EP1725909A1 - Electro-optical light modulation element, display and medium - Google Patents

Abstract

The present invention relates to an electro-optical light modulation element and to electro-optical displays and display systems, such as e.g. television screens and computer monitors, containing elements of this type. The light modulation elements according to the invention contain a mesogenic modulation medium which is in the cybotactic nematic phase during operation of the light modulation elements and are particularly distinguished by very short response times in addition to good contrast and low viewing-angle dependence. The mesogenic modulation media used in the electro-optical light modulation elements are likewise a subject-matter of the present invention.

Description

Electro-optical light modulation element, display and medium

Field of the invention

The present invention relates to light modulation elements and to displays containing same. The light modulation elements use modulation media which have a cybotactic nematic phase at the operation temperature of the elements.

The present invention relates to electro-optical light modulation elements and to electro-optical displays and display systems containing elements of this type, such as, for example, television screens and computer monitors. The light modulation elements according to the invention contain a mesogenic modulation medium which is in the cybotactic nematic phase during operation of the light modulation elements and are particularly distinguished by very short response times in addition to good contrast and low viewing-angle dependence.

The present invention furthermore relates to liquid crystalline media having a cybotactic nematic phase and to the use thereof as modulation media in light modulation elements.

Object and prior art

Conventional electro-optical liquid-crystal displays are known in general terms. They are operated at a temperature at which the modulation medium is in a mesophase, typically and predominantly in the nematic phase. But there are also displays which use liquid crystals in a smectic phase, e.g. in the chiral smectic C (Sc) phase, like SSFLC (surface stabilised ferroelectric liquid crystal) displays or in the smectic A (S_A) phase, e.g. thermally addressed displays. Especially SSFLC and anti- ferroelectric displays show extremely good, fast response, however, they are rather difficult to produce. Furher, anti-ferroelectric displays are generally very sensitive to mechanical shock. The most widespread displays are TN ("twisted nematic") and STN ("super twisted nematic") displays. The liquid-crystal cells in these displays have electrodes on the substrates on the two opposite sides of the liquid-crystal medium. The electric field is thus essentially perpendicular to the liquid- crystal layer. The first-mentioned displays in particular are used in combination with TFT (thin film transistor) addressing for displays having a large information content and high resolution, for example in laptop and notebook computers. Use has recently increasingly been made, in particular in desktop computer monitors, of liquid-crystal displays of the IPS (]n-βlane switching, for example DE 40 00 451 and EP 0 588568) type or alternatively of the VAN (vertically aligned nematic) type. VAN displays are a variant of the ECB (electrically controlled birefringence) displays. In a modern variant of the ECB displays, the MVA (multidomain vertically aligned) displays, a plurality of domains are stabilised per addressed electrode, and in addition a special optical compensation layer is used. These displays, like the TN displays already mentioned, use an electric field perpendicular to the liquid-crystal layer. In contrast thereto, IPS displays generally use electrodes on only one substrate, i.e. on one side of the liquid-crystal layer, i.e. are characterised by a significant component of the electric field parallel to the liquid-crystal layer.

In common electro-optical switching elements exploiting electro-optical effects in mesogenic modulation media the response time depends on the relevant viscosity coefficient of the modulation medium. The response time for switching off (τ_0ff) depends on the respective elastic constant or combination of elastic constants. For example in the almost ubiqitous TN displays the response time for switching off is proportional to the rotational viscosity of the liquid crystal (γi) and inversely proportional to the respective term of the elastic constants (K) as shown by the following formula: τ₀ff γι / κ

wherein ki : splay elastic constant, k₂ : twist elastic constant and k₃ : bend elastic constant.

The present invention had the object of developing light modulation elements which switch particularly quickly. These light modulation elements should have the lowest possible layer thickness in order to allow to realise the smallest possible response time of the modulation media in order to be able to be employed as elements in FPDs (flat panel displays), such as, for example, flat panel screens for computers. In case, however, that the diffraction efficiency should be optimised, the cell gap should be chosen to be comparatively large. The displays should furthermore be addressable by means of a simple electrode configuration and have a relatively low operating voltage. In addition, they should have good contrast with low yiewing-angle dependence for use in electro-optical displays.

Liquid crystals having a nematic phase may show a cybotactic order. In this phase the molecules have a short-range smectic-like arrangement, as defined in Pure and Applied Chemistry, 73, (2001), pp, 845 to 895, see especially point 3.1.2, p. 855. Liquid crystals having a cybotactic nematic phase have been investigated with respect to their general properties e.g. by Bobadova-Parvanova, P. et al. Cryst. Res. Techno!., 35, (2000), pp. 1321-1330 and by Lϋning, J. et al., Macromolecules, 34, (2001), pp. 1128- 1130. Stojadinovich, S. et al., Physical Review E, 66, (2002), pp. 060701 -1 to 060701-4 investigated the dynamic behaviour of a nematic phase of a bent-core liquid-crystal showing a cybotactic order also referred to as a cybotactic phase here. In that reference dynamic light scattering experiments have been performed to determine the dynamics of the relaxation processes in this phase. However electro-optical applications are not envisaged at all.

Thus, there is a demand for improved light modulation elements, in particular for use in displays having short response times, as are necessary, for example, in multimedia applications. Present invention

Surprisingly, it has been found that light modulation elements which

- have at least one element for polarisation of the light,

- have an electrode arrangement which is able to generate an electric field, and

- contain a mesogenic medium,

- which is in the cyboactic nematic phase at least at one of the operation temperatures of the light modulation element

enable the production of excellent displays.

Preferably these light modulation elements

- are operated at a temperature at which the mesogenic medium is in the cybotactic nematic phase.

In general, the contrast, the viewing angle of the contrast and the operation voltage of these light modulation elements are excellent. Most strikingly, however, the response times of these light modulation elements are, in particular, very small.

The mesogenic medium is used as modulation medium of the light modulation element. In the present application, the term mesogenic media is applied to media which have a mesophase, are soluble in a mesophase or induce a mesophase. The mesophase is a smectic or, preferably, a nematic phase. Most preferably the media according to the present invention, respectively the media used according to the present invention exhibit a cybotactic nematic phase.

The medium used for investigating the mesogenic properties of materials, i.e. compounds components, like e.g. pre-mixtures and media, which do not have a mesophase of their own is preferably the nematic mixture ZLI-4792 from Merck KGaA, Darmstadt, Germany. The mesogenic materials preferably have a clearing point, extrapolated from 10% solution in this mixture, of 0°C or above, particularly preferably of 20°C or above and very particularly preferably of 40°C or above.

The light modulation elements according to the invention preferably contain a mesogenic modulation medium which is in the cybotactic nematic phase at the operating temperature or at least at one of the operation temperatures of the light modulation elements.

This medium is advantageously located on or below one substrate. In general, the mesogenic medium is located between two substrates. If the mesogenic medium is located between two substrates, at least one of these substrates is light-transparent. The light-transparent substrate or the light-transparent substrates may, for example, consist of glass, quartz or plastic. If a non-light-transparent substrate is used, this may consist, inter alia, of a metal or a semiconductor. These media can be used as such or can be located on a support, for example a ceramic.

If the mesogenic modulation medium itself is a polymeric medium, the use of a second substrate can, if desired, be omitted. Polymeric mesogenic media can even be produced in self-supporting form. In this case, no substrate is necessary at all.

The light modulation elements according to the present invention have an electrode structure which generates an electric field having a significant component, preferably a predominant component, perpendicular to the main plain of layer of the mesogenic medium. This electrode structure may be designed in the conventional form. It preferably comprises active addressing elements in order to allow addressing of the displays by means of an active matrix. Active matrix displays of this type use a matrix of active addressing elements preferably one each assigned to each one of the individual light modulation elements. These active addressing elements have a non-linear current-voltage characteristic line, such as, for example, TFTs or MIM (metal insulator metal) diodes. The electrodes may consist of transparent material, such as, for example, indium tin oxide (ITO). In this case, it may be advantageous and in some cases even necessary to cover part or parts of the light modulation element by means of a black mask. This allows areas in which the electric field is not effective, as well as areas where constituents of the light modulation elements, e.g. the active addressing elements, are light sensitive, to be screened off and the contrast, respectively the performance of the light modulation elements thus improved. However, the electrodes may also consist of opaque material, usually metal. In this case, the use of a separate black mask may, if desired, be omitted.

In a preferred embodiment, the sub-structures of the electrode structure are located on the two opposite sides of the mesogenic medium. In this case, the corresponding parts of the electrodes are preferably located at he opposite sides of the light modulation medium perpendicular to one another or at least overlapping each other to a significant extent, preferably to a predominant extent and most preferably essentially completely viewed perpendicularly from the main plane of the light modulation element.

The operating temperature of the light modulation element is preferably above the transition temperature of the modulation medium, generally from the solid phase or, preferably, from a smectic phase into the cybotactic phase, in general in the range from 0.1 ° to 40° preferably to 30° above this transition temperature, preferably in the range from 0.1 ° to 20° above this transition temperature and particularly preferably in the range from 0.1° to 10° above this transition temperature .

The light modulation elements according to the invention contain at least one element for polarisation of the light. In addition, they preferably contain a further optical element. This further optical element is either a second element for polarisation of the light, a reflector or a transflector or a combination of one or more of these elements.

The optical elements are arranged in such a way that the light, on passing through the mesogenic medium of the light modulation element, passes at least once through at least one polarising element both before entering the mesogenic medium and after exiting from the mesogenic medium.

In a preferred embodiment of the light modulation element in accordance with the present invention, the mesogenic medium is located between two polarisers, i.e. a polariser and an analyser. Two linear polarisers are preferably used. In this embodiment, the absorption axes of the polarisers are preferably crossed and preferably form an angle of 90°.

The light modulation element according to the invention optionally contains one or more birefringent layers. It preferably contains one λ/4 layers or a plurality of λ/4 layers, preferably one λ/4 layer. The optical retardation of the λ/4 layer is preferably about 140 nm.

The layer thickness (d) of the mesogenic modulation medium is preferably from 0.5 μm to 20 μm, particularly preferably from 0.6 μm to 10 μm, particularly preferably from 0.8 μm to 5 μm and very particularly preferably from 1 μm to 3 μm.

The light modulation element according to the present invention may additionally contain one or more further optical elements, such as birefringent layers (for example compensation layers), diffuser layers and elements for increasing the brightness and/or the light yield, or viewing- angle dependence, where this list is not exhaustive.

The contrast of the light modulation elements according to the present invention is well comparable with that of respective light modulation elements operating in the same cell structure, respectively electro-optical mode. If it is found to be slightly inferior in cases, it can be easily improved by applying respectively higher operation voltages. Their outstanding properties, however, are their improved response times.

The light modulation elements according to the present invention my be, and preferably are, constructed like conventional light modulation elements operating in the TN mode, i.e. like typical TN cells. Preferably they have an optical retardation in the range of 0.35μm to 0.50μm, positive contrast and the absorption axis of the polarisers perpendicular to the preferential alignment of the nematic, respectively cybotactic nematic liquid crystals at the adjacent substrate.

Preferably the mesogenic modulation media for non-display applications are non chiral. In such switching devices, such as spatial light modulators (SLM), high-speed electro-optic shutters for the communication technology, as described in Chang, M.-S., Panarin, Yu.P., and Farrell, G. SPIE 2002, the switching time is much shorter than that of those typically used in display application.

The response times of the light modulation elements according to the present invention however are very short. They are generally at values of 15 ms or less, preferably at 12 ms or less and particularly preferably at 10 ms or less.

Switching between different grey shades of the light modulation elements according to the preset invention is also comparatively faster than in conventional light modulation elements using the same electro-optical mode.

Electro-optical displays according to the present invention contain one or more light modulation elements according to the present invention. In a preferred embodiment, these are addressed by means of an active matrix.

In another preferred embodiment, the light modulation elements according to the present invention are addressed in so-called "field sequential mode". Here, the switching elements may be illuminated successively with light of different colours synchronously to the addressing. In order to produce the pulsed coloured light, a colour wheel, stroboscope lamps or flash lamps, for example, can be employed.

Electro-optical displays according to the present invention may, in particular if they are used for television screens, computer monitors or the like, contain a colour filter for the display of coloured images. This colour filter advantageously consists of a mosaic of filter elements of different colours. Typically, an element of the colour filter mosaic of a colour is assigned to each electro-optical light modulation element.

In a preferred embodiment of the present invention mesogenic media in accordance with the present invention have a nematic phase besides the cybotactic nematic phase. However, it is also possible to use media in which the temperature range of the nematic phase is so narrow that in practical terms a transition from the cybotactic nematic phase into the isotropic phase takes place.

In a preferred embodiment of the present invention mesogenic media in accordance with the present invention have a cybotactic nematic phase which extends over a range having a width of 10° or more, preferably of 30° or more and most preferably of 50° or more. Preferably this range of the cybotactic nematic phase is extending around and preferably centered around the operation temperature of the light modulation element, preferably around ambient temperature and most preferably at a temperature of about 20°C.

In the case that the operating temperature of the light modulation elements is different from the ambient temperature, the temperature range of the cybotactic phase encompasses and preferably is centered around the operating temperature. In several cases the operation temperature of the light modulation elements is higher than the ambient temperature. This is e.g. generally the case in of displays having back-lighting. In particular in the case of projection displays, the temperature during operaition may rise up to 60°C or even more. Here, the clearing point of the media according to the present invention, respectively the transition temperature T(Nc,l), is preferably in the range from 10°C to 70°C and particularly preferably in the range from 20°C to 50°C.

The cybotactic nematic phase of the mesogenic modulation media according to the present invention is preferably stable over a temperature range of 1° or more, preferably of 3° or more, more preferably of 5° or more, most preferably of 10° or more and in particular preferably of 20° or more. The mesogenic modulation media according to the present invention preferably do not crystallize down to temperatures of 10°C or lower, particularly preferably down to temperatures of 0° or lower, and very particularly preferably down to temperatures of -20°C or lower.

In many cases, mesogenic mixtures showing a cybotactic nematic phase allow the determination of macroscopic parameters such as e.g. Δn, Δε, γi, and kj directly in this cybotactic nematic phase, as if these mixtures were still in the ordinary nematic phase.

The presence of the cybotactic phase for the mesogenic modulation media can be detected by several means like e.g. by X-ray scattering, differential scanning calorimetry (DSC), the observation of different divergence behaviour between the viscosity constants, especially of γi, and the elastic constants of the medium and by observation of the response times in electro-optical cells, e.g. in TN cells.

In DSC the presence of the cybotactic nematic phase typically is obvious from the presence of a transition with an unusually broad peak with low heat flow.

Preferably the presence of the cybotactic nematic phase for the mesogenic modulation media is be detected by observation of the response times in TN cells and/or by the observation of different divergence behaviour between the viscosity constants and the elastic constants of the medium.

According to the present invention preferably both the. last two independent methods are used to detect the presence of the cybotactic nematic phase used.

The cybotactic nematic phase is characterised by larger elastic constants compared to the nematic phase. With decreasing temperature the rotationel viscosity (γ-i) of the media increases following the exponential dependence according to Arrhenius' law. This behaviour even extends to temperatures below the temperature of the transition from the nematic phase into the cybotactic nematic phase. In contrast to this behaviour, the elastic constants of the media increase significantly upon occurrence of the cybotactic nematic phase. This effect is especially pronounced for k₃. Typically the elastic constants are 50 % or even up to three times larger than those of the media in the nematic phase, while viscosity still follows the Arrhenius' law.

Preferably the liquid crystalline media in the cybotactic nematic phase have elastic constants, preferably ki or k₃, most preferably k₃, which are larger by 20 % or more, preferably by 50 % or more and most preferably by 200 % or more, than those of comparative nematic media which are in the nematic phase at the given temperature.

Preferably the liquid crystalline media, preferably in the cybotactic nematic phase, have large elastic constants at the operating temperatue, preferably at 20°C. Preferably ki is 11 pN or more, more preferably 13 pN or more, more preferably 15 pN or more, still more preferably 20 pN or more and most preferably 25 pN or more and/or k₃, preferably is 14 pN or more, more preferably 16 pN or more, still more preferably 20 pN or more, even more preferably 30 pN or more and most preferably 40 pN or more.

The liquid crystalline media preferably have, preferably in the cybotactic nematic phase, a small ratio of the rotational viscosity and the elastic constants, preferably the elastic constant k₃. Preferably γ-|/k₃ is 4.0 mPa-s/pN or less, more preferably 3.5 mPa-s/pN or less, more preferably 3.0 mPa-s/pN or less, more preferably 2.5 mPa-s/pN or less and most preferably 2.0 mPa-s/pN or less.

The mesogenic media in accordance with the present invention preferably have a birefringence (Δn) at 20°C in the range of 0.06 or more to 0.35 or less, particularly preferably of 0.065 or more to 0.30 or less, very particularly preferably of 0.07 or more to 0.25 or less. In practical terms, however, it is generally 0.530 or less and usually 0.450 or less.

For the light modulation elements according to the present invention, it is alternatively possible to use either mesogenic modulation media which have a positive dielectric anisotropy (Δε) in the mesophase or those which have a negative dielectric anisotropy. Preference is given to the use of mesogenic modulation media which have a positive dielectric anisotropy (Δε) in the mesophase.

If the mesogenic modulation media have a positive dielectric anisotropy, this has a value of preferably 3.5 or more, particularly preferably 4.0 or more and very particularly preferably 5.0 or more at 1 kHz and 20°C.

If the mesogenic modulation media have negative dielectric anisotropy, this has a value of preferably -2.5 or less, particularly preferably -3.0 or less and very particularly preferably -4.0 or less.

The mesogenic media in accordance with the present invention preferably consist of several compounds, preferably of from two to 40 compounds, particularly preferably from five to 30 compounds and very particularly preferably from seven to 25 compounds.

The mesogenic media according to the invention of positive dielectric anisotropy in accordance with the present invention preferably comprise

- a dielectrically neutral component A, preferably comprising one or more dielectrically neutral biphenyl compounds of formula I

wherein

R¹¹ is alkyl or alkoxy with 1 to 15 C atoms or alkoxyalkyl, alkenyl, alkenyloxy with 1 to 15 C atoms, preferably alkyl . or alkoxy, and

R¹² is alkenyl with 2 to 15, preferably 2 to 7, C atoms,

- optionally a dielectrically positive component, component B, preferably comprising of one or more compounds with positive dielectric anisotropy bearing a terminal haloge n atom or a terminal halogenated group, preferably of formula II

wherein

R^ is n-alkyl or n-alkoxy, each having from 1 to 7 carbon atoms, or alkenyl, alkenyloxy, alkynyl or alkoxyalkyl, each having from 2 to 7 carbon atoms, preferably n- alkyl or alkenyl, prefe rably n-alkyl, vinyl or 1-E-alkenyl, preferably R³¹ is n-alkyl and R³² is vinyl or 1-E-alkenyl,

t each, independently of one another, are

and more preferably at least one is, m ost preferably at least two are

X c2 is halogen, preferably F or Cl, or fluorinated alkyl or fluorinated alkoxy, each with 1 to 4 C-atoms, or fluorinated alkenyl or fluorinated alkenyloxy, each with 2 to 4 C-atoms, preferably F, Cl, OCF₃ or OCF₂H, more preferably F or Cl and most preferably F,

Y²¹, Y²² and Y²³ are, independently of each other, H or F, preferably, however, if Y²¹ and/or Y²² is F, Y²³ is H and preferably Y²¹ is F,

Z²¹ and Z²² are each, indepenently of one another, is a single bond, -CH₂-CH₂-, -CO-O-, trans -CH=CH-, -CH=CF-, -CF=CH-, -CF=CF-, -CH=CH-CO-0-, -CF=CF-CO-O-, -CF=CH-CO-O-, -CH=CF-CO-O-, -CF₂-0-, -O-CF₂- , -CH2-O-, -O-CH₂- or -C≡C-, preferably a single bond, -CH2-CH₂- or -CF2-O-, more preferably Z²¹ is a single bond and Z ₇22 is a single bond or -CF₂-0-, and is 0, 1 or 2, preferably 0 or 1 , most preferably 1 , optionally a second dielectrically neutral component, component C comprising one or more compounds of formula III

wherein

R .31^> and R 32 are each, indepenently of one another, n-alkyl or n- alkoxy, each having from 1 to 7 carbon atoms, or alkenyl, alkenyloxy, alkynyl or alkoxyalkyl, each having from 2 to 7 carbon atoms, preferably n-alkyl or alkenyl, preferably n-alkyl, vinyl or 1 -E-alkenyl, preferably R³¹ is n-alkyl and R³² is vinyl or l-E-alkenyl,

each, independently of one another, are

and more preferably at least one is, most preferably at least two are

Z³ and Z³² are each, indepenently of one another, is a single bond, -CH₂-CH₂-, -CO-O-, trans -CH=CH-, -CH=CF-, -CF=CH-, -CF=CF-, -CH=CH-CO-0-, -CF=CF-CO-O-, -CF=CH-CO-0-, -CH=CF-CO-0-, -CF₂-0-, -O-CF₂- , -CH₂-O-, -O-CH₂- or -O≡C-, preferably a single bond, -CH2-CH₂- or trans -CH=CH-, preferably a single bond or trans -CH=CH-, preferably a single bond and

m is 0, 1 or 2, preferably 0 or 1 , preferably 0, from which compounds of formula I are excluded. Component A of these media preferably comprises one or more compounds of the formula I and particularly preferably consists predominantly and very particularly preferably consists virtually co pletely of one or more compounds of the formula I.

Preferred compounds of formula I are those wherein R¹¹ is alkyl with 1 to 8 C atoms. Very preferably R¹¹ is methyl, ethyl or propyl. in particular methyl.

Further preferred compounds of formula I are those wherein R² is vinyl, 1 E-propenyl, 1 E-butenyl, 3E-butenyl or 3E-pentenyl, in particular 3E-butenyl or 3E-pentenyl.

Very preferred compounds of formula I are compounds selecte from the group of formulae la and lb

wherein

Alkyl is an alkyl group with 1 to 8 C atoms, in particular methyl,

Alkenyl is an alkenyl group with 2 to 7 C atoms, prefe rably vinyl or 1 E-alkenyl, in particular vinyl or 1 E-propenyl, and

R 12a is H, methyl, ethyl or n-propyl, in particular methyl. Component B of these media preferably comprises one or more compounds of the formula II and particularly preferably it consists predominantly and very particularly preferably it consists virtually completely of one or more compounds of the formula II.

Very preferred compounds of formula II are compounds selected from the group of formulae 11-1 to 11-12

wherein the parameters have the respective meanings given under formula II above and preferably

R is alkyl or alkenyl with 1 , respectively 2 to 7 C-atoms,

X² F, Cl or fluorinated alkyl or fluorinated alkoxy, each with 1 to 4 C-atoms, most preferably F.

Very preferred compounds of formula II are compounds selected from the group of formulae 11-1 a to 11-12d

wherein the parameters have the respective meanings given under formula II and its respective sub-formulae above. Component C of these media preferably comprises one or more compounds of the formula 111 and particularly preferably consists predominantly and very particularly preferably consists virtually completely of one or more compounds of the formula III.

Very preferred compounds of formula III are compounds selected from the group of formulae 111-1 -1 o 111-3-4

wherein the parameters have the respective meanings given under formula HI and its respective sub-formulae above and _γ ³ⁱ _tQ γ³s _{are each(} indepenently of one another, H or F and preferably, R³¹ and R³² are each, indepenently of one another, n-alkyl, n-alkoxy, each having from 1 to 7 carbon atoms, or alkenyl, having from 2 to 7 carbon atoms, preferably n-alkyl or alkenyl, more preferably n-alkyl, n-alkoxy, vinyl or 1-E- alkenyl, most preferably R³¹ is n-alkyl or alkenyl and R³² is n- alkyl, n-alkoxy, vinyl or 1 -E-alkenyl and at most half of the substituents Y³¹ to Y³⁸ present are F, more preferably none, one two or three and most preferably none, one or two, of these substituents are F. Very preferred compounds of formula III are compounds selected from the group of formulae 111-1 -1 a to lll-3-3c

wherein the parameters have the respective meanings given under formula III and its respective sub-formulae above and in particular preferably

in formula lll-1 -1 a

R³¹ is n-alkyl, or alkenyl and R³² is n-alkyl, alkenyl or n-alkoxy,

in formula 111-2-1 a

R³¹ is n-alkyl, or alkenyl and

R 32 is n-alkyl.

In a referred embodiment of the present invention the media according to the invention preferably comprise one or more compounds selected from the group consisting of the compounds of the formulae 1-1 to 1-3, which are sub-formulae of the formula I,

wherein p is an integer from 0 to 10, preferably from 1 to 7, preferably 1 or 2, q is an integer from 0 to 10, preferably from 0 to 3, preferabyl 0 or 1 , k is an integer from 0 to 8, preferably from 0 to 4, preferabyl 0 or land o is an integer from 1 to 10, preferably rfom 1 to 5, preferabyl 1 , 2 or 3.

The mesogenic media of positive dielectric anisotropy in according to the present invention particularly preferably consist predominantly of components A to C.

In a preferred embodiment, the mesogenic media of positive dielectric anisotropy in accordance with the present invention comprise one or more components selected from the group consisting of components B and C, preferably selected from the group consisting the compounds of formulae 11-1 a, 11-1 c, 11-1 d, ll-2a, ll-2c, ll-3d, ll-5a, ll-5b, ll-6b, 11-10a, 11-10b and ll-12d and lll-1 -1 a, lll-1-2a, 111-2-1 a, lll-2-3b, lll-3-2a, lll-3-2c, lll-3-3a and lll-3-3b.

In an embodiment of the present invention the medium has a negative dielectric anisotropy. These media according to the present invention preferably comprise

- a component A' consisting of one or more compounds having a highly negative dielectric anisotropy of -5 or less, optionally a component B' consisting of one or more compounds having moderately negative dielectric anisotropy of from -1.5 to < -5, - optionally a component C consisting of one or more dielectrically neutral compounds having a dielectric anisotropy of from -1.5 to +1.5, and

- optionally a component D' consisting of one or more compounds having a positive dielectric anisotropy of greater than +1.5.

The mesogenic medium according to the present invention may comprise further additives and chiral dopants in the usual concentrations. The total concentration of these further constituents is in the range from 0% to 10%, preferably in the range from 0.1% to 6%, based on the entire mixture. The concentrations of the individual compounds of these are in the range from 0.1 to 3%. The concentrations of these compounds and similar constituents of the mixture are not taken into account when specifying the concentration ranges of the other mixture constituents.

The modulation media are obtained in the usual manner from the compounds. The compounds employed in smaller amounts are advantageously dissolved in the compounds employed in larger amounts. If the temperature during the mixing operation is increased above the clearing point of the predominant component, the completeness of the dissolution can easily be observed. However, the media according to the invention can also be prepared in other ways, for example by using pre- mixtures. Pre-mixtures which can be employed are, amongst others, mixtures of homologous compounds and/or eutectic mixtures. However, the pre-mixtures may also already be usable modulation media themselves. This is the case in so-called two-bottle or multi-bottle systems.

In the present application, the following applies, unless explicitly stated otherwise.

Dielectrically positive compounds have a Δε of > 1.5, dielectrically neutral compounds have a Δε in the range from -1.5 < Δε < 1.5 and dielectrically negative compounds have a Δε of < -1.5. The same definitions also apply to components of mixtures and to mixtures. The same definition also applies to components and media. The dielectric anisotropy Δε of the compounds, as well as of those components, which can not be investigated as such, is determined at 1 kHz and 20°C by extrapolation of the values of a 10% solution of the respective compound in a host mixture to a proportion of the respective compound of 100%. The capacitances of the test mixtures are determined both in a cell having homeotropic edge alignment and in a cell having homogeneous edge alignment. The layer thickness of the two cell types is about 20 μ . The measurement is carried out using a rectangular wave having a frequency of 1 kHz and an effective voltage (rms, root mean square) of typically from 0.2 V to 1.0 V. However, in every case, the voltage used is lower than the capacitive threshold of the mixture investigated in each case.

For dielectrically positive compounds, the mixture ZLI-4792 is used and for dielectrically neutral and dielectrically negative compounds, the mixture ZLI-3086, both from Merck KGaA, Germany, is used as host mixture.

The term threshold voltage in the present application means the optical threshold and is indicated for a relative contrast of 10% (V₁₀). The mid-grey voltage and the saturation voltage are likewise determined optically and indicated for a relative contrast of 50% and 90% respectively. If the capacitive threshold voltage (V₀), also known as the Freedericks threshold, is indicated, this is stated explicitly.

The indicated ranges of values preferably include the limit values.

The concentrations are given in % by weight and are based on the complete mixture. Temperatures are indicated in degrees Celsius (°C) and temperature differences in differential degrees Celsius (°). All physical properties were determined as described in "Merck Liquid Crystals, Physical Properties of Liquid Crystals", Version of Nov. 1997, Merck KGaA, Germany, and are indicated for a temperature of 20°C. The optical anisotropy (Δn), also known as the birefringence, is determined at a wavelength of 589.3 nm. The dielectric anisotropy (Δε) is determined at a frequency of 1 kHz. In a list of possible alternatives, where only the plural is indicated, this also means the singular.

In connection with details on the composition of the media or their components,

- "comprise" means that the concentration of the respective material mentioned, i.e. of the component or of the compound, in the reference unit, i.e. the medium or the component, is preferably 10% or more, particularly preferably 20% or more and very particularly preferably 30% or more,

- "consist predominantly of" means that the concentration of the said material in the reference unit is preferably 50% or more, particularly preferably 60% or more and very particularly preferably 70% or more, and

- "consist virtually completely of" means that the concentration of the said material in the reference unit is preferably 80% or more, particularly preferably 90% or more and very particularly preferably 95% or more.

The dielectric properties, electro-optical properties (for exam pie the threshold voltages) and the response times were determined in test cells produced at Merck KGaA, Darmstadt, Germany. The test cells for the determination of Δε had a layer thickness of 22 μm and a circular electrode of indium tin oxide (ITO) having an area of 1.13 cm² and a protective conducting ring. For homeotropic alignment for the determin-ation of n cells having a homeotropically aligning polyimide alignment I ayer were used. Alternatively, lecithin (Merck KGaA) can be used as alignment agent. The cells for the determination of εi had alignment layers of the polyimide AL-3046 from Japan Synthetic Rubber, Japan. The capacita-nces were measured using a HP 4274A LCR Meter of Hewlett Packard , USA with a rectangular wave and an effective voltage of generally 0.3 V_rms. The electro-optical investigations were carried out with white light. The characteristic voltages were determined with perpendicular observation. In the present application, particularly in the examples described below, the structures of the chemical compounds are indicated by means of abbreviations. The meanings of the respective abbreviations are shown in Tables A and B below. All groups C_nH_2n+1 and C_mH_2rtH_ι are straight-chain alkyl groups having n and m carbon atoms respectively. Table B is self- evident per se since it indicates in each case the complete abbreviation for a formula of homologous compounds. In Table A, only the abbreviations for the core structures of the compound types are sho /vn. The abbrevia- tions for the respective individual compounds are com posed of the respectively pertinent abbreviations of these for the core of the compound and the abbreviation for the groups R¹, R², L¹ and L², shown in the following table, attached by means of a dash.

Abbreviation Rⁱ R2 Lⁱ l_2

nOm nH₂n+1 OCmHa +i H H nO.m OC_nH_2n+ι _mH2 +l H H n C_nH₂n+1 CN H H nON.F.F OC_nH_2n+ι CN F F nOF OC_nH_2n+ι F H H nCI C_nH₂n+1 Cl H H

nOCF₃ C_nH₂n+1 OCF₃ H H nOCF₃.F.F C_nH₂n+l OCF3 F F nOCF₂ C_nH₂n+1 OCHF₂ H hi nOCF₂.F C_nH_2n+i OCHF₂ H F nOCF₂.F.F C_πH_2n+ι OCHF₂ F F nS.F C_nH₂n+1 NCS H F rOsN C_rH2r+rO-C₃H2s- CN H HI nAm C_nH_2n+i COOC_mH2_m+ι H H Table A:

PYP PYRP

PPYRP

BCH CCP

CPTP

CEPTP

D EPCH

HP ME

PCH PDX

PTP BECH

EBCH

EHP ET

Table B:

C_nH_2n+1-(0) ^C _nH_2n+ (0) 0-C_mH_2m+1

PCH-n(O)mFF PY-n(O)-Om

YY-n(O)-Om

YY-Vn(O)-OmV

CCP-n(O)mFF

CPY-n(O)-m

CYY-n-(O)m

CCYY-n-(O)m

PTP-n(O)mFF

CPTP-n(O)mFF

CGP-n-X (X = particularly F, Cl, CN, NCS)

Inm

CGU-n-X (X = particularly F, Cl, CN, NCS) C15

CB15

^Cn^H2n+1 \ / — \°7 — \° / — \ / ~^{C H}2 +1 CBC-nm

CBC-nmF

CHE

ECBC-nm

GP-nO-N

CP-V-N

CCP-V-m

CCP-nV-m

CCP-V2-m

CCP-nV2-m CPP-V-m

CPP-nV-m

CPP-V2-m CPP-nV2-m

CCG-V-F

CCG-nV-F

G3n ^{K3 n}

M3n

T3n

BB3-n

PGIP-n-N

PVG-n-S

PVG-nO-S

UPP-n-S

C_nH_2n+1— < O < O _C≡C— ( O V-C_mH_2m+1

PPTUI-n-m CPU-n-S

CGU-n-S

PTG-n-S

C„H "2-n+Γ1 < O >— C---≡C ( O V- NCS

PTU-n-S

PPVP-n-S

PPVG-n-S

PPVU-n-S

PTG-n(O)-N

^Cn 2n+1 -(O)- o ^'-C ( O )— CN

PTU-n(O)-N

PU-n-AN

GZU-n(O)-N

UZU-n(O)-N

^Cn^H2n+1 ^C: ≡C— < O ) — CO— O ( O )-CN

GZU-nA-N

UZU-nA-N

UVZG-n-N

PWZU-3-N CUZU-n-N

CCZU-n-F

CCQU-n-F

PUQU-n-F

PGU-n-F PGP-n-m

UM-n-N

DU-n-N CC-n-V CC-n-Vm

PP-n-2V

PP-n-2Vm

The mesogenic media in accordance with the present application preferably comprise

- two or more, preferably four or more, compounds selected from the group consisting of the compounds of Tables A and B. Examples

The examples described below illustrate the present invention without restricting it in any way. They furthermore indicate to the person skilled in the art the properties and in particular the property combinations that can be achieved by means of the present invention.

Example 1

A liquid-crystal mixture of the following composition is prepared and investigated.

The following table (table 1) shows the temperature dependence of some characteristic physical properties of this mesogenic mixture. Table 1 : Temperature dependence of physical properties of example 1

Remaks: n.d.: not determined.

An electro-optical test cell of the TN type with 90° twist containing a light modulation element containing the liquid-crystal mixture is produced. The substrates consists of glass. Substrates with rubbed AL-3046 as alignment layer are used. The layer thickness of the cell, i.e. of the modulation medium, is 4.72 μm. The optical retardation (d-Δn) is 0.50 μm.

A first polariser was used before the cell and a second polariser (analyser) is used after the cell. The absorption axes of the two polarisers form an angle of 90° to one another. The angle between the axis of maximum absorption of the polarisers and rubbing direction at the respective adjacent substrate of the cell is 0°. The voltage-transmission characteristic line was determined using a DMS 703 electro-optical measurement station from Autronic-Melchers, Karlsruhe, Germany. The operating temperature is varied from 20°C to 22°C.

The electro-optical modulation element is characterised by extremely short response times. It has been operated switching a rectangular waveform with a frequency of 60Hz from 0 V to a voltages of 5 V_rms.

It is easily seen that in case of the mixture of comparative example 1 T₀ff significantly increases with decreasing temperature, while for the mixture of example 1 in Ex.1 it remains almost constant (and is even smallest at 20°C). Table 2: Temperature dependence of response times τ_0ff of examplel and of comparative examples 1 and 2

Remaks: n.d.: not determined, V₀p = 5 V to 0 V

Table 3: Temperature dependence of elastic constants of example 1 and of comparative examples 1 and 2

Remaks: n.d.: not determined,

As can be seen in table 3, the elastic constants ki and k₃ of the mixture of comparative example 1 increases only slightly with decreasing temperature, whereas in case of of the mixture of example 1 a drastic increase both of ki and k₃ is observed. This might explain the favourable behaviour of the response time τ_0ff of example 1. Comparative Example 1

A liquid-crystal mixture of the following composition is prepared and investigated.

This liquid crystal medium shows nearly the same values for the physical properties T(N, I), Δn, Δε and γ-ι at 20°C as example 1. However, this medium is not showing a cybotactic nematic phase. It was found to have much smaller elastic constants than example 1. E.g. at a temperature of 20°C k₃ is smaller by a factor of 2.6 than that of the medium of example 1 and ki is smaller by a factor of 1.9.

The response time for switching off ( τ_0ff) of an electro-optical modulation element in the TN mode containing this medium at 20°C is rather fast with 9.7 ms. This value, however, is much larger than that of the element of example 1 with 6.4 ms, compare Table 2. Comparative Example 2

A liquid-crystal mixture of the following composition is prepared and investigated.

This liquid crystal medium, like that of comparative example 1 , has values of the physical properties T(N, I), Δn, Δε and γi ( the latter three at 20°C) which are very similar to those of example 1. Here Δn is a little higher than that of the medium of example 1 and γi is a little higher, too.

Response time of the respective light modulation element is 12.8 ms and thus even larger than that of the elememt of comparative example 1 (compare table 1). Example 2

A liquid-crystal mixture of the following composition is prepared and investigated.

Remaks: n.d.: not determined.

The following table (table 4) shows the temperature dependence of some characteristic physical properties of this mesogenic mixtu re.

An electro-optical test cell is prepared as described under example 1. Now however the call gap used is 3.2 μm. This cell, like that of example 1 , has very favourable properties. It is especially characterised by very fast response times, in particular at a temperature in the range from 10°C to 14°C. Table 4: Temperature dependence of physical properties of example 2

Remaks: n.d.: not determined.

Claims

Patent Claims

1. Electro-optical light modulation element comprising - an electrode arrangement, - at least one element for polarisation of light, and - a mesogenic modulation medium, characterised in that the light modulation element is operated at a temperature at which the mesogenic modulation medium is in the cybotactic nematic phase.

2. Light modulation element according to Claim 1 , characterised in that the electrode arrangement is able to generate an electric field having a significant component perpendicular to the surface of the mesogenic modulation medium.

3. Light modulation element according to at least one of Claims 1 and 2, characterised in that the mesogenic modulation medium has a nematic phase.

4. Light modulation element according to at least one of Claims 1 to 3, characterised in that the electrode arrangement during operation of the light modulation element generates an electric field having a significant component parallel to the plane of the mesogenic modulation medium.

5. Light modulation element according to at least one of Claims 1 to 4, characterised in that the light during passage through the light modulation element in each case passes through at least one element for polarisation of the light before passing through the mesogenic modulation medium and after passing through the mesogenic modulation medium.

6. Light modulation element according to at least one of Claims 1 to 5, characterised in that the electrode comprises at least two parts at least one part each of which arrangement is located on either of the two sides of the layer of the mesogenic modulation medium.

7. Light modulation element according to at least one of Claims 1 to 6, characterised in that it contains an additional birefringent layer.

8. Electro-optical display comprising one or more light modulation elements according to at least one of Claims 1 to 7.

9. Electro-optical display according to Claim 8, characterised in that the display is addressed by means of an active matrix.

10. Electro-optical display system containing one or more electro-optical displays according to at least one of Claims 8 and 9.

11. Electro-optical display system according to Claim 10, characterised in that it can be used as a television monitor and/or as a computer monitor.

12. Use of a light modulation element according to at least one of Claims 1 to 6 for the display of information.

13. Use of an electro-optical display according to at least one of Claims 8 and 9 in an electro-optical display system.

14. Use of an electro-optical display system according to at least one of Claims 10 and 11 for the display of video signals.

15. Mesogenic modulation medium, characterised in that it has a cybotactic nematic phase.

16. Mesogenic modulation medium according to Claim 15 for use in a light modulation element according to at least one of Claims 1 to 6.

7. Use of a mesogenic modulation medium according to Claim 15 in an electro-optical light modulation element.