Synthesis and physical properties of novel fluorinated liquid crystalline compounds

Article abstract of DOI:10.1080/15421400590955352

Most twisted nematic liquid crystal displays (TN-LCD) uses liquid crystal mixtures with a positive dielectric anisotropy. In contrast, the recently introduced vertically aligned liquid crystal display (VA-LCD), which offers superior picture quality with a

Full text of DOI:10.1080/15421400590955352

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Synthesis and Physical
Properties of Novel Fluorinated
Liquid Crystalline Compounds
Hye-Won Keum a , Su-Dong Roh a , Yun Sun Do a
Jae-Ho Lee a & Yong-Bae Kim a
,
a Liquid Crystal Research Center, Department of
Chemistry, Division of Natural Sciences , Konkuk
University 1 , Seoul, Korea
Published online: 31 Aug 2006.
To cite this article: Hye-Won Keum , Su-Dong Roh , Yun Sun Do , Jae-Ho Lee
& Yong-Bae Kim (2005) Synthesis and Physical Properties of Novel Fluorinated
Liquid Crystalline Compounds, Molecular Crystals and Liquid Crystals, 439:1, 189/
[2055]-199/[2065], DOI: 10.1080/15421400590955352
To link to this article: http://dx.doi.org/10.1080/15421400590955352
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Mol. Cryst. Liq. Cryst., Vol. 439, pp. 189=[2055]–199=[2065], 2005
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400590955352
Synthesis and Physical Properties of Novel Fluorinated
Liquid Crystalline Compounds
Hye-Won Keum
Su-Dong Roh
Yun Sun Do
Jae-Ho Lee
Yong-Bae Kim
Liquid Crystal Research Center, Department of Chemistry, Division of
Natural Sciences, Konkuk University 1, Seoul, Korea
Most twisted nematic liquid crystal displays (TN-LCD) uses liquid crystal
mixtures with a positive dielectric anisotropy. In contrast, the recently introduced
vertically aligned liquid crystal display (VA-LCD), which offers superior picture
quality with a wide viewing angle, high contrast, and video-compatible switching
times, is based on a liquid crystal mixture with negative dielectric anisotropy. We
introduce here the synthetic route and physical and electro-optical properties of the
liquid crystalline compounds with an ester group as linkage subsistent and with
an alkyl chain as lateral subsistent. Also, we present physical and electro-optical
properties of their mixture, and the LC-series for VA-LCD. Synthetic compounds
exhibit broad range nematic phases up to 60 K degree and large negative dielectric
anisotropies. And the viscosity of the LC-series is low in comparison with that of
homologues with a lateral isothiocyanate group. The threshold voltage (Vth) was
lower than that of a conventional mixture, enough so for it to be operated at a
driving voltage. And the rising time of some LC-series improved to be faster than
conventional LC mixtures. Our novel LC compounds are suitable materials for the
improvement of the dielectric anisotropy of vertically aligned liquid crystal
mixtures.
Keywords: fluorinated liquid crystalline compounds; negative dielectric constant;
threshold voltage; transition temperature; viscosity
This work performed on advanced backbone IT technology development and syn-
thesis of domestic-made LC project was supported by the Ministry of Information &
Communication, Ministry of Commerce, Industry & Energy of the Republic of Korea
and Dong-Jin Semichem Co., Ltd.
Address correspondence to Yong-Bae Kim, Liquid Crystal Research Center, Depart-
ment of Chemistry, Division of Natural Science, Konkuk University 1, Hwayangdong,
Kwanjin-ku, Seoul 143-701, Korea. E-mail: yongbae@kkucc.konkuk.ac.kr
189=[2055]

190=[2056]
H. W. Keum et al.
INTRODUCTION
Thin film transistor liquid crystal displays (TFT-LCDs) are widely
used information displays, especially in laptop computers, monitors,
and TVs due to their excellent resolution. However, TFT-LCDs have
several limitations, such as a narrow viewing angle and slow screen
update time. Various methods to solve the viewing angle problem,
such as the addition of birefringence films, the domain divided twisted
nematic [1,2], the in-plane-switching (IPS) mode [3,4], and vertical
alignment mode [5,6] have been proposed. Among these, the newly
developed VA-LCD is expected to show improved wide-viewing angle
and screen update time [7,8].
With the rapid change in the development of LCDs, demand for new
liquid crystalline materials is increasing. For example, aliphatic ter-
tiary nitrile compounds have a large negative dielectric anisotropy
but a low phase transition temperature due to a decreased l=d. And
the voltage holding ratio (VHR) values are insufficient for active
matrix liquid crystal displays (AMLCDs) because the nitrile substitu-
ent is a property of conductivity and ion salvation [9,10]. And the
laterally difluorinated phenylbicyclohexanes have large negative
dielectric anisotropy, a high phase transition temperature and a suf-
ficient VHR, but also a slow response time due to increased rotational
viscosity [11]. Today, the applied materials use for VA modes are
mainly 2,3-difluoro sub unit liquid crystalline compounds having low
rotational viscosity and broad phase transition temperature range
[12,13]. Super fluoro-substituted materials (SFMs) are known for their
larger dipole moment, high stability, and high voltage holding ratio,
low threshold voltage and low viscosity [14]. For this reason, many
types of SFMs are in use and their properties have been investigated,
despite the fact that they have low dielectric and optical anisotropy,
which are known to deter high-speed response. In this study, we
report the synthesis of novel fluorinated LCs with large negative
dielectric anisotropy, characterizing their physical properties. In
addition, we discuss the physical constants of new LCs in relation to
the calculated molecular parameters.
SYNTHESIS
The synthetic procedure that is typically applied in preparing negative
dielectric liquid crystalline compounds with a fluorine group is
depicted in Figures 1 and 2. All homologues were prepared using the
same methods. In Figure 1, the core structure (biphenyl group) was
successfully introduced by the palladium-catalyzed cross-coupling

Properties of Novel Fluorinated Liquid Crystalline Compounds
191=[2057]
FIGURE 1 Synthetic method of an LC compound with Ester group as linking
substitute.
reaction between boronic acid and aryl bromide [15], followed by the
deprotection of alcohol and esterification reactions. In Figure 2, the
core structure (phenylcyclohexyl group) was introduced by a carbon-
carbon coupling reaction from the trans-alkylcylohexanone and
organolithium reagent generated from the corresponding halide
derivatives, followed by dehydration, the palladium-catalyzed cross-
coupling reaction and reduction reactions. The pure trans compounds
FIGURE 2 Synthetic route for an alkyl chain as terminal substitute.

192=[2058]
H. W. Keum et al.
were obtained in the final stage of the successive recrystallization
process.
MEASUREMENTS
1
The chemical structures of products were confirmed by H and 13C
NMR spectroscopy (Bruker Avance 400 Spectroscope) and GC=Mass
spectrometer (Hewlett Packard 6890 series GC system and MSD).
The liquid crystalline phase and phase transition temperature were
observed through a polarizing microscope (Olympus BX51, BX60),
equipped with a heating stage and a controller (Mettler FP90, STC
200D), and Differential Scanning Calorimeter (Perkin-Elmer DSC-7=
TAC-7). The dielectric anisotropy (De) was determined by the capaci-
tive method, using an impedance analyzer (Hewlett Packard LF
impedance analyzer 4172 A) at a frequency of 1 kHz. The optical
birefringence (Dn) was measured by using an Abbe’s refractometer
and the flow viscosity (g) was obtained with a cannon micro viscometer
at 20ꢀC. All the measurements were carried out at a fixed tempera-
ture, 20ꢀC, on 15 wt% solutions of the compound in standard host
mixture (MJ 961213) and extrapolated to 100% concentrations, taking
into account the variation of the physical parameters of the mixture
under investigation. The molecular geometry was calculated using the
semi empirical quantum chemical AM1 method (MOPEC 7.0 package).
RESULTS AND DISCUSSION
The chemical structures and abbreviations are listed in Table 1. Phase
transition temperature data are given, in Table 2 and the calculated
molecular parameters, such as the molecular dipole moment (l),
TABLE 1 Chemical Structures and Abbreviations for
the Biphenyl Cyclohenxane Liquid Crystal
Abbreviation
Structure
nCcooPFFPFFF
nCPFFPFFn
R3CPPFFPFF

Properties of Novel Fluorinated Liquid Crystalline Compounds
193=[2059]
TABLE 2 Phase Transition Temperatures and Length-to-Breadth Ratios of
Biphenylcyclohenxane LC
Phase transition temperatures (ꢀC)
n
X
m
Y
l
Z
Abbreviation
3CPFFPFF3
3CPFFPFF5
5CPFFPFF3
5CPFFPFF5
3CcooPFFPFFF
5CcooPFFPFFF
3CPPFFPFF
Cr
Sm
N
Iso DHNI
3
F
3
F
5
F
5
F
3
F
5
F
3
0
F
0
F
0
F
0
F
1
F
1
F
0
0
Heat
.
.
.
.
.
.
.
.
.
.
.
.
.
.
–
22.7
–
11.3
43.2
14
22.3
0.8
–
32.3
–
18.6
–
–
–
–
–
.
.
.
–
–
–
–
–
–
–
–
–
–
.
.
–
–
.
.
.
.
.
.
.
.
43.9
.
.
.
.
.
.
.
.
.
.
.
.
.
.
–
C3H7 Cool
Heat
C5H11 Cool
Heat
C3H7 Cool
Heat
C5H11 Cool
–
34.5
–
–
0
59.3
61.3
79.4
–
62.8
63.1
108.6
108.2
109.8
109.8
190.2
190.8
0.362
–
0
–
78.6
45.3
–
63.0
–
72.4
–
130.2
–
0
0.432
0.206
0.579
0.371
0
F
0
F
1
Heat
Cool
Heat
Cool
Heat
Cool
F
F
H
111.8
anisotropy of molecular polarizability (Da) and the angle b between the
molecular dipole moment and the long molecular axis of the molecule,
and measured physical properties are summarized in Table 3.
As you can see in Tables 2, 3 CcooPFFPFF, 5CcooPFFPFF,
3CPFFPFF5, 5CPFFPFF3, and 5CPFFPFF5 were found to have enantio-
tropic nematic phases, whereas 3CPFFPFF3 compounds were non-meso-
genic. For the 5CPFFPFF3 compounds, the smectic C phase was observed
in the heating and cooling processes. It is known that thermal stability is
TABLE 3 Molecular Parameters from Molecular Modeling Calculations and
the Measured Physical Properties of Biphenyl Cyclohexane Liquid Crystal
Abbreviation
l1 (Debye) Da1 (a.u) Da=M1
b1
De2
Dn2
g2 (mm2=s)
3CPFFPFF3
3CPFFPFF5
5CPFFPFF3
5CPFFPFF5
3CcooPFFPFFF
5CcooPFFPFFF
3CPPFFPFF
4.822
4.475
4.438
4.508
5.469
5.369
5.018
181.8
162.5
163.3
167.3
166.1
170.3
287
0.4638 84.9 ꢁ4.43 0.0918
0.3869 84
83.35
66.41
56.08
49.88
44.35
45.48
47.41
ꢁ5.56 0.1095
0.3888 85.9 ꢁ4.43 0.1128
0.3734 81.8 ꢁ5.30 0.1197
0.4032 54.7 ꢁ3.83 0.1071
0.3874 40.6 ꢁ3.43 0.1093
0.6737 72.3 ꢁ5.56 0.1415
1For the calculations, we used the semi-empirical quantum method (AM1 in the
MOPAC 7.0).
2Extrapolated values from 15 wt% host solutions.

194=[2060]
H. W. Keum et al.
greatly influenced by molecular parameters, such as rigidity, length-to-
breadth ratio, packing density, polarizability and enhancement of polarity
by conjugation. Thesefactors canexplaintheeffectof fluorinesubstitution
on the thermal stability of liquid crystal molecules. And 3CcooPFFPFF
and 5CcooPFFPFF have high phase transition temperatures due to the
increase in l=d with the introduction of the ester group.
As shown in Table 3, the dielectric anisotropy values vary according
to core structure, terminal group and alkyl spacer length. The Eq. (1)
can be explained by the Maier-Meier theory [16] which is based on the
Onsager model. The dielectric anisotropy is given by
ꢀ
ꢁ
NFh
Fl2
De ¼
Da þ
ð1 ꢀ 3 cos2 bÞ ꢁ S
ð1Þ
eo
2kBT
Here, S is the order parameter, N is the molecular packing density,
h is the cavity field factor, F is the Onsager reaction field factor, and l
is the dipole moment forming an angle b with the major inertial axis of
the molecule. n is the average refractive index of the substance. Dn is
determined by molecular polarizability anisotropy (Da), order para-
meter (S), and crystal density (q), as follows.
Dn ¼ ð2p=3nÞðn2 þ 2ÞNAðDa=MÞqS
ð2Þ
From Figure 3, we can deduce that it is a typical behavior for Dn
to increase with an increase in De in each group because of the
relationship between two parameters. As shown in Eqs. (1) and (2),
dielectric anisotropy, De, and birefringence, Dn, are in proportion to
the anisotropy of molecular polarizability, Da, and order parameter
S. Figure 4 shows the viscosity tendency of LC compounds.
Introduction of the fluorine group as a lateral substituent in the
phenyl ring, affords a lower kinetic viscosity compared with homolo-
gues with the isothiocyanate-substituent (Fig. 4). This is explained
by the fact that the molecular polarizability anisotropy (Da) of
the isothiocyanato group with three non-bonding electron pairs is
higher than that of fluorine. This factor enhances the Van der Waal’s
attraction in molecules.
As shown in Table 4, LC-006, LC-007, LC-008 and LC-009 have
faster rising times than the host due to increase negative dielectric
anisotropy and low threshold voltage. In the case of falling time,
LC-006, LC-007, LC-008 and LC-009 have slower falling times than
the host due to increase kinetic viscosity and decrease elastic con-
stants. LC-010 and LC-011 have slower falling times nevertheless,
having large elastic constants due to large kinetic viscosity.
Especially, LC-012 has the fastest rising time (6.3 msec) and falling

Properties of Novel Fluorinated Liquid Crystalline Compounds
1
9
5
=
[
2
0
6
1
]
FIGURE 3
R e l a t i o n s hi p b et w e en Dn a n d De f o r n e w L C s f o r t h e e x p e r i m en t s .
FIGURE 4
C
.
o
m
p
a
r
i
o
s
f
o
n
o
f
u
f
o
l
r
o
i
w
n
v
e
i
s
c
(
o
s
d
i
t
)
y
b
a
e
t
n
w
d
e
e
n
L
L
C
C
o
o
f
f
i
s
o
i
t
s
h
o
i
t
o
h
c
i
i
o
a
c
n
y
a
a
t
n
e
a
a
n
d
L
C
l
o
f
f
l
u
o
r
i
n
e
L
C
f
l
r
e
t
e
(
b
u
e
)
.

196=[2062]

Properties of Novel Fluorinated Liquid Crystalline Compounds
197=[2063]
time (9.26 msec) than all other mixtures and hosts because of increase
negative dielectric anisotropy and low kinetic viscosity.
Experimental
The trans-2,3,20,30-tetrafluoro-40-alkyl-4-(4-alkyl-cyclohexyl)-biphenyl
were prepared according to the synthetic route in Figure 2. The
representative procedures for the propyl derivative are described below.
1-(2,3-Difluoro-phenyl)-4-propyl-cyclohexanol (2-1)
A solution of 1,2-difluorobenzene (9.2 g, 0.08 mol) in dry THF
(200 mL) was cooled to ꢁ78ꢀC, and then 2.5 M n-BuLi (38.4 mL
0.096 mol) was slowly added to this solution. After maintaining it at
ꢁ78ꢀC for 4 hrs, 4-propylcyclohexanone (11.3 mL, 0.08 mol), which
was dissolved in THF (100 mL), was added; the reaction mixture
was warmed to room temperature and neutralized with 10% hydro-
chloric acid. The mixture was washed with saturated sodium chloride
aqueous solution and Ether. The organic layer was dried over anhy-
drous MgSO4 and removed in vacuo to give a yellow liquid product
(20.03 g, 0.079 mol, yield 98%). Mass: 254(Mþ) 236, 193, 179, 165
1,2-Difluoro-3-(4-propylcyclohex-1-enyl)-benzene (2-2)
The solution of compound (2-1) (10.4 g, 0.041 mol) and p-toluenesul-
fornic acid 1-hydrate (7.78 g, 0.0041 mol) in toluene (250 mL) were
placed in a three-neck flask. After the mixture was heated under
reflux at 115ꢀC for 6hrs, the product was washed with saturated
sodium bicarbonate and water. Then, the organic layer was dried over
anhydrous MgSO4 and the solvent was evaporated in vacuo to give a
dehydration product in a yellow solid form. The product was purified
by column chromatography (hexane) to give a colorless liquid product
(8.12 g, 0.034 mol 84% yield). Mass: 236(Mþ) 193, 179, 127
2,3-Difluoro-1-iodo-4-(4-propyl-cyclohex-1-enyl)-benzene (2-3)
A solution of 1,2-difluoro-3-(4-propylcyclohex-1-enyl)-benzene (10 g,
0.042 mol) in dry THF (200 mL) was cooled to ꢁ78ꢀC, and then 2.5 M
n-BuLi (20.2 mL, 0.05 mol) was slowly added. After maintaining at
ꢁ78ꢀC for 4 hrs, 2 M I2 solution (21 mL, 0.05 mol), which was dissolved
in dried THF, was added; the reaction mixture was warmed to room
temperature and neutralized with 10% hydrochloric acid. The product
was extracted into ether (twice), and the combined ethereal extracts
were washed with water and dried (MgSO4). The organic layer
was dried over anhydrous MgSO4 and removed in vacuo to give a
dark brownish crude product. The product was purified by column

198=[2064]
H. W. Keum et al.
chromatography (hexane) to produce a yellow liquid product (12.92 g,
0.0357 mol, yield 85%). Mass: 362(Mþ), 305, 266, 165 1H NMR
(400 MHz, CDCl3): 0.7(2H, m), 1.86 (2H, m), 2.23 (3H, m), 5.96 (1H, s),
6.75(1H, m), 7.36(1H, m)
2,3,2 0,3 0-Tetrafluoro-40-propyl-4-(4-propyl-cyclohex-1-enyl)-
biphenyl (2-4)
A solution of 2,3,20,30-Tetrafluoro-4-(4-propyl-cyclohex-1-enyl)-bi-
phenyl (4.16 g, 0.012 mol) in dry THF (200 mL) was cooled to ꢁ78ꢀC,
and then 2.5 M n-BuLi (12.4 mL, 0.015 mol) was slowly added. After
it was maintained at ꢁ78ꢀC for 2.5 hrs, iodopropane (3 mL,
0.03 mol), which was dissolved in THF (10 mL), was added; the reac-
tion mixture was warmed to room temperature and neutralized with
10% hydrochloric acid. The product was extracted into ether (twice),
the organic layer was dried over anhydrous MgSO4 and removed in
vacuo to give a brownish crude product. The product was purified by
column chromatography (hexane) to produce a colorless liquid product
(3.9 g, 0.01 mol, yield 83%). Mass: 390 (Mþ), 347, 333, 294
Trans-2,3,2 0,3 0-Tetrafluoro-40-propyl-4-(4-propyl-
cyclohexyl)-biphenyl (2-5)
The reaction mixture of compound (2-4) (3.3 g, 0.0084 mol) and
palladium on activated carbon (0.5 g, 10% Pd) in ethanol (150 mL)=
toluene (50 mL) was maintained for 12 h under an H2 stream (press-
ure: 60 psi). The Pd=C was filtered off and the residue was evaporated
in vacuo to yield a colorless liquid product. The product was purified
by column chromatography (hexane) and separated by recrystalli-
zation (Ethanol and E.A.) to give trans-2,3,20,30-tetrafluoro-40-propyl-
4-(4-propyl-cyclohexyl)-biphenyl (1.36 g, 40% yield). Mass: 392(Mþ .
363,294,265 H NMR (400 MHz, CDCl3): 0.8(3H,m), 0.98 (3H, m), 1.0
(2H,m), 1.1 (9H,m), 1.32(9H,m), 1.5(4H,m), 1.67(2H,m), 1.85(4H,m),
2.87(1H,t), 6.9(4H,m)
)
1
CONCLUSION
The development of vertically aligned thin film transistor liquid
crystal displays (VA TFT-LCDs) has created a greater demand for the
development of new and improved liquid crystalline materials. In this
study, we synthesized novel LC compounds, trans-2,3,20,30-tetrafluoro-
4-alkyl-40-(4-alkyl-cyclohexyl)-biphenyl and 4-alkyl-cyclohexane car-
boxylic acid 2,3,20,30,40-pentafluoro-biphenyl-4-yl ester and 2,3,20,
30,tetrafluoro-400-(4-propyl-cyclohexyl) triphenyl measured the physical
properties for the purpose of high phase transition temperature, large

Properties of Novel Fluorinated Liquid Crystalline Compounds
199=[2065]
negative dielectric constants, and low viscosity. We found that the
synthesized LC showed relatively low viscosity and large negative
dielectric anisotropy and the threshold voltage Vth was low enough to
operate in TFT-LCD at the driving voltage. And LC-012 is the fastest
response time than host because of increase negative dielectric
anisotropy and low kinetic viscosity. The proposed LC compounds are
deemed to be suitable materials for the improving dielectric anisotropy
of vertically aligned liquid crystal displays (VA-LCDs).
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