DE4007039A1 - Positive dielectric, electro-optical liq. crystal system - contg. chiral dopant to control slope of characteristic graph and improve on-off contrast - Google Patents

Abstract

Electro-optical liq. crystal (LC) system comprises, between 2 electrodes opt. deposited on substrates, a position dielectric LC mixed with an optically transparent medium (A). The LC molecules in the 'off' position adopt an irregular orientation and the refractive index of LC and matrix are about the same and/or the quotient of mass of LC to that of (A) is 1.5 or more. In one of the 'off' and 'on' positions, the system has a lower transmission than when in the other position, regardless of the polarisation of the incident light. The new feature is that the LC medium includes at least one chiral dopant (I). USE/ADVANTAGE - These systems are esp. useful in architectural structures (windows, room dividers, sun roofs, etc.) but can also be used in display (esp. matrix-display--matrix-display) devices. They provide (1) optimum slope of the electro-optical characteristic graph (i.e. to match the driving electronics) and/or (2) high contrast between opaque and transparent states, with low haze in the transparent state.

Description

The invention relates to electro-optical liquid crystal systems according to the preamble of claim 1.

The optically transparent medium can differ from one another contain limited liquid crystal microdroplets or form a spongy, 3-dimensional network, in whose pores, to a greater or lesser extent merge into each other, the liquid crystal is. By the expression liquid crystal microdroplets small, separate liquid crystal compartments elements marked, which are by no means a ball have to have a shape, but regularly or may be irregularly shaped and / or deformed.

Contains the optically transparent medium liquid crystal Microdroplets, it is referred to below as the matrix net; on the other hand there is a spongy, 3-dimensional networked structure before, the medium is through the expression Network characterized.

NCAP and PDLC films (NCAP = nematic curvilinearly aligned phases, PDLC = polymer dispersed liquid crystal) Examples of electro-optic liquid crystal systems, where the liquid crystal is in the form of microdroplets is embedded in the matrix. NCAP films get used to Lich obtained in that the encapsulated polymer  Material such as B. polyvinyl alcohol, the liquid crystal mixture and a carrier material, such as. B. water, in be mixed intimately in a colloid mill. Subsequently the carrier material z. B. removed by drying. A corresponding method is in US 44 35 047 wrote. In contrast, the z. B. in EP 02 72 582 and US 46 88 900 described production of PDLC films the liquid crystal mixture first with monomers or Oligomers of the matrix-forming material homogeneously ver mixes. The mixture is then polymerized and induced phase separation, with between TIPS (temperature-induced phase separation), SIPS (solvent induced phase separation) and PIPS (polymerization-induced phase separation) is distinguished (Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt. 157 [1988] 427).

The PN system described in EP 03 13 053 (PN = polymer Network) has a spongy network structure optically transparent medium. The proportion of the liquid crystal on the mass of the light-modulating layer generally greater than 60% and in particular lies between 70-90%. To manufacture the PN systems usually a mixture of the liquid crystal, Monomers or oligomers of the 3-dimensional network forming material and a polymerization initiator, especially a photoinitiator, between 2 with elec brought troden provided substrate plates and then z. B. polymerized by exposure to light.

The liquid crystal generally has a positive dielectric anisotropy Δε and a relatively high optical anisotropy. In microdroplet matrix systems, one of the refractive indices of the liquid crystal, usually the ordinary refractive index n _o, is chosen so that it more or less coincides with the refractive index n _{M of} the polymeric matrix. In network systems, it is not absolutely necessary to adjust the refractive indices because of the usually much higher proportion of liquid crystal in the light-modulating layer, but it can be done to increase the light throughput and the contrast.

One observes on these electro-optic liquid crystal Systems an electrically switchable light scattering effect. There is no voltage on the electrodes between which the matrix or network usually sand arranged wich-like, laid out, becomes striking Light on the statistically aligned liquid crystal Molecules are very scattered, the system is opaque. When a voltage is applied, the liquid crystal molecules parallel to the field and perpendicular to the E-vector of the light passing through.

In microdroplet matrix systems, an optically isotropic medium sees perpendicularly to incident light due to the adaptation of n _o and n _M, and the system appears transparent. Adjustment is required to avoid light scattering at the matrix / liquid crystal droplet phase boundary. Another embodiment is described in EP 02 72 585, in which the refractive index n _x , which the liquid crystal has with a completely statistical orientation, is matched to the refractive index of the matrix n _M. In this case, the system is transparent in the field-free state, and it is converted to the opaque state by applying a voltage.

In network systems there is an adjustment of the refraction indices not absolutely necessary because of the high Liquid crystal portion of the mass of the light modulators the layer the scatter at the phase boundary network Liquid crystal is obviously less strong. The System appears without when switched on  Transparent refractive index adjustment. With network werk systems is to achieve the lowest possible Transmission in the non-switched state use of liquid crystals with high optical anisotropy prefers.

Conventional microdroplet matrix systems in particular often have an insufficient contrast between opaque and transparent state. High Δ ⁿ values of the liquid crystal on the one hand ensure strong light scattering in the opaque state, but on the other hand they cause a clouding ("haze") of the system in the transparent state, especially when viewed obliquely ("off-axis haze"). Conversely, with low Δ n values of the liquid crystal, the off-axis haze is less, but at the same time the light scatter in the opaque state is greatly reduced. However, optimization by continuous variation of Δ n is generally not possible, since one of the two refractive indices of the liquid crystal or of n _{x has} to be adjusted simultaneously to the refractive index of the matrix n _M.

In network systems, this effect is due to the higher Liquid crystal portion and which is not absolutely necessary adjustment of the refractive indices i. a. less difficult weighing. In some cases, e.g. B. when using a relatively low proportion of liquid crystals and / or Ver use of a liquid crystal with very high birefringence However, it can also be a significant problem with PN systems Impairment of the contrast.

While electro-optic liquid crystal systems according to the Preamble of claim 1 initially for archi in particular tectonic applications (windows, room dividers, sun roofs etc.) have been discussed, display  Applications, for example for large and small-area displays systems, projection systems, GH display systems, shutters and in particular also matrix displays with high informa of interest.

So describe e.g. B. Z. Yaniv et al. in Japan Display '89, 15.1, 572-575 an with an active TFT (Thin Film Tran sistor) Matrix controlled PDLC display with 240 × 372 Pixels. EP 03 37 711 describes a device with an active varistor Matrix controlled NCAP system specified.

Especially for displays that have an active matrix be controlled, the liquid crystal must be next to one high clearing point, a wide nematic range, one high UV and temperature stability, fast switching times and a low threshold voltage in particular a not too high value for the steepness of the electro have optical characteristic to the representation of To enable grayscale.

The liquid crystals used so far meet in particular especially the demand for a favorable value for the Slope of the electro-optical characteristic is insufficient, so that the possibilities of driver electronics are often only unsatisfactorily used and often not sufficient Number of grayscale results, which is also bad can be coordinated.

The invention was based on the object of electro-optical To provide liquid crystal systems that the up led requirements and especially one optimized with regard to the respective driver electronics Slope of the electro-optical characteristic and / or a high contrast between opaque and transparent condition if the haze is low, have a clear condition.  

Surprisingly, it has now been found that this task can be solved if the liquid crystal is a dopant contains component consisting of one or more chiral Dopants exist.

The invention thus relates to an electro-optical Liquid crystal system;

• - Which between 2 electrodes, if necessary are applied to substrate plates, one dielectric positive liquid crystal and a contains another optically transparent medium,
• - Its liquid crystal molecules when switched off Irregular orientation,
• - in which one of the refractive indices of the liquid crystal essentially corresponds to the refractive index of the matrix n _M and / or in which the quotient of the mass of the liquid crystal and the mass of the optically transparent medium is 1.5 or more,
• - Which one in one of the two switching states reduced transmission compared to the other state has, wherein the liquid crystal has a doping comm contains component consisting of one or more chiral Dopants exist.

The structure of the electro-optical liquid according to the invention crystal systems corresponds to that for such systems usual design. The term common design is here at broad and encompasses all variations and modes fications.  

So z. B. in PDLC or NCAP films by the Visible medium formed matrix in which the liquid crystal mixture microdispersed or microencapsulated is sandwiched between conductive electrodes arranges.

The electrodes are i. a. on substrate plates from z. B. Glass, plastic or similar applied; if necessary, the matrix but can also be provided directly with electrodes, so that the use of substrates can be dispensed with can.

The liquid crystal is in network systems in the pores of the spongy, 3-dimensional network. The network is usually between electrodes provided substrates arranged to escape the To prevent liquid crystal.

Both PN systems and microdroplet matrix systems can be operated reflectively or transmissively, so that at least one electrode and, if available, the associated substrate are transparent. Both systems usually do not contain polarizers, which means a significantly higher light throughput results. Farther no orientation layers are required, which is a considerable technological simplification in production of these systems compared to conventional liquid crystal systems such as B. TN or STN cells.

The matrix or the 3-dimensional network are based on especially on isotropic thermoplastics, thermosets and Elastomers. The systems obtained can depend on Visible use be flexible, elastic or rigid.  

A system based on a thermoplastic polymer can at temperatures greater than the glass temperature the matrix, easily by the action of a mechanical Tension can be deformed. This can e.g. B. with micro droplet matrix systems are used to make one deliberately deformed shape of the droplets by cooling the matrix to temperatures below the glass temperature freeze.

While flexible and / or elastic systems are preferred based on thermoplastics and / or elastomers preferably thermosetting for the production of rigid systems Polymers used. This can occur during curing be mechanically deformed, being in the cured Matrix z. B. the shape and arrangement of the microdroplets is fixed.

Especially for the production of the matrix or the network preferred polymers are e.g. B. in US 39 35 337, US 46 88 900, US 46 71 618, US 46 73 255, US 44 35 047, EP 03 13 053 and EP 02 72 585 discloses.

In addition, other transparent materials can also be used lien such. B. inorganic glass monoliths (US 48 14 211), other inorganic materials (see e.g. Japanese Patent Application 3 03 325/1988) or other materials can also be used.

Preferred embodiment of the electro according to the invention optical liquid crystal systems are NCAP films, PDLC Films and produced by modified processes Micro droplet matrix systems. Manufacturing process these films are e.g. B. in US 39 35 337, US 46 88 900, US 46 73 255, US 46 71 618, US 44 35 047 and EP 02 72 595 described.  

Another preferred embodiment of the Invention modern electro-optical systems are the network systems, their manufacture z. B. is described in EP 03 13 053.

However, such embodiments of FIG Invention comprises, in which the transparent medium Structure that is between the network structure one side and the microdroplet matrix configuration tion is on the other side.

The electro-optical systems according to the invention include also "inverse microdroplet matrix configurations", where the transparent medium in the form of individual e.g. B. spherical particles in the liquid crystal dis is perforated. Such an arrangement is e.g. B. in GB 14 42 360 described.

There are also others that are not explicitly mentioned here Embodiments of the invention include.

The thickness d of the electro-optical systems d is usually chosen to be small in order to achieve the lowest possible threshold voltage V _th . So z. B. reported in US 44 35 047 layer thicknesses of 0.8 and 1.6 mm, while for the layer thickness in US 46 88 900 values between 10 microns and 300 microns and in EP 03 13 053 between 5 microns and 30 microns are given . The electro-optical systems according to the invention only have layer thicknesses d that are significantly larger than a few mm in exceptional cases; layer thicknesses d ≦ 2 mm are preferred.

The threshold voltage is also affected by the size of the micro droplets or the mesh size of the network. In general, smaller microdroplets cause a higher threshold voltage V _th , but shorter switching times t _on or t _off (US Pat. No. 4,673,255). Experimental methods for influencing the mean droplet size are e.g. B. in US 46 73 255 and in JL West, Mol. Cryst. Liq. Cryst. Inc. Nonlin. Opt., 157 (1988) 427. In US 46 73 255 mean drop diameters between 0.1 microns and 8 microns are given, while z. B. a matrix based on a glass monolith, has pores with a diameter between 15 and 2000 Å. For the mesh size of the network of PN systems, a preferred range between 0.5 and 2 µm is specified in EP 03 13 053. In J. Appl. Phys. 66 (11), p. 5278 report Fuh, A. et al. about other relationships between the density ratio of polymer to liquid crystal, curing speed, droplet size and optical steepness.

A major difference between the elek trooptic CMEM systems to the previously common systems However, the liquid crystal used mixture based on a nematic liquid crystal based on positive dielectric anisotropy and a Contains doping component consisting of one or more chiral dopants.

By adding a doping component, the steepness of the electro-optical characteristic curve can be influenced in the electro-optic liquid crystal systems according to the invention and can be specifically matched to the driver electronics. Furthermore, the doping with at least one chiral dopant increases the light scattering in the opaque state, while the "haze level" in the transparent state is not influenced or is influenced only to an insignificant extent. Thus, the contrast between the opaque and the visible state is significantly improved by the use of the liquid crystal mixture according to the invention. By adding the doping component other properties of the liquid crystal or the system such as. B. the clearing point T _C , the viscosity η , the dielectric anisotropy Δε , the optical anisotropy Δ n , in the case of microdroplet matrix systems adapted to the refractive index of the matrix, the refractive index of the liquid crystal, the threshold voltage V _th and also only other properties influenced to a small and / or insignificant extent.

The slope of the electro-optical characteristic can, for. B. to be defined according to

where V ₉₀ and V ₁₀ are the voltages required to achieve a 90% or 10% transmission; but other definitions of the slope γ are also possible.

Chiral dopants can be used which induce a positive (right-handed) or negative (left-handed) sense of rotation of the cholesteric helix structure in the liquid crystal mixture. The pitch or pitch p of the cholesteric helix structure is inversely proportional to the concentration c of the chiral dopant when using a chiral dopant. The proportionality factor is the reciprocal of the helical twisting power HTP of the chiral dopant. The following applies:

If one specifies the concentration c of the chiral dopant in mass percentages of the liquid crystal mixture, the helical twisting power HTP has the dimension of a reciprocal length. The helical twisting power depends on the composition of the liquid crystal mixture and the temperature. When using several, e.g. B. of 2 chiral dopants, the reciprocal of the total pitch pitch p often results from a linear overlay of the contributions of the individual dopants, z. B.

where the index is the two different dopants indicates.

Chiral dopants can have one or more Have chirality centers. Chiral dopants with 1 or 2 chiral centers are preferred.

Chiral components that make up the structural element

have wherein

V is an alkyl group with 1-5 C atoms, unsubstituted or substituted (C₈-C₁₈) aryl or (C₅-C₁₂) cycloalkyl,
Y is a single bond, -O-, -COO-, -OOC-, or - (CH₂) _n -, and
n 0, 1, 2 or 3

mean are preferred. Y is particularly preferably -O-, -COO-, -OOC- or - (CH₂) -, but especially -O-, -COO- or -OOC-. V is preferably CH₃-, CH₂-CH₂-

especially CH₃- and

Further preferred are those chiral components that have structural element II,

wherein

S each independently of one another is an alkyl or alkoxy group with 1-5 C atoms, CN or halogen,
T each independently of one another -O-, -COO-, -OOC-, -CH₂CH₂- or a single bond, and

mean.

is preferred

especially however

S is preferably CH₃, CH₃-CH₂, Cl or CN and very special ders preferred, CH₃, Cl or CN.

In addition to chiral dopants containing a structure element of formula I or II, other chiral Dopants, e.g. B. from the enumerated in WO 86/02937 Compound classes of 2-hydroxycarboxylic acids, amino acids, Terpenes, steroids, sugar or polycarboxylic acids selected can be, or other chiral compounds be used.

It's I. a. always possible by adding a doping comm component at the same time both high contrast between opaque and transparent condition and a low haze level in the transparent state as well as an adjustment the steepness of the electro-optical characteristic to the Trei  to reach via electronics. If this is not the case succeeds, the specialist can in the specific case with regard decide on the intended application without further ado, which of the two described properties of the electro optical system should be improved or optimized.

In order to achieve a high contrast and a low haze level in the transparent state, the quotient of the pitch p of the liquid crystal mixture and the mean droplet diameter w in the case of microdroplet matrix systems or the quotient of p and the mean mesh size w * in the case of network systems is preferred chosen between 0.01 and 10, but in particular less than 5 and very particularly less than 1. The interval between 0.01 and 2.5, in particular between 0.01 and 1 and very particularly preferably between 0.01 and 0.8, is particularly preferred for p / w and p / w * . The quotients p / w and p / w * can be changed by varying w or w * (see, for example, the literature cited above) and / or p .

By adding a doping component, the slope of the electro-optical characteristic γ of systems according to the invention can be varied within a wide range and adapted to any driver electronics. A drop in the slope compared to that of the undoped system is generally observed when adding a doping component; however, steepness can also increase. For a given combination of a liquid crystal system according to the preamble of claim 1 and driver electronics, the person skilled in the art can select a doping component from the large pool of known dopants without inventive step in such a way that the system is adapted as well as possible to the driver electronics and at the same time a high contrast between transparent and opaque state and a low haze level is achieved in the transparent state.

The dopants are preferably selected such that the proportion of the doping component in the liquid crystal mixture is not too great and is between 0.01-25 percent by mass, but in particular 0.2-10 and very particularly 0.5-5 percent by mass. If the concentration of the chiral dopant (dopants) is not too large and in particular does not exceed 5 and especially 3 percent by mass, the remaining parameters of the liquid crystal or the electro-optical system such as, for. B. the birefringence Δ n , the dielectric anisotropy Δε , the clearing point T _C , the viscosity η , the threshold voltage V _th only changed to a minor and / or negligible extent. In the case of microdroplet matrix systems, the refractive index of the liquid crystal mixture which is matched to the refractive index n _{M of} the matrix in particular undergoes only a minor and / or insignificant change. The concentration of the chiral component can be kept low, especially for dopants with high HTP . Therefore, in particular those dopants are used for the doping component of systems according to the invention for which | HTP | <10 µm ^-1 , but especially | HTP | <15 µm ^-1 in the liquid crystal mixture.

It was found that the temperature dependence of the slope γ of the electro-optical characteristic curve is related to the temperature dependence of the pitch of the doping component. The temperature dependence of the slope is generally greater, the greater the temperature dependence of the pitch. Particularly in "outdoor" applications, there can be considerable temperature fluctuations of up to 50 ° C. The resulting fluctuations in contrast and changes in the slope of the characteristic curve can significantly degrade the electro-optical properties of the system.

There are therefore preferred embodiments of the electro-optical systems according to the invention which have a not too strong temperature dependence of the pitch of the liquid crystal mixture. Particularly preferred are those liquid crystal mixtures which contain at least one chiral component for which the absolute value of the temperature dependence of the helical twisting power HTP in the liquid crystal mixture is less than 5 · 10 ^-1 µm ^-1 K ^-1 , but in particular less than 3 · 10 ^-1 µm ^-1 K ^-1 . Chiral components are also particularly preferred

Electro-optical systems whose liquid crystal contains at least 2 chiral dopants are very particularly preferred, the temperature dependence of the helical twisting power HTP of these dopants in the liquid crystal mixture being matched to one another in such a way that the given temperature dependence of the pitch p of the liquid crystal mixture is largely compensated for. In particular, there are systems for which the absolute value of the relative change in the pitch per

is less than 4 · 10 ^-3 K ^-1 , but in particular less than 2 · 10 ^-3 K ^-1, is preferred. Systems in which the chiral dopants additionally have the same helical direction of rotation are very particularly preferred.

Liquid crystals whose Doping component 1-10, in particular, however, no longer contains 7 and very particularly 1-5 dopants.

The rivers used in the systems according to the invention Sig crystals are preferably based on compounds of the formulas III-VI.  

wherein

R each independently of one another is an alkyl group with 1-15 C atoms, in which one or two non-adjacent CH₂ groups can also be replaced by -O-, -CO- and / or -CH = CH,
S is a single bond, -COO- or -OOC-,

mean.

The proportion of the best from compounds of formulas III-VI base mixture to those in the invention electro-optic liquid crystal systems used Liquid crystals is preferably 15% -85%, ins special however 25% -85%. The basic mix is based particularly preferred on the following smaller group of compounds, where R has the meaning given above Has.  

Liquid crystals based on based on the mixtures 1-9 described below. These mixtures consist of 2 or more compounds, that fall under 2, 3 or 4 formulas that come from the group of the formulas III, IV, V and VI can be selected. The Mixtures 1-9 preferably contain 2-40, in particular however 2-38 and especially 2-35 connections. Ange is the mass fraction of these compounds in the Liquid crystal. The mass fraction can be within the specified limits for optimal adaptation to each display type can be varied.

The electro-optical liquid crystal according to the invention systems are preferably characterized in that none or only one during the manufacture of the system little interaction of the components of the liquid crystal mix with the polymeric carrier material polar groups comes about.

The liquid used in the systems according to the invention Crystals can contain other ingredients that precede can be selected from nematic or nemato gene (monotropic or isotropic) substances, in particular Substances from the classes of azoxybenzenes, benzylidene anilines, biphenyls, terphenyls, phenyl- or cyclohexyl benzoates, cyclohexane carboxylic acid phenyl or cyclohexyl esters, phenyl or cyclohexyl esters of cyclohexylbenzoe acid, phenyl or cyclohexyl ester of cyclohexylcyclo hexane carboxylic acid, cyclohexylphenyl ester of benzoic acid, cyclohexane carboxylic acid or cyclohexylcyclohexane carboxylic acid, phenylcyclohexanes, cyclohexylbiphenyls, Phenylcyclohexylcyclohexenes, cyclohexylcyclohexanes, Cyclohexylcyclohexenes, cyclohexylcyclohexylcyclohexenes, 1,4-bis-cyclohexylbenzenes, 4,4'-bis-cyclohexylbiphenyls, Phenyl- or cyclohexylpyrimidines, phenyl- or cyclo hexylpyridines, phenyl or cyclohexyl dioxanes, phenyl or cyclohexyl-1,3-dithiane, 1,2-diphenylethane, 1,2- Dicyclohexylethane, 1-phenyl-2-cyclohexylethane, 1-cyclo hexyl-2- (4-phenyl-cyclohexyl) ethane, 1-cyclohexyl-2-bi  phenylylethane, 1-phenyl-2-cyclohexylphenylethane, opp optionally halogenated stilbene, benzylphenyl ether, Tolanes and substituted cinnamic acids. The 1,4-phenylene Groups in these compounds can also be fluorinated.

The most important as further components of the in the electro-optical systems according to the invention used Leave liquid crystals in question are characterized by the formula 1, 2, 3, 4 and 5:

R′-L-E-R ′ ′ (1)

R′-L-COO-E-R ′ ′ (2)

R′-L-OOC-E-R ′ ′ (3)

R′-L-CH₂CH₂-E-R ′ ′ (4)

R′-L-C≡C-E-R ′ ′ (5)

In formulas 1, 2, 3, 4 and 5, L and E denote that can be the same or different, each independently one bivalent residue from the other from -Phe-, -Cyc-, -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -Pyr-, -Dio-, -G-Phe- and -G-Cyc- and their mirror images formed Group, where Phe unsubstituted or fluorine sub substituted 1,4-phenylene, cyc trans-1,4-cyclohexylene or 1,4-cyclohexenylene, pyr pyrimidine-2,5-diyl or Pyridine-2,5-diyl, dio 1,3-dioxane-2,5-diyl and G 2- (trans- 1,4-cyclohexyl) ethyl, pyrimidine-2,5-diyl, pyridine-2,5- mean diyl or 1,3-dioxane-2,5-diyl.

Preferably one of the residues L and E is Cyc, Phe or Pyr. E is preferably Cyc, Phe or Phe-Cyc. Preferential the liquid crystals according to the invention contain one or more components selected from the ver bonds of formula 1, 2, 3, 4 and 5, wherein L and E are selected from the group Cyc, Phe and Pyr and one or more components selected at the same time from the compounds of the formulas 1, 2, 3, 4 and 5, in which  one of the residues L and E is selected from the group Cyc, Phe and Pyr and the other residue is selected from the group -Phe-Phe-, -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-, and optionally one or more components selected from the compounds of the formulas 1, 2, 3, 4 and 5, wherein the residues L and E are selected from the Group -Phe-Cyc-, -Cyc-Cyc-, -G-Phe- and -G-Cyc-.

R 'and R' 'in the compounds of the sub-formulas 1a, 2a, 3a, 4a and 5a each independently Alkyl, alkenyl, alkoxy, alkenyloxy or alkanoyloxy with up to 8 carbon atoms. Most of these ver Bonds R 'and R' 'are different from each other, whereby one of these residues is mostly alkyl or alkenyl. In the Connections of sub-formulas 1b, 2b, 3b, 4b and 5b be interpret R '' - CN, -CF₃, F, Cl or -NCS; R has the specified for the compounds of the sub-formulas 1a to 5a Meaning and is preferably alkyl or alkenyl. But also other compounds of the proposed substituents in the compounds of formulas 1, 2, 3, 4 and 5 in use. Many such substances or mixtures of which are commercially available. All of these substances are by methods known from the literature or by analogy available for this.

The in the electro-optical liquid according to the invention contain liquid crystal used in crystal systems preferably in addition to components from the group of ver Bindings 1a, 2a, 3a, 4a and 5a (group 1) also compo nents from the group of compounds 1b, 2b, 3b, 4b and 5b (Group 2), the proportions of which are preferably as follows are:

Group 1: 0 to 60%, in particular 5 to 50%,
Group 2: 0 to 60%, especially 5 to 40%.

The electro-optic liquid crystal Systems can be created by creating an equal or alternate voltage can be switched. However, one is preferred AC voltage is used, which is an effective alternation voltage amplitude between 1 and 240 volts and a AC frequency between 10 Hz and 10 kHz points. Amplitudes between 2 and 220 volts and frequencies between 20 and 120 Hz. Quite the amplitude of the changes is particularly preferred voltage between 2 and 130 V.

The dielectric anisotropy of the liquid crystal mixture used is positive Δε <0 and preferably Δε <3. Very high threshold voltages are observed for smaller values of the dielectric anisotropy Δε . Values Δε <5, in particular Δε <10 and very particularly Δε <15 are particularly preferred .

The production of the liquid crystal which can be used according to the invention Mixtures are carried out in a conventional manner. As a rule, the desired amount is less Quantity of components used in the main inventory partially constituent components solved, useful for elevated temperature. It is also possible to find solutions to the Components in an organic solvent, e.g. B. in Mix acetone, chloroform or methanol and that Remove solvent after thorough mixing, for example by distillation.

The liquid crystals described can be added by suitable additives be modified so that they are electro in all optical systems according to the preamble of claim 1 can be used.  

Such additives are known to the expert and in the Literature described in detail. So z. B. pleo chroitic dyes for the production of colored electro optical systems or substances to change the dielectric anisotropy, optical aniostropy, the viscosity and / or the temperature dependence elec Trooptic parameters of the liquid crystals added will. Such substances are e.g. B. in H. Kelker, R. Hatz, Handbook of Liquid Crystals, Verlag Chemie, Weinheim, 1980 and in DE-OS 22 09 127, 22 40 864, 23 21 632, 23 38 281, 24 50 088, 26 37 430, 28 53 728 and 29 02 177.

Electro-optical liquid crystal systems in which the Liquid crystal pleochroic dyes in one Weight percentage range from 0-25%, especially 0-20% and especially 0-15% are preferred.

The expert can further select from the large pool of nematic or nematogenic substances additives to the liquid crystal mixtures described so that the double refraction Δ n and / or the ordinary refractive index n _o and / or other refractive indices and / or the viscosity and / or the dielectric anisotropy and / or further parameters of the liquid crystal can be optimized with regard to the respective application.

To increase the clearing point, the person skilled in the art Systems used liquid crystal according to the invention mixtures z. B. add clarifying substances such. B.

where R⁴ and R³ each independently represent alkyl, methoxy, alkoxycarbonyl or alkanoyloxy with 1-15 carbon atoms. The person skilled in the art will preferably choose the concentration of the additives in such a way that in particular Δ n and / or n _o and / or another refractive index and / or Δε to be adapted to the particular design of the electro-optical system according to the invention is only acceptable and / or small and / or in particular negligible extent is influenced.

If the system according to the invention with an AC voltage controlled, is the use of a highly viscous liquid crystal mixture required, otherwise otherwise in particular low to medium frequencies a flickering display results. A person skilled in the art can increase the viscosity the liquid crystal mixture highly viscous liquid crystal connections or in particular one or more Side chain polymers, as described in DE 39 19 942 is to clog. Will the electro-optical system oppose it e.g. B. ver as a matrix display with high information content uses, in particular, low-viscosity liquid crystal mixtures preferred to achieve short switching times. The professional can be nematic or out of the large pool nematogenic compounds of low viscosity select how B.

where R⁴ and R³ have the meaning given above. The person skilled in the art will select the substances used to modify the viscosity and their concentration so that other decisive parameters of the liquid crystal mixture, such as, for. B. Δ n , Δε and when using viscous lowers especially T _{c is influenced} only to an acceptable and / or small and / or insignificant extent.

If necessary, highly nematogenic substances such. B.

can be added, the expert taking into account that through such additions other parameters and in particular the solubility of the liquid crystal mixture in the for the polymer used did not change too much will.

To modify the birefringence, the expert can, for. B. in liquid crystal mixtures containing compounds of formulas II-IV, the relative proportion of these compounds in the mixture vary; for mixtures with a very high Δ n z. B. in particular 4-alkyl or alkoxy-4 'or 4''- cyano-biphenyls or -terphenyls are used ver.

Liquid crystal mixtures with very high dielectric anisotropy Δε are generally required to achieve low threshold voltages . The expert can increase Δε compounds such. B.

clog. These connections are extremely strong dielectric positive; the propyl homolog (alkyl = C₃H₇) z. B. a value Δε = 50. The person skilled in the art will choose the concentration of such additives in such a way that the liquid crystal mixture is optimally matched to the particular design of the electro-optical system; in particular, he will take care that Δ n and / or n _o and / or one or more further refractive indices of the liquid crystal are influenced only to an acceptable and / or small and / or negligible extent.

Due to the additives described, the liquid crystal modified with regard to the respective application and be optimized. What is decisive, however, is that the liquid crystal a doping component containing one or several dopants is added, whereby

• - Change the slope of the electro-optical characteristic changed and adapted to the driver electronics, and or
• - high contrast between opaque and transparent Condition and a low haze level in the transparent Condition is achieved

without that with regard to the respective application optimized properties of the liquid crystal too strong and / or changed to an unacceptable extent the. In addition, you can

• - The temperature dependence of the steepness of the electro optical characteristic and / or
• - Contrasts caused by temperature fluctuations stanchions

by using a suitable doping component be tied.

The following examples are intended to illustrate the invention, without limiting it.  

It means:

K: crystalline solid state,
S: smectic phase (the index indicates the phase type),
N: nematic phase,
Ch: cholesteric phase,
I: Isotropic phase.

The number between two symbols gives the conversion temperature in degrees Celsius.

The percentages given are percentages by mass.

Example 1

1.1.1 A liquid crystal consisting of the following links

51.0% 4-pentyl-4'-cyanobiphenyl
25.0% 4-heptyl-4'-cyanobiphenyl
16.0% 4-octoxy-4'-cyanobiphenyl
8.0% 4-pentyl-4 ′ ′ - cyanoterphenyl

is used to make a microdroplet matrix System with the curable by UV radiation Adhesive NOA 65 (Norland Products) in the ratio 1.6: 1 stirred at room temperature until one clear solution is obtained that along with Spacers between 2 transparent, with Glass substrates provided with electrode layers brought. The glass substrates become together squeezed, creating an even film is obtained by a 1-minute UV Radiation is cured. The film thickness is 17.9 µm.  

In Fig. 1, the value measured for this system electro-optical characteristic curve is reproduced. The transmission T = I / I _{o is carried} on the ordinate, where I is the intensity let through for the applied voltage and I _{o is} the maximum intensity, and the effective value of the applied alternating voltage (50 Hz) is indicated on the abscissa .

1.1.2 A liquid crystal consisting of the following links

49.9% 4-pentyl-4'-cyanobiphenyl
24.4% 4-heptyl-4'-cyanobiphenyl
15.2% 4-octoxy-4'-cyanobiphenyl
7.8% 4-pentyl-4 ′ ′ - cyanoterphenyl
2.3% (S) -1,2-bis- (4- (trans-4-pentylcyclohexyl) benzoyloxy) -1-phenylethane (chiral dopant ZLI-4571, E. Merck, Darmstadt, FRG)

is used in the production of a microdroplet matrix system according to the method described in 1.1. The film thickness is 15.9 µm. In FIG. 2, the value measured for this system electro-optical characteristic curve is reproduced.

1.1.3 A comparison of the steepness of the electroopti the characteristic curve of 1.1.1 and 1.1.2 described systems shows that the slope by adding (S) -1,2-bis- (4- (trans-4- pentylcyclohexyl) benzoyloxy) -1-phenylethane  to the liquid crystal is significantly reduced. The system containing a doping component from 1.1.2 shows a high contrast between opaque and transparent condition and one low haze level in the transparent state.

1.2 The ones described in Examples 1.1.1 and 1.1.2 Liquid crystals are used for manufacturing of a microdroplet matrix system with epicote 828 and Capcure 3-800 (Miller Stephenson Company) stirred in a ratio of 1: 1: 1 at room temperature, until a clear solution is obtained; the stirring time is kept as short as possible because the solution at room temperature after about ½ h is hardened. The solution comes with Spacers between 2 transparent, with Glass layers provided with electrode layers brings, which are compressed, whereby an even film is obtained. To Accelerating the curing process can Films can be heated to a temperature of up to 100 ° C.

A comparison of the steepness of the electro optical characteristic of the micro obtained droplet matrix systems shows that the steep units by adding (S) -1,2-bis- (4- (trans- 4-pentylcyclohexyl) benzoyloxy) -1-phenylethane to the liquid crystal can be significantly reduced. The system containing a doping component shows a high contrast between opaque and transparent condition and a low haze level in the transparent state.

1.3 The ones described in Examples 1.1.1 and 1.1.2 Liquid crystals are used for manufacturing a microdroplet matrix system with 15 g  20% aqueous PVA solution at room temperature Stirred at 2000 rpm for 2 minutes. The the solution obtained is degassed for 24 hours and together with spacers in a thin layer on a layer provided with an electrode layer Glass substrate applied. The arrangement will Dried at 85 ° C for 1 h before a second glass sub provided with an electrode layer strat is pressed, whereby a uniform ger film is obtained. The system thus obtained is dried at 85 ° C for a further 24 h.

A comparison of the slopes of the electro-optical Characteristic curve of the micro obtained droplet matrix systems shows that the steepness by adding (S) -1,2-bis- (4- (trans-4- pentylcyclohexyl) benzoyloxy) -1-phenylethane for Liquid crystal can be significantly reduced. The has a system containing doping component a high contrast between opaque and trans parental condition and a low haze level in the transparent state.

1.4 The rivers described in Examples 1.1 and 1.2 Sig crystals are used for manufacturing a network system with Trimethylpropan-tri acrylate as a polymerizable compound and 2-hydroxy-2-methyl-1-phenylpropan-1-one (Daro cure 1173, commercial product from E. Merck, Darm city) as a photoinitiator in relation 80: 19.8: 0.2 stirred and added of spacers between 2 with electrodes layers of glass. To Curing of the polymer became the system obtained at a defined speed (3 m / min) through the radiation field of a halogen lamp (70 W / cm).  

A comparison of the steepness of the elektroopti characteristic curve of the microdroplets obtained in the process Matrix systems shows that the slopes by adding (S) -1,2-bis- (4- (trans-4- pentylcyclohexyl) benzoyloxy) -1-phenylethane to the liquid crystal can be significantly reduced. The system containing a doping component exhibits a high contrast between opaque and transparent condition and a low haze-level in the transparent state.

Example 2

2.1 A liquid crystal consisting of the following links

51% 4'-pentyl-4-cyanobiphenyl
25% 4'-heptyl-4-cyanobiphenyl
16% 4'-octoxy-4-cyanobiphenyl
8% 4 ′ ′ - pentyl-4-cyanoterphenyl
chiral dopants are added, selected from the group of compounds comprising C1, C2 and C3

C1 (S) -4 - ((1-methyl-heptyl) -oxy-carbonyl) - phenyl-4-hexoxy-benzoate (chiral dopant ZLI-811, E. Merck, Darmstadt, FRG)
C2 (R) -4 - ((1-methyl-heptyl) -oxy-carbonyl) - phenyl-4-hexoxy-benzoate (chiral dopant ZLI-3786, E. Merck, Darmstadt, FRG)
C3 () -1,2-bis- (4- (trans-4-heptylcyclohexyl) benzoyloxy) -1-phenylethane (described in EP 01 68 043)

The chiral dopants C1, C2 and C3 have the following temperature dependency in the liquid crystal Helical Twisting Power HTP on:

There are 3 doped liquid crystal mixtures M1-M3 by adding one or more Dopants made to the liquid crystal.

The mixture M3 has a particularly low temperature dependence of the pitch p .

2.2 There will be those described in Example 1 Process of electro-optical systems, on the one hand the mixture M3 and on the other contain the undoped liquid crystal. A Comparison shows that the steepness of the electro optical characteristic curve by adding the doping component is changed in each case. The the systems containing doped liquid crystal are characterized by a high contrast between opaque and transparent condition, due to a low haze-level in the transparent state and above due to a low temperature dependence the steepness of the curve and the contrast featured.

Claims (15)

1. electro-optic liquid crystal system,

• which contains a dielectric positive liquid crystal and another optically transparent medium between two electrodes, which may be applied to substrate plates,
• - whose liquid crystal molecules have an irregular orientation when switched off,
• in which one of the refractive indices of the liquid crystal essentially corresponds to the refractive index of the matrix n _M and / or in which the quotient of the mass of the liquid crystal and the mass of the optically transparent medium is 1.5 or more,
• which in one of the two switching states, regardless of the polarization of the incident light, has a reduced transmission compared to the other state,

characterized in that the liquid crystal contains a doping component consisting of one or more chiral dopants. 2. Electro-optical system according to claim 1, characterized in that at least one chiral component the structural element has, wherein
V is an alkyl group with 1-5 C atoms, unsubstituted or substituted (C₈-C₁₈) aryl or (C₅-C₁₂) cycloalkyl,
Y is a single bond, -O-, -COO-, -OOC- or - (CH₂) - _n , and
n 0, 1, 2 or 3
mean. 3. Electro-optical system according to at least one of claims 1-2, characterized in that at least one chiral component is a structural element has, wherein
S each independently of one another is an alkyl or alkoxy group with 1-5 C atoms, CN or halogen,
T each independently of one another -O-, -COO-, -OOC-, -CH₂CH₂- or a single bond, and mean. 4. System according to at least one of claims 1-3, characterized in that at least one chiral dopant has an absolute value of the helical twisting power HTP in the respective liquid crystal mixture of at least 2 µm ^-1 . 5. Electro-optical system according to at least one of claims 1-4, characterized in that at least one chiral component has an absolute value of the temperature dependence of the helical twisting power HTP in the respective liquid crystal mixture of less than 5 · 10 ^-1 µm K ^-1 . 6. Electro-optical system according to at least one of claims 1-5, characterized in that the liquid crystal mixture contains at least 2 chiral components, the temperature dependence of the helical twisting power HTP of these components in the liquid crystal mixture being matched to one another such that the temperature dependence of the choles steric pitches of the liquid crystal mixture is largely compensated. 7. Electro-optical system according to claim 6, characterized characterized in that the chiral components have the same helical direction of rotation.   8. System according to at least one of claims 1-7, characterized in that the liquid crystal one or more compounds selected from the compounds of formula II-V, wherein
R each independently of one another is an alkyl group having 1-15 C atoms, in which one or two non-adjacent CH₂ groups can also be replaced by -O-, -CO- and / or -CH = CH-,
Y is a single bond, -COO- or -OOC-, mean. 9. System according to at least one of claims 1-8, characterized in that the liquid crystal has a dielectric anisotropy Δε <3. 10. System according to at least one of claims 1-9, characterized in that the liquid crystal contains at least one pleochroic dye. 11. System according to at least one of claims 1-10, characterized in that the liquid crystal in Form of microdroplets in the optically transparent Medium is embedded. 12. System according to at least one of claims 1-10, characterized in that the optically transparent Medium forms a 3-dimensional network in which The pores contain the liquid crystal. 13. Liquid crystal, characterized in that it has the liquid crystal in an electro-optical system identical to at least one of claims 8-10 is. 14. Use of a liquid crystal containing one Doping component consisting of one or more chiral Dopants consist in an electro-optical System according to at least one of claims 1-12.