcia, E.; Niessink-Trotter, J.; Richard, M.; Shao, R.; Nakata,
M.; Clark, N. A. Presented at the 22nd International Liquid Crystal
Conference, ILCC 2008, June 29-July 4, 2008, Jeju, Korea; Abstracts
I, p 228. Moreover, we found that its homologue with a diethylamino
donor shows a blue shift of >60 nm for the maximum absorption
wavelength in comparison to 2c, suggesting a smaller ꢁ value for the
analogue than that for 2c. Taking into account these factors, we can
deduce that the practical r22 value for 2c should be larger than 5 pm/
V.
(32) (a) Ortega, J.; Gallastegui, J. A.; Folcia, C. L.; Etxebarria, J.; Gimeno,
N.; Ros, M. B. Liq. Cryst. 2004, 31, 579. (b) Araoka, F.; Thisayukta,
J.; Ishikawa, K.; Watanabe, J.; Takezoe, H. Phys. ReV. E 2002, 66,
021705. (c) Folcia, C. L.; Alonso, I.; Ortega, J.; Etxebarria, J.; Pintre,
I.; Ros, M. B. Chem. Mater. 2006, 18, 4617. (d) Etxebarria, J.; Ros,
M. B. J. Mater. Chem. 2008, 18, 2919–2916. Note that big differences
for d coefficients of similar bent-core compounds were reported by
different groups, and the reasons are still unclear.
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18392 J. AM. CHEM. SOC. VOL. 131, NO. 51, 2009