Photo-isomerizable derivatives of phenylethanediol and cinnamic acid: Useful compounds for single-layer R, G, and B cholesteric color filters

Article abstract of DOI:10.1080/15421400600700299

This article describes the synthesis of derivatives of (R)-phenylethanediol and cinnamic acid. These molecules are photo-isomerizable and contain two acrylate groups that can form densely cross-linked cholesteric polymer films. The Z-isomers formed by irradiation of the E-isomeric cinnamic acid derivatives exhibit a much lower helical twisting power than that of the E-isomers. In this way, the reflection wavelength of cholesteric layers made with these molecules can be increased by irradiation. After polymerization, the cholesteric films are thermally stable and hence these molecules are suitable for use in cholesteric color filters that find application in liquid-crystalline displays.

Full text of DOI:10.1080/15421400600700299

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Photo-Isomerizable Derivatives
of Phenylethanediol and
Cinnamic Acid: Useful
Compounds for Single-Layer
R, G, and B Cholesteric Color
Filters
J. Lub a , M. P. Ezquerro a & B. Malo a
a Philips Research Laboratories , Eindhoven, the
Netherlands
Published online: 31 Jan 2007.
To cite this article: J. Lub , M. P. Ezquerro & B. Malo (2006) Photo-Isomerizable
Derivatives of Phenylethanediol and Cinnamic Acid: Useful Compounds for Single-
Layer R, G, and B Cholesteric Color Filters, Molecular Crystals and Liquid Crystals,
457:1, 161-180, DOI: 10.1080/15421400600700299
To link to this article: http://dx.doi.org/10.1080/15421400600700299
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Mol. Cryst. Liq. Cryst., Vol. 457, pp. 161–180, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400600700299
Photo-Isomerizable Derivatives of Phenylethanediol
and Cinnamic Acid: Useful Compounds for Single-Layer
R, G, and B Cholesteric Color Filters
J. Lub
M. P. Ezquerro
B. Malo
Philips Research Laboratories, Eindhoven, the Netherlands
This article describes the synthesis of derivatives of (R)-phenylethanediol and
cinnamic acid. These molecules are photo-isomerizable and contain two acrylate
groups that can form densely cross-linked cholesteric polymer films. The Z-isomers
formed by irradiation of the E-isomeric cinnamic acid derivatives exhibit a much
lower helical twisting power than that of the E-isomers. In this way, the reflection
wavelength of cholesteric layers made with these molecules can be increased by
irradiation. After polymerization, the cholesteric films are thermally stable and
hence these molecules are suitable for use in cholesteric color filters that find
application in liquid-crystalline displays.
Keywords: cholesteric liquid crystals; color filters; photo isomerisation; photo
polymerisation
1. INTRODUCTION
Considerable attention has been given to cholesteric liquid-crystalline
materials because of their ability to reflect circularly polarized light [1].
The development of photo-isomerisable chiral molecules in particular
is of interest [2]. Such materials find application in the production of
cholesteric color filters for use in liquid-crystalline displays because
of the color change induced by ultraviolet (UV) light. Cholesteric films
made with menthone derivative 1 (Fig. 1), for instance, change the
color by means of E-Z photo-isomerization. The reflection wavelength
Address correspondence to J. Lub, Philips Research Laboratory, Prof. Holstlaan 4,
5656 AA Eindhoven, the Netherlands. E-mail: johan.lub@philips.com
161

162
J. Lub et al.
FIGURE 1 Structures of the chiral compounds.
(k) of a cholesteric layer made from a mixture of 1 and nematic
molecules is, therefore, defined by [3]
ꢁ1
ꢀ
ꢀ
k ¼ n ꢀ p ¼ nðHTPE xE þ HTPz xzÞ
;
ð1Þ

Phenylethanediol and Cinnamic Acid Derivatives
163
where n is the mean refractive index of the cholesteric mixture; p is
the pitch of the helix of the cholesteric structure; xE is the weight frac-
tion of 1 (the E-isomer) present in the cholesteric mixture; xZ is the
weight fraction of photo-isomerized material (Z-isomer, obtained
through photo-isomerization of 1); HTPE is the so-called helical twist-
ing power, which is a property of 1, that is, the reciprocal of the pitch of
the helix for xE ¼ 1; and HTPZ is the helical twisting power of the Z-
isomer of 1. The change in conformation upon E-Z photo-isomerization
of 1 causes the reflection wavelength to increase, because the HTPE of
this compound is approximately 19 mmꢀ1, whereas HTPZ is approxi-
mately 3 mmꢀ1 [4].
Thermal and UV stable films are required to produce cholesteric
color filters. Therefore, compound 1 was added to a mixture of nematic
di-acrylates and a photo-initiator [5,6]. When a substrate provided
with a rubbed alignment layer is coated with the mixture, a blue
reflecting film is formed. The photo-isomerization reaction was
effected in a single UV irradiation step with the aid of a grayscale
mask, with patterned areas having 100%, 0%, and intermediate
transmission, leading to the red, blue, and green pixels, respectively
[5,7]. The colors can be optimized through use of the correct
irradiation dose for the green and red pixels. Subsequent photo-poly-
merization of the film containing the pixelated reflection colors leads
to the formation of a stable cross-linked material. The process con-
ditions were chosen to prevent the two photochemical processes (i.e.,
isomerization and polymerization from interfering with each other
[5]. Photopolymerization will not start if air is present because of
the inhibitive effect of oxygen on the acrylate polymerization reaction.
The combination of this effect and the relatively low UV intensity
needed for isomerization results in ready formation of the colors in
air without any noticeable polymerization. The photopolymerization
reaction was effected in an inert atmosphere (nitrogen or argon), in
which it proceeded at a much higher rate than the isomerization pro-
cess. This led to rapid fixation of the patterned cholesteric structure
with no changes in its optical properties. The cross-linking resulted
in a material that is stable under UV light and at elevated tempera-
tures. These cholesteric color filters reflect colors of high color purity
and appear to be interesting materials for new generations of liquid-
crystalline displays (LCDs) [8,9].
However, the manufacture of LCDs imposes even more severe
demands on the stability of the cholesteric color filter, as it involves
high-temperature treatments (typically 200ꢁC), such as Indium Tin
Oxide (ITO) deposition directly onto the color filter, followed by the
application and baking of a polyimide film on top of the ITO. During

164
J. Lub et al.
these processes, degradation of the filters was observed. This degradati-
on is partly attributable to the thermal instability of the menthone-
derived chiral moiety in 1 and partly attributable to the fact that this
chiral compound is a mono-acrylate and hence leads to the formation
of polymer networks with relatively low cross-link density upon photopo-
lymerization [10,11]. In previous publications, we reported several alter-
native structures for compound 1, in which the menthone moiety was
replaced by various derivatives of cyclohexanone [12–14]. This previous
research led to a better understanding of the structure–property
relationship of these chiral materials, but most of them are either diffi-
cult to synthesize or exhibit HTP values that are too low for them to be
used in color-filter manufacturing. We therefore searched for other mole-
cules that are more stable. In addition, we anticipated that it would be
advantageous to functionalize these new compounds with two acrylate
groups rather than one. The increase in cross-link density would
improve the thermal stability of cholesteric color filters.
Di-esters of phenylethanediol such as 2 are compounds with HTP
values of up to 40 mmꢀ1 [15]. Compounds containing two polymerizable
groups with these structures were applied in cholesteric mixtures that
formed polymeric networks reflecting visible light after photopolymer-
ization [16,17]. Di-acrylates derived from isosorbide as chiral moiety
and cinnamic acid as isomerizable moiety have been prepared and
shown to be suitable for use in cholesteric color-filter manufacturing
[11,18]. Thus, the cinnamic moiety is a useful isomerizable group.
To study the suitability of structures derived from phenylethanediol
and cinnamic acid for use in cholesteric color filters, we decided to
synthesize and investigate the properties of compounds 3, 4, 5, and
6 (Fig. 1). These compounds are all structural isomers of each other.
By comparing the properties of these compounds, the effect of the posi-
tions of both the phenyl group and the isomerizable cinnamate group
in the molecule on the helical twisting power and on the photo-induced
change in the helical twisting power can be studied.
Chiral compounds derived from menthone, such as 1, or from iso-
sorbide, described earlier for use in cholesteric color filters, form
left-handed and right-handed helices, respectively [4,11]. It is very dif-
ficult to obtain cholesteric layers with an inverse helix because the
mirror enantiomers of the chiral starting compounds, menthone and
isosorbide, either cannot be obtained or are very difficult to prepare.
For complex optical stacks, having freedom of choice in terms of the
handedness of the helix may be advantageous [19]. Both enantiomers
of the compounds described here are easily accessible because the
materials originate from the readily available (R) and (S) mandelic
acid. Thus, cholesteric layers with right-or left-handed helix layers

Phenylethanediol and Cinnamic Acid Derivatives
165
can be made with equal ease using these types of compounds. Mole-
cules 3, 4, 5, and 6 are suitable for replacing the menthone-derived
compound 1 in cholesteric mixtures. Some properties of cholesteric
films made with these new di-acrylates are presented.
2. EXPERIMENTAL
2.1. Materials and Methods
Literature procedures were used to obtain 4-(6-acryloyloxy-hexyloxy)-
benzoic acid (7a) [20], 4-(6-acryloyloxyhexyloxy)cinnamic acid (7b) [21],
4-(4-(6-acryloyloxyhexyloxy)benzoyloxy)benzoic acid, (10a) [11,18],
and (R)-2-phenyl-2-(tetrahydro-pyran-2-yloxy)-ethanol (14) [22].
RM82 {(4-(6-acryloyloxyhexyloxy)benzoyloxy)-2-methylphenyl 4-(6-
acryloyloxyhexyloxy)benzoate} and RM257 {(4-(3-acryloyloxypropyloxy)
benzoyloxy)-2-methylphenyl 4-(3-acryloyloxypropyloxy)benzoate} were
obtained from Merck. Darocur1. 4265 photo-initiator was obtained
from Ciba Specialty Chemicals. All the other chemicals were obtained
from Aldrich.
Absorption UV spectra were recorded in an acetonitrile solution
using a Unicam UV2-100 spectrometer. The transmission spectra of
the cholesteric films were recorded using a Perkin-Elmer spectrometer
equipped with a combination of a linear polarizer and an achromatic
k=4 film.
NMR spectra were recorded with a Bruker DPX300 spectrometer
in a deuterated dichloromethane or deuterated chloroform solution.
1
The H and 13C NMR data were fully consistent with the required
structures and confirmed the purity of all the final products. The spec-
1
tra were interpreted with the aid of 2D 13Cꢀ H correlation spectra.
The isomerization measurements were done in dichloromethane.
Irradiation of the NMR tubes was performed with a PL10 light source
(Philips, kmax ¼ 365 nm) or a TL08 light source (Philips, broadband,
k ¼ 350 nm).
FTIR spectra were recorded in a chloroform solution with an ATI
Mattson Genesis II spectrometer. Compounds 3, 4, 5, and 6 showed
similar spectra with the following peaks (in cmꢀ1): 2939 (CH2), 1732
=
=
(C O, benzoate, cinnamate, and acrylate), 1637 (C C), 1605 and
1511 (aromatic rings), 1408 (CH2-acrylate), 1250 (O-aromatic ring),
=
and 984 (CH CH2).
Matrix Assisted Laser Desorption Ionisation Time of Flight (MALDI-
TOF) mass spectra were recorded on a Voyager-De Pro machine using
a-cyano-4-hydroxycinnamic acid as the matrix. Calculated for
C49H52O12 (compounds 3, 4, 5, and 6) 832.35; found 832.32 ꢂ 0.03.

166
J. Lub et al.
2.2. Synthesis
2.2.1. Synthesis of Ethoxymethyl 4-hydroxycinnamate (8b)
Chloromethyl ethyl ether (6.3 ml, 100 mmol) in 40 ml of dichloro-
methane was added dropwise to a solution of 16 g (100 mmol) of
4-hydroxycinnamic acid and 14 ml (100 mmol) of triethylamine in
80 ml of dichloromethane. The mixture was stirred overnight at room
temperature. The mixture was extracted subsequently with 60 ml of
water, with 60 ml of a 2.4 M HCl solution and with 60 ml of a saturated
solution of sodium bicarbonate. After drying over magnesium sul-
phate, the solvent was evaporated, and 18.7 g of clear oil (84%), that
slowly crystallized, were obtained.
2.2.2. Synthesis of 4-(4-(6-Acryloyloxyhexyloxy)benzoyloxy)-
cinnamic Acid 10b
N,N-Dicyclohexyl carbodiimide (15.7 g, 75 mmol) was added to a
mixture of 18.7 g (75 mmol) of ethoxymethyl 4-hydroxycinnamate
(8b), 22.2 g (75 mmol) of 4-(6-acryloyloxy-hexyloxy)-benzoic acid (7a),
0.93 g (7.6 mmol) of 4-N,N-dimethylaminopyridine, and 200 ml of
dichloromethane cooled in an ice-water bath. After removing the ice-
water bath, the mixture was stirred at room temperature overnight.
The mixture was filtered through a thin layer of silica and evaporated.
The intermediate ethoxymethyl 4-(4-(6-acryloyloxyhexyloxy)benzoy-
loxy)cinnamate (9b) obtained after crystallization from ethanol was
dissolved in 60 ml of hot ethanol together with 0.75 g (3 mmol) of
pyridinium 4-toluenesulfonate. After stirring for 13 h at 60ꢁC, 12.7 g
of a white solid (40%) crystallized upon cooling to room temperature.
2.2.3. Synthesis of (R)-2-Hydroxy-2-phenylethyl 4-(4-(6-
Acryloyloxyhexyloxybenzoyloxy)benzoate (12a)
N,N0-Dicyclohexyl carbodiimide (2.1 g, 10 mmol) was added to a
mixture of 2.2 g (10 mmol) of (R)-2-phenyl-2-(tetrahydro-pyran-2-
yloxy)-ethanol (14), 4.1 g (10 mmol) of 4-(4-(6-acryloyloxyhexyloxy)
benzoyloxy)benzoic acid (10a), 0.12 g (1.0 mmol) of 4-N,N-dimethyla-
minopyridine, and 50 ml of dichloromethane stirred in an ice-water
bath. After 2.5 h the ice bath was removed, and the mixture was stir-
red at room temperature for 24 h. The reaction mixture was filtered
over a thin silica layer and evaporated. The crude intermediate (R)-
2-(tetrahydropyran-2-yloxy)-2-phenylethyl 4-(4-(6-acryloyloxyhexylox-
ybenzoyloxy)benzoate (11a) obtained as an oil was dissolved in 35 ml
of ethanol. Then 0.25 g (1.0 mmol) of pyridinium 4-toluensulfonate
was added, and the mixture was heated at 60ꢁC for 15 h. A white solid
(3.2 g, 60%) precipitated upon cooling to 5ꢁC.

Phenylethanediol and Cinnamic Acid Derivatives
167
2.2.4. Synthesis of (R)-2-Hydroxy-2-phenylethyl 4-(4-(6-
Acryloyloxyhexyloxybenzoyloxy)cinnamate (12b)
This product was obtained as a solid with 55% yield, in the
same manner as described for (R)-2-hydroxy-2-phenylethyl 4-(4-(6-
acryloyloxyhexyloxybenzoyloxy)benzoate (12a) starting from 4-(4-(6-
acryloyloxyhexyloxy)benzoyloxy)cinnamic acid (10b) instead of
4-(4-(6-acryloyloxyhexyloxy)benzoyloxy)benzoic acid (10a).
2.2.5. Synthesis of (R)-2-Hydroxy-2-phenylethyl 4-(6-
Acryloyloxyhexyloxy)benzoate (16a)
N,N0-Dicyclohexyl carbodiimide (4.1 g, 20 mmol) was added to a mix-
ture of 4.4 g (20 mmol) of (R)-2-phenyl-2-(tetrahydro-pyran-2-yloxy)-
ethanol (14), 5.8 g (20 mmol) of 4-(6-acryloyloxyhexyloxy)benzoic acid
(7a), 0.24 g (2.0 mmol) of 4-N,N-dimethylaminopyridine, and 80 ml of
dichloromethane stirred in an ice-water bath. After 2.5 h, the ice bath
was removed, and the mixture was stirred at room temperature for
24 h. The reaction mixture was filtered over a thin silica layer and eva-
porated. The crude intermediate (R)-2-(tetrahydropyran-2-yloxy)-
2-phenylethyl 4-(6-acryloyloxyhexyloxy)benzoate (15a) obtained as
an oil was dissolved in 50 ml of ethanol. Then 1.0 g (4.1 mmol) of pyr-
idinium 4-toluensulfonate was added, and the mixture was heated
at 60ꢁC for 15 h. After evaporation of the ethanol, the mixture was
purified by column chromatography (silica=dichloromethane–ethyl
acetate ¼ 94:6). The product (4.2 g, 51%) was obtained as an oil.
2.2.6. Synthesis of (R)-2-Hydroxy-2-phenylethyl 4-(6-
Acryloyloxyhexyloxy)cinnamate (16b)
This product was obtained as a solid with 30% yield, in the same
manner as described for (R)-2-hydroxy-2-phenylethyl 4-(4-(6-acryloyloxy-
hexyloxybenzoyloxy)benzoate (12a) starting from 4-(6-acryloyloxy-
hexyloxy)cinnamic acid (7b) instead of 4-(4-(6-acryloyloxyhexyloxy)
benzoyloxy) benzoic acid (10a).
2.2.7. Synthesis of 2-(4-(6-Acryloyloxyhexyloxy)
cinnamoyloxy)-2-(R)-phenylethyl 4-(4-(6-
Acryloyloxyhexyloxy)benzoyloxy)benzoate (3)
N,N0-Dicyclohexyl carbodiimide (1.2 g, 6 mmol) was added to a
mixture of 3.2 g (6 mmol) of (R)-2-hydroxy-2-phenylethyl 4-(4-(6-
acryloyloxyhexyloxybenzoyloxy)benzoate (12a), 1.9 g (6 mmol) of
4-(6-acryloyloxyhexyloxy)cinnamic acid (7b), 0.07 g (0.6 mmol) of
4-N,N-dimethylaminopyridine, and 40 ml of dichloromethane stirred
in an ice-water bath. After 2.5 h, the ice bath was removed, and
the mixture was stirred at room temperature for 24 h. The reaction

168
J. Lub et al.
mixture was filtered over a thin silica layer and evaporated. The pro-
duct (4.0 g, 80%) was obtained after crystallization from ethanol.
1H NMR (d in ppm, relative to TMS, J in Hz): 8.13 (d, 2H, J ¼ 9.0,
Hk), 8.08 (d, 2H, J ¼ 9.0, Hm), 7.69 (d, 1H, J ¼ 16.2, Hv), 7.48 (d, 2H,
J ¼ 7.2, Hp), 7.47 (d, 2H, J ¼ 8.7, Hw), 7.40 (m, 3H, HrþHs), 7.27
(d, 2H, J ¼ 9.0, Hl), 6.97 (d, 2H, J ¼ 9.0, Hj), 6.88 (d, 2H, J ¼ 8.7,
Hx), 6.42 (dd, 2H, J1 ¼ 17.3, J2 ¼ 1.5, Ha), 6.39 (d, 1H, J ¼ 16.2, Hu),
6.32 (dd, 1H, J1 ¼ 7.5, J2 ¼ 4.5, Ho), 6.13 (dd, 2H, J1 ¼ 17.3, Hc),
5.82 (dd, 2H, J1 ¼ 10.5, J2 ¼ 1.5, Hb), 4.67 (dd, 1H, J1 ¼ 7.5,
J2 ¼ 12.1, Hn), 4.61 (dd, 1H, J1 ¼ 4.5, J2 ¼ 12.1, Hn,), 4.18 (t, 4H,
J ¼ 6.4, Hd), 4.05 (t, 2H, J ¼ 6.4, Hi), 3.98 (t, 2H, J ¼ 6.4, Hy), 1.82
(q, 4H, J ¼ 6.4, Hh), 1.72 (q, 4H, J ¼ 6.4, He), 1.49 (m, 8H, Hf þ Hg).
13C NMR (d in ppm, relative to TMS): 166.7 (C3 þ C26), 165.9 (C19),
164.7 (C14), 164.1 (C10), 161.5 (C32), 155.4 (C15), 145.8 (C28), 137.2
(C21), 132.8 (C12), 131.7 (C17), 131.0 (C1), 130.3 (C30), 129.1 (C23),
129.0 (C2 þ C24), 127.6 (C18), 127.2 (C29), 127.1 (C22), 122.3 (C16),
121.5 (C13), 115.2 (C27), 114.8 (C11 þ C31), 73.5 (C25), 68.5 (C9), 68.3
(C33), 67.2 (C20), 64.9 (C4), 29.4 (C8 þ C34), 28.9 (C5), 26.1 (C6 þ C7).
2.2.8. Synthesis of 2-(4-(6-Acryloyloxyhexyloxy)cinnamoyloxy)-
1-(R)-phenylethyl 4-(4-(6-Acryloyloxyhexyloxy)benzoyloxy)
benzoate (4)
This compound was obtained as a solid with 65% yield, in the same
manner as described for 2-(4-(6-acryloyloxyhexyloxy)cinnamoyloxy)-2-
(R)-phenylethyl 4-(4-(6-acryloyloxyhexyloxy)benzoyloxy)benzoate (3)
starting from 4-(4-(6-acryloyloxyhexyloxy)benzoyloxy)benzoic acid (10a)
and (R)-2-hydroxy-2-phenylethyl 4-(6-acryloyloxyhexyloxy)cinnamate
(16b).
1H NMR (d in ppm, relative to TMS, J in Hz): 8.19 (d, 2H, J ¼ 9.0,
Hk), 8.13 (d, 2H, J ¼ 9.0, Hm), 7.63 (d, 1H, J ¼ 16.2, Hv), 7.50 (d, 2H,
J ¼ 7.2, Hp), 7.44 (d, 2H, J ¼ 8.7, Hw), 7.40 (m, 3H, Hr þ Hs), 7.30 (d,
2H, J ¼ 9.0, Hl), 6.97 (d, 2H, J ¼ 9.0, Hj), 6.87 (d, 2H, J ¼ 8.7, Hx),
6.42 (dd, 2H, J1 ¼ 17.3, J2 ¼ 1.5, Ha), 6.33 (dd, 1H, J1 ¼ 8.3,
J2 ¼ 3.8, Hn), 6.28 (d, 1H, J ¼ 16.2, Hu), 6.13 (dd, 2H, J1 ¼ 17.3, Hc),
5.82 (dd, 2H, J1 ¼ 10.5, J2 ¼ 1.5, Hb), 4.64 (dd, 1H, J1 ¼ 8.3,
J2 ¼ 11.8, Ho), 4.55 (dd, 1H, J1 ¼ 3.8, J2 ¼ 11.8, Ho), 4.18 (t, 4H,

Phenylethanediol and Cinnamic Acid Derivatives
169
J ¼ 6.4, Hd), 4.05 (t, 2H, J ¼ 6.4, Hi), 3.98 (t, 2H, J ¼ 6.4, Hy), 1.82 (q,
4H, J ¼ 6.4, Hh), 1.72 (q, 4H, J ¼ 6.4, He), 1.49 (m, 8H, Hf þ Hg).
13C NMR (d in ppm, relative to TMS): 167.3 (C26), 166.7 (C3), 165.4
(C19), 164.7 (C14), 164.1 (C10), 161.4 (C32), 155.4 (C15), 145.7 (C28),
137.8 (C21), 132.8 (C12), 131.8 (C17), 131.0 (C1), 130.3 (C30), 129.1
(C23 þ C24), 129.0 (C2), 127.8 (C18), 127.2 (C29), 127.1 (C22), 122.3
(C16), 121.5 (C13), 115.2 (C27), 114.8 (C11 þ C31), 74.7 (C25), 68.5 (C9), 68.3
(C33), 66.4 (C20), 64.9 (C4), 29.4 (C8 þ C34), 28.9 (C5), 26.1 (C6 þ C7).
2.2.9. Synthesis of 2-(4-(6-Acryloyloxyhexyloxy)benzoyloxy)-2-
(R)-phenylethyl 4-(4-(6-Acryloyloxyhexyloxy)benzoyloxy)-
cinnamate (5)
This compound was obtained as a solid with 40% yield in the same
manner as described for 2-(4-(6-acryloyloxyhexyloxy)cinnamoyloxy)-
2-(R)-phenylethyl 4-(4-(6-acryloyloxyhexyloxy)benzoyloxy)benzoate (3)
starting from 4-(6-acryloyloxyhexyloxy)benzoic acid (7a) and (R)-2-
hydroxy-2-phenylethyl 4-(4-(6-acryloyloxyhexyloxybenzoyloxy)cinna-
mate (12b).
1H NMR (d in ppm, relative to TMS, J in Hz): 8.14 (d, 2H, J ¼ 9.0,
Hk), 8.05 (d, 2H, J ¼ 8.7, Hw), 7.67 (d, 1H, J ¼ 16.2, Hv), 7.54 (d, 2H,
J ¼ 9.0, Hm), 7.47 (d, 2H, J ¼ 7.2, Hp), 7.40 (m, 3H, Hr þ Hs), 7.22
(d, 2H, J ¼ 9.0, Hl), 6.97 (d, 2H, J ¼ 9.0, Hj), 6.91 (d, 2H, J ¼ 8.7,
Hx), 6.42 (dd, 2H, J1 ¼ 17.3, J2 ¼ 1.5, Ha), 6.40 (d, 1H, J ¼ 16.2, Hu),
6.31 (dd, 1H, J1 ¼ 8.3, J2 ¼ 3.8, Ho), 6.13 (dd, 2H, J1 ¼ 17.3,
J2 ¼ 10.5, Hc), 5.82 (dd, 2H, J1 ¼ 10.5, J2 ¼ 1.5, Hb), 4.64 (dd, 1H,
J1 ¼ 8.3, J2 ¼ 12.1, Hn), 4.53 (dd, 1H, J1 ¼ 3.8, J2 ¼ 12.1, Hn,), 4.18
(t, 4H, J ¼ 6.4, Hd), 4.05 (t, 2H, J ¼ 6.4, Hi), 4.01 (t, 2H, J ¼ 6.4, Hy),
1.82 (q, 4H, J ¼ 6.4, Hh), 1.72 (q, 4H, J ¼ 6.4, He), 1.49 (m, 8H, Hf þ Hg).

170
J. Lub et al.
13C NMR (d in ppm, relative to TMS): 166.9 (C26), 166.7 (C3), 165.8
(C19), 165.0 (C14), 164.0 (C10), 163.5 (C32), 153.1 (C15), 144.8 (C28),
137.3 (C21), 132.8 (C12), 132.2 (C17), 132.1 (C18), 131.0 (C1), 129.7
(C30), 129.1 (C23), 129.0 (C2 þ C24), 127.1 (C22), 122.8 (C16), 122.5 (C29),
121.6 (C13), 117.9 (C27), 114.7 (C11), 114.6 (C31), 74.1 (C25), 68.5 (C9),
68.4 (C33), 66.7 (C20), 64.9 (C4), 29.4 (C8 þ C34), 28.9 (C5), 26.1 (C6 þ C7).
2.2.10. Synthesis of 2-(4-(6-Acryloyloxyhexyloxy)benzoyloxy)-
1-(R)-phenylethyl 4-(4-(6-Acryloyloxyhexyloxy)benzoyloxy)-
cinnamate (6)
This compound was obtained as a solid with 70% yield in the same
manner as described for 2-(4-(6-acryloyloxyhexyloxy)cinnamoyloxy)-
2-(R)-phenylethyl 4-(4-(6-acryloyloxyhexyloxy)benzoyloxy)benzoate (3)
starting from 4-(4-(6-acryloyloxyhexyloxy)benzoyloxy)cinnamic acid
(10b) and (R)-2-hydroxy-2-phenylethyl 4-(6-acryloyloxyhexyloxy)-
benzoate (16a).
1H NMR (d in ppm, relative to TMS, J in Hz): 8.14 (d, 2H, J ¼ 9.0,
Hk), 7.96 (d, 2H, J ¼ 8.7, Hw), 7.73 (d, 1H, J ¼ 16.2, Hv), 7.58 (d, 2H,
J ¼ 9.0, Hm), 7.49 (d, 2H, J ¼ 7.2, Hp), 7.40 (m, 3H, Hr þ Hs), 7.24 (d,
2H, J ¼ 9.0, Hl), 6.97 (d, 2H, J ¼ 9.0, Hj), 6.88 (d, 2H, J ¼ 8.7, Hx),
6.42 (dd, 2H, J1 ¼ 17.3, J2 ¼ 1.5, Ha), 6.50 (d, 1H, J ¼ 16.2, Hu), 6.33
(dd, 1H, J1 ¼ 7.4, J2 ¼ 4.5, Hn), 6.13 (dd, 2H, J1 ¼ 17.3, J2 ¼ 10.5,
Hc), 5.82 (dd, 2H, J1 ¼ 10.5, J2 ¼ 1.5, Hb), 4.64 (dd, 1H, J1 ¼ 7.4,
o
o
0
J2 ¼ 12.1, H ), 4.58 (dd, 1H, J1 ¼ 4.4, J2 ¼ 12.1, H ), 4.18 (t, 4H,
J ¼ 6.4, Hd), 4.05 (t, 2H, J ¼ 6.4, Hi), 4.00 (t, 2H, J ¼ 6.4, Hy), 1.82 (q,
4H, J ¼ 6.4, Hh), 1.72 (q, 4H, J ¼ 6.4, He), 1.49 (m, 8H, Hf þ Hg).
13C NMR (d in ppm, relative to TMS): 166.7 (C3), 166.4 (C26), 166.3
(C19), 165.0 (C14), 164.0 (C10), 163.5 (C32), 153.1 (C15), 144.9 (C28),
137.1 (C21), 132.8 (C12), 132.2 (C18), 132.1 (C17), 131.0 (C1), 129.7 (C30),
129.1 (C23), 129.0 (C2 þ C24), 127.2 (C22), 122.8 (C16), 122.3 (C29),
121.6 (C13), 118.2 (C27), 114.7 (C11), 114.6 (C31), 74.1 (C20), 68.5 (C9),
68.4 (C33), 66.7 (C25), 64.9 (C4), 29.4 (C8 þ C34), 28.9 (C5), 26.1 (C6 þ C7).
2.3. Cholesteric Film Formation
Cholesteric layers with a thickness of about 3.5 mm were made by spin-
coating at 800 rpm a solution containing 43% (w=w) solid material in

Phenylethanediol and Cinnamic Acid Derivatives
171
xylene onto a glass plate coated with a rubbed alignment layer. The
alignment layer induces a planar orientation of the liquid-crystalline
molecules. The solid material consisted of 16–20 wt% of one of the
chiral compounds 3, 4, 5, or 6, 1% of Darocur1 4265 and 79–83% of
a 1:4 mixture of RM82 and RM 257.
The color change in the films before polymerization was effected in
air by UV irradiation. A Philips HPA 400 S lamp that emits a broad
spectrum in the near UV region was used. After irradiation and heat-
ing at 70ꢁC for 1 min the film was polymerized in a nitrogen atmos-
phere using the same lamp. The polymerization process was
completed by heating for 90 min at 150ꢁC in a nitrogen atmosphere.
3. RESULTS AND DISCUSSION
3.1. Synthesis of 3, 4, 5, and 6
To obtain the final products, the two different hydroxyl groups of (R)-
phenylethanediol had to be esterified selectively with two different
acids. For this purpose, THP-protected derivative 14 was used. This
compound was made starting from ethyl (R)-mandelate via intermedi-
ate 13 (see Scheme 1) using a known procedure [22]. By subsequent
esterification with acid 10a or acid 10b followed by deprotection of
11a or 11b and then esterification with 7b or 7a, products 3 or 5 were
prepared, respectively. Acids 10a and 10b were prepared starting
from 4-hydroxybenzoic acid and 4-hydroxycinnamic acid, respectively.
These acids were protected as ethoxymethyl esters 8a and 8b before
esterification with the acrylate-containing acid 7a. Deprotection of
9a and 9b was performed under very mild acidic conditions. Product
4 (see Scheme 2) was made in the same manner as 3, but the esterifi-
cation sequence was reversed. In the same way, 6 was made with the
same acids used for the preparation of 5.
3.2. Properties and Photochemistry of 3, 4, 5, and 6
All four new compounds are crystalline powders that form isotropic
phases upon melting. Comparison of both types of isomers reveals that
3 and 5 have higher melting points than 4 and 6, respectively (see
Table 1). The central phenyl group in the moiety derived from pheny-
lethanediol in compounds 3 and 5 is connected to the group derived
from 10a and 10b, respectively, by an even number of atoms. In the
case of the other two compounds, this connection is made by an odd
number of atoms. This odd–even effect probably explains these differ-
ences in melting points.

172
J. Lub et al.
SCHEME 1 Synthesis of compounds 3 and 5.

Phenylethanediol and Cinnamic Acid Derivatives
173
SCHEME 2 Synthesis of compounds 4 and 6.
To study the photo-isomerization of these compounds, solutions in
deuterated dichloromethane were irradiated and analyzed by 1HNMR
spectroscopy. The E-Z isomerization was followed as a function of
irradiation time. The conversion was determined by integration of
the new signals originating from the Z-isomer. In the case of 3 and
TABLE 1 Physical Properties of Menthone Derivative 1 and (R)-Phenyl-
ethanediol Derivatives 3, 4, 5, and 6 and Conversion to the Z-Isomers after
1
Irradiation, Measured by H-NMR Spectroscopy
Conversion (%)
measured from
NMR spectra
HTP (mmꢀ1
(after polymerization)
)
Compound
Mp (ꢁC)
1
3
4
5
6
76
81
53
97
80
ꢀ20
ꢀ19
ꢀ25
ꢀ18
ꢀ21
97
30
30
38
38

174
J. Lub et al.
5, a shift of about 0.4 ppm of the olefinic protons (Hu, see Experi-
mental) was used. In the case of 4 and 6, the shift of the signals of
the olefinic protons of the Z-isomers were obscured by the signals of
the acrylic group. In this case, the shift of one of the protons in the aro-
matic ring of the cinnamate moiety (0.1 ppm for Hx and 0.15 ppm for
Hl, respectively) was used. Irradiating with a PL-10 lamp that emits
light around 365 nm resulted in a 12% conversion of compounds 3
and 4 and no conversion of compounds 5 and 6.
Figure 2 shows the UV spectra of 3 and 5 in acetonitrile solution.
These spectra are identical to those of 4 and 6, respectively. This shows
that the position of the phenyl group in the moiety derived from pheny-
lethanediol does not have an effect on the UV spectra of these
compounds, but the position of the cinnamate moiety does. The extinc-
tion around 365 nm, especially of compounds 5 and 6, is very low,
which might explain the low conversion or even the lack of conver-
sion of the photo-isomerization reaction. Comparison of the UV spectrum
of 5 with that of 3 reveals a blue shift. The band at approximately
311 nm in Fig. 2 can be assigned to the cinnamate moiety of compound
3. This band has shifted to about 276 nm in the case of compound 5.
This shift can be explained by the fact that the electron-donating
alkoxy group, substituted in the para position of the cinnamate group
FIGURE 2 Absorption UV spectra of a solution of 3 (a) and 5 (b) in acetonitrile.

Phenylethanediol and Cinnamic Acid Derivatives
175
of compound 3, is replaced by a less electron-donating arylcarbonyloxy
group in compound 5. These spectral differences explain why 5 and 6
convert less than 3 and 4 upon irradiation with a PL-10 lamp.
The use of a TL-08 lamp emitting broadband around 350 nm,
resulted in a much higher conversion in all cases because the emitted
light coincides better with the absorption bands. The conversions for
the four compounds are shown in Table 1. Compounds 3 and 4 show
a conversion of 30% when they reach a photostationary state. The dif-
ference–UV spectra for several irradiation times between the irra-
diated and nonirradiated compound 3 are shown in Fig. 3. Similar
spectra were obtained with compound 4. Thus, the position of the phe-
nyl group in the moiety derived from phenylethanediol did not obser-
vably affect the UV spectra and the photochemistry. Moreover, the
isosbestic point at 269 nm indicates a clean isomerization process with-
out side reactions, even after prolonged irradiation. The NMR spectra
also indicate a clean isomerization reaction, because the NMR spectra
did not change after forming the photostationary state.
FIGURE 3 Absorption UV difference spectra of a solution of 3 in acetonitrile
after irradiation for 1 min (a), 2 min (b), 4 min (c), 8 min (d), and 256 min (e)
with a Tl-08 lamp.

176
J. Lub et al.
Table 1 demonstrates that compounds 5 and 6 show a 38% conver-
sion of the photo-isomerization reaction when they reach a photosta-
tionary state. Contrary to compounds 3 and 4, significant yellowing
of the NMR solution occurred after prolonged irradiation, suggesting
the occurrence of side reactions. However, the NMR spectra showed
only minor changes upon prolonged irradiation. The difference–UV
spectra for several irradiation times between the irradiated and non-
irradiated compound 5 are shown in Fig. 4. These spectra clearly show
a deviation from the isosbestic point (inset, Fig. 4) and an increase in
the absorption above 350 nm upon prolonged irradiation. Similar
results were obtained with compound 6. Thus another photochemical
process is operative, resulting in yellowing of the material. It is
possible that the benzoyloxycinnamate group in compounds 5 and 6
is more susceptible to a photo-Fries rearrangement [23] than the ben-
zoyloxybenzoate group in compounds 3 and 4. This may explain the
absence of by-products in the case of 3 and 4. Concerning application
in cholesteric color filters, these side reactions will not cause problems
because they were only observed long after the formation of the photo-
stationary state.
FIGURE 4 Absorption UV difference spectra of a solution of 5 in acetonitrile
after irradiation for 2 min (a), 4 min (b), 8 min (c), 16 min (d), 32 min (e), and
256 min (f) with a Tl-08 lamp.

Phenylethanediol and Cinnamic Acid Derivatives
177
3.3. Helical Twisting Power and Irradiated Cholesteric Layers
To determine the helical twisting power of the four compounds in cho-
lesteric polymeric films, films consisting of mixtures of one of these
compounds with nematic di-acrylates were copolymerized. For this
purpose, cholesteric layers were made by spincoating a mixture of
one of the chiral compounds with the nematic di-acrylates RM82
and RM257 and the photo-initiator in xylene on a rubbed polyimide
layer. The advantage of using these nematic di-acrylates is that they
do not crystallize rapidly, which makes it possible to handle the liquid
film at room temperature (i.e., in the supercooled state). After heating
at 70ꢁC for about 1 min, good alignment was obtained and photopoly-
merization was performed at room temperature under nitrogen, fol-
lowed by thermal polymerization at 150ꢁC. From the transmission
spectra, such as the one shown in Fig. 5 obtained with compound 5,
the reflection wavelength was determined as the center of the reflec-
tion band. The helical twisting power was then calculated according
to Eq. (1), using a mean refractive index of about 1.6 of these materials
after polymerization [24]. The HTP values are presented in Table 1
FIGURE 5 Transmission spectra of a 3.5-mm-thick cholesteric polymeric film
consisting of a copolymer of RM82, RM257, and 19% of isomerizable compound
5, without irradiation before polymerization (a), irradiation before polymeriza-
tion (b), and (c) with irradiation times increasing in the order b<c. The dotted
lines are the spectra of the same polymeric films, obtained after heating at
200ꢁC for 6 h in a nitrogen atmosphere.

178
J. Lub et al.
and were found to be around that of the menthone compound 1, thus
sufficient to generate a small pitch to obtain blue reflecting layers. The
differences in HTP values between the four compounds are reversed in
order of the melting points. This may point to some relationship with
the odd–even effect referred to previously. However, it is still difficult
to understand what molecular interactions determine the helical
pitch, and because the differences are not very big, no real struc-
ture–property relationship can be deduced from these observations.
To generate the green and red colors, irradiation was performed in
air on the wet blue monomer film before polymerization. Then the
70ꢁC step was performed, followed by photopolymerization and the
thermal (post-)polymerization step at 150ꢁC, as described for the blue
reflecting film. In all cases, the increase in pitch upon irradiation was
big enough to have the colors disappear into the infrared region. This
means that the conversion of 30% and 38% shown in Table 1 for com-
pounds 3 or 4 and 5 or 6, respectively, is sufficient for a pitch change of
at least 50%. However, care should be taken when comparing the
NMR experiments in dichloromethane with the experiment with the
films. The different environments (i.e., dichloromethane solution ver-
sus wet film) and experimental conditions may influence the isomeri-
zation reaction. Future experiments where the properties of the
Z-isomers are determined or where the irradiated films are analyzed
before polymerization may give a better insight into the relationship
between the conversion of the isomerization reaction and the helical
pitch change. The most important conclusion from these experiments,
however, is that all four compounds have HTP values that are high
enough and have changes in the HTP values upon irradiation that
are sufficient for the manufacturing of cholesteric colour filters.
Figure 5 (normal lines) shows the transmission spectra of choles-
teric polymeric films made with 5. The same spectra were obtained
using one of the other three chiral compounds. The green and red col-
ors were obtained by optimizing different irradiation times for these
colors. In all cases, the reflected light was left-handed circularly polar-
ized, which is in accordance with the observation of the left-handed
helix in cholesteric mixtures with model compound 2 that was also
derived from R-phenylethanediol [15].
The same polymer films were analyzed by spectroscopy after heat-
ing them at 200ꢁC for 6 h. The spectra after this thermal treatment
are also presented in Fig. 5 (dotted lines). Comparison of the spectra
before and after the treatment reveals that no significant change in
reflection wavelength or reflection intensity can be observed, thus
these polymeric films are thermally very stable. Similar cholesteric
films made with the menthone-derived compound 1 show blue shifts

Phenylethanediol and Cinnamic Acid Derivatives
179
of up to 10 nm and a decrease in reflection intensity when thermally
treated [10,18]. Thus, the films made with the new phenylethanediol-
derived compounds exhibit a higher thermal stability, which is impor-
tant when these films are processed further in the manufacturing of
reflective LCDs [10].
Although the mechanism responsible for the thermal degradation of
the films made with menthone derivative 1 has not yet been eluci-
dated, the improved thermal stability of the films prepared from 3,
4, 5, and 6 can be attributed to the higher cross-link density of the
polymerized film, the greater stability of the chemical structure of
the newly synthesized molecules, or a combination of both effects. To
distinguish between these two effects, mono-acrylates derived from
R-phenylethanediol and mono-acrylates with a structure similar to
the di-acrylates RM82 and RM257 could be prepared and used.
Mono-acrylates with structures similar to 3, 4, 5, and 6 could be pre-
pared fairly easily using non-acrylate-containing acids instead of the
acids 7a, 7b, 10a, and 10b in Schemes 1 and 2. Future research will
deal with these types of materials.
4. CONCLUSIONS
Chiral isomerisable di-acrylates derived from (R)-phenylethanediol
and cinnamic acid were prepared. The helical twisting power of these
compounds blended with nematic di-acrylates is sufficiently high to
form cholesteric films that reflect in the blue part of the visible spec-
trum. Upon irradiation of the E-isomeric cinnamic derivatives, the
Z-isomer is formed. Because the HTP of these isomers is lower than
that of the E-isomers, the color of the films changes. In this way, color
patterning to form color filters can be carried out. The colored films
can be handled at room temperature. After photopolymerization, ther-
mally very stable films are obtained that will not lose their optical
properties upon high-temperature processing during later steps in
LCD manufacturing. A combination of the higher cross-link density
of the polymerized film and the greater stability of the chemical struc-
ture probably account for the improved stability of the cholesteric
films made with these new compounds compared with those made
before with mono-acrylates derived from menthone. These compounds
are very suitable for cholesteric color filter manufacture.
ACKNOWLEDGMENTS
We thank R. T. Wegh and W. P. N. Nijssen (Philips Research) for help-
ful discussion and help with the synthesis, respectively.

180
J. Lub et al.
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