High birefringence nematic liquid crystals for display and telecom applications

Article abstract of DOI:10.1080/15421400490434829

Several compounds with high birefringence and, having tolane structure were synthesized. For a high polarizability, isothiocyanato (NCS) terminal group was introduced. Another goal of this study was reduction of melting point and smectic phases by introduction of fluorine as lateral substituent. The transition temperatures for all the synthesized compounds and the refractive and dielectric indices for one of the compounds have been determined. Electro-optic (EO) measurements were also performed.

Full text of DOI:10.1080/15421400490434829

This article was downloaded by: [UOV University of Oviedo]
On: 30 October 2014, At: 01:27
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954
Registered office: Mortimer House, 37-41 Mortimer Street, London W1T 3JH,
UK
Molecular Crystals and Liquid
Crystals
Publication details, including instructions for
authors and subscription information:
http://www.tandfonline.com/loi/gmcl20
High Birefringence Nematic
Liquid Crystals for Display and
Telecom Applications
Carmen Catanescu a , L.-C. Chien a & Shin-Tson Wu b
a Liquid Crystal Institute , Kent State University ,
Kent, Ohio, USA
b School of Optics/CREOL , University of Central
Florida , Orlando, Florida, USA
Published online: 18 Oct 2010.
To cite this article: Carmen Catanescu , L.-C. Chien & Shin-Tson Wu (2004) High
Birefringence Nematic Liquid Crystals for Display and Telecom Applications, Molecular
Crystals and Liquid Crystals, 411:1, 93-102, DOI: 10.1080/15421400490434829
To link to this article: http://dx.doi.org/10.1080/15421400490434829
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the
information (the “Content”) contained in the publications on our platform.
However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness,
or suitability for any purpose of the Content. Any opinions and views
expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the
Content should not be relied upon and should be independently verified with
primary sources of information. Taylor and Francis shall not be liable for any
losses, actions, claims, proceedings, demands, costs, expenses, damages,
and other liabilities whatsoever or howsoever caused arising directly or

indirectly in connection with, in relation to or arising out of the use of the
Content.
This article may be used for research, teaching, and private study purposes.
Any substantial or systematic reproduction, redistribution, reselling, loan,
sub-licensing, systematic supply, or distribution in any form to anyone is
expressly forbidden. Terms & Conditions of access and use can be found at
http://www.tandfonline.com/page/terms-and-conditions

Mol. Cryst. Liq. Cryst., Vol. 411, pp. 93=[1135]–102=[1144], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490434829
HIGH BIREFRINGENCE NEMATIC LIQUID
CRYSTALS FOR DISPLAY AND TELECOM
APPLICATIONS
Carmen Otilia Catanescu and L.-C. Chienꢀ
Liquid Crystal Institute, Kent State University, Kent,
Ohio 44242, USA
Shin-Tson Wu
School of Optics=CREOL, University of Central Florida, Orlando,
Florida 32816, USA
Several compounds with high birefringence and having tolane structure were
synthesized. For a high polarizability, isothiocyanato (NCS) terminal group
was introduced. Another goal of this study was reduction of melting point
and smectic phases by introduction of fluorine as lateral substituent. The
transition temperatures for all the synthesized compounds and the refractive
and dielectric indices for one of the compounds have been determined.
Electro-optic (EO) measurements were also performed.
Keywords: birefringence; electro-optics; nematic; synthesis; tolanes
INTRODUCTION
Certain properties of display applications such as response time, viewing
angle, voltage holding ratio (VHR), power consumption, operation
temperature ranges still need to be improved without affecting general
properties in a negative way [1–3]. Liquid crystal (LC) single compound
cannot achieve all the desired parameters, therefore LC mixtures are
employed. Usually, LC mixtures consist of about 10–20 individual LC
compounds designed for particular applications.
LC materials with high dielectric anisotropy (De) can be realized by
introducing fluorine atoms in the compound’s structure. Fluorinated LCs
also contribute to the decreasing of optical anisotropy which leads to a
This work was supported by DARPA=ALCOM grants.
ꢀCorresponding author. E-mail: lcchien@lci.kent.edu
93=[1135]

94=[1136]
C. O. Catanescu et al.
better viewing angle. An appropriate selection of the fluorine atom’s
position can lower the viscosity, reduce melting points, depress the
temperature of smectic phase formation and improve solubility and
stability [2,4,5]. For almost all display applications, a faster response time
is required. The response time is directly proportional to the rotational
viscosity and the square of the cell thickness, d. As the product of cell
thickness d and the birefringence Dn is fixed for a certain set of cell para-
meters improved response time can be achieved by increasing the Dn value
of the mixtures.
In general, long wavelength light has been used for telecom applications
because of lowest absorption and light efficiency. To steer a k ¼1.55 mm
laser beam, the required 2p phase change (d ¼ 2pdDn=k) leads to
dDn ꢁ 1.6. In order to achieve fast response time while keeping high manu-
facturing yield, the cell gap is normally kept at d ꢁ 4 mm. Thus, LC mixtures
with Dn ꢂ 0.4 and low viscosity, low optical loss, wide nematic range, excel-
lent thermal stability, and low operating voltage are particularly desirable.
In the case of telecom optical switch applications, for k ¼ 1.55 mm and
90ꢃ TN cell, Dn should be approximately 0.35, if d ¼ 4 mm.
In the search for new materials with improved properties several tolanes
having fluorine atoms and NCS terminal group were synthesized based on
the fact that tolane substances have proved to have a better combination of
physical properties than other materials [6]. The saturated alkyl chain was
linked with the rigid core through oxygen or sulfur atom. Just three
compounds exhibit nematic phase, all other compounds having crystalline
structure. Four LC mixtures were formulated using the new tolane
compounds having ether linkage. Electro-optical properties and UV
stability of some of the synthesized compounds were investigated.
EXPERIMENTAL
Confirmation of the structures of the intermediates and the final products
was obtained by 1H-NMR spectroscopy using a Varian Gemini 200 MHz
spectrometer with tetramethylsilane as internal standard. IR spectra were
recorded using a Nicolet Magna 550 FT-IR spectrometer (NaCl crystal win-
dow). Thin-layer chromatography was performed using aluminium sheets
silica gel 60 F254 (Merck) and were examined under UV light. Column chro-
matography was performed under gravity using Merck’s Silicagel 60.
Melting points of final products were determined using a Thomas-
Hoover Capillary Melting Point Apparatus. Transition temperatures were
determined using a Mettler FP52 heating stage and FP5 temperature
control unit in conjunction with a Nikon polarizing optical microscope,
and the transitions were confirmed by DSC analysis (Perkin Elmer Pyris 1,
Perkin Elmer TAC 7=DX, Perkin Elmer Intracooler 2P). Heating and

High Birefringence Nematic Liquid Crystals
95=[1137]
cooling cycles were run at rates of 2ꢀC=min under nitrogen atmosphere
with samples measured in closed lid aluminium pans. Mesophase type
was assigned by visual comparison (under the microscope) with known
phase standards.
The dielectric anisotropies (De ¼ e ꢁ e ) of compound 5 were mea-
k
?
sured by the capacitance method at a frequency of 1 kHz (k ¼ 633 nm)
using a computer-controlled APT III instrument (manufactured by Display-
tech). The measurements were carried out at a fixed temperature
T ¼ 295 K, 20% of the compound being dissolved in a Merck LC mixture
(ZLI-3086 with De ꢂ 0) as a host.
The birefringence (Dn ¼ ne ꢁ no) of the same compound was measured
using a He-Ne laser (k ¼ 633 nm) at T ¼ 295 K, 20% of it being dissolved in
a low birefringence LC host (ZLI-14600-100). In both cases, the De and Dn
values were extrapolated to 100% concentration. The UV spectrum of
compound 7 was recorded on a Hewlett-Packard HP-8453 diode-array
spectrophotometer using chloroform as solvent. The UV stability of the
NCS based tolanes (15% of compound 7 mixed with E48 Merck LC mixture
- designated as LC1) was investigated using a metal halide lamp that illumi-
nate a 4 mm anti-parallel cell for 600 sec (ꢂ4.8 J=cm2esposure dosage).
Two anti-parallel cells with PI 2555 as the alignment layer and 3.7 mm
and 4.1 mm thickness were prepared for the electro-optical studies. The
transmittance vs. applied voltage and transmittance vs. time curves were
recorded using a HeNe laser beam (k ¼ 633 nm), under parallel polarizor
and analyzor and sample at 45ꢀ (rubbing direction).
SYNTHESIS
Ten novel compounds were prepared according to the procedure detailed
in Scheme 1. All homologues were synthesized using the same method,
but the general experimental procedures will be published elsewhere. Alky-
loxy=sulfanil bromobenzenes (a) were synthesized using Williamson’s
etherification in near-quantitative yields. Compound b was converted into
the alkyne c via compound b by using the 2-methylbut-3-yn-2-ol method.
Palladium-catalysed coupling reaction of alkyne with the 4-bromo-2,
6-difluoro-aniline gave compound d in low yields, but its conversion in
the final compound (e) gave quantitative yields. We present here the
experimental method just for the final product with n ¼ 4 (1-(4-Butyloxy-
phenyl)-2-(3,5-difluoro-4-isothiocyanato phenyl) ethyne)-compound 5.
1-(4-Butyloxyphenyl)-2-(3,5-difluoro-4-isothiocyanato phe-
nyl) ethyne 5e
A solution of compound 5d (2.4 g; 7.97 mmol) in chloroform (30 cm3) was
added dropwise at 0–5ꢀC to a stirred, cooled (0ꢀC) mixture of water

96=[1138]
C. O. Catanescu et al.
SCHEME 1 Synthesis of NCS based tolanes.
(18 cm3), chloroform (9 cm3), calcium carbonate (1.25 g; 12.5 mmol), and
thiophosgene (0.65 cm3; 85 mmol). Then the mixture was heated at reflux
for 16 h and poured into water (100 cm3), and the separated organic layer
was quenched with hydrochloric acid (45 cm3; 1%), and dried over sodium
sulfate anh. The solvent was removed in vacuo. The product was purified
by column chromatography (hexane) and recrystallized from hexane to
1
give white crystals. Yield 83.82%. H-NMR (CDCL3) d, ppm 0.98 (3H, t),
1.47 (2H, sext.), 1.78 (2H, quint.), 3.97 (2H, t), 6.89 (2H, sext.), 7.094
(2H, sext.), 7.45 (2H, sext.). IR (film in nujol) mmax=cmꢄ1 722, 1051,
1175, 1255, 1380, 1466, 1518, 1611, 2032, 2098, 2216, 2854, 2920, 2966.
RESULTS AND DISCUSSION
DSC thermograms for compounds 1,3,5,7,9 are shown in Figure 1. Tran-
sition temperatures and extrapolated physical properties for materials
1–10 are reported in Table I. As one can see, just three compounds (with
ether linkage) exhibit mesomorphic properties in a narrow nematic
temperature range. All the synthesized materials show relatively high DH
and low melting points, the longer the alkyl chain, the lower the clearing
point. Compounds with n ¼ 2, 3 have higher melting points than all other
substances, but with longer chain one can observe the classical ‘‘odd-even’’

High Birefringence Nematic Liquid Crystals
97=[1139]
FIGURE 1 DSC thermograms of compounds 1,3,5,7,9.
effect of alkyl chain length upon the melting point. Low melting points
usually gives a good solubility in LC host.
The exchange of oxygen atom with sulfur one led to a decrease in
transition temperatures of all homologues, in some cases with even 22ꢃC
(compound 8). This effect appears due to the change in the L=D ratio
of the compounds containing sulfur atom because of its bigger volume
compared with the oxygen atom.
TABLE I Transition Temperatures and Some Physical Properties
No.
n
X
DH (kcal=mol)
Phase transitions (ꢃC)
Physical properties
1
2
2
2
3
3
4
4
5
5
6
6
O
S
5.93
7.91
7.79
7.73
6.67
6.95
7.07
6.33
8.57
9.04
K 95.4 I
K 89.6 I
K 86.8 I
3
O
S
4
K 80.8 I
K 61.6 N (59.7)
K 58.4 I
5
O
S
Dn ¼ 0.41 De ¼ 18
6
7
O
S
K 49.5 N 56.6 I
K 34.4 I
8
9
10
O
S
K 54.7 N 65.1 I
K 55.5 I

98=[1140]
C. O. Catanescu et al.
FIGURE 2 DSC thermograms of LC mixtures.
The birefringence of compound 5 was measured using the guest-host
method to extrapolate its Dn value, 20% of the compound being dissolved
in ZLI-14600-100; Dn ¼ 0.41 was found. The dielectric anisotropy of the
same compound was found to be De ¼ 18. Both properties have relatively
large values due to the presence of NCS and fluoro groups.
Three binary and one ternary LC mixtures were formulated using
compounds 1, 5, 7 and 9. Their nematic behavior was predicted using a
computer program based on the Schroder van Laar equation and compared
with DSC thermograms of the mixtures as shown in Figure 2 and presented
in Table 2.
TABLE II Mesophase Behavior of the LC Mixtures
Mixture composition
(wt%)
Predicted nematic
range (ꢀC)
Observed nematic
rangeꢁ (ꢀC)
C2=C4 ¼ 34.5=65.5
C2=C5 ¼ 23=77
54.0–73.4
41.2–66.4
46.3–74.0
30.0–66.0
52.2–62.6
35.7–57.0
46.2–63.8
31.8–59.9
C2=C6 ¼ 26.1=73.9
C2=C5=C6 ¼ 15.8=48.9=35.3
ꢁDSC data.

High Birefringence Nematic Liquid Crystals
99=[1141]
The Schroder van Laar equation relates the melting point (T) of a
mixture to the mole fraction (xA) of component A[3]. In Eq. (1) HA and
TA are the latent heat of melting and melting point of the pure component,
R is the gas constant.
ꢀ
ꢁ
HA
R
1
1
ln xA ¼
ꢄ
ð1Þ
TA
T
For a mixture of N components there are N such equations, which when
solved for the condition shown in Eq. (2) give the temperature and mole
fractions of the eutectic composition This method occasionally predicts
the mole fractions of the eutectic mixture fairly well, but because ideal
behavior rarely exists it can only be used as a guide.
N
X
The clearing point of mixtures can be calculated using Eq. (3)
xj ¼ 1
ð2Þ
i¼A
N
X
Tc ¼
xiTci
ð3Þ
i¼A
where xi is the mole fraction of the component in the mixture and Tci is the
clearing point of the component. However, this relationship holds best
when compounds of similar polarity are mixed.
The observed nematic range was approximately in the same range with
the predicted one. In these mixtures one can see that the mesophase was
expanded or even induced compared with the pure compounds.
All mixtures are enantiotropic liquid crystals; the last three ones (C2-C5,
C2-C6 and C2-C5-C6) have very low crystallization temperature, below
room temperature and the mesophase range is quite large (around 40ꢃC
on cooling). All four mixtures show two peaks when melting or crystallizing
and that could mean a smectic phase, but it wasn’t confirmed, but usually,
when mixing two polar compounds, a smectic phase is induced.
The differences between the estimated values of the nematic range and
experimental data are 8–10ꢃC in average, with the exception of mixture
C2=C5 where this difference is smaller (ꢁ4ꢃC) – the experimental TNI value
is 35.7ꢃC instead of predicted 41.2ꢃC.
Electro-optical properties as transmittance vs. applied voltage and
transmittance vs. time were performed for compound 7 (15% of it being
dissolved in commercial Merck LC mixture E48) – abbreviated LC1.
In order to be able to compare the LC1 mixture with the host LC mixture
– abbreviated LC2 – we used anti-parallel rubbed cells with cell gap of
3.7 mm for the LC1 mixture and 4.1 mm for LC2, respectively. The results

100=[1142]
C. O. Catanescu et al.
FIGURE 3 a) T-V curve for LC1 (abbreviated E48=C5ONCS in the figure’s legend)
and LC2; b) T-t curve for LC1 and LC2.
are shown in Figure 3. The switching times were measured by applying
a square-wave AC voltage of 1.3 V for LC1 and 1.45 V for LC2 mixture,
respectively at 1 kHz. The switching-on time or rising time (son or srise
)
and switching-off time or falling time (soff or sfall) were defined as the
switching times between 10% and 90% transmittance.
Table III summarizes the results of electro-optical studies for these
mixtures. As one can see, the LC1 mixture containing compound 7 has
smaller values of threshold voltage (V10), saturation voltage (V90), rising
time (srise), falling time (srise), and contrast ratio (V90=V10) compared with
the host LC, yet the differences are too small. This might probably be
because of the viscosity of the tolane products (they are crystalline com-
pounds at room temperature). These results serves though as a basis for
further improvements in molecular design.
The UV spectrum of compound 7 was recorded in chloroform as a dilute
solution and it is shown in Figure 4. Two major absorption peaks are
observed at 326 and 343 nm.
TABLE III E-O Properties
LC1
LC2
V10(V)
90(V)
V90=V10
0.937
1.196
1.276
50.91
25.04
1.018
1.333
1.309
54.05
28.68
V
srise (ms)
sfall (ms)

High Birefringence Nematic Liquid Crystals
101=[1143]
FIGURE 4 UV absorption spectrum of compound 7 in chloroform.
We expose the cell containing LC1 mixture at a UV dosage of
ꢁ4.8 J=cm2, the results being presented in Figure 5, a and b. Analyzing this
figure one may say that the NCS based tolane compound is stable at this UV
dosage.
As one can see, the electro-optical properties of the irradiate sample cell
looks almost the same with those of the non-irradiate cell. The T-V curve
(Fig. 5a) of LC1 after UV irradiation shows lower transmittance at higher
voltage, although the change in transmittance value is irrelevant within
the high voltage region. The same trend is observed on the T-t curve (son
curves) (Fig. 5b) at longer time. Further investigation should be made to
fully understand this phenomenon.
FIGURE 5 a) T-V curve for LC1 before and after UV irradiation; b) T-t curve for
LC1 before and after UV irradiation.

102=[1144]
C. O. Catanescu et al.
CONCLUSIONS
We synthesized a series of new NCS based tolane substances that have high
birefringence and dielectric anisotropy, good solubility in LC host, relatively
low melting points and fairly good UV stability. The mixture studies reveals
that a wider nematic range can be obtained. In addition, we also observed a
smectic phase by mixing polar compounds. LC devices can use thinner cell
gap to achieve fast switching time with similar optical response because of
the intrinsic physical properties (high birefringence and high dielectric
anisotropy) of the dopant in a LC mixture.
REFERENCES
[1] Tarumi, K., Bremer, M., & Geelhaar, T. (1997). Annu. Rev. Mater. Sci., 27, 423.
[2] Geelhaar, T. (1998). Liq. Cryst., 24(1), 91.
[3] Scheffer, T. & Nehring, J. (1995). Liquid Crystals, Applications and uses (B. Bahadur,
World Scientific), 1, 231.
[4] Tarumi, K., Bartmann, E., Geelhaar, T., Schuler, B., Ichinose, H., & Numata, H. (1995). Asia
Display ‘95, 559.
[5] Nakajima, S., Sugiyama, Y., Ichinose, H., Numata, H., & Naemura, S. (2000). SID’00 Digest,
242.
[6] Ichinose, H., Nakajima, S., Sugiyama, Y., Numata, H., & Naemura, A.
[7] Manabe, Journal of the SID 9=4, 287 (2001).