Guest-host liquid crystalline composition

A novel nematic liquid crystalline composition is provided. A novel liquid crystalline chiral compound, (+)5-(2-methyl) butyl-2(4-cyanophenyl)-1, 3-dioxane, is also provided. When admixed together, these novel components form a phase-change cholesteric liquid crystalline composition which is highly useful as the host medium in a guest-host electrooptical display as a result of its low electrical threshold and optical saturation voltages, steep saturation curve, relatively low optical birefringence, low viscosity and broad liquid crystalline mesomorphic range. The host medium in conjunction with a suitable pleochroic dye provides an improved guest-host composition which exhibits a contrast of greater than 3:1 in certain displays, especially those having liquid crystal alignment layer means providing a moderate tilt angle of 10.degree.-40.degree. at the substrate/guest-host composition interface.

FIELD OF THE INVENTION 
This invention relates to improved electrooptical displays especially of 
the guest-host type and to liquid crystalline admixtures and to chiral 
additives useful as host media in such displays for providing improved 
contrast at low applied voltage in the multiplexed mode. 
BACKGROUND OF THE INVENTION 
The family of electrooptical display devices known generally as guest-host 
devices have high potential utility for information display purposes such 
as digital watches or clocks, calculators and other instruments. The 
typical guest-host device includes a pair of flat, parallel transparent 
substrates carrying transparent electrode segments on their facing 
surfaces and a mixture of nematic liquid crystal host compounds and a 
guest dichroic dye compound sealed between the substrates and electrodes. 
In this arrangement, the guest dye molecules tend to assume the 
orientation of the host liquid crystal molecules relative to the spaced 
substrates. The construction and operation of such guest-host 
electooptical display devices are well known as shown in the Helmeier U.S. 
Pat. No. 3,551,026 issued Dec. 29, 1970; the Ushiyama U.S. Pat. No. 
4,241,339 issued Dec. 27, 1980; the Suzuki et al. U.S. Pat. No. 4,257,682 
issued Mar. 24, 1981; and the Togashi U.S. Pat. No. 4,266,859 issued May 
12, 1981. 
In one type of guest-host display, the host liquid crystal molecules and 
therefore the guest dye molecules are aligned with their long axis 
parallel (homogenous) to the spaced substrates in the unactivated (off) 
state. However, when an electric field is generated across the electrode 
segment, the liquid crystal molecules align perpendicular 
(homeotropically) to the substrates as do the guest dye molecules. Since 
the dichroic dye molecules absorb only light whose electric vector lies 
along the long dye axis, the homeotropically aligned dye molecules absorb 
little light and the liquid crystaldye mixture between activated electrode 
segments appears essentially colorless or transparent to the viewer of 
incident light. Of course, homogenously aligned areas of the mixture 
appear colored or dark as a result of the perpendicular orientation of the 
dye molecules to the incident light. A display having light or colorless 
digits or symbols on a dark or color background is thereby provided. 
However, guest-host display devices of this type suffer from a serious 
drawback in that, at best, the homogenously aligned dye molecules will 
absorb only 50% of the light incident upon the device, thereby resulting 
in poor display contrast. This limitation is due to the fact that only one 
polarization direction of the incident light has its electric vector 
aligned along the long axis of the dye molecule while the other 
polarization direction has its electric vector aligned transverse to the 
long dye axis. One attempted solution to this drawback has been to use 
well-known substrate surface alignment techniques such as rubbing to 
induce a 90.degree. twist (helix) in the long axis of the homogenously 
aligned liquid crystal molecules from one substrate to the other much as 
in the well known twisted nematic liquid crystal electrooptical display 
devices, for example, see the Taylor and White U.S. Pat. No. 3,833,287 
issued Sept. 3, 1974 and Coates and Gray U.S. Pat. No. 4,145,114 issued 
Mar. 20, 1979. The purpose of this helical molecular structure is to 
ensure that no matter what the orientation of the electric vector of the 
incident light, there will be a dye molecule at some distance between the 
spaced substrates with its long axis parallel to the vector to effect 
absorption. Thus, absorption of 90% or more of the incident light can be 
effected. Unfortunately, however, as is well known in conventional twisted 
nematic liquid crystal devices, the host liquid crystal exhibits a 
positive birefringence and tends to act as an optical waveguide so that 
the polarization of light transmitted through the device is twisted. 
Guest-host displays made with positive birefringent liquid crystal 
compounds are optically equivalent to a non-twisted homogenously aligned 
guest-host device with the attendant poor contrast. 
The possibility of utilizing a host liquid crystal or mixtures thereof with 
minimal birefringent properties in such twisted guesthost display devices 
in order to increase contrast was initially proposed by Taylor in the 
Journal of Applied Physics 45(11), November 1974 at page 4,721. However, a 
practical mixture of liquid crystal compounds with low enough 
birefringence has not up to the present time been known or developed by 
prior art workers. The cyclohexyl-cyclohexane compounds first synthesized 
by Eidenschink et al., Angew. Chem. 133, p. 90 (1978) probably have low 
enough birefringent properties for twisted guest-host displays but 
mixtures containing these compounds are generally smectic rather than 
nematic at room temperature and also have low dielectric anisotropy which 
results in unacceptable slow response times during display operation 
and/or higher operating voltage. The phenyl cyclohexanes disclosed in the 
Eidenschink et al. U.S. Pat. No. 4,130,502 issued December 1978 require 
higher voltage levels and provide lower contrast which are not 
satisfactory for multiplexed electrooptical displays. 
A copending patent application U.S. Ser. No. 136,855 filed Apr. 3, 1980, 
now U.S. Pat. No. 4,322,354, in the name of Howard Sorkin and of common 
assignee herewith discloses liquid crystal compounds of the formula: 
##STR1## 
where R is alkyl, alkoxy, aryl, aryloxy, carboxy or carboxy ester. These 
compounds have a very low electrical threshold voltage of approximately 
0.6 volt, and relatively low optical birefringence of .DELTA. n equal to 
0.1 East German Pat. Nos. 139,852 and 139,867 disclose dioxane compounds 
of the general formula 
##STR2## 
and liquid crystalline admixtures containing such compounds. 
A copending patent application U.S. Ser. No. 135,381 filed Mar. 28, 1980, 
now U.S. Pat. No. 4,298,528, in the name of Nicholas Sethofer and of 
common assignee herewith describes liquid compounds of the formula: 
##STR3## 
when R and R.sup.1 can be the same or different straight chain alkyl or 
alkoxy group. These compounds exhibit extremely low optical birefringence 
values of .DELTA. n equal to 0.05 and in some cases 0.005. Liquid 
crystalline compounds having the formula: 
##STR4## 
where R and R.sup.1 are as described are also disclosed in the referenced 
application as well as pending patent applications U.S. Ser. No. 219,672 
filed Dec. 24, 1980, now U.S. Pat. No. 4,323,504, and U.S. Ser. No. 
226,298 filed Jan. 19, 1981, now U.S. Pat. No. 4,323,473, and are useful 
in raising the clearing point of liquid crystalline compositions. 
Also, a copending patent application U.S. Ser. No. 219,673 filed Dec. 24, 
1980, now U.S. Pat. No. 4,325,830, in the name of Nicholas Sethofer and of 
common assignee herewith discloses three ring liquid crystalline compounds 
of the formula: 
##STR5## 
where R.sub.1 is typically an alkyl group and R.sub.2 is typically an 
alkyl, alkoxy, nitro or cyano group and ring N can be a benzene or 
cyclohexyl ring. These compounds are also useful in raising the clearing 
point of liquid crystalline compositions. 
U.S. Pat. No. 4,200,580 issued Apr. 29, 1979 to Ying Yen Hsu and of common 
assignee herewith discloses compounds of the formula: 
##STR6## 
where R.sub.1 is a straight chain alkyl of 1 to 10 carbon atoms and 
R.sub.2 is alkyl, alkoxy, acyloxy, alkyl carbonato having 1 to 10 carbons, 
CN or NO.sub.2. 
A copending patent application U.S. Ser. No. 212,303 filed Dec. 3, 1980, 
now U.S. Pat. No. 4,313,878, in the name of the same inventor and also of 
common assignee herewith describes liquid crystalline compounds of the 
formula: 
##STR7## 
where R.sub.1 and R.sub.2 are as described in the U.S. Pat. No. 4,200,580. 
Compounds of the 1,3 dioxane type having pharmaceutical use are disclosed 
in the Rhodes et al. U.S. Pat. No. 4,085,222 issued Apr. 18, 1978. These 
compounds, however, do not exhibit liquid crystalline behavior and are not 
useful in electrooptic displays. 
Chiral or chiral containing additives for liquid crystalline compositions 
are also known. For example, the Coates et al. U.S. Pat. No. 4,195,916 
issued Apr. 1, 1980 illustrates chiral esters and their use in 
electrooptic displays. The Gray et al. U.S. Pat. No. 4,219,256 issued Aug. 
26, 1980 discloses compounds of the cyanophenyl-alkyl substituted bicyclo 
(2.2.2) octane type where the alkyl substituent may contain a chiral 
center. Three-ring compounds, in particular trans-4-alkylcyclohexane-1- 
carboxylic acid esters and ester derivatives of 1-carboxy4-alkyl 
substituted bicyclo (2.2.2) octane where the alkyl group may include a 
chiral center are taught in the Coates et al. U.S. Pat. No. 4,113,647 
issued Sept. 12, 1978 and the Gray et al. U.S. Pat. No. 4,261,652 issued 
Apr. 14, 1981, respectively. 
Commonly used chiral additives such as those of the cholesteryl nonanoate 
type produce a short helical molecular pitch in liquid crystalline 
mixtures but exhibit a weak dielectric anisotropy. Certain known optically 
active compounds such as 4-cyano-4.sup.1 - (2-methyl) butylbiphenyl (CB-15 
available from BDH, Ltd.) produce only a moderate helical pitch and 
exhibit only moderate dielectric anisotropy which properties have not been 
adequate for low voltage, multiplexed operation. 
What is still needed is a liquid crystalline host composition having 
substantially lower optical birefringence than currently available 
mixtures along with other required properties and improved chiral 
additives compatible with the host composition to provide a guest-host 
electrooptic display with improved contrast, e.g. a contrast ratio of 3:1 
at voltages of about 3 volts. 
SUMMARY OF THE INVENTION 
One object of the invention is to provide a host liquid crystalline 
composition having low optical birefringence, low electrooptical threshold 
voltage, low saturation voltage and steep saturation curve. 
Another object of the invention is to provide a novel chiral compound 
useful as an additive in liquid crystalline compositions, especially those 
provided herein. 
Still another object of the invention is to provide a phasechange 
cholesteric guest-host liquid crystalline composition incorporating the 
novel chiral additive and one or more pleochroic dyes and exhibiting 
improved contrast such as at least a 3:1 contrast ratio in at least the 
biplexed mode of operation at voltages of 3 volts or below. 
Still another object of the invention is to provide a guest-host 
electrooptical display device which includes constrast-enhancing alignment 
means. 
The host liquid crystalline composition of the invention includes the 
following compounds with the specified formulas: 
(I) 
##STR8## 
where R.sub.1 and R.sup.1 are alkyl especially straight chain, more 
especially R.sub.1 is ethyl and R.sup.1 is n-pentyl group, 
(II) 
##STR9## 
where R.sub.2 and R.sup.1.sub.2 are alkyl, especially straight chain, 
more especially where R.sub.2 is ethyl and R.sup.1.sub.2 is n-pentyl 
group. 
(III) 
##STR10## 
where R.sub.3 is alkyl, especially straight chain, more especially 
n-butyl, n-pentyl, n-hexyl and n-heptyl groups. 
(IV) 
##STR11## 
where R.sub.4 and R.sup.1.sub.4 are alkyl, especially straight chain, 
more especially where R.sub.4 is n-propyl and R.sup.1.sub.4 is either 
n-pentyl or n-heptyl groups. 
(V) 
##STR12## 
where R.sub.5 is alkyl, especially straight chain, more especially either 
of n-propyl, n-butyl or n-pentyl groups. 
Compounds I-V are present in the trans isomer configuration. 
In a particular preferred embodiment of the host nematic liquid crystalline 
composition, compound I is present in an amount of 5 to 25 weight percent; 
compound II, 5 to 25 weight percent; compounds III total 40 to 80 weight 
percent, compounds IV total 10 to 40 weight percent and compounds V total 
5 to 15 weight percent. 
In a particular more preferred embodiment of the host liquid crystalline 
compositions, the compounds are present as follows: 
______________________________________ 
Weight 
% 
______________________________________ 
5-ethyl-2-(4-pentylcyclohexyl)-1,3-dioxane 
5-25 
5-ethyl-2-[4-(pentylcyclohexyl)cyclohexyl]-1,3-dioxane 
5-25 
5-butyl-2-(4-cyanophenyl)-1,3-dioxane 
10-30 
5-pentyl-2-(4-cyanophenyl)-1,3-dioxane 
10-30 
5-hexyl-2-(4-cyanophenyl)-1,3-dioxane 
10-30 
5-heptyl-2(4-cyanophenyl)-1,3-dioxane 
10-30 
5-propyl-2-[4-(4-pentylcyclohexyl)-phenyl]-1,3-dioxane 
5-25 
5-propyl-2-[4-(4-heptylcyclohexyl)-phenyl]-1,3-dioxane 
5-25 
4-cyanophenyl-4'-(5-propyl-1,3-dioxan-2-yl) 
1-5 
4-cyanophenyl-4'-(5-butyl-1,3-dioxan-1-yl) 
1-5 
4-cyanophenyl-4'-(5-pentyl-1,3-dioxan-2-yl) 
1-5 
______________________________________ 
##STR13## 
where * represents an asymmetric carbon atom. This chiral compound is 
useful as an additive to liquid crystalline compositions, especially host 
liquid crystalline compositions described above, in additive amounts of 
about 1 to 10 weight percent. 
The present invention also provides an improved phase-change cholesteric 
guest-host electrooptical composition as a result of including in the 
aforementioned host composition the chiral additive described above along 
with one or more pleochroic dyes. 
The present invention also provides an improved guest-host electrooptical 
display device having contrast enhancing alignment means on the display 
substrate means, preferably in combination with the inventive cholesteric 
guest-host composition.

DESCRIPTION OF PREFERRED EMBODIMENTS 
The compounds I through VI set forth hereinabove may be prepared as 
follows: 
##STR14## 
where R are alkyl chains, preferably straight chain with C.sub.2 through 
C.sub.5 and the optically active 2-methylbutyl isomer (in compound VI) and 
##STR15## 
are: 
##STR16## 
Exact synthetic procedures for the types of compounds I through VI can be 
found in the following pending U.S. patent applications, the teachings of 
each of which are incorporated herein by reference: 
Compound I--U.S. Ser. No. 135,381 filed Mar. 28, 1980, now U.S. Pat. No. 
4,298,528. 
Compound II--U.S. Ser. No. 219,672 filed Dec. 24, 1980, now U.S. Pat. No. 
4,323,504, and Ser. No. 226,298 filed Jan. 19, 1981, now U.S. Pat. No. 
4,323,473. 
Compound III--U.S. Ser. No. 136,855 filed Apr. 3, 1980, now U.S. Pat. No. 
4,322,354. 
Compound IV--U.S. Ser. No. 219,673 filed Dec. 24, 1980, now U.S. Pat. No. 
4,325,830. 
Compound V--U.S. Ser. No. 212,303 filed Dec. 3, 1980, now U.S. Pat. No. 
4,313,878. 
An illustration of the preparation of the novel chiral compound is provided 
in the following Example: 
EXAMPLE 
(+) 5-(2-methyl) butyl-2(4-cyanophenyl)-1,3-dioxane 
(A) 
##STR17## 
where 
##STR18## 
The alkylation of a malonic acid ester was carried out with potassium 
carbonate in dimethyl formamide, reaction mixture being stirred for one 
week at room temperature to yield about 90% of compound II. Optically 
active (2-methyl)-butyl bromide was purchased from Aero Chemical Co., 
Newark, New Jersey. For full description of above synthetic step see U.S. 
Pat. No. 4,298,528 issued Nov. 3, 1981 to N. Sethofer. Reduction of alkyl 
malonic ester was carried out with lithium aluminum hydride in diethyl 
ether, by the method described in Fieser & Fieser: "Reagents for Organic 
Syntheses," Vol. 1, p. 584. 
(B) 
##STR19## 
where 
##STR20## 
Final step in the (+) 5-(2-methyl) butyl-2-(4-cyanophenyl)-1-3-dioxane 
synthesis consists of condensation reaction, where aldehyde and 1.2 molar 
excess of optically active diol are refluxed with the catalytical amount 
of p-toluenesulfonic acid in benzene or toluene. Reaction is completed 
when calculated amount of water is collected in the attached Dean-Stark 
trap, e.g., on the average of 40 to 60 minutes. Reaction mixture is then 
cooled, washed first with 10% NaOH solution in water, then several times 
with water, layers separated and solvent evaporated. 
Reaction yields mixture of trans and cis isomers of desired dioxanes, and 
on the average, about 5 to 10% of other impurities. The latter can be 
easily removed by crystalization from methanol. Separation of isomers 
(usually in 3:1 ratio for trans-cis in raw material) can be accomplished 
either by repeated crystalization from hexanes or by employing of 
chromatographic methods. 
Compound IV, i.e., 4-cyanobenzaldehyde, was purchased from Aldrich Chemical 
Co., catalog No. C8-960-9. 
The host liquid crystalline composition of the present invention is 
illustrated by means of the following example which is included for 
purposes of illustration rather than limitation: 
EXAMPLE 
______________________________________ 
Weight 
% 
______________________________________ 
##STR21## 10.0 
##STR22## 8.0 
##STR23## 20.0 
##STR24## 14.0 
##STR25## 17.0 
##STR26## 14.0 
##STR27## 8.0 
##STR28## 5.0 
##STR29## 1.5 
##STR30## 1.3 
##STR31## 1.2 
______________________________________ 
Typically, host liquid crystalline mixtures for multiplexable guesthost 
displays require the following characteristics, namely, low optical 
birefringence, extremely low electrooptical threshold and saturation 
voltage, broad temperature range and its own high order parameter in order 
to assure low voltage operation of the final guesthost complex in 
multiplexed displays. The admixture shown in Example I exhibits melting 
point of about -20.degree. C. (crystalline to nematic transition 
temperature), clearing point of 72.5.degree. C. (nematic to isotropic 
transition temperature); .DELTA..eta. of 0.097 (optical birefringence) at 
22.degree. C., viscosity about 30 cp at 25.degree. C. The electrooptic 
characteristics of this admixture in a display with a 11 to 12 micron 
plate spacing and with 30.degree./12.degree. SiO evaporation angle 
alignment layer (i.e. about 1.degree./20.degree. molecular surface tilt 
angle) were as follows: 
V.sub.10 (10% saturation)=0.95 V 
V.sub.90 (90% saturation)=1.5 V 
response time (ON)=90 ms 
response time (OFF)=100 ms 
The above electrooptical data were obtained using known twisted nematic 
type cells, i.e. with 90.degree. twist and 2 crossed polarizers. FIG. 1 is 
a representation of the electrooptical properties of the nematic admixture 
of Example I. 
Preparation of the complexed guest-host mixture which is suitable for low 
voltage, multiplexed operation with contrast exceeding 3:1 ratio requires 
selection of a chiral additive which will have both an extremely large 
positive dielectric anisotropy and will produce a short molecular helical 
pitch when mixed with a nematic liquid crystal. The aforementioned 
commonly used chiral additives, such as cholesteryl momanoate (CHN) type 
or "optically active compounds" as 4-cyano- 4.sup.1 (2-methyl)- 
butylbiphenyl(CB-15) have not proved satisfactory in meeting all of these 
characteristics, as mentioned already hereinabove. 
Therefore, one of the objects of present invention is to provide an 
improved chiral addition for the hereinabove described mixture. Applicant 
have discovered that the compound 
(+)5-(2-methyl)butyl-2(4-cyanophenyl)-1,3-dioxane(OPDX) possesses the 
required charcteristics e.g. dielectric anisotropy (.alpha..SIGMA.) is 
greater than +17, melting point is 58.8.degree. C., monotropic 
(1-CH)57.6.degree. C.; .alpha.H=5.0 K CAL/MOLE, and cholesteric pitch of 
approximately one micron when present in an amount of 7 weight percent in 
the above described nematic composition (Example I). 
FIG. 2 is a plot of the electrooptical properties of a phasechange 
cholesteric-nematic liquid mixture obtained by introducing 3 weight 
percent of the above described chiral compound to the composition of 
Example I. This data were determined in a twisted nematic type display 
(90.degree. twist) with a 11 to 12 micron plate spacing and with 
30.degree./12.degree. SiO evaporation angle alignment layer, i.e. a 
1.degree./20.degree. molecular surface tilt angle. Two crossed polarizers 
were used. FIG. 2 shows that under identical alignment conditions the 
electrical threshold was raised by 1 volt with 3% OPDX addition, while 
ratio V.sub.SAT and V.sub.TH remains virtually unchanged. 
A guest-host electrooptical composition is provided by further introducing 
at least about 0.3 weight percent, preferably about 0.8 to about 1.2 
weight percent of a pleochroic dye into the phase-change cholesteric 
nematic composition comprising Example I plus the novel chiral additive 
(i.e. 3 weight percent). Preferably, a pleochroic dye is used having an 
order parameter of greater than 0.7 as determined by conventional 
techniques. A preferred guesthost composition employs about 0.8 weight 
percent of the dye compound: 
##STR32## 
This dye compound is described more fully by Uchida et. al. in Mol. Cryst. 
Liq. Cryst., Vol. 34, (Lett.), pp. 150-158 (1977). Of course, those 
skilled in the art will appreciate that other known pleochroic dyes or 
mixtures of dyes can be employed. 
FIG. 3 is a graph depicting the electrooptical properties of the preferred 
guest-host composition described above, i.e. Example I plus chiral 
additive (3 w/o) and dye (0.8 w/o). This data was generated using a 
display like that used to generate the electrooptical data of FIGS. 1 and 
2 with the exception that no polarizers were used or required. Also, as in 
previous displays, the properties were measured in the transmission mode. 
The display exhibited light digits on a blue background with a contrast 
greater than 3 at a voltage of 3 volts. Some contrast increase beyond 3 
volts can be attributed to homogeneous surface alignment; i.e. 30.degree. 
SiO evaporation on one of the display plate surfaces. 
FIG. 4 is a schematic view of a basic structure of an electrooptical device 
of guest-host type according to the invention. The device can be driven in 
direct drive or multiplexed mode. Particular test device was driven in 
biplexed mode, respective rms voltages being 0.9 V and 2.4 VAC, frequency 
32 Hz. 
The device consists of a cell with glass substrate 1 and spacer 3 (in this 
particular case 12 microns). Transparent electrodes 2 (typically indium 
oxide) and substrate 1 are covered with surface aligning SiO layer 4 which 
arranges the surface liquid crystal dye molecular layer so that a 
10.degree. to 40.degree. tilt (measured relative to the substrate surface) 
is achieved (in this particular case tilt angle was 20.degree. to 
23.degree.). FIG. 5 is an enlarged view showing the tilt angle of the 
molecules relative to the substrate. 
The guest-host composition 5 (in this particular case the preferred mixture 
described above produces a helical structure in the off state, thus 
intensifying the color of the background by exposing more dye molecules 
for light absorption. The helix also controls the electrooptical threshold 
and steepness of the saturation curve (see FIG. 6) thus allowing the 
device to be multiplexed. Upon applying the required electric field 
between the electrodes 2, the cholesteric structure of layer 5 is 
disrupted and molecules align perpendicularly with their long axes to the 
substrate e.g. shown by reference numeral 6 in FIG. 4. Above described 
surface molecular pre-tilt of 20.degree. to 23.degree. assisted to a great 
extent in creating more complete homeotropic alignment under the electric 
field, thus diminishing coloration of energized segments and increasing 
the contrast of the device. Commonly used homegenous surface alignment (or 
rather low tilt angle up to 4.degree.) of the prior art permits residula 
layers of liquid crystal and dye molecules to remain in the parallel 
direction to the glass substrate even after electrical field is applied, 
thus diminishing the contrast ratio at low voltages. 
The electrooptical device of the invention can be employed in both 
transmissive and reflective modes. For the latter, Eastman White 
Reflectance coating (commercially available) was used as diffusing 
reflector. Other diffusing reflecting materials can be used by those 
skilled in the art.