Nematic liquid crystals with a tetrafluoroethylene bridge in the mesogenic core structure

Article abstract of DOI:10.1021/ja010024l

A dramatic increase of the clearing temperatures of liquid crystals based on bis(cyclohexyl)ethane 1 by 50 to 70 K can be achieved by the perfluorination of the central ethylene link. Conformational analysis indicates that this effect is due to the increased rigidity of the mesogenic core structure and to the suppression of conformers with a bent shape. Materials based on bis(cylohexyl)tetrafluoroethane 2 might play a crucial role as materials for the next generation of active matrix LCDs with reduced power consumption.

Full text of DOI:10.1021/ja010024l

5414
J. Am. Chem. Soc. 2001, 123, 5414-5417
Nematic Liquid Crystals with a Tetrafluoroethylene Bridge in the
Mesogenic Core Structure
Peer Kirsch,*,† Matthias Bremer,*,† Florian Huber,† Harald Lannert,† Andreas Ruhl,†
Max Lieb,‡ and Tilman Wallmichrath†
Contribution from the Merck KGaA, Liquid Crystals DiVision, Frankfurter Strasse 250,
D-64271 Darmstadt, Germany, and Institute for Inorganic Chemistry I, Ruhr-UniVersity of Bochum,
UniVersita¨tsstrasse 150, D-44801 Bochum, Germany
ReceiVed January 2, 2001
Abstract: A dramatic increase of the clearing temperatures of liquid crystals based on bis(cyclohexyl)ethane
1 by 50 to 70 K can be achieved by the perfluorination of the central ethylene link. Conformational analysis
indicates that this effect is due to the increased rigidity of the mesogenic core structure and to the suppression
of conformers with a bent shape. Materials based on bis(cylohexyl)tetrafluoroethane 2 might play a crucial
role as materials for the next generation of active matrix LCDs with reduced power consumption.
During the past few years active matrix liquid crystal
currently known liquid crystals in this birefringence range is
that they show no tendency to form the desired nematic
mesophase but mostly smectic B (SB) phases.1
displays1,2 (AM-LCD, or thin film transistor LCD, TFT-LCD)
have rapidly developed toward the technological standard for
flat panel displays. The widespread use of AM-LCDs in portable
devices, such as notebook computers, PDAs (personal digital
assistants), or video games, makes it desirable to reduce the
power consumption of the display as far as possible, to extend
the battery lifetime. The largest contributor to the power
consumption of a typical AM-LCD is the backlight. Use of
reflective displays without backlight can therefore cut the power
consumption by more than 70%.3 An additional reduction can
be obtained by decreasing the operating voltage of the display
driving circuitry to 3.3 or 2.5 V from currently 5 or 4 V. The
latter option also allows a more compact device design and
results in a significant reduction of production costs.
Scheme 1. 1,2-Bis(cyclohexyl)ethane Derived Liquid
Crystals (1) and Their 1,2-Bis(cyclohexyl)tetrafluoroethane
Analogues (2)
Our approach to improve this situation was the systematic
modification of liquid crystals based on 1,2-bis(cyclohexyl)-
ethane (1)7 as a mesogenic core structure. Due to the lack of
any multiple bonds contributing to the molecular polarizability,8
this type of basic structure seemed a promising candidate for
obtaining low birefringent materials with a significantly im-
proved mesophase behavior. Assuming that one of the major
structural determinants of the mesoscopic properties of 1 is the
relatively flexible ethylene link between the two cyclohexane
rings, we tried to increase the conformational rigidity of this
bridge by perfluorination.9 The resulting material 2 was expected
to have at least a significantly higher clearing point than its
On the side of the liquid crystal materials, a reduction of the
driving voltage requires a decrease of the threshold voltage of
the electrooptical response, which can be most conveniently
achieved by using a more polar material, i.e., by increasing the
dielectric anisotropy (∆ꢀ) of the liquid crystal.4 An additional
requirement for displays based on a reflective twisted nematic
(r-TN) mode is very low birefringence (∆n).3
Generally, polar liquid crystals become more sensitive toward
ionic trace contaminations with increasing dielectric anisotropy
(∆ꢀ).5 This results in a reduced voltage holding ratio (VHR),
which is manifesting itself as visible flicker and contrast loss
of the display.6 The extremely low birefringence (∆n ) 0.05-
0.065) required for reflective displays is creating an additional
technological challenge: a common characteristic of nearly all
(4) The dielectric anisotropy is defined as ∆ꢀ ) ꢀ| - ꢀ , the birefringence
as ∆n ) n| - n , where the suffix | stands for parallel and perpendicular
to the nematic phase director, which can be approximated by the long
molecular axis of a liquid crystal molecule. The correlation between ∆ꢀ,
the dipole moment µ, and the angle â between the molecular dipole and
the director is the following: ∆ꢀ ∼ ∆R - F(µ2/2kT)(1 - 3 cos2â)S; ∆R is
the anisotropy of the polarizability, S the order parameter: (a) Maier, W.;
Meier, G. Z. Naturforsch. 1961, 16a, 262-267. (b) Demus, D.; Pelzl, G.
Z. Chem. 1981, 21, 1-9. (c) Hirschmann, H.; Reiffenrath, V. Applica-
tions: TN, STN Displays. In Handbook of Liquid Crystals; Demus, D.,
Goodby, J., Gray, G. W., Spiess, H.-W., Vill, V., Eds.; Wiley-VCH:
Weinheim, 1998; Vol. 2A, pp 199-229.
* Correspondence may be addressed to either author. Fax: +(49)6151-
72-2593. E-mail: peer.kirsch@merck.de or matthias.bremer@merck.de.
† Merck KGaA.
‡ Ruhr-University of Bochum.
(1) (a) Kirsch, P.; Bremer, M. Angew. Chem. 2000, 112, 4384-4405;
Angew. Chem., Int. Ed. 2000, 39, 4216-4235. (b) Pauluth, D.; Geelhaar,
T. Nachr. Chem. Tech. Lab. 1997, 45, 9-15.
(5) Bremer, M.; Naemura, S.; Tarumi, K. Jpn. J. Appl. Phys. 1998, 37,
L88-L90.
(2) Kobayashi, S.; Hori, H.; Tanaka, Y. Active Matrix Liquid Crystal
Displays. In Handbook of Liquid Crystal Research; Collings, P. J., Patel,
J. S., Eds.; Oxford University Press: New York, Oxford, 1997; pp 415-
444.
(3) (a) Raynes, P. Freiburger Arbeitstagung Flu¨ssigkristalle; Freiburg,
March 24-26, 1999; Vol. 28. (b) Kimura, H. EKISHO (J. Jpn. Liq. Cryst.
Soc.) 1999, 3 (1), 17-24.
(6) Sasaki, A.; Uchida, T.; Miyagami, S. Jap. Display ‘86 1986, 62.
(7) (a) Praefcke, K.; Schmid, D.; Heppke, G. Chem.-Ztg. 1980, 104, 269.
(b) Gray, G. W.; Carr, N. Secretary of State for Defense, GB Patent
8104527, 1981 [Chem. Abstr. 1983, 98, 63386].
(8) Vuks, M. F. Opt., Spectrosc. 1966, 20, 361.
(9) Bartmann, E.; Poetsch, E.; Finkenzeller, U.; Rieger, B. (Merck
KGaA), DE Patent 4015681 A1, 1990 [Chem. Abstr. 1992, 116, 185190].
10.1021/ja010024l CCC: $20.00 © 2001 American Chemical Society
Published on Web 05/15/2001

Nematic Liquid Crystals
J. Am. Chem. Soc., Vol. 123, No. 23, 2001 5415
Scheme 2. Synthesis of Symmetrically Substituted
of aryl-substituted liquid crystals with positive (13, 15, 17) and
with negative dielectric anisotropy (18) were synthesized by
similar methods as depicted in Scheme 3. The newly synthesized
materials were fully characterized by 1H and 19F NMR, and by
mass spectrometry. The purity was confirmed to be higher than
99.8% by GC and HPLC. The physical properties of the liquid
crystals in comparison with some of their nonfluorinated
analogues1,14 are summarized in Table 1. For better comparabil-
ity in the application oriented development of liquid crystal
mixtures, the “virtual” clearing points (TNI,extr), obtained by
extrapolation from the standard host mixture ZLI-4792, are also
listed in Table 1.15
Compared with their conventional ethylene-linked analogues,
the most striking difference of the new materials is the
dramatically increased real and “virtual”15 clearing temperatures,
and for the purely aliphatic systems their improved tendency
to form nematic mesophases (e.g. 1a f 2a). The increase of
the virtual clearing temperatures compared to that of the
ethylene-linked analogues ranges from 50 to 70 K. Remarkably,
in contrast to our earlier attempts to increase the clearing point
of highly fluorinated liquid crystals by axial fluorination of the
cyclohexane moiety,16 the fluorination of the central ethylene
link leads only to an insignificant increase of the rotational
viscosity (γ1). Less favorably, the fluorination also induces a
tendency to form smectic phases for the aryl-substituted liquid
crystals 13, 15, 17, and 18. With the single exception of the
pair 12 f 13 the birefringence (∆n) of the materials is slightly
decreased. The dielectric anisotropy (∆ꢀ) is essentially not
affected.
Especially the purely aliphatic, very low birefringent sub-
stance classes 2 and 11 are interesting for application in
reflective TFT displays, due to their favorable combination of
nematogenic mesophase behavior (with the exception of 2d),
high clearing points, and low rotational viscosity (γ1). Materials
such as 13, 15, 17, or 18 might be useful in dielectrically positive
(13, 15, 17) or negative (18) liquid crystal mixtures leading to
reduced threshold voltage and strongly improved clearing point,
without sacrificing switching time.
A comparative systematic conformational analysis on the
model systems 1,2-bis(trans-4-methylcyclohexyl)tetrafluoro-
ethane (19) and 1,2-bis(trans-4-methylcyclohexyl)ethane (20)
indicates that the unexpected extent of improvement of the
application relevant parameters can be attributed to the increased
rigidity of mesogenic core. Using the SPARTAN program17
and the MMFF94 force field,18 9 conformers of 19 and 14
conformers of 20 were found within 5 kcal‚mol-1. The
enthalpies were then recomputed by single point calculations
using density functional theory with the functional B3LYP19
Bis(cyclohexyl)tetrafluoroethane Derivativesa
a Conditions: (a) 1. Na suspension, Me3SiCl, toluene; 40 °C; 2.
NH4NO3, catalyst Cu(OAc)2, HOAc; reflux, 5 h (37%). (b) DAST neat,
catalyst ZnI2; 60 °C (13%). (c) SF4, catalyst HF, CH2Cl2; -196 f 70
°C, 2 d (16%).
Scheme 3. Model Synthesis of Unsymmetrically Substituted
Bis(cyclohexyl)tetrafluoroethane Derivativesa
a Conditions: (a) 1. MeOCH2PPh3 Cl-, KOtBu, THF; -10 °C f
+
room temperature; 2. 98% HCOOH, toluene; room temperature, 18 h;
3. catalyst NaOH, MeOH; room temperature, 1 h (53%). (b) 1. (4-
Propylcyclohexylmethyl)triphenylphosphonium bromide, LDA, THF;
-10 °C f room temperature (54% crude product); 2. MePhSO2Na,
HCl, toluene; reflux, 16 h (88%). (c) N-Methylmorpholine-N-oxide,
catalyst OsO4, H2O, dioxane; reflux, 18 h (65%). (d) 10 equiv of DMSO,
3 equiv of (CF3CO)2O, THF, CH2Cl2, NEt3; -60 f 5 °C, 18 h (75%).
(e) SF4, catalyst HF, CH2Cl2; -196 ° f 120 °C, 48 h (17%).
analogue 1. In addition, the higher lipophilicity induced by the
fluorination should reduce the ability of these materials to
solvate ionic impurities.5 The synthesis of the structurally simple,
symmetrically substituted derivatives of 1 started with an acyloin
condensation of the corresponding ethyl 4-alkylcyclohexyl-
carboxylates, followed by oxidation of the acyloin to the
R-diketone.10 Since it was not possible to fully convert the
R-diketone to the tetrafluoroethylene group by use of diethyl-
aminosulfurtrifluoride (DAST), the more reactive fluorination
reagent sulfurtetrafluoride had to be used. Since the acyloin
condensation only yields symmetric derivatives of 2, a more
generally applicable method was developed (Scheme 3): The
E-vinylene linked bicyclohexane derivatives were subjected to
a Sharpless dihydroxylation11 and subsequently oxidized under
Swern conditions,12 furnishing the R-diketone intermediate.
Unexpectedly, it was found that the dihydroxylation only
proceeds from the E-olefin while the Z-olefin is completely
unreactive under the same conditions. The fluorinated com-
(14) Goto, Y.; Ogawa, T.; Sawada, S.; Sugimori, S. Mol. Cryst. Liq.
Cryst. 1991, 209, 1-7.
(15) The phase transition temperatures are given in °C, the γ1 values in
mPa‚s. C ) crystalline, SX ) smectic X, N ) nematic, I ) isotropic.
Usually, the “virtual” parameters TNI,extr, ∆ꢀ, ∆n, and γ1 were determined
by linear extrapolation from a 10% w/w solution in the commercially
available Merck mixture ZLI-4792 (TNI ) 92.8 °C, ∆ꢀ ) 5.3, ∆n ) 0.0964).
The extrapolated values are corrected empirically for differences in the order
parameter. In some marked cases the following mixtures were used for
extrapolation: ZLI-2857 (TNI ) 82.3 °C, ∆ꢀ ) -1.4, ∆n ) 0.0776) and
ZLI-1132 (TNI ) 71.0 °C, ∆ꢀ ) 12.8, ∆n ) 0.1406). For the pure substances
the mesophases were identified by optical microscopy, and the phase
transition temperatures by differential scanning calorimetry (DSC).
(16) Kirsch, P.; Tarumi, K. Angew. Chem. 1998, 110, 501-506; Angew.
Chem., Int. Ed. Engl. 1998, 37, 484-489.
13
pounds were obtained by reaction of the R-diketone with SF4
in reasonable yields.
For a comparative full evaluation of this novel class of
materials, aliphatic three-ring materials (11) and some examples
(10) Tietze, L.-F.; Eicher, T. Reaktionen und Synthesen im organisch-
chemischen Praktikum; Georg Thieme Verlag: Stuttgart, New York, 1981;
pp 233-235.
(11) (a) Akashi, K.; Palermo, R. E.; Sharpless, K. B. J. Org. Chem. 1978,
43, 2063. (b) VanRheenen, V.; Kelly, R. C.; Cha, D. Y. Tetrahedron Lett.
1976, 1973. (c) Ray, R.; Matteson, D. S. Tetrahedron Lett. 1980, 449.
(12) Huang, S. L.; Omura, K.; Swern, D. Synthesis 1978, 297.
(13) Burmakov, A. I.; Stepanov, I. V.; Kunshenko, B. V.; Sedova, L.
N.; Alekseeva, L. A.; Yagupolskii, L. M. J. Org. Chem. USSR 1982, 18,
1009-1013.
(17) SPARTAN, Version 5.1; Wavefunction Inc., 18401 Von Karman
Avenue, Suite 370, Irvine, CA 92612.
(18) Halgren, T. A. J. Comput. Chem. 1996, 17, 490.
(19) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang,
W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.

5416 J. Am. Chem. Soc., Vol. 123, No. 23, 2001
Kirsch et al.
Table 1. Overview of the Physical Properties15 of the Newly Synthesized 1,2-Bis(cyclohexyl)tetrafluorethane Derivatives in Comparison to
Some of Their Nonfluorinated Analogues or, as for the Pair 1c/2d, Homologuesa
no.
R1
R2
mesophases
TNI,extr
∆ꢀ
∆n
γ1
1a
1b
1c
2a
2b
2c
2d
11a
11b
12
13
14
C3H7
C5H11
C3H7
C3H7
C5H11
C5H11
C2H5
C5H11
C5H11
C3H7
C3H7
C3H7
C3H7
C3H7
C3H7
C3H7
C3H7
C5H11
CF3
C3H7
C5H11
C3H7
CF3
C 36 S? 26 SB 73 I
C 44 SB 108 I
C ? SB 41 I
C 28 SB 93 N 126.5 I
C 33 SB 138 N 139.8 I
C 43 SB 115 N 136.1 I
C 52 SB 116 I
C 29 SB 270 I
C -5 SB 283 I
C 39 N 104.3 I
C 95 SB 121 N 178.1 I
C 45 N 81.7 I
C 70 SG 95 SB 102 N 168 I
? S? 38 N 107.0 I
C 35 SB 154 N 189.1 I
C 107 SB 123 N 193.5 I
55a
78.8
-39.1
122.3
140.7
134.2
3.9
273.5
277.4
105.7
150.0
67.7
118.1
93.8
0.5
6.1
0.048
0.058
0.044
0.041
0.055
0.054
0.052
0.044
0.067
0.079
0.076
0.060
0.084
0.074
0.086
134
68
138
0.1
0.8
6.2
0.7
0.4
5.5
5.4
9.4
9.0
110
C3H7
C5H11
247
267
15
16
17
18
7.7
145.2
175.4
7.7
-2.9b
418
a Extrapolated from the Merck mixture ZLI-1132. b Extrapolated from ZLI-2857.
Scheme 4. The Five Lowest Energy Conformers of
1,2-Bis(trans-4-methylcyclohexyl)tetrafluoroethane (19)a
systems 19 and 20 seems to be appropriate to give a representa-
tive description of the substance class as a whole. All deviations
from an elongated, rodlike shape of the mesogenic core structure
can be expected to degrade the phase behavior of its liquid
crystalline derivatives.
The 1,2-bis(cyclohexyl)tetrafluoroethane derived liquid crys-
tals are a new class of liquid crystals, which exhibit clearly
superior properties with regard to application in AM-LCD.
Compared to ethylene-linked analogues, the new materials show
a dramatic increase of the clearing temperatures by 50-70 K
without any significant concomitant increase of the rotational
viscosity (γ1). In purely cycloaliphatic systems an improved
tendency to form a nematic mesophase was found. Conforma-
tional analysis indicates that these surprisingly pronounced
effects are due to the increased rigidity of the fluorinated
ethylene link and to the suppression of unfavorably bent
conformers. The combination of these properties and their very
low birefringence might place this type of liquid crystal at a
focal point in the development of the next generation of full-
a The relative energies for the corresponding conformers of 1,2-
bis(trans-4-methylcyclohexyl)ethane (20) are in parentheses [kcal‚mol-1
at B3LYP/6-31G*//MMFF94].
(20) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A.
D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi,
M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.;
Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick,
D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.;
Ortiz, J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi,
I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, Revision A.6;
Gaussian, Inc.: Pittsburgh, PA, 1998.
and the 6-31G* basis set as implemented in the GAUSSIAN98
program.20 The five lowest conformers for 19 are shown in
Scheme 4. The lowest conformer of 20 with a bent shape is
only 1.0 kcal‚mol-1 higher than the global minimum whereas
for 19 the corresponding conformer is at 2.7 kcal‚mol-1, and
should therefore be only marginally represented in the conformer
equilibrium. Preliminary calculations of the pair 14/15 (R )
CH3) indicate similar behavior. Therefore, use of the model

Nematic Liquid Crystals
J. Am. Chem. Soc., Vol. 123, No. 23, 2001 5417
color energy saving AM-LCDs for battery-powered devices,
such as mobile phones with video-capability, sub-notebook PCs,
and video games.
(MITI) supported by the New Energy and Industrial DeVelop-
ment Organization (NEDO).
Supporting Information Available: Exemplaric synthetic
procedures and analytical data for 9, 10, and 2d (PDF). This
material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment. We thank Dr. J. Krause, J. Haas, H.
Heldmann, D. Klement and B. Schuler for the physical
evaluation of the new substances. A part of this work was
performed under the management of the Association of Super-
AdVanced Electronics Industries (ASET) in the R & D program
of the Japanese Ministry of International Trade and Industry
JA010024L