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.
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