A new high twisting power material for use as a single asymmetric dopant in cholesteric displays with a temperature independence of the helical twisting power

Article abstract of DOI:10.1080/15421400490435936

The synthesis of a novel chiral non-racemic TADDOL derivative is presented and discussed. When the material is placed in a suitable nematic host it forms a chiral nematic phase which has an independence of the helical twisting power (HTP) with temperature

Full text of DOI:10.1080/15421400490435936

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A New High Twisting Power
Material for Use as a Single
Asymmetric Dopant in
Cholesteric Displays with a
Temperature Independence of
the Helical Twisting Power
a
a
Alexander J. Seed , Margaret E. Walsh , J.
b
b
William Doane & Asad Khan
a
Department of Chemistry , Kent State University ,
Kent, Ohio, USA
b
Kent Displays Incorporated , Kent, Ohio, USA
Published online: 18 Oct 2010.
To cite this article: Alexander J. Seed , Margaret E. Walsh , J. William Doane &
Asad Khan (2004) A New High Twisting Power Material for Use as a Single Asymmetric
Dopant in Cholesteric Displays with a Temperature Independence of the Helical
Twisting Power, Molecular Crystals and Liquid Crystals, 410:1, 201-208, DOI:
10.1080/15421400490435936
To link to this article: http://dx.doi.org/10.1080/15421400490435936
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Mol. Cryst. Liq. Cryst., Vol. 410, pp. 201=[729]–208=[736], 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490435936
A NEW HIGH TWISTING POWER MATERIAL FOR
USE AS A SINGLE ASYMMETRIC DOPANT IN
CHOLESTERIC DISPLAYS WITH A TEMPERATURE
INDEPENDENCE OF THE HELICAL TWISTING
POWER
Alexander J. Seed and Margaret E. Walsh
Department of Chemistry, Kent State University, Kent,
Ohio 44242-0001, USA
J. William Doane and Asad Khan
Kent Displays Incorporated, 343, Portage Blvd, Kent,
Ohio 44240, USA
The synthesis of a novel chiral non-racemic TADDOL derivative is presented
and discussed. When the material is placed in a suitable nematic host it forms
a chiral nematic phase which has an independence of the helical twisting
power (HTP) with temperature. The observed HTP is approximately twice as
large as HTPs found in conventional chiral additives, and the independence
ꢀ
of the HTP is seen over a 90C temperature range. The HTP as a function of
temperature may be modified by the addition of both liquid crystalline and
non liquid crystalline achiral additives. The material is unique in that it is
also soluble in commercial nematic host materials and has proven to be
extremely robust in both thermal and photochemical testing.
Keywords: asymmetric; chiral nematic; cholesteric displays; HTP; synthesis; TADDOL
INTRODUCTION
Bistable cholesteric display devices are excellent candidates for low power
consumption devices that offer high brightness and broad viewing angles [1].
This work is supported in part by the National Science Foundation grant DMI-0091522. The
authors would like to acknowledge the work of Mr. Ross Armbruster and Dr. Mahinda Gangoda
for measurements and physical characterization. AJS would like to express his thanks for the
generous financial support of Kent State University and Kent Displays Inc.
Address correspondence to Alexander J. Seed, Department of Chemistry, Kent State
University, Kent, OH 44242-0001, USA.
201=[729]

2
02=[730]
A. J. Seed et al.
The chiral nematic media used in such devices are often generated by
the inclusion of one or more chiral non-racemic dopants [2] into a
suitable nematic host material, causing the nematic mixture to adopt
a helical structure that reflects light according to Bragg’s law. The
addition of such asymmetric dopants will clearly affect the transition
temperatures, viscosity, dielectric properties etc. of the host material,
and usually in an adverse way. In order to minimize such problems it
is desirable to add a very small concentration of the chiral dopant
which must therefore possess a very large HTP in order to obtain a
pitch in the visible region.
The wavelength (k) of selective reflection is equal to the product np
n is the average refractive index of the chiral nematic host and p is the
(
pitch of the chiral nematic helix). Since the reciprocal of the pitch length
ꢁ
1
is related to the concentration x of the chiral dopant by p ¼ bx (b is
the HTP) it is clear that a tighter helical twist is generated as the concen-
tration x is increased. Current asymmetric dopants have relatively low
values of HTP and as a consequence are typically used at concentrations
of 20% or higher in a nematic host material. Apart from the detrimental
changes vide supra, the chiral non-racemic dopant is usually the most
expensive chemical component adding an additional concern.
In order for the display to be of practical use it is essential that the
display color remains constant over a wide temperature operating range.
To obtain an independence of the HTP with temperature it is usually
necessary to mix several chiral dopants which adds further cost and
complexity to the system and it is clear that this is far from an optimum
situation [3,4].
Recently we have synthesized and evaluated the first example of an
asymmetric dopant that appears to fulfill all of the desired criteria for an
ideal chiral additive. The single enantiomer material is derived from
commercially available and inexpensive tartaric acid and has a HTP of
ꢁ
1
3
1 mm . In addition, the material exhibits a near independence of the
ꢀ
HTP with temperature over a 90C range. Rigorous testing has shown that
mixtures containing this dopant are both thermally and photochemically
stable.
SYNTHESIS
The preparation of 2 (Scheme 1) used the method of Freedman [5] with
crystallization from acetonitrile providing a pure sample in excellent yield
(
78%).
The preparation of 4 was initially attempted using five equivalents of the
Grignard derived from 2 according to the method of Beck et al. [6].

A New High Twisting Power Material
203=[731]
0
0
SCHEME 1 Synthesis of (4R,5R)-2,2-dimethyl-a,a,a ,a -tetrakis[6-(benzyloxy)-
-naphthyl]-1,3-dioxolan-4,5-dimethanol.
2
However, the Grignard proved extremely difficult to form even when using
sonication and 1,2-dibromoethane as activators. Instead we chose to use
an excess of the organolithium derived from 2 which proved to be a con-
venient replacement for the Grignard. Since this work we have successfully
synthesized numerous TADDOL derivatives [7,8] using such organolithium
methodology. Due to the steric crowding of TADDOL 4 it was found that
formation of the desired product was difficult and low yields were
accompanied by the formation of byproducts that were present due to
incomplete reaction. However, this problem can always be resolved by
careful chromatographic purification.

2
04=[732]
A. J. Seed et al.
2-Benzyloxy-6-bromonaphthalene (2)
6
0
-Bromo-2-naphthol (1) (39.18 g, 0.1756 mol), benzyl bromide (27.31 g,
.1597 mol), anhydrous potassium carbonate (44.14 g, 0.3194 mol), and
butan-2-one (500 mL) were heated under reflux for 3.6 days (for con-
venience). The potassium salts were filtered off and washed with diethyl
ether (200 mL). The filtrate was washed with potassium hydroxide (5%,
5
00 mL) and the layers were separated. The aqueous layer was extracted
with diethyl ether (2 ꢂ 250 mL) and the combined ether extracts were
washed with water (500 mL) and dried (MgSO ). The drying agent was
4
filtered off and the solvent was removed in vacuo to yield a tan solid.
The solid was mixed with acetonitrile and filtered. The off-white solid
obtained was crystallized from acetonitrile and dried in vacuo (P O ,
2
5
ꢀ
paraffin wax, 72 h). Yield ¼ 39.34 g (79%), mp ¼ 114.3–114.5C, purity
1
(
GC) ¼ 100%. H NMR (300 MHz, CDCl ) d 7.91(d, J ¼ 2.0 Hz, 1H),
3
7
7
5
.66(d, J ¼ 9.0 Hz, 1H), 7.59(d, J ¼ 9.0 Hz, 1H), 7.52–7.49(m, 1H),
.49–7.33(m, 5H), 7.24(dd, J ¼ 8.0, 3.0 Hz, 1H), 7.18(d, J ¼ 3.0 Hz, 1H),
ꢁ
1
.
.16(s, 2H). IR (neat) mmax 2942, 1623, 1585, 1495 cm
0
-naphthyl]-1,3-dioxolan-4,5-dimethanol (4)
0
(
2
4R, 5R)-2,2-dimethyl-a,a,a ,a -tetrakis[6-(benzyloxy)-
n-Butyllithium (22.3 mL, 56 mmol, 2.5 M in hexane) was added dropwise to
a stirred solution of 2 (19.95 g, 63.70 mmol) and anhydrous THF (555 mL)
ꢀ
ꢀ
at ꢁ75C to ꢁ77C under dry argon (the solution gradually turned yellow
during the addition). After the addition was complete the reaction mixture
was stirred at ꢁ77C for an additional 45 min. A solution of 3 (2.90 g,
ꢀ
1
3.3 mmol) in anhydrous THF (27 mL) was added dropwise to the reaction
ꢀ
mixture at ꢁ75 to ꢁ77C and the bright red solution obtained was stirred
ꢀ
at ꢁ77C for 2 h and then allowed to warm gradually to room temperature.
ꢀ
After 36 h at room temperature, the reaction was cooled to ꢁ5C and a
saturated solution of ammonium chloride (470 mL) was added dropwise
ꢀ
at 0 to ꢁ5C. The mixture was stirred for 5 h at room temperature and
the layers were separated and the aqueous layer extracted with dichloro-
methane (2 ꢂ 225 mL). The combined organic extracts were washed with
sodium bicarbonate (50 mL saturated solution plus 750 mL water) and
dried (MgSO ). The drying agent was filtered off and the solvent was
4
removed in vacuo to yield an orange-brown semi-solid. The crude material
was purified twice by column chromatography using the following eluent
ꢀ
systems: (i) [petroleum ether (40–60C) : dichloromethane, 1:1, followed
by a gradual increase in polarity using additional dichloromethane], (ii)
ꢀ
[ethyl acetate : petroleum ether (40–60C), 1:4, then increasing to 30%
ethyl acetate]. The product was crystallized from cyclohexane to give an

A New High Twisting Power Material
205=[733]
off-white powder which was dried in vacuo (P O , paraffin wax, 72 h).
2
5
ꢀ
1
Yield ¼ 1.577 g (11%), mp ¼ 118.3–124.7C. H NMR (500 MHz, CDCl ) d
3
8
.04(app. s, 2H), 7.77(app. s, 2H), 7.74(d, J ¼ 9.3 Hz, 2H), 7.62(d,
J ¼ 8.8 Hz, 2H), 7.60–7.55(m, 4H), 7.50–7.45 (m, 4H), 7.43–7.27(m, 20H),
7
5
.24–7.18(m, 4H), 7.14(2H, dd, J ¼ 10.0 Hz, 3.3), 7.04(2H, d, J ¼ 3.3 Hz),
1
.17(s, 4H), 5.10(s, 4H), 4.89(s, 2H), 4.14(s, 2H), 1.12(s, 6H). C NMR
3
(
CDCl ) d 157.29, 157.26, 140.94, 138.80, 137.16, 137.07, 133.93, 130.37,
3
1
1
2
1
1
28.89, 128.23, 128.03, 127.81, 127.76, 127.12, 127.06, 126.61, 126.23,
25.98, 119.29, 119.25, 110.0, 107.08, 81.69, 78.74, 70.32, 70.22,
7.70 ppm. IR (neat) m
262, 1218 cm . HRMS calcd for C H O Na 1113.433688 amu, found
3316, 3032, 2986, 2925, 1632, 1603, 1502,
þ
max
ꢁ
1
75 62 8
113.42915 amu, Anal. Calcd. for C H O : C, 82.54; H, 5.73. Found:
ꢀ
75 62 8
D
C, 82.38; H, 6.00, Optical rotation: [a] 2
ꢀ
¼ ꢁ68.0 (CHCl , c ¼
3.5 C
3
0
.050 g=mL).
PHYSICAL STUDIES
With TADDOL 4 in hand we evaluated the twisting power of 4 and its
temperature dependence compared to other TADDOL derivatives with
the following general structure (Fig. 1)
Comparison with 5 (Aldrich) showed that although 4 had a lower HTP
ꢁ
1
(
31 mm ), this was offset by a near temperature independence of the
ꢀ
HTP over a 90C range. In comparison, 5 had a steep negative dependence
of helical twist with temperature (Fig. 2).
Different nematic hosts were evaluated and the temperature depen-
dence and magnitude of the HTP of 4 were found to vary widely as would
be expected for the differing solvent-solute interactions (Fig. 3). It is clear
that the TADDOL dopants will require structural tailoring to provide a
temperature independence of HTP for various nematic hosts. This tailoring
is straightforward using methodology analogous to that described above
and promising TADDOLs are now being systematically evaluated and will
provide us with unique insight into chirality transfer in such systems [9].
FIGURE 1 Generic TADDOL structure.

2
06=[734]
A. J. Seed et al.
FIGURE 2 Peak reflection wavelength as a function of temperature for compounds
ꢃ
4
and 5 in KLC-1R. The ( ) denotes the point of attachment to the dioxolane ring.
FIGURE 3 Peak reflection wavelength as a function of temperature for 4 in KLC-
R, KLC-50R, and KLC-6R. Physical properties of the three nematic mixtures are
also shown. The clearing points of the mixtures of 4 in the above nematic host mate-
1
ꢀ
ꢀ
ꢀ
ꢃ
rials are 79 C, 47 C, and 88 C respectively. The ( ) denotes point of attachment to
the dioxolane ring.

A New High Twisting Power Material
207=[735]
FIGURE 4 Peak reflection wavelength as a function of temperature for 4 in
nematic mixture KLC-1R, and in KLC-1R plus the denoted achiral additives.
ꢃ
Additives are present in the mixture at a concentration of 4% by weight. The ( )
denotes point of attachment to the dioxolane ring.
A temperature independence of the HTP may also be attained by the
addition of other chiral non-racemic or achiral additives. As part of our
exploratory program we have evaluated the use of inexpensive achiral
0
additives (Fig. 4) such as undecylcyanide (UCN) and 4-cyano-4 -pentylbi-
phenyl (5CB).
Achiral additives again provide a pathway for altering the solvent-solute
interactions although low concentrations of these additives must be used to
avoid detrimental physical property changes vide supra.
CONCLUSION
We have synthesized the first commercially useful soluble and stable single-
enantiomer dopant that forms a chiral nematic phase (when placed in a
suitable nematic host material) with a temperature independence of the
HTP. A major program of work is underway to uncover further TADDOL
dopants that display such characteristics and to use this work to study
the phenomenon of chirality transfer in liquid crystalline systems.
REFERENCES
[1] Crawford, G. P. (2000). IEEE Spectr., 37, 40.
[2] Kuball, H.-G., Weib, B., Beck, A. K., & Seebach, D. (1997). Helv. Chim. Acta., 80, 2507.

2
08=[736]
A. J. Seed et al.
[
[
[
[
[
[
3] Moia, F. (1989). In Freiburger arbeitstagung fl u¨ ssigkristalle, Freiburg: Germany, Vol. 18.
4] Gerber, P. R. (1980). Phys. Lett. A, 78, 285.
5] Freedman, H. H. & Dubois, R. A. (1975). Tetrahedron Lett., 38, 3251.
6] Beck, A. K., Gysi, P., Vecchia, L. L., & Seebach, D. (1999). Org. Synth., 76, 12.
7] Seed, A. J., Walsh, M. E., Doane, J. W., & Khan, A., Unpublished Results.
8] Khan, A., Doane, J. W., Seed, A., & Falana, O. (2002). In 2002 NSF design, service and
manufacturing grantees and research conference, Engineering communications and
marketing, college of engineering, Iowa State University: San Juan, Puerto Rico.
9] Solladi e´ , G. & Zimmermann, R. G. (1984). Angew. Chem. Int. Ed. Engl., 23, 348.
[