Preparation of a novel cross-linked polymer for second-order nonlinear optics

Article abstract of DOI:10.1021/ja961309e

A novel cross-linked pyroelectric polymer with pronounced second-order nonlinear optical properties has been prepared. A multistep synthesis with several selective transformations including a kinetic resolution (transesterification) with the highly enantioselective Candida antarctica lipase B, yielded the monomer 4''-{(R)-(-)-2[(10-acryloyloxy)decyl]oxy}-3-nitrophenyl 4-{4'-[(11-acryloyloxy)undecyloxy]phenyl}benzoate A2c which displayed a ferroelectric chiral smectic C phase with large spontaneous polarization (175 nC/cm²). The monomer was poled and subsequently cross-linked by in-situ photopolymerization in the surface-stabilized ferroelectric liquid crystalline state. The cross-linked pyroelectric polymer exhibited an electro-optical coefficient (r₂₂-r₁₂) of 15-35 pm/V.

Full text of DOI:10.1021/ja961309e

8542
J. Am. Chem. Soc. 1996, 118, 8542-8548
Preparation of a Novel Cross-Linked Polymer for Second-Order
Nonlinear Optics
Mikael Trollsås,† Christian Orrenius,‡ Fredrik Sahle´n,† Ulf W. Gedde,†
Torbjo1rn Norin,‡ Anders Hult,*,† David Hermann,§ Per Rudquist,§
Lachezar Komitov,§ Sven T. Lagerwall,§ and Jan Lindstro1m
Contribution from the Departments of Polymer Technology and Organic Chemistry,
Department of Chemistry, Royal Institute of Technology, S-100 44 Stockholm, Sweden,
Physics Department, Chalmers UniVersity of Technology, S-412 96 Go¨teborg, Sweden, and
National Defense Research Establishment, S-172 90 Stockholm, Sweden
ReceiVed April 22, 1996X
Abstract: A novel cross-linked pyroelectric polymer with pronounced second-order nonlinear optical properties has
been prepared. A multistep synthesis with several selective transformations including a kinetic resolution (trans-
esterification) with the highly enantioselective Candida antarctica lipase B, yielded the monomer 4′′-{(R)-(-)-2-
[(10-acryloyloxy)decyl]oxy}-3-nitrophenyl 4-{4′-[(11-acryloyloxy)undecyloxy]phenyl} benzoate A2c which displayed
a ferroelectric chiral smectic C phase with large spontaneous polarization (175 nC/cm2). The monomer was poled
and subsequently cross-linked by in-situ photopolymerization in the surface-stabilized ferroelectric liquid crystalline
state. The cross-linked pyroelectric polymer exhibited an electro-optical coefficient (r22 - r12) of 15-35 pm/V.
1. Introduction
Ferroelectric liquid crystals (FLC) are known to exhibit
relatively low second-order nonlinear optical susceptibility
(ø(2)).1 However, in 1990, Walba et al.2 synthesized a low molar
mass compound I specially designed for second-order nonlinear
optics, and this compound showed a second harmonic coefficient
(d22) of 0.6 ( 0.3 pm/V in the chiral smectic C (SC*) phase.3
The NLO chromophores aligned in the direction of the polariza-
tion, perpendicular to the long axis direction of the molecule,
were later improved by Schmitt et al.4 by the addition of an
electron-donating amino group in the para position to the
electron-accepting nitro group (II).
Figure 1. Structure of low molar mass ferroelectric liquid crystals
suitable for second-order nonlinear optics.
The thermal and mechanical stability of these NLO materials
were improved by Zentel et al.5 when they synthesized a
ferroelectric liquid-crystalline side-chain polymer based on I.
Recently Keller et al.6 presented a similar ferroelectric main-
chain polymer designed for nonlinear optics.
chromophore was ferroelectric.8 The mixture with 30% NLO
active chiral monomer, aligned and photo-cross-linked into a
pyroelectric polymer III, displayed a small Pockels constant
(r22 - r12 ) 0.15 pm/V) and a clear second harmonic signal9
(d-coefficient ) 0.4 pm/V). The chiral monomer had one
polymerizable group, and the NLO chromophore itself was
therefore not cross-linked. Later this mixture was modified in
attempts to increase the NLO activity.10
The thermal and mechanical stability and the long-term
properties of NLO materials are improved by cross-linking.7
We therefore developed a cross-linkable monomer exhibiting a
SC mesomorphism over a wide temperature range. A mixture
based on this monomer and a chiral monomer with a NLO
In this paper, we report the preparation and characterization
of a new material with a higher concentration of NLO
chromophores. The densely cross-linked material is based on
a single monomer A2c. The new synthesis presented was
possible through the use of the enantioselective biocatalyst
Candida antarctica lipase B. The monomer displayed a large
spontaneous polarization (175 nC/cm2) in the chiral smectic C
phase at 23 °C. The in-situ photopolymerization at that
temperature gave a pyroelectric polymer (poly (A2c)) with more
† Department of Polymer Technology, Royal Institute of Technology.
‡ Organic Chemistry, Department of Chemistry, Royal Institute of
Technology.
§ Chalmers University of Technology.
National Defense Research Establishment.
X Abstract published in AdVance ACS Abstracts, August 15, 1996.
(1) Drevensek, I.; Blinc, R. Condensed Matter News 1992, 1, 14.
(2) Walba, D. M.; Ros, M. B.; Clark, N. A.; Shao, R.; Johnson, K. M.;
Robinson, M. G; Liu, J. Y.; Doroski, D. Mol. Cryst. Liq. Cryst. 1991, 198,
51.
(3) Liu, J. Y.; Robinson, M. G.; Johnson, K. M.; Walba, D. M.; Ros, M.
B.; Clark, N. A.; Shao, R.; Doroski, D. J. Appl. Phys. 1991, 70, 3426.
(4) Schmitt, K.; Herr, R.-P.; Schadt, M.; Fu¨nfschilling, J.; Buchecker,
R.; Chen, X. H.; Benecke, C. Liq. Cryst. 1993, 14, 1735.
(5) Kapitza, H.; Zentel, R.; Twieg, R. J.; Nguyen, C.; Vallerien, S. U.;
Kremer, L. F.; Wilson, C. G. AdV. Mater. 1990, 11, 539.
(6) Keller, P.; Shao, R.; Walba, D. M. Brunet, M.; Liq. Cryst. 1995, 18,
915.
(8) Trollsås, M.; Sahle´n, F.; Gedde, U. W.; Hult, A.; Hermann, D.;
Rudquist, P.; Komitov, L.; Lagerwall, S. T.; Stebler, B.; Lindstro¨m, J.;
Rydlund, O. Macromolecules 1996, 29, 2590.
(9) Hult, A.; Sahle´n, F.; Trollsås, M.; Lagerwall, S. T.; Hermann, D.;
Komitov, L.; Rudquist, P.; Stebler, B. results presented at the European
Liquid Crystal Winter Conference, Bovec, Slovenia, March 1995, Liq. Cryst.
1996, 20, 23.
(7) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. ReV. 1994, 94,
31.
(10) Sahle´n, F.; Trollsås, M.; Hult, A.; Gedde, U. W.; Hermann, D.;
Komitov, L.; Rudquist, P.; Lagerwall, S. T. Manuscript in preparation.
S0002-7863(96)01309-1 CCC: $12.00 © 1996 American Chemical Society

A Cross-Linked Pyroelectric Polymer
J. Am. Chem. Soc., Vol. 118, No. 36, 1996 8543
PW 1830 generator. Heating of the samples was controlled by a
resistive oven in the Statton camera. Photopolymerization was
performed by illumination with an Osram Ultra-Vitalux lamp (300 W).
Gas chromatography was performed on Varian 3300 and Carlo Erba
Fractovap instruments. Chrompack Cp-cyclodextrin-B-2,3,6-M-19 (25
m, 0.25 mm i.d., 0.25 µm film) and Astec Chiraldex G-TA (10 m,
0.25 mm i.d., 0.25 µm film) columns were used for enantiomeric excess
determinations. Trifluoroacetate derivatives of the free alcohol were
injected. Absolute configuration was assigned by optical rotation
measurement (Perkin Elmer 241 polarimeter) and comparison with
literature data. Spontaneous polarizations were measured by the
capacitance bridge method,12 reading the hysteresis loop from a
Tektronix 2430 oscilloscope.
Figure 2. Structure of a cross-linked pyroelectric polymer (III) with
large second-order nonlinear optical susceptibility.
2.3. Synthesis of Monomers. (4′-Benzyloxyphenyl)benzoate (3).
4-Benzyloxy phenol (24.6 g, 124.9 mmol) was dissolved in CH2Cl2
(500 mL) before Et3N (37.9 g, 375 mmol) was added, and the solution
was cooled to 0 °C. Benzoyl chloride (22.8 g, 162 mmol) was then
added dropwise while stirring vigorously. After 20 h the reaction
mixture was poured into water and extracted three times with CH2Cl2.
The organic phases were separated and combined, washed three times
with water (NH4Cl, 10% (w/w)), dried with MgSO4, and filtered. The
resulting solid was obtained after evaporation of the solvent. Twice
repeated recrystallization from ethanol gave a white crystalline powder.
Yield: 35.8 g (94%). 1H NMR (CDCl3): d ) 5.09 (s, 2H, -OCH2-
), 7.02 (d, 2H, 2′-H and 6′-H), 7.14 (d, 2H, 3′-H and 5′-H), 7.40 (m,
5H, 2′′-H, 3′′-H, 4′′-H, 5′′-H and 6′′-H), 7.53 (m, 2H, 3-H and 5-H),
7.63 (m, 1H, 4-H), 8.20 (d, 2H, 2-H and 6-H).
(4′-Hydroxyphenyl)benzoate (4). 3 (20.0 g, 304 mmol) was
dissolved in ethyl acetate (200 mL) at 70 °C, Pd/C (2.5 g, 10%) was
added, and the system was evacuated from air and filled with H2(g).
The reaction began immediately and consumed about 1400 mL of H2
in about 1 h. The reaction was stopped, and the catalyst was then
filtered. The solvent was removed by distillation which resulted in
white crystals. Yield: 13.8 g (98%). 1H NMR (CDCl3): d ) 5.17 (s,
1H, -OH), 6.83 (d, 2H, 3′-H and 5′-H), 7.05 (d, 2H, 2′-H and 6′-H),
7.51 (m, 2H, 3-H and 5-H), 7.62 (m, 1H, 4-H), 8.21 (d, 2H, 2-H and
6-H).
(4′-Hydroxy-3′-nitrophenyl)benzoate (5). 4 (7.15 g, 33.4 mmol)
was dissolved in diethyl ether (180 mL) at room temperature. NaNO3
(2.84 g, 33.4 mmol) and La(NO3)3‚6H2O (0.15 g, 0.33 mmol) in water
(15 mL) were mixed with concentrated HCl (10 mL). The acidic salt
solution was added to the diethyl ether solution and stirred for 3 h at
room temperature. The colorless suspension turned yellow and then
orange-brown. The reaction product was poured into water and
extracted three times with diethyl ether. The organic phases were
separated and combined, washed three times with water, dried with
MgSO4, and filtered. A solid was obtained after evaporation of the
solvent, which after recrystallization from ethanol gave yellow needle
crystals. Yield: 6.60 g (76%). 1H NMR (CDCl3): d ) 7.23 (d, 1H,
5′-H, Jo ) 9 Hz), 7.50 (dd, 1H, 6′-H, Jm ) 3 Hz, Jo ) 9 Hz), 7.53 (m,
2H, 3-H and 5-H), 7.67 (m, 1H, 4-H), 8.03 (d, 1H, 2′-H, Jm ) 3 Hz),
8.18 (d, 2H, 2-H and 6-H), 10.36 (s, 1H, -OH).
(()-2-Hydroxy-9-decene (6). A suspension of LiBH4 (2.72 g, 124
mmol) in 175 mL of anhydrous diethyl ether was vigorously stirred at
room temperature under an Ar(g) atmosphere for 0.5 h. 1,2-Epoxy-
9-decene (12.8 g, 83 mmol) and anhydrous MeOH (4.0 g, 124 mmol)
in diethyl ether (50 mL) were added dropwise, and the mixture was
refluxed for 3 h. The reaction was subsequently quenched by the
addition of a slightly acidic (HCl) aqueous solution. The solution was
extracted with diethyl ether (3 × 150 mL). The organic phases were
separated and combined, washed three times with water, dried (MgSO4),
and filtered. The solvent was evaporated to give an oil. Yield: 6.33
g (49%). 1H NMR (DMSO): d ) 1.00 (d, 3H, -C(CH3)H-), 1.24-
1.30 (m, 10H, -CH2(CH2)4CH2- and -C(CH3)HCH2-), 1.97-2.02
(m, 2H, -CH2CHdCH2), 3.56 (t, 1H, -OH), 4.27 (d, 1H, -OC(CH3)-
HCH2-), 4.93 (d, 1H, -CHdCH2, cis), 5.00 (d, 1H, -CHdCH2, trans),
5.73 (m, 1H, -CHdCH2).
Figure 3. Structure of the new cross-linked pyroelectric polymer (poly-
(A2c)), displaying a Pockels effect which is 100-200 times larger than
the Pockels effect observed in III.
than a hundred-fold higher NLO activity (Figure 3). The
Pockels constant (r22 - r12) was determined to be 15-35 pm/
V. However, the second harmonic signal has not yet been
measured.
Keller et al.6 used the stereospecific ring-opening of (S)-
(-)-methyloxirane by Grignard reagents catalyzed by copper
(I) iodide in the synthesis of bifunctional main-chain monomers.
The approach developed here, yielded similar intermediates in
large enantiomeric excess (>99.5% ee).
2. Experimental Section
2.1. Materials. 2,4,6-Trimethylbenzoyldiphenyl phosphine oxide
(Lucirin), purchased from BASF, was used as photoinitiator. The lipase,
Candida antarctica (component B), Novozym preparation SP 435,
containing 1% (w/w) protein with an activity typically of 25 000 LU/
g, was kindly supplied by NOVO-Nordisk A/S, Denmark. 10-Undecen-
1-ol, 4-benzyloxy phenol and S-methyl thioacetate were purchased from
Lancaster. La(NO3)3‚6H2O was purchased from Alfa products (Johnson
Matthey). All other substances were purchased from Aldrich and were
used as delivered. The syntheses of ethyl-4-(4-hydroxy)phenyl benzoate
(1) and 4′-(11-undecenyloxy)-4-biphenylcarboxylic acid (2) have been
described elsewhere.11
2.2. Techniques. Chemical characterization of the synthesized
substances was made by 1H NMR spectroscopy on a Bruker 400 MHz
and when necessary by FT-IR spectroscopy using a Perkin-Elmer
1760X. A Perkin-Elmer DSC-7 differential scanning calorimeter was
used for the assessment of thermal transitions. In all cases, heating
and cooling rates were 20 °C min-1 unless otherwise stated. A Leitz
Ortholux POL BK II optical polarized microscope (magnification 100×)
equipped with a Mettler Hot Stage FP 82 and an FP 80 central processor
was used for the determination of the thermal transitions of the
anisotropic textures. Small-angle X-ray scattering (SAXS) patterns
were recorded by a Statton camera, using Cu KR radiation from a Philips
(S)-(+)-2-Hydroxy-9-decene (7). The enzyme preparation (0.65
g) was added to a 100 mL three-necked predried round-flask containing
(11) Sahle´n, F.; Trollsås, M.; Hult, A.; Gedde, U. W. Chem. Mater. 1996,
8, 382.
(12) Andersson, G. Ph.D. Thesis, Department of Physics, Chalmers
University of Technology, Go¨teborg, Sweden, 1992.

8544 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Scheme 1 Synthesis of A2c
Trollsa˚s et al.
6 (5.00 g, 32 mmol) and freshly distilled CH2Cl2 (50 mL) at 30 °C.
The reaction was initiated by the rapid addition of the acyl donor,
S-methyl thioacetate (5.00 g, 55.5 mmol), and left overnight. The
volatile leaving group, thiomethane, was evaporated through drying
tubes in order to avoid hydrolysis of the acyldonor. The conversion
was stopped after 13 h, and the reaction was quenched by filtering off
the enzyme. Ees and eep were found to be >99.5 and 97.2%,
respectively (GC as described above). According to c ) ees/(eep +
ees),13 this gives a conversion of 51% and an enantiomeric ratio E >
400.14 Isolated yields were 40% (2.01 g) and 44% (2.76 g), and optical
rotations were [R]D ) +8.7° (c 0.1, CH2Cl2) and [R]D ) -2.4° (c 0.1,
CH2Cl2) for the remaining alcohol 7 and acetate, respectively. The
literature provides absolute configurations for 2-octanol, and the
Kazlauskas rule15 allows the prediction of the absolute configuration
(S)-(+)-7.
a mixture of 5 (2.29 g, 8.86 mmol) and triphenylphosphine (TPP) (2.32
g, 8.86 mmol) in diethyl ether (10 mL). The mixture was stirred for
48 h at room temperature and then put into the freezer. Triphenylphos-
phine-oxide precipitated and was filtered off. The solvent was removed,
and the remaining product was purified by column chromatography
(silica gel, hexane/CH2Cl2 as eluent). The final product was a slightly
yellow oil. Yield: 2.70 g (81%). 1H NMR (CDCl3): d ) 1.32-1.65
(m, 11H, -CH2(CH2)4CH2- and -C(CH3)H-), 1.77 (m, 2H,
-C(CH3)HCH2-), 2.03 (m, 2H, -CH2CHdCH2), 4.47 (m, 1H, -OC-
(CH3)HCH2-), 4.94 (d, 1H, -CHdCH2, cis), 5.00 (d, 1H, -CHdCH2,
trans), 5.81 (m, 1H, -CHdCH2), 7.11 (d, 1H, 5′-H, Jo ) 9 Hz), 7.37
(dd, 1H, 6′-H, Jm ) 3 Hz, Jo ) 9 Hz), 7.53 (m, 2H, 3-H and 5-H),
7.67 (m, 1H, 4-H), 7.72 (d, 1H, 2′-H, Jm ) 3 Hz), 8.18 (d, 2H, 2-H
and 6-H).
4-[(R)-(-)-2-(9-Decenyl)oxy]-3-nitrophenol (9). 8 (2.55 g, 6.77
mmol) and KOH (1.14 g, 20.3 mmol) were stirred in ethanol (40 mL)
at 50 °C for 3 h. The mixture was acidified with HCl and extracted
three times with CH2Cl2. The combined organic phases were washed
{4′-[(R)-(-)-2-(9-Decenyl)oxy]-3′-nitrophenyl}benzoate (8).
7
(1.32 g, 8.86 mmol) and diethylazodicarboxylate (DEAD) (1.54 g, 8.86
mmol) were dissolved in diethyl ether (10 mL) and added dropwise to

A Cross-Linked Pyroelectric Polymer
J. Am. Chem. Soc., Vol. 118, No. 36, 1996 8545
Table 1. Thermal Transitions of Monomer A2c and the
Intermediate 11
2H, 2′-H and 6′-H), 7.70 (d, 2H, 2-H and 6-H),7.73 (d, 1H, 2′′-H, Jm
) 3 Hz), 8.20 (d, 2H, 3-H and 5-H). Anal. Calcd for C46H59NO10:
C, 70.30; H, 7.57; N, 1.78. Found: C, 70.35; H, 7.64; N, 1.68.
thermal transitions (°C)
2.4. Polar Orientation in the Surface Stabilized Ferroelectric
Liquid Crystal (SSFLC) Cell. Cells of a conventional sandwich type,
consisting of two parallel glass substrates kept 2 µm apart by evaporated
SiOx spacers, were used for the ferroelectric, poling, polymerization,
and nonlinear optical experiments. The substrates were prepared from
ITO-coated glass sheets (Balzers Baltracon) on which an electrode
pattern was formed. An insulating layer of SiOx about 1000 Å thick
was deposited onto the electrodes. The uniform bookshelf alignment
of the liquid crystal material in the cell (smectic layers being essentially
perpendicular to the plates) was achieved by using a thin unidirectionally
rubbed polyimide aligning layer deposited on top of the insulating layer.
The liquid-crystalline substance was introduced into the cell in the
SA* or in the isotropic phase by capillary forces. The cell was inserted
into a Mettler FP 52 hot stage with the temperature controlled to an
accuracy of (0.1 °C, and the liquid crystalline substances were
examined in a polarizing microscope (crossed polarizers).
At the temperatures (≈23 °C) at which the ferroelectric electro-
optic response indicated that the SC* phase was fully developed, a dc
electric field (E ≈ 100 V/µm) was applied in order to orient the
spontaneous polarization in the whole cell in one direction. This gave
a unique direction to the optical axis, tilted with respect to the smectic
layer normal and thus resulting in the formation of a ferroelectric mono-
domain between the electrodes. The uniformity of the orientation was
examined in the polarizing microscope, and the applied dc field was
sufficient to cause full extinction of the transmitted light when the
optical axis of the cell was either parallel to or perpendicular to the
transmission direction of the polarizer.
2.5. Thermal Stabilization by in-Situ Photopolymerization. After
obtaining a ferroelectric monodomain structure for the monomer/photo
initiator mixture at the temperature (≈23 °C) at which the ferroelectric
electro-optic response in the SC* phase was fully developed, the cell
was irradiated with UV-light, while keeping the dc-field on, for about
10 min, during which the liquid-crystalline mixture polymerized. The
ratio of photo-initiator to monomer was 1:200 (0.5 mol %). The low
concentration of photo-initiator did not destabilize the SC* phase. In
order to avoid undesired photopolymerization of the liquid-crystalline
material in the cell during the preparation and investigative procedures,
the work was performed under yellow light.
compd
heating
cooling
A2c
11
k 29 SC* 33 SA* 39 i
k 28 SC* 58 SA* 71 i
i 34 SA* 29 SC* 8 k
i 63 SA* 50 SC* 3 k
three times with NaHCO3 (aqueous, 10%), once with water, dried over
MgSO4, and evaporated to give a slightly yellow oil which solidified
upon standing. Yield: 1.84 g (98%). 1H NMR (CDCl3): d ) 1.28-
1.50 (m, 11H, -CH2(CH2)4CH2- and -C(CH3)H-), 1.73 (m, 2H,
-C(CH3)HCH2-), 2.03 (m, 2H, -CH2CHdCH2), 4.33 (m, 1H, -OC-
(CH3)HCH2-), 4.94 (d, 1H, -CHdCH2, cis), 5.00 (d, 1H, -CHdCH2,
trans), 5.25 (s, 1H, ArOH), 5.81 (m, 1H, -CHdCH2), 6.96 (d, 1H,
5-H, Jo ) 9 Hz), 7.02 (dd, 1H, 6-H, Jm ) 3 Hz, Jo ) 9 Hz), 7.28 (d,
1H, 2′-H, Jm ) 3 Hz).
4′′-[(R)-(-)-2-(9-Decenyl)oxy]-3-nitrophenyl 4-[4′-(11-Undecen-
yloxy)phenyl]benzoate (10). 9 (1.84 g, 6.24 mmol), 2 (2.45 g, 6.69
mmol), dicyclohexyl carbodiimide (DCC) (1.71 g, 8.28 mmol), and
4-(dimethylaminopyridine) (DMAP) (0.12 g, 0.96 mmol) were dissolved
in CH2Cl2. The reaction was stirred for 48 h, then cooled, filtered,
and purified by column chromatography (silica gel, hexane/CH2Cl2 as
eluent). The final product was liquid-crystalline (Table 1). Yield:
3.13 g (78%). 1H NMR (CDCl3): d ) 1.32 (m, 20H, -CH2(CH2)4-
CH2- and -CH2(CH2)6CH2-), 1.37 (d, 3H, -C(CH3)H-), 1.60 (m,
2H, -OC(CH3)HCH2-), 1.77 (m, 2H, ArOCH2CH2-), 2.03 (m, 4H,
-CH2CHdCH2), 4.01 (t, 2H, ArOCH2-), 4.51 (m, 1H, -OC(CH3)-
HCH2-), 4.94 (d, 2H, -CHdCH2, cis), 5.00 (d, 2H, -CHdCH2, trans),
5.81 (m, 2H, -CHdCH2), 6.99 (d, 2H, 3′-H and 5′-H), 7.09 (d, 1H,
5′′-H, Jo ) 9 Hz), 7.40 (dd, 1H, 6′′-H, Jm ) 3 Hz, Jo ) 9 Hz), 7.60 (d,
2H, 2′-H and 6′-H), 7.70 (d, 2H, 2-H and 6-H), 7.73 (d, 1H, 2′′-H, Jm
) 3 Hz), 8.20 (d, 2H, 3-H, and 5-H).
4′′-[(R)-(-)-2-(10-hydroxydecyl)oxy]-3-nitrophenyl 4-(4′-(11-Hy-
droxyundecyloxy)phenyl)benzoate (11). 10 (3.08 g, 4.78 mmol) was
dissolved in THF (15 mL, anhydrous) and stirred for 30 min under an
Ar(g) atmosphere. 9-BBN (20.1 mL, 10.05 mmol) (THF, 0.5 M) was
added. After 3 h of stirring, EtOH (3 mL) and subsequently H2O2 (3
mL, 30% in H2O) were added. The mixture, heated to 50 °C for 1 h,
was poured into water. The aqueous phase was extracted three times
with CH2Cl2, and the organic phases were combined and dried with
MgSO4. The solvent was removed by evaporation, and the remaining
product was purified by column chromatography (silica gel, hexane/
CH2Cl2 as eluent). The final product was liquid-crystalline, but no
thermal characterization was performed. Yield: 1.10 g (34%). 1H
NMR (CDCl3): d ) 1.32 (m, 24H, -CH2(CH2)5CH2- and -CH2(CH2)7-
CH2-), 1.37 (d, 3H, -C(CH3)H-), 1.58 (m, 6H, -CH2CH2OH and
-OC(CH3)HCH2-), 1.77 (m, 2H, ArOCH2CH2-), 3.63 (t, 4H, -CH2-
OH), 4.01 (t, 2H, ArOCH2-), 4.51 (m, 1H, -OC(CH3)HCH2-), 6.99
(d, 2H, 3′-H and 5′-H), 7.09 (d, 1H, 5′′-H, Jo ) 9 Hz), 7.40 (dd, 1H,
6′′-H, Jm ) 3 Hz, Jo ) 9 Hz), 7.60 (d, 2H, 2′-H and 6′-H), 7.70 (d, 2H,
2-H and 6-H),7.73 (d, 1H, 2′′-H, Jm ) 3 Hz), 8.20 (d, 2H, 3-H and
5-H).
2.6. Measurement of the Pockels Effect. The Pockels effect was
measured in transmission by a crossed polarizer method.16 The electro-
optic modulation of the phase difference between the incoming optical
field components parallel to and perpendicular to the plane of incidence
was converted into intensity modulation by a polarization analyzer. A
Soleil-Babinet compensator placed between the crossed polarizers was
adjusted so that the transmitted intensity was half the maximum
intensity, in order to achieve the maximum linear detection range. The
experiments were carried out by applying a 375 Hz sinusoidal voltage
with a maximum peak-to-peak voltage of 20 V over the 2 µm electrode
gap.
4′′-{(R)-(-)-2-[(10-Acryloyloxy)decyl]oxy}-3-nitrophenyl 4-{4′-
[(11-Acryloyloxy)undecyloxyphenyl}benzoate (A2c). Acryloyl chlo-
ride (0.37 g, 4.01 mmol), dissolved in dry THF (10 mL) was added
dropwise to a stirred solution of 11 (1.05 g, 1.54 mmol) and
triethylamine (0.47 g, 4.63 mmol) in dry THF (10 mL) at 0 °C. After
12 h, the reaction mixture was poured into a NH4Cl solution (15%).
The aqueous phase was extracted three times with CH2Cl2. The
collected organic phases were combined and dried with MgSO4.
Evaporation of the solvent gave a residue which was purified by column
chromatography (silica gel, hexane/EtOAc as eluent) and recrystallized
from EtOH to give a slightly brown liquid crystalline product (Table
1). Yield: 0.40 g (33%). [R]D ) -13.5° (c 0.02, CH2Cl2). 1H NMR
(CDCl3): d ) 1.32 (m, 24H, -CH2(CH2)5CH2- and -CH2(CH2)7CH2-),
1.37 (d, 3H, -C(CH3)H-), 1.68 (m, 6H, -CH2CH2OCO- and -OC-
(CH3)HCH2-), 1.77 (m, 2H, ArOCH2CH2-), 4.01 (t, 2H, ArOC-
H2-), 4.15 (t, 4H, -CH2CH2OCO-), 4.51 (m, 1H, -OC(CH3)HC-
H2-), 5.82 (d, 2H, CH2dCH-, cis), 6.12 (d, 2H, CH2dCH-), 6.37
(d, 2H, CH2dCH-, trans), 6.99 (d, 2H, 3′-H and 5′-H), 7.09 (d, 1H,
5′′-H, Jo ) 9 Hz), 7.40 (dd, 1H, 6′′-H, Jm ) 3 Hz, Jo ) 9 Hz), 7.60 (d,
3. Results and Discussion
3.1. Design of the Monomer. The monomer (A2c) was
designed to generate a cross-linkable ferroelectric liquid crystal
intended for second-order nonlinear optics. Compound I which
is a ferroelectric liquid crystal for second-order nonlinear optics
was therefore substituted with two acrylate groups. The cross-
linkable monomer A2c requires a bifunctional nonracemic chiral
spacer 7.
3.2. Synthesis of the Monomer. The synthesis of 4′′-{(R)-
(+)-2-[(10-acryloyloxy)decyl]oxy}-3-nitrophenyl 4-{4′-[(11-
acryloyloxy)undecyloxy]phenyl}benzoate A2c is outlined in
Scheme 1. In the synthesis of this compound, several selective
reactions have been used.
The syntheses of 1 and 2 have been reported elsewhere.11
La(NO3)3 was used as catalyst in the aromatic ortho nitration
of the phenol 4. The two-phase reaction gives almost exclu-

8546 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Trollsa˚s et al.
sively ortho nitration when para-substituted phenols are used,
1
which was confirmed by H NMR.17
The 1,2-epoxy-9-decene was reduced in an anti-Markovnikov
fashion to yield the racemic secondary alcohol 5. This was
done with a LiBH4-MeOH reducing system described by Soai
et al.18 The alcohol was then resolved kinetically with S-methyl
thioacetate as acyl donor and the Candida antarctica lipase B
as catalyst. The remaining substrate 7 was isolated in a 40%
yield (50% theoretical maximum) with high enantioselectivity.
The enantiomeric excess of the alcohol, ees, was >99.5% (only
one enantiomer detected) and that of the product, eep, was 97.2%
as determined by chiral GC. This gave a conversion (c ) ees/
{ees + eep})13 of 51% which is optimal for maximizing the
optical purity and chemical yield of the remaining substrate. 7
showed positive optical rotation, and the absolute configuration
of the alcohol was predicted to be (S) by 2-octanol analogy
and the empirical rule of lipase selectivity.15
Figure 4. Polarization (P) obtained on cooling as a function of
temperature for A2c.
The nitro-substituted phenol 5 was coupled to the chiral
alcohol 7 in the Mitsunobo synthesis using diethylazodicar-
boxylate (DEAD)/triphenylphosphine (TPP) which proceeds by
inversion of the configuration, converting the S-alcohol to an
R-ester. The chiral moiety 8 was then deprotected by KOH.
8 was subsequently coupled with 2 using dicyclohexylcar-
bodiimide (DCC) as dehydrating agent and dimethylaminopyr-
idine (DMAP) as catalyst. The product 10 was hydroborated
with 9-BBN and subsquently oxidized by H2O2 using a modified
procedure first reported by Sahle´n et al.11 The bifunctional
monomer A2c was finally prepared by the esterification of the
diol 11 using acryloyl chloride.
3.3. Liquid-Crystalline Properties of the Monomer.
3.3.1. Thermal Characterization. The thermal transitions of
the monomer A2c and of the intermediate substance 11 are
shown in Table 1. The DSC thermogram of monomer A2c
revealed two different mesophases. On cooling A2c from the
isotropic state, a focal-conic fan texture typical of “smectics”,
probably SA*, was formed at 34 °C. Further cooling caused a
phase transition, as observed in the DSC, which was not
observed in the polarized microscope. X-ray scattering and
electro-optical methods indicated that this transition involved
SA* and SC* phases. At 8 °C, the focal-conic fan texture started
to transform into a texture which was identified as a crystalline
phase. The liquid-crystalline phases of A2c appeared both on
cooling and heating, i.e., they were enantiotropic. However,
on heating, the liquid-crystalline phases were present in only
a narrow temperature range. The monomer A2c was thermally
stable and did not polymerize spontaneously on heating to 140
°C.
a sharp ring was reflected representing a layer thickness of 33
Å. Assuming that the layer structure is unchanged the calculated
tilt angle is θ ) 29.7° for the SC* phase.
3.3.2. Ferroelectric Response. The cell containing the pure
monomer A2c was slowly cooled from the SA* to the SC* phase,
preserving the uniform alignment achieved in the SA* phase.
A2c showed a pronounced ferroelectric response in the SC*
phase (Figure 4). The behavior of the monomer was according
to expectation where the polarization (P) was greatly increased
by lowering the temperature.
The ferroelectric switching at 22 °C demonstrated in Figure
5a (lower curve) followed the applied triangular ac field (upper
curve) in a manner typical of the SC* phase. The center curve
in Figure 5a shows the current response of the cell with its
characteristic peak at the time of the molecular switch. Figure
5b displays a typical hysteresis loop of the spontaneous
polarization in the SC* phase at 26 °C. The amplitude of the
hysteresis loop was measured from the oscilloscope and was
used to calculate the spontaneous polarization according to ref
12.
The A2c monomer has a maximum spontaneous polarization
P ) 175 nC/cm2, which is six times higher than that in the
monomer mixture used for the preparation of the cross-linked
polymer, III. This increase was due to the higher concentration
of chiral monomers compared to that in the mixture. These
dipoles are responsible for the spontaneous polarization and most
probably also for the second-order nonlinear optical response.
An increase in the spontaneous polarization can only occur if
chiral molecules are used.10 The spontaneous polarization is
in the same range as reported for compounds I and II.3,4
The spontaneous polarization depends on the tilt angle θ
which influences the length of the helical pitch po. Monomer
A2c has a large tilt angle θ ) 33° as observed in the microscope
and was also found to have a long response time τr ≈ 100 µs.
The tilt angle θ is of the order of the tilt angle calculated from
the layer-distances observed by X-ray scattering. The pitch was
estimated to be po ≈ 0.4 µm, since selective reflection of blue
light was observed at room temperature. The short pitch
corresponds well to the relatively long response time observed.
3.4. Properties of the Cross-Linked Liquid Crystal. The
texture of the sample was preserved during the polymerization
(Figure 6). No change in the initially uniform bookshelf texture
or in the position of the optical axis could be observed in the
microscope as a result of illumination. The tilt angle θ ) 33°
was thus preserved during the process. The cross-linked
polymers did not exhibit any ferroelectric response. The absence
of ferroelectric switching indicated that the uniform molecular
X-ray scattering patterns of the SA* phase of A2c consisted
of diffuse wide-angle reflections and a sharp inner ring
reflection, corresponding to a layer thickness of 38 Å. Simple
calculations indicate that the 38 Å layer thickness is consonant
with a monolayer smectic A phase. The length of the mesogenic
group including the ether oxygens is 16 Å. The length of two
spacer groups assuming a trans content of 70% is 16 Å.19 The
repeating distance of a perfectly smectic monolayer is the sum
of the length of one mesogenic group, two spacer groups, and
two acrylate groups (7 Å), i.e., 40 Å, which is only slightly
more than the 38 Å obtained by X-ray diffraction.
The X-ray scattering of the low temperature phase of A2c
consisted of a diffuse wide-angle reflection corresponding to
an intermesogenic distance of about 4 Å, and, at small angles,
(13) Rakels, J.; Straathof, A; Heinen, J. Enzyme Microb. Technol. 1993,
15, 1051.
(14) Chen, C.; Sih, C. Angew. Chem., Int. Ed. Engl. 1989, 28, 695.
(15) Kazlauskas, R. J.; Weissfloch, A. N.; Rappaport, A. T.; Cuccia, L.
A. J. Org. Chem. 1991, 56, 2656.

A Cross-Linked Pyroelectric Polymer
J. Am. Chem. Soc., Vol. 118, No. 36, 1996 8547
Figure 7. The modulated refractive index (Pockels effect), measured
as the difference in light intensity (ac) as a function of voltage in poly-
(A2c) at room temperature.
Figure 8. Configuration of the surface-stabilized ferroelectric liquid
crystal (SSFLC) cell utilized in the NLO experiments (ITO ) indium
tin oxide).
1.556, according to the m-line method.16 The material is
assumed to be almost uniaxial, ny ) nx ) no and nz ) ne. If
the isotropic refractive index is approximated by n ) (2no +
ne)/3 and the birefringence is approximately 0.1 (visual inspec-
tion), the ordinary refractive index, no ) 1.52, is obtained.
In Figure 7, ∆I/I (I is the intensity transmitted through the
set-up when E ) 0) is plotted as a function of the electric field.
The modulated intensity shows a linear dependence on the
applied E field, as expected for the Pockels effect (Figure 7).
At an incidence angle of 26°, values of the difference between
the electro-optical coefficients (r22 - r12) of 15-35 pm/V were
obtained. This is of the same order as that of organic crystals.20
The dominant coefficient is expected to be r22, since it is directed
along the polar axis.
Figure 5. Photographs taken from the oscilloscope; (a, top) The optical
and current response (bottom and middle) to a triangular electric field
(top) (22°) and (b, bottom) the hysteresis loop used for measuring the
spontaneous polarization of A2c in the SC* phase (26°).
In the SSFLC-cell utilized for the experiments, the smectic
layers have a bookshelf organization (Figure 8). Polyimide,
rubbed in the z-direction, orients the smectic layers parallel to
the xy-plane, perpendicular to the z-axis. This geometry makes
only two orientations of the molecules possible; alignment of
the molecules with their effective dipole moment in either the
positive or the negative y-direction. The molecular dipole
moments were forced to align in one of the two possible
directions by the applied electric field, which after polymeri-
zation gave the polymer a C2 symmetry. The effective global
dipole moment of the polymer network had its orientation
parallel to the y-axis, perpendicular to the glass/electrode plates.
The magnitude of the electro-optical coefficient was, as
expected, greatest in the direction of the polar axis.
Figure 6. Polarized photomicrograph of the polar aligned and cross-
linked poly (A2c).
tilt in the SC* phase became fixed during the polymerization
and that the macroscopic polarization of the sample (in a
direction perpendicular to the cell glass plates) was made
permanent.
The polymer network formed showed good thermal stability.
Heating above the isotropization temperature of A2c caused no
changes in the molecular tilt of the network, nor were any
textural changes revealed in the optical microscope.
3.4.1. Nonlinear Optical Properties. The refractive index
of the isotropic material polymerized on a glass substrate was
The Pockels effect depends on the material’s macroscopic
susceptibility ø(2) and is observed as a modification of its
refractive index when a low frequency electrical field is
applied.21 The applied electric field induces a change in the
(16) van der Vorst, C. P. J. M.; van Weerdenburg, C. J. M. Nonlinear
Optical Properties of Organic Materials III, SPIE 1990, 1337, 246.
(17) Ouertani, M.; Girard, P.; Kagan, H. B. Tetrahedron Lett. 1982, 23,
4315.

8548 J. Am. Chem. Soc., Vol. 118, No. 36, 1996
Trollsa˚s et al.
refractive index according to
In order to simplify the analysis, the term r52 corresponding
to a tilt of the refractive index ellipsoid due to the applied electric
field was neglected. Measuring the second harmonic generation
(SHG) in a ferroelectric liquid crystal, Schmitt and co-workers4
found a ratio of approximately 10:1 between the second-order
nonlinear optic coefficients d22 and d25. If the ratio between
the electro-optic coefficients in this material is the same as that
observed by Schmitt et al.4 between the second harmonic
coefficients, neglecting r52 will not seriously affect the results.
If this assumption is valid, the increase in the uncertainty in
(r22 - r12) will be less than 10%.
The linearity in Figure 7 could in principal also arise from a
motion of the optical axis in the electric ac field. This would
be the case if the monomers were free to move as in the
ferroelectric state. The sample was therefore examined in other
areas where the material was not made permanent in a polar
organization. These areas showed no detectable modulation of
the intensity. The Pockels coefficient obtained is therefore
believed to be of true electronic origin, but this has to be
confirmed by measurement of the second harmonic generation.
If the nonlinear optic response has a purely electronic origin,
the coefficients r and d are directly inter-related:21
1
∆
) rE
(1)
2
( )
n
where r is the electro-optic tensor. For a material with C2
symmetry, the tensor is given by
r12
r22
r32
0
0
0
r41
0
0
0
(2)
r43
0
r52
(
)
0
r61
0
r63
0
Assuming Kleinman degeneracy to be valid, the number of
independent tensor components is reduced to four: r41
r52 ) r63, r32 ) r43, and r12 ) r61. Thus, the modulation of
the birefringence causes a modulation of the phase retarda-
tion, ∆Φ. If the modulated phase retardation is small, it
can be related to the detected intensity variations by
)
-4π
n4
r )
d
(5)
1
2
∆I ) Imax∆Φ
(3)
The values of the d-coefficients, expected from SHG, should
be 6.4-14.9 pm/V, calculated for a refractive index of 1.52.
If the incoming polarized light propagates in the xy-plane
with the electric field vector in the same plane, the phase
retardation, modulated by an applied electric field parallel
to the y-axis, is
Conclusions
A new pyroelectric polymer exhibiting a large Pockels effect
(r22 - r12 ) 15-35 pm/V) was prepared by in-situ photo-cross-
linking of a novel ferroelectric monomer. The large nonlinear
optical activity, in the range of organic single crystals, is very
encouraging, but it has to be confirmed by measurement of the
second harmonic generation. The magnitude of the signal makes
the new material interesting for use in electro-optic devices.
3
πnï
2π
λ
l
d
∆Φ )
∆nl ≈
(r22 - r12) sin2 θ
V
(4)
λ
where l is the wavelength, l is the length in the polymer
traversed by the light, d is the distance between the
electrodes, and V is the applied voltage.
Acknowledgment. Financial support from the Swedish
Board for Technical and Industrial Development (NUTEK;
Grant 86-03476P) and from the Swedish Natural Science
Research Council (NFR; Grant K-KU 1910-305) is gratefully
acknowledged. The provision of the enzyme by NOVO-Nordisk
A/S, Denmark, is gratefully acknowledged.
(18) Soai, K.; Ookawa, A. J. Org. Chem. 1986, 51, 4000.
(19) Hellermark, C.; Gedde, U. W.; Hult, A.; Boeffel, C.; Boyd, R. H.;
Liu, F. Macromolecules 1996, in press.
(20) Kondo, T.; Morita, R.; Ogasawara, N.; Umegaki, S.; Ito, R. Jpn. J.
Appl. Phys. 1989, 28, 1622.
(21) Prasad, P. N.; Williams, D. J. Introduction to Nonlinear Optical
Effects in Molecules & Polymers; John Wiley & Sons: New York, 1991.
JA961309E