Liquid crystals based on hypervalent sulfur fluorides: Pentafluorosulfuranyl as polar terminal group

Article abstract of DOI:10.1002/(SICI)1521-3773(19990712)38:13/14<1989::AID-ANIE1989>3.0.CO;2-K

The somewhat 'exotic' pentafluorosulfuranyl functionality is one of the strongest electron-withdrawing groups with a purely inductive effect owing to its chemical stability and the resulting high dipole moment of its aromatic derivatives, this group was crucial to the design and synthesis of a new class of highly polar liquid crystals (such as 1) for application in active matrix displays. A computational model was developed with the aim to understand and predict the electrooptic properties of liquid crystals based on hypervalent sulfur fluorides.

Full text of DOI:10.1002/(SICI)1521-3773(19990712)38:13/14<1989::AID-ANIE1989>3.0.CO;2-K

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have been deposited with the Cambridge Crystallographic Data
Center as supplementary publication no. CCDC-112935. Copies of the
data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK (fax: (‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
In designing liquid crystals the most efficient way to
increase De is by using a polar terminal group with the
maximum possible dipole moment. The current limit for
AMD-compatible materials is achieved with the trifluoro-
methyl group (e.g. 1a: De ˆ 8.6). Unfortunately, the much
[20] D. M. P. Mingos, D. J. Wales, Introduction to Cluster Chemistry,
Prentice Hall, New York, 1990.
[21] J.-T. Sheu, C.-C. Lin, I. Chao, C.-C. Wang, S.-M. Peng, Chem.
Commun. 1996, 315.
[22] G. E. Herberich in Comprehensive Organometallic Chemistry II,
Vol. 1 (Eds.: E. Abel, F. G. A. Stone, G. Wilkinson), Pergamon,
Oxford, 1995, p. 197.
[23] M. A. Curtis, E. J. Houser, M. Sabat, R. N. Grimes, Inorg. Chem. 1998,
37, 102.
[24] P. L. Arnold, F. G. N. Cloke, P. B. Hitchcock, J. F. Nixon, J. Am. Chem.
Soc. 1996, 118, 7630.
[25] X. Wang, M. Sabat, R. N. Grimes, J. Am. Chem. Soc. 1995, 117, 12227.
[26] M. Stephan, J. H. Davis, Jr., X. Meng, K. J. Chase, J. Hauss, U.
Zenneck, H. Pritzkow, W. Siebert, R. N. Grimes, J. Am. Chem. Soc.
1992, 114, 5214.
more polar and nematogenic materials based on a terminal
cyano group (e.g. 1b, PCH-3: De ˆ 21.1) cannot be used for
active matrix diplays. The cyano group tends to solvate
ubiquituous ionic impurities[4] resulting in a low voltage
holding ratio[5] and observable flicker and contrast loss in the
display panel.
To further increase the dielectric anisotropy (De) of the
materials, a polar ring such as 1,3-dioxane or 1,3-dithiane can
be introduced. Fluorine substituents in ortho-position to the
terminal group also result in an increase of De, but at the cost
of a drop of the clearing points by 30 ± 40 K per lateral
fluorine atom.[6, 7] For polar two-ring materials in particular
this latter option leads to unacceptably low clearing points.
Following and anticipating the technological trend to lower
driving voltage, we concentrated on the identification and
evaluation of a new, more polar terminal group that is
compatible with active matrix technology and does not
require lateral fluorination to obtain an acceptably high
dielectric anisotropy (De). Our interest was especially focused
on two-ring materials with low rotational viscosity (g1).[3]
Other prerequisites for practical display applications are high
chemical and photochemical stability of the liquid crystalline
materials.
[27] J. W. Merkert, J. H. Davis, Jr., W. E. Geiger, R. N. Grimes, J. Am.
Chem. Soc. 1992, 114, 9846.
[28] O. J. Scherer, H. Swarowsky, G. Wolmershäuser, W. Kaim, S.
Kohlmann, Angew. Chem. 1987, 99, 1178; Angew. Chem. Int. Ed.
Engl. 1987, 26, 1153.
[29] A. W. Duff, K. Jonas, R. Goddard, H.-J. Kraus, C. Krüger, J. Am.
Chem. Soc. 1983, 105, 5479.
[30] H. Suzuki, H. Omori, D.-H. Lee, Y. Yoshida, M. Fukushima, M.
Tanaka, Y. Moro-oka, Organometallics 1994, 13, 1129.
Liquid Crystals Based on Hypervalent Sulfur
Fluorides: Pentafluorosulfuranyl as Polar
Terminal Group**
A most interesting candidate as a highly polar head group
for liquid crystals is the pentafluorosulfuranyl group.[8] This
functional group is known to be even more stable than
trifluoromethyl against hydrolytic agents, also at elevated
temperatures. In addition, the dipole moment m of 3.44 D of
pentafluorosulfuranylbenzene[9] (compared to 2.60 D for
benzotrifluoride) indicates that liquid crystals with very high
dielectric anisotropy can be obtained. A semiempirical cal-
culation[10] (PM3) on 4-(propylcyclohexyl)pentafluorosulfur-
anylbenzene (1c) gave a DePM3 value of 22.2, exceeding even
that of the cyano-based based PCH-3.
The chemistry of pentafluorosulfuranylbenzene derivatives
was first studied thoroughly at the beginning of the 1960s by
W. A. Sheppard,[9] and its use for liquid crystals was the
subject of a preliminary investigation in our group at the end
of the 1980s.[11] These studies were impeded by the incon-
venient synthesis of pentafluorosulfuranylbenzene derivatives,
which was based on a two-step reaction of aromatic disulfides
with silver difluoride in the severely ozone-depleting solvent
1,1,2-trichloro-1,2,2-trifluoroethane (ªFreon 113º). The re-
cent introduction of the direct fluorination of deactivated
aromatic disulfides made the precursor 2 commercially
available in kilogram quantities.[12] This finally enabled us to
develop a suitable synthesis, systematically screen pentafluoro-
sulfuranyl-derived liquid crystals, and compare them with
conventional cyano- and fluoro-based materials.
Peer Kirsch,* Matthias Bremer, Michael Heckmeier,
and Kazuaki Tarumi
Recently, active matrix liquid crystal displays (AM-LCD or
thin film transistor LCD, TFT-LCD) have been evolving
rapidly towards becoming the major technology for flat panel
diplays.[1] To save costs in the mass production of display
panels, there is a strong tendency to reduce the driving voltage
of active matrix LCD. On the materials side, the consequence
of this trend is the demand for liquid crystals with a higher
dielectric anisotropy (De) in order to obtain an electrooptical
response.[2, 3]
[*] Dr. P. Kirsch, Dr. M. Bremer, Dr. M. Heckmeier, Dr. K. Tarumi
Merck KGaA, Liquid Crystals Division
D-64271 Darmstadt (Germany)
Fax: (‡49)6151-72-2593
E-mail: peer.kirsch@merck.de
[**] We thank A. Hahn and A. Ruhl for experimental assistance, and Dr. J.
Krause, J. Haas and H. Heldmann for the physical evaluation of the
new substances. For the X-ray structure we are indebted to Dr. K.
Merz (Anorganische Chemie I, Universität Bochum). 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
(MITI) supported by the New Energy and Industrial Development
Organization (NEDO). The German Bundesministerium für Bildung
und Forschung (01B621/1) provided additional support.
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1989

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The synthesis of all the pentafluorosulfuranyl-substituted
liquid crystals started from 4-nitropentafluorosulfuranylbenz-
ene (2), which was hydrogenated to the aniline 3 and
subsequently converted into the bromide 4 under Sandmeyer
conditions (Scheme 1).[9b] Compound 4 is a versatile synthetic
typical liquid crystal core structures, we synthesized also the
three-ring materials 15 ± 17.[13] Due to their higher clearing
points, fluorinated three-ring liquid crystals
have a much stronger tendency to display
mesophases than the two-ring materials.[14] To
our surprise, the experimentally determined
dielectric anisotropies of the newly synthe-
sized pentafluorosulfuranyl derivatives (1c:
Deexp ˆ 12.0) were far lower than predicted by
our otherwise very reliable method[10] (1c:
DePM3 ˆ 22.2). Even if our quite ambitious
initial target was not met, the physical
evaluation (Table 1) shows that pentafluoro-
sulfuranyl-based liquid crystals are still the
most polar class of liquid crystals that is
Scheme 1. Synthesis of pentafluorosulfuranyl-based two-ring liquid crystals: a) H2, 5% Pd/C,
THF; b) 1. HBr, NaNO2; 58C; 2. CuBr; room temperature !808C (46%); c) 1. tBuLi, Et2O;
708C; 2. B(OMe)3; 70 ! 208C; 3. HOAc, H2SO4, H2O2 (30%); 20 !358C, 1 h (58%);
d) 4-propylcyclohexylcarbonyl chloride, pyridine, 0.1 equiv dimethylaminopyridine, CH2Cl2
(24%); e) 1. tBuLi, Et2O; 788C; 2. N-formylpiperidine; 408C !room temperature (76%);
f) 2-(trimethylsiloxymethyl)-1-trimethylsiloxypentane, cat. Me3SiOTf, CH2Cl2; 788C, 30 min
(67%); g) 2-thiomethylpentane-1-thiol, BF3 ´ Et2O; room temperature, 1 h (28%); h) 1. tBuLi,
Et2O; 788C; 2. 4-propylcyclohexanone; 788C !room temperature; 3. cat. TsOH, toluene;
azeotropic removal of water (55%); i) H2, 5% Pd/C, THF (25% pure trans isomer); j) 4-
propylphenylboronic acid, cat. [Pd(PPh3)4], toluene, 2n NaOH; room temperature, 2 d (23%);
k) 1-ethynyl-4-propylbenzene, cat. [Pd(PPh3)4], pyrrolidine; room temperature, 18 h (33%).
compatible with active matrix display tech-
nology. The dielectric anisotropy can even be
enhanced with a concomitant slight increase
of the clearing point, if in the phenylcyclo-
hexane derivative 1c the cyclohexane ring is
replaced by a 1,3-dioxane (8: Deexp ˆ 20.3) or
a 1,3-dithiane unit (9: Deexp ˆ 22.3). The
polarities of 8 and 9 are even in the range of
typical cyano-functionalized compounds such
as 1b (PCH-3). The trifluoromethyl-substi-
tuted liquid crystals are slightly less polar than the penta-
fluorosulfuranyl derivatives.
building block which can be easily converted into a variety of
liquid crystals[13] either directly or via the aldehyde 7 or the
phenol 5. The purity of the newly synthesized compounds was
verified to be at least 99.5% by GC or HPLC.
A less favourable property of the pentafluorosulfuranyl-
based liquid crystals is their relatively high rotational viscosity
The only unexpected ªcomplicationº during the syntheses
was the sensitivity of 4 towards n-butyllithium. In Sheppardꢁs
original paper[9b] no significant side reactions under Grignard
conditions were reported. Nevertheless, our attempts to
metalate 4 with n-butyllithium resulted in an immediate dark
coloration of the solution, even at 788C. After workup only
the products 13 and 14 were isolated (Scheme 2). Apparently,
the pentafluorosulfuranyl group with sulfur in its highest
oxidation state was undergoing a redox reaction. Fortunately,
the initial problem could be solved by using tert-butyllithium,
which produced no unwanted side reactions.
Table 1. Physical properties of the pentafluorosulfuranyl derivatives and
some selected conventional fluorinated liquid crystals.
Entry
(Meso)phases
TNI,extr
De
Dn
g1
[8C]
[mPas]
1a
1b
1c
1d
6
C 22 I
C 45 N 46.1 I
C 11 I
C 31 I
C 70 I
C 69 I
C 93 I
C 29 I
C 68 I
C 68 I
C 121 I
C 109 N (87.8) I
C 90 N 152.8 I
90.0
10.1
96.8
54.0
31.6
79.9
66.4
84.9
16.8
84.5
97.8
94.8
132.9
8.6[b]
21.1
12.0
4.3[b]
12.4
20.3
22.3
14.2
16.4
15.8
11.6
14.3
12.3
0.036[b]
0.136
0.087
0.1[b]
0.091
0.091
0.096
0.124
0.224
0.161
0.094
0.154
0.087
±
116
133
±
±
145
±
±
±
±
8
9
To evaluate the influence of the pentafluorosulfuranyl
group on the actual, not extrapolated, mesophase behavior of
10
11
12
15
16
17
612
442
±
[a] Clearing points (TNI,extr), dielectric anisotropies (De), birefringences
(Dn), and rotational viscosities (g1) are extrapolated from the Merck liquid
crystal mixture ZLI-4792.[14] C ˆ crystalline, N ˆ nematic, I ˆ isotropic.
Numbers in parentheses denote monotropic phase transitions. [b] Values
were extrapolated from ZLI-1132.
Scheme 2. Products isolated from the reaction of 4 with nBuLi: a) nBuLi,
THF; 788C (13: 6%, 14: 4%).
1990
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(g1), which is in the range of that of the cyano materials (1c vs
1b). Another disadvantage with regard to technical applica-
tion is the relatively high melting point of most of the newly
synthesized materials.
Why are the predicted dielectric anisotropies so much
higher than the experimental data? An X-ray crystal structure
of 8[15] was supposed to help us identify the possible reasons
and develop a more reliable prediction method. The X-ray
structure (Figure 1) shows that the equatorial fluorine atoms
large (mai ˆ 6.75 D, Deai ˆ 33.3, Dnai ˆ 0.025 for 8). Therefore,
on the supposedly realistic HF/6-31G*-optimized structure of
8 we calculated a PM3 single point in order to obtain the
dipole moment and the polarizabilities required for the
calculation of De and Dn.[2, 10] This method finally led to a
good correlation of experimental with calculated electro-
optical parameters, and a reliable computational model for
hypervalent sulfur fluorides became available (Table 2). A
calculation by the same method predicts for 1c a Decalc value
of 13.0 and a Dncalc of 0.083, also in excellent agreement with
the experimentally determined data (Deexp ˆ 12.0, Dnexp
ˆ
0.087, see also Table 1.)
Table 2. Comparison of calculated and experimentally determined param-
eters for 8.[a]
Parameter
Experimental PM3//PM3 6-31G*//6-31G* PM3//6-31G*
Figure 1. X-ray crystal structure of 8.[15] For selected bond lengths and
angles see Table 2.
Car S [Š]
S Feq [Š][b]
S Fax [Š]
1.8066(18)
1.5827(13)
1.5791(12)
1.833
1.590
1.609
95.6
7.07
35.9
0.091
1.799
1.583
1.577
92.57
6.75
±
±
±
±
Car-S-Feq [8][b] 92.31(7)
m [D]
De
±
20.3
0.091
5.58
21.3
0.088
of the pentafluorosulfuranyl group are staggered relative to
the ortho hydrogen atoms of the benzene moiety, resulting in
local C2v symmetry. The only significant difference with regard
to the molecular dipole moment between the PM3-optimized
33.3
0.025
Dn
[a] The experimental bond lengths and angles are based on the X-ray
structure; the dielectric anisotropy (De) and birefringence (Dn) were
extrapolated from ZLI-4792.[14] The calculated data were obtained by three
different methods: PM3//PM3: PM3 structure optimization, polarizabil-
ities, and dipole moment.[10] 6-31G*//6-31G*: Ab initio (HF/6-31G*)
structure optimization, polarizabilities, and dipole moment.[17b] PM3//6-
31G*: HF/6-31G* structure optimization, subsequent PM3 single point for
polarizabilities and dipole moment as required for the electrooptical data.
[b] Average values for all four Feq atoms.
structure and the X-ray structure is the Car-S-Feq angle: aPM3
ˆ
95.68 vs aX-ray ˆ 92.38. At first glance we thought that this
difference was too small to account for the deviation of ten
units from our predicted De.
Intuitively, one expects that materials such as 1c and 8
derive their high dielectric anisotropy mostly from the one
sulfur± fluorine bond directed along the long molecular axis
of the liquid crystal.[2] The other four S Feq dipoles are
directed like a ring symmetrically in nearly perpendicular
directions, due to the octahedral geometry at the hypervalent
sulfur atom. Therefore, they should not contribute very much
to the molecular dipole moment. The electronegativities
(EN)[16] are 4.1 for fluorine, 2.44 for sulfur, and 2.5 for carbon.
Accordingly, under the assumption of similar polarizability of
the aromatic part of the molecule, the dipole moments of 1d
(C F dipole; DEN ˆ 1.6; bond length ꢀ136 pm) and 1c
(S Fax dipole; DEN ˆ 1.66; bond length ꢀ158 pm) should
differ by no more than ꢀ20%. Contrary to the expectations
based on this rather crude model, the measured De values are
4.3 for 1d and 12.0 for 1c. This big difference in polarity is too
large to be explained only by the stronger-than-usual polar-
ization due to the hypervalent character of only one axial
S Fax bond.
An ab initio model study on pentafluorosulfuranylbenzene
indicates the reason for the grossly overestimated dipole
moments m and De values as predicted from PM3-optimized
structures: The dipole moment depends strongly on the Car-S-
Feq angle a, which yields readily to steric pressure (Figure 2).
A deformation from its minimum (ꢁ92.58) by 18 leads to an
additional dipole moment of roughly 0.3 D and requires less
So what is the reason for the strong dipole moment and the
high dielectric anisotropy of aromatic pentafluorosulfuranyl
derivatives? We approached the problem from two sides: our
first goal was a better structure prediction, especially for the
presumably sensitive Car-S-Feq angle a. The second goal was to
find out more about the dependence of the molecular dipole
moment on a.
The best fit between X-ray data and a calculated structure
was obtained when the structure was optimized on the HF/6-
31G* level.[17a] While the Car-S-Feq angle a was very close to
the experimental value, the dipole moments and the polar-
izabilities based on ab initio calculations tended to be far too
Figure 2. Correlation between dipole moment m, the deformation energy,
and the twist angle a for pentafluorosulfuranylbenzene, calculated by an ab
initio (HF/6-31G*) and a DFT method (B3LYP/6-31G*).[17b] ˆ HF/
^
~
&
*
6-31G*,
ˆ HF/B3LYP/6-31G*,
ˆ dipole HF/6-31G*,
ˆ dipole
B3LYP/6-31G*.
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1
[9] a) W. A. Sheppard, J. Am. Chem. Soc. 1960, 82, 4751 ± 4752; b) W. A.
Sheppard, J. Am. Chem. Soc. 1962, 84, 3064 ± 3071; c) W. A. Sheppard,
J. Am. Chem. Soc. 1962, 84, 3072 ± 3076.
[10] a) M. Bremer, K. Tarumi, Adv. Mater. 1993, 5, 842 ± 848; b) M. Klasen,
M. Bremer, A. Götz, A. Manabe, S. Naemura, K. Tarumi, Jpn. J. Appl.
Phys. 1998, 37, L945 ± L948.
than 1 kcalmol
for moderate deformations. A density
functional theory calculation (DFT; B3LYP/6-31G*) predicts
slightly lower strain energies also for larger deformations.
The calculations in combination with the X-ray structure
data indicate that the relatively high dipole moment m of the
aromatic pentafluorosulfuranyl group is linked to two fea-
tures. First, a twist of the equatorial fluorine atoms by roughly
2.58 out of the equatorial plane results in an additional
component to the overall dipole moment in the direction of
the long molecular axis. Second, the S F bonds are highly
polarized due to the hypervalence of the sulfur atom.
An interesting point with regard to materials design is the
theoretical possibility to ªtuneº the dipole moment of the
pentafluorosulfuranyl group by pushing the ªsoftº equatorial
fluorine atoms into the desired direction with specific steric
pressure of neighboring groups.
The pentafluorosulfuranyl group as a polar terminal group
for liquid crystals leads to a new class of liquid crystals with
the highly attractive combination of very strong dielectric
anisotropy (De) and reasonably high extrapolated clearing
points. A computational model was developed with the aim to
understand and predict the electrooptic properties of liquid
crystals based on hypervalent sulfur fluorides. Due to its
chemical robustness and its polar but lipophilic character, the
pentafluorosulfuranyl group surely also has some unexplored
potential as a structural component not only liquid crystals
but also in polymers and pharmaceuticals.
[11] V. Reiffenrath, R. Eidenschink, G. Weber (Merck KGaA), DE-B
3721268, 1987 (WO-A 8810251 A1) [Chem. Abstr. 1993, 110, 240329].
[12] M. P. Greenhall, presentation FRx C-2 at the 15th International
Symposium on Fluorine Chemistry, Vancouver, Canada, 1997.
[13] Some general methods for the synthesis of liquid crystals: a) E.
Poetsch, Kontakte (Darmstadt) 1988, 15 ± 28, and references therein;
b) P. Kirsch, E. Poetsch, Adv. Mater. 1998, 10, 602 ± 605.
[14] The application-oriented evaluation of materials that do not show any
mesophases, such as nearly all highly fluorinated two-ring materials, is
centered around ªvirtualº clearing temperatures, electrooptic param-
eters, and viscosities. These data are obtained by extrapolation from a
standardized nematic host mixture: TNI,extr, De, Dn, and g1 were
determined by linear extrapolation from a 10% w/w solution in the
commercially available Merck mixture ZLI-4792 (TNI ˆ 92.88C, De ˆ
5.27, Dn ˆ 0.0964). For the pure substances the mesophases were
identified by optical microscopy and the phase transition temper-
atures by differential scanning calorimetry (DSC).
[15] 8: Purity by HPLC: 99.9%; m.p. 698C (n-heptane); 1H NMR
(300 MHz, CDCl3, 303 K): d ˆ 0.90 ± 0.95 (m, 3H), 1.04 ± 1.13 (m,
2H), 1.27 ± 1.48 (m, 2H), 2.05 ± 2.20 (m, 1H), 3.48 ± 3.55 (m, 2H),
4.18 ± 4.26 (m, 2H), 5.43 (s, 1H), 7.56 (d, 2H, J ˆ 10.6 Hz), 7.75 (d, 2H,
J ˆ 10.6 Hz); 19F NMR (280 MHz, CDCl3, 303 K): d ˆ 29.8 (quint, 1F,
‡
J ˆ 155 Hz), 51.3 (d, 4F, J ˆ 155 Hz); MS (EI): m/z: 332 [M ], 313
‡
[M
F]. Single crystals for the X-ray structure analysis were
obtained by crystallization from n-heptane. Crystal structure data
Å
for 8 (C13H17F5O2S): triclinic, P1, a ˆ 7.3426(6), b ˆ 8.8105(7), c ˆ
12.1867(9) Š, a ˆ 80.275(6), b ˆ 79.740(6), g ˆ 68.448(6)8, Vˆ
1
716.91(10) Š3, Z ˆ 2, 1calcd ˆ 1.539 gcm
,
R(F) ˆ 0.0386 for 2546
observed independent reflections (3.428 ꢂ 2Wꢂ 50.248). Crystallo-
graphic data (excluding structure factors) for the structure reported
in this paper have been deposited with the Cambridge Crystallo-
graphic Data Center as supplementary publication no. CCDC-112480.
Copies of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: (‡44)1223-
336-033; e-mail: deposit@ccdc.cam.ac.uk).
Received: December 18, 1998
Revised version: February 15, 1999 [Z12797IE]
German version: Angew. Chem. 1999, 111, 2174 ± 2178
Keywords: hypervalent compounds ´ liquid crystals ´ meso-
gens ´ molecular modeling ´ sulfur
[16] a) H. O. Pritchard, H. A. Skinner, Chem. Rev. 1955, 55, 745 ± 786;
b) A. L. Allred, J. Inorg. Nucl. Chem. 1961, 17, 215 ± 221; c) J. Hinze,
Fortschr. Chem. Forsch. 1968, 9, 448 ± 485.
[17] a) The calculations were done with Spartan 5.0: Wavefunction Inc.,
18401 Von Karman Avenue, Suite 370, Irvine, CA 92612; b) for the
calculations on pentafluorosulfuranylbenzene Gaussian 94 was used:
M. J. Frisch, G. W. Trucks, H. B. Schlegel, P. M. W. Gill, B. G. Johnson,
M. A. Robb, J. R. Cheeseman, T. Keith, G. A. Petersson, J. A.
Montgomery, K. Raghavachari, M. A. Al-Laham, V. G. Zakrzewski,
J. V. Ortiz, J. B. Foresman, J. Cioslowski, B. B. Stefanov, A. Nanayak-
kara, M. Challacombe, C. Y. Peng, P. Y. Ayala, W. Chen, M. W. Wong,
J. L. Andres, E. S. Replogle, R. Gomperts, R. L. Martin, D. J. Fox, J. S.
Binkley, D. J. Defrees, J. Baker, J. P. Stewart, M. Head-Gordon, C.
Gonzalez, J. A. Pople, Gaussian, Pittsburgh PA, 1995.
[1] D. Pauluth, T. Geelhaar, Nachr. Chem. Tech. Lab. 1997, 45, 9 ± 15.
[2] The dielectric anisotropy is defined as De ˆ e e , the birefringence
k
?
as Dn ˆ nk n?, where k stands for parallel and ? perpendicular to
the nematic phase director, which is approximated by the long
molecular axis. The correlation between De, the dipole moment m, and
the angle b between the molecular dipole and the director is the
following: De ꢀ Da F(m2/2kB T)(1 3cos2b)S; Da is the anisotropy
of the polarizability, F the reaction field factor, and S the order
parameter: a) W. Maier, G. Meier, Z. Naturforsch. A 1961, 16, 262 ±
267; b) D. Demus, G. Pelzl, Z. Chem. 1981, 21, 1 ± 9.
[3] W. H. de Jeu, Physical Properties of Liquid Crystalline Materials,
Gordon & Breach, New York, 1980.
[4] M. Bremer, S. Naemura, K. Tarumi, Jpn. J. Appl. Phys. 1998, 37, L88 ±
L90.
[5] A. Sasaki, T. Uchida, S. Miyagami, Jpn. Display ꢀ86, 1986, 62.
[6] a) K. Tarumi, M. Bremer, T. Geelhaar, Annu. Rev. Mater. Sci. 1997, 27,
423 ± 441; b) E. Bartmann, Ber. Bunsen-Ges. Phys. Chem. 1993, 97,
1349 ± 1355.
[7] An approach to compensate for the low clearing points of polar
fluorinated liquid crystals is reported in our previous communications:
a) P. Kirsch, K. Tarumi, Angew. Chem. 1998, 110, 501 ± 506; Angew.
Chem. Int. Ed. 1998, 37, 484 ± 489; b) P. Kirsch, M. Heckmeier, K.
Tarumi, Liq. Cryst. 1999, 26, 449 ± 452.
[8] Reviews on pentafluorosulfuranyl derivatives: a) D. Lentz, K. Seppelt
in Chemistry of Hypervalent Compounds (Ed.: K. Akiba), Wiley, 1999,
Chapter 10, p. 295; b) R. Winter, G. L. Gard, ACS Symp. Ser. 1994,
555, 128.
1992
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