TECHNICAL FIELD The present invention is directed generally to liquid crystals, and, more particu- larly, to improved low voltage operation of liquid crystal devices.

BACKGROUND ART Liquid crystal display elements utilize the optical anisotropy and the dielectric anisotropy of liquid crystal materials. Various display modes and various driving methods for driving the display modes are well-known.

The properties of liquid crystal materials used for these liquid crystal display devices are various, but any liquid crystal materials have in common stability to moisture, air, heat, light, etc. Further, it is required for the devices that the liquid crystal phases have a temperature range as broad as possible, around room temperature, have a low viscosity, and, in the display device, have a quick response rate, a high contrast, and a comparatively low driving voltage. In addition, it is necessary that the liquid crystal materials have an adequate dielectric anisotropy (Ag). However, a single liquid crystal compound satisfying these characteristics apparently has not yet been found. Thus, it is common to blend several different liquid crystal compounds and non-liquid crystal compounds to form a mixture that adequately meets the needs of the specific application.

A few high dielectric anisotropy dye compounds have been reported in litera- ture. Examples include: 1. nitro-amino azobenzenes (I) and nitro-amino-tolanes (II) with structures shown below:

(Ib published by S. T. Wu et al, Asia Display, pp. 567-70 (1995) ; and 2. bicyano-amino polyene dyes (III) with structures shown below: (III)

published by S. T. Wu et al, Japanese Journal of Applied Physics, vol. 37, pp. L1254- 1256 (1998).

A general problem with these dyes is that their absorption is too large in the visible region, which causes these dyes to be colored. As a result, light transmission is greatly reduced. This is particularly undesirable for some displays and electro-optic modulators where high transmittance is required. Further, some of these dyes do not provide low voltage operation of the liquid crystal mixture.

Thus, there is a need for dyes that are colorless, have viscosities suitable for liquid crystal applications, approximately 20 mm/s, and support low voltage opera- tion, with a threshold voltage Vt,, of less than 2 Vlms.

DISCLOSURE OF INVENTION In accordance with the present invention, a dopant is provided that is colorless, has a viscosity approximately the same as that of liquid crystal compositions, and sup- ports low voltage operation. The dopants have one of three structures: (1) biphenyl; (2) diphenyl-diacetylene; and (3) double tolane, and in each case, have an amino group (secondary or tertiary) attached at one end of the molecule, with at least one polar group at the other end of the molecule. Schematically, the generic structure may be illustrated as: where ltl is an alkyl having from 1 to 12 carbon atoms, R2 is either hydrogen or, inde- pendently of Rl, an alkyl having from 1 to 12 carbon atoms, M is a polar group, X and Y are independently hydrogen or a polar group not necessarily the same as M, and (A) is (1) a single bond (biphenyl); (2)-C-C-C-C-(diphenyl-diacetylene) ; or (3)

(double tolane), where Z is hydrogen, F, or alkyl.

Also in accordance with the present invention, one or more of the foregoing col- orless dopants is added to a liquid crystal composition for supporting low voltage op- eration.

The teachings of the present invention provide new molecular structures of colorless dopant compounds for low voltage liquid crystal operation. These com- pounds exhibit an extraordinarily large dielectric anisotropy and relatively low vis- cosity. Therefore, adding a few percent of such dopant to a liquid crystal mixture re- duces the operating voltage significantly while retaining fast response time. The low voltage operation enables use of a low cost electronic driver.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1, on coordinates of phase change (7c) and voltage (V"5), is a plot de- picting the concentration effect of a nitro-amino-tolane compound on the threshold voltage of a liquid crystal mixture designated PTTP-24/36 ;

Figure 2, on coordinates of optical density and wavelength (nm), depicts the optical density of a 6 p, m homogeneous cell containing 1% nitro-amino tolane dye dissolved in a liquid crystal mixture designated ZLI-2359; and Figure 3, on coordinates of optical density and wavelength (in nm), is a plot of an absorption curve of 4-dimethyl amino-4'-fluoro biphenyl, showing that it is a col- orless compound.

BEST MODES FOR CARRYING OUT THE INVENTION Reference is now made in detail to a specific embodiment of the present inven- tion, which illustrates the best mode presently contemplated by the inventor for practic- ing the invention. Alternative embodiments are also briefly described as applicable.

Low voltage operation of a liquid crystal (LC) device is highly desirable be- cause it enables a low cost electronic driver to be used. Low threshold LC mixtures will find useful application in optical phased arrays for agile beam steering and flat panel display devices.

In the agile beam steering employing a liquid crystal phase modulator, a ho- mogeneous alignment is a preferred approach. Based on the Freedericksz transition, the threshold voltage, Vff"of a homogeneously aligned liquid crystal cell is governed by the splay elastic constant (Kll) and dielectric anisotropy (As) of the liquid crystal material as Vth = #(K11/##)1/2. A simple way to reduce Vll is to use liquid crystals having high As values, that is, polar liquid crystals with a large dipole moment, such as a cyano group. The shortcoming of this approach are twofold: (1) these polar liq- uid crystals normally possess a high melting temperature and narrow nematic range, and (2) the dielectric anisotropy of these polar liquid crystals is usually lower than 20.

The push-pull effect has been investigated in some dye molecules, e. g., the ni- tro-amino-azo dyes (I) and nitro-amino-tolane dyes (II) as shown below; see, S. T. Wu et al, Applied Physics Letters, vol. 64, pp. 2191-2193 (1994) and the Asia Display reference, supra : (I) (II) where Rl is an alkyl group or an alkoxy group having from 1 to 12 carbon atoms (CnH2n+, or OCnH2n+1) where n = 1-12) or an alkenyl group or an alkenyloxy group having from 2 to 12 carbon atoms (CnHn-1 or OCnH2n-1) and R2 is H or, independently of R, is an alkyl group or an alkoxy group having from 1 to 12 carbon atoms or an alkenyl group or an alkenyloxy group having from 2 to 12 carbon atoms.

The dielectric anisotropy of these compounds (1) and (II) is around 60. Thus, adding a few percent of such compound to a binary nonpolar diphenyl-diacetylene liquid crystal mixture PTTP-24136 would reduce the threshold voltage noticeably.

This mixture is disclosed by S. T. Wu et al, Applied Physics Letters, vol. 61, pp. 630- 632 (1992) and in U. S. Patent No. 5,338,451, issued August 16,1994. The addition

of (II) to PTTP-24/36 is disclosed and claimed in related application Serial No.

, filed [PD-99E115].

Diphenyl-diacetylene liquid crystals are known to exhibit a high birefringence, low viscosity, wide nematic range, and small heat fusion enthalpy. They have been used extensively in optical phased arrays. The structure of a diphenyl-diacetylene liq- uid crystal is shown below. where R,, and R. are alkyl groups, independently having from 1 to 12 carbon atoms.

These compounds are abbreviated as PTTP-nm, where"PTTP"is an abbre- viation for phenyl-triple bond-triple bond-phenyl. A binary mixture has been formu- lated using PTTP-24 and PTTP-36 at 1: 1 ratio, designated PTTP-24/36, mentioned above.

Figure 1 depicts the effect of doping the PTTP-24/36 eutectic mixture with 5 and 10% of compound (II) with Rl=csHll and R2=H. The threshold voltage is re- duced from 3.85 to 2.0 and 1.5 Vms for the 5% and 10% concentration, respectively.

The visco-elastic coefficient (y,/K") of these two guest-host mixtures were measured to be ~19 ms/llm2, which is nearly the same as the host PTTP-24/36 binary mixture.

Thus, doping such dye will lower the operating voltage while keeping response time unaffected.

However, compounds (I) and (II) are dyes; their colors are red and orange, re- spectively. Their absorption tail extends to the visible spectral region as shown in

Figure 2 for the 1% tolane dye in a ZLI-2359 liquid crystal host mixture. ZLI-2359 is a mixture of bicyclohexanes available from Merck & Co. This UV-transparent host mixture was used for the absorption measurement of the tolane. The cell gap used for this measurement was-6 urn. The peak absorption of the nitro-amino-tolane is cen- tered at about 420 nm and extends to nearly 500 nm. While these compounds may find use in particular applications, it is desired in other applications to use colorless compounds.

To eliminate absorption in the visible region while maintaining the desirable large dielectric anisotropy and low viscosity, three series of strong polar, but colorless compounds, have been devised: (1) biphenyls ; (2) diphenyl-diacetylenes, and (3) double tolanes. Schematically, the generic structure may be illustrated as: where R, is an alkyl having from 1 to 12 carbon atoms, R2 is either hydrogen or, inde- pendently of Rl, an alkyl having from 1 to 12 carbon atoms, M is a polar group, X and Y are independently hydrogen or a polar group not necessarily the same as M, and (A) is (1) a single bond (biphenyl) ; (2)-C=C-C=C- (diphenyl-diacetylene); or (3)

(double tolane), where Z is hydrogen, F, or alkyl. Each class of compounds is dis- cussed separately below.

1. Biphenyls Biphenyl liquid crystals, such as disclosed by G. W. Gray et al, Electronics Letters, vol. 9, pp. 130-131 (1973), have been used extensively for flat panel displays, such as wrist watches. Here, the biphenyl core is combined with the push-pull agents.

Two types of electron donor (CN and CF3) are listed below as examples. Other polar group, such as OCF3, CHF2 etc., can be considered as well; see, e. g., A. 1. Pavlu- chenko et al, Molecular Crystals and Liquid Crystals, vol. 209, pp. 225-235 (1991).

Alternatively, yet another polar group, F, may be employed. where R, and R2 are as defined above.

Both CN and CF3 are strong polar groups. The dipole moment of CN is somewhat stronger than that of CF3. However, the CP3 group possesses a lower vis- cosity, higher resistivity, and shorter absorption wavelength, and is more favorable than the CN group from a colorless viewpoint. High resistivity is required for the thin-filin-transistor (TFT) based liquid crystal displays. The side chain RI and R, make important contributions to the melting temperature and viscosity of these com- pounds. A longer side chain would normally lead to a lower melting temperature and higher viscosity. To further enhance dielectric anisotropy, another polar group, such as F or Cl, can be substituted in the X and/or Y positions (3 and/or 5 positions). This lateral substitution not only lowers the melting temperature but increases AE. Due to steric hindrance considerations, the polar group in the X and/or Y positions is smaller than the group in the para, or 4, position.

Examples of biphenyl compounds have been prepared and their melting points (m. p.) and heats of enthalpy (AH) determined. The results are listed in the Table be- low for the various compounds.

Table. Biphenyl dopants and their properties. Cmpd | Rl R2 M X m. p., °C AH, KcaUmol 1-CH3-CH3 CN H H 219. 5 7. 10 2-C2Hs-C2H5 CN H H 159* 7. 28 3-CH3-CH3 F H H 169 7. 28 4-C2Hs-C2Hs F F F 86. 6* 6. 8 Note: * The extrapolated As value (from 10% concentration in PTTP-24/36 host) for Compounds #2 and #4 is 26 and 25, respectively.

Figure 3 shows the absorption spectrum of 1 wt% of the third biphenyl com- pound in the above Table (R, = R : = CH3 ; M = F; X = Y = H) in the liquid crystal

mixture designated ZLI-2359, using a 6 J, m celt. The compound is seen to be color- less in the visible spectrum.

2. Diphenyl-Diacetylenes In the alkyl-alkyl diphenyl-diacetylene structure, the absorption extends to -340 nm; see, e. g., S. T. Wu et al, Journal of Applied Physics, vol. 68, pp. 78-85 (1990). Adding an amino group would extend the absorption to-400 m-n. Thus, the cyano group, which is a strong electron acceptor, cannot be used. Under this circum- stance, the CF3 is a better choice in order to maintain colorless.

3. Double Tolanes The absorption wavelength of the double tolanes is only slightly longer than that of diphenyl-diacetylenes ; see, e. g., S. T. Wu et al, Applied Physics Letters, vol. 74, pp. 344-346 (1999). Thus, the following molecular structure is likely to be col- orless.

To lower the melting temperature of such a highly conjugated molecule and improve its solubility, the side chain (Rl and R2) length may be varied, or the hydro- gen (s) in the lateral position (s) (X and/or Y) may be replaced by a polar (e. g., F or Cl) group, as with the diphenyl-diacetylenes above. Further, the substitution of Z with a polar group (F or Cl) or an alkyl (Cl, H2nl) group may be used to lower the melting temperature substantially, see, e. g., S. T. Wu, Japanese Journal of Applied Physics, vol. 39, pp. L38-41 (2000). These lateral groups serve to increase the molecular breadth and weakening the intermolecular associations. As a result, the melting tem- perature is lowered significantly.

4. Further Considerations The amount of the colorless dopant added to the liquid crystal composition, which comprises at least one liquid crystal compound, is in an effective amount to achieve the elimination of absorption of light by the liquid crystal composition in the visible region while maintaining a desirable large dielectric anisotropy and low vis- cosity. Preferably, the amount of the dopant added is within the range of about 1 to 20 wt% of the total liquid crystal composition, and more preferably, about 5 to 10 wt%.

The purposes of the addition of the colorless dopants of the present invention include: (1) reduction of the operation voltage of liquid crystal devices; (2) maintenance of high transmittance in the visible region; and (3) retention of fast response time.

The advantages provided by the colorless dopants of the present invention in- clude: (1) these compounds are colorless (their electronic absorptions appear in the ultraviolet region); (2) they possess a very large dielectric anisotropy;

(3) their viscosity is relatively low; and (4) they help reduce operation voltage for both polar and nonpolar LC mixtures.

INDUSTRIAL APPLICABILITY The colorless dopants of the present invention are expected to find use in the reduction of operating voltages of liquid crystal devices while retaining fast response time.